JPH0510848B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a proximity switch with improved object detection response speed.

Prior Art and Its Problems A high frequency oscillation type proximity switch has a detection coil in its detection head, an oscillation circuit is configured with the detection coil as an oscillation coil, and objects are detected based on a decrease in the oscillation output. This oscillation circuit is the detection coil L
By adjusting the shape, winding value, and other circuit constants, the speed when oscillation stops changes. However, if a nearby object approaches and the conductance of the detection coil increases, the oscillation will eventually stop. Therefore, the response speed of the proximity switch can be considered to be the total time of the oscillation start speed and oscillation stop speed. Generally, an oscillation circuit has a very slow rise (start) speed of oscillation, but a relatively fast stop speed. The time τ for oscillation to start and grow to reach an amplitude level at which an object can be detected is given by the following equation.

τ=2C/Δg×lnVo/Vs…(1) Vo…Output inversion level Vs…Amplitude level at the start of oscillation C…Capacitance of the resonant capacitor Δg…Amount of change in conductance from the oscillation starting point Generally, when oscillation stops, amplitude level at that time
Vs is a noise level, for example, about several mV. When the shape of the detection coil becomes larger, the response speed becomes slower, and there is a problem in that it is not possible to accurately detect the proximity of objects that change rapidly. Therefore, a device equipped with a starting signal generator to provide a starting signal to the oscillation circuit has been proposed (Japanese Patent Application Laid-Open No. 1327/1983), but the starting signal must be matched to the oscillation frequency, resulting in a complicated circuit configuration. It had the disadvantage of becoming.

Furthermore, in an environment such as a resistance welding machine where a large current of tens of thousands of amperes flows and a strong alternating current magnetic field is applied, the ferrite core of the detection coil becomes saturated, the loss of the detection coil increases, and oscillation stops. Therefore, there is a problem in that a high frequency oscillation type proximity switch cannot be used under such an environment.

Purpose of the Invention The present invention has been made in view of the problems of the conventional proximity switch. It is an object of the present invention to provide a proximity switch which can be configured as magnetic field resistant so that it can be used in

Structure and Effects of the Invention The present invention provides a proximity switch having an oscillation circuit and a detection circuit that detects an object by reducing the oscillation output, the oscillation circuit being an oscillation circuit that improves the oscillation gain based on the output from the outside. There is a first rectifier circuit that rectifies the output of the oscillation circuit and smoothes it with a predetermined time constant, and an oscillation gain control signal for the oscillation circuit that rectifies the output of the oscillation circuit and smoothes it with a smaller time constant than the first rectifier circuit. and a comparison circuit that detects a decrease in output by comparing the output of the first rectifier circuit with a reference level and uses it as an object detection signal, and the oscillation circuit has an oscillation gain control. The feature is that the gain is improved based on the signal.

According to the present invention having such features, the output of the oscillation circuit is divided into two circuit parts, one for signal detection and the other for gain adjustment when oscillation decreases, and a rectifier circuit is provided for each, thereby achieving a predetermined discharge. The object detection signal is obtained and the gain is adjusted using the time constant. Therefore, by reducing the time constant for gain adjustment, it is possible to quickly adjust the gain in response to changes in the oscillation amplitude. Also, the usage environment of the proximity switch,
For example, an object detection signal can be output by selecting an appropriate discharge time constant depending on whether noise resistance or magnetic field resistance is required, and an easy-to-use proximity switch can be provided.

DESCRIPTION OF THE EMBODIMENTS FIG. 2 is a circuit diagram showing an embodiment of the proximity switch oscillation circuit according to the present invention. This oscillation circuit is designed to continue oscillating with a small amplitude even after the oscillation output decreases when an object approaches and the object is detected. In this figure, a capacitor C is connected in parallel with a detection coil L provided in front of the proximity switch to form a resonant circuit. And this LC
A current is supplied to the resonant circuit from a constant current source 2 via a power supply 3, and its common connection terminal is applied to a transistor 4 to amplify the current. transistor 4
The collector of is connected to the collector of a PNP transistor 5, and the transistors 5 and 6 form a current mirror circuit CM1. Further, the emitter of the transistor 4 is grounded via a variable resistor 7 that determines the collector current. The collector of transistor 6 is connected to the emitter of transistor 8. The transistor 8 is a multi-collector transistor having four collectors as shown in the figure, and its three collector terminals 8a are commonly connected.
Feedback is provided to the LC resonant circuit. The other collector terminal 8b is connected to the collector-base common terminal of the NPN transistor 9.
Transistor 9 constitutes a current mirror circuit CM2 together with transistor 10, and transistor 1
It is assumed that the emitter of transistor 9 is formed on the IC chip with n times the area of the emitter of transistor 9. The collector of the transistor 10 is connected to the base-collector common connection terminal of the multi-collector transistor 11. The other collector terminal of multi-collector transistor 11 is connected to the LC resonant circuit. Furthermore, the base and collector of transistor 9 are connected to the collector terminal of transistor 12. The transistor 12 is operated by a signal given when the oscillation output increases,
It controls the operation of the current mirror circuit CM2.

