JPH039650B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a high frequency oscillation type proximity switch.

Generally, this type of proximity switch is constructed as shown in FIG. In this figure, a proximity switch 10 is composed of an IC circuit 1 forming a proximity sensor circuit, and a transistor 20 added because the output current capacity is insufficient with only this IC circuit. This IC circuit has an oscillation circuit 2, a comparator 3, an integration circuit 4, a comparator 5, an output circuit 6, a constant voltage circuit 7, and a power supply reset circuit 8, and includes a detection coil, a resonance coil 16, and a variable sensitivity adjustment circuit. A resistor 12, a bypass capacitor 14, a power supply reset capacitor 15, etc. are attached externally.

The oscillation circuit 2 includes a detection coil 11 and oscillates, and when an object approaches the detection coil 11, the oscillation amplitude is attenuated or the oscillation is stopped. And this change in oscillation amplitude is caused by comparator 3.
The detected output is further integrated by an integrating circuit 4, and this integrated output is compared with a level comparison voltage provided in a comparator 5, and the output state of the output circuit 6 is controlled by the output of the comparator.

The conventional oscillation circuit 2 was constructed as shown in FIG. In this figure, 21 is an LC resonant circuit, 22 is an external variable resistor, 31 is a constant current source, 32 is a level shift circuit, and 33 to 35 are transistors.

The voltage appearing at the upper end of the LC resonant circuit 21 is shifted by the level shift circuit 32 to the operating point which is the active region of the transistor 33, and the transistor 33 is configured as an emitter follower circuit with the resistor 22 as an emitter resistor. ing. Also,
The AC voltage appearing at the upper end of the LC resonant circuit 21
When V _LC is assumed, the alternating current component I _C of the collector current of the transistor 33 becomes I _C =V _LC /R22 (1). This current is transferred to the LC resonant circuit 2 by a current mirror circuit consisting of transistors 34 and 35.
The signal is returned to 1 and oscillated. That is, L.C.
The feedback current to the resonant circuit is expressed by equation (1), and if R22 changes by, for example, 1%, the feedback current also changes by 1%. Needless to say, the proximity switch performs a switching operation by detecting that when the detected object approaches the coil of the LC resonant circuit 21, the loss of the coil increases and the oscillation amplitude becomes smaller, or that the oscillation stops. It's on. Moreover, as a function of the proximity switch, it is necessary to turn the switch on and off when the object to be detected approaches the coil at a certain distance. However, since the coils used in LC resonant circuits vary widely, a method is generally used to adjust the gain on the circuit side in order to adjust the operating distance. Therefore, the resistor R22 is not installed inside the IC, but is an external component. After connecting the LC resonant circuit and the IC, the resistor R22 is trimmed so that the object to be detected is brought close to a predetermined distance and the switching operation is performed. are doing. After that, potting and filling with a filler are performed for assembly, but since the high frequency voltage oscillated by the LC resonant circuit appears at both ends of the resistor R22, the impedance Zi due to stray capacitance due to potting or filling increases. This is not negligible compared to the above, and there is a drawback that the operating distance of the switch deviates from a predetermined distance. Furthermore, this stray capacitance cannot be predicted depending on the material and method of potting or filling, and the behavior of the capacitance of the resonant capacitor due to temperature is also unstable, making the operation of the proximity switch unstable.

The present invention aims to eliminate such conventional drawbacks, and one embodiment of the present invention will be described below with reference to the drawings.

In other words, in FIG. 3, among the transistors 35 and 36 constituting the current mirror, the diode-connected transistor 36 is constructed from a group of n transistors connected in parallel or a transistor having an area ratio n times larger than the transistor 35. The rest of the structure is the same as that shown in FIG. 2, so a description thereof will be omitted.

Therefore, the AC component of the feedback current in Fig. 3
I _R is I _R = V _LC /nR'22 ...(2), and in order to obtain the same feedback current as in equation (1), that is, to obtain the same gain, R'22 = R22 /
Therefore, the value of the external resistor R22 can be small.

Here, in the conventional example and the present invention, it is assumed that impedance Zi due to stray capacitance is added to R22 and R'22.

In the conventional example (Figure 2), R22 = R _X I _R = I _C = V _LC / (1/ _Rx + ₁ /Zi) ^-1 = _{V LC /} R Only I _C changes.

On the other hand, _{in the} present invention ( _{Fig .} 3 ₎ , ^R22 =1/ _nR ), and I _C changes by 1/n・R _X /Zi.

In this way, the change in I _R due to Zi becomes 1/n.

Figure 4 shows a circuit in which a Wilson type current mirror is added and a transistor 37 is added to prevent the large effect of changes in β due to temperature due to the low current amplification factor β of the PNP transistor. There are known circuits that have been devised to compensate for the shortcoming of the small β of the PNP transistor, and it can be seen that the present invention can be easily combined with these circuits.

As described above, in this invention, the current capacity of the mirrored transistor of the current mirror circuit is made larger than the current capacity of the mirrored transistor, so the value of the sensitivity adjustment resistor R22 can be reduced. Therefore, the influence of the impedance of the stray capacitance attached to both ends can be made relatively small. Therefore, even if potting and filling are performed after adjusting the switch sensitive distance, the sensitive distance does not change, and the temperature This has the advantage that the influence of stray capacitance changes due to changes is greatly reduced, and switching operation is stable over temperature.

[Brief explanation of the drawing]

Fig. 1 is a block circuit diagram of a general proximity switch, Fig. 2 is a circuit diagram of a conventional oscillation circuit, Fig. 3 is a circuit diagram of an oscillation circuit of a proximity switch according to the present invention, and Fig. 4 is a circuit diagram of a conventional oscillation circuit. FIG. 2 is a circuit diagram showing an example. 2...Oscillation circuit, 21...LC resonance circuit, 22...Resistance for sensitivity adjustment, 31, 33, 34, 35...Transistor.

Claims (1)

[Claims] 1. An oscillation circuit including a resonant circuit including an oscillation coil and a resonant capacitor, and a change in the oscillation amplitude of the oscillation circuit being guided to an output circuit via an integrating circuit, wherein the oscillation circuit includes a resonant circuit including the oscillation coil and a resonant capacitor. a level shift circuit that level-shifts the voltage of the resonant circuit; a voltage-current conversion circuit that converts the voltage obtained from the level shift circuit into a current; and a current mirror circuit that supplies the resonant circuit, the voltage-current conversion circuit having a resistor that determines a conversion coefficient for converting voltage to current,
A proximity switch characterized in that the current is greater than the feedback current.