JP2550620B2 - High frequency oscillation type proximity switch - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a high-frequency oscillation type proximity switch with improved response speed for object detection.

[Prior art and its problems]

(Prior Art) Conventionally, an oscillating circuit as shown in, for example, FIG. 10 is used for a proximity switch of a high frequency oscillation type. In this oscillation circuit, the collector of one transistor 1 of the current mirror circuit 3 composed of transistors 1 and 2 is connected to a resonance circuit composed of a coil L and a capacitor C1. The emitters of the transistors 1 and 2 have emitter resistors R1 and R2 having the same resistance value connected to the power supply terminal. A resistor R3, diodes 4, 5 and a resistor R4 are further connected to the resonance circuit from the power source end. The collector of the transistor 2 is connected to the bases of the transistors 1 and 2, and is grounded via the transistor 6 and the resistor R5. A transistor 7 is provided at the connection point between the resistor R3 and the diode 4.
The base of is connected. The transistor 7 is an emitter follower type transistor having an emitter resistance R6, and its output is given to the base of the transistor 6. The oscillation output from the emitter of the transistor 7 is detected by the transistor 8 connected to the emitter follower, and the output is given to the comparator 9. A predetermined threshold level is set in the comparator 9, and an object detection signal is output from the output circuit 10 to the outside when the threshold level is exceeded.

In such a conventional high frequency oscillation type proximity switch, the current flowing in the resonance circuit is converted into a voltage by the transistor 7, and the emitter follower output is fed back to the transistor 6. Therefore, the current flowing through the transistor 6 is positively fed back to the resonance circuit via the current mirror circuit 3 to continue the oscillation. Then, as shown in FIGS. 11 and 12, when an object approaches the coil L of the resonance circuit and the distance l to the object decreases, the conductance g _{a of the} resonance circuit increases (g _a = g
_a1 ), the conductance g _{a of the} resonant circuit becomes smaller if there is no object (g _a = g _a2 ). If the change in the conductance g _a of this resonance circuit becomes larger than the negative conductance g _x0 of the oscillation circuit that is determined by the reciprocal of the resistance R5, the oscillation stops, and if it becomes smaller than this conductance g _x0 , the oscillation occurs. Can be detected.

(Problems to be Solved by the Invention) In such a conventional proximity switch, as shown in FIG. 12, oscillation is stopped when an object approaches, and oscillation is restarted when the object moves away. Therefore, the response speed of the proximity switch can be considered to be the total time of the oscillation start speed and the oscillation stop speed. In general, the oscillation circuit has a very slow rising (starting) speed and a relatively high stopping speed. Therefore, there is a problem that the response speed of the proximity switch is limited by the oscillation start speed.

Therefore, for example, in Japanese Utility Model Laid-Open No. 60-145742, an oscillation circuit is proposed in which the amplitude of the oscillation output is discriminated at a predetermined level, and when the oscillation output decreases, a switching circuit for amplitude adjustment is operated to continue oscillation. ing. With such an oscillation circuit, oscillation can be continued when an object approaches, but since the resistance value of the oscillation state control resistor of the resonance circuit is switched by switching operation, oscillation can be continued with a constant amplitude. Instead, the amplitude changes depending on the position of the object. Further, there is a problem that it takes a predetermined time to start the oscillation from the oscillation state in which the level is relatively low, and the response speed may not be so fast.

[Object of the Invention]

The present invention has been made in view of the problems of the conventional high-frequency oscillation type proximity switch, and it is a technical object to keep oscillation at a constant level even when an object is in proximity.

[Constitution and effect of the invention]

