Drum brake device

A drum brake device produces a stable braking force at a predetermined boosting ratio in response to a brake operation force input thereto, even if the friction characteristics of the interface of the brake drum and the brake shoe varies, without increasing the size of the wheel cylinder. The drum brake device includes an input detector that detects a hydraulic braking pressure output from a master cylinder, and produces a signal representative of the detected hydraulic braking pressure as an input force to the drum brake device; an output detector that detects an anchor reaction force generated at the time of braking and produces a signal representative of the detected anchor reaction force as an output force of the drum brake device; electromagnetic valves capable of controlling the supply of a hydraulic braking pressure supplied from the master cylinder to a wheel cylinder; and a control circuit that controls the operations of the electromagnetic valves in accordance with output signals of the input detector and the output detector so that the output force is limited to a force equal to or smaller than a force defined by a predetermined boosting ratio.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a drum brake device which is based on
 feedback control, and is able to constantly produce a stable braking force
 irrespective of the state of a brake friction surface, and is further
 adaptable for use in both an anti-lock braking system and a traction
 control system.
 2. Discussion of the Related Art
 Drum brake devices are widely employed in vehicle braking systems. A major
 reason for the drum brake device's use is its ability to automatically
 amplify a braking force (referred to as a self-amplifying function), as is
 observed in the uni-servo or duo-servo brake devices. This self-amplifying
 ability lessens the required brake operation force (e.g., a brake pedal
 depression force). However, this self-amplifying operation of the drum
 brake device is unstable. A proportional constant, which defines a
 relationship between a pedal depressing force and a final braking force,
 is very sensitive to the friction coefficient of the interface between the
 brake drum and the brake shoe. In cases where the inner surface of the
 brake drum is rusted after the vehicle travels in the rain, the wheels
 will be abruptly locked by applying an extremely small depression to the
 brake pedal.
 To remove this defect, it is essential that the drum brake device have the
 ability to constantly produce a stable braking force at a predetermined
 boosting ratio in response to a brake operation force, even if the
 frictional characteristic of the interface between the brake drum and the
 brake shoe varies. One solution to this problem was proposed in
 JP-B-2-46424. The technique of this publication will be described with
 reference to FIGS. 2 and 3.
 As shown, a drum brake device 1 is made up of a pair of brake shoes 3 and
 4, a wheel cylinder 6, a link member 8, a master cylinder 12, and a fluid
 passage 13. In the drum brake device 1, the wheel cylinder 6 is improved
 to produce a stable braking force. The brake shoes 3 and 4 are oppositely
 disposed within an inner space of a cylindrical brake drum 2. The wheel
 cylinder 6, disposed between the opposed ends of brake shoes 3 and 4, is
 used for expanding the ends of the brake shoes 3 and 4 so that they come
 into engagement with the inner surface of the brake drum. The link member
 8 mutually links the other ends of the brake shoes 3 and 4, and receives
 an anchor reaction force of one of the brake shoes 3 and 4 and transmits
 it to the other brake shoe. The master cylinder 12 generates a hydraulic
 braking pressure that corresponds in magnitude to a brake operation force
 F1 (a depression force applied to a brake pedal 10). The hydraulic braking
 pressure generated by the master cylinder 12 is introduced into the wheel
 cylinder 6 through the fluid passage 13.
 Details of the wheel cylinder 6 are illustrated in FIG. 3. As shown, a
 cylinder body 17 includes a first cylinder 17a and a second cylinder 17b.
 These cylinders (17a and 17b) are integrally formed, with the former (17a)
 being located below the latter (17b). The first cylinder 17a contains a
 slidable drive piston 15. The second cylinder 17b contains a slidable
 control piston 16. A hydraulic braking pressure is transmitted from the
 fluid passage 13 to a pressure chamber 20 via a control chamber 19. The
 control chamber 19 is formed in the second cylinder 17b while the pressure
 chamber 20 is formed in the first cylinder 17a. The tip of the drive
 piston 15 contacts the primary shoe 3, and presses the primary shoe 3
 against the brake drum 2 by a thrust force P1. Thrust force P1 corresponds
 to the hydraulic braking pressure supplied to the pressure chamber 20.
