Steering control apparatus for vehicle

Target rotational speed of servo motor 31 is set based on steering speed .theta.hv detected by steering speed sensor and transmission ratio G set according to a running condition, and speed of servo motor is controlled so that rotational speed .theta.pv of servo motor detected by rotational speed sensor may agree with the target rotational speed.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a steering control apparatus for vehicle
 provided with a transmission ratio varying mechanism for varying a
 transmission ratio between a steering amount of a steering wheel and a
 turn amount of wheels.
 2. Related Background Art
 An example of the steering control apparatus provided with the transmission
 ratio varying mechanism of this type is disclosed in Japanese Laid-open
 Patent Application No. Sho 63-227472, for example. As disclosed in this
 application, the transmission ratio varying mechanism is provided with a
 driving motor and is constructed in such structure that a prescribed gear
 mechanism connects an input shaft connected to the steering wheel side to
 an output shaft connected to the tie rod side and that the transmission
 ratio of rotations between the input shaft and the output shaft can be
 changed by driving the gear mechanism by the driving motor.
 On this occasion, the driving motor is controlled so that a rotational
 position of the driving motor detected by a rotational angle sensor
 approaches a target position computed based on the steering angle of the
 steering wheel and the transmission ratio.
 SUMMARY OF THE INVENTION
 Since the position control of the driving motor was carried out based on
 deviation from the target position as described above, the driving motor
 might be driven by the degree corresponding to remaining deviation even in
 the cases where steering operation was stopped after abrupt steering
 operation and where abrupt switching-back operation of the steering wheel
 was carried out. An example thereof is illustrated in FIG. 8. When the
 abrupt switching-back operation of the steering wheel is carried out, the
 target rotational position of the driving motor varies as indicated by a
 solid line. In contrast with it, at a point of time t1 when the steering
 wheel is switched back, the deviation e1 due to a delay in follow-up
 remains with respect to an actual rotational position of the driving motor
 indicated by a dashed line. As the steering direction of the steering
 wheel is switched back after this point, steering torque is also switched
 so that the direction of the steering torque becomes opposite to the
 rotating direction of the driving motor. Therefore, the steering torque
 comes to act in the direction to assist the rotation of the driving motor.
 This steeply increases the rotating speed of the driving motor. At time t2
 the rotational position of the driving motor agrees with the target
 rotational position. Although the control of the driving motor is desired
 to stop there, a large overshoot e2 appears at time t3 because of the
 inertia of the driving motor and the rotational speed of the driving motor
 increased. This phenomenon resulted in giving a driver such incompatible
 steering feeling that the steering direction of the steering wheel is not
 coincident with the steered direction of the wheels.
 The present invention has been accomplished in order to solve this problem
 and an object thereof is to provide a steering control apparatus for
 vehicle that can improve the steering feeling by adequately suppressing
 occurrence of the above-stated overshoot due to the inertia of motor.
 The present steering control apparatus for vehicle is a steering control
 apparatus for vehicle adapted to change a transmission ratio between a
 steering amount and a turn amount by driving a transmission ratio varying
 mechanism by an actuator, the steering control apparatus for vehicle
 comprising: transmission ratio setting means for setting the transmission
 ratio according to a running condition of a vehicle; steering speed
 detecting means for detecting a steering speed of a steering wheel;
 driving speed setting means for setting a target driving speed for the
 actuator, based on the transmission ratio set by the transmission ratio
 setting means and the steering speed detected by the steering speed
 detecting means; and speed control means for controlling a speed of the
 actuator to the target driving speed set by the driving speed setting
 means.
 The speed control means carries out the speed control of the actuator
 according to increase or decrease of the steering speed during
 manipulation of the steering wheel. For example, in the preceding example
 of FIG. 8, this control decreases the driving speed of the actuator with
 decreasing steering speed toward the time to and then nulls the driving
 speed of the actuator at the point of the time t1 at which the steering
 speed is zero. This adequately suppresses occurrence of the overshoot due
 to the inertia of motor.
