Hydraulic power system

A hydraulic power system of the type in which a hydraulic fluid of a controlled flow rate and/or controlled pressure is supplied from a reservoir to a load line communicating with a hydraulic actuator and vice versa, comprising:

• a reversible hydraulic pump motor having a rotary shaft, said pump motor operating as a hydraulic pump for supplying said fluid at a flow rate corresponding to a rotational speed of said rotary shaft rotating in a forward direction when said fluid is supplied to said load line from said reservoir, said pump motor operating as a hydraulic motor for rotating said rotary shaft in a reverse direction at a rotational speed corresponding to a flow rate of said fluid when said fluid is supplied to said reservoir from said load line,
• a variable-speed servomotor having a driving shaft coupled to said rotary shaft in a torque transmitting manner, said driving shaft being rotatable in either of said forward and reverse directions with a rotational speed and rotational direction corresponding to a driving current supplied to said servomotor,
• pressure detecting means for generating a first electric signal corresponding to a pressure of said fluid in said load line,
• rotational speed detecting means for generating a second electric signal corresponding to the rotational speed of said driving shaft,
• signal command means for generating a preprogrammed pressure signal and a preprogrammed flow command signal,
• signal processing means for preferentially generating a speed command signal of a magnitude corresponding to said flow command signal by a limiter operation when a pressure difference signal corresponding to the difference between said pressure command signal and said first signal is higher than a predetermined limiting level and for preferentially generating a speed command signal of a magnitude corresponding to said pressure difference signal when said pressure difference signal is equal to or lower than said limiting level, and
• rotational speed control means responsive to said speed command signal and said second signal for controlling the magnitude of said driving current supplied to said servomotor through a closed feedback loop for the rotational speed in such a manner that the rotational speed of said servomotor corresponds to said speed command signal,
• wherein said rotational speed control means comprises means for generating a rotational speed difference signal corresponding to the difference between said speed command signal and said second signal, torque limiter means for limiting upper and lower limits of said rotational steed difference signal to come into a predetermined range, and current control means for receiving said rotational speed difference signal limited by said torque limiter means as a control input and said second signal as a feedback signal to feedback control the magnitude of said driving current supplied to said servomotor.