JPH0334255B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a proximity switch oscillation circuit that detects the presence or absence of metal in the vicinity.

A conventional proximity switch using this type of oscillation circuit is 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 with a predetermined level and determines the magnitude of the oscillation amplitude, and 3 is an output circuit that amplifies the output of the amplitude comparison circuit and outputs it to the outside. be.

To explain the configuration of the oscillation circuit 1, 4 is a detection coil, 5 is a capacitor that forms a parallel resonant circuit together with the detection coil 4, 6 is a transistor connected as an emitter follower, 7 is an emitter resistor thereof,
Reference numerals 8 and 9 indicate two transistors constituting a current mirror circuit that outputs a current equal to the collector output current of transistor 6, and reference numeral 10 indicates a bias circuit that applies a constant DC base bias to transistor 6.

Next, the operation of the circuit configured in this manner will be explained. When the base potential of transistor 6 rises,
The emitter potential rises, and the current flowing through the emitter resistor 7, that is, the emitter current of the transistor 6 increases. Correspondingly, the collector current increases by an amount approximately equal to the emitter current, and the transistor 6 having the emitter follower configuration serves as a voltage-current conversion circuit that converts the base potential into the collector current. The collector current of transistor 6 is
A current mirror circuit consisting of transistors 8 and 9 is driven, and a current equal to the collector current of transistor 6 is output from transistor 9. Since the base of transistor 6 and the collector of transistor 9 are connected, the base point A of transistor 6
When the potential at point A is increased, a current is output from transistor 9 to point A corresponding to the potential at point A. Therefore, if the impedance of the bias circuit 10 is zero, the admittance expected from the parallel resonant circuit consisting of the detection coil 4 and the capacitor 5 will be negative conductance, and the negative conductance value is approximately equal to the reciprocal of the emitter resistance 7.

Now, since the LC resonant circuit always has a loss,
Even if an oscillating voltage is generated between the terminals due to external noise or noise generated by circuit elements, it will always be attenuated. However, in the above circuit, a negative conductance is connected in parallel to this resonant circuit, so the loss of the resonant circuit, that is, the power consumed by the conductance component, is canceled out by the power supplied by the negative conductance of the circuit. In effect, a resonant circuit with zero loss or negative loss is constructed, and the generated oscillating voltage continues or grows, and the circuit enters an oscillating state. The oscillation amplitude grows until it enters a nonlinear region due to circuit saturation, etc., and is fixed at a constant value due to the nonlinearity of negative conductance.

When the circuit is in an oscillating state as described above, an alternating magnetic field is generated from the detection coil 4 toward the outside.
When a metal is located near the detection coil 4, an alternating magnetic field from the coil 4 passes through the inside of the metal, and this magnetic field induces an eddy current in the metal due to changes in magnetic flux. Power consumption occurs inside. Since all the power consumed in the metal is supplied from the detection coil, from the perspective of the oscillation circuit, the proximity of the metal is equivalent to adding conductance in parallel to the detection coil. Therefore, the total conductance of the oscillation circuit is the negative conductance of the active circuit (-Go) and the conductance corresponding to the loss of the LC resonant circuit itself (G tank).
It is the sum of the conductance (Gmetal) caused by the proximity of metals, and −Go + Gtank +
Becomes Gmetal. When this value is negative, the circuit continues to oscillate, but when it becomes positive, the loss in the oscillation circuit becomes positive, and the circuit stops oscillating while performing damped oscillation.

The conductance (Gmetal) caused by the proximity of metals is a function of the position of the detection coil 4 and the metal to be detected, and its value increases as the distance approaches, so the negative conductance (-Gsens = −Go + Gtenk) smaller than
When there is a metal at the position corresponding to Gmetal, oscillation continues, and when Gmetal becomes larger than Gsens in the vicinity, oscillation stops.

As described above, the function of the oscillation circuit is to set the oscillation state in the absence of metal while the oscillation continues, and to stop the oscillation when metal is present. Sustaining oscillation,
The position of the stop boundary is determined by the above-mentioned −Gsens, so if the loss Gtank of the LC resonant circuit is constant,
By selecting the negative conductance Go of the active circuit, that is, the value of the emitter resistor 7, the oscillation continuation and stop positions can be arbitrarily set.

