JPH0357650B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a high frequency oscillation type proximity switch;
This invention relates to a magnetic field-resistant proximity switch that can be used particularly in strong alternating magnetic fields.

Prior art and its problems A high-frequency oscillation type proximity switch has a detection coil in the detection head, and an oscillation circuit is configured with the detection coil as an oscillation coil, and the object is detected based on the decrease in oscillation output due to the proximity of the object. ing. However, in an environment such as a resistance welder where a large current of tens of thousands of amperes flows and a strong alternating magnetic field is applied, the permeability of the ferrite core of the detection coil becomes saturated and oscillation stops. Therefore, there is a problem in that a high frequency oscillation type proximity switch cannot be used in such an environment.

Purpose of the Invention The present invention solves the problems of conventional proximity switches, and provides a magnetic field-resistant proximity switch that can be used even in environments where strong alternating magnetic fields are applied. be.

Structure and Effects of the Invention The present invention provides a high frequency oscillation type proximity switch having an oscillation circuit and a detection circuit that detects an object by a decrease in the oscillation output, and a comparison circuit that compares the oscillation amplitude with a predetermined level and detects a decrease in the oscillation amplitude. , an oscillation control circuit that changes the resistance value of the oscillation adjustment resistor of the oscillation circuit based on the comparison circuit output to facilitate oscillation, and an oscillation control circuit that discriminates the oscillation output of the oscillation circuit at a predetermined level and when the oscillation is stopped. oscillation output discrimination means for providing an output; an integration circuit that integrates the output of the oscillation output discrimination means and rapidly discharges an integrating capacitor during discharging; and output means that shapes the waveform of the output of the integration circuit and outputs it as a switch output. It is characterized by:

According to the present invention having such characteristics, oscillation can be easily started by changing the resistance value of the adjustment resistor of the oscillation circuit when oscillation is stopped. Therefore, even if a strong alternating current magnetic field is applied to the proximity switch and the oscillation of the proximity switch stops, oscillation occurs only near the zerox point and the oscillation can be continued intermittently. Therefore, it becomes possible to detect an object based on the cessation of intermittent oscillation. Therefore, it is possible to detect an object without malfunction even in a place where a high magnetic field is applied, such as near a welding machine or the like where a large current flows, which was impossible to detect with a conventional proximity switch.

DESCRIPTION OF THE EMBODIMENTS FIG. 1 is a block diagram showing an embodiment of a proximity switch for magnetic field resistance according to the present invention. In this figure, an oscillation circuit 1 is connected to a detection coil 2 and a resistor VR3 for adjusting the oscillation output. resistance
VR3 is a variable resistor that adjusts the detection distance of the proximity switch. The oscillation output of the oscillation circuit 1 is given to the smoothing circuit 4. The smoothing circuit 4 converts the oscillation amplitude into a DC voltage at a level corresponding to the magnitude of the oscillation amplitude, and its output is given to a Schmitt circuit 5 and a comparator 6. The Schmitt circuit 5 discriminates the output of the smoothing circuit 4 and converts it into a square wave, and supplies the output to the output circuit 7. The comparator 6 has one input terminal connected to a reference power source having a voltage Vref, and is a comparator that compares the output of the smoothing circuit 4 with the reference power source, and provides a comparison output to the transistor 8. Transistor 8 is a switching transistor, and as shown in the figure, its emitter is grounded and its collector is connected in parallel to resistor R3 for adjusting the detection distance via resistor R9. It is inserted in parallel. On the other hand, the output circuit 7 outputs an "L" level output based on the output of the Schmitt circuit 5, and its output terminal is connected to the resistor R1.
0 and a capacitor C11. The time constant of the integrating circuit is selected to be equal to or greater than the commercial AC cycle, for example, 20 mS or more. A diode 12 is connected in parallel to both ends of the resistor R10 of the integrating circuit, and is configured to instantly discharge the capacitor C11 when the output of the output circuit 7 drops. The output of this integrating circuit is further connected via a second Schmitt circuit 13 to an output circuit 14. The output of the output circuit 14 is output to the outside as a proximity switch output.

Next, the operation of this proximity switch will be explained with reference to the waveform diagrams of FIGS. 2 and 3. FIG. 2a is a waveform diagram showing the oscillation output of the oscillation circuit 1, which oscillates with a constant intensity when no object exists around the detection coil. At this time, the output circuit 7 outputs an "L" level output due to the output of the Schmitt circuit 5, and the capacitor C11 of the integrating circuit
is not charged. Also, since it is higher than the output reference voltage Vref given to the comparator 6 by the smoothing circuit 4, the transistor 8 is in the off state, and the resistor R
9 is not connected in parallel to the detection distance adjustment resistor VR3. As shown in FIG. 2b, if a nearby object approaches only between times t1 and t3, the oscillation circuit 1 stops oscillating, and the amplitude change is converted into a square wave by the Schmitts circuit 5 and output to the output circuit 7. As shown in Figure 2c, from time t1 to
An output signal that is at the "H" level with a width up to t3 is obtained. This signal is then applied to an integrating circuit comprising a resistor R10 and a capacitor C11, and integration is performed as shown in FIG. 2d. The integral output has a rising edge.
Due to the time constant of CR, the curve gradually rises as shown in the figure, but since it is short-circuited by the diode 12 when the capacitor C is discharged, an integrated output that falls at time t3 is obtained. Since this output is added to the Schmitt circuit 13 and outputted by the output circuit 14, the time is determined as shown in FIG. 2f.
The output of the proximity switch is obtained having a range from time t2, which is slightly later than t1, to time t3.