 The steering control apparatus may further comprise limiting means for
 limiting the target driving speed to the magnitude within a predetermined
 range.
 When the target driving speed of the actuator is limited by the limiting
 means, for example when the limit is defined by the operation limit speed
 of the actuator, the speed of the actuator is always controlled within the
 range not more than the operation limit speed, which can prevent
 occurrence of the overshoot with reliability.
 The steering control apparatus for vehicle may further comprise: driving
 speed detecting means for detecting the driving speed of the actuator; and
 failure determining means for determining whether a failure occurs, based
 on the driving speed detected by the driving speed detecting means and the
 target driving speed limited by the limiting means.
 Since the target driving speed is limited to the magnitude in the
 predetermined range by the limiting means, the deviation between the
 target driving speed and the driving speed detected becomes smaller than
 predetermined deviation. Therefore, the failure determining means
 determines that a failure occurred in a control system, for example, when
 this deviation is greater than a predetermined value.
 The present invention will become more fully understood from the detailed
 description given hereinbelow and the accompanying drawings which are
 given by way of illustration only, and thus are not to be considered as
 limiting the present invention.
 Further scope of applicability of the present invention will become
 apparent from the detailed description given hereinafter. However, it
 should be understood that the detailed description and specific examples,
 while indicating preferred embodiments of the invention, are given by way
 of illustration only, since various changes and modifications within the
 spirit and scope of the invention will become apparent to those skilled in
 the art from this detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments of the present invention will be described with reference to
 the accompanying drawings.
 FIG. 1 shows the structure of steering apparatus 100 according to an
 embodiment.
 An input shaft 20 is connected through a transmission ratio varying
 mechanism 30 to an output shaft 40 and a steering handle (steering wheel)
 10 is connected to the input shaft 20. The output shaft 40 is connected to
 a rack shaft 51 through a gear system 50 of the rack-and-pinion type and
 wheels FW1, FW2 are connected to the both ends of the rack shaft 51. The
 input shaft 20 is provided with a steering speed sensor 21 for detecting
 the steering speed of the steering wheel 10.
 The transmission ratio varying mechanism 30 connects the input shaft 20 to
 the output shaft 40 through a gear mechanism and is constructed so as to
 drive this gear mechanism by servo motor 31 as an actuator, thereby
 permitting change in the transmission ratio of rotations between the input
 shaft and the output shaft. This servo motor 31 is equipped with a
 rotational speed sensor 32 for detecting the rotational speed of the servo
 motor 31.
 Steering control device 70 receives supply of detection results including
 the steering speed .theta.hv detected by the steering speed sensor 21, the
 rotational speed .theta.pv of the servo motor 31 detected by the
 rotational speed sensor 32, and the vehicle speed V detected by vehicle
 speed sensor 60, sets the transmission ratio based on these detection
 results, and controls the rotational speed of the servo motor 31 according
 to the transmission ratio thus set.
 FIG. 2 shows the structure of the steering control device 70. The steering
 control device 70 is equipped with transmission ratio setting section 71,
 target speed limiting section 72, compensator 73, and failure determining
 section 74.
 The transmission ratio setting section 71 searches a map indicating the
 relationship between vehicle speed V and transmission ratio G, based on
 the speed V detected by the speed sensor 60, to set the transmission ratio
 G (G=steering input amount/turned output amount) according to the speed V.
 Based on the transmission ratio G thus set and the steering speed
 .theta.hv detected by the steering speed sensor 21, the target rotational
 speed .theta.mv of the servo motor 31 is computed according to
 .theta.mv=G..theta.hv and the result of this computation is supplied to
 the target speed limiting section 72.
 The target speed limiting section 72 has a function as a limiter to limit
 the target rotational speed .theta.mv of the servo motor 31 to below the
 maximum rotational speed which was preset. This maximum rotational speed
 is set based on the performance characteristics of the servo motor 31. For
 example, where the servo motor 31 has the load torque (T)-number of
 rotation (N) characteristics illustrated in FIG. 3, the maximum reaction
 (load torque) from road surfaces is preliminarily assumed to be Tmax, and
 then the maximum number of rotation (rotational speed) is obtained as Nmax
 from the graph of FIG. 3. The target speed limiting section 72 limits the
 target rotational speed .theta.mv to within the range of .+-.Nmax as
 illustrated in FIG. 4 and outputs a value subject to the limitation, as
 .theta.smv.