The conventional proximity switch oscillator circuit is constructed as described above, but the emitter follower-continuous transistor 6 that converts the oscillating voltage of the LC resonant circuit into a current is
During oscillation, there is a large signal operation from the cut-off region to the saturation region, and the emitter output voltage changes from the base input voltage to the base-emitter voltage.
Since V _BE is subtracted, the emitter current varies from almost zero to the maximum current determined by the power supply voltage and emitter resistance. By the way, there is a nonlinear relationship between V _BE and emitter current I _E of a transistor, where V _T is the thermal voltage and I _S is the reverse saturation current, as V _BE = V _T lnI _E /I _S. In the above emitter follower circuit, the relationship between the base input voltage and the emitter output voltage, that is, the emitter output current, is nonlinear, and the collector current is also nonlinear with the base input voltage.Strictly speaking, the voltage-current conversion characteristic of an emitter follower-connected transistor is linear. cannot be obtained. Now, since the current mirror circuit consisting of transistors 8 and 9 outputs a current equal to the collector current of transistor 6, the output current from transistor 9 has a nonlinear relationship with the base potential.
As a result, the admittance looking from the LC resonant circuit to the active circuit side becomes a negative conductance with poor linearity. FIG. 2 is a diagram showing the voltage-current characteristics of the negative conductance of the conventional circuit, and the broken line B in the figure shows the desired linear negative conductance characteristic,
Solid line C is the characteristic of the conventional circuit. In the conventional circuit, linearity is maintained near the operating point (V _Q , I _Q ), but linearity disappears as you move away from the operating point, and the negative conductance (slope of the characteristic curve) becomes smaller. Therefore, in the conventional circuit, as the amplitude of oscillation increases, the effective negative conductance decreases.

Considering actual metal detection, the position of the metal to be detected corresponds to the value Gsens (=Go - Gtank) obtained by subtracting the loss of the resonant circuit from the negative conductance Go of the circuit. Normally, Gsens is about several tens of percent of Gtank, but especially in high-sensitivity areas where distant metals are detected, it becomes a small value of several percent, and a small change in negative conductance Go appears as a large change in Gsens. Therefore, the value of negative conductance Go must always be kept at a stable constant value, otherwise the detection position of the metal will change. The presence or absence of oscillation is usually detected by comparing whether the oscillation amplitude or its detected and smoothed value is higher than a predetermined level, but in conventional circuits, the value of Go changes depending on the oscillation amplitude, so oscillation The value of gsens at that time differs depending on the comparison level of the amplitude comparator 2, and the metal detection position changes depending on the comparison level. FIG. 3 shows this situation.

In Figure 3, the horizontal axis is the oscillation amplitude, the comparison level,
The vertical axis is the conductance, the curve D is the negative conductance of the conventional circuit, the shaded area is the loss of the resonant circuit, a and b are the comparison levels, Goa and Gob are the negative conductance values corresponding to the comparison levels, respectively. Goo is the negative conductance value near the operating point of the circuit. From this figure, Gsens at comparison level a is Goa - Gtank, and comparison level b
Gsens at the time becomes Gob-Gtank, and the value of Gsens at b is smaller than that at a, and oscillation will stop even under the influence of metal located far away.

The nonlinearity of a transistor is highly dependent on the ambient temperature, and as a result, the nonlinearity of the negative conductance of the circuit also changes greatly depending on the temperature. The nonlinear range also depends on the power supply voltage. Therefore, in conventional circuits, the detection conductance remains constant over a wide temperature range and power supply voltage fluctuations.
In order to provide Gsens, the oscillation amplitude comparison level must be adjusted to the nonlinearity of the transistor, which is practically impossible, and is problematic for high-precision position detection and for use in small high-sensitivity regions of Gsens. It was hot.

This invention was made in view of the above circumstances.
It is an object of the present invention to provide an oscillation circuit for improving the nonlinearity of the negative conductance of the oscillation circuit and for configuring a proximity switch having constant detection sensitivity characteristics independent of temperature, power supply voltage, etc.

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 4, 11 is a differential amplifier,
Its output terminal is connected to the base of transistor 6, its inverting input terminal is connected to the emitter of transistor 6, and its non-inverting input terminal is connected to point A.

The operation of an embodiment of the present invention configured as described above will be explained. The oscillating voltage generated in the LC resonant circuit enters the non-inverting input terminal of the differential amplifier 11. If this non-inverting input is Vi, the circuit gain of differential amplifier 11 is Av, and the emitter output of transistor 6 is Ve, then 100% current feedback from the emitter of transistor 6 is applied to the inverting input of the differential amplifier. The emitter output voltage Ve at that time is expressed as Ve=Av/1+AvVi−V _BE /1+Av, and the effect of the base-emitter voltage V _BE of the transistor 6 on the emitter output voltage is as follows.
It is reduced to 1/(A+1), and at the same time its nonlinearity becomes 1/(A+1). Since the voltage gain of the differential amplifier 11 is sufficiently large, the nonlinearity caused by V _BE is improved to the point where it can be ignored in practice, and the emitter voltage of the transistor 6 completely follows Vi. The emitter current of transistor 6 is equal to the emitter potential.
Since it is Ve divided by emitter resistance 7,
Since the voltage changes linearly with respect to Vi, and the collector output current also changes linearly at that time, a voltage-current converter with good linearity with respect to the input voltage Vi is configured. Next, this collector current is faithfully fed back to the inverting input side of the differential amplifier 11 by the current mirror circuit consisting of transistors 8 and 9, so the admittance looking into the active circuit from the LC resonant circuit side becomes a conductance with good linearity. , its conductance value is the reciprocal of the emitter resistance.

