Question

a) What is the main drawback with regard to the use of AC coils in hydraulic proportional valves? b) Distinguish between force-controlled and stroke-controlled hydraulic proportional solenoids

Answer 1

a)the main drawback with ac coils in hydraulic valves is power loss due to Eddy currents which form due to electromagnetic induction in the armature. Another drawback is the risk of vibration, which can be mitigated by using well-engineered solenoid valves which make use of appropriate shading rings.

b)i.Force-controlled Proportional Solenoid.

The solenoid force is controlled by the change in current I in the force-controlled solenoid without the armature of the solenoid performing a measurable stroke.Due to current feedback in the electrical amplifier, the solenoid current and therefore the solenoid force are kept constant even if resistance changes.The main feature of the force-controlled proportional solenoids is the characteristic force-stroke curve.The solenoid force remains constant over a defined stroke range at constant current.The stroke for the solenoid shown in this example is approx. 1 5 mm. The solenoid is used in this range.The force-controlled solenoid is of compact design due to the short stroke. In view of this short stroke, the force-controlled solenoid is used particularly for pilot-operated proportional directional and pressure control valves with the solenoid force being converted Into hydraulic force. The proportional solenoid is a controllable “wet pin” DC linear solenoid contained in an oil bath.

ii.Stroke-Controlled Proportional Solenoid.

In the case of the stroke-controlled solenoid the position of the armature is controlled by a closed-loop control circuit and maintained irrespective of the counter-pressure. provided it is within the rated working range of the solenoid.With the stroke-controlled solenoid, the spools of proportional directional, flow as well as pressure control valves can be directly operated, and be controlled in any stroke position The stroke ot the solenoid is between 3 and 5 mm depending on the size.As already mentioned, the stroke-controlled solenoid is primarily used for directly operated 4-way proportional valves.In conjunction with the electrical feedback, the hysteresis and the repetition error of the solenoid are maintained with very tight tolerances. In addition, any flow forces. which occur at the valve spool are compensated (or {relatively small solenoid force in relation to the interfering forces).In the case of pilot operated valves, the controlled hydraulic pressure is applied to a relatively large control area. The available positioning forces are therefore considerably greater and the percentage effect of interfering forces is not so marked. For this reason, pilot operated proportional valves can be implemented without electrical feedback.