Abstract

The present invention controls the position of a center mounted spool valve (192) with an externally mounted vacuum controlled actuator (301). The actuator position is preferably controlled by a pulse width modulated or variable force solenoid (302), which modulates the amount of vacuum going to the actuator (301). A microprocessor (208) reads the phase angle and adjusts the duty cycle or current based on the error signal of the control loop (450). In a preferred embodiment, a position sensor (304) further controls the position of the spool valve (192). The position sensor (304) creates an inner loop (400) with position feedback on the position of the actuator (301) and spool valve (192), while the outer loop controls the phase angle. Added to the spool valve position is an offset to move the spool valve (192) to its steady state or null position (410).