JP2009244049A - Cv converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a CV converter with a simple circuit configuration for reducing jitter noise and high-frequency noise. <P>SOLUTION: The CV converter is configured so that a change in the capacitance of a capacitive element is converted to a voltage and by giving an alternating constant current to a counter electrode of a device to be measured, by adding the voltage of both ends of the counter electrode after the balance adjustment and by further adding an alternating constant current converted to a voltage and by performing an AM detection thereof. Preferably the device to be measured is an electrostatic capacitor sensor. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention generally relates to a CV converter that converts capacitance to voltage. More specifically, the present invention relates to a CV converter used for signal processing of a capacitive sensor.

  As a sensor for detecting acceleration, angular velocity, force sense, pressure, and the like, a capacitive sensor having advantages such as high sensitivity and low power consumption has attracted attention (see Patent Document 1). Taking the acceleration sensor as an example, the basic principle of the capacitive sensor will be described with reference to FIG. The capacitance type sensor includes a fixed substrate 10, a movable substrate 12 made of a flexible material disposed to face the fixed substrate 10 at a predetermined interval, and a weight attached to the lower center surface of the movable substrate 12. 14 and a case 16 that houses the fixed substrate 10, the movable substrate 12, and the weight 14 (see FIG. 3A). The fixed substrate 10 is a disk-shaped substrate whose periphery is fixed to the case 16, and a disk-shaped fixed electrode 1a is disposed on the lower surface thereof (see FIG. 3B). The movable substrate 12 is also a disk-shaped substrate whose periphery is fixed to the case 16, and fan-shaped displacement electrodes 12a to 12d and a disk-shaped displacement electrode 12e are arranged on the upper surface thereof (FIG. 3 (c). )reference).

An action point P is defined at the center of gravity of the weight 14, and the action point P is defined as an origin, and an X axis is defined in the right direction in FIG. In a state where no external force is applied to the action point P, as shown in FIG. 3A, the fixed electrode 10a and the displacement electrodes 12a to 12e are parallel to each other at a predetermined interval. Now, it is assumed that a force + F _X in the X-axis direction acts on the action point P when acceleration is applied to the sensor. Then, a bending moment is applied to the movable substrate 12 by F _X , which causes the movable substrate 12 to bend. As a result, the interval between the fixed electrode 10a and the displacement electrode 12a increases, and the interval between the fixed electrode 10a and the displacement electrode 12c. Becomes smaller (see FIG. 3D). Moreover, the force -F _X in the X-axis direction to the point P, Y-axis direction force + F _Y, -F _Y
In the case of acting, the same change occurs in the distance between the fixed electrode 10a and the displacement electrodes 12a to 12d.

  Capacitors increase in capacitance when the distance between the opposing electrodes decreases, and decrease in capacitance when the distance between the electrodes increases, but with capacitive sensors, the change in capacitance between each electrode is measured. Then, the acceleration applied to the sensor is detected based on the measured value. Note that the same principle is used when detecting angular velocity, force sense, pressure, and the like other than acceleration.

  Conventionally, a switched capacitor method has been the mainstream as a method of a CV converter that changes a capacitance change of a capacitance type angular velocity / acceleration / force sensor / pressure sensor into a voltage. A switched capacitor type CV converter mainly comprises a digital circuit for sequence control, an analog switch, a phase detection circuit, a low-pass filter (LPF), and the like.

Japanese Patent Laid-Open No. 4-019568 JP 2003-042879 A

  However, the conventional switched capacitor CV converter has a problem that jitter noise and harmonic noise are likely to occur in the digital circuit for sequence control, the analog switch, and the phase detection circuit. Further, there has been a problem that the circuit becomes complicated if the generation of such noise is suppressed. In particular, in an angular velocity sensor, since the sensitivity is as small as about 10 aF / deg / s, it is essential to reduce noise.

  The present invention has been developed in view of such a situation, and an object of the present invention is to provide a CV converter capable of reducing jitter noise and high frequency noise with a simple circuit configuration.

  The CV converter according to claim 1 of the present invention applies an AC constant current to the counter electrode of the device to be measured, and balances and adds the voltages at both ends of the counter electrode to the AC constant current. What is converted into a voltage is further added, and AM detection is performed on the added value to convert a change in capacitance of the capacitive element into a voltage.

