Self-sensing of a solenoid valve via phase detection

This article presents a self-sensing method suitable for high-resolution positioning of a switching-solenoid valve. An external capacitor is connected to the solenoid coil, which is excited by a signal with a near-resonant frequency. At this frequency the phase between the input and output signals is most sensitive to variations of the coil inductance, which is dependent on the core position. Displacement of the iron core is estimated by observing the phase angles. Experimental results on an industrial solenoid indicate satisfactory repeatability of the self-sensing scheme. To explain for the frequency-dependent characteristic of the measured inductance and the phase-frequency curves, an equivalent circuit for the solenoid is proposed and analyzed. Preliminary results on closed-loop position control are also reported.