The actual metal detection operation is the same as the conventional circuit, so the explanation will be omitted.

Now, in a circuit that eliminates the nonlinearity of negative conductance due to the nonlinearity of V _BE , the linearity of negative conductance, which was maintained only near the operating point in the past, extends over the entire operating range of the differential amplifier 11. Therefore, unlike the conventional circuit, the negative conductance -Go of the circuit does not change depending on the oscillation amplitude. Therefore, the comparison level of the subsequent oscillation amplitude comparator that detects the presence or absence of oscillation does not need to be set strictly, but may be set to an appropriate level within the operating range of the differential amplifier.

FIG. 6 is a diagram showing the characteristics of the present invention, where the horizontal axis shows the distance between the detection coil and the metal, and the vertical axis shows the oscillation amplitude and comparison level. In the figure, E is a curve showing the amplitude characteristics of the conventional circuit, and F is a curve showing the characteristics of an embodiment of the present invention.

When the distance between the detection coil and the metal is short, the equivalent conductance of the metal is larger than Gsens (= Go - Gtatk), as explained above, and the circuit does not oscillate, but when the distance exceeds _L1 ,
As Gmetal becomes smaller, oscillation begins. By the way, in the conventional circuit, as the amplitude increases, Go decreases, that is, Gsens decreases, so the oscillation amplitude is fixed at the point where Gsens = Gmetal, but in the embodiment of the present invention, the change in Go due to the amplitude is Therefore, the oscillation grows to a large amplitude. When the distance is further increased, the amplitude hardly changes in the embodiment, but in the conventional circuit, the oscillation amplitude is determined at the position of Go corresponding to the distance. When detecting the presence or absence of oscillation in such a state of oscillation operation, in the conventional circuit, at the oscillation amplitude comparison levels e1 and e2, ΔL
Errors occur, so the comparison level must always be constant under constant temperature and power supply voltage.When temperature and power supply voltage change, the slope and other aspects of the curve E will change, so ,
It is necessary to move the comparison level. On the other hand,
In the embodiment of the present invention, oscillation and stoppage at distance L1 can always be detected at any comparison level within the range of f. Further, even if the comparison level changes due to changes in temperature or power supply voltage, the detection distance L1 is not affected in any way, so metal detection can be performed with high accuracy and sensitivity over a wide temperature range.

In the above embodiment, the emitter follower stage transistor is composed of one element, but a Darlington transistor or a field effect transistor may be used instead. By using this Darlington transistor or field effect transistor, the difference between the current flowing through the emitter resistance (source resistance) and the collector current (drain current) that drives the current mirror circuit is only a small base current, and the base is grounded. The slight nonlinearity of negative conductance due to the current amplification factor α is also improved.
As described above, according to the present invention, by connecting a differential amplifier before an emitter follower-connected transistor and applying sufficient current feedback, the nonlinearity between the V _BE and the emitter current of the transistor that performs voltage-current conversion can be reduced. Since the non-linearity of the negative conductance due to Adjustment is possible, and it is possible to configure a high-precision, high-sensitivity proximity switch that is independent of temperature and power supply voltage.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a conventional proximity switch, Fig. 2 is a characteristic diagram of the negative conductance of the circuit shown in Fig. 1, Fig. 3 is a diagram showing the relationship between the oscillation amplitude and negative conductance, and Fig. 4 is a diagram showing the relationship between the oscillation amplitude and negative conductance. FIG. 5 is an electrical circuit diagram showing an embodiment of the present invention, and FIG. 5 is an explanatory diagram of the operating characteristics of FIG. 4. In the figure, 1 is an oscillation circuit, 2 is an oscillation amplitude comparison circuit, and 3
4 is an output circuit, 4 is a detection coil, 5 is a resonant capacitor, and 10 is a bias circuit. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A transistor with an emitter follower connection, its emitter resistor, a current mirror circuit that outputs a current corresponding to the collector current of the transistor, and a non-inverting input terminal whose inverting input terminal is connected to the emitter of the transistor. a differential amplifier whose output terminal is connected to the output of the current mirror circuit and whose output terminal is connected to the base of the transistor; a parallel resonant circuit consisting of a detection coil and a capacitor connected to the output terminal of the current mirror circuit; Proximity switch oscillator circuit equipped with a bias circuit to set the DC operating point. 2. The proximity switch oscillation circuit according to claim 1, wherein a Darlington connection transistor or a field effect transistor is used as the emitter follower transistor.