(Means for Solving the Problems) The present invention relates to a resonance circuit, a current mirror circuit whose first terminal is connected to the resonance circuit, and a second terminal of the current mirror circuit, which responds to an oscillation output. A first feedback circuit for positively feeding back a positive feedback current from the first terminal of the current mirror circuit to the resonant circuit by causing a current value to flow in the second terminal of the current mirror circuit, A first detection circuit for detecting the oscillation output of the oscillation circuit, a comparator for detecting an object by comparing the detection output of the first detection circuit with a predetermined level, and an object detection signal by the comparison output of the comparator A high frequency oscillation type proximity switch having an output circuit for outputting a current mirror circuit connected to the second terminal of the current mirror circuit as a resistor for controlling the conductance of the oscillation circuit,
A voltage control resistor circuit that continuously changes its resistance value according to a control voltage, and an oscillation output of an oscillation circuit is converted into a voltage signal and is supplied to a second terminal of the current mirror circuit, so that A second feedback circuit for positively feeding back a positive feedback current from a terminal to the resonance circuit, a second detection circuit for detecting the oscillation output of the oscillation circuit to obtain a DC voltage corresponding to the amplitude, and the second detection circuit A small-amplitude oscillation control circuit having a transistor to which the detection output of the circuit is applied to the base, which is inverted and amplified to give the output as a control voltage to the voltage controlled resistance circuit, is attached to the oscillation circuit. .

(Operation) According to the present invention having such a feature, an oscillation circuit having a resonance circuit that is current-driven by a current mirror circuit and a first feedback circuit that positively feeds back to the resonance circuit via the current mirror circuit is provided. A first detection circuit for converting the oscillation output into a voltage, a comparator, and an output circuit are provided. A small amplitude oscillation control circuit is connected in parallel with this oscillation circuit. The small-amplitude oscillation control circuit detects the amplitude level of oscillation by the second detection circuit and converts it into a voltage, which is inverted and amplified by the transistor to control the feedback current of the second feedback circuit, so that the detected voltage is When it becomes larger, the feedback current is reduced. Therefore, the conductance of the oscillating circuit with the small-amplitude oscillation control circuit follows the change in the conductance of the resonance circuit corresponding to the approach of the object, and the oscillating operation with a constant small amplitude even when the object is approaching. Can continue. When the object moves away, the small-amplitude oscillation control circuit stops operating, and its amplitude changes based on the position of the object. Then, the oscillating output is detected, and the amplitude level is discriminated by the comparator in which a certain threshold value is set, so that an object in proximity is detected.

(Effects of the Invention) Therefore, according to the present invention, when an object is in proximity, the amplitude of the oscillation circuit hardly changes regardless of its position, and can be maintained at a substantially constant low level. When the object moves away, the oscillation sharply rises and the amplitude can be increased to a large amplitude level in an extremely short time. As described above, according to the present invention, the oscillation can be continued at a constant level even when an object is approaching with a simple configuration in which a small-amplitude oscillation control circuit is added to the oscillation circuit, and the response speed of object detection is improved. You can In the present invention, the first detection circuit for detecting an object, the first feedback circuit, the second detection circuit used for continuing oscillation with a small amplitude, and the second feedback circuit are independent, During large-amplitude oscillation in which the amplitude oscillation control circuit is not operating, the oscillation circuit can be either class A or class B operation, and the degree of freedom in circuit selection can be increased.

[Explanation of Example]

(Structure of Embodiment) FIG. 1 is a circuit diagram showing a proximity switch having an oscillation circuit which operates in class B according to the first embodiment of the present invention. In this figure, the same parts as those of the conventional example are denoted by the same reference numerals. Also in this embodiment, the current mirror circuit 3 including the transistors 1 and 2 is provided in the resonance circuit including the oscillation coil L and the capacitor C1.
Is connected, and its oscillation output is converted into a voltage signal by the transistor 7 connected in the emitter follower type, which is further given to the base of the transistor 6 and positively fed back to the resonance circuit via the current mirror circuit 3 by its collector current. There is. Here, the resistors R3 to R6, the diodes 4 and 5, and the transistors 6 and 7 form a first feedback circuit 11 which controls the positive feedback current of the current mirror circuit 3 by the oscillation output. Also current mirror circuit 3
Uses the collector of the transistor 1 as the first terminal and the collector of the transistor 2 as the second terminal. The voltage across resistor R6 is applied to the base of transistor 8. The transistor 8 has a resistor R at the emitter as in the conventional example described above.
7, R8 is connected and operates as an emitter follower. A capacitor C2 is connected in parallel with the resistor R8 to form a first detection circuit 12 for detecting the oscillation output and converting it to a DC level according to its amplitude. The output of the detection circuit 12 is given to the comparator 9 in which a predetermined threshold level is set. The comparator 9 detects the proximity of an object when the voltage changes to a level lower than the threshold level,
The output is output to the outside as an object detection output via the output circuit 10.

