JP4158346B2 - Proximity sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a proximity sensor.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a high-frequency oscillation type is known as a proximity sensor that detects a detected object made of a metal body (conductor), a magnetic body, or the like. This type of proximity sensor is based on the fact that when a metal body comes close to the detection coil constituting the LC resonance circuit, eddy current loss occurs due to electromagnetic induction and the effective resistance (impedance) of the detection coil changes. Yes, this change is taken out as an output of the signal processing circuit.
[0003]
FIG. 6 shows a circuit diagram of a conventional high-frequency oscillation type proximity sensor. The conventional high-frequency oscillation type proximity sensor includes an LC resonance circuit 14 including a parallel circuit of a detection coil 7 and a capacitor 8 grounded at one end, and an LC An npn-type transistor for level shift for level-shifting the amplitude of the oscillation voltage output from the LC resonance circuit 14 by connecting the emitter to the other end of the LC resonance circuit 14 via the terminal 33 and short-circuiting between the collector and the base 2, a signal processing circuit 13 connected to the base of the transistor 2 and inputted with a level-shifted oscillation voltage, a constant current source 6 connected to the collector of the transistor 2, and a feedback current I 1 according to the level-shifted oscillation voltage Is positively fed back to the LC resonance circuit 14 and the feedback resistor 9 for adjusting the feedback current I1 output from the oscillation circuit 26 is level-shifted. The vibration voltage by detecting a signal processing circuit 13 generates an on-off output.
[0004]
The oscillation circuit 26 includes a constant voltage circuit 10 connected to a power supply 40, a pnp type transistors 4 and 5 constituting a current mirror circuit by connecting a constant voltage Vcc output from the constant voltage circuit 10 to each emitter, a base Is connected to the base of the transistor 2, the collector is connected to the collector of the transistor 5, the emitter is connected to the feedback resistor 9, and an npn transistor 3 constituting an emitter follower, and the emitter of the transistor 3 toward the power supply line The diode 11 is connected in the forward direction, and the diode 12 is connected in the forward direction from the GND line of the constant voltage circuit 10 toward the emitter of the transistor 3.
[0005]
The transistor 2, the constant current source 6, and the oscillation circuit 26 form an IC 1 integrated on the same semiconductor chip. The IC 1 includes a power supply terminal 30 connected to the power supply 40, a GND terminal 31 connected to the GND level, A SET terminal 32 connected to the feedback resistor 9 and an LC terminal 33 connected to the LC resonance circuit 14 are provided.
[0006]
Next, the operation of this circuit will be described.
[0007]
An oscillation voltage having a frequency determined by the inductance value of the detection coil 7 and the capacitance value of the capacitor 8 is generated at both ends of the LC resonance circuit 14, and the oscillation is continued by positively feeding back the feedback current I1 from the transistor 4 of the IC1. I am letting. When the amplitude of the oscillation voltage is shifted by the base-emitter voltage Vbe of the transistor 2 that is diode-connected by short-circuiting the emitter and the base, and input to the base of the transistor 3, the amplitude of the oscillation voltage depends on the current input to the base of the transistor 3. The collector current of the transistor 3 flows, and the feedback current I1 is positively fed back to the LC resonance circuit 14 by the action of the current mirror circuit composed of the transistors 4 and 5 according to the collector current. At this time, the transistor 3 is configured as an emitter follower, and the current value of the feedback current I1 is controlled according to the emitter potential of the transistor 3 (the potential of the feedback resistor 9).
[0008]
Here, when a metal body comes close to the detection coil 7, an eddy current loss occurs due to electromagnetic induction action, and the impedance of the detection coil 7 changes, so that the oscillation voltage of the LC resonance circuit 14 decreases, and the signal processing circuit 13 The amplitude of the voltage level-shifted by the input transistor 2 is also reduced, and the proximity of the conductor can be detected by the signal processing circuit 13 switching the output according to the magnitude of the input amplitude.
[0009]
The diodes 11 and 12 are electrostatic protection elements that are generally used in ICs to prevent electrostatic breakdown. When a high voltage due to static electricity or the like is applied to the SET terminal 32, the diodes 11 and 12 correspond to positive and negative applied voltages. The diode 11 or the diode 12 conducts and the applied voltage is clamped to prevent destruction of the IC 1. The diode 11 is generally connected between the power supply potential of the power supply 40 which is the highest potential in the IC 1 and the SET terminal 32. For example, if the output voltage Vcc of the constant voltage circuit 10 is sufficiently stable, It is also possible to connect between the voltage Vcc and the SET terminal 32.
