JPS5932932Y2 - Inductance minute change detection device - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an improvement in a sensor that detects minute instantaneous changes in inductance.

Conventional methods for detecting minute changes in inductance include (1) a method in which a bridge circuit is used to detect inductance changes as voltage changes, (2) a method in which inductance is configured as a resonant circuit and detected as a change in oscillation frequency, ( 3) Similarly, there is a method of detecting it as a deviation in the tuning voltage of a resonant circuit.

Now, assuming that the original inductance is Lo and the Watanabe component is ΔL, the detection sensitivity in each of these methods is ideally determined as follows.

That is, in the bridge method of 1, ′△”'L o,
In the frequency method of 2, it is proportional to r-.

However, in the case of 3, the effective resistance component Ro included in the resonant circuit
is ideally extremely small, the tuning degree of the circuit is Q = ω
It is proportional to OLO/R0 (ω0 is the resonant frequency), and ts L
7, the change ratio increases compared to Lo.

However, in either method, the change is 10 (-60
db) In the following cases, the stability of detection sensitivity becomes extremely poor in practical use.

Therefore, the present invention proposes a new detection device that can detect extremely minute changes in inductance of -60 db or less with high sensitivity and compensate for its stability.The features are as follows. be.

In a method of detecting a change in inductance as a frequency change, the ratio △f of the change △f to the original frequency fo
In contrast to the conventional method of detecting an electrical quantity (for example, voltage) proportional to /fo, a reference frequency generation circuit is newly provided and operated differentially with the original frequency fo, and the ratio of the change to the output, that is, △ By detecting f/(fo-fs), higher sensitivity is achieved than the conventional detection ratio Δf/f.

Ru.

If fs is set to a value close to fo, considerably high sensitivity can be obtained.

Further, in response to fluctuations in fo, the amount of change is negatively fed back to the reference frequency generating circuit, and tracking control is performed to stabilize (fo-fs) so that it becomes constant.

Normally, fluctuations in fO change slowly depending on the ambient temperature, etc., and the control speed may be slow enough to follow it.

Therefore, signal components that change instantaneously can be stably detected without being affected by natural fluctuations (hereinafter referred to as drift).

From the above configuration, the sensor of the present invention has a higher sensitivity of 120 db or more compared to the conventional sensor, and is extremely effective in practice.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the accompanying drawings.

In the figure, 1 is a detection coil, and 2 is an oscillation circuit, which oscillates according to the constant of the detection coil 1.

3 is a buffer amplifier, and 4 is a low-pass filter that attenuates high-frequency noise frequencies induced by disturbances having a frequency fx or higher corresponding to the constant of the detection coil.

5 is a phase comparator and the frequency f from the detection coil
This circuit mixes the reference frequency fs from the voltage controlled oscillator 17.

6 is a low-pass filter that attenuates the high frequency component of (fo+fs) in the output of the phase comparator;
This is a circuit that extracts only the low frequency component of fs).

7 is a buffer amplifier, 8 is a low-pass filter for converting frequency changes into voltage changes, and the cutoff frequency is (f
o-fs) set smaller than the frequency, and set it to the most efficient part of the voltage attenuation vs. frequency (fo-fs) a
Throw.

9 is a buffer amplifier; 10 is a rectifier circuit that converts a voltage corresponding to (fo-fs) amplitude-modulated by the low-pass filter 8 into direct current.

11 is a differential amplifier which inputs the voltage eo corresponding to the above-mentioned (fo fs) and the reference voltage es divided by the variable resistor VR1 from the constant voltage power source E, and cancels them out with each other.
Normally this output is regulated to O volts.

That is, in the circuit systems 1 to 11 above, the detection coil 1
If the oscillation frequency fO of the differential amplifier 11 has no drift due to ambient temperature and the reference frequency fs is constant, the differential amplifier 11
The output of the differential amplifier 11 is normally adjusted to O volts, but when an object approaches the detection coil 1 and causes a frequency change Δf, the differential amplifier 11 generates a voltage Δe proportional to Δf.

12 is a differential amplifier, which connects a constant voltage power supply E to a variable resistor VR2.
If the voltage Δe is thicker than the set voltage ec divided by , the signal voltage Es1g is output.

On the other hand, when a drift occurs in the detection coil 1, a drift voltage ed is similarly generated at the output of the differential amplifier 11, but this voltage is amplified and detected by the saturation amplifier 13 when it is smaller than the aforementioned set voltage ec.

14 is an attenuator that limits the input to the integrating circuit 15 in the subsequent stage;
Along with the free running number R-C of the integrating circuit, the voltage controlled oscillator 1 in the subsequent stage
Adjust the control speed of 7.

Reference numeral 16 denotes an adder circuit that adds the control voltage from the integrating circuit 15 to the bias voltage divided from the low voltage power supply E by the variable resistor VR3.