As shown in the figure, the oscillator circuit according to this embodiment is the conventional oscillator circuit to which a small-amplitude oscillation control circuit 20 shown by a chain line is added. The small amplitude oscillation control circuit 20 is powered by Vcc
Between the resistor and the hot end of the resonant circuit as well as resistors R3 to R4
R9, diodes 21, 22 and resistor R10 are connected in series, and similarly to the transistor 7, the collector of the transistor 2 is connected to the power supply terminal, and the base is connected to the common connection point of the resistor R9 and the diode 21. Further, a series connection body of the transistor 24, the resistor R11 and the FET 25 is connected between the collector of the transistor 2 and the ground terminal. The transistor 23 is an emitter follower-connected transistor having an emitter resistor R12, and its output is given to the bases of the transistor 24 and the transistor 26. The transistor 26 has a collector connected to the power supply terminal, an emitter connected to a series circuit including a resistor R13 and a capacitor C3, and a resistor R14 connected in parallel with the capacitor C3.
Is connected. At the common connection point of resistors R13 and R14, resistor R15
The base of the transistor 27 is connected via. The transistor 27 has an emitter grounded and a collector connected to a power supply terminal via a resistor R16, and supplies the collector output to the FET 25 as a voltage control signal V _G. Here, the resistor R11 and the FET 25 constitute a voltage control resistor circuit 28 that changes the resistance value according to a control voltage, and the resistors R9 to R12, the diodes 21, 22 and the transistors 23, 24 and the voltage control resistor circuit 26 are current mirror circuits. A second feedback circuit 29 for positively feeding back the current of 1 by the oscillation output is configured. Further, the transistor 26, the resistors R13 and R14 and the capacitor C3 constitute a second detection circuit 30 which detects the oscillation output and obtains a DC voltage proportional to the amplitude. Then, the output is given as the control voltage of the voltage controlled resistance circuit 28 by the transistor 27. The small-amplitude oscillation control circuit 20 added to these oscillating circuits supplies a positive feedback voltage corresponding to the amplitude to the FET 25 when an object approaches and the conductance of the resonant circuit becomes large. It is intended to continue the oscillation.

Next, the operation of this embodiment will be described. When this oscillator circuit is operated, the current flowing through the resonance circuit is the first feedback circuit.
It is converted into a voltage signal by the transistor 7 of 11, and the voltage signal flows through the transistor 6 to one transistor 2 of the current mirror circuit 3. Similarly, the current flowing in the resonance circuit is converted into a voltage signal by the transistor 13 of the second feedback circuit 29, and the voltage is applied to the base of the transistor 24. This voltage is also applied to the base of transistor 26, and the capacitor is held only while transistor 26 is on.
C3 is charged. Therefore transistor 26 and capacitor
C3 operates as a kind of detection circuit, and a DC voltage corresponding to the amplitude of the high frequency is obtained across the capacitor C3. When the detected voltage V _d is small and is 0.6 V or less, the voltage applied to the transistor 27 is also 0.6 V or less. Therefore, the transistor 27 is off when the oscillation of the resonance circuit is stopped or when the amplitude is very small, and the FE
The control voltage V _G given to T25 has a high value equal to the power supply voltage. Therefore, the resistance value of the voltage controlled resistance circuit 28 is the resistance R11.
It is a value determined only by. Therefore, when a slight high-frequency voltage flows in the resonance circuit, the current positive feedback is performed with a high amplification factor and is fed back to the resonance circuit. Here, the control voltage V _G for the detection voltage V _d detected by the transistor 26 and obtained in the capacitor C3 is
As shown in the figure, it is equal to the power supply voltage Vcc below 0.6V,
If it is not higher than that, the base current starts to flow through the transistor 27 and drops sharply to a level close to zero. Then, the combined resistance R _x of the voltage control resistance circuit 28 changes as shown in FIG. 3 corresponding to the detection voltage V _d . Therefore, the conductance g _x of the voltage control resistor circuit 28 is the combined resistance R _x shown in FIG.
Therefore, as shown in FIG. 4, the FET 25 rapidly increases below the detection voltage V _do at which the FET 25 is turned off, and the detection voltage changes to zero when V _do becomes larger. The collector current flowing out of the transistor 2 of the current mirror circuit 3 is the sum of the currents flowing in the first feedback circuit 11 connected in parallel with the small amplitude oscillation control circuit 20 and also includes the conductance component due to the resistor R5. The negative conductance g _x ′ of the oscillator circuit is the sum of the conductance g _x and the reciprocal of the resistor R5, that is, Becomes FIG. 5 shows the relationship between the negative conductance g _x ′ of the oscillator circuit and the detected voltage V _d .