 The tip of the control piston 16, which is in contact with the end of the
 secondary shoe 4, receives an anchor reaction force P2 from the secondary
 shoe 4 while the base end of the control piston 16 receives a hydraulic
 braking pressure supplied to the control chamber 19. When an urging force
 corresponding to the anchor reaction force P2 exceeds an urging force P3
 caused by the hydraulic braking pressure, the control piston 16 is
 displaced toward the control chamber 19. Further, a valve body 24 is
 provided within the control chamber 19. The valve body is used for opening
 and closing a communicating passage 22 which communicates the control
 chamber 19 with the pressure chamber 20. When the control piston 16 is
 displaced toward the control chamber 19, the communicating passage 22 is
 closed with the valve body 24.
 When the urging force P3 (caused by the hydraulic braking pressure from the
 master cylinder 12) is imparted or input to the control piston 16, and the
 anchor reaction force P2 is varied to a force defined by a predetermined
 boosting ratio, the control piston 16 is displaced toward the control
 chamber 19 to stop the supply of the hydraulic braking pressure to the
 pressure chamber 20. As a result, the thrust force P1 of the drive piston
 15 is kept constant, a further increase of the anchor reaction force is
 prevented, and the braking force is stabilized.
 In the conventional drum brake device mentioned above, the mechanism for
 controlling the anchor reaction force is incorporated into the wheel
 cylinder 6. However, this type of drum brake device has the following
 problem. The structure of the brake device increases the size of the wheel
 cylinder 6. The increase of the cylinder size makes it difficult to
 assemble the wheel cylinder 6 into the brake device of a small-size
 vehicle that has a brake drum with a small inner space. Additionally, the
 increased cylinder size further increases the weight of the brake device
 with the wheel cylinder assembled thereinto. For this reason, the
 conventional drum brake device which employs the unique mechanism for
 achieving braking force stabilization has found limited use in small-size
 vehicles. It is almost impossible to apply a common conventional drum
 brake device to various types of vehicles. Specifically, vehicles differ
 in body weight and in anchor reaction forces of the drum brake devices
 assembled thereinto. Accordingly, an object of the present invention is to
 provide a drum brake device which produces a stable braking force at a
 predetermined boosting ratio in response to a brake operation force input
 thereto, even if the frictional characteristics of the interface of the
 brake drum and the brake shoe varies. Additionally, the aforementioned
 object should be realized without increasing the device size. Furthermore,
 the drum brake device should be suitably applicable to a small-size
 vehicle having a brake drum with a small inner space.
 SUMMARY OF THE INVENTION
 Additional features and advantages of the invention will be set forth in
 the description which follows, and in part will be apparent from the
 description, or may be learned by practice of the invention. The
 objectives and other advantages of the invention will be realized and
 attained by the structure particularly pointed out in the written
 description and claims hereof as well as the appended drawings.
 To achieve these and other advantages and in accordance with the purpose of
 the present invention, as embodied and broadly described, in one aspect of
 the present invention there is provided a drum brake device, comprising a
 brake drum; a pair of brake shoes oppositely disposed within the brake
 drum, each of the pair of brake shoes having a first end and a second end;
 a wheel cylinder for expanding the brake shoes, the wheel cylinder being
 disposed between the first ends of the brake shoes, and the second ends of
 the brake shoes being connected; a master cylinder outputting a hydraulic
 braking pressure; a fluid passage for introducing the hydraulic braking
 pressure output from the master cylinder to the wheel cylinder, the
 hydraulic braking pressure being dependent on a brake operation force; an
 input detector, coupled to the fluid passage, that detects the hydraulic
 braking pressure output from the master cylinder and produces a first
 output signal representative of the detected hydraulic braking pressure,
 the hydraulic braking pressure being an input force to one of the pair of
 brake shoes; an output detector that detects an anchor reaction force
 generated by the other one of the pair of brake shoes during braking and
 produces a second output signal representative of the detected anchor
 reaction force, the detected anchor reaction force being an output force
 of the other one of the pair of brake shoes; electromagnetic valves,
 coupled to the fluid passage, that control the supply of the hydraulic
 braking pressure output from the master cylinder to the wheel cylinder;
 and a control circuit that controls the operations of the electromagnetic
 valves in accordance with the first and second output signals so that the
 output force is limited to a force equal to or smaller than the input
 force multiplied by a predetermined boosting ratio.
