JPS59929B2 - proximity switch circuit - Google Patents

Description

[Detailed description of the invention] This invention relates to proximity switch circuits.

When a metal object approaches a detection coil through which an alternating current is passed, an eddy current is generated in the metal object, reducing the inductance of the detection coil and increasing loss resistance.

Proximity switch circuits utilize this principle.

A part of the inductance of the LO oscillation circuit is formed by a detection coil, and the oscillation circuit or oscillation is started or stopped according to changes in the inductance or loss resistance of the detection coil, and the metal body is detected.

This type of proximity switch circuit has conventionally been constructed using discrete elements.

Therefore, attempts have been made to use oscillation circuits with as few parts as possible to reduce the size.

For example, Bartley oscillation circuits and Colpitts oscillation circuits were used.

Incidentally, in recent years, there has been a demand for further miniaturization of proximity switch circuits, and there has also been a demand for simpler wiring work from the viewpoint of cost.

Therefore, there is a demand for a proximity switch circuit to be integrated into an IC.

In this case, it is possible to use a Bartley oscillation circuit or a Colpitts oscillation circuit, as in the case of configuring it with discrete elements, but ■ The performance of the oscillation circuit depends on a single transistor, so there will be large variations ■ Detection coil The problem arises that the external wiring becomes complicated because the oscillation capacitor becomes three terminals (Bartley oscillation circuit) or two oscillation capacitors (Colpitts oscillation circuit).

The advantage of using ICs is that complex circuits can be easily realized, so there is no need to sacrifice performance in order to reduce the number of parts.

In a proximity switch circuit, the oscillation frequency of the oscillation circuit is high, and the oscillation circuit is kept close to its oscillation limit. In other words, the characteristics of the amplifier circuit that makes up the oscillation circuit, such as the amplification factor and feedback factor, are stabilized. It is important to reliably start or stop oscillation in response to changes in the characteristics of the detection coil.

It is also important that the oscillation circuit be functionally simple.

For example, using only one LC resonant circuit.

The oscillation frequency can be set using this resonant circuit.

In other words, it is possible to configure various proximity switch circuits by simply changing the resonant circuit (it is versatile).

Oscillation should be stopped and started only in response to changes in the inductance and loss resistance of the detection coil.

It can be adjusted by changing the resistance.

When integrated into an IC, it is important to satisfy the above requirements and realize a higher quality proximity switch circuit, even if the configuration becomes somewhat complicated.

Furthermore, when implementing an IC, the use of coupling capacitors is restricted and each circuit is directly connected.

Therefore, it is desired to stabilize the DC level of the oscillation circuit.

This is because fluctuations in the DC level may lead to malfunction or failure to detect.

In view of the above, this invention is ideal for IC implementation, can maximize the various advantages of IC implementation, can achieve high quality, and can reduce hysteresis (operating distance and return distance) regardless of the resonant circuit. The object of the present invention is to provide a proximity switch circuit that allows easy adjustment of the distance difference.

One embodiment of this invention will be described below.

In FIG. 1, an IC circuit 1 includes an amplifier circuit 11, an AC amplitude level detection circuit 12, and a switching circuit 13.

A detection coil 21, a capacitor 22, and a variable resistor 3 are attached to this IC circuit 1 as external components.

The amplifier circuit 11 includes an emitter follower transistor 111.
113, a common base transistor 112, an attenuator transistor 114, a reference voltage source 115, and the like.

This amplifier circuit 11 uses emitter follower transistors 111 and 113 in the input stage and the output stage, thereby achieving high input impedance and low output impedance.

Amplification is mainly performed by the common base transistor 112, and its amplification factor is determined by the ratio of the resistance value of the resistor 116 and the resistance value of the resistor 117.

In IC circuits, the variation in resistance value is as large as 30% (in the case of a base diffused resistor), but the variation in the ratio is small.

Therefore, stable amplification gain can be obtained.

The level detection circuit 12 includes comparators 121 and 123 and an integrating circuit 122.

