Electrically operated braking system having a device for operating electric motor of brake to obtain relationship between motor power and braking torque

An electrically operated braking system of a motor vehicle, including a brake having a friction member movable to be forced onto a rotor rotating with a vehicle wheel and thereby braking the wheel. The friction member is forced onto the rotor by an electric motor operated by an electric power supplied from an electric power source. A controller determines an amount of the electric power to be supplied to the motor, depending upon an operating amount of the brake operating member, and a relationship estimated by a relationship estimating and utilizing device. The relationship estimating and utilizing device obtains an actual value of the electric power supplied to the motor during operation of the brake while the vehicle is running, and an actual value of a braking torque applied from the brake to the wheel during the brake operation, and estimate a relationship between the electric power to be supplied to the wheel on the basis of the actual values obtained. The estimated relationship is then utilized to control the braking torque to be generated in response to operation of the brake operating member.

This application is based on Japanese Patent Applications Nos. 9-203454 and
 9-341290 filed July 29 and Dec. 11, 1997, respectively, the contents of
 which are incorporated hereinto by reference.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an electrically operated braking system of
 a motor vehicle, which includes a brake employing an electric motor as a
 drive source.
 2. Discussion of the Related Art
 An electrically operated braking system of a motor vehicle generally
 includes (a) a brake operating member such as a brake pedal, which is
 operated by an operator of a motor vehicle, (b) an electric power source
 such as a battery, (c) a brake including an electric motor which is
 operated by an electric power supplied from the electric power source, to
 generate a drive force for forcing a friction member onto a rotor rotating
 with a wheel of the vehicle, for thereby braking the wheel, and (d) a
 controller which determines an amount of the electric power to be supplied
 from the electric power source to the electric motor, depending upon an
 operating amount of the brake operating member, for thereby controlling an
 operation of the brake.
 A friction coefficient of friction members such as brake pads or brake
 linings used in a brake generally varies due to gradual deterioration or
 wearing of the friction members, or due to temperature or humidity of the
 friction members. Further, the friction members have different friction
 coefficient values due to variations associated with the manufacture. In
 an electrically operated braking system as described above, a change in
 the friction coefficient of the friction members will cause a change in
 the relationship between the amount of electric power actually supplied
 from the electric power source to the electric motor of the brake and the
 braking torque actually applied from the brake to the corresponding wheel.
 In other words, the relationship between the amount of electric power
 supplied to the motor and the braking torque generated by the brake is not
 necessarily held constant.
 However, the conventional electrically operated braking system does not
 obtain and utilize the actual relationship between the supplied amount of
 electric power and the braking torque generated by the brake. That is, the
 conventional system uses a predetermined nominal relationship between the
 electric power amount and the braking torque, to determine the amount of
 electric power to be supplied to the electric motor, depending upon the
 operating amount of the brake operating member, on the assumption that the
 relationship is held constant. Therefore, the conventional electrically
 operated braking system is not capable of accurately controlling the
 braking torque of the brake in relation to the operating amount of the
 brake operating member.
 SUMMARY OF THE INVENTION
 It is an object of this invention to provide an electrically operated
 braking system which obtains an actual relationship between the amount of
 electric power supplied to an electric motor and the braking torque
 generated by the brake, and utilizes the obtained relationship.
 The above object may be achieved according to any one of the following
 modes or embodiments of the present invention, which are numbered to
 indicate possible combinations of various features of the invention:
 (1) An electrically operated braking system of a motor vehicle having a
 wheel, comprising: (a) a rotor rotating with the wheel; (b) a brake
 operating member which is operated by an operator of the motor vehicle;
 (c) an electric power source; (d) a brake including a friction member
 movable to be forced onto the rotor, and an electric motor which is
 operated by an electric power supplied from the electric power source, to
 generate a drive force for forcing the friction member onto the rotor and
 thereby braking the wheel; and (e) a controller which determines an amount
 of the electric power to be supplied from the electric power source to the
 electric motor, depending upon an operating amount of the brake operating
 member, for thereby controlling an operation of the brake, the braking
 system being characterized by further comprising (e) a relationship
 estimating and utilizing device for obtaining an actual value of the
 electric power supplied from the electric power source to the electric
 motor during an operation of the brake while the motor vehicle is running,
 and an actual value of a braking torque applied from the brake to the
 wheel during the operation of the brake. The relationship estimating and
 utilizing device is adapted to estimate a relationship between the
 electric power to be supplied to the electric motor and the braking torque
 to be applied to the wheel, on the basis of the actual values obtained,
 and utilize the relationship. The relationship is formulated such that the
 braking torque to be applied to the wheel is changed with a change in the
 electric power to be supplied to the electric motor.
 In the braking system constructed according to the above mode (1) of the
 present invention, the relationship estimating and utilizing device is
 capable of obtaining the relationship between the electric power to be
 supplied to the electric motor of the brake and the braking torque to be
 applied from the brake to the wheel. The relationship estimating and
 utilizing device is further adapted to utilize the obtained relationship
 for various purposes, for instance, for controlling the brake and
 providing the vehicle operator with information helpful to operate the
 vehicle.
 The electric power to be applied to the electric motor may be expressed by
 a voltage or current. The electric motor of the brake may be a ultrasonic
 motor or DC motor. The operating amount of the brake operating member may
 be expressed as an operating force acting on the brake operating member,
 or an operating stroke or distance of the brake operating member. The
 operation of the brake while the vehicle is running may be a normal
 operation initiated by an operation of the brake operating member by the
 vehicle operator, or a special operation which is performed without an
 operation of the brake operating member, for the sole purpose of
 estimating the above-indicated relationship.
 (2) An electrically operated braking system according to the above mode
 (1), wherein said relationship is a pattern of change of said braking
 torque with a change of said electric power, and said relationship
 estimating and utilizing device comprises relationship estimating means
 for selecting one of a plurality of candidate patterns which corresponds
 to a combination of the actual values obtained during the operation of the
 brake while the motor vehicle is running.
 (3) An electrically operated braking system according to the above mode
 (2), wherein said relationship estimating means includes pattern memory
 means for storing the above-indicated plurality of candidate patterns, and
 pattern selecting means for selecting the above-indicated one of the
 plurality of candidate patterns stored in the pattern memory means.
 (4) An electrically operated braking system according to any one of the
 above modes (1)-(3), wherein the relationship estimating and utilizing
 device obtains the above-indicated relationship from a plurality of
 different relationships between the electric power to be supplied to the
 electric motor and the braking torque to be applied to the wheel.
 In the braking system according to the above mode (4), the control load of
 the relationship estimating and utilizing device is made smaller than in
 the braking system in which the relationship is estimated in a continuous
 fashion.
 (5) An electrically operated braking system according to any one of the
 above modes (1)-(4), wherein the controller includes the relationship
 estimating and utilizing a device, and the relationship estimating and
 utilizing device includes relationship utilizing means for determining
 desired value of the braking torque on the basis of the operating amount
 of the brake operating member, and determining the value of the electric
 power to be supplied to the electric motor, on the basis of the determined
 desired value of the braking torque and according to the estimated
 relationship.
 In this braking system, the electric power to be supplied to the electric
 motor during an operation of the brake is determined according to the
 actual relationship between the electric motor and the braking torque, as
 well as depending upon the operating amount of the brake operating member,
 so that the brake can be accurately controlled so as to provide the
 braking torque corresponding to the operating amount of the brake
 operating member, irrespective of a variation in the friction coefficient
 of the friction members used in the brake.
 (6) An electrically operated braking system according to any one of claims
 1-5, wherein the relationship estimating and utilizing device includes
 means for supplying a predetermined amount of the electric power from the
 electric power source to the electric motor for a predetermined length of
 time to thereby activate the brake while the motor vehicle is running
 without an operation of the brake operating member, and obtaining the
 actual values of the electric power and the braking torque during
 activation of the brake.
 In the braking system according to the above mode (6), the brake is
 activated for the sole purpose of estimating the relationship, while the
 brake operating member is not in operation, that is, while the vehicle is
 running under a condition that does not require a normal operation of the
 brake. Since the relationship is estimated in this condition without an
 operation of the brake operating member, the accuracy of estimation of the
 relationship is made relatively high.
 In a usual run of the motor vehicle, the relationship estimating and
 utilizing device is operated to activate the brake for estimating the
 relationship, prior to a normal operation of the brake by an operation of
 the brake operating member by the vehicle operator. Accordingly, the
 estimated actual relationship between the electric power to be supplied to
 the electric motor of the brake and the braking torque to be generated by
 the brake can be utilized to control the brake upon initial activation of
 the brake during the vehicle run.
 While the actual value of the braking torque generated by the brake with
 the predetermined amount of the electric power supplied to the electric
 motor is obtained while the brake operating member is not in operation,
 the estimation of the relationship on the basis of the above-indicated
 predetermined amount of the electric power and the obtained braking torque
 may be effected either immediately after the actual braking torque value
 is obtained during running of the vehicle without an operation of the
 brake operating member, or during the next normal operation of the brake
 initiated by an operation of the brake operating member.
 (7) An electrically operated braking system according to the above mode
 (6), wherein the means for supplying the predetermined amount of the
 electric power to the electric motor for the predetermined length of time
 while the vehicle is running without an operation of the brake operating
 member comprises a brake operation detecting sensor for detecting an
 operation of the brake operating member, and a vehicle run detecting
 sensor for detecting a running of the vehicle, and activates the brake by
 supplying the predetermined amount of the electric power to the electric
 motor when the above sensors detect that the vehicle is running without an
 operation of the brake operating member.
 (8) An electrically operated braking system according to the above mode (6)
 or (7), wherein the motor vehicle has a front left wheel and a front right
 wheel, and the braking system comprises the brake for each of the front
 left and right wheels, and wherein the means for supplying the
 predetermined amount of the electric power to the electric motor while the
 vehicle is running without an operation of the brake operating member is
 adapted to concurrently activate the brakes for the two front wheels.
 In this braking system wherein the brakes for the front left and right
 wheels are concurrently activated while the vehicle is running without an
 operation of the brake operating member, the same amounts of the electric
 power are preferably applied to the electric motors for the two front
 brakes to generate the same braking torque, so as to avoid generation of a
 yaw moment acting on the vehicle body during activation of the front
 brakes by the relationship estimating and utilizing device. This
 arrangement is effective to prevent yawing of the vehicle during
 activation of the front brakes by the relationship estimating and
 utilizing device, which may be felt unusual or abnormal by the vehicle
 operator.
 (9) An electrically operated braking system according to any one of the
 above modes (6)-(8), wherein the motor vehicle has a front wheel and a
 rear wheel, and the braking system comprises the brake for each of the
 front and rear wheels, and wherein the means for supplying the
 predetermined amount of the electric power to the electric motor while the
 motor vehicle is running without an operation of the brake operating
 member is adapted to activate the brakes for the front and rear wheels at
 different times.
 In the braking system according to the above mode (9) wherein the
 deceleration value of the vehicle body during activation of the brake for
 each of the front and rear wheels is made lower than in the braking system
 wherein the brakes for the front and rear wheels are activated
 concurrently. This arrangement makes it possible to estimate the
 relationship without the vehicle operator feeling unusual or uncomfortable
 with excessive deceleration of the vehicle while the vehicle is running
 without an operation of the brake operating member. Further, the
 predetermined amount of the electric power to be supplied to the electric
 motor of the brake for each of the front and rear wheels can be increased
 to improve the accuracy of estimation of the relationship, because the two
 brakes for the front and rear wheels are activated at different times,
 with the relatively low deceleration value of the vehicle body during
 activation of each brake.
 (10) An electrically operated braking system according to any one of the
 above modes (6)-(9), wherein said means for supplying the predetermined
 amount of the electric power to the electric motor while the vehicle is
 running without an operation of the brake operating member is operated to
 activate the brake only once during a run of the motor vehicle, to obtain
 the actual value of the braking torque to be applied to the wheel.
 (11) An electrically operated braking system according to the above mode
 (10), further comprising a vehicle start member which is operated by the
 vehicle operator when the run of the vehicle is initiated, and wherein the
 means for supplying the predetermined amount of the electric power to the
 electric motor determines, upon operation of the vehicle start member,
 that the run of the vehicle has been initiated.
 (12) An electrically operated braking system according to any one of the
 above modes (1)-(11), wherein the relationship estimating and utilizing
 device comprises standard relationship utilizing means for provisionally
 utilizing a predetermined standard relationship stored in a memory, before
 said relationship is estimated for the first time during a run of the
 motor vehicle on the basis of the actual values of the electric power and
 braking torque which are obtained during activation of the brake without
 an operation of the brake operating member.
 (13) An electrically operated braking system according to any one of the
 above modes (1)-(11), wherein the relationship estimating and utilizing
 device stores in a memory the relationship which is estimated during each
 run of the motor vehicle, and before the relationship is estimated during
 a present run of the vehicle, provisionally utilizes the relationship
 which was obtained during the preceding run of the motor vehicle and which
 is stored in the memory.
 In the braking system according to the above mode (13), the stored
 relationship obtained during the preceding run of the vehicle is
 provisionally utilized until the relationship is obtained during the
 present run of the vehicle. The relationship which was actually obtained
 during the preceding run of the vehicle is more reliable than the standard
 relationship which is not actually obtained.
 (14) An electrically operated braking system according to any one of the
 above modes (1)-(5), wherein the relationship estimating and utilizing
 device comprises means for obtaining the actual values of the electric
 power supplied to the electric motor and the braking torque generated by
 the brake activated while the vehicle is running with an operation of the
 brake operating member, and estimates the relationship on the basis of the
 above-indicated actual values.
 The braking system according to the above mode (14) may include the feature
 according to any one of the above modes (11), (12) and (13).
 (15) An electrically operated braking system according to any one of the
 above modes (1)-(14), further comprising a driver connected between the
 electric power source and the electric motor so that the electric power is
 supplied from the electric power source to the electric motor through the
 driver according to an external control command applied to the driver, and
 the relationship estimating and utilizing device comprising electric power
 estimating means for estimating the actual value of the electric power
 supplied from the electric power source to the electric motor, on the
 basis of the external control command, and for estimating the relationship
 on the basis of the estimated actual value of the electric power.
 The braking system according to the above mode (15) does not require an
 electric power sensor for detecting the electric power actually supplied
 to the electric motor, since the actual value of the electric power
 supplied to the electric motor is estimated from the control command
 applied to the driver. Where the relationship estimating and utilizing
 device comprises the means for suppling the predetermined amount of the
 electric power to the electric motor while the vehicle is running without
 an operation of the brake operating member, as described above with
 respect to the mode (5) of this invention, this means applies the control
 command to the driver. Where the relationship estimating and utilizing
 device comprises means for obtaining the actual values of the electric
 power supplied to the electric motor and the braking torque generated by
 the brake activated while the vehicle is running with an operation of the
 brake operating member, as described above with respect to the mode (14)
 of this invention, the controller for the brake applies the control
 command to the driver.
 (16) An electrically operated braking system according to any one of the
 above modes (1)-(14), wherein the relationship estimating and utilizing
 device comprises an electric power sensor for detecting the electric power
 actually supplied to the electric motor, and estimates the relationship on
 the basis of the actual value of the supplied electrical power detected by
 the electric power sensor.
 The value of the electric power represented by the control command applied
 to the driver is not necessarily equal to the value of the electric power
 actually supplied to the electric motor. Further, the value of the
 electric power actually supplied to the electric motor may vary with a
 change in the load of the electric motor, even when the same control
 command is applied to the driver. Therefore, the actual value of the
 electric power supplied to the electric motor may not be estimated with
 high accuracy in the braking system according to the above mode (15). In
 this respect, the braking system according to the above mode (16) in which
 the actually supplied electric power is detected by the sensor permits
 improved accuracy of estimation of the actual relationship between the
 electric power to be supplied to the electric motor and the braking torque
 to be generated by the brake.
 (17) An electrically operated braking system according to any one of the
 above modes (1)-(16), wherein the relationship estimating and utilizing
 device comprises a braking torque sensor for detecting the actual value of
 the braking torque applied from the brake to the wheel, and estimates the
 relationship on the basis of the actual value of the braking torque
 detected by the braking torque sensor.
 (18) An electrically operated braking system according to any one of the
 above modes (1)-(16), wherein the relationship estimating and utilizing
 device includes vehicle deceleration detecting means for detecting a
 deceleration value of the motor vehicle, and obtains the actual value of
 the braking torque on the basis of the deceleration value detected by the
 vehicle deceleration detecting means.
 In the braking system according to the above mode (18), the actual value of
 the braking torque is obtained on the basis of the detected deceleration
 value of the vehicle, based on a fact that the vehicle deceleration value
 increases with an increase in the actual braking torque. In this respect,
 it is also noted that the vehicle deceleration value can be detected more
 easily than the actual braking torque. That is, the actual value of the
 braking torque can be obtained relatively easily on the basis of the
 detected vehicle deceleration value.
 The vehicle deceleration detecting means may be adapted to directly detect
 the deceleration value of the vehicle body, or may be a combination of a
 vehicle speed sensor for detecting the running speed of the vehicle, and
 means for calculating the deceleration value of the vehicle based on the
 detected vehicle running speed. Alternatively, the vehicle deceleration
 detecting means may comprise wheel speed sensors for detecting rotating
 speeds of a plurality of wheels of the motor vehicle, vehicle speed
 estimating means for determining as an estimated vehicle running speed a
 highest one of the wheel speeds detected by the respective wheel speed
 sensors (on the basis of a fact that the highest wheel speed is closest to
 the actual running speed of the vehicle), and vehicle deceleration
 calculating means for calculating the deceleration value of the vehicle on
 the basis of the estimated vehicle running speed.
 (19) An electrically operated braking system according to any one of the
 above modes (1)-(16), wherein the relationship estimating and utilizing
 device comprises vehicle deceleration detecting means for detecting a
 deceleration value of the motor vehicle while a gradient of a road surface
 on which the motor vehicle is running is substantially zero, and obtains
 the actual value of the braking torque on the basis of the deceleration
 value detected by the vehicle deceleration detecting means.
 While the vehicle deceleration value increases with an increase in the
 actual braking torque, as described above, the vehicle deceleration value
 may vary with the gradient of the road surface, even when the actual
 braking torque is kept constant. Based on this fact, the vehicle
 deceleration detecting means used in the braking system according to the
 above mode (19) is adapted to detect the vehicle deceleration value while
 the road surface gradient is substantially zero, and the relationship
 estimating and utilizing means is adapted to obtain the actual braking
 torque based on the vehicle deceleration value detected while the road
 surface gradient is substantially zero. Thus, the actual braking torque
 can be obtained with high accuracy based on the detected vehicle
 deceleration value.
 (20) An electrically operated braking system according to any one of the
 above modes (1)-(16), wherein the relationship estimating and utilizing
 device includes a wheel speed sensor for detecting a rotating speed of the
 wheel, obtains a deceleration value of the wheel on the basis of a rate of
 change of the rotating speed of the wheel detected by the wheel speed
 sensor, and obtains the actual value of the braking torque on the basis of
 the deceleration value of the wheel obtained.
 In the braking system according to the above mode (20), the actual value of
 the braking torque is obtained on the basis of the detected deceleration
 value of the vehicle wheel, based on a fact that the wheel deceleration
 value increases with an increase in the actual braking torque. In this
 respect, it is also noted that the wheel deceleration value can be
 detected more easily than the actual braking torque. That is, the actual
 value of the braking torque can be obtained relatively easily on the basis
 of the detected wheel deceleration value.
 Where the braking system comprises a plurality of brakes for respective
 wheels, the deceleration value of the vehicle is influenced by the braking
 effects provided by all or some of the brakes which are activated.
 Therefore, it is difficult to accurately obtain the actual braking effect
 of the brake for each wheel based on the deceleration value of the
 vehicle. In the braking system according to the above mode (20), however,
 the actual braking torque generated by the brake for each wheel can be
 accurately obtained on the basis of the rate of change of the rotating
 speed of the wheel. According to this arrangement, the relationship
 between the electric power to be supplied to the electric motor and the
 braking torque to be generated by the brake can be estimated for each of
 the plurality of wheels, so that the estimated relationship can be
 utilized for controlling the brake for each wheel, depending upon the
 specific condition of each brake.
 (21) An electrically operated braking system according to the above mode
 (6) or (7), wherein the brake is provided for each of a plurality of
 wheels provided for the motor vehicle, and the means for supplying the
 predetermined amount of the electric power to the electric motor while the
 motor vehicle is running without an operation of the brake operating
 member is adapted to activate the brakes for the respective wheels at
 different times, detect the deceleration values of the motor vehicle
 during the activation of the respective brakes, and obtain the actual
 value of the braking torque of each brake on the basis of the vehicle
 deceleration value detected during the activation of that brake.
 In the braking system according to the above mode (21) wherein the brakes
 for the respective wheels are activated at different times during running
 of the motor vehicle without an operation of the brake operating member,
 the vehicle deceleration values are detected during the activation of the
 respective brakes. In this arrangement, the vehicle deceleration value
 detected during activation of a given one of the brakes reflects only the
 actual braking torque generated by that brake. Thus, the actual braking
 torque of each brake can be accurately obtained based on the vehicle
 deceleration value detected during operation of that brake, and the
 relationship can be obtained for each of the brakes.
