JP2002014174A - Capacitance sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable stable oscillation of an oscillating circuit, while miniaturizing a detection electrode, by ensuring that a Schmitt trigger inverter circuit surely follows the changes in voltage caused by the charge and discharge of a detection electrode. SOLUTION: At high level, the detection electrode 16 is charged with the output of the Schmitt circuit 14 via a feedback resistance 15, causing a rise in the voltage of the detection electrode 16. At low level, the output of the Schmitt circuit 14 is discharged from the detection electrode 16 via the feedback resistance 15, resulting in the decrease in the voltage of the detection electrode 16. In this case, a voltage-dividing means 17 divides and imparts the voltage of the detection electrode 16 to the Schmitt circuit 14, so that the upper limit threshold of the Schmitt circuit 14 is raised and lowering the lower limit threshold of the circuit in virtual sense. This results in the enlargement of the hysteresis of the Schmitt circuit 14 in virtual sense and a decrease in the oscillating frequency of the oscillating circuit 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the presence / absence of an object to be detected based on the oscillation state of an oscillation circuit which oscillates in response to a voltage change caused by charging / discharging of a detection electrode which generates a capacitance with the object to be detected. The present invention relates to a capacitance sensor that detects

[0002]

Conventionally, as this type of capacitance sensor, there is one disclosed in Japanese Patent No. 3044938. As shown in FIG. 10, a detection electrode 1 approaching an object to be detected and a detection electrode 1
Oscillating circuit 4 comprising a Schmitt trigger / inverter circuit (hereinafter abbreviated as Schmitt circuit) 2 for receiving an output voltage from the inverter and a feedback resistor 3 interposed between an output terminal from the Schmitt circuit 2 and the detection electrode 1. Is mainly composed.

FIG. 11A shows an oscillation state of the oscillation circuit 4.
In the state where the output from the Schmitt circuit 2 is at a high level, the output terminal of the Schmitt circuit 2
, The detection electrode 1 is charged, so that the voltage of the detection electrode 1 increases. When the voltage of the detection electrode 1 exceeds the upper threshold set in the Schmitt circuit 2, the output of the Schmitt circuit 2 is inverted from a high level to a low level. Then, since the detection electrode 1 is discharged to the output terminal of the Schmitt circuit 2 via the feedback resistor 3, the voltage of the detection electrode 1 decreases. When the voltage of the detection electrode 1 falls below the lower threshold set in the Schmitt circuit 2, the Schmitt circuit 2
Is inverted from low level to high level. As a result of the above operations being repeated, the oscillation operation of the oscillation circuit 4 is performed.

When an object to be detected approaches the detection electrode 1, the capacitance of the detection electrode 1 increases, so that the oscillation frequency of the oscillation circuit 4 decreases as shown in FIG. Thus, the presence or absence of the detected object or the distance to the object can be detected based on the oscillation frequency of the oscillation circuit 4. That is, the oscillation frequency f in the oscillation circuit 4 is
It is determined based on the product (time constant CR) of the capacitance C of the detection electrode 1 and the resistance value R of the feedback resistor 3.

More specifically, the capacitance C of the detection electrode 1 is, specifically, the dielectric constant ε of a substance (air) interposed between the detection electrode 1 and an object to be detected, the electrode area S of the detection electrode 1, the detection electrode 1 From the distance dx to the object to be detected as follows. C = ε (S / dx) However, the dielectric constant ε of the detection electrode 1 and the electrode area S of the detection electrode 1 are determined by the material and shape of the detection electrode 1 and are fixed values.

Therefore, according to the above equation, when the distance dx from the detection electrode 1 to the object to be detected changes, the capacitance C of the detection electrode 1 changes. The presence or absence of an object or the distance to the detected object can be obtained.

In recent years, downsizing of the sensor has been demanded, and downsizing of the capacitance sensor is desired. However, downsizing of the capacitance sensor means that the electrode area of the detection electrode 1 is small. Since S is to be reduced, the capacitance C of the detection electrode 1 is reduced. Therefore, when the resistance value R of the feedback resistor 3 is fixed, the smaller the capacitance C of the detection electrode 1 is, the higher the oscillation frequency of the oscillation circuit 4 is.

However, when the oscillation frequency of the oscillation circuit 4 increases, the Schmitt circuit 2 cannot follow the voltage change due to the charging and discharging of the detection electrode 1, and as a result, the oscillation of the oscillation circuit 4 becomes unstable, and There is a problem that accurate detection cannot be performed.

In this case, the resistance value R of the feedback resistor 3 is increased by an amount corresponding to the decrease in the capacitance C of the detection electrode 1 so that the Schmitt circuit 2 can follow a voltage change caused by charging and discharging of the detection electrode 1. But the feedback resistor 3
If the resistance R of the Schmitt circuit 2 becomes excessively large, the resistance R of the feedback resistor 3 approaches the input impedance of the Schmitt circuit 2, so that the voltage of the sensing electrode 1 is divided by the input impedance of the Schmitt circuit 2 and the feedback resistance 3. The Schmitt circuit 2 will not be fed back enough,
There is a possibility that the oscillation circuit 4 may be unable to oscillate.