[0010]
However, in the conventional example, the LC terminal 33 is not provided with a diode for electrostatic protection like the SET terminal 32. This is because the oscillation voltage waveform applied to the LC terminal 33 connected to the LC resonance circuit 14 is a sine wave based on the GND potential, so that an electrostatic protection diode connected between the LC terminal 33 and the GND potential is provided. This is because the oscillation voltage is clamped and does not oscillate or becomes an oscillation voltage with a very small amplitude.
[0011]
For the above reasons, in proximity sensor ICs, a diode for electrostatic protection is generally not provided at the LC terminal 33, and instead, countermeasures against static electricity are taken by measures such as increasing the size of the transistor, but the chip area is increased. In addition, adverse effects such as deterioration of frequency characteristics occur.
[0012]
The conventional proximity sensor is used for a pachinko ball detector on a pachinko machine, and FIG. 7A is a view showing a cross section of a pachinko ball passage passage portion of the pachinko ball detector.
[0013]
The detection coil 7 is wound along the coil bobbin 23 around the passage hole 25 through which the pachinko ball B passes, and the detection coil 7 and the coil bobbin 23 are accommodated in a casing 24 made of an insulating material such as a synthetic resin. In this way, when the detection coil 7 is wound around the passage hole 25, the passage of the pachinko ball B can be detected with high sensitivity, and is stable regardless of the location in the passage hole 25 through which the pachinko ball B has passed. Detection can be performed.
[0014]
When the pachinko ball B passes through the passage hole 25, there is a capacitance Cbc between the pachinko ball B and the detection coil 7 due to the opposing metals. Generally, this capacitance Cbc is approximately 5 pF. In order to increase the sensitivity of the detection coil 7, the thickness of the coil bobbin 23 on the side of the passage hole 25 may be reduced, and the distance between the detection coil 7 and the pachinko ball B may be shortened. The value of the capacitance Cbc increases.
[0015]
7 (b) is a pachinko balls B, a capacitance Cbc, and the detection coil 7 is a circuit diagram showing the relationship between the capacitor 8. By the way, the pachinko ball B is charged to a considerably high potential (for example, about 10 KV) due to the influence of friction during polishing. The capacitance Cbc is about 5 pF, and the capacitance C8 of the capacitor 8 is about several hundred pF. In this conventional example, C8 = 500 pF. Here, if the electrostatic potential is 10 KV, a voltage of about 100 V is induced at the LC terminal 33 by voltage division between the electrostatic capacitance Cbc and the electrostatic capacitance C8. The LC terminal 33 is not provided with an electrostatic protection diode for preventing electrostatic breakdown, and the voltage of 100 V is often sufficient to thermally destroy the IC 1. It was the cause of the failure. Further, even if the IC1 is not completely destroyed, there is a possibility that the leakage current of the IC1 gradually increases.
[0016]
Actually, in addition to voltage induction due to electrostatic induction, there is also electromagnetic induction voltage induction due to the passage of a charged pachinko ball B, but it is considered that voltage induction due to electrostatic induction is more dominant.
[0017]
[Problems to be solved by the invention]
As described in the prior art, there is a problem that the IC is destroyed when a voltage induced by the proximity of a charged conductor is applied to the connection terminal of the LC resonance circuit.
[0018]
The present invention has been made in view of the above reasons, and an object of the present invention is to provide a proximity sensor that protects against an induced voltage generated by the proximity of a charged conductor.
[0019]
[Means for Solving the Problems]
The invention according to claim 1 is characterized in that an LC resonant circuit having one end of a parallel circuit of a detection coil and a capacitor grounded, and an emitter connected to the other end of the LC resonant circuit, the amplitude of a voltage output from the LC resonant circuit is leveled. A level-shifting transistor that shifts, and an oscillation circuit that positively feeds back a feedback current corresponding to the amplitude of the level-shifted voltage to the LC resonance circuit, and at least the level-shifting transistor and the oscillation circuit are the same IC Integrated and formed in a circuit, and the IC circuit is between a connection terminal for connecting the LC resonance circuit and a power supply potential of the IC circuit, and between the connection terminal and a ground potential of the IC circuit. Ri Na connecting the electrostatic protection element clamps the voltage to at least one, is connected between the ground potential of the connecting terminal and the IC circuit The electrical protection element is a diode connected in series in three stages so as to be forward from the ground potential of the IC circuit toward the connection terminal, and the forward voltage of the diode connected in series in the three stages is The sum is larger than the amplitude of the voltage output from the LC resonance circuit .