 (22) An electrically operated braking system according to the above mode
 (6) or (7), wherein the brake is provided for each of a front wheel and a
 rear wheel provided for the motor vehicle, and the means for supplying the
 predetermined amount of the electric power to the electric motor while the
 motor vehicle is running without an operation of the brake operating
 member is adapted to concurrently activate the brakes for the respective
 wheels, detect the deceleration value of the motor vehicle during the
 concurrent activation of the brakes, obtain the actual values of the
 braking torque values of the brakes for the front and rear wheels on the
 basis of the detected vehicle deceleration value, and estimate the
 relationship on the basis of the obtained actual braking torque value of
 each brake.
 In the braking system according to the above mode (22) wherein the brakes
 for the front and rear wheels are concurrently activated, the required
 time of activation of the brakes can be reduced as compared with the
 required time where the brakes are activated at different times.
 Since the brakes for the front and rear wheels are concurrently activated,
 the vehicle deceleration value detected is influenced by both of the
 actual braking torque values of the front and rear brakes. However, the
 actual braking torque values of the front and rear brakes can be estimated
 by calculation based on the detected vehicle deceleration value, and the
 relationship for each of the front and rear wheels is estimated on the
 basis of the actual braking torque value obtained for each wheel, although
 the vehicle deceleration value detected during the concurrent activation
 of the front and rear brakes is influenced by the actual braking torque
 values of the respective brakes.
 (23) An electrically operated braking system according to the above mode
 (6) or (7), further comprising first inhibiting means for inhibiting the
 relationship estimating and utilizing device from operating the brake to
 obtain the relationship while the motor vehicle is running under a
 condition in which the operation of the brake is likely to be felt unusual
 or uncomfortable by the operator of the motor vehicle.
 In the braking system according to the above mode (23) of this invention,
 the actual braking torque of the brake during activation of the brake
 without an operation of the brake operating member can be obtained without
 the vehicle operator feeling unusual with the activation of the brake.
 (24) An electrically operated braking system according to the above mode
 (23), wherein the first inhibiting means includes means for inhibiting the
 relationship estimating and utilizing device from operating the brake when
 the motor vehicle is running at a speed lower than a predetermined
 threshold.
 In the braking system according to the above mode (24), the operation of
 the relationship estimating and utilizing device without an operation of
 the brake operating member is inhibited when the vehicle running speed is
 lower than the predetermined lower limit. This arrangement is based on a
 finding that the vehicle operator is more likely to feel unusual or
 uncomfortable with the activation of the brake during running of the
 vehicle at a relatively low speed, than at a relatively high speed.
 (25) An electrically operated braking system according to the above mode
 (24), wherein said means for inhibiting the relationship estimating and
 utilizing device from operating the brake when the vehicle is running at a
 speed lower than a predetermined threshold includes vehicle speed
 detecting means for detecting the running speed of the vehicle, and
 inhibits the operation of the relationship estimating and utilizing means
 from operating to activate the brake when the detected vehicle running
 speed is lower than the predetermined threshold.
 In the braking system according to the above mode (25), the vehicle speed
 detecting means may be adapted to directly detect the running speed of the
 vehicle. Alternatively, the vehicle speed detecting means comprises wheel
 speed sensors for detecting rotating speeds of respective wheels of the
 vehicle, and vehicle speed estimating means for determining as an
 estimated vehicle running speed a highest one of the wheel speeds detected
 by the respective wheel speed sensors on the basis of a fact that the
 highest wheel speed is closest to the actual running speed of the vehicle.
 (26) An electrically operated braking system according to any one of the
 above modes (23)-(25), wherein said first inhibiting means comprises means
 for inhibiting the relationship estimating and utilizing means from
 operating the brake when the vehicle is turning.
 In the braking system according to the above mode (26), the operation of
 the relationship estimating and utilizing means is inhibited during
 turning of the vehicle, based on a finding that the operation of the brake
 during turning of the vehicle is likely to cause the vehicle to have a
 behavior which is felt unusual or comfortable by the vehicle operator.
 (27) An electrically operated braking system according to the above mode
 (26), wherein the means for inhibiting the relationship estimating and
 utilizing means from operating the brake when the vehicle is turning
 includes a vehicle turning sensor for detecting turning of the vehicle,
 and inhibits the operation of the relationship estimating and utilizing
 device to activate the brake when the turning of the vehicle is detected
 by the vehicle turning sensor.
 (28) An electrically operated braking system according to any one of the
 above modes (1)-(27), further comprising second inhibiting means for
 inhibiting the relationship estimating and utilizing device from at least
 utilizing the relationship while the motor vehicle is running under a
 condition in which the relationship estimating and utilizing device is not
 likely to accurately estimate the relationship.
 In the braking system according to the above mode (28), the relationship
 estimating and utilizing device is inhibited from at least utilizing the
 relationship while the motor vehicle is running under a condition in which
 the relationship estimating and utilizing device is not likely to
 accurately estimate the relationship. This arrangement is based on a
 finding that the relationship cannot always be estimated with high
 accuracy. Thus, the present arrangement assures increased reliability of
 the relationship estimating and utilizing device. Where the estimated
 relationship is utilized to control the brake, the relationship estimated
 in the above-indicated running condition of the vehicle is not utilized
 for controlling the vehicle, whereby the accuracy of control of the brake
 is improved.
 (29) An electrically operated braking system according to the above mode
 (28), wherein the second inhibiting means includes means for inhibiting
 the relationship estimating and utilizing device from at least utilizing
 the relationship while a drive force for driving the motor vehicle is
 being changed.
 In the braking system according to the above mode (28), the relationship
 estimating and utilizing device is inhibited from at least utilizing the
 relationship while the motor vehicle is running with a drive force being
 changed. This arrangement is based on a finding that the relationship is
 not likely to be estimated accurately while the drive force for driving
 the vehicle is being changed. The vehicle drive force may be changed due
 to a change in the output of a drive power source of the vehicle, or in
 the speed ratio of a power transmission of the vehicle as described below.
 (30) An electrically operated braking system according to the above mode
 (29), wherein the motor vehicle includes a drive power source for driving
 the motor vehicle, and an accelerator operating member which is operated
 by the operator of the motor vehicle to increase an output of the drive
 power source for accelerating the motor vehicle, and wherein said means
 for inhibiting the relationship estimating and utilizing device from at
 least utilizing the relationship comprises means for inhibiting the
 relationship estimating and utilizing device from at least utilizing the
 relationship while said accelerator operating member is in operation.
 In the braking system according to the above mode (30), the relationship
 estimating and utilizing device is inhibited from at least utilizing the
 relationship during operation of the accelerator operating member. This
 arrangement is based on a finding that the relationship is not likely to
 be estimated accurately while the accelerator operating member is in
 operation. The drive power source may be an engine such as an internal
 combustion engine, or an electric motor, or a combination of an engine and
 an electric motor.
 The accelerator operating member may be considered to be "in operation",
 when the accelerator operating member is placed in one of the following
 states: a first transient state in which the accelerator operating member
 is operated to increase the output of the drive power source to accelerate
 the motor vehicle; a steady state in which the accelerator operating
 member is held at the same position to maintain the output of the drive
 power source at the same value; and a second transient state in which the
 accelerator operating member is operated to reduce the output of the drive
 power source to decelerate the motor vehicle. However, the accelerator
 operating member may be considered to be "in operation" only when the
 accelerator operating member is placed in the first transient state, or in
 the first or second transient state. In other words, the relationship
 estimating and utilizing device may be inhibited from at least utilizing
 the relationship when the accelerator operating member is in one of the
 first and second transient states and the steady state, or in one of the
 first and second transient states, or in the first transient state.
 The operating state of the accelerator operating member may be detected by
 either a switch for detecting whether the accelerator operating member is
 placed in a non-operated position or an operated position, or a position
 sensor capable of continuously detecting an amount of operation of the
 accelerator operating member from the non-operated position. The switch is
 usually used for detecting a moment at which the accelerator operating
 member is operated from the non-operated position to an operated position,
 or returned from the operated position to the non-operated position.
 However, this switch cannot detect a change in the operating position of
 the accelerator operating member, or the operating amount of the
 accelerator operating member. That is, while the switch is capable of
 detecting whether the accelerator operating member is in operation or not,
 but is not capable of detecting whether the accelerator operating member
 is placed in the first transient state (vehicle accelerating state) or the
 second transient (vehicle decelerating state), or held in the steady state
 (same operating position). On the other hand, the position sensor is
 capable of detecting whether the accelerator operating member is placed in
 the first transient or accelerating state or in the second transient or
 decelerating state. The above-indicated switch may be a switch for
 detecting an operation of an accelerator pedal as the accelerator
 operating member. The above-indicated sensor may be a sensor for detecting
 an amount of operation of the accelerator pedal, or a sensor for detecting
 an angle of opening of a throttle valve provided in an intake valve of an
 engine (internal combustion engine) which is provided as the drive power
 source. The throttle valve may be operated according to only an operation
 of the accelerator operating member, or according to selectively an
 operation of the accelerator operating member or a control command applied
 to an electrically operated throttle actuator provided for automatically
 operating the throttle valve. To accurately detect a change in the output
 of the engine, the throttle valve sensor for detecting the opening angle
 of the throttle valve is preferably used.
 (31) An electrically operated braking system according to the above mode
 (29), wherein the motor vehicle includes an engine for driving the
 vehicle, and a fuel supply device for supplying a fuel to a combustion
 chamber of the engine, and wherein the means for inhibiting the
 relationship estimating and utilizing device from at least utilizing the
 relationship while the drive force for driving the engine is being changed
 comprises means for inhibiting the relationship estimating and utilizing
 device from at least utilizing the relationship when the fuel supply
 device is switched between an operated state thereof in which the fuel is
 supplied to the combustion chamber and a non-operated state thereof in
 which the fuel is not supplied to the combustion chamber.
 In the braking system according to the above mode (31), the relationship
 estimating and utilizing device is inhibited from at least utilizing the
 relationship when the fuel supply device is switched between the operated
 and non-operated state. This arrangement is based on a finding that the
 relationship is not likely to be estimated accurately when the fuel supply
 device is switched between the operated and non-operated states.
 In a certain type of motor vehicles, the fuel supply device may be switched
 between the operated and non-operated states, even while the accelerator
 operating member is not in operation. In this type of motor vehicle
 wherein the relationship is not likely to be estimated accurately due to a
 change in the output of the engine even while the accelerator operating
 member is not in operation, the arrangement according to the above mode
 (31) is effective to prevent utilization of the relationship estimated
 while the engine output is changing.
 (32) An electrically operated braking system according to any one of the
 above modes (29)-(31), wherein the motor vehicle includes a drive power
 source, and a power transmission for transmitting the drive force of the
 drive power source to the wheel at a selected one of a plurality of speed
 ratios, and wherein the means for inhibiting the relationship estimating
 and utilizing device from at least utilizing the relationship comprises
 means for inhibiting the relationship estimating and utilizing device from
 at least utilizing the relationship while the power transmission is in the
 process of a shifting action to change the speed ratio.
 In the braking system according to the above mode (32), the relationship
 estimating and utilizing device is inhibited from at least utilizing the
 relationship while the power transmission is in the process of a shifting
 action. This arrangement is based on a finding that the relationship is
 not likely to be estimated accurately in the process of a shifting action
 of the power transmission.
 (33) An electrically operated braking system according to the above mode
 (32), wherein the means for inhibiting the relationship estimating and
 utilizing device from at least utilizing the relationship while the power
 transmission is in the process of a shifting action comprises a shift
 sensor for detecting the shifting action of the power transmission.
 (34) An electrically operated braking system according to any one of the
 above modes (28)-(33), wherein the second inhibiting means includes means
 for inhibiting the relationship estimating and utilizing device from at
 least utilizing the relationship while the motor vehicle is turning.
 In the braking system according to the above mode (34), the relationship
 estimating and utilizing device is inhibited from at least utilizing the
 relationship during turning of the vehicle. This arrangement is based on a
 finding that the relationship is not likely to be estimated accurately
 while the vehicle is turning.
 (35) An electrically operated braking system according to the above mode
 (34), wherein the means for inhibiting the relationship estimating and
 utilizing device from at least utilizing the relationship while the motor
 vehicle is turning includes a vehicle turning sensor for detecting a
 turning of the motor vehicle.
 (36) An electrically operated braking system according to any one of the
 above modes (28)-(35), wherein the second inhibiting means comprises means
 for inhibiting the relationship estimating and utilizing device from at
 least utilizing the relationship while the motor vehicle is running on a
 bad road surface.
 In the braking system according to the above mode (36), the relationship
 estimating and utilizing device is inhibited from at least utilizing the
 relationship during running of the vehicle on a bad road surface. This
 arrangement is based on a finding that the relationship is not likely to
 be estimated accurately while the vehicle is running of a bad road
 surface.
 The bad road surface may be a graveled road surface, a Belgian brick- or
 stone-paved road surface, a non-paved road surface, or any other bumpy
 road surface.
 (37) An electrically operated braking system according to the above mode
 (36), wherein the means for inhibiting the relationship estimating and
 utilizing device from at least utilizing the relationship while the motor
 vehicle is running on a bad road surface includes means for detecting the
 bad road surface.
 (38) An electrically operated braking system according to any one of the
 above modes (28)-(37), wherein the second inhibiting means comprises means
 for inhibiting the relationship estimating and utilizing device from at
 least utilizing the relationship while a slip ratio of the wheel is higher
 than a predetermined threshold.
 (39) An electrically operated braking system according to the above mode
 (38), wherein the means for inhibiting the relationship estimating and
 utilizing device from at least utilizing the relationship while the slip
 ratio of the wheel is higher than a predetermined threshold includes means
 for detecting that the slip ratio is higher than the predetermined
 threshold.
 (40) An electrically operated braking system according to any one of the
 above modes (1)-(39), wherein said relationship estimating and utilizing
 device comprises braking torque obtaining means for obtaining the actual
 value of the braking torque for obtaining the actual value of the braking
 torque on the basis of a rate of change of the deceleration value of the
 motor vehicle or a rate of change of the deceleration value of the wheel.
 Where the actual value of the braking torque of the brake is obtained on
 the basis of the deceleration value of the motor vehicle or the drive
 wheel, the obtained actual value of the braking torque is likely to be
 influenced by the overall drive force of the vehicle or the drive force of
 the drive wheel. Where the actual value of the braking torque is obtained
 on the basis of the rate of change of the vehicle or wheel deceleration
 value, the obtained actual value of the braking torque is relatively less
 likely to be influenced by the drive force. In the braking system
 according to the above mode (40), therefore, the actual value of the
 braking torque can be obtained with a reduced influence by the drive force
 of the vehicle or drive wheel.
 (41) An electrically operated braking system according to any one of the
 above modes (1)-(40), wherein the brake includes a self-servo mechanism
 operated such that a friction force between the friction member and the
 rotor is increased by the friction force.
 Referring to the graph of FIG. 8, there are indicated a plurality of I-T
 relationships between an electric current I (electric power) to be
 supplied to the electric motor and the braking torque T in the brake
 including a self-servo mechanism, which relationship correspond to
 respective different values .mu. of a friction coefficient of the friction
 member. It will be understood from the graph that the rate of increase of
 the braking torque T with an increase in the electric current I is higher
 when the friction coefficient .mu. of the friction member is relatively
 high than when it is relatively low. Where the brake includes the
 self-servo mechanism, therefore, the need of estimating the actual
 relationship between the electric power and the braking torque and
 utilizing the estimated relationship for controlling the brake is
 relatively high. This need is satisfied in the braking system according to
 the above mode (41), wherein the braking torque generated by the brake can
 be accurately controlled in relation to the operating amount of the brake
 operating member, even where the brake is provided with the self-servo
 mechanism.
 (42) An electrically operated braking system according to the above mode
 (41), wherein the brake including the self-servo mechanism includes a drum
 brake which has brake linings as the friction member and a drum as the
 rotor.
 (43) An electrically operated braking system according to the above mode
 (41) or (42), wherein the brake including the self-servo mechanism
 includes a disc brake which has brake pads as the friction member and a
 disc as the rotor.
 (44) An electrically operated braking system according to any one of the
 above modes (1)-(39), wherein the relationship estimating and utilizing
 device obtains the actual value of the braking torque on the basis of a
 change in a running condition of the motor vehicle due to the operation of
 the brake.
 The "change in a running condition of the motor vehicle" indicated above
 may include a change in the deceleration value of the motor vehicle.
 (45) An electrically operated braking system according to any one of the
 above modes (1)-(39), wherein the relationship estimating and utilizing
 device comprises means for obtaining the actual value of the braking
 torque on the basis of a change in a rotating state of the wheel due to
 the operation of the brake.
 The "change in a rotating state of the wheel" indicated above may include a
 change of the deceleration value of the wheel, and a rate of change of the
 deceleration value of the wheel.
 (46) An electrically operated braking system according to any one of the
 above modes (1)-(45), wherein the brake includes a support member for
 supporting the friction member in frictional contact with the rotor so as
 to prevent the friction member from being rotated with the rotor, and the
 relationship estimating and utilizing device includes a force switch which
 is interposed between the friction member and the support member, so as to
 receive a force from the friction member in frictional contact with the
 rotor, the force switch being selectively placed in one of two states,
 depending upon whether the force received from the friction member is
 larger than a predetermined threshold which is not zero, the relationship
 estimating and utilizing device utilizing an output of the force switch to
 obtain the actual value of the braking torque.
 (47) An electrically operated braking system according to the above mode
 (46), wherein the rotor is a disc having a friction surface, and the
 friction member is a brake pad which is movable into frictional contact
 with the friction surface, the force switch being disposed in a position
 at which a spacing between the brake pad and the support member decreases
 with an increase in an amount of rotation of the brake pad with the disc.
 (48) An electrically operated braking system according to the above mode
 (46) or (47), wherein the relationship estimating and utilizing device
 further includes a pressing-force-related quantity sensor whose output
 varies continuously with a quantity relating to a pressing force generated
 by the electric motor to force the friction member onto the rotor, the
 relationship estimating and utilizing device using the output of the
 pressing-force-related quantity sensor as a quantity relating to the
 actual value of the electric power supplied to the electric motor.
 (49) An electrically operated braking system according to the above mode
 (48), wherein the the relationship estimating and utilizing device further
 includes a braking force estimating device for estimating the braking
 torque to be applied to the wheel, on the basis of the output of the
 pressing-force-related quantity sensor and according to a predetermined
 relationship between the output and the braking torque, the braking force
 estimating device compensating the predetermined relationship on the basis
 of the output when the force switch is switched from one of the two states
 to the other states.
 (50) An electrically operated braking system according to the above mode
 (49), wherein the braking force estimating device includes relationship
 compensating means for compensating the predetermined relationship, on the
 basis of a difference between an actual value of the output and a nominal
 value of the output when the force sensor is switched from one of the two
 states to the other.
 (51) An electrically operated braking system of a motor vehicle having a
 wheel, comprising: (a) a rotor rotating with the wheel; (b) a brake
 operating member which is operated by an operator of the motor vehicle;
 (c) an electric power source; (d) a brake including a friction member
 movable to be forced onto the rotor, and an electric motor which is
 operated by an electric power supplied from the electric power source, to
 generate a drive force for forcing the friction member onto the rotor and
 thereby braking the wheel; and (e) a controller which determines an amount
 of the electric power to be supplied from the electric power source to the
 electric motor, depending upon an operating amount of the brake operating
 member, for thereby controlling an operation of the brake, the braking
 system characterized by further comprising (f) a relationship estimating
 and utilizing device for obtaining an actual value of a physical quantity
 relating to the electric power supplied from the electric power source to
 the electric motor during an operation of the brake while the motor
 vehicle is running, and an actual value of a physical quantity relating to
 a braking torque applied from the brake to the wheel during the operation
 of the brake, for estimating a relationship between the electric power to
 be supplied to the electric motor and the braking torque to be applied to
 the wheel, on the basis of the actual values obtained, and for utilizing
 the relationship, the relationship being formulated such that the braking
 torque to be applied to the wheel being changed with a change in the
 electric power to be supplied to the electric motor.
 According to the present invention, there is also provided:
 (52) An electrically operated brake for a motor vehicle having a wheel,
 comprising a rotor rotating with the wheel, a friction member movable to
 be forced onto the rotor, an electric motor operated to generate a drive
 force for forcing the friction member onto the rotor, and a self-servo
 mechanism which is operated such that a friction force between the
 friction member and the rotor is increased by the friction force, the
 electrically operated brake being characterized by further comprising a
 biasing mechanism interposed between the friction member and a stationary
 member which supports the friction member, the biasing mechanism biasing
 the friction member in a direction for moving the friction member away
 from the rotor.