For the reasons described above, the oscillation circuit 4 of the electrostatic capacity sensor allows the Schmitt circuit 2 to follow the voltage change due to the charging and discharging of the detection electrode 1 without adjusting the resistance value R of the feedback resistor 3. In practice, it is difficult to reduce the size of the capacitance sensor when the capacitance C of the detection electrode 1 is reduced as a result of such restrictions.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to invert the output of a Schmitt trigger inverter circuit in response to a voltage change due to charging and discharging of a detection electrode, thereby making the oscillation circuit oscillate. In this configuration, the Schmitt trigger / inverter circuit ensures that the oscillation circuit stably oscillates by following the voltage change due to the charging and discharging of the detection electrode while miniaturizing the detection electrode. To provide.

[0012]

According to the present invention, there is provided an oscillation circuit whose oscillation state changes in accordance with the capacitance generated between a sensing electrode and an object to be detected, and based on the oscillation state of the oscillation circuit. In the capacitance sensor that detects the presence or absence of the detected object, the oscillation circuit switches the output from a high level to a low level when the input voltage exceeds the upper threshold, and when the input voltage falls below the lower threshold. Schmitt trigger to switch output from low level to high level
An inverter circuit, a feedback resistor interposed between the output of the Schmitt trigger inverter circuit and the detection electrode,
Voltage dividing means for applying the voltage of the detection electrode to the Schmitt trigger / inverter circuit in a divided state (claim 1).

According to this invention, when the output from the Schmitt trigger inverter circuit is at the high level, the detection electrode is charged from the output terminal of the Schmitt trigger inverter circuit via the feedback resistor. Voltage rises. Further, when the output from the Schmitt trigger inverter circuit is at a low level, the detection electrode is discharged to the output terminal of the Schmitt trigger inverter circuit via the feedback resistor, so that the voltage of the detection electrode decreases.

Here, since the voltage dividing means applies the voltage of the detection electrode to the Schmitt trigger inverter circuit in a divided state, the hysteresis width of the Schmitt trigger inverter circuit can be virtually enlarged. This means lowering the oscillation frequency of the oscillation circuit,
Even if the size of the detection electrode is reduced, it is possible to prevent the oscillation frequency of the oscillation circuit from increasing and to stably oscillate the oscillation circuit.

In the above arrangement, the voltage dividing means is constituted by a voltage dividing circuit which divides the voltage of the detection electrode between a predetermined reference voltage and the voltage of the detecting electrode. According to such a configuration, since the voltage of the detection electrode is applied to the Schmitt trigger inverter circuit in a state of being divided between the predetermined reference voltage and the voltage, the voltage dividing means can be implemented with a simple configuration. .

The reference voltage of the voltage dividing circuit is set to a low level in a state where a high level is output from the Schmitt trigger inverter circuit, and the reference voltage of the voltage dividing circuit is set in a state in which a low level is output from the Schmitt trigger inverter circuit. It is desirable to provide reference voltage changing means for setting the reference voltage to a high level (claim 3).

According to this configuration, when the Schmitt trigger inverter circuit outputs a high level, the detection electrode is charged from the output terminal of the Schmitt trigger inverter circuit via the feedback resistor. Voltage rises.

At this time, since the reference voltage of the voltage dividing circuit is at the low level by the reference voltage changing means, the voltage divided by the voltage dividing circuit is a value obtained by dividing the voltage of the detection electrode with the low level. Then, when the divided voltage given by the voltage dividing circuit exceeds the upper threshold set in the Schmitt trigger inverter circuit, the output of the Schmitt trigger inverter circuit becomes low level.

In this case, in order for the divided voltage provided by the voltage dividing circuit to exceed the upper limit threshold set in the Schmitt trigger inverter circuit, the voltage of the detection electrode needs to greatly exceed the upper limit threshold. This means that the upper threshold of the Schmitt trigger inverter circuit has virtually increased.

When the output of the Schmitt trigger inverter circuit becomes low level, the detection electrode is discharged to the output terminal of the Schmitt trigger inverter circuit via the feedback resistor, and the voltage of the detection electrode decreases.

At this time, the reference voltage of the voltage dividing circuit is at a high level by the reference voltage changing means, so that the voltage divided by the voltage dividing circuit is a value obtained by dividing the voltage of the detection electrode with the high level. . Then, when the divided voltage given by the voltage dividing circuit falls below the lower threshold set in the Schmitt trigger inverter circuit, the output of the Schmitt trigger inverter circuit becomes high level.

In this case, in order for the divided voltage given by the voltage dividing circuit to fall below the lower limit threshold set in the Schmitt trigger inverter circuit, the voltage of the detection electrode needs to fall significantly below the lower limit threshold. This means that the lower threshold of the Schmitt trigger inverter circuit has virtually decreased.

As described above, in the configuration in which the voltage dividing means comprises a voltage dividing circuit, the hysteresis width of the Schmitt trigger inverter circuit can be effectively expanded by providing the reference voltage changing means.