FIG. 1 is a block diagram showing the overall structure of a proximity switch according to the present invention. In this figure, the oscillation output of the oscillation circuit 1 shown in FIG.
0,21. Comparison circuits 20, 21
are given reference voltages Vre1 and Vref2 that define different threshold levels, respectively, and convert the input signal into a square wave. Comparison circuit 2
Smoothing capacitors 22 and 23 having different capacitances are connected to the output terminals 0 and 21, respectively. The comparator circuit 20 and the capacitor 22 and the comparator circuit 21 and the capacitor 23 each constitute a rectifier circuit that converts a signal at a predetermined level into a DC level. The comparator circuit 20 detects a decrease in the oscillation output, and its output is further provided to a comparator circuit 24. The comparison circuit 24 discriminates the applied comparison value signal at a predetermined threshold level and outputs an object detection output to the outside via the output circuit 25. On the other hand, the comparison circuit 21 compares the oscillation output with a predetermined level, and the time constant of the smoothing capacitor 23 connected to its output terminal is set small. The smoothed output terminal is configured to be applied to the transistor 12 of the oscillation circuit 1 described above.

Next, the operation of this embodiment will be explained. FIG. 3 is a waveform diagram showing the waveforms of various parts with respect to the distance L between the detection coil and the proximate object according to this embodiment. In this figure, when the adjacent object is sufficiently far away, the coil L is in a state with almost no loss. In the oscillation circuit 1, the voltage of the LC resonant circuit is applied to the transistor 4, where the current is amplified.
A collector current flows through 4. Therefore, a mirror current flows through the other transistor 6 of the current mirror circuit CM1, and the current is divided by the transistor 8. Therefore, approximately 3/4 of the collector current of the transistor 6 is fed back to the LC resonant circuit via the collector terminal 8a of the transistor 8. When the nearby object is far away and the oscillation level is high, an output is obtained from the comparator circuit 21, smoothed by the smoothing capacitor 23,
As shown in 3c, an oscillation gain control signal is applied to the transistor 12 of the oscillation circuit 1. Therefore, transistor 12 is turned on and current mirror circuit CM2
does not operate, and no current is fed back to the LC resonant circuit through transistor 11. Therefore, the current value fed back to the LC resonant circuit is only the current at the collector terminal 8a of the transistor 8.

Now, when an object approaches, the oscillation output of the oscillation circuit 1 decreases rapidly, as shown in FIG. 3a. When the object approaches distance L1, the oscillation output decreases and comparison circuit 2
If the output of 0 becomes less than the reference value level Vref1, the comparison circuit 20 will no longer provide a signal to the comparison circuit 24. Therefore, the comparator circuit 24 detects this and outputs an object detection signal to the circuit 25 as shown in FIG. 3b.
Give it to the outside via. Furthermore, if the object approaches and the oscillation level decreases, the output of the comparison circuit 21 will also decrease as shown in FIG. 3c, and the oscillation gain control signal applied to the transistor 12 will decrease. Therefore, the current mirror circuit CM2 formed by the transistors 9 and 10 gradually becomes active, and the collector current of the collector terminal 8b of the transistor 8 flows into the current mirror circuit CM2.
The multi-collector transistor 11 is driven through. When transistor 12 is completely off, the area of the emitter of transistor 10 is n times larger than that of transistor 9.
The current at the collector terminal 8b of the transistor 8 is amplified by the current mirror circuit CM2. The other collector current of the multi-collector transistor 11 is fed back to the LC resonant circuit. By doing so, the gain of the oscillation circuit 1 is improved, and oscillation can be continued at a minute level even if a nearby object approaches the proximity switch even further, as shown in FIG. 3a.

Since the capacitor 23 connected to the output of the comparison circuit 21 has a small capacitance and a small time constant, it is possible to quickly change the gain in response to fluctuations in the oscillation amplitude. Furthermore, the response speed of object detection can be changed by the capacitance of the capacitor 22 connected to the output of the comparison circuit 20.
That is, as shown in equation (1) above, the oscillation start response time τ depends on the initial state amplitude, and the initial amplitude level
If Vs is high, the oscillation rise time will be significantly shortened. Therefore, as shown in FIG. 3a, by continuing oscillation at a low level even after a nearby object approaches and outputs an object detection output, the rise speed of oscillation can be improved.

Therefore, when used in a place with little noise, the response speed can be improved by reducing the capacitance of the capacitor 22 provided at the output end of the comparator circuit 20 and reducing the smoothing time constant.
Furthermore, if it is necessary to improve the noise resistance characteristics, the influence of noise can be removed by using a capacitor with a relatively large value. Furthermore, the proximity switch can also be used in an environment where a large current flows and a strong alternating current magnetic field is applied, such as in a resistance welding machine. In this case, as the AC magnetic field approaches the zero-crossing point, oscillation is likely to occur, so oscillation starts abruptly. Therefore, for example, when an alternating current magnetic field of 60 Hz is applied, a burst waveform in which oscillation is intermittent at twice that frequency, 120 Hz, is obtained from the oscillation circuit 1. Therefore, the capacitance of the capacitor 22 at the output end of the comparison circuit 20 is further increased to increase the smoothing time constant.
Since the presence or absence of this burst oscillation can be detected by comparing it with a predetermined threshold level, a magnetic field resistant type proximity switch can be constructed.

