JP3391010B2 - Proximity switch - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high frequency oscillation type proximity switch, and more particularly to a proximity switch using a positive feedback type oscillation circuit.

[0002]

2. Description of the Related Art FIG. 7 shows a conventional proximity switch using a positive feedback oscillator circuit. In FIG. 7, the constant current circuit 2 outputs a predetermined constant current to the anode of the biasing diode 3. The anode of the biasing diode 3 is connected to the base of the oscillating transistor 4, and the cathode thereof is connected to the resonance circuit 1 in which the detection coil 1A and the capacitor 1B whose impedance changes depending on the proximity of the object to be detected are connected in parallel. To be done. The emitter of the oscillation transistor 4 is connected to the negative power supply terminal 15 via the resistor 5, the collector is connected to the base and collector of the transistor 6A, and is connected to the positive power supply terminal 14 via the transistor 6A. The transistor 6A forms a current mirror circuit 6 together with the transistor 6B, and a current substantially equal to the collector current of the oscillation transistor 4 is positively fed back to the resonance circuit 1. At the base of the oscillation transistor 4,
Level discrimination circuit 1 for detecting whether or not a detected object approaches
2 input terminals are connected, the output terminal of the level discrimination circuit 12 is connected to the base of the output transistor 13, and the emitter of the output transistor 13 is the negative power supply terminal 15
The collector is connected to the signal terminal 16.

The oscillation voltage of the resonance circuit 1 is converted into a current by the oscillation transistor 4 and is positively fed back through the current mirror circuit 6 to continue oscillation. A DC bias voltage is applied to the base of the oscillation transistor 4 by the biasing diode 3, and the oscillation voltage is superimposed around this DC bias voltage. When the object to be detected is not close to the detection coil 1A, the impedance of the detection coil 1A is high and the oscillation voltage applied to the base of the oscillation transistor 4 is high. The level discriminating circuit 12 discriminates that the oscillation voltage is higher than the set level, and at this time, the signal is not output and the output transistor 13 is off. When the object to be detected approaches, the impedance of the detection coil 1A decreases, and the oscillation voltage applied to the base of the oscillation transistor 4 decreases. The level discrimination circuit 12 discriminates that this voltage is lower than the set level and outputs a signal, and the output transistor 13 is turned on. The off or on signal of the output transistor 13 is output from the signal terminal 16 as a detection signal for detecting the approach of the detected object.

However, this proximity switch has a problem that the detection distance changes depending on whether the object to be detected is a magnetic material or a non-magnetic material. FIG. 8 and FIG. 9 show an example of measuring the resonance frequency and impedance of the resonance circuit 1 with respect to the distance between the detection coil and the detected body when the detected body is a magnetic body and a non-magnetic body. In FIG. 8, the resonance frequency of the resonance circuit 1 is maintained at a substantially constant resonance frequency F ₀ with respect to the resonance frequency F ₀ when there is no detection object, even when the magnetic detection object approaches. But
When the non-magnetic object to be detected approaches, its resonance frequency increases. Further, as shown in FIG. 9, the impedance of the resonance circuit 1 rapidly decreases when the magnetic object to be detected approaches the impedance Z ₀ when the object to be detected does not exist, but is nonmagnetic. When the body to be detected approaches, it decreases relatively slowly. Since the magnitude of the impedance of the resonance circuit 1 determines the magnitude of the oscillation voltage applied to the base of the oscillating transistor, if the impedance level of the resonance circuit 1 corresponding to the discrimination level of the level discrimination circuit is Zl, If the impedance of the resonance circuit 1 is lower than Zl, a detection signal for detecting the approach of the object is output. That is, the detection distance L of the non-magnetic material
₁ is different from the detection distance L _{2 of the} magnetic substance, and the detection distance L _{1 in} the case of a non-magnetic substance is shorter than the detection distance L _{2 in} the case of a magnetic substance.