Actually, the conductance g _a of the resonance circuit changes as shown by the broken line in FIG. 6 as the object approaches. At a position where the object is close to the coil L, the conductance g _{a of the} resonance circuit, that is, the loss of the coil is large and it is difficult to oscillate, but if there is noise in the resonance circuit, it is high amplification as described above. It is amplified at a rate and positive feedback is applied to the resonant circuit. Therefore, the oscillation changes so that the rising amplitude increases, but if the amplitude increases, the detection voltage V _d
Becomes larger and the transistor 27 gradually turns from the off state to the on state. Therefore, the control voltage V _G decreases and FE
The resistance value of T25 increases. Therefore, in reality, the negative conductance g _x ′ of the oscillator circuit, which is a combination of the voltage control resistor circuit 28 and the original negative resistance value of the oscillator circuit, follows the conductance g _{a of the} resonance circuit. It changes as shown. Therefore, as shown in FIG. 7, if the distance l from the proximity switch to the object is shorter than the object detection distance l _on , the amplitude of the oscillation circuit becomes constant and the detection voltage V _d becomes constant regardless of the position of the object. Even if the object comes close to the coil, the oscillation continues at a low level. Here, if the value of the resistor R11 is decreased, oscillation continues at a low constant amplitude until the object contacts the detection coil L, but if the resistance value of the resistor R11 is increased, a certain position as shown by the broken line in FIG. The oscillation will stop at. In FIG. 7, Vref represents the threshold level of the comparator 9. Thus, in this embodiment, the oscillation is continued even when the object is extremely close to the detection distance. Then, when the object moves away from the l _on point, the conductance g _{a of the} resonant circuit becomes a fixed negative conductance of the oscillator circuit.
When it becomes smaller than g _xon ′, the negative conductance g _x of the small amplitude oscillation control circuit 20 becomes zero and the amplitude cannot be controlled to a constant value. As a result, the amplitude value suddenly increases and enters the class B operating region. Therefore, FIG. 8 (a), (b)
When the object moves to a point at a position l ₁ closer than the detection distance l _on and a point at a distance l ₂ farther than l _on from each other in an extremely short time, the conductance g of the resonant circuit is
The changes in _a1 and g _a2 and the negative conductance g _xon ′ of the oscillation circuit are as shown in the figure. Here, V _d1 and V _{d2 in} FIG. 8B indicate amplitude values at the distances l ₁ and l ₂ .
For this reason, the oscillation amplitude sharply increases as the object moves away, but since the oscillation rises from a certain level, its rise is extremely fast and high-speed response can be realized.

