To prevent positional deviation accompanying excessive actuation or discharge of a working pressure after positioning by selectively opening/ closing solenoid valves so as to stop a rod in four positions and positioning the rod according to actuation order of the solenoid valves and a pressure receiving area difference of a cylinder.
Passages 16a-16d always communicating a second pressure chamber 14b and a fourth pressure chamber 14d together are arranged, and a first, second and third solenoid valves 15a-15c are arranged. An end stopper 11 is arranged, while a maximum stroke L1 of a piston 5 and a maximum stroke L2 of an intermediate stepped piston 8 are set so that L1=L2/2 is satisfied, and a rod 4 can be stopped in four positions, in each of which the stroke of the rod 4 is zero, L1, L2, or L1+L2. When the solenoid valves 15a-15c are turned off, pressure chambers 14a-14d are opened to the atmosphere, so that the rod 4 can be moved within a range of the sum of the maximum stroke L1 of the piston 5 and the maximum stroke L2 of the intermediate stepped piston 8. In this process, the solenoid valves 15a-15c are selectively turned on so as to feed compressed air, and the rod 4 can be properly positioned without generating any drift.
OKAMOTO ISAO
IMAZATO KAZUNARI
SANWA SEIKI MFG CO LTD