Hydraulic drive system

A hydraulic driving system comprising:

• a hydraulic cylinder including a cylinder tube and a cylinder rod, the cylinder tube having a first end part and a second end part, the cylinder rod having a proximal end part inserted inside the cylinder tube such that the proximal end part can move reciprocally between the first end part and the second end part, the proximal end part of the cylinder rod partitioning inside of the cylinder tube into a first chamber disposed between the first end part and the proximal end part and a second chamber disposed between the proximal end part and the second end part, the cylinder rod being configured to move in an expansion direction such that the first chamber expands when hydraulic fluid is supplied to the first chamber and exhausted from the second chamber, the cylinder rod being configured to move in a contraction direction such that the first chamber contracts when hydraulic fluid is supplied to the second chamber and exhausted from the first chamber;
• a main pump switchable between
• a state of supplying hydraulic fluid to the first chamber and sucking in hydraulic fluid from the second chamber, and
• a state of supplying hydraulic fluid to the second chamber and sucking in hydraulic fluid from the first chamber;
• a hydraulic-fluid path connecting the first chamber and the main pump, the hydraulic-fluid path also connecting the second chamber and the main pump, the hydraulic-fluid path forming a closed circuit between the main pump and the hydraulic cylinder;
• a charge pump configured to replenish hydraulic fluid in the hydraulic-fluid path, the charge pump having a maximum discharge flow rate that is smaller than a maximum suction flow rate of the main pump;
• a stroke position detector configured to detect a stroke position of the proximal end part of the cylinder rod inside the cylinder tube;
• an operating member configured to be manipulated by an operator to operate the hydraulic cylinder;
• an operation detector arranged and configured to detect at least an operation direction of the operating member and to feed a detection signal to the pump controller;
• and
• a pump controller arranged to receive a signal indicating the stroke position from the stroke position detector, the pump controller being programmed to execute a flow rate reduction control when a distance from the stroke position detected by the stroke position detector to a stroke end where the proximal end part of the cylinder rod contacts the first end part or the second end part of the cylinder tube is equal to or smaller than a distance from a prescribed reduction start position to the stroke end, the flow rate reduction control being configured to reduce a suction flow rate of the main pump as the stroke position approaches a the stroke end from the reduction start position,
• the flow rate reduction control being configured to control the suction flow rate in accordance with one of an expansion operation flow rate reduction characteristic and a contraction operation flow rate reduction characteristic in accordance with a moving direction of the cylinder rod, each of the expansion operation flow rate reduction characteristic and the contraction operation flow rate reduction characteristic prescribing a change in the suction flow rate with respect to the stroke position during the flow rate reduction control, each of the expansion operation flow rate reduction characteristic and the contraction operation flow rate reduction characteristic having a reduction portion in which the suction flow rate becomes smaller as the stroke position approaches the stroke end, a change rate of the suction flow rate in the reduction portion not changing regardless of a suction flow rate before execution of the flow rate reduction control,
• the pump controller being further programmed to
• determine whether the cylinder rod is moving in the expansion direction or the contraction direction based on detection results of the stroke position detector,
• determine whether the operating member is being operated in a direction corresponding to the expansion direction of the hydraulic cylinder or the contraction direction of the hydraulic cylinder based on the detection signal from the operation detector, and
• determine that the cylinder rod is performing an expansion operation or a contraction operation upon the moving direction of the cylinder rod matching the operation direction of the operating member,
• control the suction flow rate in accordance with the expansion operation flow rate reduction characteristic during the flow rate reduction control when the hydraulic cylinder is performing the expansion operation, and
• control the suction flow rate in accordance with the contraction operation flow rate reduction characteristic during the flow rate reduction control when the hydraulic cylinder is performing the contraction operation.