[0005]

In the proximity switch described above, there is a difference in detection distance depending on whether the object to be detected is a magnetic material or a non-magnetic material. This can cause problems in production lines with mixed bodies. An object of the present invention is to reduce the difference in the detection distance that occurs when the object to be detected is a magnetic material and a non-magnetic material.

[0006]

In order to achieve the above-mentioned object, the present invention provides a first resonance circuit including a detection coil whose impedance changes as an object to be detected approaches, and a first resonance circuit of the first resonance circuit. An oscillating transistor for converting a voltage into a current, a first transistor connected to the oscillating transistor, a current mirror circuit together with the first transistor, and the oscillating transistor flowing through the first transistor. A second transistor that positively feeds back the output current to the first resonance circuit, a bias diode connected between the oscillation transistor and the first resonance circuit, and a predetermined constant for the bias diode. A constant current circuit that outputs a current, a biasing diode, and the first resonance circuit are connected in parallel, and both of the first resonance circuit and the first resonance circuit are connected. So that and a second resonant circuit having a resonance frequency lower than the frequency. Alternatively, a first resonance circuit including a detection coil whose impedance changes as an object to be detected approaches, an oscillation transistor that converts the voltage of the first resonance circuit into a current, and the oscillation circuit are connected to the oscillation transistor. Forming a current mirror circuit with the first transistor and the first transistor;
A second transistor that positively feeds back the output current of the oscillation transistor flowing in the first transistor to the first resonance circuit, and a bias connected between the oscillation transistor and the first resonance circuit. Diode for
A constant current circuit that outputs a predetermined constant current to the bias diode, and a detection coil that is connected in parallel with the bias diode and the first resonance circuit and that has an impedance that changes as an object to be detected approaches A second resonance circuit including a second resonance circuit, and including a detection coil of the second resonance circuit when a magnetic detection target and a non-magnetic detection target approach the detection coil of the second resonance circuit. Impeda
Nsu within equal frequency respectively, so as to set the resonance frequency of the first resonant circuit.

[0007]

According to the first aspect of the invention, the second resonance circuit is connected in parallel with the biasing diode and the first resonance circuit and has a resonance frequency lower than the resonance frequency of the first resonance circuit including the detection coil. Since the non-magnetic object to be detected approaches the detection coil of the first resonance circuit,
Since the resonance frequency of the resonance circuit increases, the impedance of the second resonance circuit decreases. Since the impedance of the second resonance circuit is reduced, the oscillation voltage applied to the base of the oscillation transistor is reduced, so that the detection distance is increased. On the other hand, even if the magnetic object to be detected approaches, the first
Since the resonance frequency of the resonance circuit is constant, the detection distance does not change, so that the detection distance can be made substantially unchanged when the detection target is a magnetic substance or a non-magnetic substance.

According to a second aspect of the present invention, in the first aspect of the invention, the first resonance circuit is a resonance circuit having a predetermined resonance frequency, and the second resonance circuit is a resonance circuit including a detection coil. The resonance characteristics of this resonance circuit change so that the resonance frequency increases when a non-magnetic object is approached, and the resonance frequency decreases when a magnetic object is approached. Because of the change, the impedance of the second resonance circuit becomes equal when the magnetic detection target and the non-magnetic detection target approach each other at the frequency at the point where these two characteristics intersect. Therefore, by setting the resonance frequency of the first resonance circuit to be equal to this resonance frequency, the oscillating voltages of the magnetic substance and the non-magnetic substance of the object to be detected become equal to each other, so that the detection distance can be made substantially unchanged. it can.

[0009]

1 is a circuit diagram showing an embodiment of a proximity switch according to the present invention. The embodiment shown in FIG. 1 differs from the conventional example shown in FIG. 7 in that the diode 3A and the resonance circuit 1 (hereinafter referred to as the first resonance circuit 1) of the conventional example shown in FIG. The point is that the second resonance circuit 9 having a resonance frequency F _c slightly lower than the resonance frequency F ₀ of the resonance circuit 1 is connected.

