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Details for Patent: 5,482,361

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Details for Patent: 5,482,361

Title: Brake system with adjustably variable front/rear axle braking force distribution
Abstract:A brake system has a variable front/rear axle braking force distribution for a racing car. An electrohydraulic braking force control device with an electronic computer stage is provided in the system to take account of the aerodynamic axle load changes occurring at high speeds. This computer stage forms, from the vehicle speed and dynamically determined magnitudes, or magnitudes assumed to be appropriate, of the coefficient of friction .mu..sub.B effective at the vehicle wheels, an optimized braking force distribution factor. The brake pressure connected into the rear wheel brakes is caused to follow up the brake pressure connected into the front wheel brakes in such a way that the rear axle braking force follows the front axle braking force.
Inventor(s): Burckhardt; Manfred (Waiblingen, DE), Kazan; Sinan (Esslingen, DE), Zimmer; Richard (Fellbach, DE)
Assignee: Mercedes-Benz AG (DE)
Filing Date:Oct 31, 1994
Application Number:08/256,248
Claims:1. A brake system with variable braking force distribution between front and rear axles of a high speed vehicle in which downward aerodynamic forces on the vehicle cause an increase in axle loads as vehicle speed increases, wherein the increase in axle load is greater at the rear axle than at the front axle, comprising front wheel brakes, rear wheel brakes, a brake unit actuatable by a brake pedal and configured to generate at least a brake pressure in the front wheel brakes and operatively associated with the front wheel brakes and the rear wheel brakes that in the event of a specified defined limiting value of an actuation force exerted by a driver of the high speed vehicle, the vehicle has a magnitude of deceleration corresponding to complete utilization of frictional force on the braked vehicle wheels, and a separate pressure source comprising a braking force control device and providing a settable outlet pressure operatively associated between the brake unit and the rear wheel brakes so as to be directly connected as brake pressure into piston pairs of brake calipers of the rear wheel brakes, the source comprising an electronic control unit operatively configured to process at least one signal representative of the vehicle speed and one at least adjustably specifiable magnitude of a friction coefficient present at the vehicle wheels or assumed to be present and set, and to continuously form a value of an optimized braking force distribution factor in accordance with the following linear relationship

in which k.sub.1, k.sub.2 and k.sub.3 are vehicle-specific constants determined from linear interpolation of the manner in which DB.sub.om depends on v and .mu..sub.B, where DB.sub.om is determined in accordance with the following relationship ##EQU20## for different values of an optimized rear axle braking force proportion, .phi..sub.om which has different values determined from a numerical evaluation of the following relationship ##EQU21## wherein .psi. represents the rear axle load proportion,

.chi. represents the height of the vehicle center of gravity in relation to the wheel base,

K.sub.a,v represents the aerodynamic upward or downward force coefficient at the front axle,

K.sub.a,h represents the aerodynamic upward or downward force coefficient at the rear axle,

.mu..sub.B represents the coefficient of friction, and

G.sub.g represents the vehicle weight, and a sensor device operatively arranged between the brake unit and the pressure source such that, during a braking operation, a signal characteristic of an instantaneous value of front axle braking force, F.sub.BV, is generated, wherein the controllable pressure source is operatively arranged to cause rear axle braking force, F.sub.BH, to follow up the front axle braking force, F.sub.Bv, in accordance with the following relationship

2. The brake system according to claim 1, having a front/rear axle brake circuit division on which the front wheel brakes are jointly connected to an outlet pressure space of the brake unit in which static pressure can be generated as a brake pressure p.sub.v, by actuation of the brake pedal, the rear wheel brakes are arranged to be acted on exclusively by the outlet pressure, p.sub.h, of the controllable pressure source, and the outlet pressure, p.sub.h, is caused to follow up the front axle brake pressure, p.sub.V, in accordance with the relationship

wherein r.sub.dyn,h represents the dynamic rolling radius of the rear wheels, and r.sub.dyn,v represents the dynamic rolling radius of the front wheels of the vehicle.

3. The brake system according to claim 2, wherein the brake unit comprises a tandem main cylinder with an outlet pressure space associated with the front wheel brakes and an outlet pressure space associated with the rear wheel brakes, the rear wheel brakes have four-piston calipers of one configuration, each with two piston pairs of one or different configuration, the piston pairs of one brake caliper being connected together with one identically dimensioned piston pair of the other brake caliper via a respective common brake conduit, one brake conduit being connected to a brake unit pressure outlet, associated with the rear wheel brakes and another brake conduit being connected to a pressure outlet of the controllable pressure source.