 Supplied to the compensator 73 is the deviation ev
 (ev=.theta.smv-.theta.pv) between the target rotational speed .theta.smv
 outputted from the target speed limiting section 72 and the rotational
 speed .theta.pv of the servo motor 31 detected by the rotational speed
 sensor 32. This compensator 73 computes Is=C(s).ev, using the transfer
 function C(s) obtained by properly setting parameters of PID control, to
 determine the control signal Is, which is supplied to the servo motor 31.
 In the function "s" stands for the Laplace operator.
 In this way the steering control device 70 carries out the speed control of
 the servo motor 31 according to the steering speed .theta.hv of the
 steering wheel 10, based on the transmission ratio set. For example,
 supposing that the abrupt switching-back operation to switch the steering
 wheel 10 back is conducted at the point of time t3 as illustrated in FIGS.
 5A and 5B, this control gradually decreases the target rotational speed
 .theta.mv of the servo motor 31 toward the time t3 and makes .theta.mv=0
 at the point of time t3. Therefore, the actual rotational speed of the
 servo motor 31 is controlled so as to gradually decrease toward the time
 t3 and to be nulled at the point of time t3. As a consequence, the
 influence of remaining deviation, which occurred upon execution of the
 positional control, can be eliminated by carrying out the control of the
 rotational speed of the servo motor 31 based on the steering speed, even
 in the cases of abrupt switching-back steering and stop of steering after
 the abrupt steering operation. This can adequately suppress occurrence of
 the overshoot due to the inertia of motor.
 The value of deviation ev is also supplied to the failure determining
 section 74. The failure determining section 74 determines whether a
 failure occurs in this control system. Since the target rotational speed
 is limited by the target speed limiting section 72 as stated previously,
 the deviation ev between the target rotational speed .theta.smv and the
 rotational speed .theta.pv of the servo motor 31 should take a value
 within a predetermined range. The failure determining section 74 thus
 compares the deviation ev with a predetermined threshold. When the value
 of the deviation ev is larger than the threshold, the failure determining
 section 74 determines that a failure occurs in this control system and
 informs the driver of the occurrence of failure by performing a process of
 turning an alarm lamp or the like on.
 Another embodiment will be described below.
 FIG. 6 shows the structure of steering apparatus 200 according to another
 embodiment. The same components as those in FIG. 1 will be denoted by the
 same reference numerals and the description thereof will be omitted.
 The input shaft 20 is equipped with a steering angle sensor 22 for
 detecting a steering angle of the steering wheel 10 and the servo motor 31
 of the transmission ratio varying mechanism 30 is equipped with a
 rotational angle sensor 33 for detecting a rotational angle of the servo
 motor 31.
 The steering control device 80 receives supply of detection signals from
 the steering angle sensor 22, from the rotational angle sensor 33, and
 from the vehicle speed sensor 60, sets the transmission ratio based on
 these detection signals, and executes the speed control of the servo motor
 31 according to the transmission ratio thus set.
 FIG. 7 illustrates the structure of the steering control device 80. The
 steering control device 80 is provided with transmission ratio setting
 section 81, target speed limiting section 82, compensator 83, N-point
 shift determining section 84, failure determining section 85,
 differentiators 86, 87, and integrators 88, 89.
 The transmission ratio setting section 81 performs the map search based on
 the vehicle speed V detected by the speed sensor 60 with reference to the
 map indicating the relationship between vehicle speed V and transmission
 ratio G to set the transmission ratio G according to the vehicle speed V.
