JPH0943068A - Signal processing circuit for sensor utilizing change in capacitance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain approximately constant detection sensitivity for external force of an electrode plane irrespective of a distance between electrodes in a signal processing circuit for a sensor where a change in capacitance is utilized. SOLUTION: A CV-conversion circuit 2 for converting capacitance to voltage is connected to a capacitance element 1 for detecting external force in a direction between electrodes, an output of this circuit is an output detected in a direction between electrodes, capacitance elements 3, 4 for detecting external force in a direction of electrode plane operate differentially and respectively connected to CV-conversion circuits 5, 6 for converting capacitance to voltage, an output of the CV-conversion circuits is made through an operational amplifier 7 to be an output for detecting external force in the direction of electrode plane, the output for detecting the external force in the direction between electrodes is connected to sensitivity adjusting circuits 8, 9, and the respective sensitivity adjusting circuits 8, 9 are connected to capacitances 3, 4 for detecting external force in the direction of electrode plane. The sensitivity adjusting circuits 8, 9 control amplitude of voltage appearing across the capacitance for detecting in the direction of electrode plane according to the external force in the direction between electrodes, thereby making the detection output in the direction of electrode plane correct irrespective of a distance between the electrodes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a signal processing circuit for a sensor that utilizes a change in electrostatic capacitance that detects force, acceleration, magnetism, etc. of another axis based on a change in a distance between a pair of electrodes. By controlling the detection sensitivity of the force in the direction of the electrode plane with the detection voltage in the direction of the distance between the electrodes, a sensor signal that utilizes the change in capacitance that makes the detection output in the direction of the electrode plane accurate regardless of the distance between the electrodes It relates to a processing circuit.

[0002]

2. Description of the Related Art Conventionally, a capacitive multi-axis force sensor is known from, for example, Japanese Patent Application No. 4-181679. This is to detect the force in the inter-electrode direction by the capacitance corresponding to the average value of the inter-electrode distance and to detect the force in the electrode plane direction by the inclination of the electrode as the inter-electrode distance moves due to the force. More specifically, when used as a capacitance type acceleration sensor, a plurality of pairs of capacitance elements are provided, each of which is provided with an electrode on each opposing surface of a fixed substrate and a flexible substrate. Set the XY plane parallel to the plane and the X of the Z axis orthogonal to this
The change in acceleration in the three-dimensional directions of the Y and Z axes is detected for each component in the X, Y, and Z axis directions based on the change in capacitance between a plurality of pairs of capacitance elements.

For example, as shown in the vertical cross section of FIG. 3, a fixed substrate 11 arranged in a frame body 10 and a flexible substrate 12 arranged in parallel with a predetermined space therebetween, and the lower surface of the fixed substrate 11 is arranged. As shown in FIG. 2, the electrodes 21 to 25 are attached to the facing surfaces of the fixed substrate 11 and the flexible substrate 12, respectively, to form the capacitance elements C _{21 to} C ₂₅ .
The lower surface of the flexible substrate 12 has an actuator 13 having an appropriate mass.
Is provided.

More specifically, here, four electrodes 21 to 25 are provided in the outer peripheral portion between the facing surfaces, and one electrode 21 to 25 is provided in the central portion to form a pair with the electrode 26 of the fixed substrate 11 to form the capacitance element C. ₂₁ ~ C
₂₅ , that is, X, which is orthogonal to the electrode surface,
Each of the two capacitive elements C ₂₁ arranged on two axes of Y
C ₂₄ and a capacitive element C ₂₅ arranged in the center of the front two axes. In the above structure, when acceleration is applied in the X-axis direction, the flexible substrate 12 having the actuator 13 is deformed, so that the electrodes 1 to 5 between the facing surfaces of the fixed substrate 11 and the flexible substrate 12 are deformed. since the distance changes, the capacitance of the capacitive element C ₂₁ -C ₂₄ is changed. Similarly, when acceleration is applied in the Z-axis direction, each capacitance element C
_The capacitance of _{21 to} C ₂₅ changes.

