Control method for chuck device

A control method of controlling a chuck device in a drive system for driving the chuck device, the chuck device including a piston displaceable in an axial direction under an effect of supply of fluid and a pair of fingers configured to be opened and closed according to the displacement of the piston, the drive system comprising a controller, a detection sensor configured to detect an axial position of the piston, and a switching valve configured to switch a state of supply of the fluid to the chuck device,

• wherein the switching valve is a servo valve whose degree of valve opening is controllable based on a control signal input from the controller, and an amount of displacement of the piston and an amount of opening and closing of the fingers are controlled by changing the degree of valve opening, the control method comprising the steps of:
• opening or closing the fingers by opening the switching valve at a first degree of valve opening to supply the fluid to a first cylinder chamber of the chuck device and to discharge the fluid from a second cylinder chamber of the chuck device so that the piston moves in one axial direction;
• reducing, while the fingers are being opened or closed, a displacement speed of the piston moving in the one axial direction by switching the valve opening to a second degree of valve opening to supply the fluid to the second cylinder chamber and to discharge the fluid from the first cylinder chamber when the detection sensor detects that the piston reaches a switching point, which is ahead of a predetermined target position of the piston in said one axial direction by a predetermined distance, such that the piston reaches the switching point before reaching the predetermined target position when moving in said one axial direction;
• and
• stopping the fingers at an intermediate position located between a fully open position and a fully closed position by opening the switching valve at a third degree of valve opening to stop supplying the fluid to the second cylinder chamber and to stop discharging the fluid from the first cylinder chamber when the piston reaches the predetermined target position.