JP3265807B2 - Capacitive sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitive sensor for detecting pressure, displacement, acceleration, and the like by utilizing a change in capacitance.

[0002]

2. Description of the Related Art FIG. 5 shows a circuit of a conventional capacitive sensor. 8, the capacitance detecting section 1 has a movable electrode 4 disposed at an intermediate position (neutral position) between the fixed electrodes 2 and 3 as shown in FIG. 8, and the movable electrode 4 is held by a beam 5, for example. , Is displaced in the Y direction by receiving acceleration. In the circuit of FIG. 5, the capacitor (capacitor) C ₁ is formed by the fixed electrode 2 and the movable electrode 4, and the capacitor (capacitor) C ₂ is formed by the fixed electrode 3 and the movable electrode 4. The capacitor C ₁ of the capacitance detecting section 1 has an amplitude e from the AC drive power supply 6.
Of which the AC voltage is applied, an AC voltage of amplitude e whose phase is inverted by the phase inverting circuit 9 is applied to the capacitor C _2. Further, a positive bias voltage U ₀ is applied to the capacitor C ₁ by the bias power supply 7, and a negative bias voltage −U ₀ having a different DC polarity is applied to the capacitor C ₂ by the bias power supply 8. Has been applied.

The output side of the capacitance detecting section 1 is connected to an inverting input terminal of an operational amplifier 10 functioning as a voltage conversion output circuit, and the non-inverting input terminal of the operational amplifier 10 is connected to ground for applying a reference voltage. .

In this type of capacitive sensor, the capacitance detecting unit 1
In a state in which acceleration is not applied to, since the movable electrode 4 is in the intermediate position of the fixed electrodes 2 and 3, equal the capacitance of the capacitor C ₁ and C _2, as a result, the current i ₁ and the capacitor C flows in the capacitor C ₁ current i ₂ flowing through the _secondary becomes equal. At this time, the output voltage of the capacitor C _1, as shown in FIG. 6, the bias voltage U ₀ as a reference, becomes a voltage in the form of an AC voltage is superimposed on the drive power source 6 to the U _0, the output voltage of the capacitor C ₂ Is a voltage in which the AC voltage of the drive power supply 6 is applied to the bias voltage −U ₀ with a phase shift of 180 °, and the output voltage of the capacitor C ₁ and the capacitor C ₂
Is zero voltage. In other words, currents i ₁ and i ₂ flow through the feedback resistor 11 of the operational amplifier 10, and a voltage V _OUT (V 2) obtained by multiplying the combined current i ₁ + i ₂ by the resistance value R _f of the feedback resistor 11. _OUT
= (I ₁ + i ₂ ) R _f ) is extracted as an acceleration detection output, but when there is no acceleration, the currents i ₁ and i
₂ is equal in magnitude and 180 ° out of phase, so i
The combined current of ₁ and i ₂ becomes zero, and a zero voltage is extracted from the operational amplifier 10 as a sensor output.

On the other hand, when acceleration is applied to the capacitance detecting section 1,
Since the movable electrode 4 is displaced in response to the direction of the acceleration, the capacitance C ₂ is changed in capacitance C ₁ and capacitor C ₂ of the capacitor C _1, i ₂ is when a relation of C _1> C ₂
I ₁ is larger than i _1, and the combined current of i ₁ and i ₂ is
A voltage V _OUT obtained by multiplying the combined current by the feedback resistance value _Rf is output as an acceleration detection signal. On the other hand, acceleration acts in the opposite direction,
When the relationship of C ₁ <C ₂ is established, as shown by the broken line in FIG. 7, the waveform of the combined current of i ₁ and i ₂ is 180 ° out of phase with the waveform of the solid line. The voltage multiplied by _f is output as the acceleration detection signal.

[0006] These relationships are summarized as follows. When acceleration is applied and C ₁ > C ₂ , the sensor output V _OUT is expressed by the equation (1).

V _OUT = R _f · ω · e | C ₁ -C ₂ | ······ (1)

Further, the acceleration acts in the opposite direction, and C ₁ <C
_{In the case of 2} , it is expressed by equation (2).

V _OUT = −R _f · ω · e | C ₁ −C ₂ |

In these equations, ω represents the angular frequency of the AC voltage of the drive power supply 6, and a current proportional to the capacitance difference between the capacitors C ₁ and C ₂ that changes with acceleration is converted by the operational amplifier 10 into a voltage. It is output.

[0011]

However, in the conventional capacitive sensor, as shown in the above equations (1) and (2), the phase of the sensor output is inverted by 180 ° depending on the direction of acceleration. Since the detection region coincides with the phase inversion region, there is a problem that the linearity near the zero point of the sensor output is deteriorated.

