JPH0139247B2 - - 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 metal protrusion is installed on the rotating body to detect the rotational speed and rotation angle position of the rotating body, and a high-frequency oscillation type proximity switch detects the metal protrusion that crosses the front surface of the proximity switch as the rotating body rotates, and generates an on-off signal. The method of outputting is well known. The characteristics of the high-frequency oscillation type proximity switch circuit are, for example, the vibration amplitude of the damped vibration waveform that goes from a steady oscillation state when the metal (object to be detected) does not approach to stopping oscillation due to the proximity of the object to be detected is set to a predetermined level comparison. When the vibration amplitude is compared with the level comparison set value, the output is inverted, thereby detecting the proximity of the object to be detected. Conversely, when the object to be detected moves away from the close state, the transient oscillation amplitude from the start of oscillation to the steady oscillation state is compared with the level comparison set value, and when the above amplitude becomes larger than the level comparison set value, the output is output. This is to detect when the object to be detected has moved away from the proximity switch. Therefore, as a practical example, it must be acknowledged that from the moment of proximity or separation until output reversal occurs, there is a time delay depending on the time constant of the oscillation output decay and growth of the oscillation circuit, as well as the initial conditions. Nakatsuta. The level comparison setting value for determining the magnitude of the amplitude is selected to be approximately 1/2 of the steady oscillation amplitude due to S/N or reliability associated with circuit stability, etc. When detecting the proximity of the detected object, the circuit attenuation The time delay is approximately equal to the vibration time constant, but this time constant can be made very small by bringing the object to be detected sufficiently close to the detection coil of the proximity switch, and can usually be reduced to a few μs, so it can be ignored in practice. It was hot. However, when the object to be detected is separated, the amplitude at which oscillation starts is determined by external noise and noise generated by the element itself. Double or 37
It takes about twice as long. The time constant of amplitude growth when the detected object is not close is the minimum when the detected object is completely separated, and its value is determined when the detection sensitivity of the detected object is set. It cannot be reduced to a small value; it is usually around 10 microseconds. Therefore, there is a very long time delay of several hundred microseconds to several milliseconds until the object to be detected moves away and 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 differs depending on the conditions at that time. In this way, conventional proximity switch circuits have a time delay in operation and fluctuations due to the time delay, so when used to detect the rotation speed of a rotating body, the detectable rotation speed is limited by the time delay. In addition, when used to detect the angular position of a rotating body, only the information when approaching is valid, and the output inverted signal when separating is disadvantageous in that the angular position signal cannot be obtained due to the above-mentioned time delay variations and fluctuations. It was hot.

Therefore, the present invention has been made to eliminate the above-mentioned drawbacks, by supplying an alternating current component current having a predetermined frequency and amplitude to the oscillation circuit of the proximity switch in advance, and after separating the object to be detected. The oscillation conditions of the proximity switch are regulated by the electromotive force generated in the oscillation circuit of the proximity switch by the above-mentioned AC component current, without depending on random noise components for the start of oscillation, thereby reducing the time delay and fluctuation of the delay time. It is an object of the present invention to provide a proximity switch circuit which improves the stability of the output signal when separated by eliminating the proximity switch circuit.

An embodiment of the present invention will be described below with reference to FIG. In the figure, 1 is an oscillation circuit, 2 is a comparison circuit that compares a predetermined level comparison set value and the oscillation amplitude of the oscillation circuit 1 and generates an output signal according to the magnitude thereof, and 3 is a predetermined frequency and a predetermined amplitude. This is an alternating current supply circuit that superimposes currents with alternating current components.

The configurations of the oscillation circuit 1 and the alternating current supply circuit will be explained below. That is, 4 is a detection coil,
5 is a capacitor forming a parallel resonant circuit together with the detection coil 4; 6 is a transistor with an emitter follower connection; 7 is an emitter resistor thereof; 8 and 9 are transistors with a current mirror connection that output a current corresponding to the collector output current of the transistor 6. ,1
0 is a first bias circuit that applies a constant DC base bias to the transistor 6. Further, 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; 16;
is a second bias circuit that applies a constant DC base bias to the transistor 13, and 14 is an emitter resistor of the transistor 13.