Preferably, the voltage dividing means includes a voltage follower circuit for receiving an output from the detection electrode (claim 4). According to such a configuration, the input impedance of the voltage dividing means can be extremely increased by the voltage follower circuit, so that the influence of the presence of the voltage dividing means on the oscillation circuit can be easily avoided.

In addition, a guard electrode for shielding a non-detection side of the detection electrode may be provided, and the guard electrode may be connected to an output terminal of the voltage follower circuit.

According to such a configuration, by connecting the output terminal of the voltage follower circuit receiving the voltage of the detection electrode and the guard electrode, the detection electrode and the guard electrode can be made to have the same potential. The guard electrode can prevent the influence of the capacitance between the electrode and the object located on the non-detection side.

In this case, in the configuration in which the voltage follower circuit is provided to receive the voltage of the detection electrode, it is not necessary to newly provide any special means for making the detection electrode and the guard electrode have the same potential. The overall configuration can be simplified.

Preferably, an air layer is interposed between the detection electrode and the guard electrode. Although the detection electrode and the guard electrode have the same potential by the voltage follower circuit, the potential difference between the input and the output is instantaneous due to the slight temporal difference between the input and output of the voltage follower circuit in the operating state of the oscillation circuit. This causes a capacitance between the detection electrode and the guard electrode, which causes erroneous detection.

However, according to the above configuration, by using air having the smallest dielectric constant between the detection electrode and the guard electrode, the capacitance generated between the detection electrode and the guard electrode can be minimized. Since the size can be reduced, the influence of the capacitance between the detection electrode and the guard electrode can be avoided as much as possible.

Further, a temperature compensating means for compensating a voltage applied to the Schmitt trigger inverter circuit by the voltage dividing means so as to cancel a fluctuation of an oscillation frequency of the oscillation circuit due to a temperature characteristic of the voltage follower circuit is provided. Desirable (claim 7).

When the temperature changes, the gain of the voltage follower circuit may fluctuate, and the oscillation frequency of the oscillation circuit may fluctuate. However, the temperature compensating means corrects the voltage applied to the Schmitt trigger inverter circuit by the voltage dividing means so as to cancel the fluctuation of the oscillation frequency of the oscillation circuit due to the temperature characteristics of the voltage follower circuit. It is possible to prevent the frequency from fluctuating.

A thermistor exhibiting a predetermined temperature characteristic may be used as the temperature compensating means. According to such a configuration, the temperature compensating means can be realized with a simple configuration.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (Summary of the Invention) An overview of the present invention will be described below with reference to FIGS. FIG. 1 schematically shows the configuration of the present invention. In FIG. 1, the capacitance sensor 11 is composed of an oscillation circuit 12 and a control means 13. The control means 13 determines the oscillation state of the oscillation circuit 12 to determine the presence or absence of the detected object or the detected object. The distance to is detected.

In the oscillation circuit 12, a Schmitt trigger / inverter circuit (hereinafter abbreviated as Schmitt circuit) 14
Is connected to the detection electrode 16 via the feedback resistor 15. The Schmitt circuit 14 has a hysteresis. When the input voltage exceeds the upper threshold, the output voltage is inverted from the high level to the low level, and when the input voltage is lower than the lower threshold, the output voltage is inverted from the low level to the high level. It is supposed to.

Here, the detection electrode 16 is connected to the input terminal of the Schmitt circuit 14 via the voltage dividing means 17.
The voltage dividing means 17 applies the voltage of the detection electrode 16 to the Schmitt circuit 14 in a divided state, and virtually expands the level between the upper and lower thresholds of the Schmitt circuit 14, that is, the hysteresis width. It is provided for. It is assumed that the input impedance of the voltage dividing means 17 is set sufficiently larger than the feedback resistor 15. As described above, the oscillation circuit 12 includes the detection electrode 16, the Schmitt circuit 14, the feedback resistor 15, and the voltage dividing unit 17.

Next, the principle by which the hysteresis width of the Schmitt circuit 14 can be virtually expanded by the voltage dividing means 17 will be described. Figure 2 shows Schmitt circuit 1
4 shows the relationship between the output of No. 4 and the voltage of the detection electrode 16 and the output of the voltage dividing means 17. In FIG. 2, when the input voltage of the Schmitt circuit 14 is low immediately after the power is turned on, the output of the Schmitt circuit 14 is at a high level.

In the state in which the output of the Schmitt circuit 14 is at the high level, the detection electrode 16 is charged from the output terminal of the Schmitt circuit 14 via the feedback resistor 15, so that the voltage of the detection electrode 16 gradually increases. To rise. In this case, the input impedance of the voltage dividing means 17 is the feedback resistance 15
The charging time constant is determined by the product of the resistance value of the feedback resistor 15 and the capacitance of the sensing electrode 16.

In a state where the voltage of the detection electrode 16 increases,
Since the voltage dividing means 17 applies the voltage of the detection electrode 16 to the Schmitt circuit 14 in a divided state, the input voltage of the Schmitt circuit 14 rises more slowly than the voltage rise of the detection electrode 16.

When the output from the voltage dividing means 17 exceeds the upper threshold SLT set in the Schmitt circuit 14, the output from the Schmitt circuit 14 is inverted from a high level to a low level. In other words, this corresponds to virtually increasing the upper threshold SLT of the Schmitt circuit 14 to SLT 'shown in FIG.

