Circuit pressure control system for hydrostatic power transmission

A circuit pressure control system for a hydrostatic power transmission including a variable-displacement hydraulic pump driven by a prime mover, a hydraulic actuator for actuating a load and a displacement adjusting mechanism for the hydraulic pump, the hydraulic pump and hydraulic actuator being connected together in a closed or a semi-closed circuit, and the displacement adjusting mechanism being actuated by a signal indicative of an operating lever manipulated variable and a signal indicative of the actual displacement of the hydraulic pump to control the speed of the hydraulic actuator, said circuit pressure control system comprising:

• means for sensing at least a higher one of a circuit pressure in discharge and suction sides of the hydraulic pump and generating a signal indicative of the sensed circuit pressure;
• means for calculating, based on said manipulated variable signal and said signal indicative of the sensed circuit pressure, a hydraulic pump displacement command which causes the displacement of the hydraulic pump to be changed at a maximum rate when the circuit pressure is below a predetermined value and causes the rate of change of the pump displacement to be reduced when the predetermined value is exceeded thereby said means for calculating the hydraulic pump displacement command including first means for converting said sensed circuit pressure signal to a value indicative of the rate of change in the displacement of the hydraulic pump based on a preset function whih predetermines the above relations between the circuit pressure and the rate of change in the displacement of the hydraulic pump, and second means for converting said value indicative of the rate of change in the pump displacement to a value indicative of the displacement of the hydraulic pump to thereby obtain said hydraulic pump displacement command;
• and
• means for comparing the displacement command with the actual displacement of the hydraulic pump and producing a signal for decreasing the difference between them and supplying such signal to said displacement adjusting mechanism.