 Based on the transmission ratio G thus set and the steering angle .theta.h
 detected by the steering angle sensor 22, a value of target rotational
 angle .theta.m indicating a target rotational position of the servo motor
 31 is computed according to .theta.m=G..theta.h. This computation result
 is supplied to the differentiator 86 to be converted to the target
 rotational speed .theta.mv of the servo motor 31.
 The target speed limiting section 82 functions as a limiter to limit the
 target rotational speed .theta.mv to the range not more than the maximum
 rotational speed set based on the performance characteristics of the servo
 motor 31, similar to the target speed limiting section 72 described
 previously. The target rotational speed .theta.mv outputted from the
 differentiator 86 is input to this target speed limiting section 82 to be
 limited to a value not more than the maximum rotational speed, and then
 the value is outputted as the target rotational speed .theta.smv.
 On the other hand, the rotational angle .theta.p of the servo motor 31
 detected by the rotational angle sensor 33 is supplied to the
 differentiator 87 to be converted to the rotational speed .theta.pv of the
 servo motor 31.
 The compensator 83 is given the deviation ev (ev=.theta.smv-.theta.pv)
 between the target rotational speed .theta.smv outputted from the target
 speed limiting section 82 and the rotational speed .theta.pv of the servo
 motor 31 outputted from the differentiator 87 and computes Is=C(s).ev to
 determine the control signal Is, which is supplied to the servo motor 31.
 The target rotational speed .theta.mv outputted from the differentiator 86
 is supplied to the integrator 88 to be converted to target rotational
 angle .theta.m of the servo motor 31. This target rotational angle
 .theta.m resulting from the conversion, and the actual rotational angle
 .theta.p of the servo motor 31 detected by the rotational angle sensor 33
 are input to the N-point shift determining section 84.
 The N-point shift determining section 84 compares this target rotational
 angle .theta.m with the value of rotational angle .theta.p, for example,
 at the timing when the deviation ev becomes zero. When this comparison
 results in disagreement between the two values, the neutral point (N
 point) of the steering wheel 10 is different from that of the wheels FW1,
 FW2. If a difference between the two values falls outside a predetermined
 range, the apparatus executes, for example, a process of turning on an
 alarm lamp for indicating the N-point shift of the steering wheel 10 to
 notify the driver of the occurrence of N-point shift. There will be no
 trouble at all in steering of the vehicle, per se, even if a small shift
 occurs between the neutral position of the steering wheel 10 and the
 neutral position of the wheels FW1, FW2.
 The target rotational speed .theta.smv outputted from the target speed
 limiting section 82 is also supplied to the integrator 89 to be converted
 to target rotational angle .theta.m' of the servo motor 31. This target
 rotational angle .theta.m' resulting from the conversion, and the actual
 rotational angle .theta.p of the servo motor 31 detected by the rotational
 angle sensor 33 are supplied to the failure determining section 85.
 The failure determining section 85 determines whether a failure occurs in
 this control system. Since the target rotational speed is limited by the
 target speed limiting section 82 described previously, the deviation
 between the target rotational angle .theta.m' obtained through the
 integrator 89 after the limitation and the actual rotational angle
 .theta.p of the servo motor 31 should take a value within a predetermined
 range. When this deviation is larger than a predetermined threshold, the
 failure determining section 85 determines that a failure occurs in this
 control system, and performs the process of switching the alarm lamp on or
 the like to inform the driver of the occurrence of failure. The failure of
 the control system is determined based on the angular position in this
 way.
 In each embodiment described above the servo motor 31 was exemplified as an
 actuator for driving the transmission ratio varying mechanism 30, but the
 actuator can also be composed of another motor, such as a stepping motor.
 In this case, the target speed limiting section 72, 82 estimates the
 maximum reaction (load torque) from the road surfaces, determines the
 maximum of the number of rotation of the motor according to the estimated
 load torque, based on the load torque (T)-number of rotation (N)
 characteristics concerning the motor employed, and limits the rotational
 speed of the motor to the range not more than the maximum determined.
