JPH0139246B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high frequency oscillation type proximity switch that detects the presence or absence of metal.

A known method is to detect the rotational speed and rotation angle position of a rotating body by installing a metal piece on the rotating body and using a high-frequency oscillation type proximity switch to detect the rotating metal piece based on the rotation of the rotating body. .

Generally, high frequency oscillation type proximity switch circuits are
The vibration amplitude of the damped vibration waveform from the steady oscillation state when the metal is not in close proximity to the oscillation stop due to the proximity of the metal is compared with a predetermined comparison level, and when the comparison level is lower than the above vibration amplitude, the output is inverted. Detect proximity of. Conversely, when the metal moves away from the close state, the transient oscillation amplitude from the start of oscillation to the steady oscillation state is compared with the above comparison level, and when the amplitude becomes larger than the comparison level, the output is inverted,
Detects separation of metal. Therefore, from the moment of actual approach or separation until the output is reversed, there is a time delay depending on the time constant of the attenuation or growth of the oscillation amplitude of the oscillation circuit and the initial conditions.

The comparison level, which is the standard when determining the magnitude of the amplitude, is based on the ease of detection and S/N from the electronic circuit side.
etc., the steady oscillation amplitude is selected to be approximately 1/2, so when detecting the proximity of metal, there is a time delay approximately equal to the time constant of the damped vibration of the circuit. If it is brought close enough, it can be very small, for example, down to several microseconds, so it can be ignored in practical terms.

However, when the metal is separated, the amplitude at which oscillation starts is determined by the frequency components within the oscillation circuit's bandwidth of external noise and noise generated by the element itself. It takes about 10 to 30 times the time constant of amplitude growth. The time constant of amplitude growth when metals are not in close proximity is the minimum when the metals are completely separated, but this value is determined when the metal detection sensitivity is set, and it is set to a small value as when metals are close to each other. For example, it is about several + μS.

Therefore, there is a very large time delay of several hundred microseconds to several milliseconds from when the metal separates until the output is reversed. Furthermore, the noise that determines the oscillation start voltage described above is a statistical quantity, and its wave height is widely distributed, so the time delay when detecting separation also has statistical properties. In other words, the delay time varies from time to time.

In this way, the proximity switch circuit has large time delays and time delay fluctuations, so when used to detect the rotation speed of a rotating object, the detectable rotation speed is limited by the maximum time delay. Ru.
Also, when used to detect the angular position of a rotating body,
Only the information at the time of approach is valid, and the output inversion signal at the time of separation fluctuates due to the above-mentioned time delay distribution, so there is a drawback that information as an angular position signal cannot be obtained.

This invention was made in view of the above circumstances.
A current having an alternating current component of a predetermined amplitude at a predetermined frequency is supplied to the oscillation circuit in advance, and the start of oscillation after the metal is separated does not depend on random noise.
By starting oscillation with the electromotive force generated in the oscillation circuit by the alternating current component of the above current, the time delay is reduced and fluctuations in the delay time are eliminated. It is an object of the present invention to provide a proximity switch circuit which enables the detection of a rotating body, and whose output signal at the time of separation is also a significant signal as angular information for detecting the angular position of the rotating body.

The present invention will be explained below with reference to an embodiment shown in FIG.

In the figure, 1 is an oscillation circuit, 2 is an amplitude comparison circuit that compares the oscillation amplitude of the oscillation circuit 1 with a predetermined comparison level and generates an output signal according to the magnitude thereof, and 3
is an alternating current supplying means for supplying a current having an alternating current component of a predetermined frequency and a predetermined amplitude to the parallel resonant circuit of the oscillation circuit 1.

To explain the configuration of the oscillation circuit 1 and the alternating current supply means 3, 4 is a detection coil, 5 is a capacitor forming a parallel resonant circuit together with the detection coil 4, and 6 is a detection coil.
is a transistor with an emitter holly connection, 7 is its emitter resistance, 8 and 9 are two transistors with a current mirror connection that output a current corresponding to the collector output current of the transistor 6, and 10 is a transistor that provides a constant DC base bias to the transistor 6. 1
This is a bias circuit. 15 is an AC voltage source having a predetermined frequency and a predetermined amplitude; 13 is a transistor that outputs the AC voltage of the AC voltage source 15 as an AC current from its collector; and 16 is a second transistor that applies a constant DC base bias to the transistor 13. 14 is an emitter resistor of the transistor 13, and 12 and 11 are two current mirror-connected circuits for supplying a current corresponding to the collector output current of the transistor 13 to a parallel resonant circuit consisting of a detection coil 4 and a capacitor 5. It is a transistor.

