The pneumatic brake servo is operated directly by the brake linkage and by a remote control in which a solenoid drives the control valve. The movement of the solenoid is computed by monitoring the voltage to the solenoid and the operating current. A monitoring circuit provides an output signal corresponding to the displacement of the solenoid. No external sensors are required. The function of the solenoid is checked by comparing the monitoring signal with a reference signal produced by a model circuit which simulates the action of the solenoid. Also monitored are the hydraulic brake pressures in the brake system.