[0020]
According to a second aspect of the present invention, there is provided the amplitude limiting circuit according to the first aspect, wherein the amplitude of the voltage level shifted by the level shift transistor is limited to less than twice the base-emitter voltage of the level shift transistor. It is characterized by that.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0022]
( Reference Example 1 )
FIG. 1 shows a circuit diagram of the proximity sensor of this reference example . The basic configuration and basic operation of the proximity sensor are substantially the same as those of the conventional example shown in FIG. 6, and the electrostatic protection diode 15, diodes 161, 162,. . . , 16n series circuit is provided. In addition, about the same structure as a prior art example, the same code | symbol is attached | subjected and description is abbreviate | omitted.
[0023]
The diode 15 is connected in the forward direction from the LC terminal 33 toward the power supply line of the power supply 40, and the diodes 161, 162,. . . , 16n are connected in the forward direction from the GND line toward the LC terminal 33.
[0024]
The diode 15 conducts when the positive amplitude of the voltage applied to the LC terminal 33 exceeds the sum of the power supply voltage and the forward voltage of the diode 15 to clamp the applied voltage.
[0025]
Diodes 161, 162,. . . 16n, the negative amplitude of the voltage applied to the LC terminal 33 is such that the diodes 161, 162,. . . , 16n, the applied voltage is clamped by conducting when the sum of forward voltages exceeds 16n.
[0026]
Thus, the diode 15, the diodes 161, 162,. . . , 16n to clamp the voltage applied to the LC terminal 33, electrostatic protection can be performed.
[0027]
Further, the diodes 161, 162,. . . , 16n is made larger than the amplitude of the oscillation voltage of the LC resonance circuit 14, so that the oscillation voltage of the LC resonance circuit 14 is not clamped, and the LC resonance circuit 14 performs normal oscillation. It can be carried out.
[0028]
( Embodiment 1 )
FIG. 2 shows a circuit diagram of the proximity sensor of the present embodiment. The basic configuration and basic operation of this proximity sensor are substantially the same as those of the reference example shown in FIG. 1, and the point where the amplitude limiting circuit 21 is provided and the diode for electrostatic protection are connected in three stages of diodes 161, 162 and 163. It is different from the point. In addition, about the same structure as the reference example 1 , the same code | symbol is attached | subjected and description is abbreviate | omitted.
[0029]
The LC resonance circuit 14 performs sinusoidal oscillation. When the amplitude of the oscillation increases and the collector potential of the level shift transistor 2 falls below the voltage −Vbe, the LC resonance circuit 14 advances from the substrate of the IC 1 toward the collector of the transistor 2. A parasitic diode (not shown) existing in the direction is conducted. Therefore, when the amplitude of the oscillation voltage exceeds 2 × Vbe, the forward voltage of the pn junction between the base and the emitter of the transistor 2 and the parasitic diode existing between the substrate of the IC1 and the collector of the transistor 2 in the negative period Since the sum of the pn junction and the forward voltage of the pn junction exceeds the IC1 substrate and the emitter of the transistor 2 and a parasitic current flows, the oscillation does not grow any further and the oscillation is practically performed. The amplitude of the voltage does not exceed 2 × Vbe.
[0030]
However, if the parasitic current flows, the current consumption of the IC 1 may increase, the oscillation waveform may be distorted or unstable, and the operation of the proximity sensor may become unstable.
[0031]
Therefore, in the present embodiment, an amplitude limiting circuit 21 for limiting the amplitude of the voltage level-shifted by the level-shifting transistor 2 to less than 2 × Vbe is provided. The amplitude limiting circuit 21 connects a series circuit of a resistor 19, an npn transistor 18 and a DC voltage source 20 between the voltage Vcc line and the GND line. The base of the transistor 18 is connected to the bases of the transistors 2 and 3. ing. Between the voltage Vcc line and the emitter of the transistor 3 is connected a pnp type transistor 17 whose base is connected to the midpoint of connection between the resistor 19 and the transistor 18.
