Motor-driven brake system, especially for rail vehicles

Claim 1

A brake-operating system for a brake mechanism having a brake rod, at least one brakeshoe operatively connected with said rod and a rotary brake member engageable by said brakeshoe, especially for a rail vehicle, said brake operating system comprising: a brake actuator connected to said rod and displaceable in one direction to cause said brakeshoe to engage said member and in an opposite direction to retract said brakeshoe from said member; an electric motor energizeable to retract said brakeshoe from said brake member and provided with a driver connected with said actuator, and a control circuit for variable electrical energization of said electric motor; a force-storing spring acting upon said brake actuator to drive said actuator in said one direction to apply said brakeshoe to said member with a braking force generated by said spring, said spring being stressed upon energization of said motor to retract said brakeshoe from said member; and a brake-force controller comprising: a brake-force sensor in the form of at least one strain-gauge measuring resistance disposed between said spring and said brakeshoe and varying in accordance with said braking force, a command-variable resistance including a photoresponsive field-effect transistor having output terminals bridged by a parallel-connected fixed resistor, and an infrared light source of variable intensity juxtaposed with said field-effect transistor for controlling the command-variable resistance and selectively energizeable to vary the command-variable resistance on command, and means connecting said resistances as respective branches in a Wheatstone bridge circuit connected to said control circuit for varying energization of said electric motor to control said braking force upon an imbalance of said bridge resulting from development of different resistance values in said branches from those at which said bridge is in balance.