 In an electrically operated brake provided with a self-servo mechanism,
 there is a general tendency that the friction member cannot be moved away
 from the rotor with a high response to a command generated to return the
 electric motor to its initial position, once the self-servo mechanism is
 activated to provide a self-servo effect. Therefore, the braking torque
 cannot be rapidly reduced to zero. In the brake according to the above
 mode (52) in which the biasing mechanism is provided, the biasing
 mechanism is effective to rapidly move the friction member away from the
 rotor, thereby permitting rapid reduction of the braking torque.
 The biasing mechanism may be adapted to hold the friction member in the
 biased state, or bias the friction member only when it is required to
 rapidly reduce the braking torque.
 (53) A braking system for a motor vehicle having a wheel, comprising: (a) a
 rotor rotating with the wheel; (b) a friction member movable to be forced
 onto the rotor, for braking the wheel; (c) a support member for supporting
 the friction member in frictional contact with the rotor so as to prevent
 the friction member from being rotated with the rotor; (d) a pressing
 device for forcing the friction member into frictional contact with the
 rotor; and (e) a force switch which is interposed between the friction
 member and the support member, so as to receive a force from the friction
 member in frictional contact with the rotor, the force switch being
 selectively placed in one of two states, depending upon whether the force
 received from the friction member is larger than a predetermined threshold
 which is not zero.
 The braking system constructed according to the above mode (53) of this
 invention provides an improvement over a conventional braking system which
 uses a braking-force-related quantity sensor for continuously detecting a
 physical quantity relating to braking force generated by a brake. This
 sensor may be a strain gage using an electrically resistive wire, or a
 piezoelectric sensor. The quantity which is detected by the sensor and
 which relates to the braking force changes over a relatively wide range.
 Accordingly, it is generally difficult for the sensor to detect the
 quantity with high accuracy over the entire range. Further, the operating
 environment of the sensor is considerably severe. Namely, the sensor may
 be subject to a considerable amount of change in the operating temperature
 and a considerably intense vibration, and is likely to be exposed to
 foreign matters such as mud, water and dust or grit. Thus, the
 braking-force-related quantity sensor used in the conventional braking
 system is not capable of detecting a quantity relating to the braking
 force, with a high degree of reliability, and does not have a satisfactory
 degree of durability.
 The force switch used according to the above mode of the present invention
 is different from a sensor in that like a commonly used switch, the force
 switch is not capable of continuously detecting a physical quantity.
 However, the force switch is less likely to be influenced by the operating
 environment, as compared with a sensor, since the force switch is simpler
 in construction and operating principle. For instance, a relationship
 between the actual value of the physical quantity and the output of the
 force sensor is less likely to be influenced by the operating environment,
 that a relationship between the actual value of the quantity and the
 output of a sensor. Accordingly, the force switch is capable of detecting,
 with high reliability and durability, whether the force which the support
 member receives from the friction member as a physical quantity relating
 to the braking force has exceeded the predetermined value or has been
 reduced to the predetermined value.
 (54) A braking system according to the above mode (53), wherein the rotor
 is a disc having a friction surface, and the friction member is a brake
 pad which is movable into frictional contact with the friction surface,
 the force switch being disposed in a position at which a spacing between
 the brake pad and the support member decreases with an increase in an
 amount of rotation of the brake pad with the disc.
 (55) A braking system according to the above mode (1) or (2), wherein the
 force switch is provided at each of a plurality of positions between the
 friction member and the support member, and the force switches at the
 different positions have respective different predetermined threshold
 values of the force received from the friction member.
 In the braking system according to the above mode (55), the quantity
 relating to the braking force can be detected in two or more steps by the
 respective force sensors.
 (56) A braking system according to any one of the above modes (53)-(55),
 wherein the force switch includes a pair of contacts which are movable
 relative to each other between two positions corresponding to the
 above-indicated two states, and one of the contacts is fixed to the
 friction member while the other contact is fixed to the support member.
 The force switch in the braking system according to the above mode (56) is
 relatively simple in construction and operating principle, and accordingly
 assures increased operating reliability and durability.
 (57) A braking system according to the above mode (56), wherein the contact
 fixed to the friction member is a movable contact, while the contact fixed
 to the support member is a stationary contact.
 (58) A braking system according to any one of the above modes (53)-(57),
 wherein the pressing device includes an electric motor as a drive source,
 and does not use a pressurized hydraulic fluid.
 (59) A braking system according to any one of the above modes (53)-(57),
 wherein the pressing device uses a pressurized hydraulic fluid.
 (60) A braking system according to any one of the above modes (53)-(59),
 wherein the friction member, support member, pressing device and force
 switch cooperate to constitute a major portion of a brake, the braking
 system further comprising a brake information obtaining device for
 obtaining brake information relating to an operation of the brake.
 In the braking system according to the above mode (60), the brake
 information may include a friction coefficient of the friction member,
 information as to whether the friction coefficient of the friction member
 is unacceptably low or not, information as to whether the brake is
 abnormal, and a friction coefficient between the wheel and the road
 surface.
 (61) A braking system according to any one of the above modes (53)-(59),
 further comprising: a pressing-force-related quantity sensor whose output
 varies continuously with a quantity relating to a pressing force by which
 the friction member is forced onto the rotor by the pressing device; and a
 friction coefficient estimating device for estimating a friction
 coefficient of the friction member, on the basis of a relationship between
 the output of the pressing-force-related quantity sensor and the
 predetermined threshold.
 In the braking system according to the above mode (61), the friction
 coefficient of the friction member can be estimated by the friction
 coefficient estimating device, so that the braking system can be
 controlled with high accuracy, by utilizing the estimated friction
 coefficient.
 (62) A braking system according to the above mode (61), wherein the
 pressing device includes a presser member which is disposed on one of
 opposite sides of the friction member remote from the rotor, so as to
 force the friction member onto the rotor, and the pressing-force-related
 quantity sensor includes a pressing-force sensor provided on the presser
 member to continuously detect a force which acts on the presser member in
 a direction of movement of the presser member toward the friction member.
 (63) A braking system according to the above mode (61), wherein the
 pressing device includes an electric motor for forcing the friction member
 onto the rotor, and the pressing-force-related quantity sensor includes an
 electric power sensor for continuously detecting an amount of electric
 power supplied to the electric motor.
 (64) A braking system according to the above mode (63), wherein the
 electric power sensor is a motor current sensor for continuously detecting
 an amount of electric current supplied to the electric motor.
 (65) A braking system according to any one of the above modes (61)-(64),
 wherein the above-indicated two states of the force sensor consists of an
 on state and an off state, the state of the force switch being changed
 from one of the on and off states to the other when the force received
 from the friction member has increased and exceeded the above-indicated
 predetermined threshold, and is changed from the above-indicated other
 state to the above-indicated one state, and wherein the friction
 coefficient estimating device estimates the friction coefficient of the
 friction member when at least one of a change from the above-indicated one
 state to the other state and a change from the above-indicated other state
 to the above-indicated one state takes place.
 (66) A braking system according to the above mode (65), wherein the
 friction coefficient estimating device estimates the friction coefficient
 of the friction member when each of the changes between the on and off
 states takes place.
 In the braking system according to the above mode (66), the friction
 coefficient is estimated at the two opportunities, namely, when the force
 switch is turned on and when the force switch is turned off. Accordingly,
 the accuracy of estimation of the friction coefficient can be improved, as
 compared with the accuracy of estimation where the friction coefficient is
 estimated only once, that is, when the force switch is turned on or turned
 off.
 (67) A braking system according to any one of the above modes (61)-(66),
 wherein said friction coefficient estimating device estimates the friction
 coefficient of the friction member, by dividing the above-indicated
 predetermined threshold by the quantity as detected by the
 pressing-force-related quantity sensor when the state of the force switch
 is changed from one of the above-indicated two states to the other.
 (68) A braking system according to any one of the above modes (61)-(67),
 further comprising a brake pad fade detecting device for detecting that
 the friction coefficient estimated by the friction coefficient estimating
 device is lower than a predetermined lower limit.
 (69) A braking system according to any one of the above modes (53)-(59),
 wherein the friction member, support member, pressing device and force
 switch cooperate to constitute a major portion of a brake, the braking
 system further comprising: a physical quantity sensor for detecting a
 physical quantity which relates to an operation of the brake and which
 excludes a force acting on the force switch, the physical quantity
 changing in relation to the force acting on the force switch, depending
 upon whether the brake is abnormal or not; and a brake failure detecting
 device for detecting whether the brake is abnormal or not, on the basis of
 a relationship between an output of the physical quantity sensor and an
 output of the force switch.
 (70) A braking system according to any one of the above modes (53)-(59),
 wherein the friction member, support member, pressing device and force
 switch cooperate to constitute a major portion of a brake, the braking
 system further comprising a road surface friction coefficient estimating
 device for estimating a friction coefficient between the wheel and a road
 surface on which the motor vehicle is running, on the basis of an output
 of the force switch.
 (71) A braking system according to any one of the above modes (53)-(70),
 wherein said pressing device includes an electric motor for forcing the
 friction member onto the rotor, and cooperates with the rotor, friction
 member, support member and force sensor to constitute an electrically
 operated brake, the braking system further comprising a mechanically
 operated brake which is operated mechanically to brake the wheel by a
 force generated by a manually operated brake operating member, and a
 manual brake control device disposed between the mechanically operated
 brake and the brake operating member, for permitting an operation of the
 mechanically operated brake when the electrically operated brake is
 abnormal, and inhibiting the operation of the mechanically operated brake
 when the electrically operated brake is normal.
 In the braking system according to the above mode (71), the electrically
 operated brake is used as a normal brake, and the mechanically operated
 brake is used as an emergency brake which is activated by the force
 generated by the brake operating member. Thus, the present braking system
 is provided with an electrically operated sub-system including the brake
 operating member and the electrically operated brake, and a mechanically
 operated sub-system including the brake operating member, the manual brake
 control device and the mechanically operated brake. Therefore, the present
 braking system is capable of braking the wheel with the braking force
 corresponding to the operating force acting on the brake operating member,
 even when the electrically operated brake is abnormal or defective.
 Accordingly, the present braking system has increased operating
 reliability and improved safety of the vehicle.
 In the present braking system, the mechanically operated brake and the
 manual brake control device may or may not use a pressurized hydraulic
 fluid.
 The electrically operated brake may be adapted to be operated according to
 an intention of the vehicle operator, which is represented by an operation
 of the brake operating member. For instance, the electrically operated
 brake may be operated according to the operating force acting on the brake
 operating member, or the operating amount or stroke of the brake operating
 member.
 In the present braking system, the friction member and the rotor may be
 commonly used for the electrically and mechanically operated brakes.
 Alternatively, the electrically and mechanically operated brakes may use
 respective sets of the friction member and rotor.
 (72) A braking system according to the above mode (71), further comprising
 a brake information obtaining device for obtaining information relating to
 the electrically operated brake, on the basis of an output of the force
 switch, and wherein the brake information obtaining device is inhibited
 from operating when the manual brake control device is operated.
 In the braking system according to the above mode (72), the information
 relating to the electrically operated brake includes information which has
 been described above with respect to the above mode (60).
 (73) A braking system according to any one of the above modes (53)-(59),
 wherein the friction member, support member, pressing device and force
 switch constitute a major portion of a brake, the braking system further
 comprising: a force-related quantity sensor whose output varies
 continuously with a quantity relating to one of a braking force generated
 by the brake and applied to the wheel and a pressing force generated by
 the pressing device to force the friction member onto the rotor; a wheel
 braking force estimating device for estimating the braking torque to be
 applied to the wheel, on the basis of the output of the force-related
 quantity sensor and according to a predetermined relationship between the
 output and the braking torque, the wheel braking force estimating device
 compensating the predetermined relationship on the basis of the output
 when the force switch is switched from one of the two states to the other
 state.
 It will be understood from the foregoing description that the force switch
 is advantageous for its comparatively high operating reliability but is
 disadvantageous for its incapability of continuous detection of the force,
 while on the other hand the force-related quantity sensor is advantageous
 for its capability of continuous detection of the force-related quantity
 but is disadvantageous for its comparatively low operating reliability.
 Thus, the force switch and the force-related quantity sensor mutually
 supplement their disadvantages, cooperating with each other to permit
 continuous detection of the force-related quantity with high reliability.
 The braking system according to the above mode (73) is based on this
 finding.
 Where the force-related quantity sensor in the braking system according to
 the above mode (73) is a pressing-force-related quantity sensor adapted to
 continuously detect a quantity relating to the pressing force generated by
 the pressing device, the quantity detected by this pressing-force-related
 quantity sensor is not influenced by a variation in the friction
 coefficient of the friction member but the output of the force switch is
 influenced by the variation. In this case, the wheel braking force
 estimating compensates the above-indicated relationship by taking account
 of both the variation in the operating characteristic of the
 pressing-force-related sensor and the variation in the friction
 coefficient of the friction member. Where the force-related quantity
 sensor is a braking-force-related quantity sensor adapted to continuously
 detect a quantity relating to the braking force applied to the wheel, the
 force detected by the force switch and the quantity detected by the
 braking-force-related quantity sensor are substantially the same physical
 quantities which are not influenced by the variation in the friction
 coefficient of the friction member. In this case, the wheel braking force
 estimating device compensates the above-indicated relationship by
 adjusting the calibration of the braking-force-related quantity sensor.
 The above-indicated relationship may be either directly compensated, or
 indirectly or eventually compensated. In the latter case, the output of
 the force-related quantity sensor may be compensated, or alternatively the
 wheel braking force provisionally estimated may be compensated on the
 basis of the output of the sensor and the predetermined relationship.
 (74) A braking system according to the above mode (73), wherein the
 force-related quantity sensor includes a pressing force sensor whose
 output continuously varies with the pressing force acting thereon.
 The operating environment of the pressing force sensor is not so severe as
 compared with that of the a braking force sensor which will be described.
 In the braking system according to the above mode (74), therefore, the
 operating reliability and durability of the pressing force sensor can be
 relatively easily improved.
 (75) A braking system according to the above mode (73), wherein the
 force-related quantity sensor includes a braking force sensor whose output
 continuously varies with the force which the support member receives from
 the friction member as the braking force applied to the wheel.
 (76) A braking system according to any one of the above modes (73)-(75),
 wherein the wheel braking force estimating device includes relationship
 compensating means for compensating the predetermined relationship, on the
 basis of a difference between an actual value of the output and a nominal
 value of the output when the force sensor is switched from one of the two
 states to the other.
 (77) A braking system according to the above mode (76), wherein the
 relationship compensating means compensates the predetermined relationship
 such that the pressing force obtained on the basis of the actual value of
 the output of the braking force sensor coincides with the actual value of
 the pressing force, even in the presence of the above-indicated
 difference.
 (78) A braking system according to any one of the above modes (73)-(77),
 further comprising a brake information estimating device for obtaining
 brake information relating to an operation of the brake, on the basis of
 the output of the force switch.
 The brake information estimating device and "brake information relating to
 an operation of the brake" in the above mode (78) are similar to those
 which have been discussed above with respect to the above mode (60) of
 this invention.
 (79) A braking system according to the above mode (78), wherein the brake
 information estimating device includes a friction coefficient estimating
 device for estimating a friction coefficient of the friction member, on
 the basis of a relationship between the output of the brake-related
 quantity sensor and the above-indicated predetermined threshold of the
 force switch when the force switch is switched from one of the two states
 to the other.
 The friction coefficient estimating device according to the above mode (79)
 is similar to that which has been discussed above with respect to the
 above mode (61).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring first to FIG. 1, there is shown an arrangement of an electrically
 operated braking system constructed according to a first embodiment of
 this invention. This electrically operated braking system is adapted for
 use on a four-wheel motor vehicle having a front left wheel FL, a front
 right wheel FR, a rear left wheel RL and a rear right wheel RR. The motor
 vehicle has a drive power source in the form of an internal combustion
 engine 10, and a power transmission in the form of an automatic
 transmission 12. The motor vehicle is run with a drive force generated by
 the engine 10 and transmitted through the automatic transmission 12 to the
 front wheels FL, FR and/or the rear wheels RL, RR.
 The front left and right wheels FL, FR have respective electrically
 operated disc brakes 22, 22 each including an electric motor 20 as a drive
 source, while the rear left and right wheels RL, RR have respective
 electrically operated drum brakes 32, 32 each including an electric motor
 30 as a drive source. In the present first embodiment of the invention,
 each of these electric motors 20, 30 is a DC motor. However, all of the
 motors 20, 30 may be ultrasonic motors. Alternatively, the motors 20 for
 the front wheels FL, FR and the motors 30 for the rear wheels RL, RR may
 be ultrasonic motors and DC motors, respectively, or vice versa.
 The motor vehicle is provided with various operator-controlled members
 including: a brake pedal 40 as a primary brake operating member (one of
 brake operating members); a parking brake pedal 42 as a parking brake
 operating member; an accelerator pedal 44 as an accelerator operating
 member; and a steering wheel 46. When the brake pedal 40 is operated, the
 electrically operated disc and drum brakes 22, 32 for the four wheels FL,
 FR, RL, RR are activated to apply a brake to the vehicle. When the parking
 brake pedal 42 is operated, the electrically operated drum brakes 32 for
 the rear left and right wheels RL, RR are activated to apply a parking
 brake to the vehicle. When the accelerator pedal 44 is operated, the drive
 force or power generated by the engine 10 is increased to accelerate the
 vehicle. When the steering wheel 46 is rotated, a steering device as well
 known in the art is activated to change the steering angles of the front
 wheels FL, FR (or front and rear wheels).
 In the present electrically operated braking system, an operation of the
 brake pedal 40 as the primary brake operating member causes a controller
 50 to energize the electric motors 20, 30 for activating the disc and drum
 brakes 22, 32 to generate braking forces for braking the four wheels.
 Thus, the force produced by the brake pedal 40 upon operation thereof by
 the vehicle operator is not used to brake the vehicle. However, the
 operating amount of the brake pedal 40 should change depending upon the
 operating force acting on the brake pedal 40, so that the vehicle operator
 is given an operating feel of the brake pedal 40 similar to that in the
 conventional hydraulically operated braking system. To this end, the brake
 pedal 40 is connected to a stroke simulator 52, so that the operating
 amount of the brake pedal 40 will change depending upon the operating
 force applied to the brake pedal 40. The stroke simulator 52 includes (a)
 a linking member 54 linked with the brake pedal 40, (b) a guide member 56
 for guiding the linking member 54, and (c) an elastic member in the form
 of a spring 58 whose elastic force changes due to contraction and
 expansion thereof as the linking member 54 is moved by the brake pedal 40.
 The operating amount (stroke) of the brake pedal 40 changes with a change
 in the elastic force of the spring 58 depending upon the operating force
 of the brake pedal 40.
 Referring to FIG. 2, the construction of the electrically operated disc
 brake 22 for the front right wheel FR is shown in detail, by way of
 example. The disc brake 22 for the front left wheel FL has the same
 construction.
 The electrically operated disc brake 22 includes a mounting bracket 100
 which is a stationary member fixed to the body of the motor vehicle, and a
 disc rotor 104 which has opposite friction surfaces 102 and which is
 rotated with the front right wheel FR. The mounting bracket 100 includes
 (a) a support portion for supporting a pair of friction members in the
 form of brake pads 106a, 106b on the opposite sides of the disc rotor 104
 such that the brake pads 106a, 106b are movable in the axial direction of
 the disc rotor 104, and (b) a torque receiving portion for receiving
 friction forces generated between the friction surfaces 102 of the disc
 rotor 104 and the brake pads 106a, 106b during frictional contact
 therebetween. Character "X" in FIG. 2 represents a rotating direction of
 the disc rotor 104 for forward running of the motor vehicle.
 The outer brake pad 106a located on the outer side (right side as seen in
 FIG. 2) of the disc rotor 104 is supported by the mounting bracket 100
 such that the outer brake pad 106a is substantially prevented from
 rotating with the disc rotor 104 during its friction contact with the
 outer friction surface 102, namely, substantially prevented from being
 "dragged" by the disc rotor 104. On the other hand, the inner brake pad
 106b on the inner side (left side as seen in FIG. 2) of the disc rotor 104
 is supported by the mounting bracket 100 such that the inner brake pad
 106b is allowed to be "dragged" by the disc rotor 104 due to friction
 contact of the pad 106b with the inner friction surface 102. Character "Y"
 in FIG. 2 represents a direction of "dragging" movement of the inner brake
 pad 106b due to its frictional contact with the disc rotor 104.
 The dragging movement of the inner brake pad 106b is prevented when the
 friction force generated between the disc rotor 104 and the brake pad 106b
 is smaller than a predetermined threshold value, and is allowed after the
 friction force has exceeded the threshold value. To this end, the inner
 brake pad 106b has a rear portion 110 which engages the mounting bracket
 100 through an elastic member in the form of a spring 112. When the
 friction force between the inner brake pad 106b and the disc rotor 104 is
 smaller than the threshold, the spring 112 does not undergo elastic
 deformation and prevents the dragging movement of the inner brake pad 106b
 with the disc rotor 104. When the friction force of the inner brake pad
 106b has exceeded the threshold, the spring 112 starts elastic deformation
 and allows the inner brake pad 106b to be dragged with the disc rotor 104.