The operation of the proximity switch circuit of the present invention having such a configuration will be explained below. Now, if the potential at the base point (A) of the transistor 6 rises, the emitter potential of the transistor 6 also rises, and the current flowing through the emitter resistor 7 increases. Correspondingly, the collector current of transistor 6 increases by an amount approximately equal to its emitter current, thereby driving a current mirror circuit consisting of transistors 8 and 9. Then,
A current corresponding to the collector current of transistor 6 is output from transistor 9, and the potential at the base point (A) of transistor 6 is increased. In other words, a current corresponding to the potential at point (A) flows from transistor 9. Therefore, if the impedance of the first 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 has negative conductance, and its negative conductance value is approximately equal to the reciprocal of the resistance value of the emitter resistor 7 of the transistor 6. On the other hand, the transistor 13 and the emitter resistor 14 constitute an emitter follower.
Therefore, a change in the potential at the base point (B) of the transistor 13 is outputted to the collector of the transistor 13 as a current equal to this changed voltage divided by the resistance value of the emitter resistor 14. 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 second bias circuit 16 and an AC current. Since the collector of the transistor 13 is connected to the base-collector connection point of the transistor 8 constituting the current mirror circuit in the oscillation circuit 1, a current corresponding to the collector current of the transistor 13 flows from the collector of the transistor 9 to the detection coil 4 and the capacitor 5. is supplied to a parallel resonant circuit consisting of: In other words, the parallel resonant circuit is supplied with a superimposed current corresponding to the potential at point (B) on the current corresponding to the potential at point (A), so that when looking at the circuit from an AC operation point of view, the detection coil 4 and capacitor 5
A negative conductance and an alternating current source are connected in parallel to the parallel resonant circuit consisting of the following. Here, since the parallel resonant circuit has a loss as is well known, even if an amplitude voltage is generated, it is always attenuated. However, in the above-mentioned oscillation circuit 1, since the negative conductance is connected in parallel to the parallel resonant circuit as described above, the loss of the parallel resonant circuit, that is, the power consumed by the conductance component is replaced by the power supplied by the negative conductance component of the circuit. Therefore, a parallel resonant circuit with a negative loss is substantially constructed, and the oscillating voltage generated between the terminals of the resonant circuit is amplified by the alternating current component of the alternating current supply circuit 3, and oscillation grows. This oscillation grows to a constant value due to nonlinearity due to circuit saturation, etc., and performs steady oscillation. When a detected object comes close to the detection coil 4 when the circuit is in an oscillation state, the power of the oscillation circuit 1 is consumed due to the proximity effect. In other words, this is equivalent to connecting a conductance due to the proximity effect in parallel to the detection coil. Therefore, if the total conductance of the oscillation circuit due to the increase in the conductance of the detection coil 4 due to the proximity effect, that is, the sum of the negative conductance of the active circuit, the loss of the parallel resonant circuit, and the conductance due to the proximity effect becomes positive, the loss Since becomes positive, the oscillation circuit performs damped oscillation and stops oscillating. Furthermore, if the object to be detected moves away, the conductance due to the proximity effect disappears, so the total conductance of the oscillation circuit 1 becomes negative and it starts oscillating again.

Above, the oscillation amplitude of the oscillation circuit 1 is the level comparison setting value (see Fig. 3) that is preset in the comparison circuit 2.
Vref). Here, the level comparison setting value is selected within a range where the oscillation amplitude is not affected by circuit saturation, etc., but it is usually set to about 1V based on S/N. Therefore, if the amplitude does not reach the comparison level, a high (hereinafter referred to as "H") level output signal is output, assuming that the detected object is close, and if the amplitude exceeds the comparison level, the detected object is detected. It is determined that they are not close, and outputs, for example, a low (hereinafter referred to as "L") level signal. here,
The responsiveness of the proximity switch circuit according to the present invention to a detected object 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, G _T is the loss of the L _C parallel resonant circuit consisting of the detection coil 4 and the capacitor 5, and -G _O is the negative conductance of the active circuit. , G _H is the conductance caused by the proximity of the object to be detected, _IS is the AC current source of the AC current supply circuit 3, and _IN is the noise current source.

Now, consider a situation where the object to be detected, which was in close proximity, momentarily separates. If this time is t = 0, the vibration amplitude V after time t is the amplitude of the component within the oscillation circuit bandwidth of the noise current _{source N} and the AC current source _S.
If _O 2 , _SO 2 , and the frequencies are respectively set as ω _N and ω _S , the equation (1) is obtained.

Here, Ψ _N and Ψ _S are constant phase terms, and ω _O is the oscillation frequency, which is approximately equal to 1/√. The first term in equation (1) is the electromotive force due to the noise current source _N , the second term is the electromotive force due to the alternating current source _S , and the third term is the growth oscillation component. The time required for this vibration amplitude to reach the amplitude comparison level (Vref in Figure 3), that is, the delay time T until the output of the amplitude comparison circuit 2 is inverted after the object to be detected is separated, is Vref≫ _NO /G _O--
If G _T , _SO ／G _O −G _T , then from equation (1), is required.