The output (oscillation output) from the amplifier circuit 11 is the converter 1
21, it is sliced at a predetermined level, and then the integration circuit 1
At 22, it is converted to direct current.

This DC signal is level-discriminated by a comparator 123 and sent to the switching circuit 13 and the base of the above-mentioned attenuator transistor 114.

The output of the switching circuit 13 is sent to the outside via the output terminal 02.

Note that the terminal 01 is a power supply terminal, and the terminal 03 is a common terminal.

The detection coil 21 and the capacitor 22 are a parallel resonant circuit 2
is formed.

This resonant circuit 2 is connected to the bases of transistors 112 and 111 via input terminals 04 and 05.

One end of the variable resistor 3 is connected to the feedback terminal 06, and the other end is connected to the input terminal 05.

The resonant circuit 2 and resistor 3 connected in this way, and the amplifier circuit 11 constitute an oscillation circuit as shown isometrically in FIG.

The oscillation frequency of this oscillation circuit is approximately equal to the resonant frequency of the resonant circuit 2.

Therefore, by changing the detection coil 21, the oscillation frequency can be changed.

Note that since the common base transistor 112 has good high frequency characteristics, high frequency oscillation can be easily realized.

Further, when the amplification factor of the amplifier circuit 11 is A1, the resonance point impedance of the resonant circuit 2 is Zo, and the resistance O value of the variable resistor 3 is Rp, the feedback factor β is Zo+Rp, and the oscillation start condition is as follows.

Therefore, adjustment can be performed using the variable resistor 3.

Further, as described above, the amplifier circuit 11 has high input impedance and low output impedance.

Since the input impedance is high, the resonance point impedance of the resonance circuit 2 does not decrease.

Also, since the output impedance is low, the output will not fluctuate due to the influence of the connected load.

Therefore, a stable feedback rate can be achieved.

Furthermore, in this oscillation circuit, transistors 111°112
The base of the sensor is directly connected to the detection coil 21 in a direct current manner.

Therefore, the base potential of the transistor 111 fluctuates in parallel to the base potential of the transistor 112 in a DC manner.

Therefore, even if the voltage of the reference voltage source 115 (comprised of a resistive voltage divider circuit and a constant voltage element, for example) changes, the DC level of the oscillation circuit remains stable.

This is due to the following reason.

For example, if the voltage of the reference voltage source 115 increases slightly, the collector potential of the transistor 112 decreases, and the emitter potential of the transistor 113 decreases.

This change is fed back to the base of transistor 111 through resistor 119 and variable resistor 3, but transistor 1
This is because the base of transistor 11 and the base of transistor 112 are DC-coupled through detection coil 21, and as a result negative feedback is provided to reference voltage source 115.

Also, in this circuit, the attenuator transistor 1
14, the approach position (operating distance) of the metal body corresponding to the on (or off) of the proximity switch circuit,
An appropriate difference (the difference between the operating distance and the return distance is called hysteresis) can be set between the separated position (return distance) of the metal body corresponding to the off (or on) state.

That is, when the metal bodies are spaced apart and the resonance point impedance of the detection coil 21 is large, the oscillation output of the oscillation circuit is large and the output of the level detector 12 is "L".

Therefore, the transistor 114 is off, and the amplification factor of the amplifier circuit 11 is A.

On the other hand, once the metal object approaches and the output of the level detection circuit 12 becomes "H", the transistor 114 is turned on and the amplification factor of the amplifier circuit 11 becomes A-ΔA.

Therefore, if the metal bodies move apart after that, a hysteresis of ΔZo will occur in the resonance point impedance.

This △Zo is calculated from the above oscillation condition formula. is given by

Here, by setting A>>△A-1 It can be done.

That is, a constant hysteresis ratio ΔZo/Z equal to ΔA/A for the resonant circuit 2 of any resonance point impedance Zo.

can be obtained.

Therefore, no matter what type of proximity switch (such as a high-sensitivity type or a low-sensitivity type resonant circuit 2), this hysteresis MD (operating distance and difference in return distance) is obtained, and resonant circuit 2
There is no need to make adjustments accordingly.

