Anti-stall controller

Claim 1

In a control system for a hydrostatic transmission of the type including a prime mover driven variable displacement pump; a fluid motor; fluid operable means for varying the displacement of the pump, the fluid operable means defining first and second fluid pressure chambers for varying the displacement of the pump in first and second opposite directions; a source of pressurized control fluid for operating the fluid operable means; supply conduit means in fluid communication with the source of control fluid, main control means operable to control the flow of control fluid from the supply conduit means to one of the first and second fluid pressure chambers through one of first and second fluid conduit means, respectively, and to control the flow of control fluid from the other of the first and second fluid chambers to a reservoir; the improvement comprising: (a) auxiliary anti-stall override control valve means defining control conduit means operable to communicate control fluid from said one of the first and second fluid conduit means to said other of the first and second fluid conduit means; (b) said auxiliary control valve means including an on-off, electro-hydraulic valve disposed in said control conduit means to prevent fluid communication therethrough in response to an electrical input signal in one condition, and to permit fluid communication therethrough in response to an electrical input signal in another condition; (c) control logic means operable in response to variations in an input condition to alternate said electrical input signal between said one condition and said another condition at a duty cycle determined by the deviation of said input condition from a normal operating condition, said input condition comprises the speed of operation of the prime mover, and said normal operating condition comprises a prime mover reference setting speed.