In the state where the output of the Schmitt circuit 14 is at the low level in this way, the detection electrode 16 is discharged to the output terminal of the Schmitt circuit 14 via the feedback resistor 15, so that the voltage of the detection electrode 16 gradually increases. descend. In this case, the discharge time constant is determined by the product of the resistance value of the feedback resistor 15 and the capacitance of the detection electrode 16.

In a state where the voltage of the detection electrode 16 decreases,
Since the voltage dividing means 17 applies the voltage of the detection electrode 16 to the Schmitt circuit 14 in a divided state, the input voltage of the Schmitt circuit 14 decreases more slowly than the voltage decrease of the detection electrode 4.

When the output from the voltage dividing means 17 falls below the lower threshold SLB set in the Schmitt circuit 14, the output of the Schmitt circuit 14 is inverted from a low level to a high level. That is, the lower threshold SLB of the Schmitt circuit 14
Is equivalent to virtually reducing to SLB 'shown in FIG.

As a result of such an operation being repeatedly performed,
Since the oscillation cycle T of the oscillation circuit 12 is longer than that of the configuration in which the voltage dividing means 17 is not provided, the Schmitt circuit 14 can reliably follow the voltage change accompanying the charging and discharging of the detection electrode 16, and Can oscillate stably.

However, the oscillation frequency of the oscillation circuit 12 changes when the object to be detected approaches the detection electrode 16, and the presence or absence of the object to be detected or the distance to the object is detected based on the oscillation frequency. It becomes possible.

According to the present invention, since the hysteresis width of the Schmitt circuit 14 is virtually expanded by providing the voltage dividing means 17, the oscillation frequency of the oscillation circuit 12 can be reduced. Therefore, the detection electrode 1
6, the oscillation frequency of the oscillation circuit 12 can be prevented from increasing, so that the Schmitt circuit 14 can follow a change in the voltage of the detection electrode 16 and stabilize the oscillation circuit 12. Oscillation.

As can be seen from FIG. 2, if the voltage at the time of either the voltage increase or the voltage decrease of the detection electrode 16 is divided by the voltage dividing means 17, the hysteresis width of the Schmitt circuit 14 is virtually reduced. Can be expanded to:

(First Embodiment) Next, a first embodiment in which the present invention is applied to a capacitance sensor having a guard electrode will be described with reference to FIGS. FIG. 3 schematically shows the entire electrical configuration. In FIG. 3, the capacitance sensor 21 includes a head unit 22 and a controller unit 2.
And 3. The head section 22 includes the detection electrode 2
Guard electrode 25 for shielding the non-detection side other than the front surface of 4
It is configured to cover.

FIG. 4 shows a cross section of the head section 22.
In FIG. 4, for example, a substantially circular detection electrode 24 and a substantially circular guard electrode 25 having a larger area than the detection electrode 24 are housed in a resin case 26. The detection electrode 24 is, for example, an O-ring. It is supported in a form separated from the guard electrode 25 by an insulator such as 27. In this case, the resin case 26 is hermetically sealed in an air-filled state. With such a configuration, an air layer 28 is interposed between the detection electrode 24 and the guard electrode 25. A shield cable 29 is connected to the detection electrode 24 and the guard electrode 25, and the shield cable 29 is connected to the controller 23.

Here, the reason why the guard electrode 25 is provided on the head section 22 will be described. That is, since the capacitance generated on the detection electrode 24 has no directivity and is generated in any direction, the capacitance of the detection electrode 24 is equal to the capacitance generated on the detection side and the capacitance on the non-detection side. Is the sum of
For this reason, although it is desired to detect only the change in the capacitance on the detection side, it is affected by the capacitance on the non-detection side, so that the detected object located on the detection side is accurately detected. May not be possible.

In view of this, a guard electrode 25 for shielding is provided on the non-detection side of the detection electrode 24, so that only the detected object located on the detection side is reliably detected. In this case, if a potential difference occurs between the detection electrode 24 and the guard electrode 25, a capacitance also occurs between the detection electrode 24 and the guard electrode 25.
5 must be at the same potential. Therefore, the detection electrode 24 and the guard electrode 25 are set to the same potential by using a voltage follower circuit 30 (see FIG. 3) provided in the controller section 23 as described later.

Next, the reason why the air layer 28 is interposed between the detection electrode 24 and the guard electrode 25 will be described. That is, the voltage follower circuit 30 of the controller unit 23
Therefore, even if the detection electrode 24 and the guard electrode 25 are set to the same potential, a potential difference is instantaneously generated between the input and the output in the voltage follower circuit 30 in the operation state of the oscillation circuit 31, and the detection A potential difference occurs between the electrode 24 and the guard electrode 25. For this reason, a capacitance is generated between the detection electrode 24 and the guard electrode 25, which causes an erroneous detection.

Conventionally, as the detection electrode 24 and the guard electrode 25, for example, electrodes formed by printing on a glass epoxy resin substrate are used, and they are held by the glass epoxy resin substrate. The relative permittivity of this glass epoxy resin substrate is the relative permittivity of air (ε _r =
It is several times larger than 1).

