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A displacement detecting apparatus of an electromagnetic actuator, including: an electromagnetic actuator including: a movable core; a drive coil which causes a displacement of the movable core; and a detection coil disposed at a position where a mutual coupling coefficient with the drive coil chang

A displacement detecting apparatus of an electromagnetic actuator, including: an electromagnetic actuator including: a movable core; a drive coil which causes a displacement of the movable core; and a detection coil disposed at a position where a mutual coupling coefficient with the drive coil changes with the displacement, the actuator transmitting a drive force of the movable core to a mechanical system; a drive portion for supplying a drive current to the drive coil on the basis of a pulse width modulation signal; and a displacement detecting portion to which an output voltage of the detection coil is inputted, for outputting a displacement detection signal, wherein the displacement detecting portion includes: a sampling-signal generating portion for generating a sampling signal in synchronism with the pulse width modulation signal; and a synchronous sampling portion for outputting the displacement detection signal by sampling the output voltage in synchronism with the sampling signal.

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1. An apparatus for detecting a displacement of an electromagnetic actuator, comprising: an electromagnetic actuator including: a movable core; a drive coil which causes a displacement of the movable core by giving a drive force to the movable core; and a detection coil which is disposed at a positi

1. An apparatus for detecting a displacement of an electromagnetic actuator, comprising: an electromagnetic actuator including: a movable core; a drive coil which causes a displacement of the movable core by giving a drive force to the movable core; and a detection coil which is disposed at a position where a mutual coupling coefficient with the drive coil changes in accordance with the displacement of the movable core, the electromagnetic actuator transmitting a drive force of the movable core to a mechanical system;a drive portion configured to supply a drive current to the drive coil, the drive current being synchronized with a pulse width modulation signal; anda displacement detecting portion to which an output voltage of the detection coil is inputted and which is configured to output a displacement detection signal corresponding to the displacement of the movable core,wherein the displacement detecting portion includes: a sampling-signal generating portion configured to generate a sampling signal in synchronism with the pulse width modulation signal; and a synchronous sampling portion configured to output the displacement detection signal by sampling the output voltage of the detection coil in synchronism with the sampling signal. 2. The apparatus according to claim 1, wherein the detection coil includes a main detection coil and an auxiliary detection coil,wherein, where a mutual coupling coefficient of the drive coil and the main detection coil changes, as a first characteristic, in accordance with the displacement of the movable core and a mutual coupling coefficient of the drive coil and the auxiliary detection coil changes, as a second characteristic, in accordance with the displacement of the movable core, the main detection coil and the auxiliary detection coil are disposed such that the first characteristic and the second characteristic differ from each other, andwherein the synchronous sampling portion is configured to sample an output voltage of the main detection coil and the auxiliary detection coil that are connected in reverse-polarity series such that polarity of the mutual coupling coefficient of the drive coil and the main detection coil and polarity of the mutual coupling coefficient of the drive coil and the auxiliary detection coil are opposite to each other. 3. The apparatus according to claim 2, wherein the main detection coil and the auxiliary detection coil are disposed with respect to the drive coil so as to be coaxial therewith, andwherein the main detection coil and the auxiliary detection coil are disposed so as to be arranged adjacent to each other in an axial direction of the drive coil. 4. The apparatus according to claim 3, wherein the main detection coil and the auxiliary detection coil are disposed such that opposite ends, in the axial direction, of a winding wire of the main detection coil and the auxiliary detection coil are located inside respective positions of opposite ends of a winding wire of the drive coil in the axial direction. 5. The apparatus according to claim 1, wherein the displacement detecting portion includes a correction portion configured to correct the displacement detection signal inputted thereto from the synchronous sampling portion, in accordance with a duty ratio of the pulse width modulation signal. 6. The apparatus according to claim 5, wherein the drive portion is configured to set the duty ratio to a prescribed value in a displacement detection period that is a part of a period in which the movable core is driven by the drive portion,wherein the sampling-signal generating portion is configured to generate, in the displacement detection period, a sampling signal which is synchronized with the pulse width modulation signal whose duty ratio is the prescribed value, andwherein the correction portion is configured to correct the displacement detection signal inputted thereto from the synchronous sampling portion, in accordance with the duty ratio set to the prescribed value. 