JPH0334256B2 - - Google Patents

Description

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

In a high-frequency oscillation type proximity switch, an oscillation circuit is configured including a detection coil, and if this oscillation circuit is set to oscillate at a high frequency, when a metal object approaches the detection coil, the conductance of this detection coil will change to an equivalent value. oscillation output decreases.
When a metal object approaches further, the oscillation stops and no output is produced. Object detection is performed by detecting changes in this oscillation output.

When a conventional proximity switch is used in a place where there are changes in the magnetic field, an induced voltage is generated in the detection coil under the influence of the magnetic field. Therefore, even if the detection object approaches the detection coil and the oscillation output decreases, the detection circuit mistakenly detects the generated induced voltage as the output voltage of the oscillation circuit, and determines that there is no object even though the object is in the detection area. This may cause malfunction.

In particular, in a sinusoidal magnetic field, even if the detection object approaches the detection coil and the amplitude of oscillation becomes small and the detection circuit no longer senses the output of the oscillation circuit, the induced voltage generated in the detection coil by the external magnetic field remains sine wave. Even if the frequency of the induced voltage is completely different from the oscillation frequency of the oscillation circuit, the detection circuit will mistakenly sense that it is an oscillation state, and even if there is an object, it will judge that there is no object, causing a malfunction.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a proximity switch which is improved so that the above-mentioned malfunctions and malfunctions do not occur, and which operates normally even in a magnetic field.

An embodiment of the present invention will be described below with reference to the drawings. As shown in FIG. 1, an auxiliary coil 12 is arranged coaxially or not coaxially near the detection coil 11, and both coils 11 and 12 are connected in series with their respective winding directions in opposite directions. Connecting. Capacitors 13 and 14 are connected in parallel to both coils 11 and 12, respectively.
These two coils 11 and 12 have the same product of effective area and number of turns. Therefore, when these two coils 11 and 12 are placed in an alternating current magnetic field, the voltages induced in both coils 11 and 12 are equal to each other, and both coils 11 and 12 are connected in the opposite winding direction. Therefore, these induced voltages cancel each other out. Note that both these coils 11,
The high frequency coupling between 12 is kept small. Further, the capacitance of the capacitors 13 and 14 is selected such that the capacitor 14 is much larger than the capacitor 13.

These coils 11, 12, capacitors 13, 1
The circuit No. 4 is connected to an oscillation circuit 21 as shown in FIG. That is, a parallel circuit of coil 11 and capacitor 13 and a parallel circuit of coil 12 and capacitor 14
Each of the parallel circuits constitutes a resonant circuit of the oscillation circuit 21. Here, as mentioned above, since the capacitance of the capacitor 14 is much larger than that of the capacitor 13, the conductance of the resonant circuit of the coil 11 and the capacitor 13 is
is significantly smaller than the conductance of the resonant circuit. Therefore, in a normal case where no external magnetic field exists, this oscillation circuit 21 will oscillate at a resonant frequency determined by the constants of the coil 11 and capacitor 13, and at this time, the impedance of the capacitor 14 at this frequency will be Since the capacitance of the auxiliary coil 12 is extremely large, it is sufficiently low, and the influence of the auxiliary coil 12 on the detection coil 11 can be almost ignored.

It should be noted that within the size of the proximity switch configured by housing these two coils 11 and 12, the actually generated magnetic field can be approximated to a parallel magnetic field, so the two coils 11 and 12 are not necessarily coaxial as described above. There is no need to place it in

In FIG. 2, when a metal object approaches the detection coil 11, the oscillation amplitude of the oscillation circuit 21 changes, this amplitude change is detected by the detection circuit 22, and the output circuit 23 outputs an object detection signal.