[0053] Here, when determining the electrostatic capacitance CS between the sensing electrode 24 and guard electrode 25, CS = ε (S / d) where, since the ε = ε ₀ · ε _r, the electrostatic capacitance CS is the electrode area S of the detection electrode 24, the distance d between the detection electrode 24 and the guard electrode 25,
Further, it can be seen that it is determined by the dielectric constant ε of the dielectric substance interposed between the detection electrode 24 and the guard electrode 25.

Incidentally, the capacitance CS is determined by the detection electrode 24.
It is desirable that the capacitance is as small as possible because of the non-detection-side capacitance. However, when the head portion 22 of the capacitance sensor 21 is downsized (thinned), the detection electrode 2
The distance d between the gate electrode 4 and the guard electrode 25 is reduced. In this case, since the capacitance CS is inversely proportional to the distance d,
As the head section 22 is made thinner, the capacitance CS becomes larger.

To cope with such a case, the detection electrode 2
It is conceivable to reduce the capacitance CS by reducing the electrode area S of No. 4; however, if the electrode area S is reduced, the capacitance CS is reduced, but the capacitance C on the detection side is accordingly reduced. Will also be smaller. Therefore, even if the distance to the detected object changes, the change in the capacitance C is small, and the influence of the non-detection-side capacitance CS itself is large. Can not.

For the above reasons, it is desirable that the capacitance CS generated between the detection electrode 24 and the guard electrode 25 be as small as possible. In this case, the capacitance CS is the detection electrode 2
4 and the guard electrode 25, there is a proportional relationship depending on the dielectric constant ε of the dielectric. Therefore, as the dielectric interposed between the detection electrode 24 and the guard electrode 25, air having the smallest dielectric constant ε is used. It is desirable to use (ε _r = 1). Therefore,
In the present embodiment, an air layer 28 is interposed as a dielectric between the detection electrode 24 and the guard electrode 25 except for the O-ring 27 as a support member.

Returning to FIG. 3, the controller 23 comprises an oscillation circuit 31 and control means 32. In the oscillation circuit 31, the non-inverting input terminal of the voltage follower circuit 30 is connected to the detection electrode 24 of the head unit 22, and the inverting input terminal is connected to the output terminal of the voltage follower circuit 30 and the guard electrode 25 of the head unit 22. Have been.

The output terminal of the voltage follower circuit 30 is connected to the input terminal of the Schmitt circuit 34 via the first voltage dividing resistor 33 and the second voltage dividing resistor 35
And an output terminal of the Schmitt circuit 34 via an inverter circuit 36 having the illustrated polarity.

The output terminal of the Schmitt circuit 34 is connected via a feedback resistor 37 to the detection electrode 24 of the head section 22 and the non-inverting input terminal of the voltage follower circuit 30. The Schmitt circuit 34 has a hysteresis. When the input voltage exceeds the upper threshold, the output voltage is inverted from the high level to the low level, and when the input voltage is lower than the lower threshold, the output voltage is inverted from the low level to the high level. It is supposed to.

Here, the first voltage dividing resistor 33 and the second voltage dividing resistor 35 are provided for applying the voltage of the detection electrode 24 to the Schmitt circuit 34 in a divided state as described later. And (the resistance value of the first voltage dividing resistor 33) /
The larger the value of (the resistance value of the second voltage dividing resistor 35), the larger the divided voltage value.

In this embodiment, the voltage follower circuit 30 described above, the first voltage dividing resistor 33, and the second voltage dividing resistor 35
And an inverter circuit 36 constitute a voltage dividing means 38,
The first voltage dividing resistor 33 and the second voltage dividing resistor 35 constitute a voltage dividing circuit 39. Further, the inverter circuit 36 corresponds to a reference voltage changing unit. In this case, the voltage dividing circuit 39
Is the output voltage of the inverter circuit 36, becomes low level when the output voltage of the Schmitt circuit 34 is at a high level, and becomes high level when the output voltage of the Schmitt circuit 34 is at a low level. Further, since the input impedance of the voltage dividing means 38 is extremely large by the voltage follower circuit 30, the time constant composed of the resistance value of the feedback resistor 37 and the capacitance of the detection electrode 24 is not affected by the voltage dividing means 38. Absent. As described above, the oscillation circuit 31
It comprises a detection electrode 24, a Schmitt circuit 34, a feedback resistor 37 and a voltage dividing means 38.

The control means 32 which receives the pulse signal from the oscillation circuit 31 having the above-mentioned configuration is mainly composed of a microcomputer, and includes a counting means 40, a comparing means 41 and an output means 4.
2 is comprised. The counting means 40 determines the oscillation frequency of the oscillation circuit 31 by counting the pulse signals output from the oscillation circuit 31 per unit time. The comparing means 41 determines the approach state of the detected object by comparing the count value counted by the counting means 40 with a predetermined value. The output unit 42 outputs a detection signal based on the detection result of the comparison unit 41. Therefore, the presence or absence of the detected object can be determined based on the detection signal from the control unit 32.

