JP2001068985A - Proximity switch - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a large current from flowing to transistors(TRs) in a current mirror circuit on the occurrence of disconnection in a transmission coil. SOLUTION: When a disconnected line detection circuit 42 detects disconnection of a transmission coil 6, a bypass circuit 43 inhibits supply of a bias current from a bias circuit 4 to an oscillation transistor(TR) 11. Thus, when the transmission coil 6 is disconnected and the oscillation of a resonance circuit 8 is stopped, the oscillation TR 11 is turned off to suppress a large current continues to flow to TRs 10a, 11 of a current mirror circuit 13.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a proximity switch which receives a magnetic flux generated by a transmitting coil by a receiving coil and detects that a detected object has approached based on a change in the magnetic flux.

[0002]

2. Description of the Related Art FIG. 4 shows an example of a conventional configuration on the transmitting side of such a high-frequency oscillation type proximity switch.
No. 0116 is disclosed. Power supply side,
A series circuit of a bias circuit 4 including a constant current source 2 and a diode 3, a resistor 5, and a transmission coil 6 is connected between the power supply buses 1a and 1b on the ground side. A resonance capacitor 7 is connected in parallel to the transmission coil 6 to form a resonance circuit 8.

One end of each of resistors 9a and 9b is connected to the power supply bus 1a, and the other end of each of the resistors 9a and 9b is connected to a PNP transistor 10a, 1 whose base is connected in common.
0b of the emitter is connected. Transistor 10a
Are connected to their bases and N
It is connected to the collector of the PN type oscillation transistor 11. The emitter of the transistor 11 is connected to a resistor 12
, And the base is connected to the anode of the diode 3. On the other hand, the collector of the transistor 10b is connected to a common connection point between the resistor 5 and the resonance circuit 8. The resistors 9a and 9b and the transistors 10a and 10b constitute a current mirror circuit 13. The above constitutes the transmission device 14.

FIG. 5 is a functional block diagram schematically showing an electrical configuration including the receiving device 15. Receiver 15
The receiving coil 16 receives the magnetic signal (magnetic flux) transmitted from the transmitting coil 6 of the transmitting device 14, converts the magnetic signal into an electric signal, and outputs the electric signal to the receiving circuit 17. The receiving circuit 17 outputs a received signal corresponding to the amount of magnetic flux received by the receiving coil 16 to the amplifier circuit 18. The amplifier circuit 18 amplifies the given received signal and outputs the amplified signal to the band-pass filter 19. Output to

[0005] The band-pass filter 19 is connected to the amplifier circuit 1.
8 removes unnecessary noise components from the amplified reception signal and outputs the result to the detection circuit 20. The detection circuit 20 compares a given level of the received signal with a preset reference level, and outputs a detection signal when the received level is higher than the reference level.

The proximity switch configured as described above is
For example, consider the case of using as a vehicle sensor in a parking lot. For example, as shown in FIG. 6, the transmitting device 14 and the receiving device 15 are arranged at predetermined intervals on the ground of the parking area 21 of the parking lot. When the vehicle 22 does not exist in the parking area 21, the receiving coil 16 of the receiving device 15 directly receives the magnetic signal transmitted from the transmitting coil 6 of the transmitting device 14 without any intermediary, and the reception level is the reference level. You are below the level.

On the other hand, when the vehicle 22 exists in the parking area 21, a part of the magnetic signal transmitted from the transmission coil 6 is transmitted to the reception coil 16 via the bottom of the vehicle 22 made of metal. Become like Therefore, the total amount of magnetic flux received by the receiving coil 16 increases, and the reception level exceeds the reference level, and the detection circuit 20 outputs a detection signal.

[0008]

When the vehicle 22, which is the detection object, approaches the proximity switch, the impedance of the transmission coil 6 changes greatly. Therefore, when the current flowing through the transmission coil 6 is small, the magnitude of the transmission magnetic flux is large. Change. In such a proximity switch of the magnetic flux transmission / reception system, the detection is performed based on the amount of magnetic flux received by the receiving coil 16, so that when the magnitude of the transmission magnetic flux changes, correct detection cannot be performed. Therefore, it is necessary to supply a large current to the transmission coil 6 so that the magnitude of the transmission magnetic flux does not change even when the impedance of the transmission coil 6 greatly changes when the vehicle 22 approaches.