[0032]
Next, the operation of the amplitude limiting circuit 21 will be described. The voltage Vlim of the DC voltage source 20 is set to a voltage less than 2 × Vbe (for example, about 1.3 × Vbe), and the base potential of the transistor 2 (the oscillation voltage of the LC resonance circuit 14 is level-shifted by the transistor 2). When the voltage) exceeds the voltage Vlim, the transistor 18 is turned on and the transistor 17 is also turned on. A current is passed through the feedback resistor 9 via the transistor 17 to reduce the feedback current I1 and limit the oscillation voltage of the LC resonance circuit 14.
[0033]
Even if the amplitude limiting circuit 21 is provided or not provided, the oscillation voltage of the LC resonant circuit 14 does not exceed 2 × Vbe due to the parasitic current described above, so that the LC terminal 33 and the GND line are not connected. It is sufficient that the series diode for electrostatic protection to be provided is a three-stage connection of the diodes 161, 162, and 163. There is no point in increasing the number of diodes beyond this.
[0034]
( Reference Example 2 )
FIG. 3 shows a circuit diagram of the proximity sensor of this reference example . The basic configuration and basic operation of this proximity sensor are substantially the same as those of Reference Example 1 shown in FIG. 1 except that the electrostatic protection diode is replaced with a series circuit of a Zener diode 22 and a diode 27. In addition, about the same structure as the reference example 1 , the same code | symbol is attached | subjected and description is abbreviate | omitted.
[0035]
The diode 15 is connected in a forward direction from the LC terminal 33 toward the power supply line of the power supply 40, and the Zener diode 22 is a Zener diode of an IC process, and generates a positive voltage on the GND line with respect to the LC terminal 33. The diode 27 is connected in the forward direction toward the LC terminal 33 in series with the Zener diode 22.
[0036]
The diode 15 conducts when the positive amplitude of the voltage applied to the LC terminal 33 exceeds the sum of the power supply voltage and the forward voltage of the diode 15 to clamp the applied voltage.
[0037]
In the series circuit of the Zener diode 22 and the diode 27, the negative amplitude of the voltage applied to the LC terminal 33 exceeds the sum of the Zener voltage (for example, about 5.5V) of the Zener diode 22 and the forward voltage of the diode 27. And the applied voltage is clamped.
[0038]
Thus, electrostatic protection can be performed by clamping the voltage applied to the LC terminal 33 by the series circuit of the diode 15, the Zener diode 22 and the diode 27.
[0039]
Furthermore, by making the Zener voltage of the Zener diode 22 larger than the amplitude of the oscillation voltage of the LC resonance circuit 14, the LC resonance circuit 14 performs normal oscillation without clamping the oscillation voltage of the LC resonance circuit 14. be able to.
[0040]
( Reference Example 3 )
4 and 5 show circuit diagrams of the proximity sensor of this reference example . The basic configuration and basic operation of this proximity sensor are substantially the same as those of Reference Example 1 shown in FIG. 1, and only one of a diode 15 and a series circuit of diodes 161 to 16n is used as an electrostatic protection diode. The prepared point is different. In addition, about the same structure as the reference example 1 , the same code | symbol is attached | subjected and description is abbreviate | omitted.
[0041]
This reference example is effective when the polarity with which the pachinko ball B is charged is clearly known. That is, static electricity is generated due to friction between the pachinko ball B, which is a metal body, and plastic, but the polarity of electric charges generated by the material such as plastic is determined. For example, if it is a friction between glass, nylon, polycarbonate and pachinko ball B, pachinko ball B is charged with a negative polarity, and if it is a friction between Teflon, vinyl chloride, polyethylene, polypropylene and pachinko ball B, pachinko ball B is Charge to positive polarity.
[0042]
When the pachinko ball B is charged with a positive polarity, as shown in FIG. 4, if the diode 15 connected in the forward direction from the LC terminal 33 toward the power supply line of the power supply 40 is provided, the LC terminal 33 is provided. The positive voltage applied to can be clamped to protect IC1.
[0043]
When the pachinko ball B is negatively charged, the diodes 161, 162,... Connected in the forward direction from the GND line toward the LC terminal 33 as shown in FIG. . . , 16n, the negative voltage applied to the LC terminal 33 can be clamped to protect the IC1.
[0044]
Therefore, if it is known in advance that this proximity sensor can be attached to a specific part, or if the material of the pachinko machine is decided, electrostatic protection for one polarity is sufficient, and only electrostatic protection for one polarity is possible. This eliminates the need to arrange extra elements on the chip of IC1.