The detection of each component of the acceleration from the change of the electrostatic capacitance is performed, for example, as an output with respect to the acceleration in the X-axis direction, the electrostatic capacitance difference between the electrostatic capacitance elements C ₂₁ and C ₂₃ (C ₂₁ -C ₂₃ ), Y
As an output for the acceleration in the axial direction, the capacitance element C ₂₂
And C ₂₄ (C ₂₂ −C ₂₄ ), the capacitance of the capacitance element C ₂₅ (C _22
25 ) or C ₂₁ + C ₂₂ + C ₂₃ + C ₂₄ .

[0006]

In a sensor such as a capacitance type acceleration sensor having such a structure, there is a problem that the output sensitivity to an external force in the plane direction changes according to the distance between the electrodes. Conventionally, no change was made to the fluctuation of the detection sensitivity of the external force on the electrode plane,
The conventional correction circuit has a problem that the change in sensitivity due to the distance between the electrodes is so large that it cannot be ignored.

The present invention can solve the above-mentioned problems in the signal processing circuit for the sensor utilizing the change of the electrostatic capacitance, and can make the detection sensitivity of the external force of the electrode plane substantially constant regardless of the distance between the electrodes. It is an object of the present invention to provide a signal processing circuit, and further to provide a configuration in which the detection sensitivity of the external force on the electrode plane can be made substantially constant with a simple circuit without significantly increasing the cost.

[0008]

As a result of various studies on a signal circuit for the purpose of constructing a structure in which the external force detection sensitivity of the electrode plane can be made substantially constant, the inventor has found that the force detection sensitivity in the electrode plane direction is the distance between electrodes. It was found that the detection output in the electrode plane direction can be made constant irrespective of the inter-electrode distance by controlling with the detection voltage of 1.

That is, the present invention has one movable part that moves by the action of an external force, a plurality of electrodes are arranged on this movable part, and the capacitive element is constituted by the electrode of this movable part and the electrode pair of the fixed part. , In the signal processing circuit for the sensor that detects the external force in the distance direction between the electrodes and the external force in the plane direction of the electrodes based on the change in the capacitance of this capacitive element, It has a sensitivity adjustment circuit by an external signal, and connects the detection signal of the external force in the inter-electrode direction to the sensitivity adjustment circuit so that the detection sensitivity of the external force in the planar direction of the electrodes can be controlled according to the external force in the inter-electrode direction. A signal processing circuit for a sensor using a change in electrostatic capacitance, which has a characteristic circuit.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing a basic configuration of a signal processing circuit of a capacitance type sensor. FIG. 4 is a graph showing the relationship between the inter-electrode distance and capacitance, and FIG. 5 is a graph showing the detection output of the external force in the electrode plane direction due to the difference in the average inter-electrode distance. FIG. 6 is a circuit diagram showing a specific example of the signal processing circuit according to the present invention, and FIG. 7 is an explanatory diagram showing an operation waveform example of the signal processing circuit of FIG. The electrostatic capacitance type sensor is intended for the electrostatic capacitance type acceleration sensor having the configuration of FIG. 2 and FIG. 3 described above.

In FIG. 1, a CV conversion circuit 2 for converting an electrostatic capacity into a voltage is connected to a capacitance element 1 for detecting an external force between electrodes, and the output of this circuit is used as a detection output in the direction between electrodes. . The electrostatic capacitance elements 3 and 4 for detecting the external force in the electrode plane direction operate differentially and are connected to CV conversion circuits 5 and 6 for converting the electrostatic capacitance into a voltage, respectively. The output of this CV conversion circuit is subtracted by the operational amplifier 7, and the output of this circuit is used as the detection output of the external force in the electrode plane direction.