 In another aspect of the present invention, there is provided a drum brake
 device, comprising a brake drum; a pair of brake shoes oppositely disposed
 within the brake drum, each of the pair of brake shoes having a first end
 and a second end; a wheel cylinder for expanding the brake shoes, the
 wheel cylinder being disposed between the first ends of the brake shoes; a
 master cylinder outputting a hydraulic braking pressure; a fluid passage
 for introducing the hydraulic braking pressure output from the master
 cylinder to the wheel cylinder, the hydraulic braking pressure being
 dependent on a brake operation force; input detecting means, coupled to
 the fluid passage, for detecting the hydraulic braking pressure output
 from the master cylinder and producing a first output signal
 representative of the detected hydraulic braking pressure, the hydraulic
 braking pressure being an input force to the drum brake device; output
 detecting means for detecting an anchor reaction force generated during
 braking and producing a second output signal representative of the
 detected anchor reaction force, the detected anchor reaction force being
 an output force of the drum brake device; electromagnetic valves, coupled
 to the fluid passage, that control the supply of the hydraulic braking
 pressure output from the master cylinder to the wheel cylinder; and a
 control circuit that controls the operations of the electromagnetic valves
 in accordance with the first and second output signals so that the output
 force is limited to a force equal to or smaller than a force defined by a
 predetermined boosting ratio of the output force to the input force.
 In yet another aspect of the present invention, there is provided a drum
 brake device, comprising a brake drum; a pair of brake shoes oppositely
 disposed within the brake drum, each of the pair of brake shoes having a
 first end and a second end, and the second ends of the pair of brake shoes
 being linked together; a wheel cylinder for expanding the brake shoes, the
 wheel cylinder being disposed between the first ends of the brake shoes; a
 master cylinder outputting a hydraulic pressure; a fluid passage for
 introducing the hydraulic braking pressure output from the master cylinder
 to the wheel cylinder, the hydraulic braking pressure being dependent on a
 brake operation force; an input detector coupled to the fluid passage, the
 input detector detecting the hydraulic braking pressure output from the
 master cylinder and producing a first output signal representative of the
 detected hydraulic braking pressure, the hydraulic braking pressure being
 an input force to the drum brake device; an output detector that detects
 an anchor reaction force generated during braking and produces a second
 output signal representative of the detected anchor reaction force, the
 detected anchor reaction force being an output force of the drum brake
 device; electromagnetic valves, coupled to the fluid passage, that control
 the supply of the hydraulic braking pressure output from the master
 cylinder to the wheel cylinder; and a control circuit that controls the
 operations of the electromagnetic valves in accordance with the first and
 second output signals so that the output force is limited to a force equal
 to or smaller than the input force multiplied by a predetermined boosting
 ratio.
 It is to be understood that both the foregoing general description and the
 following detailed description are exemplary and explanatory and are
 intended to provide further explanation of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 is a diagram showing an embodiment of the drum brake device of the
 present invention. As shown, a drum brake device 30 is made up of a pair
 of primary and secondary shoes (32 and 33), a wheel cylinder 35, a link
 member 36, a master cylinder 37, a fluid passage 39, and braking-force
 stabilizing mechanism 40. The brake shoes 32 and 33 are oppositely
 disposed within an inner space of a cylindrical brake drum 31. The wheel
 cylinder 35, disposed between opposed ends of the brake shoes 32 and 33,
 is used for expanding the ends of the brake shoes 32 and 33 so as to come
 into engagement with the inner surface of the brake drum. The link member
 36 mutually links the other ends of the brake shoes 32 and 33, and
 receives an anchor reaction force of one of the brake shoes 32 and 33 and
 transmits it to the other brake shoe. The master cylinder 37 generates a
 hydraulic braking pressure corresponding in magnitude to a brake operation
 force F1 (a depression force applied to a brake pedal 38). The hydraulic
 braking pressure generated by the master cylinder 37 is introduced into
 the wheel cylinder 35 through the fluid passage 39.
 The braking-force stabilizing mechanism 40 controls the anchor reaction
 force. The braking-force stabilizing mechanism 40 includes input detector
 42, output detector 44, electromagnetic valves 45 and 46, and a control
 circuit 48. The input detector 42, which is coupled to the fluid passage
 39, detects a hydraulic braking pressure supplied to the wheel cylinder 35
 (which expands the brake shoes 32 and 33), produces a signal
 representative of the detected hydraulic braking pressure, and inputs it
 as an input force to the control circuit 48. The output detector 44 is in
 contact with the anchor end of the secondary shock 33 which is located
 closer to the wheel cylinder. Output detector 44 detects an anchor
 reaction force generated at the time of braking and produces a signal
 representative of the detected anchor force and outputs it as an output
 force to the control circuit 48. The electromagnetic valves 45 and 46,
 coupled to the fluid passage 39, are used for controlling a hydraulic
 braking pressure supplied to the wheel cylinder 35. The control circuit 48
 controls the operations of the electromagnetic valves 45 and 46 (the
 electromagnetic valve 46 will be described in detail later) in accordance
 with the output signals of the input detector 42 and the output detector
 44.
