Cylinder control

The ports of a double-acting cylinder are connected to working ports 2 and 4 of a 5/2 valve, and the valve coil to a PLC output. The position sensors are wired to PLC inputs.

Rest position

A spring-return valve sends air to port 2 when the coil is de-energised, and to port 4 when the coil is energised. Which end of the cylinder each port is connected to decides whether the rest position is retracted or extended. In a power cut, too, the cylinder returns to its rest position, as long as there is compressed air.

Position sensors

The magnet in the piston activates a sensor when the piston is level with it. B1 is on when the cylinder is retracted, and B2 when it is extended. The program waits for the sensor signal before the next movement, so the step sequence moves on only when the movement is complete.

Common mistakes

• Working ports 2 and 4 are connected the wrong way round. Then the cylinder is extended in its rest position.
• The program moves on to the next movement after a set time and does not wait for the position sensor. If the cylinder sticks, the program does not notice.