On the other hand, the detection output of the external force in the inter-electrode distance direction is connected to the sensitivity adjusting circuits 8 and 9, and the sensitivity adjusting circuits 8 and 9 respectively detect the electrostatic capacitances 3 for detecting the external force in the electrode plane direction.
4 is connected. The sensitivity adjusting circuits 8 and 9 are configured to control the amplitude of the voltage appearing at both ends of the electrostatic capacitance for detecting the electrode plane direction in accordance with the external force between the electrodes.

Next, details of the operation will be described. Generally, in the capacitance type sensor, the external force and the inter-electrode distance d are in a proportional relationship, and the electrode distance d and the electrostatic capacitance C are in an inversely proportional relationship as shown in FIG. In FIG. 4, the electrostatic capacitance component changed by applying a constant force at the point of the inter-electrode distance d ₁ shown by the solid line is C ₁ . Similarly, at the point of the inter-electrode distance d ₂ indicated by the broken line, the electrostatic capacitance component changed by applying the same force is C ₂ . Comparing C ₁ and C ₂ , it can be seen that the value of C _{1 having} a short distance between the electrodes becomes large. In other words, the detected capacitance value differs depending on the distance between the electrodes even with the same applied force,
This is the error in the detected voltage.

Assuming that the movable part electrode is subjected to a force between the electrodes,
It moves about ± 10% with respect to the average inter-electrode distance d ₀ indicated by the one-dot chain line, and at this point, 5% of the average inter-electrode distance in the inter-electrode plane direction.
When a force equivalent to that is applied, the difference in sensitivity in the electrode plane direction between the maximum point and the minimum point between the electrodes is as follows.
Capacitance C = (dielectric constant ε × electrode area S) / distance d between electrodes
Therefore, if the permittivity and the electrode area are fixed and the capacitance for detecting the force in the inter-electrode direction is K _CZ, and the capacitance for detecting the force in the inter-electrode plane direction is K _CX , the inter-electrode distance Detect the force of.

The capacitance C ₀ of the average distance between the electrodes for detecting the force of the distance between the electrodes is C ₀ = K _CZ ÷ d ₀ , and the capacitance C at the minimum point of the distance between the electrodes is the external force in the direction between the electrodes.
₁ is C ₁ = K _CZ ÷ (0.9 × d ₀ ), and the capacitance C at the maximum point of the inter-electrode distance due to the external force in the inter-electrode direction.
₂ is C ₂ = K _CZ ÷ (1.1 × d ₀ ).

Capacitance C when an external force in the electrode plane direction is applied to the state of C ₀ for detecting the force in the electrode plane direction
₇ , C ₇ = K _CX ÷ (1.0 × d ₀ −0.05 × d ₀ ) = K _CX ÷ (0.95 × d ₀ ), and an external force in the electrode plane direction was applied to the state of C ₀ . The electrostatic capacitance C ₈ at this time is C ₈ = K _CX ÷ (1.0 × d ₀ + 0.05 × d ₀ ) = K _CX ÷ (1.05 × d ₀ ), in the electrode plane direction in the state of C _1. The electrostatic capacitance C ₃ when the external force is applied is C ₃ = K _CX ÷ (0.9 × d ₀ −0.05 × d ₀ ) = K _CX ÷ (0.85 × d ₀ ), C ₁ The electrostatic capacitance C ₄ when an external force in the electrode plane direction is applied to the state is C ₄ = K _CX ÷ (0.9 × d ₀ + 0.05 × d ₀ ) = K _CX ÷ (0.95 × d ₀ ), the electrostatic capacitance C ₃ when an external force in the electrode plane direction is applied to the state of C ₂ is C ₅ = K _CX ÷ (1.1 × d ₀ −0.05 × d ₀ ) = K _CX ÷ (1.05 × d _0), the capacitance C ₄ when the external force of the electrode plane direction is applied to the state of C ₂ is, C ₆ = K _{CX (1.1 × d 0 + 0.05 ×} d 0) = K CX ÷ (1.05 × d 0).