 In the embodiment, the input detector 42 takes the form of a pressure
 sensor which senses a hydraulic braking pressure in the fluid passage 39
 and outputs it in the form of an electrical signal. The output detector 44
 takes the form of a strain gauge which detects a quantity of a movement of
 the secondary shoe 33 when it is angularly moved by the received anchor
 reaction force. The electromagnetic valve 45 is a 2-position switch valve
 for opening and closing the fluid passage 39. The electromagnetic valve 46
 is a 2-position switch valve for opening and closing a by-pass passage 49
 connecting to the fluid passage 39. One end of the by-pass passage 49
 communicatively connects to a part of the fluid passage 39, located
 between the input detector 42 and the master cylinder 37. The other end of
 the by-pass passage 49 communicatively connects to a part of the fluid
 passage 39, located between the electromagnetic valve 45 and the wheel
 cylinder 35.
 A damper cylinder 50, a pump 53, an accumulator 54, and an orifice 55 are
 coupled to the by-pass passage 49. The damper cylinder 50 receives a fluid
 flowing from the fluid passage 39 into the by-pass passage 49 via the
 electromagnetic valve 46. The pump 53 returns the operation fluid from the
 damper cylinder 50 to the fluid passage 39 in response to a control signal
 received from the control circuit 48 when the brake pedal is released from
 its depressed state. The accumulator 54 dumps a pulsation of the operation
 fluid generated when the pump is driven. The orifice 55 restricts a flow
 of the operation fluid flowing from a part of the fluid passage 39, which
 is located closer to the master cylinder 37, to the by-pass passage 49.
 The control circuit 48 processes the output signals of the input detector
 42 and the output detector 44 in accordance with a procedure as an
 implementation of a predetermined algorithm. The control circuit 48
 produces control signals in accordance with the result of the processing,
 and outputs them to the electromagnetic valves 45 and 46 and the pump 53.
 Further, the control circuit 48 controls the electromagnetic valves 45 and
 46 so that the anchor reaction force P2 detected by the output detector 44
 is limited to a force equal to or smaller than a force defined by a
 predetermined boosting ratio of the thrust force P1 (input from the wheel
 cylinder 35 to the primary shoe 32). The thrust force P1 of the wheel
 cylinder 35 depends on the output signal of the input detector 42.
 More specifically, when the thrust force P1 is input from the wheel
 cylinder 35 to the primary shoe 32, and the anchor reaction force P2 is
 varied to a force defined by a predetermined boosting ratio, the control
 circuit 48 drives the electromagnetic valve 45 to close the fluid passage
 39 and to stop the supply of the hydraulic braking pressure to the wheel
 cylinder 35. As a result of stopping the supply of the hydraulic braking
 pressure from the fluid passage 39 to the wheel cylinder 35, an increase
 of the anchor reaction force is restricted.
 In cases where a further decrease of the anchor reaction force is required
 in a state that the electromagnetic valve 45 stops the hydraulic braking
 pressure supply to the wheel cylinder 35, the electromagnetic valve 46 is
 opened to set up a communication between the wheel cylinder 35 and the
 damper cylinder 50. Then, the hydraulic braking pressure is released from
 the wheel cylinder 35 to the damper cylinder 50, whereby the hydraulic
 braking pressure in the wheel cylinder 35 is reduced.
 An operation of the drum brake device 30 thus constructed will be
 described. The input detector 42 detects a hydraulic braking pressure
 (that depends on a brake operation force) supplied to the wheel cylinder
 35, produces an output signal representative of an input force to the
 brake device, and applies it to the control circuit 48. The output
 detector 44 detects an anchor reaction force generated at the time of
 braking, produces an output signal representative of an output force of
 the brake device, and applies it to the control circuit 48. The control
 circuit 48 monitors a ratio of the input force and the output force while
 receiving those force representative signals, controls the operations of
 the electromagnetic valves 45 and 46 in accordance with the ratio, and
 hence controls the hydraulic braking pressure supply to the wheel cylinder
 35. Thus, the anchor reaction force or the output force is limited to a
 force equal to or smaller than a force defined by a predetermined boosting
 ratio.