In addition, if you particularly want to adjust the hysteresis MD, use △A/A.
This adjustment can be made by simply changing .

In this way, since hysteresis is provided in the resonance point impedance, the device will not oscillate or stop due to external noise or a slight drift in the circuit element constants.

Note that the present invention is not limited to the above-mentioned one embodiment.

For example, as shown in FIG. 3, in addition to the detection coil 41 constituting the resonance circuit 4, a detection coil 6 connected in series to a variable resistor 5 is provided.

The detection coils 41 and 6 may be housed in the legs 71 and 72 of the groove-shaped head 7, respectively, as shown in FIG.

In this way, when the metal object passes between the legs 71 and 72, the mutual inductance between the detection coils 41 and 6 changes, making it possible to detect this metal object.

In this case, non-magnetic materials such as aluminum can also be easily detected.

As described above, the proximity switch circuit according to the present invention has the first
a first emitter follower transistor 111 whose base is connected to the input terminal 05; a common base transistor 112 whose base is connected to the second input terminal 04 and the reference voltage source 115; and both transistors 111, 11.
a common emitter resistor 116 connecting the emitters of the two emitters, and a second emitter follower transistor 11 connecting its base to the collector of the common base transistor 112.
3 and an attenuator transistor 114 whose collector is connected to the emitter of the second emitter follower transistor 113 via a plurality of series-connected resistors 119 and 120; and the plurality of resistors 119. The IC circuit 1 includes a circuit 12 that opens and closes the attenuator transistor 114 according to the output taken out from the connection point 120, and a capacitor 22 and a capacitor 22 are connected between the first input terminal 05 and the second input terminal 04. A resonant circuit 2 consisting of a parallel circuit of a detection coil 21 is connected, and an output appearing at a feedback terminal 06 connected to the connection point of the plurality of resistors 119 and 120 is connected to the first input terminal 05 via a resistor 3. The amplification factor of the amplification circuit 11 is changed by feeding back and causing the amplification circuit 11 to oscillate, and opening and closing the attenuator transistor 114 according to the oscillation amplitude.

Therefore, no matter what type of proximity switch circuit is configured, such as changing the resonant circuit 2 to a high-sensitivity type or a low-sensitivity type, the change in amplification factor is equal to the change in the amplification factor depending on whether the attenuator transistor 114 is turned on or off. A ratio of the hysteresis can be obtained, and if the hysteresis is the same, there is no need to make adjustments according to the resonance circuit 2.

Further, especially when it is desired to adjust the hysteresis, the adjustment is simple because it is only necessary to change the ratio of the amplification factor between when the attenuator transistor 114 is on and when it is off.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, Fig. 2 is a block diagram for explaining Fig. 1, Fig. 3 is a circuit diagram showing a modified example, and Fig. 4 is an explanation of Fig. 3. FIG. 1...IC circuit, 11...amplifier circuit,
12... AC amplitude level detection circuit, 13...
... Switching circuit, 2, 4 ... Resonance circuit, 21, 41, 6 ... Detection coil, 22, 42
... Capacitor, 3, 5 ... Variable resistor, 7 ... Groove head.

Claims (1)

[Claims] 1 a first emitter follower transistor having its base connected to a first input terminal; a common base transistor having its base connected to a second input terminal and a reference voltage source; and a common emitter resistor connecting the emitters of both said transistors. , a second emitter follower transistor whose base is connected to the collector of the common base transistor; and an attenuator transistor whose collector is connected to the emitter of the second emitter follower transistor via a plurality of resistors connected in series. and a circuit that opens and closes the attenuator transistor according to the output taken out from the connection point of the plurality of resistors, and a capacitor between the first and second input terminals. A resonant circuit consisting of a parallel circuit of a detection coil and a detection coil is connected, and an output appearing at a feedback terminal connected to a connection point of the plurality of resistors is fed back to the first input terminal via a resistor to operate the amplifier circuit. oscillate,
A proximity switch circuit characterized in that the amplification factor of the amplifier circuit is changed by opening and closing the attenuator transistor according to the oscillation amplitude.