Vehicle brake pressure modulator

Abstract: An electric motor-driven brake pressure modulator (104) has an auxiliary field winding (22,24) connected in series-parallel relationship with a permanent magnet field DC motor (138) to produce different speed/torque performance characteristics depending on the direction of motor rotation. In forward motor rotation when the brake pressure is to be increased, the magneto-motive force of the auxiliary field winding (22,24) aids the magneto-motive force of the field magnets (14) to produce a higher flux density in the working air gap of the motor (138). This maximizes the torque characteristic of the motor (138) for the development of adequate braking pressure. In the reverse direction of motor rotation when the brake pressure is to be decreased, the magneto-motive force of the auxiliary field winding (22,24) opposes the magneto-motive force of the field magnets (14) to lower the flux density in the working air gap of the motor. This maximizes the speed characteristic of the motor to quickly reduce the braking pressure.

Description: This invention relates to apparatus for modulating the braking pressure applied to a vehicle brake for use, for example, in a motor vehicle anti-lock braking system. A known electric motor-driven anti-lock braking system, is depicted in Figure 1. Referring to Figure 1, the braking system comprises a hydraulic boost unit 100, a wheel brake 102, an electric motor-driven hydraulic pressure modulator 104, and an electronic controller 106 for operating the modulator 104 with current from the vehicle storage battery 108. The boost unit 100 develops hydraulic pressure in line 120 in relation to the force applied to an operator manipulated brake pedal, the line 120 being connected to the brake 102 via modulator 104 and brake line 122. Brake 102 is depicted as a disk brake caliper which develops braking force on the wheel rotor 126 in relation to the hydraulic pressure in brake line 122. The modulator 104 comprises an actuator 130 axially displaceable in the modulator bore 132, a check ball 134 resiliently seated on a ball seat 136 disposed between the brake lines 120 and 122, and a bi-directional DC motor 138 coupled to the actuator 130 via a reduction gearset 140 and a ball screw 142 to control the axial displacement of actuator 130. Energization of the motor 138 is controlled by the electronic controller 106 in response to a signal on line 144 indicative of the angular velocity of rotor 126. When the controller 106 energizes the motor 138 for rotation in a forward direction, the ball screw 142 extends into the bore 132, thereby extending actuator 130 to unseat the check ball 134. This opens the communication between brake lines 120 and 122, and represents the normal or quiescent state of the anti-lock brake system. When the controller 106 energizes the motor 138 for rotation in the opposite, or reverse, direction, the ball screw 142 retracts actuator 130 within the bore 132, permitting spring 146 and the fluid pressure in brake line 120 to seat the check ball 134 on the ball seat 136, thereby isolating the brake line 122 from the brake line 120. In this condition, the brake fluid in line 122 backfills the modulator bore 132, relieving the fluid pressure developed at brake 102. In anti-lock operation, the brake pressure in line 122 is modulated by repeatedly reversing the direction of rotation of motor 138 to effect a dithering movement of the actuator 130 in the bore 132. When the onset of wheel lock, that is an incipient wheel lock condition, is detected, the controller 106 causes the motor 138 to rotate in the reverse direction to retract the actuator 130; when recovery of the wheel is detected by measuring the recovery of the wheel speed, the controller 106 causes the motor 138 to rotate in the forward direction to extend the actuator 130, thereby to increase the brake pressure. DE-A-3,322,422 discloses a brake pressure modulator, having the features of the pre-characterising portion of claim 1, in which a two-stage control of brake pressure is effected on detection of wheel slip. The first stage involves the cutting of a hydraulic link between the relevant wheel brake and a hydraulic cylinder coupled to the brake pedal, thereby to keep the brake pressure constant. If this stage fails to stop wheel slip, the second stage is actuated, in which an electrical motor causes a piston to move in a chamber to reduce hydraulic pressure on the wheel brake. When wheel traction has been regained, the above two stages are carried out in reverse manner to re-apply hydraulic pressure to the wheel brake. During the anti-lock operation described above, optimum braking performance requires different motor speed/torque characteristics depending on the direction of motor rotation. When the actuator 130 is being retracted (reverse direction of rotation), the torque requirement is relatively low, but the speed requirement is relatively high in order to enable quick relief of the brake pressure. When the actuator 130 is being extended (forward direction of rotation), the speed requirement is relatively low, but the torque requirement is relatively high in order to develop adequate pressure in brake line 122. Unfortunately, the speed/torque characteristics of a conventional DC electric motor are substantially the same in both directions, and some design compromises have had to be made in order to provide acceptable performance in both the forward and reverse directions of motor rotation. Of course, this has involved some sacrifice in the anti-lock braking performance. The present invention seeks to provide an improved vehicle brake pressure modulator. Accordingly, an aspect of the present invention provides apparatus for modulating the braking pressure applied to a vehicle brake characterised by the features in the characterising portion of claim 1. An embodiment which includes a conventional DC motor having permanent magnet field poles, such as the motor 138 of Figure 1, can