Patent ID: 8146481

Claim:
An actuator having a piston slidably arranged in a cylinder and dividing the cylinder into a first chamber and a second chamber, and controlling pressures of gaseous or liquid fluids in the first and second chambers to control a position of the piston in a sliding direction while changing bias pressure common to the gaseous or liquid fluids in the first and the second chambers to change rigidity caused to the piston by the bias pressure, the actuator comprising: first and second control valves arranged between a fluid pressure source and the first and second chambers and configured to adjust fluid pressure supply to the first and second chambers in a non-step manner; first and second discharge valves configured to allow the fluid in the first and second chambers to flow in a discharge direction from the first and second chambers toward the atmosphere or low pressure source side; a first control device carrying out feedback control of pressure to be supplied to the first or second chamber such that a difference between a target position of the piston and a detected position of the piston becomes smaller; and a second control device carrying out feedback control of bias pressure to be commonly supplied to the first and second chambers such that the difference between the target position of the piston and the detected position of the piston becomes the smallest, wherein each of the first control device and the second control device serves as a control device for controlling either one of the first control valve and the second control valve.