Electromagnetic actuator

Abstract

An electromagnetic actuator for use with a fuel injection pump, for example, has a position detector for detecting the operating position of a plunger which is axially movable. The electromagnetic actuator includes a core mounted on one end of the plunger, a drive coil disposed around the plunger and energizable to displace the plunger axially under magnetic forces, a detection coil disposed closely to the drive coil coaxially therewith, for receiving the core therein in response to axial displacement of the plunger, whereby the inductance of the detection coil can vary depending on the length of the portion of the core which is positioned within the detection coil, an LC oscillator including an inductor and a capacitor, for producing an alternating current whose frequency is determined by the inductance of the inductor and the electrostatic capacitance of the capacitor, the detection coil being at least part of the inductor, whereby the oscillation frequency of the LC oscillator varies as the inductance of the detection coil varies, and a frequency-to-voltage converter for converting the frequency of an output signal from the LC oscillator into a corresponding DC voltage signal which is indicative of an operating positon of the plunger. The electromagnetic actuator is small in size and capable of actuating the fuel injection pump under accurate control.