  The CV converter according to claim 2 of the present application is the CV converter according to claim 1, wherein the balance adjustment is performed by multiplying a voltage at both ends of the counter electrode by a predetermined ratio and converting the voltage into a voltage. It is characterized by this.

  The CV converter according to claim 3 of the present application is configured so that a predetermined coefficient is multiplied when the AC constant current is converted into a voltage in the CV converter of claim 1 or 2. It is a feature.

  A CV converter according to claim 4 of the present invention is the CV converter according to any one of claims 1 to 3, wherein the device is a capacitive sensor. It is.

  The CV converter of the present invention does not generate jitter noise and can have a simple circuit configuration as compared with a switched capacitor type or a synchronous detection type. In the CV converter of the present invention, it is easy to adjust the balance of the capacitive element. Furthermore, in the CV converter of the present invention, by performing AM detection by adding a carrier wave, it is possible to determine the sign of the difference between the capacitive elements and to avoid a dead zone during AM detection.

  Next, with reference to the drawings, a CV converter according to a preferred embodiment of the present invention will be described in detail by taking as an example a case where it is used for signal processing of a capacitive sensor. FIG. 1 is a block diagram for explaining the basic concept of a CV converter according to a preferred embodiment of the present invention.

The CV converter is provided with a first capacitor C ₁ and a second capacitor C ₂ that serve as capacitive elements. These capacitors C ₁ and C ₂ serve as counter electrodes of the sensor used for measurement.

One end of the first capacitor C ₁ is connected to one end of the first AC voltage generation source 20a (voltage e ₁ ) via the first resistance element 22a (resistance value R ₁ ). One end of the second capacitor C ₂ is connected to one end of the second AC voltage generation source 20b (voltage e ₂ ) via the second resistance element 22b (resistance value R ₂ ). Thus, a current i ₁ ≒ e ₁ / R ₁ flowing through the first capacitor C _1, the current i ₂ ≒ e ₂ / R ₂ of the second through the capacitor C _2.

The first AC voltage generation source 20a and the second AC voltage generation source 20b are for supplying a signal voltage to the signal processing circuit, and are 180 ° out of phase with each other. That is, the signal voltage of the first AC voltage source 20a is
e ₁ = A × sin (ωt)
Then,
The signal voltage of the second AC voltage source 20b is
e ₂ = −A × sin (ωt)
It is represented by Here, A is the voltage amplitude of the signal, and ω is the angular frequency.

The other end of the first AC voltage source 20a is connected to a first end of the capacitor C _1, the other end of the second AC voltage source 20b is also connected to the second end of the capacitor C _2. The other end of the first AC voltage source 20a and has the other end is connected to the second ac voltage source 20b, the other end a second end of the capacitor C ₂ of the first capacitor C ₁ is also connected Yes. As a result, at the contact 0 in FIG. 1, charging and discharging are repeated so that the voltages cancel each other and become zero.

The first end of the capacitor C ₁ is, via a first buffer 24a, is connected to the first amplifier 26a, one end of the second capacitor C ₂ is also through the second buffer 24b, connected to the second amplifier 26b Has been. The reason why the first buffer 24a and the second buffer 24b are provided is to reduce impedance.

In the first amplifier 26a, an AC predetermined ratio alpha ₁ is multiplied by the constant current is converted to a voltage v _1, the second amplifier 26b, converted alternating current predetermined ratio alpha ₂ is multiplied by the constant current to a voltage v ₂ Is done. The first amplifier 26a and the second amplifier 24b are provided in order to adjust the balance between the _two capacitors C ₁ and C ₂ . In other words, the capacitor contains a specific error (it is theoretically possible to remove the specific error by trimming with a laser beam, but a complicated process is required), and without adjusting the balance. When the capacitors C ₁ and C ₂ are used, a situation in which addition of force or the like to the sensor is detected even when force or the like does not act on the sensor due to an inherent error of the capacitor. In order to avoid the occurrence of such a situation, balance adjustment is performed.

One end of the first AC voltage generation source 20a is connected to the third amplifier 28. In the third amplifier 28, the AC constant current is multiplied by a predetermined ratio α _C to be converted into a voltage v _C , and the first amplifier 26a. The voltage is added in the adder 30 of the operational amplifier together with the voltages v ₁ and v ₂ that are balanced through the second amplifier 26b. That is, in the adder 30, v ₁ −v ₂ + v _C = v.