[0009] In the above state, the wheels of the vehicle 22 may come into contact with the transmitting device 14. Then
Transmission coil 6 arranged inside a case (not shown)
Therefore, it is expected that the transmission coil 6 will be disconnected.

When the transmission coil 6 is disconnected, the oscillation operation of the resonance circuit 8 is stopped. Therefore, the potential at the point A shown in FIG. The bias current (base current) continues to be supplied. Then, a relatively large current continues to flow through the transistors 10a and 11 of the current mirror circuit 13, and there is a problem that the transistors 10a and 11 generate heat.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a proximity switch that can prevent a large current from flowing through a transistor of a current mirror circuit when a transmission coil is disconnected. is there.

[0012]

In order to achieve the above object, a proximity switch according to the present invention comprises a resonance circuit including a transmission coil, and an oscillation transistor for converting an oscillation voltage output by the resonance circuit into a current. A current mirror circuit that feeds a current back to the resonance circuit in accordance with the current converted by the oscillation transistor to perform an oscillation operation; a bias circuit that supplies a bias current to the oscillation transistor; When the object to be detected approaches the receiving coil that receives the magnetic flux,
A detection circuit that outputs an object detection signal when detecting that the magnetic flux received by the reception coil has changed, and a disconnection detection circuit that detects disconnection of the transmission coil based on the stop of the oscillation operation of the resonance circuit. And a current supply inhibiting means for inhibiting supply of a bias current to the oscillation transistor when the disconnection detection circuit detects a disconnection (claim 1).

Specifically, the current supply prohibiting means is configured as a bypass circuit for bypassing a bias current when the disconnection detecting circuit detects a disconnection (claim 2). The disconnection detection circuit is configured as an interruption circuit that interrupts a bias current when the disconnection is detected.

With this configuration, when the disconnection detecting circuit detects the disconnection of the transmission coil and outputs a disconnection detection signal, the current supply prohibiting means includes a bypass circuit (claim 2) or a cutoff circuit (claim 3). For example, the supply of the bias current to the oscillation transistor is prohibited. That is, even if the transmission coil is disconnected and the oscillation operation of the resonance circuit is stopped, the bias current is not supplied to the oscillation transistor, so that a large current is prevented from continuously flowing through the transistor of the current mirror circuit.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described below with reference to FIGS. Note that the same parts as those in FIG. 4 are denoted by the same reference numerals and description thereof is omitted, and only different parts will be described below. A common connection point between the resistor 5 and the transmission coil 6 is an NPN through a capacitor 32 and a resistor 33.
Connected to the base of a transistor 34 of the type.

The base of transistor 34 is connected to power supply bus 1b via a resistor 35, and the emitter is directly connected to power supply bus 1b. The collector of the transistor 34 is connected to the anode of the indicator light 36 composed of an LED, is connected to the power bus 1a via the resistor 37, and is further connected to the power bus 1b via the capacitor 38. Have been.

The cathode of the indicator light 36 is connected to the base of an NPN transistor 39, and is connected to a resistor 4
0 is connected to the power supply bus 1b. The emitter of the transistor 39 is directly connected to the power supply bus 1b, and the collector is connected to the base of the transistor 11 via the diode 41 in the reverse direction.

In the above, a circuit centered on the transistor 34, the indicator light 36 and the capacitor 38 constitutes a disconnection detecting circuit 42, and a circuit centered on the transistor 39 and the diode 41 constitutes a bypass circuit 43. ing. That is, the disconnection detection circuit 4 is added to the conventional transmission device 15.
The configuration in which the transmission device 44 and the bypass circuit (current supply prohibiting means) 43 are added constitutes the transmission device 44 of the present embodiment. still,
The configuration on the receiving device side is exactly the same as the receiving device 15 shown in FIG.

