Direct and Indirect Control of Single Action Cylinder

In a relay-controlled system, electrically operated directional control valves can either be activated directly or indirectly. This example considers the control of a single-action cylinder via a 3/2 directional control valve.

Direct Control

The graphic below shows the electrical component of an electro-pneumatic control system that directly controls a cylinder (the pneumatic component of the system is not illustrated). Pressing the button S closes circuit 1 and solenoid Q1.1 of the directional control valve is supplied with power. The magnet is drawn in, switching the valve to its operating position causing the cylinder to extend. When the button is released, the circuit opens, the electromagnet releases and the valve returns to its rest position.

Indirect Control

The circuit below illustrates indirect control of a cylinder (with its more complex circuitry). The current to the coil of the solenoid is not directly applied via the switch S but indirectly via an intermediate relay K

Indirect control has the following advantages over direct control:

Control circuit 1 and main circuit 2 can operate at different voltages (e.g. 24 V and 220 V).

The current in the solenoid of the valve can be greater than the maximum permissible for the switch S.

One switch can simultaneously control more than one valve (by means of a relay with an appropriate number of contacts).

Extensive links between signals from various switches can be implemented.