Next, the operation of the circuit configured in this manner will be explained. When the potential at the base point (A) of transistor 6 rises, its emitter potential rises, and the current flowing through emitter resistor 7, that is, the emitter current of transistor 6 increases. Correspondingly, the collector current increases by an amount almost equal to the emitter current, driving the current mirror circuit composed of transistors 8 and 9, so that transistor 9 outputs a current corresponding to the collector current of transistor 6. Since the base of transistor 6 and the collector of transistor 9 are connected, when the potential at the base point (A) of transistor 6 is increased, the potential at point (A) is higher than that of transistor 9 in response to the potential at point (A). Current is output. Therefore, if the impedance of the bias circuit 10 is zero, the admittance looking from the side of the parallel resonant circuit consisting of the detection coil 4 and the capacitor 5 to the connection point between the base of the transistor 6 and the collector of the transistor 9 of the oscillation circuit 1 is negative. The negative conductance value thereof is approximately equal to the reciprocal of the resistance value of the emitter resistor 7 of the transistor 6.
On the other hand, since the transistor 13 constitutes an emitter follower together with the emitter resistor 14, a change in the potential at the base point (B) of the transistor 13 is expressed as a current of the value obtained by dividing this changing voltage by the resistance value of the emitter resistor 13. output to the collector. Therefore, by driving the base point (B) of the transistor 13 with the AC voltage source 15, the collector of the transistor 13 outputs a DC operating point current from the bias circuit 16 and an AC current.
This current output is supplied by a current mirror circuit made up of transistors 11 and 12 to a parallel resonant circuit made up of a detection coil 4 and a capacitor 5 of the oscillation circuit 1.

As described above, in one embodiment of the present invention, when looking at the circuit from an AC perspective, a negative conductance and an AC current source are connected in parallel to a parallel resonant circuit consisting of a detection coil 4 and a capacitor 5. become.

Now, since the LC parallel resonant circuit has loss,
Even if an oscillating voltage occurs, it will always be attenuated. However, in the above oscillation circuit 1, the negative conductance is connected in parallel to the LC parallel resonant circuit, so the loss of the LC parallel resonant circuit, that is, the power consumed by the conductance component, is the power supplied by the negative conductance component of the circuit. This effectively constitutes an LC parallel resonant circuit with negative loss.
Due to the alternating current component of the alternating current supply means 3, the oscillating voltage generated between the terminals is amplified and oscillation grows. This oscillation grows to a constant value, which is nonlinear due to circuit saturation, etc., and becomes steady oscillation.

When a metal approaches the detection coil 4 while the circuit is in an oscillating state, the proximity effect causes the power of the oscillation circuit to be consumed. In other words, this is equivalent to connecting a conductance due to the proximity effect in parallel to the detection coil. In this way, the increase in the conductance of the detection coil 4 due to the proximity effect makes the total conductance of the oscillation circuit, that is, the negative conductance of the active circuit, the loss of the LC parallel resonant circuit, and the conductance due to the proximity effect, positive. In this case, since the loss becomes positive, the oscillation circuit undergoes damped oscillation and oscillation stops. If the metal is separated, the conductance due to the proximity effect disappears, so the oscillation circuit 1 has a negative total conductance and starts oscillating again.

An amplitude comparison circuit 2 compares the oscillation amplitude of the oscillation circuit 1 with a predetermined comparison level (Vref). Here, the amplitude comparison level is selected within a range where the oscillation amplitude is not affected by circuit saturation, etc., but is usually set to about 1V for ease of comparison and S/N ratio.
Therefore, if the amplitude does not reach the comparison level, it is assumed that there is metal in the vicinity, and an output signal of, for example, "high level (H)" is output, and if the amplitude exceeds the comparison level, it is determined that there is no metal in the vicinity, for example. Outputs a “low level (L)” signal.

As described above, one embodiment of the present invention detects the proximity of metal.Here, the responsiveness of this circuit to metal detection will be explained using the AC equivalent circuit shown in FIG.

In Figure 2, L is the inductance of the detection coil 4, C is the capacitance of the capacitor 5, GT is the loss of the LC parallel resonant circuit consisting of the detection coil 4 and the capacitor 5, -G _p is the negative conductance of the active circuit, and G _n is the conductance caused by the proximity of metals, _Is is the alternating current component of the current supply means 3, and _Io is the noise current.

Now, consider a situation in which metals that were in close proximity are momentarily separated. Assuming that this time is t=o, the vibration amplitude V after time t is the amplitude of the component within the noise current source I _o and the alternating current source I _s parallel resonant circuit bandwidth.
If I _op , I _sp , and frequencies are respectively set as ωn and ωs, the following equation is obtained.