4. The brake system according to claim 3, wherein the brake unit and cross-section of cylinders of the wheel brakes are configured to fixedly match for stable dynamic vehicle braking behavior over a braking range from partial braking range to full braking range.

5. The brake system according to claim 1, wherein the outlet pressure of the controllable pressure source is set by controlling rotational speed of an electric drive motor of a high-pressure pump.

6. The brake system according to claim 1, wherein the pressure source further comprises one of a pressure supply pump driven with a constant high rotational speed and a pressure reservoir kept at a high pressure level, and the rear axle brakes are selectively connectable thereto by pulse-controllable inlet and outlet valves.

7. The brake system according to claim 1, wherein the sensor device comprises at least one pressure pick-up to monitor brake pressure in the front wheel brakes and generate an output signal characteristic of the outlet pressure of the brake unit connectable into at least one of the front wheel brakes.

8. The brake system according to claim 1, wherein a position pick-up is provided to generate, as representative of the outlet pressure of the brake unit, an output signal characteristic of one of a pedal position and a position of the brake unit piston which is displaceable by the brake pedal.

9. The brake system according to claim 1, wherein a pressure pick-up is provided for recording an actual value of the pressure connected into the rear wheel brakes and is operatively connected to the pressure outlet of the pressure source.

10. The brake system according to claim 1, wherein a computer stage is operatively associated with the system to continuously evaluate wheel peripheral speeds and of at least one driven and one free-running vehicle wheel to form a signal characteristic of a parameter in which drive slip of the vehicle, .lambda..sub.a, is given by the following relationship

where v.sub.ant is the driven wheel velocity, and v.sub.f is the free-running wheel velocity, and vehicle longitudinal acceleration is represented by z and, for values of the parameter d.lambda..sub.a /dz, which are located within a range between 0.4 and 0.6, and to determine the friction coefficient, .mu..sub.B, associated with values of the vehicle longitudinal acceleration z in accordance with the following relationship

the friction coefficient, .mu..sub.B, being applied by the computer stage to determine the optimum rear axle braking force proportion, .phi..sub.om, and the optimized braking force distribution factor, DB.sub.om, and thereby provided optimized braking force distribution to the high speed vehicle.

11. The brake system according to claim 1, wherein the vehicle is equipped with an anti-lock brake system for stable dynamic deceleration behavior, and a vehicle deceleration Z.sub.ABS, in relation to the gravitational constant, g, which equals 9.81 m/s.sup.2, which occurs with a response of the anti-lock brake system on all vehicle wheels, is applied as the friction coefficient, .mu..sub.B, used for the determination of the optimum braking force distribution factor, DB.sub.om.

12. The brake system according to claim 1, wherein the vehicle is equipped with a longitudinal acceleration sensor arranged to generate electrical output signals characteristic of at least one of accelerations and decelerations occurring on the vehicle.

13. A brake system with variable axle braking force distribution between front and rear axles of a high speed vehicle in which downward aerodynamic forces on the vehicle cause an increase in axle loads as vehicle speed increases, wherein the increase in axle load is greater at the rear axle than at the front axle, comprising front wheel brakes, rear wheel brakes, a brake unit, actuatable by a brake pedal and configured to generate at least a brake pressure connectable into the front wheel brakes, and operatively connected with the front and rear wheel brakes such that in the event of a specified defined limiting value of an actuation force exerted by a driver of the high speed vehicle, the vehicle has a deceleration of a magnitude corresponding to complete utilization of frictional force on the braked vehicle wheels, a separate pressure source comprising a braking force control device with an electrically controllable outlet pressure connectable into a control pressure space of the brake unit and thereby additively superimposed on brake pressure generatable by actuation of the pedal alone, which brake pressure is connectable into the rear wheel brakes, the pressure source comprising an electronic control unit operatively configured to process at least one signal representative of the vehicle speed and one at least adjustably specifiable magnitude of a friction coefficient present at the vehicle wheels or assumed to be present and set, and to continuously form a value of an optimized braking force distribution factor in accordance with the linear relationship

in which k.sub.1, k.sub.2 and k.sub.3 are vehicle-specific constants determined from linear interpolation of the manner in which DB.sub.om depends on v and .mu..sub.B, where DB.sub.om is determined in accordance with the following relationship ##EQU22## for different values of an optimized rear axle braking force proportion, .phi..sub.om, which has different values determined from a numerical evaluation of the following relationship ##EQU23## wherein .psi. represents the rear axle load proportion,

