Method for controlling variable displacement pump

A method for controlling a variable displacement pump, driven by an engine, a pump discharge flow rate of which can be adjusted from the outside, the variable displacement pump having an actuator or actuators connected thereto through one or more closed-center-type directional control valves, the method comprising:

• detecting a real discharge flow rate of the variable displacement pump and an operation amount of the directional control valves;
• determining a first virtual discharge pressure from the real discharge flow rate of the variable displacement pump based on a characteristic curve defining a relation between discharge pressure and discharge flow rate of the variable displacement pump;
• using the real discharge flow rate of the variable displacement pump as an actuator flow rate necessary for the actuators, determining a virtual bleed-off flow rate based on a virtual bleed-off area of the closed-center-type directional control valves determined according to the operation amount, and determining a second virtual discharge pressure of the variable displacement pump so that a flow rate value obtained by subtracting the actuator flow rate and the virtual bleed-off flow rate from a virtual discharge flow rate of the variable displacement pump converges to zero;
• and
• controlling the variable displacement pump based on a smaller value of the first virtual discharge pressure and the second virtual discharge pressure
• wherein a plurality of the variable displacement pumps are connected to the engine, wherein one or more of the actuators are connected to each of the variable displacement pumps through one or more of the closed-center-type directional control valves, wherein horse power is distributed to the variable displacement pumps, and a distribution ratio of the horse power of the engine to each of the variable displacement pumps is predetermined or determined according to the operation amount of each of the closed-center-type directional control valves, and the first virtual discharge pressure for each of the variable displacement pumps is determined from the distributed horse power and the real discharge flow rate of each of the variable displacement pumps;
• wherein for each of the variable displacement pumps, the method includes subtracting from the distributed horse power a value obtained by multiplying the real discharge flow rate by the smaller of the discharge pressure of the first virtual discharge pressure and the second virtual discharge pressure to calculate excess horse power for each of the variable displacement pumps, and adding the excess horse power for a first one of the variable displacement pumps to the distributed horse power for at least one of the other of the variable displacement pumps.