  The voltage v thus added is subjected to AM detection in the AM detection circuit 32.

  Next, how the CV converter configured as described above operates will be described with reference to FIG.

FIGS. 2A to 2C are a series of diagrams showing a state in which the capacitances of the first and second capacitors C ₁ and C ₂ do not change (in other words, no force acts on the sensor). is there. That is, FIG. 2A shows the voltages v ₁ , v ₂ , and v _C, but the capacitances of the capacitors C ₁ and C ₂ do not change, so the amplitudes of the voltages v ₁ and v ₂ are the same. And v ₁ −v ₂ cancels to zero. Therefore, v = v ₁ −v ₂ + v _C = v _C , only the carrier voltage remains, the amplitude remains constant (see FIG. 2B), and when AM detection is performed, the voltage V becomes a constant value. (See FIG. 2C).

FIG. 2D to FIG. 2F show a state in which the capacitance of the first capacitor C ₁ changes (in other words, a force acts on the sensor to change the capacitance of C ₁ ). FIG. That is, since the capacitance of the capacitor C ₁ increases from time t ₀ , the voltage v ₁ > v ₂ is satisfied (see FIG. 2D). Therefore, v = v ₁ −v ₂ + v _C increases from time t ₀ (see FIG. 2E), and when AM detection is performed, the voltage V increases from time t ₀ (see FIG. 2F). Thereby, the addition of force to the sensor is detected.

Figure 2 (g) ~ FIG 2 (i) is (in other words, a force to the sensor acts, the capacitance of C ₂ changes) the second electrostatic capacitance of the capacitor C ₂ is changed indicates the state FIG. That is, the voltage v ₁ for the time t ₀ increases the capacitance of the capacitor C ₂ becomes <v ₂ (see FIG. 2 (g)). Therefore, v = v ₁ −v ₂ + v _C decreases from time t ₀ (see FIG. 2 (h)), and when AM detection is performed, voltage V decreases from time t ₀ (see FIG. 2 (i)). Thereby, the addition of force to the sensor is detected.

Even when the capacitances of both the first capacitor C ₁ and the second capacitor C ₂ change, the same operation as described above is performed.

Note that the direction of the force applied to the sensor can be determined by adding the voltage v _C obtained by multiplying the AC constant current by a predetermined ratio α _C in the third amplifier 28 to v ₁ −v _2. (See FIG. 2 (f) (i)).

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the invention described in the claims, and these are also included in the scope of the present invention. Needless to say, it is something.

For example, in the above-described embodiment, the case where it is used for signal processing of a capacitive sensor has been described as an example. However, the CV converter of the present invention is also used when detecting changes in other capacitive elements. May be used. In the third amplifier 28, the AC constant current is multiplied by a predetermined ratio α _C. However, the carrier wave may be added without multiplying the ratio α _C.

It is a block diagram for demonstrating the basic concept of the CV converter which concerns on preferable embodiment of this invention. It is a series of figures for demonstrating operation | movement of the CV converter of FIG. 3A and 3B are a series of diagrams showing a capacitive sensor, where FIG. 3A is a side sectional view of the capacitive sensor, and FIG. 3B is taken along line 3b-3b in FIG. FIG. 3C is a view seen along line 3c-3c in FIG. 3A, and FIG. 3D is a side sectional view showing a state in which acceleration is applied to the sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fixed substrate 10a Fixed electrode 12 Movable substrate 12a-12e Displacement electrode 14 Weight 16 Case 20a, 20b AC power source 22a, 22b Resistive element 24a, 24b Buffer 26a, 26b, 28 Amplifier 30 Adder 32 AM detector circuit

Claims (4)

An AC constant current is applied to the counter electrode of the device to be measured, the voltage at both ends of the counter electrode is balanced and added, and the AC constant current converted into a voltage is further added to AM. A CV converter configured to convert a change in capacitance of a capacitive element into a voltage by detection. The CV converter according to claim 1, wherein the balance adjustment is performed by multiplying a voltage at both ends of the counter electrode by a predetermined ratio to convert the voltage into a voltage. 3. The CV converter according to claim 1, wherein a predetermined coefficient is multiplied when the AC constant current is converted into a voltage. 4. The CV converter according to any one of claims 1 to 3, wherein the device is a capacitive sensor.

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