The present invention relates to motor vehicle brake control apparatus including antiskid brake control and load-dependent brake control subsystems which act through regulating valve devices to influence the front and/or rear wheel brake pressure independent of the operator controlled brake pressure. In apparatus of the above type, the load-dependent brake control subsystem typically comprises sensors for measuring the brake pressure produced by the operator's brake valve device, sensors for measuring the actual pressure delivered to the wheel brake cylinders (since this latter pressure varies from the first mentioned pressure during operation of the wheel skid control), sensors for measuring the axle load and an electronic logic control circuit which produces appropriate output signals in response to which the regulating valve devices are controlled to adjust the wheel braking pressure to the wheel axle load, thereby permitting the operator to apply normal brake forces irrespective of the vehicle load.
Wheel antiskid control systems are well known in a great variety of forms to also influence wheel brake pressure in accordance with existing coefficients of friction between the vehicle wheels and the roadway. In one such exemplary form, the operator-controlled wheel brake pressure is supplied, held constant or vented via the regulating valve device in response to voltage signals produced by an electronic control portion of the antiskid system.
While it is known to combine the electronic antiskid control system with a mechanically-operated load-dependent brake control, such an arrangement is extremely costly to construct, is complicated and accordingly prone to malfunction.
Accordingly, systems have been developed wherein the load-dependent brake control employs a form of electronic sensing and control similar to that of the electronic antiskid control. However, these arrangements are also expensive in that they require a complicated electro-pneumatic valve design to permit actuation in accordance with different analog voltage levels. Furthermore, these arrangements suffer from a lack of accuracy due to tolerances in the magnetic force of attraction and possible voltage fluctuations.