Since the detected force is proportional to the electrostatic capacitance, the coefficient for changing the electrostatic capacitance in the electrode plane direction into a voltage is K.
Let _VX be the detection voltage V _X1 in the electrode plane direction of d ₁ and d ₂ ,
And V _X2 is given by the following formula.

[0018]

[Equation 1]

[0019]

[Equation 2]

[0020]

(Equation 3)

Comparing the detected voltages, it can be seen that there is a large difference despite the same external force. Basically,
Since the external force of 5% is applied in the inter-electrode plane direction and two electrostatic capacitances are detected by the operating method, the first coefficient on the right side of the above equation is a true value of 0.05 × 2 = 0.100. Therefore,
It can be seen that the detection error is + 24% at the point of d ₁ and -17% at the point of d ₂ . This state is shown in FIG. 5 as the inter-electrode direction detection output due to the difference in the average inter-electrode distance.

According to the present invention, the above V _X1 and V _X2 can be corrected by the average inter-electrode distance as follows. _Letting K _CZ be a coefficient for converting the electrostatic capacitance in the inter-electrode distance direction into a voltage, the detection voltage V _Z0 of the average inter-electrode distance at the point of d ₀ is V _Z0 = K _VZ × C ₀ = K _VZ × K _CZ The detection voltage V _Z1 of the average inter-electrode distance at the points ÷ d ₀ and d ₁ is V _Z1 = K _VZ × C ₁ = K _VZ × K _CZ ÷ (0.9 × d ₀ ), the average of the points d ₂ The detection voltage V _{Z2 of the} distance between the electrodes is V _Z2 = K _VZ × C ₂ = K _VZ × K _CZ ÷ (1.1 × d ₀ ).

[0023] In the value obtained by multiplying a predetermined proportionality constant K _ZH to the detected voltage, the gain of the inter-electrode plane direction of the detection voltage is 1 the correction coefficient of a point d _0, when the average distance between the electrodes is narrow small, When it is wide, control it so that it becomes large. When the detection voltage after the correction of V _X1 and V _X3, becomes the following equation.

[0024]

(Equation 4) _{However, V Z1 -V Z0 = (K} CZ × K CZ ÷ d 0) × (1 ÷ 0.9-1) = (K VZ × K CZ ÷ d 0) × 0.111

Here, V _X1 is basically a double change amount due to the operating system with respect to (V _Z1 -V _Z0 ), and corresponding to this, K _ZH × (K _VZ × K _CZ ÷ K _ZH so that d ₀ ) is 2.
If V _x3 is adjusted, V _X3 becomes the following formula.

[0026]

(Equation 5)

If the detected voltage after correction of V _X2 is V _X4 ,
It becomes the following formula.

[0028]

(Equation 6) _{However, V Z2 -V Z0 = (K} VZ × K CZ ÷ d 0) × (1-1 ÷ 1.1) = (K VZ × K CZ ÷ d 0) × -0.091

Here, in the same manner as described above, K _ZH × (K _VZ × K _CZ
If K _ZH is adjusted so that ÷ d ₀ ) _becomes 2, V _X4 is obtained.

[0030]

(Equation 7)

As described above, according to the present invention, the detection sensitivity of the force in the electrode plane direction can be corrected to an error of about 1% even if the average inter-electrode distance is different.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A signal processing circuit shown in FIG. 6 will be described as an embodiment of the present invention. A clock pulse having a constant amplitude is externally applied to the electrostatic capacitance elements 1, 3 and 4 for detecting the external force between the electrodes. The fixed capacitance 2b, which is substantially equal to the electrostatic capacitance for detecting the external force in the inter-electrode direction, is connected in parallel in the same circuit, and the pulse having the opposite phase to the pulse applied to the electrostatic capacitance for detection is applied with a constant amplitude.