Next, the specific structure of this embodiment will be explained based on FIG.
and a main body portion 40 housed in a cylindrical or square cylindrical metal case 41, and both metal cases 31 and 41 are coupled. The metal case 31 includes a detection coil 11, an auxiliary coil 12, capacitors 13, 14, an oscillation circuit 21, and a detection circuit 22.
(See FIG. 2) is mounted on a printed circuit board 32;
A cylindrical or rectangular metal cylinder 33 is housed therein. The metal cylinder 33 covers the circuit section for electrostatic shielding, and is placed between the detection coil 11 and the auxiliary coil 12 to reduce high frequency coupling between the two coils 11 and 12. The reason for reducing this high frequency coupling is as follows. At the oscillation frequency determined by the resonant circuit of the coil 11 and the capacitor 13, the capacitor 1
The impedance of 4 can be regarded as approximately zero. Therefore, when the magnetic flux generated from the detection coil 11 passes through the auxiliary coil 12, eddy current flows through the auxiliary coil 12 through the capacitor 14, causing eddy current loss.In order to reduce this loss, both coils 11, This is because it is necessary to reduce the high frequency coupling between the two. Therefore, this metal cylindrical body 33
The coil is made of aluminum or brass, which has low eddy current loss, and is placed between the coils 11 and 12 to reduce high frequency coupling between the coils 11 and 12. The external magnetic field has a low frequency and the metal cylinder 3
3 acts on both coils 11 and 12 regardless of the presence or absence of the voltage, and generates the same induced voltage as described above. A printed circuit board 42 on which an output circuit 23 (see FIG. 2) is mounted is housed in a metal case 41 of the main body 40, and a cable 43 for power supply and signal output is drawn out from the rear end.

Although not shown in FIG. 3, it is possible to fill each metal case 31, 41 with a synthetic resin and harden it to firmly hold the internal circuit etc. and improve insulation and sealing. preferable. Also,
It is not necessary to provide a wall between the metal cases 31 and 41.

According to the present invention, an auxiliary coil that generates an induced voltage substantially the same as a voltage induced in the detection coil by an external magnetic field is disposed near the detection coil, and the induced voltage generated in the auxiliary coil is the same as the induced voltage of the detection coil. By connecting them in series in opposite winding directions so as to cancel each other out, it is possible to realize a proximity switch that does not malfunction due to induced voltage generated in the detection coil by an external magnetic field. Furthermore, according to the present invention, the capacitance of the second capacitor connected in parallel to the auxiliary coil is made much larger than the capacitance of the first capacitor connected in parallel to the detection coil, so that the oscillation frequency is the same as that of the detection coil. The impedance of the second capacitor at this oscillation frequency is determined by the first capacitor, and since the impedance of the second capacitor at this oscillation frequency is sufficiently low, the parallel circuit of the auxiliary coil and the second capacitor will not have a negative effect on the detection operation by the detection coil. .

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of one embodiment of the present invention, FIG. 2 is a block diagram showing a circuit of the same embodiment, and FIG. 3 is a sectional view showing the structure of the same embodiment. 11...Detection coil, 12...Auxiliary coil, 2
DESCRIPTION OF SYMBOLS 1... Oscillation circuit, 22... Detection circuit, 23... Output circuit, 30... Detection head part, 33... Metal cylinder, 40... Main body part, 31, 41... Metal case.

Claims (1)

[Claims] 1. A detection coil, a first capacitor connected in parallel to the detection coil to form a resonant circuit, and arranged in the vicinity of the detection coil so as to reduce high-frequency coupling with the detection coil, so that the detection coil is connected in parallel to the detection coil to reduce high frequency coupling with the detection coil. an auxiliary coil that generates substantially the same voltage as the voltage induced in the coil; a second capacitor that is connected in parallel to the auxiliary coil to form a resonant circuit and has a capacitance much larger than the capacitance of the first capacitor; A proximity switch comprising an oscillation circuit connected to a circuit in which the detection coil and the auxiliary coil are connected in series in opposite winding directions, and a detection circuit for detecting a change in the output of the oscillation circuit.