Devices that convert electrical input signals into mechanical movements (or forces/torques) that in turn actuate hydraulic control elements. They therefore form the actual interface between the electrical signal circuit and the hydraulic power circuit. Of the various possibilities, four have become more widespread (Figure E 25):
Moving coil:
Operates based on the electrodynamic principle. It combines low hysteresis and good linearity with very good dynamic behaviour. A disadvantage is the low stroke work.
Torque motor:
In the rest position there is an equilibrium between the magnetic fields in the two coils and the twisting tube. On operation the armature is deflected until an equilibrium is established between the solenoid force and the returning force of the twisting tube.
The dynamic performance of the torque motor is better than the moving coil. It is suitable for very high requirements on the dynamic performance.
Proportional solenoid:
solenoid on which due to the special arrangement of the magnetic flux a force proportional to the current is applied to the armature over a certain adjustment travel. Cheap, but dynamic performance not as good the torque motor. It is therefore only used for feedback controls in special cases.
Stepper motor:
Electric motor on which the rotor moves in small, defined steps that are controlled by electrical pulses on the stator winding. The number of angular steps of the rotor corresponds to the number of pulses and the rotational speed corresponds to the pulse frequency.
The low power of stepper motors (approx. 100 W) can be hydraulically amplified in electro-hydraulic linear or torque amplifiers. The result is simple but precise positioning drives without external distance measuring systems.
Figure E 25: Electro-hydraulic signal converters