Next, the operation of the present embodiment will be described with reference to FIG. During a normal operation in which the resonance circuit 8 is oscillating, an oscillation voltage waveform output by the resonance circuit 8 is observed at a point A as shown in FIG. Since the oscillation transistor 11 is biased in class B by the bias circuit 4, it is turned on when the polarity of the oscillation voltage amplitude at the point A is positive and the base potential is about 0.7 V or more, and in other cases. Becomes OFF. When the transistor 11 is turned on, the current mirror circuit 13 operates, the current is fed back to the resonance circuit 8, and the oscillation operation is continued. Further, while the transistor 11 is OFF, the current mirror circuit 13 does not operate, and no current is fed back to the resonance circuit 8.

On the other hand, the transistor 3 of the disconnection detection circuit 42
4 performs the ON / OFF operation at a timing slightly delayed from the ON / OFF timing of the oscillation transistor 11 (see FIG. 2B). Since the base side of the transistor 34 forms a differentiating circuit by the capacitor 32 and the resistor 35, the transistor 34 is turned on when the base potential increases due to an AC change component when the resonance voltage polarity of the resonance circuit 8 is positive. Becomes And the capacitor 38
Is charged via the resistor 37 while the transistor 34 is OFF, and the charged charge is discharged when the transistor 34 is ON.

Therefore, the time constants of the resistor 37 and the capacitor 38 are adjusted so that the resonance cycle of the
During the period of (3), the charging potential of the capacitor 38 (potential at point B)
Is set so as not to reach the operating potential (forward voltage) of the indicator light 36, the indicator light 36 maintains the non-conductive state (OFF) during the normal operation as shown in FIGS. It does not light up. When the indicator light 36 is off, the transistor 39 is also turned off as shown in FIG.
F, the bypass circuit 43 does not operate.

Here, it is assumed that the transmission coil 6 is disconnected at time T1. Then, the resonance of the resonance circuit 8 stops, the potential at the point A becomes constant at the charged potential of the capacitor 7, and the AC change component is not applied to the base of the transistor 34, so that the transistor 34 remains OFF. Then, the capacitor 38 continues to be charged, reaches the operating potential of the indicator light 36, and turns on the indicator light 36.
The disconnection of the transmission coil 6 is notified.

When the indicator light 36 is turned on, the transistor 39 is also turned on. Then, the current of the constant current source 2 flows between the collector and the emitter of the transistor 39 having the smallest resistance, does not flow to the base of the oscillation transistor 11, and the transistor 11 is turned off. Therefore, the operation of the current mirror circuit 13 also stops.

As described above, according to the present embodiment, when the disconnection detecting circuit 42 detects the disconnection of the transmitting coil 6, the bypass circuit 43 bypasses the bias current supplied from the bias circuit 4 to thereby control the oscillation transistor. Since the supply of the bias current to the transistor 11 is prohibited, when the transmission coil 6 is disconnected and the oscillation operation of the resonance circuit 8 is stopped, the transistor 11 is turned off and the bias current is not supplied to the oscillation transistor 11. Therefore, it is suppressed that a large current continues to flow through the transistors 10a and 11 of the current mirror circuit 13. Therefore, transistors 10a, 1
Element 1 can be prevented from being destroyed without excessively generating heat.

FIG. 3 shows a second embodiment of the present invention. The same parts as those in the first embodiment are denoted by the same reference numerals, and the description thereof will be omitted. Only the different parts will be described below. In the second embodiment, a PNP transistor 45 is arranged between the power supply bus 1a and the constant current source 2, and the bias circuit 4
A is configured. Further, the diode 41 is omitted, and the collector of the transistor 39 is connected to the power supply bus 1a via the resistor 46 and to the base of the NPN transistor 47.

The emitter of the transistor 47 is the power supply bus 1
The collector is connected to the power supply bus 1a via a series circuit of resistors 48 and 49. The common connection point of the resistors 48 and 49 is the transistor 4
5 bases. The transistor 45,
A circuit centered on 47 and 39 constitutes a cutoff circuit (current supply inhibiting means) 50.

Next, the operation of the second embodiment will be described. When the transistor 39 is off, the base potential increases and the transistor 47 is turned on. Therefore, the base current also flows through the transistor 45 and the transistor 47 is turned on. Therefore, since the transistor 45 is ON during the normal operation, the bias current is supplied from the constant current source 2 to the oscillation transistor 11, and the oscillation operation of the resonance circuit 8 is continued.