Now, suppose that a high alternating current magnetic field is applied to the proximity switch at time t4 due to the operation of a welding machine, etc., as shown in Figure 2e, then the magnetic field strength will increase as shown in Figure 3 with an enlarged time axis. If it is large, the magnetic permeability of the detection coil 2 will be saturated and the conductance will rise, resulting in a state similar to that of a close object, and the oscillation will stop rapidly immediately after time t4. At this time, the output of the smoothing circuit 4 is given to the comparator 6, and the transistor 8 is driven by the output of the comparator 6, and a resistor R9 is connected in parallel to the resistor VR3 for adjusting the detection distance.
will be connected. This increases the conductance of the detection coil, making it easier to start oscillation. Therefore, as shown in FIGS. 3a and 3b, as the AC magnetic field approaches the zero-crossing point, oscillation is likely to occur, and oscillation starts rapidly. Therefore, when an alternating current magnetic field of 60 Hz is applied, a burst waveform that oscillates intermittently at 120 Hz, which is twice that frequency, is obtained. The waveform after time t4 in FIG. 2a shows this state. Since this burst waveform is shaped and applied to the output circuit 7, the output circuit 7
As shown in the figure, a signal that goes to L level is obtained every burst oscillation. However, in the integrating circuit, CR is applied to charging.
It takes a certain period of time determined by the time constant of , while the discharge occurs instantaneously, so as shown in FIG. 2d, the integrated output hardly increases and no output is obtained from the output circuit 14. At this time, if a nearby object approaches at time t5, oscillation cannot occur even near the zero cross of the alternating current magnetic field, and the oscillation circuit 1 stops oscillating. Therefore, the output of the output circuit 7 does not fall to the L level,
Since the output of the integrating circuit also rises as shown in FIG. 2d, a detection signal as shown in FIG. 2f is obtained from the output circuit 14. Then, at time t6, when there are no nearby objects, the oscillation circuit 1 will oscillate intermittently again, so
The output of the integrating circuit becomes L level, and no output is generated from the output circuit 14. In this way, in the present invention,
By operating the oscillation circuit at the zero-crossing point of the alternating current magnetic field, it is possible to operate the proximity switch even under a high magnetic field. Then, when the alternating magnetic field is no longer applied at time t7, the oscillation circuit 1 begins to oscillate continuously, and as in the case described above, the oscillation is stopped by a nearby object, making it possible to detect the object.

In this embodiment, the resistance value of the detection distance adjustment resistor is changed based on the stoppage of the oscillation output, but the response speed of the oscillation can be increased by changing the feedback resistance of the oscillation circuit according to the comparison output. It is also possible to configure it using an oscillation circuit.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing one embodiment of the proximity switch according to the present invention, Fig. 2 is a waveform diagram showing the waveforms of each part, and Fig. 3 is an enlarged time axis of the waveforms of each part at the time when an alternating magnetic field is applied. FIG. 1... Oscillator circuit, 2... Detection coil, R3, R
9, R10...Resistor, 4...Smoothing circuit, 5,13
...Schmitt circuit, 6...Comparator, 7,
14... Output circuit, C11... Capacitor, 12
……diode.

Claims (1)

[Scope of Claims] 1. A high frequency oscillation type proximity switch having an oscillation circuit and a detection circuit that detects an object by a decrease in oscillation output, comprising: a comparison circuit that compares the oscillation amplitude with a predetermined level and detects a decrease thereof; an oscillation control circuit that changes the resistance value of an oscillation adjustment resistor of the oscillation circuit based on the output of the comparison circuit to facilitate oscillation; oscillation output discrimination means for providing an output; an integration circuit that integrates the output of the oscillation output discrimination means and rapidly discharges an integrating capacitor during discharge; and output means that shapes the waveform of the output of the integration circuit and outputs it as a switch output. A proximity switch characterized in that it has. 2. The proximity switch according to claim 1, wherein the oscillation control circuit has a resistor connected in parallel to the detection distance adjustment resistor and turned on and off by the output of the comparison circuit. 3. The proximity switch according to claim 1, wherein the integrating circuit is an integrating circuit having an integrating capacitor and a resistor, and a diode for discharging the integrating capacitor is provided at both ends of the resistor.