The operation of this proximity switch is shown in FIGS.
This will be described with reference to the characteristic diagram shown in FIG. Figure 3 shows the second resonance circuit
Resonance of the first resonant circuit 1 showing the impedance of the path 9
Frequency is F₀And the resonance frequency F of the second resonance circuit 9
_cIs the resonance frequency F of the first resonance circuit 1.₀Slightly lower than
The impedance of the second resonance circuit 9 at the time of resonance is Z _c
Has become. Here, the magnetic body to be detected is the detection coil
When approaching 1 A, the resonant frequency of the first resonant circuit is
As shown in FIG. 4, the resonance frequency F₀And almost never changes
(FIG. 4 is a characteristic diagram similar to FIG. 8), the second resonance
The impedance of path 9 is constant Z₀Maintained at, but non
When the magnetic object to be detected approaches the detection coil 1A,
The resonance frequency of the first resonance circuit 1 rises as shown in FIG.
For example, the resonance frequency F_dSecond resonant circuit when rising to
The impedance of 9 drops to Zd.

Here, the second resonance circuit 9 includes the diode 3
Since A and the first resonance circuit 1 are connected in parallel,
When the impedance of the second resonance circuit 9 decreases due to the approach of the non-magnetic object to be detected, the oscillation voltage applied to the base of the oscillation transistor 4 decreases, and the detection distance increases. On the other hand, when the magnetic object to be detected approaches,
Since the impedance of the first resonance circuit 1 is maintained constant, the detection distance does not change. Therefore, in FIG. 5 (this FIG. 5 is a characteristic diagram similar to FIG. 9), the detection distance L ₁ when the detection target is a non-magnetic material is long, and the detection distance L ₁ when the detection target is a magnetic material. Approaching ₂ That is, the detection distances when the object to be detected is a magnetic material and when it is a non-magnetic material are changed in the same direction. By appropriately determining the resonance frequency F _c of the second resonance circuit 9 and the impedance Z _c at resonance with respect to the resonance frequency F ₀ of the first resonance circuit 1, it is possible to detect whether the detection target is a magnetic substance or a non-magnetic substance. It is possible to make the detection distance almost unchanged.

FIG. 2 is a circuit diagram showing another embodiment of the proximity switch of the present invention. The embodiment shown in FIG. 2 is different from the embodiment shown in FIG. 1 in that a coil 10A and a capacitor 10C are used instead of the second resonance circuit in which the capacitor 9B is connected in parallel to the coil 9A shown in FIG. The second resonance circuit 10 in which the capacitor 10B is connected in parallel is provided in the series circuit. The operation of this embodiment is the same as the operation of the embodiment shown in FIG. 1, but the capacitor 10C connected in series to the coil 10A is provided.
When the resonance frequency F _c of the resonance circuit 10 and the impedance Z _c at resonance are adjusted, the number of adjustment elements is increased and fine adjustment is facilitated.

Further, in FIG. 1 or 2, instead of the detection coil 1A of the first resonance circuit, the coil 9A of the second resonance circuit 9 or the coil 10 of the second resonance circuit 10 is used.
A can be used as a detection coil whose impedance changes with the approach of the object to be detected (in this case,
The impedance of the detection coil 1A does not change due to the approach of the object to be detected).