The phase of the sensor output is inverted in the direction of the acceleration. In any case of the acceleration in any direction, the sensor output is a sine-wave AC voltage based on zero voltage. For this purpose, a synchronous detection circuit is required, and the circuit configuration becomes complicated, and the sensor cost becomes high.

The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide a capacitive sensor having a simple configuration in which the linearity of a sensor output near a zero point is improved and a synchronous detection circuit is omitted. To provide.

[0014]

The present invention is configured as follows to achieve the above object. That is, the present invention provides two variable capacitors formed by disposing a movable electrode between two fixed electrodes, a driving power supply for applying an AC voltage to the two variable capacitors, and two variable capacitors. A bias power supply for applying a DC bias voltage having different polarities to each other, and a voltage conversion output circuit for converting the capacitance difference between the two variable capacitors into a voltage and outputting the voltage. An offset direct-current power supply is provided that applies an electrostatic force that makes the capacitance unbalanced to the movable electrode and maintains the capacitance of one capacitor always larger than the capacitance of the other capacitor. ing.

[0015]

In the present invention having the above-mentioned structure, by applying the offset voltage of the offset DC power supply, an electrostatic force due to the offset voltage is applied to the movable electrode, and the movable electrode becomes
And the capacitances of the two capacitors are forcibly unbalanced, so that even if the object to be detected such as acceleration acts and the movable electrode is displaced, The relationship that the capacitance of the capacitor on the side is larger than the capacitance of the capacitor on the other side is maintained, and the phase of the sensor output does not reverse according to the direction in which the detection target acts, such as acceleration, and the signal of the sensor output The magnitude of the change of the detection target and the direction of the change are detected based on the magnitude of.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. In the description of the present embodiment, the same components as those of the conventional example are denoted by the same reference numerals, and the description thereof will not be repeated. FIG. 1 shows a circuit configuration of an embodiment of a capacitive sensor according to the present invention.

The present embodiment is different from the conventional example in that a positive DC offset voltage is connected to the connection between bias power supplies 7 and 8 for applying DC bias voltages having different polarities to the capacitors C ₁ and C _2. Offset power supply
12 and the offset adjustment means 13 is provided on the non-inverting input terminal side of the operational amplifier 10. The other configuration is the same as that of the conventional example. The offset power supply 12
Is composed of a DC power supply, the positive pole of which is connected to the connection of the bias power supplies 7 and 8, and the negative pole of the offset power supply 12 is connected to the ground. Further, the offset adjusting means 13 is constituted by a sliding resistor, and slides the sliding terminal in contact with the sliding resistor to adjust the voltage applied to the non-inverting input terminal of the operational amplifier, so that the offset voltage component ΔU Is removed and the sensor output V _OUT is output.

In this embodiment, the offset power supply 12
By offset DC voltage U is applied to the connecting portion of the bias power source 7, 8, as shown in FIG. 2, the DC voltage applied to the capacitor (capacitor) C ₁ is U ₀ + .DELTA.U
Next, the DC voltage applied to the capacitor (capacitor) C ₂ becomes -U ₀ + ΔU. The offset voltage ΔU is equal to the voltage U _{0 of the} bias power supply and the amplitude e of the AC voltage of the drive power supply 6.
Is set higher than the voltage obtained by adding
As shown in, the voltage across the capacitor C ₁ and C ₂ is always greater than zero voltage, the AC voltage waveform to ride lower bias voltage (-U ₀ + ΔU), that intersects the zero voltage However, the voltage is always set to be higher than zero voltage.

When the offset voltage ΔU is applied, the voltage U ₀ + ΔU applied to the capacitor C ₁ and the bias voltage −U ₀ + ΔU applied to the capacitor C ₂ become asymmetric voltages, and this asymmetric voltage difference As a result, the movable electrode 4 is displaced by receiving an electrostatic force on the fixed electrode 2 side, and the movable electrode 4 becomes a neutral position at a position closer to the fixed electrode 2 side, and the capacitance C _{1 of the} capacitor C ₁ always becomes the capacitance of the capacitor C ₂ . C ₂
(C ₁ > C ₂ ).

[0020] In this embodiment, the movable electrode 4 by the acceleration as in the conventional example is displaced in response to the direction of acceleration, the capacitance of the capacitor C ₁ and C ₂ is changed, the capacitor C ₁ and C ₂ Is converted into a voltage by the operational amplifier 10 and output. That is, C ₁ and C by acceleration
_The current i ₁ flowing through the capacitor C ₁ according to the capacitance change of
And the current i ₂ flowing through the capacitor C ₂ change, and offset adjustment is performed from the voltage generated by multiplying the combined current (i ₁ + i ₂ ) of i ₁ and i ₂ by the resistance value R _f of the feedback resistor 11. V _OUT from which the offset component ΔU has been removed by the circuit is taken out from the output terminal of the operational amplifier 10 as an acceleration detection signal.