 In the present embodiment, the rear portion 110 of the inner brake pad
 106b is associated with a stop 114 which is adapted to abut on the
 mounting bracket 100, for thereby limiting the distance of the dragging
 movement of the brake pad 106b after the friction force of the inner brake
 pad 106b has exceeded the threshold. Thus, the stop 114 prevents an
 excessive degree of a self-servo effect which will be described.
 The disc brake 22 further includes a caliper 120 which is movable in the
 axial direction of the disc rotor 104 but is not rotatable about the axis
 of rotation of the disc rotor 104. The caliper 120 is slidably supported
 by a plurality of pins (not shown) which are fixed to the vehicle body so
 as to extend in the axial direction of the rotor 104. The caliper 120
 includes (a) a reaction portion 126 located on the outer side of the disc
 rotor 104, for abutting contact with the outer surface of the outer brake
 pad 106a, (b) a presser portion 128 located on the inner side of the disc
 rotor 104, for abutting contact with the inner surface of the inner brake
 pad 106b, and (c) a connecting portion 130 connecting the reaction and
 presser portions 126, 128 together.
 The presser portion 128 accommodates the electric motor 20, and carries a
 presser member 134 which is linked with the motor 20 through a motion
 converting mechanism in the form of a ballscrew mechanism 132 such that
 the presser member 134 and the motor 20 are coaxial with each other. The
 presser member 134 is supported by the presser portion 128 such that the
 presser member 134 is not rotatable about the axis of rotation of the
 motor 20, but is movable in the axial direction of the motor 20. A rotary
 motion of the motor 20 is converted by the ballscrew mechanism 132 into a
 linear motion of the presser member 134 in the axial direction of the
 motor 20, so that a drive force generated by the motor 20 is applied to
 the inner brake pad 106b, and also to the outer brake pad 106a through the
 caliper 120, whereby the outer and inner brake pads 106a, 106b are forced
 onto the opposite friction surfaces 102 of the disc rotor 104.
 The outer brake pad 106a has a backing plate 140 whose thickness is
 constant in the rotating direction X. On the other hand, the inner brake
 pad 106b has a backing plate 140 whose thickness continuously decreases in
 the direction Y of the dragging movement, that is, in the forward running
 direction of the vehicle. Described in detail, the backing plate 140 of
 the inner brake pad 106b has a slant back surface 142 which is remote from
 the inner friction surface 102 of the disc rotor 104 and which is inclined
 with respect to the inner friction surface 102. Thus, the presser member
 134 is adapted to contact at its front end with the slant back surface 142
 of the inner brake pad 106b. Further, the presser member 134 is provided
 at its front end face with means for facilitating movement of the inner
 brake pad 106b relative to the presser member 134 while the front end face
 of the presser member 134 is in contact with the slant back surface 142 of
 the backing plate 140. This arrangement makes it possible to provide a
 wedge effect between the inner brake pad 106b and the presser member 134
 during the dragging movement of the inner brake pad 106b, so that the
 dragging movement of the inner brake pad 106b provides a self-servo effect
 in the disc brake 22. In this embodiment, the axis of the motor 20
 (presser member 134) is perpendicular to the slant back surface 142 of the
 inner brake pad 106b.
 The above-indicated means for facilitating the relative movement between
 the inner brake pad 106b and the presser member 134 includes a plurality
 of balls 144 which are arranged on the front end face of the presser
 member 134 along a circle coaxial with the motor 20, at a substantially
 equal angular interval. The balls 144 are held on the front end face such
 that the balls 144 may roll in contact with the slant back surface 142.
 Thus, the balls 144 serve as a thrust bearing 146 through which the
 presser member 134 comes into contact with the inner brake pad 106b, so
 that the thrust bearing 146 reduces the friction between the front end
 face of the presser member 134 and the slant back surface 142 of the inner
 brake pad 106b. The balls 144 may be replaced by rollers.
 There will be described an operation of the electrically operated disc
 brake 22 of FIG. 2.
 When the brake pedal 40 is depressed by the vehicle operator, the motor 20
 is operated to advance the presser member 134 from its non-operated
 position, for forcing the outer and inner brake pads 106a, 106b against
 the respective friction surfaces 102 of the disc rotor 104, so that the
 front wheel FR is braked with the friction forces generated between the
 disc rotor 104 and the brake pads 106a, 106b.
 When the friction force between the inner brake pad 106b and the disc rotor
 104 is smaller than a predetermined elastic or biasing force of the spring
 112, the dragging movement of the inner brake pad 106b is prevented by the
 spring 112, so as to prevent the self-servo effect. In an initial period
 of operation of the disc brake 22 with a relatively small operating force
 acting on the brake pedal 40, the friction force between the inner brake
 pad 106b and the disc rotor 104 is smaller than the elastic force of the
 spring 112, the front right wheel is braked by only the drive force
 generated by the motor 20.
 When the friction force of the inner brake pad 106b becomes larger than the
 elastic force of the spring 112, the spring 112 allows the inner brake pad
 106b to be dragged with the disc rotor 104. With the movement of the slant
 back surface 142 relative to the presser member 134, the distances between
 the slant back surface 142 and the friction surface 102 at the points of
 contacts between the balls 144 and the slant back surface 142 increase,
 with a result of an increase in the force by which the brake pads 106a,
 106b are forced onto the disc rotor 104. Therefore, when the brake pedal
 40 is depressed with a relatively large force (e.g., a force enough to
 achieve vehicle deceleration of about 0.3-0.6 G), there arises a wedge
 effect between the inner brake pad 106b and the presser member 134 which
 contact each other at the slant back surface 142, so that the front right
 wheel FR is braked by both the drive force of the motor 20 and the
 self-servo effect owing to the dragging movement of the inner brake pad
 106b.
 When the stop 114 has come into abutting contact with the mounting bracket
 100 with a further increase in the friction force between the inner brake
 pad 106b and the disc rotor 104, a further dragging movement of the brake
 pad 106b is prevented or inhibited by the stop 114, whereby an excessive
 degree of the self-servo effect is prevented.
 Referring next to FIG. 3, the construction of the electrically operated
 drum brake 32 for the rear left wheel RL is shown in detail, by way of
 example. The drum brake 32 for the rear right wheel RR has the same
 construction.
 The electrically operated drum brake 32 includes a stationary member in the
 form of a substantially circular backing plate 200 fixed to the vehicle
 body, and a drum 204 which has an inner circumferential friction surface
 202 and which rotates with the rear right wheel RR. The backing plate 200
 has an anchor member in the form of an anchor pin 206 fixed to a
 relatively radially outer portion thereof at a given circumferential
 position thereof. At another circumferential position of the backing plate
 200 which is diametrically opposite to the circumferential position at
 which the anchor pin 206 is fixed, there is disposed a connecting link in
 the form of an adjuster 208 of a floating type not directly fixed to the
 backing plate 200. A pair of friction members in the form of a pair of
 brake shoes 210a, 210b are disposed between and so as to connect the
 anchor pin 206 and the adjuster 208, such that the brake shoes 210a, 210b
 face the inner friction surface 202 of the drum 204. Each of the brake
 shoes 210a, 210b has an arcuate shape. The brake shoes 210a, 210b are
 fixed by respective hold-down devices 212a, 212b to the backing plate 200
 such that the brake shoes 210a, 210b are movable in a plane parallel to
 the backing plate 200. The backing plate 200 has a central opening through
 which a rear axle shaft extends so as to be rotatable.
 Each of the brake shoes 210a, 210b is operated connected at one end thereof
 to the corresponding end portion of the adjuster 208, and is held at the
 other end in abutting engagement with the anchor pin 206, so that the shoe
 210a, 210b is pivotable about the anchor pin 206. An adjuster spring 214
 is connected to the end portions of the brake shoes 210a, 210b operatively
 connected to the adjuster 208, so that the end portions are biased by the
 adjuster spring 214 toward each other. A return spring 215a, 215b is
 connected to the other end portions of the brake shoes 210a, 210b, so that
 these end portions are biased by the return spring 215a, 215b toward the
 anchor pin 206. The arcuate brake shoes 210a, 210b have respective arcuate
 brake linings 216a, 216b held at their outer surfaces such that the brake
 linings 216a, 216b face the circumferential friction surface 202 of the
 drum 204. With friction contact of these brake linings 216a, 216b with the
 friction surface 202, there arise friction forces between the brake
 linings 216a, 216b and the drum 204. The adjuster 208 is provided for the
 purpose of changing the radial distance between the friction surface 202
 and the inner arcuate surfaces of the brake shoes 210a, 210b, so as to
 maintain a desired radial clearance between the friction surface 202 and
 the surfaces of the brake linings 216a, 216b, irrespective of gradual
 wearing of the brake linings.
 Each brake shoe 210a, 210b consists of a rim 220 and a web 222. A lever 230
 is pivotably connected at one end thereof to a lever support member in the
 form of a pin 232 fixed to the web 222 of the brake shoe 210a. The lever
 230 and the web 222 of the other brake shoe 210b have respective cutouts
 which engages respective opposite ends of a strut 236 serving as a power
 transmitting member. The lever 230 and the strut 236 enable both of the
 brake shoes 210a, 210b of the present drum brake 32 to provide a
 self-servo effect during both forward and backward runs of the vehicle.
 Namely, the present drum brake 32 is a duo-servo type drum brake. In the
 present embodiment, the lever 230 is connected to the brake shoe 210a
 which functions as a secondary brake shoe during the forward running of
 the vehicle. However, the lever 230 may be connected to the brake shoe
 210b which functions as a primary brake shoe during the forward running of
 the vehicle.
 The present electrically operated drum brake 32 is activated by pivotal
 movement of the lever 230 about the pin 232 at its one end when the
 parking brake pedal 42 is operated, as well as when the brake pedal
 (primary brake pedal) 40 is operated. To this end, not only a primary
 brake cable 240 but also a parking brake cable 242 are connected to the
 other end of the lever 230. Each of these brake cables 240, 242 consists
 of a strand of a plurality of wires, and is accordingly flexible. A
 compression coil spring 244 is connected at its one end to the
 above-indicated other end of the lever 230 and at the other end to the
 backing plate 200, as in the conventional hydraulically operated braking
 system. The spring 244 extends coaxially with the parking brake cable 242.
 The primary brake capable 240 is connected to a shoe expanding actuator 250
 attached to the backing plate 200. As shown in enlargement in FIG. 4, the
 shoe expanding actuator 250 includes the electric motor 30 indicated
 above, a speed reducer 252 whose input shaft is connected to the output
 shaft of the motor 30, and a ballscrew mechanism 254 whose input member is
 connected to an output shaft of the speed reducer 252. The end of the
 primary brake cable 240 remote from the lever 230 is connected to an
 output member of the ballscrew mechanism 254. A rotary motion of the motor
 30 is converted by the ballscrew mechanism 254 into a linear movement of
 the primary brake cable 240. In FIG. 4, reference numerals 256 and 258
 denote brackets, while reference numerals 260 and 262 denote mounting
 screws for mounting the brackets 256, 258 to the backing plate 200.
 The ballscrew mechanism 254 includes an externally threaded member 264 as
 the input member, a nut 266 as the output member, and a plurality of balls
 through which the externally threaded member 264 and the nut 266 engage
 each other. The nut 266 engages a stationary housing 267 such that the nut
 266 is not rotatable and is axially movable relative to the housing 267. A
 rotary motion of the externally threaded member 264 is converted into a
 linear or axial motion of the nut 266. The nut 266 has an output shaft 268
 fixed to its one end remote from the externally threaded member 264, such
 that the output shaft 268 is coaxial with the nut 266. The externally
 threaded member 264, nut 266 and output shaft 268 are protected against
 exposure of their engaging portions to dust or other foreign matters, by
 the housing 267 and an elastic dust boot 270.
 The primary brake cable 240 is connected to the output shaft 268 through an
 externally threaded member 272 and a nut 274. The externally threaded
 member 272 is formed so as to extend from the end of the output shaft 268
 remote from the ballscrew mechanism 254, while the nut 274 engages the
 externally threaded member 272 and is connected to the primary brake cable
 240. A lock nut 276 is screwed on the externally threaded member 272 so as
 to lock the nut 274.
 The shoe expanding actuator 250 constructed as described above is operated
 in one direction to pull the primary brake cable 240 upon operation of the
 brake pedal 40, so that the lever 230 is pivoted about the pin 232 such
 that the end portion of the lever 230 to which the primary brake cable 240
 is connected is moved toward the brake shoe 210b. As a result, the two
 brake shoes 210a, 210b are moved away from each other.
 After the shoe expanding actuator 250 is operated in the reverse direction
 and returned to its initial non-operated position, the brake shoes 210a,
 210b are moved toward each other by a shoe contracting mechanism in the
 form of a primary brake return spring 280, against a self-servo effect.
 The primary brake return spring 280 is a compression coil spring 280 which
 is connected at its one end to the lever 230 and at the other end to a
 stationary portion of the actuator 250. The compression coil spring 280 is
 disposed coaxially with the primary brake cable 240. Upon releasing of the
 brake pedal 40, the actuator 250 is returned to the initial position, and
 the lever 230 is pivoted to be returned to its initial non-operated
 position under the biasing force of the primary brake return spring 280.
 However, the spring 280 serving as the shoe contracting mechanism is not
 essential, and may be eliminated, particularly where the adjuster spring
 214 and the shoe return springs 215a, 215b have relatively large elastic
 forces.
 As shown in FIG. 1, the parking brake cable 242 is connected, at its end
 remote from the lever 230, to a parking control 284, which is mechanically
 operated by the parking brake pedal 42 so as to pull the parking brake
 cable 242 for pivoting the lever 230 in the shoe expanding direction for
 moving the two brake shoes 210a, 210b away from each other.
 When the brake pedal 40 is operated, the shoe expanding actuator 250 is
 operated to pull the primary brake cable 240 for pivoting the lever 230 in
 the above-indicated shoe expanding direction. In this case, the parking
 brake cable 242 becomes slack. When the parking brake pedal 42 is
 operated, the parking control 284 is operated to pull the parking brake
 cable 242 for pivoting the lever 230 also in the shoe expanding direction.
 In this case, the primary brake cable 240 becomes slack. Since the two
 brake cables 240, 242 which are both connected to the lever 230 and pulled
 at different times are flexible, the operation of one these brake cables
 is not influenced or disturbed by the other brake cable.
 While the structural arrangement of the present electrically operated
 braking system has been described, there will next be described a control
 arrangement of the braking system.
 Referring back to FIG. 1, the braking system employs a controller 50 which
 is principally constituted by a computer 300 incorporating a central
 processing unit (CPU), a read-only memory (ROM) and a random-access memory
 (RAM). The controller 50 is adapted to receive various inputs, namely,
 output signals of various sensors and switches including an operating
 force sensor 302, a brake pedal operation detecting switch 304, an
 accelerator pedal operation detecting switch 306, a steering angle sensor
 308, a vehicle deceleration sensor 310, a vehicle speed sensor 312, four
 wheel speed sensors 314 and a motor current sensor 316.
 The operation force sensor 302 generates an output signal indicative of an
 operation force F which acts on the brake pedal 40. The brake pedal
 operation detecting switch 304 generates an output signal indicating
 whether the brake pedal 40 is in operation or not. The accelerator pedal
 operation detecting switch 306 generates an output signal indicating
 whether the accelerator pedal 44 is in operation or not. The steering
 angle sensor 308 generates an output signal indicative of a rotation angle
 .theta. of the steering wheel 46. This sensor 308 serves as a sensor for
 detecting whether the vehicle is turning or not. The vehicle deceleration
 sensor 310 generates an output signal indicative of a deceleration value G
 of the motor vehicle in the running or forward direction. The vehicle
 speed sensor 312 generates an output signal indicative of a running speed
 V of the motor vehicle. The four wheel speed sensors 314 generate
 respective output signals indicative of rotating speed Vw of the
 respective wheels FL, FR, RL, RR. The motor current sensor 316 is
 connected to coils of the motors 20, 30 of the disc and drum rakes 22, 32,
 and generates output signals indicative of electric currents I which are
 actually applied to the respective coils of the motors 20, 30 from, a
 battery 320 through a driver 322. The output signals of the motor current
 sensor 316 are voltage signals representative of the electric current
 values I.
 The controller 50 provides output signals including current control signals
 to be applied to the above-indicated driver 322. Upon operation of the
 brake pedal 40, the controller 50 applies the current control signals to
 the driver 322 so that the electric current values I to be applied from
 the battery 320 through the driver 322 to the respective motors 20, 30 of
 the brakes 22, 32 are controlled based on the current control signals. The
 output signals provided by the controller 50 also include signals to be
 applied to an engine output control device and a transmission control
 device. The engine output control device includes a throttle valve control
 device, a fuel supply control device and an ignition timing control
 device, while the transmission control device includes various
 solenoid-operated valves. The engine output control device and the
 transmission control device are controlled according to the signals from
 the controller 50, so as to control the wheel drive forces to be applied
 to the drive wheels, for preventing excessive amounts of spinning of the
 drive wheels during starting or acceleration of the vehicle, that is, for
 effecting a so-called "traction control" of the vehicle.
 The ROM of the computer 300 stores various programs such as those for
 executing a brake control routine illustrated in the flow chart of FIG. 5
 and a friction coefficient estimating routine illustrated in the flow
 chart of FIG. 6. The ROM is also used for storing data tables or
 functional equations which represent F-T relationship patterns and I-T
 relationship patterns.
 Each of the F-T relationships is a relationship between the operating force
 F acting on the brake pedal 40 and a braking torque T applied to each
 wheel by operation of the corresponding brake 22, 32. An example of the
 F-T relationship patterns is indicated in the graph of FIG. 7. Each of the
 I-T relationships is a relationship between the electric current I to be
 applied to each motor 20, 30 and the braking torque T applied to each
 wheel by operation of the corresponding brake 22, 32. The I-T
 relationships are obtained by experiments or by calculation. Examples of
 the I-T relationship patterns are indicated in the graph of FIG. 8. The
 F-T relationship patterns and I-T relationship patterns generally differ
 for the front wheels FL, FR and the rear wheels RL, RR, and are therefore
 stored in the ROM in relation to the front wheels and the rear wheels.
 Each I-T relationship pattern indicates a change of the braking torque
 value T with a change in the electric current value I. This I-T
 relationship pattern is stored for each of different friction coefficient
 values .mu. of the friction members used in the brakes 22, 32. That is,
 the ROM stores a plurality of I-T relationship patterns corresponding to
 the respective different friction coefficient values .mu.. The friction
 members consist of the brake pads 106a, 106b of the disc brakes 22 for the
 front wheels FL, FR, and the brake linings 216a, 216b of the drum brakes
 32 for the rear wheels RL, RR.
 Before explaining in detail the brake control routine of FIG. 5 and the
 friction coefficient estimating routine of FIG. 6, the brake control and
 the friction coefficient estimation will first be briefly described.
 Referring to FIG. 9, there is schematically illustrated a relationship
 between the controller 50, motor 20, 30, friction member 330 and wheel
 332. The controller 50 receives the operating force F acting on the brake
 pedal 40 operated by the vehicle operator. Depending upon the operating
 force F, the controller 50 determines the electric current I to be applied
 to the motor 20, 30. In response to the electric current I supplied, the
 motor 20, 30 generates a drive force D for forcing the friction member 330
 onto the disc rotor 104 or drum 204. The friction member 330 having a
 specific friction coefficient .mu. cooperates with the disc rotor 104 or
 drum 204 to apply a braking torque T to the wheel 332, based on the drive
 force D generated by the motor 20, 30. As a result, the vehicle is given a
 deceleration value G, while the wheel 332 is given a deceleration value
 Gw.
 In the brake control, the electric current I to be applied to the motor 20,
 30 is determined on the basis of the operating force F applied to the
 brake pedal 40. Described more specifically, a desired braking torque T*
 for each wheel 332 is determined on the basis of the operating force F and
 according to the appropriate F-T relationship pattern. On the basis of the
 thus determined desired braking torque T*, the electric current I to be
 applied to the wheel is determined according to the I-T relationship
 pattern.
 The friction coefficient .mu. of the friction member 330 is estimated by
 activating the motor 20, 30 of the brake 22, 32 under predetermined
 conditions, namely: when the brake pedal 40 is not in operation; when the
 accelerator pedal 42 is not in operation; when the vehicle is running
 straight, that is, is not turning; when the vehicle is not running on a
 bad road surface; when the vehicle is not running at a speed lower than a
 predetermined threshold; and when the automatic transmission 12 is not in
 the process of a shifting action. When the vehicle running speed V is zero
 (when the vehicle is stopped), the controller 50 determines that the
 vehicle is running at a speed lower than the predetermined threshold, the
 brake 22 is not operated to estimate the friction coefficient .mu. of the
 friction member 330, even when the brake pedal 40 is not in operation.
 That is, the estimation of the friction coefficient .mu. is effected while
 the vehicle is coasting straight at a relatively high speed on a good road
 surface.