.chi. represents the height of the vehicle center of gravity in relation to the wheel base,

k.sub.a,v represents the aerodynamic upward or downward force coefficient at the front axle,

k.sub.a,h represents the aerodynamic upward or downward force coefficient at the rear axle,

.mu..sub.B represents the coefficient of friction, and

G.sub.g represents the vehicle weight, and

a sensor device operatively arranged between the brake unit and the pressure source such that, during a braking operation, a signal characteristic of an instantaneous value of front axle braking force, F.sub.BV, is generated, and the pressure source is operatively arranged to cause the rear axle braking force, F.sub.BH, to follow up the front axle braking force, F.sub.BV, in accordance with the following relationship

14. The brake system according to claim 13, wherein the brake unit comprises a tandem main cylinder in which the front wheel brakes are connected to a primary outlet pressure space thereof and the rear wheel brakes are connected to a secondary outlet pressure space thereof, a housing of the tandem main cylinder having a first bore step forming a boundary fixed to a housing of the secondary outlet pressure space and being bounded therein by an outer piston flange of a secondary piston relative to an unpressurized follow-up space which is movably sealed by an inner piston flange against an inner section of the bore step and which is bounded by an intermediate wall of the housing against a central bore step beginning at a second bore step accommodating a primary piston, a cross-sectional area A.sub.1 of the second bore step being somewhat larger than a cross-sectional area A.sub.3 of the central bore step, an actuating piston is guidably displaceably arranged in a pressure-tight manner in the central bore step and forms an axially movable, inner boundary between the primary outlet pressure space and an unpressurized follow-up space axially bounded by the intermediate wall connected to the housing, the actuating piston having a push-rod configured axially to penetrate the central follow-up space and displaceably to pass through a central hole of the intermediate wall in a pressure-tight manner and further configured to be axially supportable on the inner piston flange of the secondary piston, and the annular space, which is axially movably bounded by the inner piston flange of the secondary piston and axially fixed relative to the housing by the intermediate wall and which is penetrated at a central region thereof by the push-rod, is connected to the pressure outlet of the controllable pressure source.

15. The brake system according to claim 14, wherein an end section of the push rod, acting on the secondary piston is displaceably arranged in a pressure-tight manner in a pocket hole of the secondary piston and is supportable on a bottom portion of the pocket hole.

16. The brake system according to claim 15, wherein the inner piston flange of the secondary piston is provided with a balance hole opening into the pocket hole centrally at the bottom portion to connect the pocket hole with the follow-up space.

17. A method for providing variable braking force distribution between front and rear axles of a high speed vehicle in which downward aerodynamic forces on the vehicle cause an increase in axle loads as vehicle speed increases, with the increase in axle load being greater at the rear axle than at the front axle, comprising the steps of

generating a brake pressure in front wheel brakes of the vehicle

processing at least one signal representative of the vehicle speed and one at least adjustably specifiable magnitude of an actual or assumed friction coefficient to continuously form an optimized braking force distribution factor value in accordance with the following linear relationship

in which k.sub.1, k.sub.2 and k.sub.3 are vehicle-specific constants determined from linear interpolation based on a relationship between DB.sub.om and v and .mu..sub.B, where DB.sub.om is determined in accordance with the following relationship ##EQU24## for different values of an optimized rear axle braking force proportion, .phi..sub.om, which has different values determined from a numerical evaluation of the following relationship ##EQU25## wherein .psi. represents the rear axle load proportion,

.chi. represents the height of the vehicle center of gravity in relation to the wheel base,

k.sub.a,v represents the aerodynamic upward or downward force coefficient at the front axle,

k.sub.a,h represents the aerodynamic upward or downward force coefficient at the rear axle,

.mu..sub.B represents the coefficient of friction, and

G.sub.g represents the vehicle weight, and during a braking operation, generating a signal characteristic of an instantaneous front axle braking force, F.sub.B, to cause a controllable pressure source to provide a rear axle braking source, F.sub.BH, for rear wheel brakes in accordance with the relationship

whereby, in the event of a specified defined limiting value of actuation force exerted by a vehicle driver on a brake pedal of the vehicle, the vehicle decelerates with utilization of substantially all frictional force on the braked vehicle wheels.

18. The method according to claim 17, wherein the provision of the rear axle braking force, F.sub.BH, comprises the step of utilizing exclusively outlet pressure, p.sub.h, of the controllable pressure source for the rear wheel brakes in accordance with the relationship

wherein r.sub.dyn,h represents the dynamic rolling radius of the rear wheels, and r.sub.dyn,v represents the dynamic rolling radius of the front wheels of the vehicle.
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