When the external force in the inter-electrode direction is zero, the waveforms at both ends of the fixed capacitance 2b and the electrostatic capacitance 1 for detecting the inter-electrode direction external force are the same, and the output of the operational amplifier U ₁ becomes zero. When an external force is applied to the inter-electrode direction, the inter-electrode direction external force detecting electrostatic capacitance 1 changes, and the voltage waveform across the electrostatic capacitance also changes, so that an output appears in the operational amplifier U ₁ . By synchronously detecting and smoothing this output voltage by the synchronous detection and smoothing circuit 2a, the inter-electrode direction external force is detected.

Basically, the same idea is applied to the detection of the external force in the electrode plane direction. The electrostatic capacitances 3 and 4 for detecting the external force in the electrode plane direction are of the differential type, and the electrostatic capacitance for detection is also included in the fixed capacitance 2b. 3, 4 are connected. Since the respective capacitances increase or decrease conversely, it is possible to obtain a detection output that is twice as large as the detection output of the external force in the inter-electrode direction. Here, the detection voltage in the inter-electrode direction is also connected to a voltage dividing circuit, and divided in a constant ratio so that the amplitude of the pulse driving the differential electrode plane direction external force detection capacitance can be varied. ing. The amplitude of this pulse is made to decrease as the inter-electrode direction external force detecting capacitance increases, and the detection output of the electrode plane direction external force described above is corrected.

In the above embodiments, the method of changing the amplitude of the driving pulse in order to change the detection sensitivity in the inter-electrode plane direction has been described, but a multiplier may be provided for the detection output of the external force in the electrode plane direction. It is possible to adopt a means of treating as gain change due to resistance value change using FET or the like.

[0036]

The signal processing circuit for an electrostatic capacitance type sensor according to the present invention has a very simple and inexpensive circuit which controls the detection sensitivity of the force in the electrode plane direction by the detection voltage in the inter-electrode distance direction. Even if the average distance between the electrodes of the multi-axis force sensor having a plurality of electrodes in the movable portion and the fixed portion is different, the detection sensitivity of the external force in the electrode plane direction can be made substantially constant, and a high performance sensor can be provided.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a basic configuration of a signal processing circuit of a capacitance type sensor.

FIG. 2 is a plan view of a capacitance type sensor.

FIG. 3 is a cross-sectional explanatory view of a capacitance type sensor.

FIG. 4 is a graph showing a relationship between a distance between electrodes and a capacitance.

FIG. 5 is a graph showing the detection output of the external force in the electrode plane direction due to the difference in the average inter-electrode distance.

FIG. 6 is a circuit diagram showing a specific example of a signal processing circuit according to the present invention.

7 is an explanatory diagram showing an example of operation waveforms of the signal processing circuit of FIG.

[Explanation of symbols]

1, 3, 4 Capacitance element 2, 5, 6 CV conversion circuit 2a, 7a Synchronous detection smoothing circuit 2b Fixed capacitance 7, U ₁ , U ₂ Operational amplifier 8, 9 Sensitivity adjustment circuit 10 Frame 11 Fixed substrate 12 Yes Flexible substrate 13 Actuator 26 Electrodes C _{21 to} C ₂₅ Capacitance element

Claims (1)

[Claims] 1. A single movable part that moves by the action of an external force is provided, a plurality of electrodes are arranged on the movable part, and a capacitive element is constituted by the electrode of the movable part and an electrode pair of the fixed part. In the signal processing circuit for the sensor that detects the external force in the distance direction between the electrodes and the external force in the planar direction of the electrode based on the change in the capacitance of the A circuit having an adjusting circuit, wherein the detection signal of the external force in the inter-electrode direction is connected to the sensitivity adjusting circuit, and the detection sensitivity of the external force in the planar direction of the electrodes can be controlled according to the external force in the inter-electrode direction. A signal processing circuit for a sensor that utilizes a change in capacitance.