7. The apparatus according to claim 5, further comprising a drive control portion configured to output, to the drive portion and the correction portion, a duty ratio determined by the drive force that the movable core should generate, wherein the drive portion includes:a pulse width modulation signal generating circuit configured to output the pulse width modulation signal in accordance with the duty ratio outputted by the drive control portion; anda drive circuit configured to supply, to the drive coil, the drive current which is synchronized with the pulse width modulation signal outputted from the pulse width modulation signal generating circuit, andwherein the correction portion is configured to correct the displacement detection signal inputted thereto from the synchronous sampling portion, in accordance with the duty ratio outputted from the drive control portion. 8. The apparatus according to claim 1, wherein the electromagnetic actuator further includes a yoke,wherein the movable core is a columnar body and is linearly displaceable along an axial direction of the columnar body by being inserted through a hollow portion of each of the drive coil and the detection coil, a drive shaft being provided on at least one end face of the columnar body in the axial direction so as to protrude therefrom, for transmitting the drive force to the mechanical system, andwherein the yoke is disposed outside the drive coil and the detection coil and has an opening portion through which the drive shaft protrudes. 9. The apparatus according to claim 1, wherein the electromagnetic actuator includes an outer stationary core, an inner stationary core, and a connecting core which connects one end of the outer stationary core and one end of the inner stationary core to each other,wherein a recess whose bottom is defined by the connecting core is formed between the outer stationary core and the inner stationary core,wherein the drive coil and the detection coil are fitted in the recess,wherein the movable core is a plate-like body and is disposed so as to be opposed to an opening portion of the recess via a clearance whose size changes in accordance with the displacement of the movable core, so that the movable core is linearly displaceable in an axial direction perpendicular to a plane of the plate-like body, andwherein a drive shaft is provided on at least one surface of the plate-like body so as to protrude therefrom in the axial direction perpendicular to the plane of the plate-like body. 10. The apparatus according to claim 1, wherein the electromagnetic actuator is constituted by a first electromagnetic actuator and a second electromagnetic actuator each of which has the drive coil,wherein the first electromagnetic actuator and the second electromagnetic actuator have a single movable core common thereto,wherein the first electromagnetic actuator and the second electromagnetic actuator are disposed so as to be arranged in a direction of a displacement of the single movable core, andwherein the first electromagnetic actuator is configured to drive the single movable core in one direction while the second electromagnetic actuator is configured to drive the single movable core in a direction opposite to the one direction. 11. The apparatus according to claim 10, wherein the detection coil is provided in at least one of the first electromagnetic actuator and the second electromagnetic actuator. 12. The apparatus according to claim 1, wherein the drive portion includes:a pulse width modulation signal generating circuit configured to output the pulse width modulation signal; anda drive circuit configured to supply, to the drive coil, the drive current which is synchronized with the pulse width modulation signal outputted from the pulse width modulation signal generating circuit, andwherein the sampling-signal generating portion is configured to generate the sampling signal in a period from rise timing to fall timing of the drive current in a rectangular wave which is synchronized with the pulse width modulation signal. 13. The apparatus according to claim 1, wherein the drive portion includes:a pulse width modulation signal generating circuit configured to output the pulse width modulation signal; anda drive circuit configured to supply, to the drive coil, the drive current which is synchronized with the pulse width modulation signal outputted from the pulse width modulation signal generating circuit,wherein the detection coil is configured to output a voltage in a rectangular wave energized by the drive current in a rectangular wave which is synchronized with the pulse width modulation signal, andwherein the synchronous sampling portion is configured to sample the output voltage of the detection coil immediately before fall timing of the voltage in the rectangular wave outputted from the detection coil. 14. The apparatus according to claim 1, wherein the drive portion is configured to set a duty ratio of the pulse width modulation signal to a prescribed value in a displacement detection period that is a part of a period in which the movable core is driven by the drive portion,wherein the sampling-signal generating portion is configured to generate, in the displacement detection period, a sampling signal which is synchronized with the pulse width modulation signal whose duty ratio is the prescribed value, andwherein the synchronous sampling portion is configured to output the displacement detection signal by sampling the output voltage of the detection coil in synchronism with the sampling signal.