DISPLACEMENT MEASUREMENT DEVICE

1 . A displacement measurement device comprising:

a metal object arranged movable in a predetermined moving direction within a predetermined moving plane;

a measurement coil having a measurement coil surface opposite to the moving plane and arranged such that an opposite area of the measurement coil surface to the moving plane is varied with a movement of the metal object;

a correction coil having a correction coil surface opposite to the moving plane and arranged such that an opposite area of the correction coil surface to the moving plane is not varied irrespective of the movement of the metal object;

an inductance detection circuit configured to measure respective inductances of the measurement coil and the correction coil; and

a calculation circuit configured to create a displacement signal according to a relative position of the metal object relative to the measurement coil by use of a measurement result from the inductance detection circuit,

wherein

the measurement coil and the correction coil are arranged such that the measurement coil surface and the correction coil surface are not overlapped with each other with regard to a plane parallel to the moving plane but a range occupied by the measurement coil in a coordinate axis extending along the moving direction and a range occupied by the correction coil in the coordinate axis are overlapped with each other.

2 . The displacement measurement device as set forth in claim 1 , wherein

the correction coil includes a first correction coil and a second correction coil which are electrically connected with each other,

the first correction coil and the second correction coil are arranged in a direction respectively perpendicular to a normal direction of the moving plane and the moving direction, and

the measurement coil is interposed between the first correction coil and the second correction coil.

3 . The displacement measurement device as set forth in claim 1 , wherein

the measurement coil includes a first measurement coil and a second measurement coil which are electrically connected with each other,

the first measurement coil and the second measurement coil are arranged in a direction respectively perpendicular to a normal direction of the moving plane and the moving direction, and

the correction coil is interposed between the first measurement coil and the second measurement coil.

4 . The displacement measurement device as set forth in claim 1 , wherein

the metal object includes a first metal object and a second metal object which are arranged opposite to each other, and

the measurement coil and the correction coil are interposed between the first metal object and the second metal object.

5 . The displacement measurement device as set forth in claim 1 , wherein

the inductance detection circuit is constituted by a single inductance detection circuit which is configured to measure the inductances of the measurement coil and the correction coil, and

the displacement measurement device further comprises a switch configured to select a destination for an input of the inductance detection circuit from the measurement coil and the correction coil.

6 . The displacement measurement device as set forth in claim 1 , wherein

the measurement coil and the correction coil are defined as patterned circuits which are formed on the same substrate.

7 . The displacement measurement device as set forth in claim 1 , wherein

the inductance detection circuit includes:

capacitors connected in parallel with the measurement coil and the correction coil, respectively;

an oscillator configured to oscillate an resonance circuit of the measurement coil and the capacitor connected to the measurement coil and an resonance circuit of the correction coil and the capacitor connected to the correction coil;

an amplitude detector configured to measure oscillation voltages of the respective resonance circuits;

a comparison unit configured to compare the oscillation voltage measured by the amplitude detector with a reference voltage;

a conductance controller configured to adjust negative conductance of the oscillator based on a comparison result from the comparison unit such that the oscillation voltage is equal to the reference voltage; and

inductance detector configured to measure the inductances of the measurement coil and the correction coil based on an adjustment result of the negative conductance from the conductance controller.

8 . The displacement measurement device as set forth in claim 1 , wherein

the calculation circuit is configured to, based on the measurement result from the inductance detection circuit, generate the displacement signal by means of multiplying the inductance of the measurement coil by a gain and modify the gain according to the inductance of the correction coil.

9 . The displacement measurement device as set forth in claim 1 , wherein

the displacement measurement device further comprises a temperature detection circuit configured to measure a temperature of the displacement measurement device or an ambient temperature of the displacement measurement device, and

the calculation circuit is configured to perform a correction process of the displacement signal based on a measurement result from the temperature detection circuit.

10 . The displacement measurement device as set forth in claim 1 , wherein

the correction coil is closer to the moving plane of the metal object than the measurement coil is.