produce different speed/torque performance characteristics depending on the direction of rotation of the motor. In the forward direction of rotation of the motor, when the actuator 130 is extended to increase the pressure in brake line 122, the magneto-motive force of the auxiliary field winding aids the magneto-motive force of the field magnets to produce a higher flux density in the working air gap of the motor. This maximizes the torque characteristic of this motor for the development of adequate braking pressure. In the reverse direction of rotation of this motor, when the actuator 130 is retracted within the bore 132, the magneto-motive force of the auxiliary field winding opposes the magneto-motive force of the field magnets to lower the flux density in the working air gap of the motor. This maximises the speed characteristic of this motor to reduce the braking pressure quickly. The result can be an anti-lock braking system in which the braking performance is maximised without the design compromise penalties of presently known systems. An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 is a system drawing of a vehicular anti-lock braking system of the general type to which the present invention is directed; Figure 2 is a schematic diagram of an embodiment of motor drive arrangement including a permanent magnet DC motor and an auxiliary field winding; Figure 3 is a magnetization curve for the motor of Figure 2, illustrating the differential performance characteristics thereof; Figures 4a and 4b are graphs of motor speed against motor torque and battery current against motor torque respectively, for the forward and reverse directions of rotation of the motor; and Figures 5a, 5b and 5c show first, second and third embodiments of motor for use as the motor of Figure 2. Referring to Figure 2, the reference numeral 10 generally designates a schematic representation of an embodiment of permanent magnet field DC motor. As with any permanent magnet field DC motor, the stator 12 includes two or more permanent magnets 14 for the motor field poles and a wound rotor 16 for the rotor poles (not shown). The brushes 18 and 20 ride on a conventional commutator (not shown) comprising a plurality of rotary conductor segments affixed to the rotor 16. The conductor segments, in turn, are connected to distributed turns of the rotor winding (not shown) to ensure that the rotor poles are magnetized in relation to the rotor position to produce torque for turning the rotor in the desired direction. An auxiliary field winding comprising two or more series-connected coils 22 and 24 is connected across the commutator brushes 18 and 20, electrically in parallel with the windings of rotor 16. In the illustrated embodiment, the stator has two permanent magnets 14, and the coils 22 and 24 are individually wound around the respective permanent magnets 14, as shown in Figures 5a-5c. In a motor having, for example, four permanent magnet field poles, the auxiliary winding would comprise four series-connected coils. In operation, the coils 22 and 24 produce a magneto-motive force (mmf) which either adds to or subtracts from the magneto-motive force produced by the permanent magnets 14. This, in turn, either increases or decreases the magnetic flux density in the working air gap between the stator 12 and rotor 16. Significantly, the magneto-motive force produced by the permanent magnets 14 is unidirectional, while the magneto-motive force produced by the coils 22 and 24 is bi-directional depending on the direction of motor current. When the motor current Im is in the direction of arrow 26, that is from terminal 28 to terminal 30, the rotor windings are energized via brushes 18 and 20 so as to cause the motor to rotate in a first, forward direction, thereby increasing the braking pressure. The sense or polarity of the coils 22 and 24 is such that during this condition, their magneto-motive force aids the magneto-motive force of the permanent magnets 14. As a result, the air gap flux density is increased, and the motor torque is maximized. When the motor current Im is in a direction opposite to arrow 26, that is from terminal 30 to terminal 28, the rotor windings are energized via brushes 18 and 20 so as to cause the motor to rotate in a second, reverse direction, thereby releasing the braking pressure. In this case, the coils 22 and 24 produce magneto-motive force of the opposite polarity to oppose the magneto-motive force of the permanent magnets 14. As a result, the air gap flux density is decreased, and the motor speed is maximized to retract the actuator 130 relatively quickly within the valve bore 132 so as to relieve the braking pressure. The motor current is supplied from a vehicle storage battery 108 via a bridge or H-switch comprising the electronic switches 32, 34, 36 and 38. The switches 32 - 38, which may be transistors, relays or other switching devices, are operated by a conventional H-switch driver circuit 40, which in turn, is controlled by the ABS electronic controller 106. The enable (EN) and forward/reverse (F/R) inputs on lines 42 and 44 are generated by the controller 106 in response to vehicle wheel speed information (WS) as described above with reference to Figure 1. The bridge switches 32 and 34 selectively connect the motor terminal 28 to the negative (ground) and positive terminals of battery 108, respectively. The bridge switches 36 and 38 selectively connect the motor terminal 30 to the negative (ground) and positive terminals of battery 108, respectively. When it is desired to increase the brake pressure, the ABS electronic controller 106 signals the H-switch driver 40 to close the bridge switches 34 and 36 to produce motor current Im in the direction of arrow 26. When it is desired to decrease the brake pressure, the ABS electronic controller 106 signals the H-switch driver 40 to close the bridge switches 32 and 38 to produce motor current Im in the opposite direction. Figures 3, 4a and 4b graphically illustrate the effect