In general, a boom type construction machine comprises a boom cylinder that rotates a boom upward and downward. JP 2011-179541A published by the Japan Patent Office proposes a regenerative generator driven by a rotational torque of a hydraulic motor that rotates by a fluid returning from the boom cylinder when the boom is descended.
The boom cylinder comprises a rod-side chamber delimited by a piston on one side thereof in a cylinder and a piston-side chamber delimited by the piston on another side thereof in the cylinder. A hydraulic fluid is selectively supplied to one of the rod-side chamber and the piston-side chamber via a change-over valve that switches depending on an operating direction of the boom cylinder. The change-over valve also connects one of the rod-side chamber and the piston-side chamber to a tank.
This regenerative generator comprises a piston-side chamber that discharges the hydraulic fluid in response to contraction of the boom cylinder. A regenerative control spool valve is disposed in a passage connecting the piston-side chamber and the change-over valve. A part of the returning fluid is supplied to the hydraulic motor via the regenerative control spool valve for regenerative power generation.
The regenerative control spool valve has an operative position for cutting off connection between the piston-side chamber and the regenerative hydraulic motor and an operative position for supplying a part of the returning fluid to the regenerative hydraulic motor. In addition, the regenerative control spool valve controls a regenerative flow rate depending on a displacement position of a spool in the course of the position switching by continuously changing a flow cross-sectional area of the regenerative passage between the regenerative control spool valve and the regenerative hydraulic motor.
According to the aforementioned configuration, a part of the returning fluid discharged from the piston-side chamber of the boom cylinder is supplied to the hydraulic motor via a regenerative spool valve, and the remaining fluid returns to the tank via the change-over valve.
In other words, a sum of the regenerative flow rate and the recirculation flow rate corresponds to a total flow rate of the returning fluid from the boom cylinder. A contraction speed of the boom cylinder is determined based on a total flow rate of the returning fluid. The total amount of the returning fluid is determined based on a control position of the change-over valve.