 To apply the drum brake devices of the invention to different types of
 vehicles of different weights, a designer need only set the boosting ratio
 in advance at values appropriate to those types of vehicles. Thus, the
 drum brake device of the invention may readily be applied to various types
 of vehicles. Further, the drum brake device can produce a stable braking
 force even in situations where the frictional characteristic of the
 interface between the cylindrical brake drum 31 and the brake shoes 32 and
 33 varies.
 In the drum brake device of the invention, the control of the anchor
 reaction force is carried out in such a manner that the hydraulic braking
 pressure supply to the wheel cylinder 35 is controlled by use of the
 electromagnetic valves 45 and 46. In other words, the special mechanism
 for controlling the anchor reaction force is not assembled into the wheel
 cylinder 35. Therefore, there is no increase of the size of the wheel
 cylinder 35. A strain gauge is located between the wheel cylinder 35 and
 the secondary shoe 33, and is used as the output detector 44 for detecting
 the anchor reaction force. Since the stain gauge is compact in size, it
 requires less space when it is attached to the brake assembly.
 From this, it is readily understood that the drum brake device of the
 invention finds application in small-size vehicles that have brake drums
 with smaller inner spaces. It is apparent that any means other than a
 strain gauge may be used as the output detector 44 so long as it has an
 ability to detect the anchor reaction force. For example, a sensor element
 capable of sensing a mechanical strength and reproducing it in the form of
 an electrical signal may be used as output detector 44. Another example is
 to place a sensor element, like the input sensor, for sensing a hydraulic
 braking pressure in the control chamber that receives the anchor reaction
 force.
 The above-mentioned embodiment obtains the input force input to the wheel
 cylinder 35 through the detection of the hydraulic braking pressure by use
 of the input detector 42. Alternatively, the input force may be obtained
 in the form of a quantity of stroke of the depressed brake pedal, which is
 proportional to a thrust force. In this case, a position sensor may be
 used for sensing the stroke quantity. The position sensor produces an
 electrical signal corresponding in amplitude to the sensed stroke
 quantity.
 It is preferable that the brake control performed by the control circuit 48
 in the drum brake device is integrated into an anti-lock braking system or
 a traction control system. If this is done, the hydraulic braking pressure
 supplied to the wheel cylinder can be controlled (i.e., increased or
 decreased) through the control of the electromagnetic valves, and in
 accordance with accidental vehicle behaviors. In this manner, the drum
 brake device stabilizes the vehicle motion.
 The above-mentioned embodiment uses two electromagnetic valves for
 controlling the hydraulic braking pressure supply to the wheel cylinder. A
 3-position switch electromagnetic valve, if used instead, will reduce the
 required number of the valves. Further, the damper cylinder 50, the pump
 53, the accumulator 54 and the orifice 55, which are coupled to the
 by-pass passage 49, may be modified and changed within the scope of the
 invention. It is also evident that the present invention is applicable to
 any other brake device having a self-amplifying function.
 As seen from the foregoing description, in the drum brake device of the
 present invention, the control circuit monitors a ratio of the input force
 detected by the input detector and the output force detected by the output
 detector, and controls the operations of the electromagnetic valves. The
 electromagnetic valves are coupled to the fluid passage connecting the
 master cylinder to the wheel cylinder so that the output force is limited
 to a force equal to or smaller than a force defined by a predetermined
 boosting ratio. The boosting force is defined in connection with the input
 force that is input from the wheel cylinder to the brake shoe.
 The drum brake device can produce a stable braking force at a predetermined
 boosting ratio even in a situation where the frictional characteristic of
 the interface between the cylindrical brake drum and the brake shoes
 varies. Therefore, stable braking torques are constantly maintained. The
 drum brake device may also operate to block vehicle behaviors in an
 accidental situation so as to stabilize the vehicle motion.
 Further, the control of the anchor reaction force is carried out in a
 manner such that the hydraulic braking pressure supply to the wheel
 cylinder is controlled by use of the electromagnetic valves. In other
 words, the special mechanism for controlling the anchor reaction force is
 not assembled into the wheel cylinder. As a result, there is no increase
 of the size of the wheel cylinder. Hence, the drum brake device of the
 invention finds application in small-size vehicles having brake drums with
 small inner spaces.