Here, Ψn, Ψs, are constant phase terms,
ωo is the oscillation frequency and is approximately equal to 1/√.
The first term in this equation is the electromotive force due to the noise current source _Io ,
The second term is the electromotive force caused by the alternating current source _Is , and the third term is the growth oscillation component. The time required for this vibration amplitude to reach the amplitude comparison level (this is referred to as Vref), that is, the delay time T until the output of the amplitude comparison circuit 2 is inverted after the metal is separated is Vref≫Ino/
(Go−Gt), Iso/(Go−Gt), then from equation (A), is required.

Now, in the conventional general proximity switch, its AC equivalent circuit diagram is the AC equivalent circuit diagram of one embodiment of the present invention shown in FIG. 2, with the AC current source removed. Therefore, when comparing the conventional circuit and the embodiment of the present invention, in the conventional circuit, the time delay (T ₁ ) becomes the equation (c) because there is no component I _sp ² of the alternating current source in the equation (a).

T ₁ = 2C / G _p − G _t lnVref / I _op (G _p − G _t ) ... (C) I _op is due to current noise, and I _op / (G _p − G _t )
is usually about 10 ^-6 to 10 ^-9 V, and Vref in equation (c) is
If it is 1V, T ₁ will be 15 to 20 times the time constant 2C/(G _p −G _t ) of amplitude growth of the LC parallel resonant circuit.

Next, in one embodiment of the present invention, by setting the amplitude of the AC source so that I _sp ≫ I _op , the delay time (this is T ₂ ) is calculated as T ₂ = 2C/G _p −G It is expressed as _t lnVref / I _sp / (G _p − G _t ) ... (d). For example, I _sp / (G _p − G _t ) is 10mv
If the alternating current component _of the current source _{I s} is set so _that The delay time _T2 is reduced to 1/3 to 1/4 of the delay time _T1 of the conventional circuit.

Furthermore, in the conventional circuit, the amplitude of the oscillation circuit is determined by the amplitude of the current noise _Io . Current noise is random noise and its wave height has a Gaussian distribution, so the logarithmic part of equation (c) is distributed around a certain value. In other words, since the value of T ₂ is not a constant value but has fluctuations, there are cases where the time delay is as large as 30 or 40 times the time constant of the amplitude growth of the oscillation circuit. Therefore, when detecting the rotation speed using the conventional circuit, the maximum usable detected rotation speed is determined by taking into account this maximum delay time.

On the other hand, in one embodiment of the present invention, the amplitude I _sp of the AC current source is determined by the AC voltage source 15 and the emitter resistor 14, so that it can always be kept at a constant value. Therefore, the delay time is constant and no fluctuation occurs at all.

FIG. 3 shows how the oscillation amplitude changes in a conventional circuit and an embodiment of the present invention. In the figure, a indicates the proximity or separation of metals, b indicates the oscillation amplitude of the conventional circuit, c indicates the output waveform of the conventional circuit, and d and e indicate the oscillation amplitude and output waveform of an embodiment of the present invention. It can be seen that the time delay has been improved from T ₁ to T ₂ .

As described above, according to the present invention, the parallel resonant circuit consisting of the detection coil and the capacitor has a frequency set at least within the bandwidth of the parallel resonant circuit and a value sufficiently larger than the amplitude of the noise that normally supports the start of oscillation. By supplying a current containing an alternating current component with an amplitude set at Since the oscillation is made to oscillate from the beginning, the time delay until the output is reversed after the metal is separated is greatly improved, and the delay time is always constant, making it possible to detect the rotation of rotating bodies rotating at high speeds. Further, in detecting the angular position of the rotating body, since the time delay is constant, there is an effect that a proximity switch can be obtained in which the signal when the metal is separated can also be used as an angular position signal.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is an AC equivalent circuit thereof, and FIG. 3 is a waveform diagram for explaining the effects of the present invention. In the figure, 1 is an oscillation circuit, 2 is an amplitude comparison circuit, and 3 is means for supplying an alternating current of a predetermined frequency and a predetermined amplitude to the parallel resonant circuit of the oscillation circuit 1.

Claims (1)

[Claims] 1. An oscillation circuit consisting of a parallel resonant circuit consisting of a detection coil and a capacitor, and an active circuit having negative conductance characteristics and outputting a current corresponding to the terminal voltage of the parallel resonant circuit and supplying it to the parallel resonant circuit. , an amplitude comparison circuit that generates an output signal depending on the magnitude of the oscillation amplitude of this oscillation circuit, and the above parallel resonant circuit, a frequency set within the bandwidth of this parallel resonant circuit and the amplitude of the noise that induces the start of oscillation. A proximity switch circuit comprising means for supplying a current containing an alternating current component with an amplitude set to a sufficiently larger value, inducing the start of oscillation by the alternating current component of the current, and preventing a delay in the start of oscillation.