Next, FIG. 4 is an overall configuration diagram showing another embodiment of the proximity switch according to the present invention. In the figure, the output of the oscillation circuit 1 is given to two rectifier circuits 30 and 31. The rectifier circuit 30 is the same as the comparison circuit 20 described above.
Similarly, the oscillation output is converted into a DC component, and a capacitor 32 having a capacitance selected in accordance with the operating environment of the proximity switch is connected to the output terminal thereof. Also, the output of the rectifier circuit 30 is connected to the comparator circuit 3.
4 and outputted as an object detection signal via the switch output circuit 36. The rectifier circuit 31 converts the oscillation output into a DC component, and has a smoothing capacitor 33 with a small capacity and a small time constant at its output terminal, similar to the capacitor 23 described above.
is connected. The output is converted into a square wave by a comparison circuit 35 having a reference level at a low oscillation amplitude, and the transistor 12 of the oscillation circuit 1 described above is operated based on the decrease in the oscillation output. In this embodiment, the oscillation gain control signal is a binary signal, and the transistor 12 of the oscillation circuit 1 is turned on and off to control the amplitude of the oscillation circuit 1.

Next, FIG. 5 is a circuit diagram showing a second embodiment of the proximity switch oscillation circuit according to the present invention. In this embodiment, the same parts as in FIG. 1 are indicated using the same reference numerals. In this embodiment, an emitter follower circuit is provided before the current amplifying transistor, and the amount of feedback current is varied by changing the emitter resistance. That is, one end of the power supply 3 is applied to the base of the transistor 41. The transistor 41 is connected in an emitter follower type, with its collector connected to a power supply, and two resistors 42 and 43 connected in series to the emitter end and grounded. A series connection body of a resistor 44 and a switching transistor 45 is connected in parallel to both ends of the resistor 43. The base of the current amplifying transistor 4 described above is connected to this common connection end, and the collector thereof is connected to a current mirror circuit consisting of transistors 5 and 6. The output of the comparator circuit 25 or 35 in FIG. 1 or 3 is then applied to the base of the transistor 45. In this oscillation circuit, when the amplitude is large, the transistor 45 becomes conductive, and the transistor 4
The emitter resistor 1 is a series connection of a resistor 42 and parallel resistors 43 and 44. However, if the amplitude decreases and the output from the comparator circuits 25, 35 stops, the transistor 45 turns off, and the resistor 43
A resistor 44 is connected in series to increase the emitter voltage ratio of the transistor 41, thereby increasing the current value of the current amplifying transistor 4. Therefore, the feedback current value provided through the current mirror circuit of transistors 5 and 6 increases, and oscillation can continue at a low level as in the previously described embodiment.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the overall configuration of the proximity switch according to the present invention, FIG. 2 is a circuit diagram showing an embodiment of the oscillation circuit of the proximity switch according to the present invention, and FIG.
4 is a block diagram showing the overall configuration of another embodiment of the proximity circuit according to the present invention. FIG. FIG. 7 is a circuit diagram showing another embodiment. 1...Oscillation circuit, 4-6, 8-12, 41, 4
5...transistor, 20, 21, 24, 34,
35... Comparison circuit, 22, 23, 32, 33...
Capacitor, 25, 36... Output circuit, 30, 3
1... Rectifier circuit, 42-44... Resistor, CM1,
CM2...Current mirror circuit.

Claims (1)

[Claims] 1. In a proximity switch having an oscillation circuit and a detection circuit that detects an object by reducing the oscillation output, the oscillation circuit is an oscillation circuit that improves oscillation gain based on an external output, a first rectifier circuit that rectifies the output of the oscillation circuit and smoothes it with a predetermined time constant; and a first rectifier circuit that rectifies the output of the oscillation circuit and smoothes it with a time constant smaller than that of the first rectifier circuit to obtain an oscillation gain of the oscillation circuit. a second rectifier circuit that provides a control signal; and a comparison circuit that detects a decrease in output by comparing the output of the first rectifier circuit with a reference level and generates an object detection signal, the oscillation circuit is a proximity switch that improves the gain based on the oscillation gain control signal. 2. The second rectifier circuit includes a comparison circuit that provides an output when the output of the oscillation circuit is equal to or higher than a predetermined value;
2. The proximity switch according to claim 1, further comprising a capacitor for smoothing the comparison output. 3. The oscillation circuit is a current feedback type oscillation circuit that includes a switching element that controls a feedback current, and changes the feedback current and oscillation gain according to the output of the second rectifier circuit. A proximity switch according to claim 1. 4. The proximity switch according to claim 1, wherein the oscillation circuit has a resistive voltage divider circuit that determines a feedback current, and changes the oscillation gain depending on the presence or absence of the voltage divider. .