Next, the operation of the above configuration will be described. In FIG. 5, which shows the output of the Schmitt circuit 34, the voltage of the detection electrode 24, and the time course of the divided voltage by the voltage dividing circuit 39, when power is supplied to the capacitance sensor 11, the power supply voltage of the Schmitt circuit 34 rises. . Since the Schmitt circuit 34 has a hysteresis, when the input voltage of the Schmitt circuit 34 is low immediately after the power is turned on, the output of the Schmitt circuit 34 is at a high level.

Here, since the detection electrode 24 forms a small-capacity capacitor even when the object to be detected is separated, the detection electrode 24 is charged from the output terminal of the Schmitt circuit 34 via the feedback resistor 37. , The voltage of the detection electrode 24 gradually increases.

Since the voltage follower circuit 30 receives the input voltage with high impedance and outputs the same voltage as the input voltage,
The output voltage of the voltage follower circuit 30 becomes the same as the voltage of the detection electrode 24.

Here, since the guard electrode 25 is connected to the output terminal of the voltage follower circuit 30, the guard electrode 2
5 has the same potential as the detection electrode 24. Therefore, the guard electrode 25 can prevent the influence of the object located on the non-detection side of the detection electrode 24.

When the output from the Schmitt circuit 34 is at a high level, the output of the inverter circuit 36 is at a low level, so that the reference voltage of the voltage dividing circuit 39 is at a low level. Thus, the voltage dividing circuit 39
A voltage obtained by dividing the voltage of the detection electrode 24 between a low level and the low level is supplied to the Schmitt circuit 34. This means that when the detection electrode 24 is charged and the voltage is rising, the upper threshold SLT of the Schmitt circuit 34 becomes SLT ′ shown in FIG.
It is equivalent to being virtually increased.

When the voltage divided by the voltage dividing circuit 39 exceeds the upper threshold SLT of the Schmitt circuit 34, the output of the Schmitt circuit 34 is inverted from the high level to the low level. As a result, the detection electrode 24 is discharged to the output terminal of the Schmitt circuit 34 via the feedback resistor 37, so that the voltage of the detection electrode 24 gradually decreases.

At this time, since the output of the inverter circuit 36 is at a high level, the reference voltage of the voltage dividing circuit 39 is at a high level. Thereby, the voltage dividing circuit 39
Supplies the Schmitt circuit 34 with a voltage obtained by dividing the voltage of the detection electrode 24 between the high level and the high level. This means
In a state where the detection electrode 24 is discharged and the voltage is reduced,
The lower limit threshold SLB of the Schmitt circuit 34 is SLB shown in FIG.
'Is virtually equivalent to being lowered.

When the voltage divided by the voltage dividing circuit 39 falls below the lower limit threshold SLB of the Schmitt circuit 34, the output of the Schmitt circuit 34 is inverted from a low level to a high level. As a result, the detection electrode 24 is charged again and the voltage rises.

As described above, when the oscillation circuit 31 oscillates, the control means 32 determines the presence or absence of the detected object based on the oscillation state of the oscillation circuit 31. In this case, the oscillation circuit 31 oscillates at a high frequency because the capacitance of the detection electrode 24 is small in the separated state of the detected object.

Here, when the detected object approaches the head section 22, the distance from the detection electrode 16 to the detected object is D.
Then, the oscillation period T is inversely proportional to the distance D between the detection electrode 16 and the detected object.

Therefore, the oscillation frequency of the oscillation circuit 31 decreases as the object to be detected approaches the detection electrode 24. Therefore, the control means 32 counts the pulse signals per unit time from the oscillation circuit 31 and counts them. When the numerical value falls below the set value, it is determined that the detected object has been detected, and a detection signal is output.

By the way, when the voltage of the detection electrode 24 is directly applied to the Schmitt circuit 34 in the configuration for miniaturizing the detection electrode 24 as in the present embodiment, the oscillation of the oscillation circuit 31 In this state, the detection electrode 24 repeats charging and discharging in a very short time. For this reason,
The Schmitt circuit 34 cannot follow the voltage change accompanying the charging and discharging of the detection electrode 24, and the oscillation of the oscillation circuit 31 becomes unstable, and the detection of the detected object may be uncertain.

However, in the present embodiment,
When the voltage of the detection electrode 24 is divided by the voltage dividing means 38, the Schmitt circuit 3 is charged.
4 and in the discharge state of the detection electrode 24, the voltage drop of the detection electrode 24 is supplied to the Schmitt circuit 34 in a divided state, so that the hysteresis width of the Schmitt circuit 34 can be virtually expanded. , Oscillation circuit 3
1 can be reduced. Therefore, the Schmitt circuit 3 detects a voltage change accompanying the charging and discharging of the detection electrode 24.
4 can reliably follow, and the oscillation circuit 31 can oscillate stably, so that the size of the detection electrode 24 can be reduced compared to the conventional example in which the voltage of the detection electrode is directly supplied to the Schmitt circuit. Even if it is attempted, the detected object can be reliably detected.