 The embodiments illustrated the examples in which the vehicle speed V was
 detected as a running condition of the vehicle and the transmission ratio
 G was set according to the vehicle speed V, but, in another embodiment,
 the apparatus can also be constructed to detect the vehicle speed V and
 the input angle .theta.h as running conditions of the vehicle and to set
 the transmission ratio G according to the vehicle speed V and input angle
 .theta.h.
 The embodiments illustrated the examples in which the target speed was
 limited by the target speed limiting section 72, but, in place of the
 target speed limiting section 72, the apparatus can also be constructed to
 provide the steering speed sensor 21 itself with such saturation
 characteristics that the output becomes saturated when the output of the
 steering speed sensor 21 exceeds a predetermined level.
 As detailed above, the steering control apparatus for vehicle employs the
 structure in which the-driving speed setting means sets the target driving
 speed of the actuator according to the steering speed of the steering
 wheel and in which the speed control means controls the speed of the
 actuator so as to match it with the target driving speed thus set. This
 controls the driving speed of the actuator according to the steering speed
 of the steering wheel, which can adequately suppress the overshoot due to
 the inertia of the motor appearing on the occasion of abrupt
 switching-back steering or the like and in turn improve the steering
 feeling, as compared with the case where the driving control of the
 actuator is carried out based on the positional deviation as before.
 As stated above, the steering control apparatus for a vehicle comprises:
 (A) a transmission ratio varying mechanism (30) capable of varying the
 amount of angular rotation of an output shaft (40) relative to the amount
 of angular rotation of an input shaft (20) connected to a steering handle
 (10); and (B) a steering control device (70) that controls the
 transmission ratio (G) by driving the mechanism (30), the device (70)
 driving the mechanism (30) so as to make the real driving speed
 (.theta.pv) of the mechanism (30) a target driving speed (.theta.mv), the
 target driving speed (.theta.mv) being based on a predetermined
 transmission ratio (G) and actual rotational speed (.theta.hv) of the
 steering handle (10).
 This predetermined transmission ratio (G) is determined using data on the
 vehicle's operational condition.
 The operational condition is defined based on signals from a vehicle
 velocity sensor (60).
 The mechanism (30) includes an actuator (31) driven by the control device
 (70).
 The actual rotational speed (.theta.hv) is detected by an actual rotational
 speed sensor (21) for the steering handle (10).
 The actual rotational speed (.theta.hv)is detected by using angular data
 (.theta.h) from a steering angle sensor (22) for the steering handle (10),
 the angular data (.theta.h) being converted to the actual rotational speed
 (.theta.hv) by calculating the differential of the angular data
 (.theta.hv).
 The target driving speed (.theta.mv) is limited to less than a
 predetermined value (Nmax).
 The defect diagnosis is performed by based on the real driving speed
 (.theta.pv) and the target driving speed (.theta.mv).
 In more detail, the above apparatus is a steering control apparatus for
 vehicle adapted to change a transmission ratio between a steering amount
 and a turn amount by driving a transmission ratio varying mechanism by an
 actuator, the steering control apparatus for vehicle comprising:
 transmission ratio setting means for setting the transmission ratio
 according to a running condition of a vehicle; steering speed detecting
 means for detecting a steering speed of a steering wheel; driving speed
 setting means for setting a target driving speed for the actuator, based
 on the transmission ratio set by the transmission ratio setting means and
 the steering speed detected by the steering speed detecting means; and
 speed control means for controlling a speed of the actuator to the target
 driving speed set by the driving speed setting means.
 Further, the steering control apparatus further comprises limiting means
 for limiting the target driving speed to the magnitude within a
 predetermined range.
 Further, the steering control apparatus further comprises: driving speed
 detecting means for detecting the driving speed of the actuator; and
 failure determining means for determining whether a failure occurs, based
 on the driving speed detected by the driving speed detecting means and the
 target driving speed limited by the limiting means.
 From the invention thus described, it will be obvious that the invention
 may be varied in many ways. Such variations are not to be regarded as a
 departure from the spirit and scope of the invention, and all such
 modifications as would be obvious to one skilled in the art are intended
 for inclusion within the scope of the following claims.