 During operation of the brake 22, 32 to estimate the friction coefficient
 .mu., the electric current I supplied to the motor 20, 30 and the vehicle
 deceleration value G are obtained, and the actual braking torque value T
 of each wheel is obtained based on the obtained vehicle deceleration value
 G. Further, one of the I-T relationship patterns which has a point located
 on or closest to a point indicative of a combination of the obtained
 actual braking torque value T and the obtained electric current I is
 selected as the presently effective I-T relationship pattern. The friction
 coefficient .mu. corresponding to the selected or presently effective I-T
 relationship pattern is determined as the estimated friction coefficient
 .mu. of the friction member 330, which is stored in the RAM.
 Although the friction coefficient .mu. of the friction member 330 is
 preferably estimated for each of the four wheels, the estimation in the
 present embodiment is effected for each of the front and rear wheel pairs
 F, R, since the disc brakes 22 for the two front wheels FL, FR use the
 same friction members in the form of the brake pads 106a, 106b, while the
 drum brakes 32 for the two rear wheels RL, RR use the same friction
 members in the form of the brake linings 215a, 216b.
 While the friction coefficient .mu. estimated in the present run of the
 vehicle is stored in the RAM, the I-T relationship pattern corresponding
 to this last estimated friction coefficient .mu. is selected to determine
 the electric current I on the basis of the determined desired braking
 torque T*. While the friction coefficient .mu. estimated in the present
 vehicle run is not stored in the RAM, the I-T relationship pattern
 corresponding to the friction coefficient .mu. which was estimated in the
 previous run of the vehicle and which is stored in the RAM is
 provisionally used to determine the electric current I, until the friction
 coefficient is estimated in the present vehicle run (until the estimation
 is updated). If the friction coefficient .mu. was not estimated in the
 previous vehicle run and is not stored in the RAM, the I-T relationship
 pattern corresponding to the predetermined standard value (stored in the
 ROM) of the friction coefficient .mu. is provisionally used to determine
 the electric current I, until the friction coefficient is estimated in the
 present vehicle run.
 Then, the brake control routine of FIG. 5 and the friction coefficient
 estimating routine of FIG. 6 will be described in detail.
 The brake control routine of FIG. 5 is executed while an ignition switch of
 the vehicle is on. The brake control routine is initiated with step S1 in
 which the operating force F acting on the brake pedal 40 is detected by
 the operation force sensor 402. Step S1 is followed by step S2 to
 determine whether the friction coefficient value .mu. estimated in the
 present run of the vehicle is stored in the RAM of the controller 50. If
 an affirmative decision (YES) is obtained in step S2, the control flow
 goes to step S3 in which the friction coefficient value .mu. estimated in
 the present vehicle run and stored in the RAM is selected as the effective
 friction coefficient value. If a negative decision (NO) is obtained in
 step S2, the control flow goes to step S4 to determine whether the
 friction coefficient value .mu. estimated in the previous vehicle run is
 stored in the RAM. If an affirmative decision (YES) is obtained in step
 S4, the control flow goes to step S5 in which the previously estimated
 friction coefficient value .mu. is selected as the effective value. If a
 negative decision (NO) is obtained in step S4, the control flow goes to
 step S6 in which the predetermined standard value of the friction
 coefficient .mu. is selected as the effective value.
 Step S3, S5 and S6 are followed by step S7 in which one of the stored I-T
 relationship patterns which corresponds to the currently selected
 effective friction coefficient value .mu. is selected as the effective I-T
 relationship pattern. Then, step S8 is implemented to determine the
 desired braking torque value T* for each wheel, on the basis of the
 detected operating force F and according to the F-T relationship pattern.
 Step S8 is followed by step S9 in which a desired value I* of the electric
 current I for each wheel is determined on the basis of the desired braking
 torque value T* and according to the currently selected effective I-T
 relationship pattern. The control flow then goes to step S10 in which the
 electric current of the determined desired value I* is applied to the
 electric motor 20, 30 of each brake 22, 32. Thus, one cycle of execution
 of the brake control routine of FIG. 5 is terminated, and the control flow
 returns to step S1.
 The friction coefficient estimating routine of FIG. 6 is also executed with
 a predetermined cycle time while the ignition switch is on. The routine is
 initiated with step S11 to effect initialization in which an ESTIMATION
 flag is reset to "0". When this flag is set at "0", it means that the
 friction coefficient .mu. has not been estimated during the present run of
 the vehicle. When the flag is set at "1", it means that the friction
 coefficient .mu. has been estimated once during the present run of the
 vehicle.
 Step S11 is followed by steps S12-S18 to determine whether the
 predetermined conditions that should be satisfied to estimate the friction
 coefficient .mu. have been satisfied. Described in detail, step S12 is
 implemented to determine whether the ESTIMATION flag is set at "0". If an
 affirmative decision (YES) is obtained in step S12, the control flow goes
 to step S13 to determine whether the brake pedal operation detecting
 switch 304 is off, that is, to determine whether the brake pedal 40 is
 placed at its non-operated position. If an affirmative decision (YES) is
 obtained in step S13, the control flow goes to step S14 to determine
 whether the accelerator pedal operation detecting switch 306 is off, that
 is, to determine whether the accelerator pedal 42 is placed at its
 non-operated position. If an affirmative decision (YES) is obtained in
 step S14, the control flow goes to step S15 to determine whether the
 vehicle is turning, that is, to determine whether the rotation angle
 .theta. of the steering wheel 46 detected by the steering angle sensor 308
 is larger than a threshold value which is close to zero. If a negative
 decision (NO) is obtained in step S15, the control flow goes to step S16
 to determine whether the vehicle is running on a bad road surface. The
 determination in step S16 is effected by determining whether the frequency
 of change of the sign of the wheel deceleration value Gw is higher than a
 predetermined threshold value. The wheel deceleration value Gw is obtained
 by obtaining a time derivative of the wheel speed Vw detected by the wheel
 speed sensor 314. If a negative decision (NO) is obtained in step S16, the
 control flow goes to step S17 to determine whether the vehicle running
 speed V detected by the vehicle speed sensor 312 is lower than a
 predetermined threshold Vo. If a negative decision (NO) is obtained in
 step S17, the control flow goes to step S19 to determine whether the
 automatic transmission 12 is in the process of a shifting action. This
 determination in step S19 is effected based on a signal received from the
 automatic transmission 12. If the affirmative decision (YES) is obtained
 in steps S12-S14 while the negative decision (NO) is obtained in steps
 S15-S18, the control flow goes to step S19. In the other cases, the
 control flow goes back to step S12.
 In step S19, a predetermined amount of electric current Io is applied to
 the electric motors 20 of the disc brakes 22 for the front left and right
 wheels FL, FR, for a predetermined time .DELTA.t. Step S19 is followed by
 step S20 in which the electric current I actually applied to the motors 20
 is detected by the motor current sensor 316. Step S20 is followed by step
 S21 in which the deceleration value G of the vehicle during activation of
 the disc brakes 22 is detected by the vehicle deceleration sensor 310.
 Then, step S22 is implemented to estimate the friction coefficient .mu. of
 the brake pads 106a, 106b of the disc brakes 22, on the basis of the
 detected electric current I and vehicle deceleration value G. Described in
 detail, the actual braking torque value T of the front disc brakes 22 is
 calculated on the basis of the detected vehicle deceleration value G. One
 of the I-T relationship patterns which has a point located on or closest
 to a point indicative of a combination of the detected electric current I
 and the calculated actual braking torque value T is selected as the
 effective I-T relationship pattern. The friction coefficient value .mu.
 corresponding to the selected effective I-T relationship pattern is
 determined as the estimated value of the friction coefficient of the brake
 pads 106a, 106b. Then, steps S23 through S26 similar to the
 above-indicated steps S19-S22 are implemented for the rear drum brakes 32,
 to estimate the friction coefficient value .mu. of the brake linings 216a,
 216b of the drum brakes 32. Step S26 is followed by step S27 in which the
 ESTIMATION flag is set to "1". Then, the control flow goes to step S12.
 However, since the ESTIMATION flag has been set to "1", the negative
 decision (NO) is subsequently obtained in step S12, and the estimation of
 the friction coefficient .mu. of the friction members 106a, 106b, 210a,
 210b is not implemented, until the present vehicle run is terminated.
 As described above, the present embodiment is adapted to effect the
 estimation of the friction coefficient .mu. of the friction members only
 once during each run of the vehicle. Once the estimation has been effected
 during the present vehicle run, the friction coefficient is not updated
 during the present vehicle run. However, the friction coefficient may be
 updated during the same vehicle run.
 The graph of FIG. 10 shows a gradual drop of the vehicle speed V as a
 result of the activation of the disc and drum brakes 22, 32 during the
 estimation of the friction coefficient .mu. of the friction members while
 the vehicle is coasting without the brake pedal 40 being depressed.
 When the conditions for initiating the estimation of the friction
 coefficient .mu. have been satisfied at point of time t1, the
 predetermined amount Io of electric current I is applied to the electric
 motors 20 of the front disc brakes 22, so that the vehicle speed V is
 reduced. The rate of reduction of the vehicle speed V, that is, the
 deceleration value G of the vehicle depends upon the friction coefficient
 .mu. of the brake pads 106a, 106b of the disc brakes 22. Where the
 friction coefficient .mu. of the brake pads 106a, 106b is relatively high,
 the vehicle is decelerated with a relatively high deceleration value G1.
 Where the friction coefficient .mu. is relatively low, the vehicle is
 decelerated with a relatively low deceleration value G2. When the
 predetermined time .DELTA.t has passed from the point of time t1, that is,
 at a point of time t2, the supply of the electric current to the electric
 motors 20 is terminated, and the motors 20 are restored to the
 non-operated state.
 At a point of time t3 short time after the point of time t2, the
 predetermined amount Io of current is applied to the electric motors 30 of
 the rear drum brakes 32. As a result, the vehicle speed V is further
 reduced. The rate of reduction of the vehicle speed V or the deceleration
 value G of the vehicle at this time depends upon the friction coefficient
 m of the brake linings 216a, 216b. Where the friction coefficient is
 relatively high, the vehicle is decelerated with a relatively high
 deceleration value G3. Where the friction coefficient is relatively low,
 the vehicle is decelerated with a relatively low deceleration value G4.
 When the predetermined time .DELTA.t has passed after the point of time
 t3, that is, at a point of time t4, the supply of the electric current to
 the electric motors 30 is terminated, and the motors 30 are restored to
 the non-operated state.
 It will be understood from the above description of the present embodiment
 of this invention that portions of the controller 50 assigned to execute
 the brake control routine of FIG. 5 and the friction coefficient
 estimating routine of FIG. 6 constitute a relationship estimating and
 utilizing device for estimating a relationship between the electric
 current I to be supplied to the electric motors 20, 30 and the braking
 torque or force to be applied from the disc and drum brakes 22, 32 to the
 wheels, on the basis of the actual value of the electric current I
 supplied from the battery 320 to the electric motors 20, 30 and the actual
 values of the braking torque T of the disc and drum brakes 22, 32, which
 actual values are detected during operations of the brakes 22, 32 while
 the vehicle is running. The values of the braking torque to be applied to
 the wheels is changed with a change in the electric current to be applied
 to the electric motors. The relationship estimating and utilizing device
 is further adapted to utilize the estimated relationship for controlling
 the brakes 22, 32. It will also be understood that the portion of the
 controller 50 assigned to execute the brake control routine of FIG. 5
 constitutes relationship utilizing means for utilizing the obtained
 relationship, while the portion of the controller 50 assigned to implement
 steps S13 and S19-S26 of the friction coefficient estimating routine of
 FIG. 16 constitutes means for estimating the relationship while the
 vehicle is running without an operation of the brake pedal 40. It will
 further be understood that the vehicle deceleration sensor 310 serves as
 means for detecting the vehicle deceleration G. It will also be understood
 that the portion of the controller 50 assigned to implement steps S15 and
 S17 of the routine of FIG. 6 constitutes first inhibiting means for
 inhibiting the relationship estimating and utilizing device from operating
 the disc and drum brakes 22, 32 to obtain the relationship, while the
 vehicle is running under a condition in which the operations of the drum
 and disc brakes 22, 32 by the relationship estimating and utilizing device
 are likely to be felt unusual or uncomfortable by the vehicle operator. It
 will further be understood that the portion of the controller 50 assigned
 to implement step S17 of the routine of FIG. 6 constitutes means for
 inhibiting the relationship estimating and utilizing device from operating
 the disc and drum brakes 22, 32 while the vehicle is running at a speed
 lower than a predetermined threshold value. It will also be understood
 that the portion of the controller 50 assigned to implement steps S14-S16
 and S18 of the routine of FIG. 6 constitutes second inhibiting means for
 inhibiting the relationship estimating and utilizing device from
 estimating and/or utilizing the relationship while the vehicle is running
 under a condition in which the relationship is not likely to be accurately
 estimated. It will also be understood that the portion of the controller
 50 assigned to implement steps S14 and S18 of the routine of FIG. 6
 constitutes means for inhibiting the relationship estimating and utilizing
 device from at least utilizing the estimated relationship while a drive
 force for driving the vehicle is changing. It will further be understood
 that the portion of the controller 50 assigned to implement step S15 of
 the routine of FIG. 6 constitutes means for inhibiting the relationship
 estimating and utilizing device from at least utilizing the estimated
 relationship while the vehicle is turning.
 Referring to FIGS. 11-22, there will be described other embodiments of the
 present invention. The same reference numerals as used in the first
 embodiment will be used to identify the same elements in these other
 embodiments, and only differences of these embodiments from the first
 embodiment will be described, to avoid redundant explanation.
 A second embodiment of this invention is adapted to concurrently activate
 the disc and drum brakes 22, 32 for the four wheels while the vehicle is
 coasting without an operation of the brake pedal 40, and the vehicle
 deceleration value G is obtained during the activation of. the brakes 22,
 32 so that the actual braking torque values T of the brakes 22, 32 are
 obtained on the basis of the obtained deceleration value G. In this
 respect, the second embodiment is ,different from the first embodiment
 which is adapted to activate the front disc brakes 22 and the rear drum
 brakes 32 at different times in steps S19 and S23, respectively.
 A friction coefficient estimating routine according to the second
 embodiment is illustrated in the flow chart of FIG. 11. In the following
 description of this routine, steps similar to those in the routine of FIG.
 6 will be described only briefly.
 The friction coefficient estimating routine of FIG. 11 is initiated with
 step S51 to effect initialization in which the ESTIMATION flag is reset to
 "0". Step S11 is followed by step S52 to determine whether the ESTIMATION
 flag is set at "0". If an affirmative decision (YES) is obtained in step
 S52, the control flow goes to step S53 to determine whether the brake
 pedal operation detecting switch 304 is off, that is, to determine whether
 the brake pedal 40 is placed at its non-operated position. If a negative
 decision (NO) is obtained in step S53, the control flow returns to step
 S52. If an affirmative decision (YES) is obtained in step S53, the control
 flow goes to step S54 to determine whether the vehicle running speed V
 detected by the vehicle speed sensor 312 is lower than a predetermined
 threshold Vo. If an affirmative decision decision (YES) is obtained in
 step S54, the control flow returns to step S52. If a negative decision
 (YES) is obtained in step S54, the control flow goes to step S55 to
 determine whether the vehicle is running under any conditions in which the
 friction coefficient values .mu. of the friction members are not likely to
 be accurately estimated. These conditions include: an operation of the
 accelerator pedal 42 to accelerate the vehicle; a turning of the vehicle;
 a running of the vehicle on a bad road surface; and a shifting action of
 the automatic transmission 12, as discussed above with respect to steps
 S14, S14, S16 and S18 of the routine of FIG. 6 of the first embodiment. If
 an affirmative decision (YES) is obtained in step S55, the control flow
 returns to step S52. If a negative decision (NO) is obtained in step S55,
 the control flow goes to step S56.
 In step S56, the front disc brakes 22 for the front wheels and the rear
 drum brakes 32 for the rear wheels are substantially concurrently or
 simultaneously activated. Step S56 is followed by step S57 in which the
 electric current I actually applied to each of the motors 20, 30 is
 detected by the motor current sensor 316. Step S57 is followed by step S58
 in which the deceleration value G of the vehicle during activation of the
 four brakes 22, 32 is detected by the vehicle deceleration sensor 310.
 Then, step S59 is implemented to estimate the friction coefficient value
 .mu. of the brake pads 106a, 106b of the disc brakes 22 and the friction
 coefficient value .mu. of the brake linings 216a, 216b of the drum brakes
 32, on the basis of the detected electric current values I and vehicle
 deceleration value G. Described in detail, the actual braking torque
 values T of the front disc brakes 22 and the actual braking torque values
 T of the rear drum brakes 32 are estimated on the basis of the detected
 vehicle deceleration value G, and depending upon a difference between the
 braking capacities of the disc and drum brakes 22, 32 and a difference
 between the load acting on the front wheels and the load acting on the
 rear wheels. A sum of the estimated braking torque values T of the front
 disc brakes 22 and a sum of the estimated braking torque values T of the
 rear drum brakes 32 are then calculated. A half of the former sum is
 determined as the actual braking torque T of each front disc brake 22,
 while a half of the latter sum is determined as the actual braking torque
 T of each rear drum brake 32. One of the I-T relationship patterns which
 has a point located on or closest to a point indicative of a combination
 of the detected electric current I and the calculated actual braking
 torque T of each front disc brake 22 is selected as the effective I-T
 relationship pattern. The friction coefficient value .mu. corresponding to
 the selected effective I-T relationship pattern is obtained as the
 estimated value of the friction coefficient of the brake pads 106a, 106b
 of each front disc brake 22. Similarly, the estimated friction coefficient
 value of the brake linings 216a, 216b of each rear drum brake 32 is
 obtained.
 As described above, the second embodiment is adapted such that the actual
 braking torque value T of the front disc brakes 22 and the actual braking
 torque value T of the rear drum brakes 32 are obtained on the basis of the
 same vehicle deceleration value G which is obtained during concurrent
 operations of the four brakes 22, 32, and the friction coefficient value
 .mu. of the brake pads 106a, 106b of the front disc brakes 22 and the
 friction coefficient value .mu. of the brake linings 216a, 216b of the
 rear drum brakes 32 are estimated independently of each other on the basis
 of the obtained actual front and rear braking torque values T.
 Step S59 is followed by step S60 to set the ESTIMATION flag to "1". Then,
 the control flow goes back to step S52.
 A third embodiment of the invention is adapted to activate the four brakes
 22, 32 one after another at different times, and detect the deceleration
 values G for the respective brakes 22, 32 and obtain the actual braking
 torque values T for the respective brakes independently of each other.
 A friction coefficient estimating routine according to the third embodiment
 is illustrated in the flow chart of FIG. 12. In the following description
 of this routine, steps similar to those in the routine of FIG. 6 will be
 described only briefly.
 The friction coefficient estimating routine of FIG. 12 is initiated with
 step S71 to effect initialization in which the ESTIMATION flag is reset to
 "0". Step S71 is followed by step S72 to determine whether the ESTIMATION
 flag is set at "0". If an affirmative decision (YES) is obtained in step
 S72, the control flow goes to step S73 to determine whether the brake
 pedal operation detecting switch 304 is off. If a negative decision (NO)
 is obtained in step S73, the control flow returns to step S72. If an
 affirmative decision (YES) is obtained in step S73, the control flow goes
 to step S74 to determine whether the vehicle running speed V is lower than
 a predetermined threshold Vo. If an affirmative decision (YES) is obtained
 in step S74, the control flow goes back to step S72. If a negative
 decision (NO) is obtained in step S54, the control flow goes to step S75
 to determine whether the vehicle is running under any conditions in which
 the friction coefficient values .mu. of the friction members are not
 likely to be accurately estimated. These conditions include: an operation
 of the accelerator pedal 42 to accelerate the vehicle; a turning of the
 vehicle; a running of the vehicle on a bad road surface; and a shifting
 action of the automatic transmission 12, as discussed above with respect
 to steps S14, S15, S16 and S18 of the routine of FIG. 6 of the first
 embodiment. If an affirmative decision (YES) is obtained in step S75, the
 control flow returns to step S72. If a negative decision (NO) is obtained
 in step S75, the control flow goes to step S76.
 In step S76, the two front disc brakes 22 and the two rear drum brakes 32
 are sequentially activated one after another, for instance, in the order
 of the disc brake 22 for the front left wheel FL, the disc brake 22 for
 the front right wheel FR, the drum brake 32 for the rear left wheel RL,
 and the drum brake 32 for the rear right wheel RR. Step S76 is followed by
 step S77 in which the values of the electric current I actually supplied
 to the motors 20, 30 are detected during sequential activations of the
 four brakes 22, 32. Step S77 is followed by step S78 in which the
 deceleration values G are detected during the sequential activations of
 the four brakes 22, 32. It is noted that while steps S76-S78 are
 sequentially and repeatedly implemented for each of the four brakes 22,
 32, although the flow chart of FIG. 12 does not explicitly show this
 arrangement. The thus detected vehicle acceleration values G accurately
 reflect the actual braking torque values T of the respective brakes 22,
 32.
