A three-position control method using a cylinder device having a piston which is interlocked with a control object, the piston being displaceable to each of a retracted position, an intermediate position, and an advanced position by receiving a thrust of a working fluid,
- a fluid control unit that controls intake and discharge of a working fluid to and from the cylinder device, and
- a displacement restriction device that restricts displacement of the piston moving from the retracted position toward the advanced position at the intermediate position, wherein
- the cylinder device is provided with a first fluid chamber into which the working fluid is introduced so that the piston is pressed toward the retracted position and a second fluid chamber into which the working fluid is introduced so that the piston is pressed toward the advanced position,
- the displacement restriction device is provided with a load receiving portion that receives a load of the piston moving toward the advanced position when the piston is displaced from the retracted position to the intermediate position, and a biasing means for biasing the load receiving portion in a direction against a load input from the piston,
- a biasing force of the biasing means is set to be greater than a thrust caused by a working pressure of the working fluid when the piston is displaced from the retracted position to the intermediate position, and to be smaller than a thrust caused by the working pressure of the working fluid when the piston is displaced from the intermediate position to the advanced position,
- the biasing means is a spring member,
- an initial load at which the spring member starts to deform is set to be greater than the thrust of the piston caused by the first working pressure and smaller than the thrust of the piston caused by the second working pressure,
- the fluid control unit includes:
- a flow path switching valve capable of selectively connecting and switching a supply flow path and a discharge flow path of the working fluid to the first fluid chamber and the second fluid chamber;
- and
- a pressure switching valve capable of selectively switching a pressure source connected to the supply flow path between a first pressure source whose pressure is the first working pressure and a second pressure source whose pressure is the second working pressure, and
- the method comprises:
- when the piston is displaced to the retracted position, introducing the working fluid into the first fluid chamber at a predetermined working pressure and discharging the working fluid from the second fluid chamber;
- when the piston is displaced from the retracted position to the intermediate position and stopped at the intermediate position, introducing the working fluid into the second fluid chamber at a first working pressure and discharging the working fluid from the first fluid chamber;
- and
- when the piston is displaced from the intermediate position to the advanced position, introducing the working fluid into the second fluid chamber at a second working pressure higher than the first working pressure and discharging the working fluid from the first fluid chamber,
- wherein the predetermined working pressure is set to be equal to the first working pressure or the second working pressure.