Now, here, the AC equivalent circuit of a conventional general proximity switch is obtained by removing the AC current source from the AC equivalent circuit of the embodiment of the present invention shown in FIG. Therefore, when comparing a conventional proximity switch circuit (not shown) and a circuit according to an embodiment of the present invention, it is found that in the conventional circuit, the component of the AC current source in equation (2) is
Since there is no _SO ² , the time delay T ₁ is expressed by equation (3).

T ₁ = 2c/G _O −G _T lnVref/ _NO / (G _O − G _T ) ...(3) _NO is due to current noise, _NO / (G _O −
G _T ) is usually about 10 ^-6 to ^{10 -9} V, and Vref in equation (3) is
If the voltage is 1V, T ₁ will be 15 to 20 times the time constant 2c/(G _O −G _T ) of the amplitude growth of the Lc parallel resonant circuit. Next, in the present invention, since the AC amplitude of the current source _S is set so that _SO ≫ _NO , the delay time T ₂
is expressed as T ₂ = 2c/G _O −G _T lnVref/ _SO /(G _O − G _T )…(4). For example, if I _SO /(G _O − G _T ) is 10 mV
If _the alternating current component of the current _{source S} is set so _that T ₂ is approximately 1/3 of the delay T ₁ of the conventional circuit.
This means that it has been shortened to 1/4.

Furthermore, in conventional circuits, the amplitude at the start of oscillation is due to current noise.
The current noise is determined by the amplitude of _N. Since the current noise is random noise and the wave height has Gaussian distribution characteristics, the logarithmic part of equation (3) is distributed around a certain value. In other words, the delay time T ₂ is not a constant value. , has fluctuations, so there are cases where the time delay is as large as 30 to 40 times the time constant of the amplitude growth of the oscillation circuit. Therefore, when detecting the rotational speed using the conventional circuit, it is necessary to determine the maximum usable detected rotational speed in consideration of the width of this delay time. On the other hand, in the case of the present invention, the amplitude _SO of the AC current source is determined by the AC voltage source 15 and the emitter resistor 14, so it can always be kept at a constant value, the delay time is also constant, and no fluctuation occurs at all.

FIG. 3 shows the oscillation waveforms of the conventional circuit and the present invention. In the figure, t is the time axis, a is the close section P or distant section L of the detected object, b is the oscillation waveform of the conventional proximity switch circuit, c is the output waveform of the conventional circuit, and d and e are the main The oscillation waveform and output waveform of the invention are shown. As is clear from the figure, the time delay has been significantly improved from _T1 to _T2 compared to the conventional proximity switch circuit.

As described above, according to the present invention, the drive current of the current mirror circuit driven by the output current of the means for generating a current corresponding to the terminal voltage of the parallel resonant circuit includes an alternating current component having a predetermined frequency and amplitude. By superimposing the current mirror circuit and supplying the combined current to the parallel resonant circuit by the current mirror circuit, the oscillation starts after separation from the object to be detected, and is not dependent on noise and always starts oscillating from a constant amplitude. The time delay from when the output is reversed after separation from the object to be detected is significantly shortened, and the delay time is also constant and there is no fluctuation, making it possible to detect the rotation of rotating objects that rotate at high speeds, and furthermore, when detecting the rotational angle position. This has the effect of providing a proximity switch with high speed and highly stable operation, such as the ability to effectively utilize the output signal when the object to be detected is separated.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a proximity switch showing an embodiment of the present invention, FIG. 2 is an AC equivalent circuit thereof, and FIG. 3 is a waveform diagram illustrating the effects of the present invention and a conventional circuit example. . 1...Oscillation circuit, 2...Comparison circuit, 3...Superimposition circuit.

Claims (1)

[Claims] 1 A current mirror circuit having a first and a second transistor and generating an output current from the second transistor determined by the drive current of the first transistor, and a parallel resonant circuit consisting of a coil and a capacitor. oscillation of an oscillation circuit comprising this voltage-current converting means, the current mirror circuit, and the parallel resonant circuit; A comparison circuit that compares the voltage with a separately provided comparison voltage, and an alternating current supply circuit that superimposes an alternating current component with a predetermined amplitude and frequency on the output current of the voltage-current conversion means that drives the first transistor. Proximity switch circuit.