 Then, step S79 is implemented to estimate the friction coefficient values
 .mu. of the brake pads 106a, 106b of the disc brakes 22 and the friction
 coefficient values .mu. of the brake linings 216a, 216b of the drum brakes
 32, on the basis of the detected electric current values I and vehicle
 deceleration values G. Described in detail, the actual braking torque
 value T of each brake 22, 32 is estimated on the basis of the
 corresponding vehicle deceleration value G. Then, One of the I-T
 relationship patterns which has a point located on or closest to a point
 indicative of a combination of the detected electric current I and the
 calculated actual braking torque T of each brake 22, 32 is selected as the
 effective I-T relationship pattern. The friction coefficient value .mu.
 corresponding to the selected effective I-T relationship pattern is
 obtained as the estimated value of the friction coefficient of the brake
 pad or lining of each brake 22, 32. Step S79 is followed by step S80 to
 set the ESTIMATION flag to "1". Then, the control flow returns to step
 S72.
 A fourth embodiment of this invention is adapted to inhibit the detection
 of the vehicle deceleration G to obtain the actual braking torque T where
 the gradient of the road surface on which the vehicle is running is higher
 than a predetermined threshold. In the first embodiment, the vehicle
 deceleration G is detected irrespective of the gradient of the road
 surface.
 Referring to FIG. 13, there is shown an arrangement of an electrically
 operated braking system according to the fourth embodiment, which includes
 a road gradient sensor 340 for detecting the gradient of the road surface.
 A friction coefficient estimating routine executed according to a program
 stored in a ROM of a computer 344 of a controller 342 of the braking
 system of the present fourth embodiment is illustrated in the flow chart
 of FIG. 14. In the following description of the routine of FIG. 14, steps
 similar to those in the first embodiment will be described only briefly.
 The friction coefficient estimating routine of FIG. 14 is initiated with
 step S81 to effect initialization in which the ESTIMATION flag is reset to
 "0". Step S81 is followed by step S82 to determine whether the ESTIMATION
 flag is set at "0". If an affirmative decision (YES) is obtained in step
 S82, the control flow goes to step S83 to determine whether the brake
 pedal operation detecting switch 304 is off. If a negative decision (NO)
 is obtained in step S83, the control flow returns to step S72. If an
 affirmative decision (YES) is obtained in step S83, the control flow goes
 to step S84 to determine whether the vehicle running speed V is lower than
 a predetermined threshold Vo. If an affirmative decision (YES) is obtained
 in step S84, the control flow goes back to step S82. If a negative
 decision (NO) is obtained in step S84, the control flow goes to step S85
 to determine whether the vehicle is running under any conditions in which
 the friction coefficient values .mu. of the friction members are not
 likely to be accurately estimated. These conditions include: an operation
 of the accelerator pedal 42 to accelerate the vehicle; a turning of the
 vehicle; a running of the vehicle on a bad road surface; and a shifting
 action of the automatic transmission 12, as discussed above with respect
 to steps S14, S14, S16 and S18 of the routine of FIG. 6 of the first
 embodiment. In the present fourth embodiment, the conditions that inhibit
 the estimation of the friction coefficient .mu. also include a condition
 that the gradient of the road surface on which the vehicle is running is
 higher than the predetermined threshold value. If an affirmative decision
 (YES) is obtained in step S85, the control flow returns to step S82. If a
 negative decision (NO) is obtained in step S85, the control flow goes to
 step S86.
 In step S86, the disc and drum brakes 22, 32 are activated in one of the
 following modes: (a) The four brakes 22, 32 are substantially concurrently
 activated, as in the second embodiment of FIG. 11; (b) the front disc
 brakes 22 are concurrently activated, and the rear drum brakes 32 are
 concurrently activated, but after or before the activation of the disc
 brakes 22, as in the first embodiment of FIG. 6; and (c) the four brakes
 22, 32 are sequentially activated, as in the third embodiment of FIG. 12.
 Step S86 is followed by step S87 in which the electric current I supplied
 to each brake 22, 32 is detected. Step S87 is followed by step S88 in
 which the deceleration value or values G is/are detected during the
 activation of the four brakes 22, 32. Then, step S89 is implemented to
 estimate the friction coefficient values .mu. of the friction members of
 the brakes 22, 32, on the basis of the detected electric current values I
 and vehicle deceleration value or values G, in one of the manners
 described with steps S26, S59 and S79. Step S89 is followed by step S90 to
 set the ESTIMATION flag to "1". Then, the control flow returns to step
 S82.
 A fifth embodiment of this invention is adapted to obtain the actual
 braking torque values T on the basis of deceleration values Gw of the
 wheels. In the preceding embodiments, the actual braking torque T is
 obtained on the basis of the detected vehicle deceleration G.
 Referring to FIG. 15, there is shown an arrangement of an electrically
 operated braking system according to the fifth embodiment, which does not
 include the vehicle deceleration sensor 310, since the vehicle
 deceleration G is not used to obtain the actual braking torque T in the
 present fifth embodiment.
 A friction coefficient estimating routine executed according to a program
 stored in a ROM of a computer 362 of a controller 360 of the braking
 system of the present fifth embodiment is illustrated in the flow chart of
 FIG. 16. In the following description of the routine of FIG. 16, steps
 similar to those in the first embodiment will be described only briefly.
 The friction coefficient estimating routine of FIG. 16 is initiated with
 step S101 to effect initialization in which the ESTIMATION flag is reset
 to "0". Step S101 is followed by step S102 to determine whether the
 ESTIMATION flag is set at "0". If an affirmative decision (YES) is
 obtained in step S102, the control flow goes to step S103 to determine
 whether the brake pedal operation detecting switch 304 is off. If a
 negative decision (NO) is obtained in step S103, the control flow returns
 to step S102. If an affirmative decision (YES) is obtained in step S103,
 the control flow goes to step S104 to determine whether the vehicle
 running speed V is lower than a predetermined is threshold Vo. If an
 affirmative decision (YES) is obtained in step S104, the control flow goes
 back to step S102. If a negative decision (NO) is obtained in step S104,
 the control flow goes to step S105 to determine whether the vehicle is
 running under any conditions in which the friction coefficient values .mu.
 of the friction members are not likely to be accurately estimated. These
 conditions include: an operation of the accelerator pedal 42 to accelerate
 the vehicle; a turning of the vehicle; a running of the vehicle on a bad
 road surface; and a shifting action of the automatic transmission 12, as
 discussed above with respect to steps S14, S14, S16 and S18 of the routine
 of FIG. 6 of the first embodiment. If an affirmative decision (YES) is
 obtained in step S105, the control flow returns to step S102. If a
 negative decision (NO) is obtained in step S105, the control flow goes to
 step S106.
 In step S106, the front disc brakes 22 for the front wheels and the rear
 drum brakes 32 for the rear wheels are substantially concurrently or
 simultaneously activated. Step S106 is followed by step S107 in which the
 electric current I actually applied to each of the motors 20, 30 is
 detected by the motor current sensor 316. Step S107 is followed by step
 S108 in which the deceleration value Gw of each of the four wheels FL, FR,
 RL, RR is calculated. The deceleration value Gw of each wheel is
 calculated by obtaining a time derivative of the rotating speed Vw of that
 wheel detected by the wheel speed sensor 314.
 The graph of FIG. 17 shows a change in the wheel speed Vw during activation
 of the brakes 22, 32. When the friction coefficient .mu. of the friction
 members of the brake 22, 32 is relatively high, the rate of reduction of
 the wheel speed Vw, that is, the deceleration value Gw of the wheel is
 relatively high. When the friction coefficient is relatively low, the
 wheel deceleration value Gw is relatively low.
 Then, step S109 is implemented to estimate the friction coefficient values
 .mu. of the friction members of the four brakes 22, 32, on the basis of
 the detected electric current values I and calculated wheel deceleration
 values Gw. Described in detail, the actual braking torque values T of the
 brakes 22, are estimated on the basis of the calculated wheel deceleration
 values Gw. One of the I-T relationship patterns which has a point located
 on or closest to a point indicative of a combination of the detected
 electric current I and the calculated actual braking torque T of each
 brake 22, 32 is selected as the effective I-T relationship pattern. The
 friction coefficient value .mu. corresponding to the selected effective
 I-T relationship pattern is obtained as the estimated value of the
 friction coefficient of each brake 22, 32. Then, step S110 is implemented
 to set the ESTIMATION flag to "1". The control flow then goes back to step
 S102.
 A sixth embodiment of the invention is different from the fifth embodiment,
 only in the friction coefficient estimating routine illustrated in the
 flow chart of FIG. 18.
 In the friction coefficient estimating routine of FIG. 18 according to the
 sixth embodiment, the friction coefficient .mu. of the friction members of
 the brakes 22, 32 is effected while the brake pedal 40 is operated. The
 routine is initiated with step S201 to effect initialization in which the
 ESTIMATION flag is reset to "0". Step S201 is followed by step S202 to
 determine whether the ESTIMATION flag is set at "0". If an affirmative
 decision (YES) is obtained in step S202, the control flow goes to step
 S203 to determine whether the brake pedal operation detecting switch 304
 is on, that is whether the brake pedal 40 is in operation. If a negative
 decision (NO) is obtained in step S203, the control flow returns to step
 S202. If an affirmative decision (YES) is obtained in step S203, the
 control flow goes to step S204 to determine whether the whether the
 vehicle is running under any conditions in which the friction coefficient
 values .mu. of the friction members are not likely to be accurately
 estimated. These conditions include: an operation of the accelerator pedal
 42 to accelerate the vehicle; a turning of the vehicle; a running of the
 vehicle on a bad road surface; and a shifting action of the automatic
 transmission 12, as in the fifth embodiment. In this sixth embodiment, the
 conditions that inhibits the estimation of the friction coefficient .mu.
 include an operation of the braking system in an anti-lock fashion, and
 stopping of the vehicle. The anti-lock control of the disc and drum brakes
 22, 32 is effected with the electric motors 20, 30 being controlled by the
 controller 50 on the basis of the output signals of the wheel speed
 sensors 314. The stopping of the vehicle is detected if the vehicle
 running speed V detected by the vehicle speed sensor 312 is lower than a
 predetermined lower limit. If an affirmative decision (YES) is obtained in
 step S204, the control flow returns to step S202. If a negative decision
 (NO) is obtained in step S205, the control flow goes to steps S205-S209,
 which are similar to steps S106-S110 of the routine of FIG. 16 in the
 fifth embodiment.
 A seventh embodiment of the present invention is different from the first
 embodiment, in that the actual braking torque values T of the brakes 22,
 32 are detected by sensors exclusively provided for this purpose.
 The arrangement of an electrically operated braking system according to the
 seventh embodiment is schematically shown in FIG. 19. This braking system
 includes (a) a first force sensor 380 provided in each front disc brake
 22, to detect the actual braking torque T, and (b) a second force sensor
 382 provided in each rear drum brake 32, to detect the actual braking
 torque T. Unlike the braking system of the first embodiment, the present
 braking system does not include the vehicle deceleration sensor 310.
 Referring to FIG. 20, there is shown the electrically operated disc brake
 22 for each front wheel, wherein the first force sensor 380 is interposed
 between the spring 112 and the mounting bracket 100. Referring next to
 FIG. 21, there is shown the electrically operated drum brake 32 for each
 rear wheel, wherein the second force sensor 382 is disposed on one of the
 brake shoes 210a, 210b, that is, the secondary shoe 210a which will
 receive a larger load than the primary shoe 210b. The second force sensor
 382 may be disposed on the anchor pin 206.
 A friction coefficient estimating routine executed according to a program
 stored in a ROM of a computer 386 of a controller 384 of the present
 braking system is illustrated in the flow chart of FIG. 22. In the
 description of the routine of FIG. 22, steps similar to those in the first
 embodiment will be described only briefly.
 The routine of FIG. 22 is initiated with step S301 to effect initialization
 in which an ESTIMATION flag is reset to "0". Step S301 is followed by step
 S302 to determine whether the whether the ESTIMATION flag is set at "0".
 If an affirmative decision (YES) is obtained in step S202, the control
 flow goes to step S303 to determine whether the brake pedal operation
 detecting switch 304 is off. If a negative decision (NO) is obtained in
 step S303, the control flow goes back to step S302. If an affirmative
 decision (YES) is obtained in step S303, the control flow goes to step
 S304 to determine whether the vehicle speed V is lower than a
 predetermined threshold Vo. If an affirmative decision (YES) is obtained
 in step S304, the control flow goes back to step S302. If a negative
 decision (NO) is obtained in step S304, the control flow goes to step S305
 to determine whether the vehicle is running under any conditions in which
 the friction coefficient .mu. is not likely to be accurately estimated.
 These running conditions include: an operation of the accelerator pedal
 42; a turning of the vehicle; a running of the vehicle on a bad road
 surface; and a shifting action of the automatic transmission 12, as
 described above with respect to the first embodiment. If an affirmative
 decision (YES) is obtained in step S305, the control flow goes back to
 step S302. If a negative decision (NO) is obtained in step S305, the
 control flow goes to step S306.
 In step S306, the disc and drum brakes 22, 32 are activated in one of the
 following modes: (a) The four brakes 22, 32 are substantially concurrently
 activated, as in the second embodiment of FIG. 11; (b) the front disc
 brakes 22 are concurrently activated, and the rear drum brakes 32 are
 concurrently activated, but after or before the activation of the disc
 brakes 22, as in the first embodiment of FIG. 6; and (c) the four brakes
 22, 32 are sequentially activated, as in the third embodiment of FIG. 12.
 Step S306 is followed by step S307 in which the electric current I
 supplied to each brake 22, 32 is detected. Step S307 is followed by step
 S308 in which the actual braking torque values T of the disc and drum
 brakes 22, 32 are detected by the first and second force sensors 380, 382,
 respectively, during the activation of the brakes 22, 32. Then, step S309
 is implemented to estimate the friction coefficient values .mu. of the
 friction members of the brakes 22, 32, on the basis of the detected
 electric current values I and actual braking torque values T. One of the
 I-T relationship patterns which has a point located on or closest to a
 point indicative of a combination of the electric current I and the
 braking torque value T of each brake 22, 32 is selected as the effective
 I-T relationship pattern. The friction coefficient .mu. corresponding to
 the selected I-T relationship pattern is obtained as the estimated
 friction coefficient value for each brake. The control flow then goes to
 step S309 to the ESTIMATION flag to "1". Then, the control flow returns to
 step S310.
 The embodiments which have been described are adapted to detect various
 physical parameters such as the electric current I supplied to each
 electric motor 20, 30, vehicle deceleration G, wheel deceleration Gw and
 actual braking torque T of the brakes 22, 32. Each of these physical
 parameters is detected for a predetermined time period. The peak value or
 an average of a plurality of values obtained in the detection period may
 be used as the detected value. Alternatively, an integral value of the
 values obtained in the detection period may be used as the detected value.
 Referring to FIGS. 23-43, further embodiments of this invention will be
 described.
 FIG. 23 shows an electrically operated braking system constructed according
 to an eighth embodiment of the invention, which includes electrically
 operated front disc brakes 522 each having the electric motor 20 described
 above, and electrically operated rear drum brakes 532 each having the
 electric motor 30 also described above. These disc and drum brakes 530,
 532 do not use a hydraulic working fluid. Like the braking system of the
 first embodiment of FIG. 1, the braking system of FIG. 23 includes the
 engine 10, automatic transmission 12, parking brake pedal 43, accelerator
 pedal 44 and steering wheel 46.
 Unlike the braking system of FIG. 1, the braking system of FIG. 23 further
 includes mechanically operated rear drum brakes 536 which are operated as
 emergency brakes, by a force produced as a result of an operation of a
 brake operating member in the form of a brake pedal 534. This drum brake
 536 does not use a hydraulic working fluid, either. Thus, each of the rear
 left and right wheels RL, RR is provided with both the electrically
 operated drum brake 532 and the mechanically operated drum brake 536.
 These drum brakes 532, 536 commonly use the same drum 204 and the same
 brake shoes 210a, 210b (brake linings 216a, 216b).
 Upon operation of the brake pedal 534, the vehicle is braked by at least
 one of the three pairs of brakes 522, 532, 536. That is, there are the
 following four cases:
 (a) Where the electrically operated disc brakes 522 and drum brakes 532 are
 both normal, the vehicle is braked by these brakes 522, 532;
 (b) Where the electrically operated disc brakes 522 are not normal while
 the electrically operated drum brakes 532 are normal, the vehicle is
 braked by only the drum brakes 532;
 (c) Where the electrically operated disc brakes 522 are normal while the
 electrically operated drum brakes 532 are not normal, the vehicle is
 braked by both the disc brakes 522 and the mechanically operated drum
 brakes 536;
 (d) Where the electrically operated disc and drum brakes 522, 532 are both
 abnormal due to abnormality of an electric power source (primary battery
 874) or an electronic control unit (ECU) 550, the vehicle is braked by
 only the mechanically operated drum brakes 536.
 In the present braking system, parking brake is applied to the front left
 and right wheels FL, FR by the disc brakes 522 upon operation of the
 parking brake pedal 42. The parking brake is applied by holding the
 electric motors 20 (ultrasonic motors) of the disc brakes 522 stationary
 with a holding torque while the parking brake pedal 42 is kept operated.
 The disc brake 522 for the front right wheel FR is shown in detail in FIG.
 24. The disc brake 533 for the front left wheel FL has the same
 construction as shown in FIG. 24. The disc brake 522 of FIG. 24 is
 identical with the disc brake 22 of FIG. 2 of the first embodiment, except
 for the configuration of brake pads 606 (more specifically, inner brake
 pad 606b) and the provision of a force switch 650 provided in a stationary
 member in the form of the mounting bracket 100. Unlike the inner brake pad
 110b of the disc brake 22 of FIG. 2, the inner brake pad 606b of the disc
 brake 522 has a backing plate 640 which has a constant thickness. When the
 outer and inner pads 606a, 606b are forced onto the friction surfaces 102
 of the disc rotor 102, the pads 606a, 606b are "dragged" or rotated with
 the disc rotor 102 in the direction X. However, the amounts of rotation of
 the pads 606a, 606b are limited by respective torque receiving portions
 610a, 610b of the mounting bracket 100. Namely, the amount of rotation of
 the outer pad 606a is limited by abutting contact of its end face with the
 outer torque receiving portion 610a, while the amount of rotation of the
 inner pad 606b is limited by abutting contact of the force switch 650 with
 the inner torque receiving portion 610b, as described below by reference
 to FIG. 25. The torque receiving portions 610a, 610b function as support
 members for supporting the friction members so as to prevent rotation of
 the friction members in the form of the brake pads 606a, 606b with the
 rotor 104 when the friction members are held in frictional contact with
 the rotor 104.
 The presser portion 134 moved by the electric motor 20 through the
 ballscrew mechanism 136 is not provided at its front end with such a
 thrust bearing as provided in the disc brake 22 of FIG. 22. The presser
 portion 134 cooperates with the ballscrew mechanism 132 to constitute a
 pressing device for forcing the brake pads 606a, 606b onto the rotor 104.
 The force switch 650 is provided in the inner torque receiving portion 610b
 which is adapted to receive a torque from the inner brake pad 606b. As
 shown in enlargement in FIG. 25, the force switch 650 consists of a
 movable member 652 and an elastic member in the form of a coned disc
 spring 654. The movable member 652 is a stepped cylindrical member
 including a large-diameter portion 656 and a small-diameter portion 658
 which both have a circular cross sectional shape and are coaxial with each
 other. The large-diameter portion 656 is received axially movably within
 the inner torque receiving portion 610b, while the small-diameter portion
 658 extends through a center opening of the coned disc spring 654. The
 coned disc spring 654 biases the movable member 652 toward the inner brake
 pad 606b. Normally, the movable member 652 is held in abutting contact
 with a stop portion 659 formed with the mounting bracket 100. In other
 words, the stop portion 659 determines the fully retracted position of the
 movable member 652. When the inner brake pad 606b is rotated with the disc
 rotor 104 in the direction Y as indicated in FIG. 25, the movable member
 652 is moved away from the fully retracted position against the biasing
 force of the spring 654.
 The small-diameter portion 658 has a movable contact 660 fixed to its end
 face. The inner torque receiving portion 610b has a stationary contact 662
 which is an elastic member. The movable contact 660 comes into contact
 with the stationary contact 662 when the movable member 652 is moved to
 its fully advanced position by the inner brake pad 606b against the
 biasing action of the coned disc spring 654. The fully advanced position
 of the movable member 652 is determined by abutting contact of the end
 face of the small-diameter portion 658 with the surface of the inner
 torque receiving portion 610b. In this arrangement, a force generated by
 friction contact of the inner brake pad 616b with the rotor 104 is
 transmitted to the inner torque receiving portion 610b through the movable
 member 652.
 The stationary contact 662 is electrically connected through a wire 664 to
 the electronic control unit 550. The wire 664 extends through the mounting
 bracket 100. The stationary contact 662 and the wire 664 are electrically
 insulated from the mounting bracket 100 by an insulator 665. On the other
 hand, the movable contact 660 is grounded through the electrically
 conductive movable member 652 and mounting bracket 100 and through a wire
 666 connected to the mounting bracket 100.
