This invention relates to a brake system for automotive vehicles and, more particularly, to such a system having a device for controlling the distribution of the braking force to the front and rear axles depending upon the static and dynamic axle load distribution and other parameters resulting from the braking behavior. Such systems also have: sensors for detecting corresponding measured values such as the rotational behavior of the wheels, the velocity of the vehicle, the longitudinal deceleration of the vehicle, etc; an electronic control unit for transforming the measured values to generate braking pressure control signals; and a braking pressure generator aggregate operable by an auxiliary drive means for influencing the braking pressure at the rear axle by means of the control signals.
Nowadays, braking force distributors or pressure reducers which are load-responsive, deceleration-responsive or which depend on the braking pressure in accordance with a predetermined characteristic line are used for a better distribution of the braking force to the front and rear axles of a vehicle. The respective axle load distribution is subject to static and dynamic influences and thus depends on the load condition of the vehicle, on the slowing-down, and on other influences. By virtue of said auxiliary drive means, on the one hand, the braking of the front and rear axles is as uniform as possible. On the other hand, it is more difficult to lock-up of the rear wheels. It has to be ensured that, in case of overbraking, or rather in case of excessive braking pressure occurring, for instance, on a slippery road surface, the rear wheels can lock only after the front wheels; otherwise, there is a high danger of skidding due to the loss of the lateral guiding force at the locked wheel. In such an arrangement there exists the disadvantage that one can achieve only a relatively rough approximation of the braking force distribution to the factual static and dynamic axle load distribution. Therefore, in order to render the dangerous lock-up of the rear wheels more difficult, the brake systems have to be rated so as to ensure that, in most situations, the major part of the braking force is received by the front wheels.
It has already been suggested to electronically control the braking force distribution, or rather the part of the braking force received by the rear wheels, in dependence on the rotational behavior of the front wheels (German Patent Application No. P 33 01 948.7 corresponding to my pending U.S. application Ser. No. 562,196 filed Dec. 16, 1983). To this end, braking pressure modulators are inserted into the pressure medium path to the rear wheel brakes. In the rest position, said braking pressure modulators block the passage of pressure medium. By means of said braking pressure modulators it is possible to electronically control the flow and, hence, the part of the braking force received by the rear wheel brakes so that there is an approximately ideal braking force distribution to the front and rear axles. In this arrangement, electromagnetically operable high-speed multidirectional valves are used as braking pressure modulators. Said valves bring about the desired braking pressure increase at the rear axle by short-time or clocked changing-over into the flow condition.
This control of the braking force distribution between front and rear axles which work without the use of auxiliary energy and, in particular, without an auxiliary drive of the pressure modulators is limited with regard to its possibilities as it only allows a pressure increase, and not a pressure reduction, to be achieved at the rear wheel brake cylinders. Research now has revealed that, despite constant hydraulic actuating pressure, there are variable wheel braking force during a braking operation. The variations in wheel braking forces are due to thermal instability of the brake pads in use today. This can result in a change in the rear axle braking force share of more than 205 after the completion of the electronically controlled adaptive metering-in of the rear axle braking pressure. To compensate for the change in the rear axle braking force share, it is desirable to be able to reduce the braking pressure at the rear axle by a corresponding amount. A braking pressure reduction at the rear wheels can also prove necessary for the purpose of preventing a lock-up in case of a reduction in the friction coefficient during the braking operation.
In order to enable such a braking pressure reduction, according to a further suggestion (German Patent Application No. P 33 06 611.6 corresponding to my co-pending U.S. application Ser. No. 846,479 filed on Mar. 31, 1986 as a continuation of application Ser. No. 581,297 filed Feb. 17, 1984) additional valves are provided in the rear axle brake circuit. With the additional valves, it is possible to remove pressure to a pressure compensation reservoir in case of an imminent lock-up. However, this procedure does not provide an increase in the braking pressure in the rear wheel brakes during the same braking operation. The volume reserve of the master brake cylinder which is rapidly depletable would also impose narrow limits on the possibility of repeated correction cycles.
In another device for electronically controlled adaptive braking force distribution between front and rear axles the braking pressure at the rear axle is generated by a separate braking pressure generator completely uncoupled from the front axle brake circuit and operated by a controlled auxiliary drive in the form of a vacuum servo drive. Thus, it is possible to meter in the rear axle braking pressure in complete independence of the front axle braking pressure. In particular, it is possible to increase and reduce the rear axle braking pressure in respect of the front axle braking pressure in repeated control cycles. The required expense in terms of equipment, however, is high as the braking pressure generator generating the full rear axle braking pressure is complicated, expensive, and bulky. Its operation requires a high auxiliary energy and this has to be taken into consideration in rating the vehicle's engine. Also, there are some objections with regard to safety because trouble in the auxiliary energy supply may result in a total failure of the rear axle brake circuit.