[0045]
From now on, as IC design rules become finer, electrostatic protection for preventing electrostatic breakdown becomes more important. If ICs with advanced design rules are not equipped with electrostatic protection elements, the static electricity that is charged to the human body when assembling or installing the proximity sensor, regardless of voltage induction by charged pachinko balls May cause electrostatic breakdown. However, the danger can be eliminated by providing a diode for electrostatic protection as described in Embodiment 1 and Reference Examples 1 to 3 .
[0046]
【The invention's effect】
The invention according to claim 1 is characterized in that an LC resonant circuit having one end of a parallel circuit of a detection coil and a capacitor grounded, and an emitter connected to the other end of the LC resonant circuit, the amplitude of a voltage output from the LC resonant circuit is leveled. A level-shifting transistor that shifts, and an oscillation circuit that positively feeds back a feedback current corresponding to the amplitude of the level-shifted voltage to the LC resonance circuit, and at least the level-shifting transistor and the oscillation circuit are the same IC Integrated and formed in a circuit, and the IC circuit is between a connection terminal for connecting the LC resonance circuit and a power supply potential of the IC circuit, and between the connection terminal and a ground potential of the IC circuit. Ri Na connecting the electrostatic protection element clamps the voltage to at least one, is connected between the ground potential of the connecting terminal and the IC circuit The electrical protection element is a diode connected in series in three stages so as to be forward from the ground potential of the IC circuit toward the connection terminal, and the forward voltage of the diode connected in series in the three stages is Since the sum is larger than the amplitude of the voltage output from the LC resonance circuit, it is possible to prevent the destruction of the IC due to the voltage induced by the proximity of the charged conductor, or the voltage due to the static electricity of the human body during assembly and attachment. effective. In addition, the voltage output from the LC resonance circuit is not clamped by an electrostatic protection element composed of diodes connected in series in three stages, and the LC resonance circuit can oscillate normally and has the least effect. There is also an effect that it can be obtained with a number of diodes.
[0047]
The invention of claim 2 is the invention of claim 1, comprising an amplitude limiting circuit for limiting the amplitude of the voltage level shifted by the level shift transistors, to less than twice the base-emitter voltage of the level shift transistor Therefore, there is an effect that it is possible to prevent malfunction caused by conduction of the parasitic diode of the level shift transistor, and to prevent increase in current consumption and unstable operation of the detector.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing Reference Example 1 of the present invention.
FIG. 2 is a circuit diagram showing Embodiment 1 of the present invention.
FIG. 3 is a circuit diagram showing Reference Example 2 of the present invention.
FIG. 4 is a first circuit diagram showing Reference Example 3 of the present invention.
FIG. 5 is a second circuit diagram showing Reference Example 3 of the present invention.
FIG. 6 is a circuit diagram showing a conventional example.
FIG. 7A is a view showing a cross section of a pachinko ball passage passage portion;
(B) It is a circuit diagram which shows the relationship between the pachinko ball and the LC resonance circuit.
[Explanation of symbols]
1 IC
2 Transistor for level shift 14 LC resonance circuit 15 Diode 161, 162,. . . , 16n Diode 26 Oscillator circuit

Claims (2)

An LC resonance circuit having one end of a parallel circuit of a detection coil and a capacitor grounded, and a level shift transistor for connecting the emitter to the other end of the LC resonance circuit and level-shifting the amplitude of the voltage output from the LC resonance circuit; An oscillation circuit that positively feeds back a feedback current according to the amplitude of the level shifted voltage to the LC resonance circuit, and at least the level shift transistor and the oscillation circuit are integrated and formed in the same IC circuit, The IC circuit applies a voltage to at least one of a connection terminal for connecting the LC resonance circuit and a power supply potential of the IC circuit, and a connection potential between the connection terminal and the ground potential of the IC circuit. Ri Na connecting the electrostatic protection element which clamps, electrostatic protection element connected between the ground potential of the connecting terminal and the IC circuit A diode connected in series in three stages so as to be forward from the ground potential of the IC circuit toward the connection terminal, and the sum of forward voltages of the diodes connected in series in the three stages is the LC resonance circuit A proximity sensor characterized in that it is larger than the amplitude of the voltage output from . 2. The proximity sensor according to claim 1, further comprising an amplitude limiting circuit that limits an amplitude of a voltage level-shifted by the level-shifting transistor to less than twice a base-emitter voltage of the level-shifting transistor.

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