On the other hand, when the transmission coil 6 is disconnected, the disconnection detection circuit 42 operates in the same manner as in the first embodiment, the indicator light 36 is turned on, and the transistor 39 is turned on. Then, both the transistors 47 and 45 are turned off, and the constant current source 2
, The bias current supplied to the base of the oscillation transistor 11 is cut off, and the oscillation transistor 11 is turned off.
Becomes Therefore, the operation of the current mirror circuit 13 also stops.

As described above, according to the second embodiment, when the disconnection detecting circuit 42 detects the disconnection of the transmission coil 6, the cutoff circuit 50 sets the path of the bias current supplied from the constant current source 2 by the transistor 45. Since the cutoff is performed, even if the transmission coil 6 is disconnected and the oscillation operation of the resonance circuit 8 is stopped, the bias current is not supplied to the oscillation transistor 11, and a large current continues to flow through the transistors 10a and 11 of the current mirror circuit 13. Is suppressed. Therefore, the same effect as that of the first embodiment can be obtained.

The invention is not limited only to the embodiments described above and shown in the drawings. For example, the object to be detected is not limited to the vehicle 22, and the present invention can be effectively applied to any proximity switch that requires a relatively large current to flow through the transmission coil.

[0031]

The present invention is as described above. When the disconnection detecting circuit detects the disconnection of the transmission coil and outputs a disconnection detection signal, the current supply inhibiting means inhibits the supply of the bias current to the oscillation transistor. (Claim 1).
Specifically, the current supply prohibiting means is configured as a bypass circuit that bypasses a bias current when the disconnection detection circuit detects a disconnection (claim 2), or the current supply prohibition means is configured as a disconnection detection circuit. (3), even if the transmission coil is disconnected and the oscillation operation of the resonance circuit is stopped, the bias current is not supplied to the oscillation transistor even if the transmission coil is disconnected. It is suppressed that a large current continues to flow through the transistor of the mirror circuit. Therefore, the transistor of the current mirror circuit does not generate excessive heat, and the element can be prevented from being destroyed.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an electrical configuration of a transmission device according to a first embodiment of the present invention.

FIG. 2 is a timing chart showing an operation waveform of each unit.

FIG. 3 is a view corresponding to FIG. 1, showing a second embodiment of the present invention.

FIG. 4 is a diagram corresponding to FIG. 1 showing a conventional technique.

FIG. 5 is a functional block diagram showing an electrical configuration including a receiving device side;

FIG. 6 is a diagram illustrating a state where the vehicle sensor is installed in a parking area of a parking lot.

[Explanation of symbols]

Reference numerals 4 and 4A denote bias circuits, 6 denotes a transmission coil, 8 denotes a resonance circuit, 11 denotes an oscillation transistor, 13 denotes a current mirror circuit, 16 denotes a reception coil, 20 denotes a detection circuit, 22 denotes a vehicle (object to be detected), and 42 denotes A disconnection detection circuit 43 is a bypass circuit (current supply prohibiting means), and 50 is a cutoff circuit (current supply prohibiting means).

Claims (3)

[Claims] 1. A resonance circuit including a transmission coil, an oscillation transistor for converting an oscillation voltage output by the resonance circuit into a current, and a resonance circuit according to the current converted by the oscillation transistor. A current mirror circuit that performs an oscillation operation by feeding back a current; a bias circuit that supplies a bias current to the oscillation transistor; a reception coil that receives a magnetic flux generated by the transmission coil; A detection circuit that outputs an object detection signal when detecting that the magnetic flux received by the reception coil has changed, and a disconnection detection circuit that detects a disconnection of the transmission coil based on the stop of the oscillation operation of the resonance circuit. When the disconnection detection circuit detects a disconnection, supply of a bias current to the oscillation transistor is stopped. Proximity switch, characterized by comprising a current supply inhibiting means for stopping. 2. The proximity switch according to claim 1, wherein said current supply inhibiting means is configured as a bypass circuit that bypasses a bias current when the disconnection detection circuit detects a disconnection. 3. The proximity switch according to claim 1, wherein the current supply prohibiting means is configured as a cutoff circuit that cuts off a bias current when the disconnection detection circuit detects a disconnection.