This bias voltage becomes a voltage that sets the initial value fs of the oscillation frequency of the voltage controlled oscillator 17.

Therefore, the voltage controlled oscillator 17 normally oscillates at fs. When the drift voltage ed occurs, it is immediately amplified by the saturation amplifier 13, detected as a voltage of +E or -E volts depending on the direction of the drift, and delayed by the integrating circuit. Until then, the voltage controlled oscillator 17 is constantly controlled on and off around the reference frequency fs.

Next, to explain its function and effect, (1) When the oscillation frequency fo of the detection coil is directly converted by a frequency-voltage conversion circuit of 8,9°10, the voltage change with respect to the frequency change △f becomes △f/fo. Only a proportional voltage change is detected, but by subtracting the original frequency fo with the reference frequency fs, the voltage change becomes △f/(fO - fs) and becomes fo
/(fo-fs) times.

For example, if the detection coil side oscillates at fo = 50KHz and the signal frequency changes by IHz, the change ratio is 50X10"-3 using the conventional method, but if the reference frequency fs
If mixed at =49KHz, the output of phase comparator 5 (f o - f s ) = I KHz,
The voltage change ratio with respect to the IHz change is 1×10 −3 , which means that the sensitivity has increased by 50 times.

The sensitivity improves as fs is set to a value closer to fo.

(2) Since the drift ED is controlled on/off by determining only its polarity by the saturation amplifier 13, it is stable in principle without any hunting factor compared to the conventional ED delay operation.

In other words, when the sum of the delay phase angle θ of the signal amplification system (in this case determined by the constant of the wave circuit of the rectifier circuit) and the delay angle θ2 of the delay circuit of the control system approaches 188, the drift of fo A positive feedback state occurs and hunting occurs, but in this case, the integrating circuit of the control system merely determines the control speed and does not become a delay element, so hunting does not occur.

This means that even if the drift fluctuates at a relatively fast period,
This means that the control speed can be followed anywhere, for example, it is also possible to compensate for circuit system drift (relatively fast) other than the detection coil drift (slow), reducing the noise level and increasing the detection sensitivity. It helps improve.

However, since the change in signal level usually changes sufficiently faster than the change in drift, the signal level does not need to be corrected at a practical level.

(3) Due to the above two main effects, it is possible to detect minute changes of -90 db or less, which is 20 db better than the conventional one.
The sensitivity was improved more than b, and a remarkable effect was obtained in practical use.

In the above explanation, an inductance type sensor has been described, but a capacitor type sensor may also be used, for example, and the present invention can be applied to all sensors that can be replaced with frequency changes.

The detection device of the present invention can be extremely effectively applied to vehicle detectors, object proximity switches, metal material discriminators, and the like.

[Brief explanation of the drawing]

The drawing is a structural block diagram of an embodiment of the present invention. 1...detection coil, 2...transmission circuit,
3... Buffer amplifier, 4... Parallel low pass filter, 5... Phase comparator, 6...
...Low pass filter, 7°°°...Buffer amplifier, 8...Low pass filter (for frequency-voltage conversion), 9...Buffer amplifier, 10...
... Rectifier circuit, 11 ... Differential amplifier, 12
... Differential amplifier, 13 ... Saturation amplifier, 14 ... Attenuator, 15 ... Integrating circuit, 16 ... Adding circuit, 17...Voltage controlled oscillator, fo...Detection coil frequency, fs.
...Reference frequency, eo... (fo-fs
)Equivalent voltage, es...Reference voltage for eo cancellation,
ed...Drift voltage, △e...△f
...Equivalent voltage, ec...Setting voltage, E
s1g...Signal voltage.

Claims (1)

[Claims for Utility Model Registration] Detection coil 12 Transmission circuit 2. The detection coil oscillation frequency fo from the low-pass filter 4 is subtracted by the reference frequency fs from the voltage-controlled oscillator 17 by the phase comparator 5, and the low-pass filter 6, low-pass filter 8. Rectifier circuit 10
outputs a DC voltage e□ corresponding to the attenuation frequency fo fs, and cancels this DC voltage eo in the differential amplifier 11. The output Δe from the equivalent differential amplifier 11 is the set voltage e. When the signal voltage Es1g from the differential amplifier 12 exceeds
in the output detection circuit; The output ed follows the output ed of the differential amplifier 11 due to the drift of the oscillation frequency fO of the detection coil 1, and the output ed becomes the set voltage e. The oscillation reference frequency fs of the voltage controlled oscillator 17 is controlled at a stage smaller than the attenuation frequency f. - Saturation amplifier for keeping fs constant 13. Attenuator 14. It has a control circuit including an integrating circuit 15 and an adder circuit 16; it detects minute frequency changes of a detection coil with high sensitivity, and is characterized in that stability against drift of the detection coil is compensated. detection equipment