of the auxiliary coils 22 and 24 on the performance characteristics of motor 10. Figure 3 depicts the first and second quadrants of a magnetization curve for motor 10. The magnetization curve represents the magnetic flux density B achieved in the stator 12 for a given magnetic field intensity H. The permanent magnets 14 produce a particular magnetic field intensity h represented by the broken vertical line 50, resulting in a normal operating flux density b. If the motor 10 is energized for forward operation, the coils 22 and 24 produce magneto-motive force which aids the permanent magnet field intensity by the amount designated by arrow 52, resulting in a forward flux density of b f which is greater than the normal flux density b. If the motor 10 is energized for reverse operation, the coils 22 and 24 produce magneto-motive force which opposes the permanent magnet field intensity by the amount designated by arrow 54, resulting in a reverse flux density of b r which is less than the normal flux density b. The differential magnetic intensity and the corresponding differential flux density produced by the coils 22 and 24 is indicated by the DELTA-H and DELTA-B, respectively. Figures 4a and 4b depict the motor speed and current, respectively, as a function of motor output torque. In each case, the forward direction characteristic is designated by the broken trace, and the reverse direction characteristic is designated by the solid trace. Referring to Figure 4a, it will be seen that the free speed of the motor in the reverse direction MS r is significantly greater than in the forward direction, MS f . This occurs because the induced electromotive force, which otherwise remains substantially constant, is proportional to the product of the air gap flux density and the motor speed. In forward operation, when the flux density is increased by the coils 22 and 24, the free speed is lower. In reverse operation, when the flux density is decreased by the coils 22 and 24, the free speed is higher. In practice, the speed advantage during reverse operation may be of the order of 1.5:1. Referring to Figure 4b, it will be seen that the stall torque at the current limit Imax in the forward direction, MT f , is substantially greater than in the reverse direction, MT r . This occurs because the stall torque is proportional to the product of the air gap flux density and the rotor current. In forward operation, when the flux density is increased by the coils 22 and 24, the torque is higher. In reverse operation, when the flux density is decreased by the coils 22 and 24, the torque is lower. In practice, the torque advantage during forward operation may be of the order of 1.5:1. Figures 5a-5c show three different embodiments of stator 12. In each case, the permanent magnets 14 are glued or otherwise secured in a motor case 60, which is equivalent to the stator 12 of Figure 2, and the coils 22 and 24 are wound around a respective magnet 14. In the embodiment of Figure 5a, the sides of the coils 22 and 24 are disposed radially outboard (behind) of the magnets 14 in axially extending open channels 62 of case 60, and the end turns are disposed in the plane of the magnets 14 at either end thereof. In the embodiment of Figure 5b, the coils 22 and 24 lie entirely behind the magnets 14 in a pair of closed case channels 64. In the embodiment of Figure 5c, the case 60 is cylindrical, and the coils 22 and 24 are simply wound around the periphery of the magnets 14. It will be apparent that a single auxiliary coil could be used instead of two serially connected coils as shown in the illustrated embodiments. In this case, the single coil would comprise the sum of the turns of the individual coils so that the total ampere-turn product of the auxiliary winding would remain the same.

Claims: Apparatus for modulating the braking pressure applied to a vehicle brake comprising an actuator (130) adapted to regulate the braking pressure by displacement thereof; and pressure control means (10) including an electric motor (138), the electric motor comprising a rotor (16), and a stator (12) having at least one permanent magnet field pole (14), the electric motor being drivingly connected to the actuator and adapted to be driven in forward and reverse directions to displace the actuator so as to increase or decrease the braking pressure respectively; characterised in that the electric motor comprises an auxiliary winding (22,24) disposed by the, or at least one, permanent magnet field pole (14) and connected electrically in parallel to the rotor (16) such that when the rotor is supplied with a first current adapted to drive the electric motor in the forward direction, the auxiliary winding produces a magneto-motive force which augments the magnetic field of the permanent magnet field pole or poles, thereby to increase the stall torque of the motor to enhance the increase in braking pressure, and when the rotor is supplied with a second current adapted to drive the motor in the reverse direction, the auxiliary winding producing a magneto-motive force which reduces the magnetic field of the permanent magnet field pole or poles, thereby to increase the free speed of the motor to enhance the decrease in braking pressure. Apparatus according to claim 1, comprising two or more permanent magnet field poles (14). Apparatus according to claim 2, wherein the auxiliary winding comprises two or more coils (22,24), each disposed by a respective permanent magnet field pole. Apparatus according to claim 1, 2 or 3, wherein the stator (12) comprises a case (60) in which the permanent magnet field pole or poles are disposed, the case including one or more channels (62) formed proximate the periphery of the permanent magnet field pole or poles, the auxiliary winding being disposed in the channel or channels.

Inventors: SAVAGE JACK WINFIELD, SAVAGE, JACK WINFIELD
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1991
Main classification: B60T8
IPC codes: B60T8, F16K31, H02K21, H02K23