FIG. 6 shows a characteristic diagram of the resonance frequency of the second resonance circuit and the impedance of the second resonance circuit in this case. In FIG. 6, when the object to be detected is not close to the coil 9A of the second resonance circuit 9 or the coil 10A of the second resonance circuit 10 (in this case, the coil is a detection coil; hereinafter referred to as the detection coil 9A or 10A) When the non-magnetic object to be detected approaches the detection coil 10A, the resonance frequency changes in a direction in which the resonance frequency increases as shown in the characteristic B. When the magnetic object to be detected approaches, As shown in C, the resonance frequency decreases and the impedance changes in the direction of decreasing. At the frequency Fe at the point where these characteristics A and B intersect, the impedance when the magnetic substance and the non-magnetic substance to be detected approach each other becomes equal to Ze. Therefore, by setting the resonance frequency F ₀ of the first resonance circuit to be equal to this frequency Fe, the oscillating voltages of the magnetic substance and the non-magnetic substance of the detected object become equal, so that the detection distance is substantially unchanged. You can

[0015]

According to the present invention, the detection distance difference between the case where the object to be detected is a magnetic material and the case where the object is a nonmagnetic material is reduced, so that the practical effect is large. Further, the conventional proximity switch is low in cost because it only needs to add some components.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a proximity switch of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the proximity switch of the present invention.

FIG. 3 is a characteristic diagram showing the operation of the proximity switch of the present invention shown in FIG.

FIG. 4 is a characteristic diagram showing changes in the resonance frequency of the first resonance circuit when the magnetic or non-magnetic substance to be detected approaches the detection coil in the proximity switch of the present invention shown in FIG.

5 is a characteristic diagram showing changes in impedance of the first resonance circuit when a magnetic or non-magnetic object to be detected approaches the detection coil in the proximity switch of the present invention shown in FIG.

FIG. 6 is a characteristic diagram showing an operation when a coil of a second resonance circuit is used as a detection coil in place of the detection coil of the first resonance circuit in the proximity switch of the present invention shown in FIG. 1 or 2.

FIG. 7 is a circuit diagram showing an example of a conventional proximity switch.

8 is a characteristic diagram showing a change in resonance frequency of a resonance circuit when a magnetic or non-magnetic object to be detected approaches the detection coil in the conventional proximity switch shown in FIG. 7.

9 is a characteristic diagram showing changes in impedance of a resonance circuit when a magnetic or non-magnetic object to be detected approaches a detection coil in the conventional proximity switch shown in FIG. 7.

[Explanation of symbols]

1 First resonance circuit 1A coil, detection coil 1B capacitor 2 constant current circuit 3 Biasing diode Transistor for oscillation 6 Current mirror circuit 9 Second resonance circuit 9A coil, detection coil 10 Second resonance circuit 10A coil, detection coil

Claims (2)

(57) [Claims] 1. A first resonance circuit including a detection coil whose impedance changes as an object to be detected approaches, an oscillation transistor for converting the voltage of the first resonance circuit into a current, and an oscillation transistor for the oscillation transistor. A second mirror that forms a current mirror circuit with the connected first transistor and the first transistor, and positively feeds back the output current of the oscillation transistor flowing in the first transistor to the first resonance circuit. A transistor, a bias diode connected between the oscillation transistor and the first resonance circuit, a constant current circuit for outputting a predetermined constant current to the bias diode, the bias diode and the first diode. The first resonance circuit is connected in parallel with the first resonance circuit.
And a second resonance circuit having a resonance frequency lower than that of the resonance circuit. 2. A first resonance circuit including a detection coil whose impedance changes when an object to be detected approaches, an oscillating transistor for converting the voltage of the first resonance circuit into a current, and the oscillating transistor. A second mirror that forms a current mirror circuit with the connected first transistor and the first transistor, and positively feeds back the output current of the oscillation transistor flowing in the first transistor to the first resonance circuit. A transistor, a bias diode connected between the oscillation transistor and the first resonance circuit, a constant current circuit for outputting a predetermined constant current to the bias diode, the bias diode and the first diode. A second resonance including a detection coil connected in parallel to the first resonance circuit and having its impedance changed by the approach of the object to be detected. And a road, when the second detected body of the detection object and the non-magnetic material of the magnetic substance in the detection coil of the resonant circuit approaches, the impedance of the second resonant circuit including the detection coil respectively A proximity switch, wherein the resonance frequency of the first resonance circuit is set to be equal.