The voltage follower circuit 30 includes a voltage dividing means 3
8 functions to increase the input impedance of the guard electrode 25 and to make the potential of the guard electrode 25 the same as that of the detection electrode 24.
0 can be used to implement both functions simultaneously,
The overall configuration can be greatly simplified, which contributes to downsizing.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIGS.
The same reference numerals are given to the same portions as those of the embodiment, and the description is omitted. The second embodiment is characterized in that the reference voltage of the voltage dividing circuit is fixed.

In FIG. 6 schematically showing the overall electric configuration, a second voltage dividing resistor 35 constituting a voltage dividing circuit 39 is connected to a 0 V line in the capacitance sensor 21.

When the voltage is increased by charging the detection electrode 24, the fluctuation is divided by the ratio of the resistance value of the first voltage dividing resistor 33 and the second voltage dividing resistor 35. When the voltage of the sensing electrode 24 rises, the upper threshold increases virtually by increasing slowly, whereas when the voltage of the sensing electrode 24 decreases, the lower threshold decreases virtually by increasing quickly. Will do. In this case, as shown in FIG. 7, the increase in the virtual upper threshold SLT 'is larger than the increase in the virtual lower threshold SLB', so that the hysteresis width of the Schmitt circuit 34 is virtually reduced as a whole. Can be expanded.

According to such an embodiment, the reference voltage of the voltage dividing circuit 39 is fixed to 0 V. Therefore, the hysteresis width of the Schmitt circuit 34 is virtually expanded, so that the oscillation frequency of the oscillation circuit 31 is increased. And the number of inverter circuits 36 can be reduced.

The second voltage dividing resistor 35 constituting the voltage dividing circuit 39 may be connected to a positive power supply voltage as a reference voltage. In this case, although both the virtual upper threshold value SLT ′ and the lower threshold value SLB ′ decrease, the decrease width of the virtual lower threshold value SLB ′ is larger than the decrease amount of the virtual upper threshold value SLT ′. Schmitt circuit 3 as a whole
4 can be enlarged. Further, a predetermined arbitrary voltage other than the power supply voltage may be selected as the reference voltage.

(Third Embodiment) Next, a third embodiment of the present invention will be described with reference to FIGS. The third embodiment is characterized in that the fluctuation of the oscillation frequency of the oscillation circuit 31 due to the temperature change is prevented.

That is, in each of the above embodiments, when the internal temperature of the capacitance sensor 21 rises, the oscillation cycle of the oscillation circuit 31 becomes shorter as shown in FIG. This is caused by the gain of the voltage follower circuit 30 used for the oscillation circuit 31 becoming higher due to temperature characteristics (however, depending on the voltage follower circuit, the gain may become lower due to temperature rise). That is, the voltage follower circuit 3
Although the gain of 0 is normally 1, the voltage follower circuit 3
When the temperature of 0 rises, the gain of the voltage follower circuit 30 becomes larger than 1, and the Schmitt circuit 34 is supplied with a voltage higher than the original input voltage. As a result, the time required for the input voltage of the Schmitt circuit 34 to exceed the upper threshold value SLT or to fall below the lower threshold value SLB is shortened, so that the oscillation cycle of the oscillation circuit 31 is shortened as shown in FIG. is there.

Normally, the oscillation cycle of the oscillation circuit 31 is short when no object is present, and is long when the object is present. When the gain of the follower circuit 30 fluctuates, a detection signal indicating that the detected object exists may not be output in spite of the presence of the detected object, so that an accurate detection operation may not be performed. There is.

Therefore, in the present embodiment, as shown in FIG. 8, a second voltage-dividing resistor 35 is connected in series with a fixed resistor 43 and a thermistor 44 (corresponding to a temperature compensating means) having a negative temperature characteristic. To be configured. In this case, the temperature characteristic of the thermistor 44 is set so as to have a characteristic that cancels the fluctuation of the oscillation cycle of the oscillation circuit 31 due to the temperature change.

When the internal temperature of the capacitance sensor 21 rises and the oscillation cycle of the oscillation circuit 31 becomes short, the thermistor 44 has a negative temperature characteristic. The resistance value decreases. As a result, the divided voltage of the voltage dividing circuit 39 decreases when the detecting electrode 24 is charged, and the divided voltage increases when the detecting electrode 24 is discharged. Therefore, the hysteresis width of the Schmitt circuit 34 is virtually expanded. (See FIG. 9B), and the oscillation cycle of the oscillation circuit 31 can be prevented from being shortened.

According to such an embodiment, even if the oscillation frequency of the oscillation circuit 31 fluctuates due to a temperature change, the fluctuation is canceled by the temperature compensation operation of the thermistor 44. The object to be detected can be reliably detected without being affected by the temperature change.

When the ambient temperature rises, the oscillation circuit 3
In the case where the oscillation frequency of 1 becomes low, a thermistor having a positive temperature characteristic is used instead of the thermistor 44 having a negative temperature characteristic.

Further, the second voltage dividing resistor 35 is connected to the fixed resistor 43
Instead of using the thermistor 44 and the thermistor 44 having a negative temperature characteristic, the first voltage dividing resistor 33 may be configured by connecting a fixed resistor and a thermistor having a positive temperature characteristic in series.

