Displacement sensor
View Patent ↗An oscillator circuit outputs an oscillator signal with a frequency corresponding to an inductance of a displacement detector coil. An oscillation cycle measurement circuit measures a cycle of the oscillator signal output from the oscillator circuit, and outputs a signal corresponding to the measured cycle. A squaring circuit calculates and outputs a square of the signal output from the oscillation cycle measurement circuit. By the calculation and output of the square of the oscillator signal cycle, a square root component of the product of inductance and capacitance components is eliminated, so that the output signal changes linearly relative to the displacement of a displacement body. As a result, the linearity of the output signal relative to the displacement of the displacement body can be improved.
1. A displacement sensor comprising:
a cylindrical displacement detector coil;
an electric conductor disposed close to an inside or outside of the displacement detector coil in a radial direction thereof, the electric conductor being freely displaceable in an axial direction of the displacement detector coil;
an oscillator that outputs an oscillation signal with a frequency corresponding to an inductance of the displacement detector coil, the inductance of the displacement detector coil linearly increasing or linearly decreasing depending on a displacement of the electric conductor from one end to the other;
an oscillation cycle meter that outputs a signal corresponding to a cycle of the oscillation signal output from the oscillator, the signal output from the oscillation cycle meter including square root component of a product of inductance and capacitance of the displacement detector coil; and
a square calculator that performs a mathematical squaring operation on the signal output from the oscillation cycle meter, and outputs a squared signal corresponding to a square of the signal output from the oscillation cycle meter, the mathematical squaring operation removing the square root component of a product of inductance and capacitance of the displacement detector coil, such that the squared signal output from the square calculator changes generally linearly relative to a displacement of the electric conductor.
2. The displacement sensor according to claim 1 , further comprising:
a temperature compensator that compensates for temperature change of the squared signal output from the square calculator.
3. The displacement sensor according to claim 2 , wherein the temperature compensator adds an offset amount having a predetermined temperature coefficient to the squared signal output from the square calculator thereby to compensate for the temperature change of the squared signal.
4. The displacement sensor according to claim 3 , wherein at least one of the temperature coefficient and the offset amount is a programmable parameter.
5. The displacement sensor according to claim 3 , wherein the temperature compensator applies a compensation temperature coefficient to the squared signal to which the offset amount has been added, so as to cancel the predetermined temperature coefficient.
6. The displacement sensor according to claim 5 , wherein the compensation temperature coefficient is a programmable parameter.
7. The displacement sensor according to claim 1 , wherein a non-magnetic body is disposed inside the displacement detector coil.
8. The displacement sensor according to claim 1 , wherein the oscillator includes an amplitude limiter that limits an amplitude of the oscillation signal to a predetermined value.
9. The displacement sensor according to claim 1 , wherein a winding of the displacement detector coil is non-uniformly distributed in the axial direction.
10. The displacement sensor according to claim 1 , wherein the displacement detector coil and the electric conductor are enclosed with a magnetic shield, such that the magnetic shield shields the displacement detector coil and the electric conductor from electromagnetic waves.
11. The displacement sensor according to claim 1 , wherein the squared signal output from the square calculator comprises a product of inductance and capacitance of the displacement detector coil.
12. The displacement sensor according to claim 1 , wherein the signal output from the oscillation signal cycle meter corresponds to 2π(LC) 1/2 , and the squared signal output from the square calculator corresponds to (2π) 2 LC, wherein L and C are the inductance and capacitance of the displacement detector coil, respectively.
13. The displacement sensor according to claim 1 , wherein the square calculator performs only the mathematical squaring operation to obtain the squared signal that changes generally linearly relative to a displacement of the electric conductor.