 The movable contact 660 is normally held away from the stationary contact
 662 with the movable member 652 held in its fully retracted position under
 the biasing force of the coned disc spring 654. Thus, the force switch 650
 is normally held in its off state. When the force which the movable member
 652 receives from the inner brake pad 606b exceeds a predetermined value
 (a pre-load given to the coned disc spring 654), the movable member 652
 starts moving with the inner brake pad 606b against the biasing force of
 the spring 654, from the fully retracted position toward the fully
 advanced position in which the movable contact 660 contacts the stationary
 contact 662, whereby the force switch 650 is turned on.
 It will be understood that the force switch 650 may be provided in the
 outer torque receiving portion 610a of the mounting bracket 100.
 Referring to FIG. 26, there is shown the electrically operated drum brake
 532 for the rear right wheel RR. The drum brake 532 for the rear left
 wheel RL has the same construction as shown in FIG. 26.
 The drum brake 532 is identical with the drum brake 32 of FIG. 3 of the
 first embodiment, except for a strut 736 provided in place of the strut
 236. This strut 736 incorporates a length adjusting mechanism including a
 screw device, which is manipulated to adjust a clearance or gap between
 the brake shoes 210a, 10b and the drum 204.
 The mechanically operated drum brake 536 for each rear wheel will be
 described.
 All the elements of the electrically operated drum brake 532 except the
 primary brake cable 240, shoe expanding actuator 250 and return spring 280
 are also used for the mechanically drum brake 536. This drum brake 536
 uses an emergency brake cable 782 and a return spring 784, which are
 similar to the parking brake cable 242 and the return spring 244 provided
 in the drum brake 32 of FIG. 3. The emergency brake cable 782 is connected
 at one end thereof to the end of the lever 230 to which the primary brake
 cable 240 of the electrically operated drum brake 532 is connected. When
 the mechanically operated drum brake 536 is activated, the lever 230 is
 pivoted to force the brake linings 216a, 216b onto the drum 204, for
 thereby braking the rear right wheel RR. The emergency brake cable 782 is
 guided through an outer tubing 786, as indicated in FIG. 23.
 The emergency brake cable 782 connected at its one end to the mechanically
 operated drum brake 536 for each rear wheel is operatively connected at
 the other end to the brake pedal 534 through a manual brake control device
 800, as shown in FIG. 23.
 The manual brake control device 800 is shown in enlargement in FIG. 27,
 together with a brake pedal device 802. The brake pedal device 802
 includes a pedal bracket 804 fixed to the vehicle body. The brake pedal
 534 is supported at a proximal end thereof by the pedal bracket 804 such
 that the brake pedal 534 is pivotable at its proximal end about an axis
 which extends in the lateral or transverse direction of the vehicle.
 Normally, the brake pedal 534 is held in its non-operated position, which
 is determined by abutting contact of the brake pedal 534 with a stop 806
 under a biasing action of a return spring 808. The brake pedal 534 is
 pivotally connected to the rear end of a push rod 312 through a clevis
 810, which serves as a pivotal link mechanism. In this arrangement, the
 push rod 312 is movable in the longitudinal or running direction of the
 vehicle. A pivotal motion of the brake pedal 534 is converted into a
 linear motion of the push rod 312.
 The manual brake control device 800 includes a housing 814 fixed to the
 vehicle body. Within a bore formed in the housing 814, there are slidably
 received a first piston 816 and a second piston 818 which are disposed
 coaxially with each other such that the pistons 816, 818 are movable
 relative to each other in the longitudinal direction of the vehicle. The
 push rod 812 engages at its front end with the rear end of the first
 piston 816. An operating force f acting on the brake pedal 534 is
 transmitted in the forward direction to the first piston 816 through the
 push rod 812. Thus, the brake pedal 534 is mechanically connected to the
 first piston 816. Between the first piston 816 and the housing 814, there
 is disposed an elastic member in the form of a spring 820, which biases
 the first piston 816 toward the push rod 812, so as to hold the brake
 pedal 534 in its non-operated position. Upon operation of the brake pedal
 534 when the electrically operated drum brake 532 is normal, the brake
 pedal 534 is moved by a distance corresponding to the operating force f
 acting on the brake pedal 534. Thus, the manual brake control device 800
 gives the vehicle operator an operating feel of the brake pedal 534 as
 obtained with a brake pedal in a hydraulically operated braking system. In
 the present eighth embodiment, the first piston 816 and the spring 320
 cooperate to constitute a pedal stroke simulator generally indicated at
 812 in FIG. 27.
 The first piston 816 has an engaging portion in the form of a projection
 822 which extends toward the second piston 818 such that the projection
 822 is coaxial with the pistons 816, 818. The second piston 818 has a
 fully retracted position determined by a stop 824. The retracted position
 of the second piston 818 or the position of the stop 824, which determines
 an initial distance between the two pistons 816, 818, is determined so
 that the projection 822 is spaced apart from the second piston 818 when
 the brake pedal 534 is placed in its non-operated position of FIG. 27, but
 is brought into abutting contact with the second piston 818 when the
 operating force f acting on the brake pedal 534 has reached a reference
 value f.sub.0. This reference value f.sub.0 is determined such that the
 operating force f equal to the reference value f.sub.0 produces a
 relatively high deceleration value G (e.g., 1.2 G) of the vehicle which is
 not usually obtained when the electrically operated disc and drum brakes
 522, 532 are both normal. If and after the operating force f exceeds the
 thus determined reference value f.sub.0 when the disc and drum brakes 522,
 532 are both normal, the second piston 818 is moved forward from the fully
 retracted position, and the mechanically operated drum brake 536 is also
 activated simultaneously, the rate of increase of the vehicle deceleration
 value G with an increase in the operating force f is raised, as indicated
 in the graph of FIG. 28.
 The second piston 818 is connected through a lever device 826 to the rear
 end of the emergency brake cable 782 of the mechanically operated drum
 brake 536 for each rear wheel, as shown in FIG. 27.
 The lever device 826 includes a lever 828 and a lever bracket 830 fixed to
 the vehicle body. The lever 828 is supported at a proximal end thereof by
 the lever bracket 830 such that the lever 828 is pivotable about the
 proximal end in a plane which includes the axis of the second piston 818.
 The lever 828 is connected at an intermediate portion thereof to the front
 end of the second piston 818 through a clevis 832, and is held in its
 fully retracted position under a biasing action of a return spring 834
 such that the clevis 832 is held in abutting contact with the front end of
 the second piston 818. The lever 828 is connected at a free end thereof to
 the end of the emergency brake cable 782 of each drum brake 536 through a
 clevis 836. In this arrangement, a forward movement of the second piston
 818 (in the left direction as seen in FIG. 27) will cause the lever 828 to
 be pivoted in the clockwise direction (as seen in FIG. 27), pulling the
 emergency brake cable 782 in the left direction as seen in FIG. 27, out of
 the outer tubing 786, whereby the movement of the second piston 818 is
 boosted into the movement of the emergency brake cable 782. Reference
 numeral 838 in FIG. 27 denotes a bracket fixed to the vehicle body for
 fixing the outer tubing 786 for the emergency brake cable 782.
 When the brake pedal 534 is operated while the electrically operated drum
 brakes 532 are normal, the electric motors 30 of the actuators 250 are
 operated to pull the primary brake cables 240 to force the brake shoes
 210a, 210b onto the drum 204. At this time, the flexible emergency brake
 cables 782 are contracted, so that the actions of the brake shoes 210a,
 210b by operation of the electrically operated drum brakes 532 are not
 disturbed by the manual brake control device 800.
 When the brake pedal 534 is operated while the electrically operated drum
 brakes 532 are not normal, the emergency brake cables 782 are pulled by
 the brake pedal 534, and the lever 230 is pivoted to force the brake shoes
 210a, 210b onto the drum 204. At this time, the flexible primary brake
 cables 240 are contracted, so that the actions of the brake shoes 210a,
 210b by operation of the mechanically operated drum brakes 536 are not
 disturbed by the electrically operated drum brakes 532.
 Thus, the primary brake cables 240 and the emergency brake cables 782 which
 are both flexible and connected to the same lever 230 are not disturbed by
 the emergency brake cables 782 and the primary brake cables 240,
 respectively, when the electrically and mechanically operated drum brakes
 532, 536 are operated at different times.
 Referring back to FIG. 23, there will be described a control system of the
 braking system according to the eighth embodiment of the invention. The
 control system includes the electronic control unit (ECU) 350, which is
 principally constituted by a computer 836 incorporating a read-only memory
 (ROM) 842 and a random-access memory (RAM) 844. To the ECU 850, there are
 connected various sensors and switches including: the above-indicated
 force switches 650 of the disc brakes 522 for the front left and right
 wheels FL, FR; a brake pedal switch 850; an operation force sensor 848; a
 parking pedal switch 851; an accelerator pedal switch 852; an accelerator
 operation amount sensor 853; a steering angle sensor 854; a yaw rate
 sensor 855; a longitudinal acceleration sensor 856; a lateral acceleration
 sensor 857; a front wheel load sensor 858; a rear wheel load sensor 859;
 four wheel speed sensors 860; four motor position sensors 862 and four
 motor current sensors 864.
 The operation force sensor 848 generates an output signal indicative of the
 operating force f acting on the brake pedal 534. The brake pedal switch
 850, which is a primary brake operation sensor, generates an output signal
 indicative of whether the brake pedal 34 is in operation. That is, the
 brake pedal switch 850 is placed in an off state when the brake pedal 534
 is not in operation, and in an on state when the brake pedal 534 is in
 operation. The parking pedal switch 851, which is a parking brake
 operation sensor, generates an output signal indicative of whether the
 parking brake pedal 42 is in operation. That is, the parking brake pedal
 switch 851 is placed in an of f state when the parking brake pedal 42 is
 not in operation, and in an on state when the parking brake pedal 42 is in
 operation. The accelerator pedal switch 852, which is an accelerator
 operation sensor, generates an output signal indicative of whether the
 accelerator pedal 44 is in operation. That is, the accelerator pedal
 switch 852 is placed in an off state when the accelerator pedal 44 is not
 in operation, and in an on state when the accelerator pedal 44 is in
 operation. The accelerator pedal operation amount sensor 853 generates an
 output signal indicative of an operating amount of the accelerator pedal
 44. The steering angle sensor 354, which is a sensor for detecting an
 angle of turn of the vehicle, generates an output signal indicative of the
 angle of rotation of the steering wheel 46. The yaw rate sensor 855
 generates an output signal indicative of a yaw rate .gamma. of the
 vehicle. The longitudinal acceleration sensor 856 generates an output
 signal indicative of a deceleration value G.sub.FR of the vehicle in the
 longitudinal direction of the vehicle. The lateral acceleration sensor 857
 generates an output signal indicative of a lateral acceleration value
 G.sub.LR of the vehicle in the lateral direction of the vehicle. The front
 wheel load sensor 858 generates an output signal indicative of a load
 W.sub.F acting on the front axle in the vertical direction, while the rear
 wheel load sensor 859 generates an output signal indicative of a load
 W.sub.R acting on the rear axle in the vertical direction. Each of the
 four wheel speed sensors 860 generates an output signal indicative of the
 rotating speed Vw of the corresponding wheel. Each of the motor position
 sensors 860 generates an output signal indicative of the angular position
 of the corresponding electric motor 20, 30. Each of the motor current
 sensors 864 generates an output signal indicative of an electric current
 supplied to the coil of the corresponding motor 20, 30.
 The ECU 550 is also connected to a first driver 866 and a second driver
 868. The first driver 866 is connected between an electric power source in
 the form of a first battery 870 and the electric motor 20 of each
 electrically operated disc brake 522. On the other hand, the second driver
 868 is connected between an electric power source in the form of a second
 battery 872 and the electric motor 30 of each electrically operated drum
 brake 532. Upon operation of the brake pedal 534, the ECU 550 applies
 control commands to the first and second drivers 866, 868, so that the
 amounts of the electric current to be supplied from the first and second
 batteries 870, 872 to the respective electric motors 20, 30 are controlled
 according to the control commands, which are determined by the operating
 force f acting on the brake pedal 534.
 The braking system also includes a primary battery 874 independent of the
 first and second batteries 870, 872. This primary battery 874 is used for
 operating all electrical components of the vehicle, except the electric
 motors 20, 30 of the brakes 522, 532. The ECU 550 is powered by the
 primary battery 874, rather than the first and second batteries 870, 872.
 The ECU 550 is further connected to an engine output control device
 (throttle control unit, a fuel supply control unit, ignition timing
 control unit, etc.) for controlling the engine 10, and a shift control
 device (solenoid-operated valves, etc.) for controlling the automatic
 transmission 12. The ECU 550 applies control commands to these engine
 output control device and shift control device to effect a traction
 control of the vehicle, namely, to control the running vehicle so as to
 avoid excessive spinning or slipping of the drive wheels.
 The ECU 550 is also connected to a brake failure indicator light 876 which
 is turned on in the event of an electrical or other failure or defect of
 the electrically operated disc and drum brakes 522, 532.
 The ROM 842 of the computer 846 stores various control programs including
 programs for executing various routines such as a brake control routine
 and a friction coefficient calculating routine.
 The brake control routine is formulated to control the brakes 522, 532 in
 various modes such as a basic control mode, an anti-lock control mode, a
 traction control mode and vehicle stability control (VSC) mode. In the
 basic control mode, the electric motors 20, 30 of the disc and drum brakes
 522, 532 are controlled so as to achieve the vehicle deceleration
 corresponding to the operating force f acting on the brake pedal 534, on
 the basis of the output signals of the operation force sensor 848, brake
 pedal switch 850, front and rear wheel load sensors 858, 859, motor
 position sensors 862 and motor current sensors 864, while monitoring the
 detected angular positions of the motors 20, 30 and the detected amounts
 of electric current supplied to the motors 20, 30, such that the braking
 force is suitably distributed to the front wheels FL, FR and the rear
 wheels RL, RR. In the anti-lock control mode, the electric motors 20, 30
 are controlled to control the braking torque values of the wheels, so as
 to avoid an excessive locking tendency of each wheel, on the basis of the
 output signals of the brake pedal switch 850, wheel speed sensors 860,
 motor position sensors 862 and motor current sensors 864. In the traction
 control mode, the electric motors 20, 30 are controlled to control the
 driving torque values of the drive wheels, so as to avoid an excessive
 spinning or slipping tendency of each drive wheel, on the basis of the
 output signals of the accelerator pedal switch 852, accelerator operation
 amount sensor 853, wheel speed sensors 860, motor position sensors 862 and
 motor current sensors 864. In the VSC mode, the electric motors 20, 30 are
 controlled to control a yaw movement of the vehicle, by controlling a
 difference between the braking forces of the left and right wheels, so as
 to avoid an excessive drift-out or spinning tendency of the vehicle, on
 the basis of the steering angle sensor 854, yaw rate sensor 855, lateral
 acceleration sensor 858, wheel speed sensors 860, motor position sensors
 862 and motor current sensors 864.
 The brake control routine is illustrated in the flow chart of FIG. 29. This
 routine is repeatedly executed while the ignition switch of the vehicle is
 held on. The routine is initiated with step S501 to determine whether the
 brake pedal 534 is in operation, that is, whether the brake pedal switch
 530 is in the on state. If an affirmative decision (YES) is obtained, the
 control flow goes to step S2 in which the braking system is controlled in
 the basic control mode according to a basic control mode sub-routine
 illustrated in the flow chart of FIG. 30, which will be described.
 The sub-routine of FIG. 30 is initiated with step S521 in which the
 operating force f acting on the brake pedal 534 is detected on the basis
 of the output signal of the operation force sensor 848. Then, step S522 is
 implemented to read the longitudinal acceleration value G.sub.FR, lateral
 acceleration value G.sub.LR, front wheel load W.sub.F, rear wheel load
 W.sub.R and yaw rate .gamma., which are represented by the output signals
 of the appropriate sensors. The control flow then goes to step S523 in
 which a desired braking torque or force F* for each wheel is determined on
 the basis of the detected values G.sub.FR, F.sub.LR, W.sub.F, W.sub.R,
 .gamma., so as to achieve an optimum or ideal front-rear distribution of
 the braking force depending upon the vehicle weight and deceleration
 values G, and so as to avoid yawing and/or lateral slipping tendency of
 the vehicle due to a large difference between the braking force of the
 left wheels and the braking force of the right wheels.
 Step S523 is followed by step S524 to read the friction coefficient .mu. of
 the inner brake pad 606b of each front disc brake 522, which is stored in
 the RAM 844. Upon power application to the computer 846, the standard
 value of the friction coefficient .mu. is stored in the RAM 844, and is
 provisionally used before the friction coefficient .mu. is calculated
 according to a friction coefficient calculating routine (which will be
 described) and stored in the RAM 844. Each time the friction coefficient
 calculating routine is executed, the friction coefficient value .mu.
 stored in the RAM 844 is updated.
 Then, the control flow goes to step S525 to determine a desired value I* of
 the electric current I to be supplied to the motor 20, 30 of each brake
 522, 532. The desired electric current value I* for the motor 30 of each
 rear drum brake 532 is determined on the basis of the determined desired
 braking force F* and according to a predetermined relationship between the
 desired braking force F* and the desired electric current I*. This
 relationship is stored in the ROM 842. The desired electric current value
 I* for the motor 30 of each front disc brake 522 is determined according
 to the following equation, based on a fact that the desired electric
 current I* corresponds to a force N by which the brake pads 606a, 606b are
 forced onto the disc rotor 104.
EQU I*=F*/(.mu..multidot.K)
 In the above equation, K represents a constant.
 Then, the control flow goes to step S526 in which each motor 20, 30 is
 activated with the determined desired electric current I* supplied
 thereto. Thus, one cycle of execution of the basic control mode
 sub-routine of FIG. 30 is terminated in step S502 of the brake control
 routine of FIG. 29.
 Step S502 of the routine of FIG. 29 is followed by step S503 to determine
 whether it is necessary to control the brakes 522, 532 in the anti-lock
 control mode, that is, whether the vehicle wheels have an excessive
 locking tendency. If a negative decision (NO) is obtained in step S503,
 one cycle of execution of the routine of FIG. 29 is terminated. If an
 affirmative decision (YES) is obtained in step S503, the control flow goes
 to step S504 in which the braking system is controlled in the anti-lock
 control mode. Step S504 is followed by step S505 to determine whether the
 anti-lock brake control becomes unnecessary. If a negative decision (NO)
 is obtained, the control flow goes back to step S504, and step S504 is
 repeatedly implemented until an affirmative decision (YES) is obtained in
 step S505, that is, until the excessive locking tendency of the wheels has
 been removed. If the affirmative decision (YES) is obtained in step S505,
 one cycle of execution of the routine is terminated.
 If a negative decision (NO) is obtained in step S501, the control flow goes
 to step S506 to determine whether it is necessary to control the brakes
 522, 532 in the traction control mode, that is, whether the drive wheels
 have an excessive spinning or slipping tendency. If an affirmative
 decision (YES) is obtained in step S506, the control flow goes to step
 S507 in which the braking system is controlled in the traction control
 mode. Step S507 is followed by step S508 to determine whether the traction
 control becomes unnecessary. If a negative decision (NO) is obtained in
 step S507, the control flow goes to step S507, and step S507 is repeatedly
 implemented until an affirmative decision (YES) is obtained in step S508,
 that is, until the excessive slipping tendency of the drive wheels has
 been removed. If the affirmative decision (YES) is obtained in step S508,
 one cycle of execution of the routine is terminated.
 If the brake pedal switch 850 is off and if the traction control is not
 necessary, that is, if a negative decision (NO) is obtained in steps S501
 and S506, the control flow goes to step S509 to determine whether the VSC
 control (vehicle stability control) is necessary, that is, whether the
 vehicle has an excessive drift-out or spinning tendency. If an affirmative
 decision (YES) is obtained in step S509, the control flow goes to step
 S510 in which the braking system is controlled in the VSC control mode.
 Step S510 is followed by step S511 to determine whether the VSC control
 becomes unnecessary. If a negative decision (NO) is obtained in step S511,
 the control flow goes back to step S510, and step S510 is repeatedly
 implemented until an affirmative decision (YES) is obtained in step S510,
 that is, until the VSC control becomes unnecessary. If the affirmative
 decision (YES) is obtained in step S511, one cycle of execution of the
 routine is terminated.
 The friction coefficient calculating routine indicated above is illustrated
 in the flow chart of FIG. 31.
 The friction coefficient calculating routine is also repeatedly executed
 while the ignition switch of the vehicle is held on. The routine is
 executed alternately for the disc brakes 522 for the front left and right
 wheels FL, FR. The routine is initiated with step S531 to determine
 whether the disc brake 522 in question is controlled in any of the
 anti-lock, traction and VSC control modes. If an affirmative decision
 (YES) is obtained in step S531, one cycle of execution of the routine is
 terminated. If a negative decision (NO) Is obtained in step S531, that is,
 if none of the anti-lock, traction and VSC controls is currently effected,
 the control flow goes to step S532.
