The SER, SEI and SEH valves modulate by the electronically controlled rotation of a step motor. The step motor drives a gear train and lead screw to position a piston.

The piston is used to modulate flow through a port, refer to Figure 10.

The motor is a two phase type driven in the bi-polar mode.

Two discrete sets of motor stator windings are powered in sequence to rotate the rotor 3.6 degrees per step.

Polarity of the drive signal reverses for each step.
The sequencing is accomplished electronically through the bi-polar drive circuit shown in Figure 7.

The drive transistors, Q1 through Q8, are electronically biased in pairs by the controller as shown in Table 1.

The SER valves have a stroke of 189a (4.8mm) and 1596 or 2500 steps of resolution. Each step yields .00012a (03mm) of travel. Small SEI valves have 1596 with .250 i.