Further, the second voltage-dividing resistor 35 is constituted by connecting in series a series circuit having different resistance values consisting of a resistor and an analog switch, and by turning on a predetermined analog switch according to a temperature change, the second voltage-dividing resistor 35 is turned on. The resistance value of the voltage dividing resistor 35 may be adjusted.

The present invention is not limited to the above embodiments, but can be modified or expanded as follows. The guard electrode 25 may be omitted. A Schmitt circuit may be used instead of the inverter circuit 36. In this case, a plurality of Schmitt circuits included in the IC can be used, and the IC can be effectively used.

An inverter circuit is interposed between the output terminal of the Schmitt circuit 34 and the control means 32, and a second voltage dividing resistor 35 is connected between the output terminal from the inverter circuit and the input terminal of the Schmitt circuit 34. You may make it.

[0093]

As is apparent from the above description, according to the capacitance sensor of the present invention, the voltage of the detecting electrode is divided by the voltage dividing means and applied to the Schmitt trigger / inverter circuit. Since the hysteresis of the trigger inverter circuit is virtually expanded,
While downsizing the sensing electrode, the Schmitt trigger / inverter circuit reliably follows the voltage change accompanying the charging / discharging of the sensing electrode, allowing the oscillation circuit to oscillate stably and reliably detect the object to be detected. It has an excellent effect that it can be done.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a capacitance sensor for explaining an outline of the present invention.

FIG. 2 is a waveform diagram of each signal.

FIG. 3 is a schematic diagram showing an electrical configuration of the capacitance sensor according to the first embodiment of the present invention.

FIG. 4 is a sectional view of a head unit.

FIG. 5 is a waveform diagram of each signal.

FIG. 6 is a view corresponding to FIG. 3, showing a second embodiment of the present invention;

FIG. 7 is a diagram corresponding to FIG. 5;

FIG. 8 is a diagram corresponding to FIG. 3, showing a third embodiment of the present invention.

FIG. 9 is a waveform diagram of each signal when the oscillation cycle fluctuates and when temperature is compensated.

FIG. 10 is an electric circuit diagram showing an oscillation circuit of a conventional capacitance sensor.

FIG. 11 is a waveform diagram of each signal at the time of detection and at the time of detection.

[Explanation of symbols]

11 is a capacitance sensor, 12 is an oscillation circuit, 14 is a Schmitt trigger inverter circuit, 15 is a feedback resistor, 16 is a detection electrode, 17 is a voltage dividing means, 21 is a capacitance sensor, 2
2 is a head unit, 23 is a controller unit, 24 is a detection electrode, 25 is a guard electrode, 28 is an air layer, 30 is a voltage follower circuit, 31 is an oscillation circuit, 32 is control means, 33 is a first voltage dividing resistor, 34 is a Schmitt circuit, 35 is a second voltage dividing resistor, 36 is an inverter circuit (reference voltage changing means),
38 is a voltage dividing means, 39 is a voltage dividing circuit, 43 is a fixed resistor,
Reference numeral 4 denotes a thermistor (temperature compensation means).

Claims (8)

[Claims] An oscillation circuit that changes an oscillation state according to a capacitance generated between a detection electrode and a detection object;
In a capacitance sensor for detecting the presence or absence of an object to be detected based on the oscillation state of the oscillation circuit, the oscillation circuit switches an output from a high level to a low level when an input voltage exceeds an upper limit threshold, and A Schmitt trigger inverter circuit that switches the output from a low level to a high level when the voltage falls below a lower threshold, a feedback resistor interposed between the output of the Schmitt trigger inverter circuit and a detection electrode, Voltage dividing means for dividing the voltage into a voltage and applying the voltage to the Schmitt trigger / inverter circuit. 2. The capacitance sensor according to claim 1, wherein said voltage dividing means comprises a voltage dividing circuit which divides a voltage of said detection electrode between a predetermined reference voltage and said voltage. 3. The reference voltage of the voltage dividing circuit is set to a low level in a state where a high level is output from the Schmitt trigger inverter circuit, and the reference voltage of the voltage dividing circuit is set in a state where a low level is output from the Schmitt trigger inverter circuit. 3. The capacitance sensor according to claim 2, further comprising a reference voltage changing unit that sets the reference voltage to a high level. 4. The capacitance sensor according to claim 1, wherein said voltage dividing means includes a voltage follower circuit for receiving an output from said detection electrode. 5. The electrostatic capacitance according to claim 4, further comprising a guard electrode that shields a non-detection side of the detection electrode, wherein the guard electrode is connected to an output terminal of the voltage follower circuit. Sensor. 6. The capacitance sensor according to claim 5, wherein an air layer is interposed between the detection electrode and the guard electrode. 7. A temperature compensating means for compensating a voltage applied to said Schmitt trigger inverter circuit by said voltage dividing means so as to cancel a fluctuation of an oscillation frequency of said oscillation circuit due to a temperature characteristic of said voltage follower circuit. The capacitance sensor according to any one of claims 4 to 6, wherein 8. The capacitance sensor according to claim 7, wherein said temperature compensating means is a thermistor exhibiting a predetermined temperature characteristic.