 Step S532 is provided to determine whether the force switch 650 is turned
 on or off, that is, whether the state of the force switch 650 is changed
 from the off state to the on state or vice versa. If the force switch 650
 is turned on, it means that the force transmitted from the inner brake pad
 606b to the force switch 650 has increased to the predetermined value. If
 the force switch 650 is turned off, it means that the force transmitted
 from the inner brake pad 606b has decreased to the predetermined value. If
 a negative decision (NO) is obtained in step S532, one cycle of execution
 of the routine is terminated. If an affirmative decision (YES) is obtained
 in step S532, the control flow goes to step S533.
 In step S533, the actual value of the electric current I.sub.A supplied to
 the electric motor 20 is detected on the basis of the output signal of the
 appropriate motor current sensor 864. Then, step S534 is implemented to
 calculate the friction coefficient .mu. of the brake pads 606 of the disc
 brake 522 in question, on the basis of the calculated motor current
 I.sub.A and an optimum braking force F.sub.0 which should act on the inner
 brake pad 606b when the force switch 650 is turned from the off state to
 the on state or vice versa. Namely, the friction coefficient .mu. is
 calculated according to the following equation:
EQU .mu.=F.sub.0 /(K.multidot.I.sub.A)
 Then, the control flow goes to step S535 in which the calculated friction
 coefficient .mu. of the brake pads 606 is stored in the RAM 844. Thus, one
 cycle of execution of the routine of FIG. 31 is terminated.
 It will be understood from the above description of the eighth embodiment
 of the invention that the motor current sensors 864 serve as a
 force-related quantity sensor for detecting a quantity relating to the
 braking force generated by the disc brake 522, and a
 pressing-force-related quantity sensor for detecting a physical quantity
 relating to the pressing force by which the friction member 606b is forced
 onto the rotor 104 by the pressing device. It will also be understood that
 a portion of the ECU 550 assigned to implement steps S531-S534 constitutes
 a friction coefficient estimating device for estimating the friction
 coefficient of the friction members 606 of the disc brakes 522. This
 friction coefficient estimating device may be considered to be a
 relationship estimating means for estimating the relationship between the
 electric current I to be applied to the electric motor 20 and the braking
 force or torque F to be generated by the brake and applied to the wheel.
 It will also be understood that a portion of the ECU 550 assigned to
 execute the routine of FIG. 30 constitutes relationship utilizing means
 for utilizing the estimated relationship, for controlling the disc brake
 522.
 There will next be described a ninth embodiment of this invention, in which
 the same reference numerals as used in the eighth embodiment will be used
 to identify the same elements.
 In the eighth embodiment, the manual brake control device 600 and the
 mechanically operated drum brakes 536 serving as the emergency brakes are
 provided for the rear left and right wheels RL, RR. In the present ninth
 embodiment, the manual brake control device 600 and the mechanically
 operated drum brakes 536 are provided for the front left and right wheels
 FL, FR. As shown in FIG. 32, the brake pedal 534 is operatively connected
 to the brake pads 606a, 606b of the electrically operated disc brakes 522
 for the front left and right wheels FL, FR, through a manual brake control
 device 900, emergency brake cables 902 and mechanically operated brakes
 906. The mechanically operated brakes 906 may be in operation when a
 friction coefficient calculating routine is executed for the disc brakes
 522. The operation of the mechanically operated brakes 906 during
 execution of the friction coefficient calculating routine may lower the
 accuracy of calculation of the friction coefficient .mu.. In view of this,
 the execution of the friction coefficient calculating routine is inhibited
 while the mechanically operated brakes 906 are in operation. In the
 present ninth embodiment, a switch 910 is provided to detect an operation
 of the mechanically operated brakes 906. The switch 910 is turned on when
 a second piston (similar to the second piston 818 of the manual brake
 control device 900 of the eighth embodiment) is moved from the fully
 retracted position. The switch 910 is held off when the second piston is
 placed in the fully retracted position.
 In the present ninth embodiment, the ROM 842 stores a program for executing
 the friction coefficient calculating routine illustrated in the flow chart
 of FIG. 33.
 The routine of FIG. 33 is initiated with step S601 to determine whether the
 switch 910 is off, namely, whether the second piston of the manual brake
 control device 910 is held in its fully retracted position. If a negative
 decision (NO) Is obtained in step S601, one cycle of execution of the
 routine is terminated. If an affirmative decision (YES) is obtained in
 step S601, the control flow goes to step S602-606 identical with steps
 S531-S535 of the routine of FIG. 31.
 A tenth embodiment of the invention will be described. This embodiment is a
 modification of the eighth embodiment.
 The tenth embodiment is adapted to execute a brake pad fade detecting
 routine as well as the brake control routine of FIG. 29 and the friction
 coefficient calculating routine of FIG. 31. The brake pad fade detecting
 routine is illustrated in the flow chart of FIG. 34.
 The brake pad fade detecting routine is executed alternately for the front
 left and right wheels FL, FR. The routine is initiated with step S801 to
 determine whether the force switch 650 is turned on or off. If a negative
 decision (NO) is obtained in step S801, one cycle of execution of the
 routine is terminated. If an affirmative decision (YES) is obtained in
 step S801, the control flow goes to step S802 to detect the motor current
 I.sub.A based on the output signal of the motor current sensor 864. Then,
 step S803 is implemented to calculate the friction coefficient .mu. of the
 brake pad 606, in the same manner as described above with respect to step
 S534 of the eighth embodiment. Step S803 is followed by step S804 to
 determine whether the calculated friction coefficient .mu. is equal to or
 smaller than a predetermined threshold .mu..sub.0. If an affirmative
 decision (YES) is obtained in step S804, the control flow goes to step
 S805 to determine that the friction coefficient .mu. of the brake pad 606
 is unacceptably low due to fading of the brake pad. In this case, the
 brake failure indicator light 876 is turned on to inform the vehicle
 operator of some failure or defect of the disc brake 522 in question. If a
 negative decision (NO) is obtained in step S804, the control flow goes to
 step S806 to determine that the friction coefficient .mu. is acceptably
 high. One cycle of execution of the routine of FIG. 34 is terminated with
 step S805 or S806.
 An eleventh embodiment of the invention will be described. This embodiment
 is another modification of the eighth embodiment.
 The eleventh embodiment is adapted to execute a brake failure detecting
 routine as well as the brake control routine of FIG. 29 and the friction
 coefficient calculating routine of FIG. 31. The brake failure detecting
 routine is illustrated in the flow chart of FIG. 34.
 The brake failure detecting routine is also executed alternately for the
 front left and right wheels FL, FR. The routine is initiated with step
 S901 to detect the operating force f based on the operating force sensor
 848. Then, step S902 is implemented to determine whether the detected
 operating force f is larger than the predetermined reference value
 f.sub.0. If an affirmative decision (YES) is obtained in step S902, the
 control flow goes to step S903 to determine whether the force switch 650
 is in the on state. The reference value f.sub.0 is determined such that
 the force switch 650 is in the on state when the operating force f is
 larger than the reference value f.sub.0, as long as the disc brake 522 is
 normal. In other words, if the force switch 650 is in the off state when
 the operating force f is larger than the reference value f.sub.0, it means
 that the disc brake 522 is abnormal. Therefore, if a negative decision
 (NO) is obtained in step S903, the control flow goes to step S906 to
 determine that the disc brake 522 is abnormal. In this case, the brake
 failure indicator light 876 is turned on. Thus, one cycle of execution of
 the routine is terminated.
 If an affirmative decision (YES) is obtained in step S903, the control flow
 goes to step S904 to determine whether the operating force f is smaller
 than a predetermined reference value f.sub.1. If an affirmative decision
 (YES) is obtained in step S904, the control flow goes to step S905 to
 determine whether the force switch 650 is placed in the off state. The
 reference value f.sub.1 is determined such that the force switch 650 is in
 the off state when the operating force f is smaller than the reference
 value f.sub.1, as long as the disc brake 522 is normal. In other words, if
 the force switch 650 is in the on state when the operating force f is
 smaller than the reference value f.sub.1, it means that the disc brake 522
 is abnormal. Therefore, if a negative decision (NO) is obtained in step
 S905, the control flow goes to step S906 to determine that the disc brake
 522 is abnormal. In this case, too, the brake failure indicator light 876
 is turned on. Thus, one cycle of execution of the routine is terminated.
 If a negative decision (NO) is obtained in step S902, the control flow goes
 to step S904. If a negative decision (NO) is obtained in step S904, one
 cycle of execution of the routine of FIG. 35 is terminated.
 It is noted that the reference value f.sub.0 is larger than the reference
 value f.sub.1, so that the affirmative decision (YES) is not obtained in
 step S904 when the affirmative decision (YES) is obtained in step S902.
 There will be described a twelfth embodiment of this invention, in which
 the same reference numerals as used in the eighth embodiment will be used
 to identify the same elements.
 The braking system according to this twelfth embodiment includes a pressing
 force sensor 930 provided on the presser member 134 of each front disc
 brake 522. This pressing force sensor 930 is adapted to continuously
 detect a force by which the presser member 134 forces the inner brake pad
 606b the onto the friction surface 102 of the disc rotor 104. The pressing
 force sensor 930 functions as a force-related quantity sensor for
 detecting a quantity relating to a quantity relating to the braking force
 generated by the disc brake 522 or the pressing force by which the
 friction member 606b is forced onto the rotor 104.
 In the present twelfth embodiment, the ROM 842 stores a program for
 executing a front brake control routine illustrated in the flow chart of
 FIG. 37.
 The front brake control routine of FIG. 37 is executed alternately for the
 front left and right wheels FL, FR. The routine. is initiated with step
 S951 to determine whether the brake pedal switch 850 is on. If a negative
 decision (NO) is obtained in step S951, one cycle of execution of the
 routine is terminated. If an affirmative decision (YES) is obtained in
 step S951, the control flow goes to step S952 to detect the operating
 force f on the basis of the operating force sensor 848. Then, step S953 is
 implemented to determine the desired braking force F* on the basis of the
 detected operating force f. Step S953 is similar to step S523 of the
 eighth embodiment of FIG. 30. Then, the control flow goes to step S954 to
 read an output S.sub.A of the pressing force sensor 930. Step S954 is
 followed by step S955 to read a conversion function g(S) stored in the RAM
 844. The conversion function g(S) is used to convert the output S.sub.A
 into an actual braking force F.sub.A. Upon power application to the
 computer 846, a standard conversion function g(S)* is stored in the RAM
 844, and is provisionally used before the conversion function g(S) is
 obtained according to a conversion function compensating routine (which
 will be described) and stored in the RAM 844. Each time the conversion
 function compensating routine is executed, the conversion function g(S)
 stored in the RAM 844 is updated.
 Then, the control flow goes to step S956 to calculate the actual braking
 force F.sub.A according to the conversion function g(S). Then, step S957
 is implemented to determine a control amount .DELTA.I of the electric
 current I to be supplied to the electric motor 20. This determination is
 effected on the basis of the calculated actual braking force F.sub.A and
 the determined desired braking force F*, more precisely, on the basis of a
 difference between the actual and desired braking force values F.sub.A and
 F*. The control amount .DELTA.I permits the actual braking force F.sub.A
 to coincide with the desired value F*. Step S957 is followed by step S958
 in which the electric motor 20 is activated according to the control
 amount .DELTA.I.
 The conversion function compensating routine indicated above is illustrated
 in the flow chart of FIG. 38.
 This routine of FIG. 38 is also executed alternately for the front left and
 right wheels FL, FR. The routine is initiated with step S1001 to determine
 whether the force switch 650 is turned on or off. If a negative decision
 (NO) is obtained in step S951, one cycle of execution of the routine is
 terminated. If an affirmative decision (YES) is obtained in step S1001,
 the control flow goes to step S1002 to read the output S.sub.A of the
 pressing force sensor 930. Step S1003 is then implemented to calculate an
 error .DELTA.S between the output S.sub.A and a nominal value S.sub.T of
 the output S.sub.A. The error .DELTA.S=S.sub.A -S.sub.T corresponds to a
 difference of an actual characteristic of the pressing force sensor 930
 from a nominal or designed characteristic, as indicated in the graph of
 FIG. 39. The characteristic is represented by a relationship between the
 pressing force f and the output S of the pressing force sensor 930. The
 nominal characteristic is represented by the standard conversion function
 g(S)* while the actual characteristic is represented by the conversion
 function g(S-.DELTA.S), as indicated in the graph of FIG. 40.
 Then, the control flow goes to step S1004 to determine whether the absolute
 value .vertline..DELTA.S.vertline. of the error .DELTA.S is larger than a
 predetermined threshold .DELTA.S.sub.0. If an affirmative decision (YES)
 is obtained in step S1004, the control flow goes to step S1005 to
 compensate the conversion function g(S)* or g(S) stored in the RAM 844.
 Namely, the conversion function g(S-.DELTA.S) is set as the conversion
 function g(S), so that the actual braking force F.sub.A is calculated
 according to the conversion function g(S-.DELTA.S) in step S956 upon next
 execution of the front brake control routine of FIG. 37. In other words,
 the f-S relationship which is used in the routine of FIG. 37 is shifted to
 the right by an amount equal to the error .DELTA.S, as indicated in the
 graph of FIG. 40. Then, the control flow goes to step S1006 to store the
 compensated conversion function g(S) in the RAM 844, that is, store the
 conversion function g(S-.DELTA.S) as the compensated or updated conversion
 function g(S). If a negative decision (NO) is obtained in step S1004, one
 cycle of execution of the conversion function compensating routine of FIG.
 38 is terminated.
 It will be understood from the above description of the twelfth embodiment
 that the conversion function g(S) represents a relationship between the
 output S.sub.A of the pressing force sensor 930 and the actual braking
 force F.sub.A when the force sensor 650 is turned on. Since the output
 S.sub.A reflects the amount of electric current I supplied to the electric
 motor 20, the conversion function g(S) is considered to represent a
 relationship between the electric current I to be supplied to the electric
 motor 20 and the braking force F to be applied to the front wheel.
 Therefore, a portion of the ECU assigned to execute the conversion
 function compensating routine of FIG. 38 is considered to constitute a
 relationship estimating device for estimating the relationship between the
 electric current I to be applied to the electric motor 20 and the braking
 force F to be applied to the front wheel. It will be understood that a
 portion of the ECU 550 assigned to execute the front brake control routine
 of FIG. 37 constitutes relationship utilizing means for utilizing the
 estimated relationship for controlling the front disc brake 522. It will
 also be understood that a portion of the ECU 550 assigned to implement
 steps S954-S956 and S1001-S1005 constitutes a wheel braking force
 estimating device for estimating the braking force F to be applied to the
 front wheel. It will further be understood that a portion of the ECU 550
 assigned to implement steps S1003 and S1005 constitutes relationship
 compensating means for compensating the above-indicated relationship.
 There will be described a thirteenth embodiment of this invention, which is
 a modification of the twelfth embodiment. The same reference numerals as
 used in the twelfth embodiment will be used in the thirteenth embodiment,
 to identify the same elements.
 In the braking system according to the thirteenth embodiment, a braking
 force sensor 950 is used in place of the pressing force sensor 930, as
 shown in FIG. 41. The braking force sensor 950 is capable of continuously
 detecting a force received from the inner brake pad 606b during activation
 of the front disc brake 522. The braking force sensor 950 is interposed
 between the force switch 650 and the inner torque receiving portion 610b
 of the mounting bracket 100, such that the coned disc spring of the force
 switch 650 is held in contact with the braking force sensor 950. The
 braking force sensor 950 may be a strain gage or a piezoelectric element,
 or consists principally of a rubber material whose electrical conductivity
 changes with a pressure applied thereto. The braking force sensor 950
 functions as a force-related quantity sensor or a braking-force-related
 quantity sensor for detecting a quantity relating to the braking force
 generated by the pressing device 20, 134, 136.
 The ROM 842 of the present braking system stores a program for executing a
 front brake control routine illustrated in the flow chart of FIG. 42.
 The front brake control routine of FIG. 42 is executed alternately for the
 front left and right wheels FL, FR. The routine is initiated with step
 S1051 to determine whether the brake pedal switch 850 is on. If a negative
 decision (NO) is obtained in step S951, one cycle of execution of the
 routine is terminated. If an affirmative decision (YES) is obtained in
 step S1051, the control flow goes to step S1052 to detect the operating
 force f on the basis of the operating force sensor 848. Then, step S1053
 is implemented to determine the desired braking force F* on the basis of
 the detected operating force f. Step S1053 is similar to step S523 of the
 eighth embodiment of FIG. 30. Then, the control flow goes to step S1054 to
 read an output S.sub.A of the braking force sensor 950. Step S1054 is
 followed by step S1055 to read a conversion function h(S) stored in the
 RAM 844. The conversion function h(S) is used to convert the output
 S.sub.A into an actual braking force F.sub.A. Upon power application to
 the computer 846, a standard conversion function h(S)* is stored in the
 RAM 844, and is provisionally used before the conversion function h(S) is
 obtained according to a conversion function compensating routine (which
 will be described) and stored in the RAM 844. Each time the conversion
 function compensating routine is executed, the conversion function h(S)
 stored in the RAM 844 is updated.
 Then, the control flow goes to step S1056 to calculate the actual braking
 force F.sub.A according to the conversion function h(S). Then, step S1057
 is implemented to determine a control amount .DELTA.I of the electric
 current I to be supplied to the electric motor 20. This determination is
 effected on the basis of the calculated actual braking force F.sub.A and
 the determined desired braking force F*, more precisely, on the basis of a
 difference between the actual and desired braking force values F.sub.A and
 F*. The control amount .DELTA.I permits the actual braking force F.sub.A
 to coincide with the desired value F*. Step S1057 is followed by step
 S1058 in which the electric motor 20 is activated according to the control
 amount .DELTA.I.
 The conversion function compensating routine indicated above is illustrated
 in the flow chart of FIG. 43.
 This routine of FIG. 43 is also executed alternately for the front left and
 right wheels FL, FR. The routine is initiated with step S1101 to determine
 whether the force switch 650 is turned on or off. If a negative decision
 (NO) is obtained in step S1101, one cycle of execution of the routine is
 terminated. If an affirmative decision (YES) is obtained in step S1101,
 the control flow goes to step S1102 to read the output S.sub.A of the
 braking force sensor 950. Step S1103 is then implemented to calculate an
 error .DELTA.S between the output S.sub.A and a nominal value S.sub.T of
 the output S.sub.A. The significance of this error .DELTA.S=S.sub.A
 -S.sub.T has been described with respect to the routine of FIG. 38.
 Then, the control flow goes to step S1104 to determine whether the absolute
 value .vertline..DELTA.S.vertline. of the error .DELTA.S is larger than
 the predetermined threshold .DELTA.S.sub.0. If an affirmative decision
 (YES) is obtained in step S1104, the control flow goes to step S1105 to
 compensate the conversion function h(S)* or h(S) stored in the RAM 844.
 Namely, the conversion function h(S-.DELTA.S) is set as the conversion
 function h(S), so that the actual braking force F.sub.A is calculated
 according to the conversion function g(S-.DELTA.S) in step S1056 upon next
 execution of the front brake control routine of FIG. 42. Step S1105 is
 similar to step S1005 of the routine of FIG. 38. Then, the control flow
 goes to step S1106 to store the compensated conversion function h(S) in
 the RAM 844, that is, store the conversion function h(S-.DELTA.S) as the
 compensated or updated conversion function h(S). If a negative decision
 (NO) is obtained in step S1104, one cycle of execution of the conversion
 function compensating routine of FIG. 43 is terminated.
 It will be understood from the above description of the thirteen embodiment
 that the conversion function h(S) represents a relationship between the
 output S.sub.A of the braking force sensor 950 and the actual braking
 force F.sub.A when the force sensor 650 is turned on. Since the output
 S.sub.A reflects the amount of electric current I supplied to the electric
 motor 20, the conversion function h(S) is considered to represent a
 relationship between the electric current I to be supplied to the electric
 motor 20 and the braking force F to be applied to the front wheel.
 Therefore, a portion of the ECU assigned to execute the conversion
 function compensating routine of FIG. 43 is considered to constitute a
 relationship estimating device for estimating the relationship between the
 electric current I to be applied to the electric motor 20 and the braking
 force F to be applied to the front wheel. It will be understood that a
 portion of the ECU 550 assigned to execute the front brake control routine
 of FIG. 42 constitutes relationship utilizing means for utilizing the
 estimated relationship for controlling the front disc brake 522. It will
 also be understood that a portion of the ECU 550 assigned to implement
 steps S1054-S1056 and S1101-S1105 constitutes a wheel braking force
 estimating device for estimating the braking force F to be applied to the
 front wheel. It will further be understood that a portion of the ECU 550
 assigned to implement steps S1103 and S1105 constitutes relationship
 compensating means for compensating the above-indicated relationship.
 While the presently preferred embodiments of the present invention have
 been described above in detail by reference to the accompanying drawings,
 it is to be understood that the present invention may be embodied with
 various changes, modifications and improvements, which may occur to those
 skilled in the art, without departing from the spirit and scope of the
 invention defined in the following claims: