Proportional Solenoid Theory
There are two critical design requirements that must be met for a proportional control solenoid. These are low hysteresis and a relatively flat force vs. stroke curve at a given applied current. Both of these greatly affect the predictability, repeatability and the command response of the solenoid.
Low hysteresis is best accomplished through the use of bearings that reduce linear friction. A flat force versus stroke curve is achieved by proper design of the armature and pole pieces.
The advantage of the flat curve is that high predictability of position and force control of the solenoid is achieved. An example of a force curve for a proportional solenoid is shown in the graph.