The output voltage V _OUT is represented by the following equation (3).

V _OUT = R _f · ω · e (C ₁ -C ₂ ) (3)

In this embodiment, when the offset voltage ΔU is applied, the capacitance of the capacitor C ₁ always maintains a larger relationship than the capacitance of the capacitor C ₂ , and as a result, regardless of the direction of acceleration. The current i ₁ flowing through the capacitor C ₁ is larger than the current i ₂ flowing through the capacitor C ₂ , and the phase of the synthesized current obtained by synthesizing the current i ₂ whose phase is different from the current i ₁ by 180 ° is the phase of the current i ₁ Matches. As a result, as shown in FIG.
Sinusoidal but when displaced to the fixed electrode 2 side, changes in the direction in which the capacitor C ₁ is increased, the capacitance of the capacitor C ₂ changes in a decreasing direction, a combined current of i ₁ and i ₂ are shown in a solid line When the acceleration is applied in the opposite direction, the combined current waveform of i ₁ and i ₂ becomes a current waveform having a small amplitude indicated by a broken line in FIG. 3, and i ₁ is independent of the acceleration direction. And i ₂ have the same phase waveform.

Therefore, in this embodiment, as shown in FIG. 4, the sensor output voltage V is independent of the acceleration direction.
_OUT becomes a positive voltage, and by detecting the sensor output voltage, the magnitude and direction of the acceleration can be obtained.

In this embodiment, the phase of the sensor output does not reverse depending on the direction of acceleration unlike the conventional example. Linearity is obtained, and highly sensitive and highly accurate acceleration detection is possible.

Further, a complicated synchronous detection circuit for detecting the direction of the acceleration is not required, so that the circuit configuration can be simplified and the sensor cost can be reduced.

The present invention is not limited to the above-described embodiment, but can adopt various embodiments. For example, in the above-described embodiment, the offset voltage of the offset power supply 12 is given by the positive value ΔU. On the contrary, the offset voltage may be given by the negative offset voltage −ΔU.

In this embodiment, the capacitive sensor has been described by taking an acceleration sensor as an example. However, the capacitive sensor of the present invention is used as a capacitive sensor for detecting an object other than acceleration, such as pressure and displacement. Applicable. In this case, the configuration of the capacitance detection unit 1 is designed according to the application.

[0029]

According to the present invention, the offset voltage is applied by the offset DC power supply so that the capacitance on one side of the two variable capacitors is always maintained larger than the capacitance on the other side. Therefore, the phase of the sensor output does not reverse depending on the direction of the action of the detection target, and the direction and magnitude of the action of the detection target can be easily detected by the sensor output. Good linearity of the output with respect to the change of the detection target in the minute detection region can be obtained, and both the detection accuracy and the detection sensitivity of the detection target can be improved.

Further, the complicated synchronous detection circuit of the conventional example is not required, and the circuit configuration can be simplified accordingly, and the sensor cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of an embodiment of a capacitive sensor according to the present invention.

FIG. 2 is an explanatory diagram showing applied voltages to respective capacitors in the sensor of the embodiment.

FIG. 3 is an explanatory diagram of a combined current waveform of a current flowing through each capacitor in the embodiment.

FIG. 4 is a characteristic explanatory diagram showing a sensor output of the embodiment in comparison with a conventional example.

FIG. 5 is a circuit diagram of a conventional capacitive sensor.

FIG. 6 is an explanatory diagram showing a voltage waveform applied to each capacitor in the circuit of FIG. 5;

7 is a waveform diagram of a composite current of the current flowing through the capacitor C ₁ of the conventional capacitive sensor and a current flowing through the capacitor C _2.

FIG. 8 is an explanatory diagram illustrating a configuration example of a capacitance detection unit.

[Explanation of symbols]

1 capacity detection portion 6 drive power 7,8 bias power source 10 operational amplifier 11 feedback resistor 12 offset power source 13 offset adjusting means C _1, C ₂ capacitors

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01D 5/24 G01L 1/14 G01R 27/26

Claims (1)

(57) [Claims] 1. Two variable capacitors formed by arranging a movable electrode between two fixed electrodes, a drive power supply for applying an AC voltage to the two variable capacitors, and two variable capacitors. In a capacitive sensor including a bias power supply for applying a DC bias voltage having different polarities and a voltage conversion output circuit for converting a capacitance difference between two variable capacitors into a voltage and outputting the voltage, a static power supply of the two capacitors is provided. An offset DC power source for applying an electrostatic force for imbalance of capacitance to the movable electrode to maintain the capacitance of one capacitor always larger than the capacitance of the other capacitor. Sensor.