(Explanation of Second Embodiment) In the above-described first embodiment, the class A operation is performed at the start of oscillation, and the oscillation circuit is switched to the class B operation when the oscillation amplitude sharply increases as the object moves away. The temperature characteristics of the circuit can be improved by performing all of the class A operations. FIG. 9 is a circuit diagram showing a configuration of a proximity switch having such an oscillation circuit. In this figure, the same parts as those in the above-described embodiment are designated by the same reference numerals. This embodiment has a small-amplitude oscillation control circuit 20 similar to that of the above-mentioned embodiment, and the first feedback circuit 11 'of the oscillation circuit has a different configuration. That is, the hot end of the resonance circuit is connected to the common connection point of the resistors R17 and R18 via the capacitor C4. The resistors R17 and R18 are voltage divider circuits that divide the power supply voltage, and their connection points are connected to the transistor 31. The bias voltage of oscillation is arbitrarily determined by this divided voltage. The transistor 31 is an emitter follower type transistor having a collector connected to a power source and having an emitter resistor R19, and its emitter end is connected to the base of the transistor 6. The terminal voltage of the resistor R19 is applied to the first detection circuit 12 similar to that of the above-mentioned embodiment. The first detection circuit 12 detects the emitter voltage of the transistor 31, and its output is given to the comparator 9 similar to that of the above-described embodiment. The comparator 9 compares a predetermined threshold level Vref with the input signal, and its output is given to the output circuit. In this embodiment, the above-mentioned first
Different from the embodiment, a transistor corresponding to the transistor 7
The base voltage of 31 is arbitrarily selected by the resistors R17 and R18, only the high frequency signal is input to the transistor 31 via the capacitor C4, and only the AC component is amplified at an arbitrary bias point. Therefore, by selecting the base voltage of the transistor 31 to be, for example, 1/2 of the power supply voltage, the class A operation can always be performed even if the amplitude becomes large, and the temperature characteristic of the proximity switch can be improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a high frequency oscillation type proximity switch according to an embodiment of the present invention, FIG. 2 is a graph showing characteristics of a control voltage V _G of a voltage control circuit with respect to a detection voltage V _d , and FIG. 3 is a detection voltage.
Graph showing the combined resistance R _x of the voltage controlled resistor circuit for V _d, Figure 4 is a graph showing the conductance g _x of the voltage controlled resistor circuit for detection voltage V _d, Fig. 5 negative oscillation circuit for detection voltage V _d A graph showing the sexual conductance g _x ′,
FIG. 6 is a graph showing the conductance g _a of the resonant circuit and the negative conductance g _x ′ of the oscillator circuit with respect to the distance 1 from the proximity switch to the object, and FIG. 7 shows the change of the detection voltage V _d with the distance 1 to the object. FIG. 8 is a time chart showing the conductance g _a of the resonance circuit, the conductance g _x ′ of the oscillation circuit, and the oscillation amplitude with respect to the position of the object, and FIG. 9 is a time chart of the proximity switch according to the second embodiment of the present application. It is a circuit diagram. Further, FIG. 10 is a circuit diagram showing an example of a conventional high-frequency oscillation type proximity switch, FIG. 11 is a graph showing a change in oscillation amplitude with respect to an object distance of the conventional proximity switch, and FIG. 12 is an object of the conventional proximity switch. 3 is a graph showing changes in the conductance g _a of the resonance circuit and the conductance g _xo of the oscillation circuit and changes in the oscillation amplitude with respect to the position of. L: coil, 1,2,6 to 8,23,24,26,27,31 ... transistor, 3 ... current mirror circuit, 9 ... comparator, 10 ... output circuit, 11 '... th 1 feedback circuit, 12 ... first detection circuit, 20 ... small amplitude oscillation control circuit, 25 ... FET, 28 ... voltage control resistance circuit, 29 ... second feedback circuit, 30 ... second Detection circuit

Claims (2)

(57) [Claims] 1. A resonance circuit, a current mirror circuit having a first terminal connected to the resonance circuit, and a second terminal of the current mirror circuit, and a current value according to an oscillation output is supplied to the current mirror circuit. An oscillation circuit having a first feedback circuit for positively feeding back a positive feedback current from the first terminal of the current mirror circuit to the resonance circuit by flowing the current to the second terminal of the oscillation circuit, and an oscillation output of the oscillation circuit. A first detection circuit for detecting, a comparator for detecting an object by comparing the detection output of the first detection circuit with a predetermined level, and an output circuit for outputting an object detection signal by the comparison output of the comparator In the high-frequency oscillation type proximity switch having ,, a resistance is connected to the second terminal of the current mirror circuit as a resistance for controlling the conductance of the oscillation circuit, and the resistance value is continuously changed according to the control voltage. A voltage control resistor circuit for converting the oscillation output of the oscillation circuit into a voltage signal and flowing the voltage signal to the second terminal of the current mirror circuit, so that a positive feedback current from the first terminal of the current mirror circuit to the resonance circuit. A second feedback circuit for inflowing and positively feeding, a second detection circuit for detecting an oscillation output of the oscillation circuit to obtain a DC voltage corresponding to the amplitude, and a detection output of the second detection circuit are given to the base. A high-frequency oscillation-type proximity switch characterized in that a small-amplitude oscillation control circuit having a transistor for inverting and amplifying its output and giving it as a control voltage to the voltage control resistor circuit is provided together with the oscillation circuit. 2. The voltage controlled resistance circuit has a field effect transistor.
High frequency oscillation type proximity switch for machine.