The solenoids which form the actuators for most of our systems are generally operated with direct current.

The simplest form is switching a supply voltage on and off. This causes the solenoid to switch and the armature to move from the stroke start position to the stroke end position. When the supply voltage is switched off, the armature returns to its initial position.

The supply voltage can be provided via the power supply system or, for example, in the form of a capacitor discharge.

After application of the voltage the current increases, and the magnetic field is built up. When the magnetic force is high enough, the armature is set in motion and moves to the stroke end position. The current then increases further, up to the so-called limiting current.

When the supply voltage is switched off, the magnetic field is suppressed. The suppression can be accelerated by means of suitable components such as a freewheeling diode or a suppressor diode. Another alternative is to feed the voltage back into the power supply system.

As the magnetic force is proportional to the current, a regulated current is used for some applications. Solenoids or solenoid valves, which operate against a force or pressure, can thus be controlled by the current strength.

In order to reduce the friction of the moving components, stopping of these components is avoided. This means that no static friction has to be overcome, allowing the hysteresis of the solenoid or valve to be considerably increased. For this purpose the current is provided as a dither signal or PWM signal (pulse width modulation).

Simple controls, for example for on-off solenoids and pumps, are usually integrated into the customer’s control unit.

Thomas has developed its own control unit to provide a more complex control. This has a modular design and can control up to 24 proportional valves, in addition to on-off valves. To enable optimal fulfillment of customer requirements, the inputs and outputs can be freely defined, and even logic links are no problem. For example, the entire pilot control on mobile machines can be fulfilled by the ECU (Electronic Control Unit).