JP3354727B2 - Capacitive acceleration sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acceleration sensor.
In particular, the present invention relates to a series capacitance type acceleration sensor that detects acceleration by a force that restrains a movable electrode at a center position, thereby improving a dynamic range and linearity.

[0002]

2. Description of the Related Art Conventionally, a weight, which is displaced by receiving an acceleration due to fine processing of a silicon substrate, is supported by a cantilever, and a movable electrode formed on both sides of the weight and a small gap is formed with respect to the movable electrode. 2. Description of the Related Art A capacitance type acceleration sensor in which a capacitance connected in series with a fixed electrode is formed is known (Japanese Patent Laid-Open No. 1-253657). Then, the sensor detects a change in the capacitance of the series connection and applies a voltage corresponding to the change to the movable electrode, so that the external force due to acceleration and the movable electrode due to the electric charge charged by the applied voltage are applied to the movable electrode. The acceleration is detected while the movable electrode is held at the intermediate position by balancing the electrostatic force generated between the movable electrode and the fixed electrode. The voltage applied to hold the movable electrode at the intermediate position is a voltage that has been pulse width modulated in accordance with the detected change in capacitance.

[0003]

However, this sensor,
There is a problem that a circuit for changing the duty ratio of the applied square wave is required, and a circuit for detecting a change in capacitance requires high-speed response, so that the circuit configuration is complicated. . The present invention has been made to solve the above problems, and an object of the present invention is to simplify a circuit configuration, improve linearity, and increase sensitivity and a dynamic range.

[0004]

In order to solve the above-mentioned problems, a configuration of the present invention is to provide a series-connected capacitive type in which voltage feedback is performed so that a movable electrode is located at the center, and acceleration is detected based on the voltage value. The acceleration sensor described above is characterized in that the acceleration sensor is constituted by a series-connected switch circuit for converting a series-connected capacitance into an equalizing resistance.

[0005] That is, the characteristic part is that the capacitance detection circuit includes a first switch, a second switch, a third switch, and a fourth switch, and a set of the first switch and the fourth switch, which are connected in series to a power supply. , A control circuit for alternately turning on and off a set of a second switch and a third switch, wherein one of the fixed electrodes is connected to a connection point between the first switch and the second switch, and the other of the fixed electrodes is a third switch. The first switch and the fourth switch are connected to a connection point between the switch and the fourth switch, and the first switch and the fourth switch are turned on, and the second switch and the third switch are turned off to charge the capacitance. A circuit configured to discharge the capacitance by turning off and turning on the second switch and the third switch, and to detect a change in capacitance of the capacitance based on a change in potential at a connection point between the second switch and the third switch. And, a feedback circuit, in accordance with the change amount with respect to the reference potential of the potential at the connection point between the second switch and the third switch, which is formed of the circuit for applying a feedback voltage to the movable electrode.

[0006]

The voltage corresponding to the deviation of the potential of the connection point between the third switch and the fourth switch from the reference potential is applied to the movable electrode. This applied voltage charges (or discharges) electric charges to the movable electrode, and the charged electric charges generate an electrostatic force that balances the external force received by the weight due to the acceleration. Transition is suppressed. The magnitude of the voltage applied to the movable electrode is a value proportional to the restraining force at the center of the movable electrode, that is, the detected acceleration.

As described above, in the circuit in which the voltage is negatively fed back to the movable electrode, a set of the first switch and the fourth switch,
The switch and the third set of switches are turned on and off alternately.
Therefore, a pair of capacitances are charged by turning on the first and fourth switches and turning off the second and third switches, and by turning off the first and fourth switches and turning on the second and third switches.
The pair of capacitances is discharged. These on / off operations are performed with a high-frequency signal. At this time, the first and second switches are connected in series by a resistance proportional to the product of the capacitance and the switching frequency on the side where the fixed electrode is connected to the connection point;
The series connection of the third and fourth switches is equivalent to a resistance proportional to the product of the capacitance on the side where the fixed electrode is connected to the connection point and the switching frequency. Therefore, the potential of the connection point between the second switch and the third switch changes according to the change in the capacitance of the series connection. Therefore, the change in capacitance is detected as the potential at the connection point between the second switch and the third switch, and a negative feedback voltage is applied to the movable electrode according to the amount of change in the potential. Since this voltage is a voltage proportional to the electrostatic force balanced with the external force, the acceleration is detected based on this voltage.

[0008]

As described above, the magnitude of the capacitance formed by the movable electrode and the fixed electrode can be detected by turning on and off the four switches connected in series.
The circuit configuration of the sensor becomes extremely simple. Further, since the movable electrode is feedback-controlled so as to be located at the center between the pair of fixed electrodes, the detection sensitivity and linearity are improved, and the dynamic range is expanded.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to specific embodiments. FIG. 1 is a sectional view showing the configuration of the acceleration sensor 1, and FIG. 2 is a plan view thereof. 2 is an upper glass substrate,
Reference numeral 3 denotes a lower glass substrate, and a finely processed silicon substrate 4 is bonded to the glass substrates 2 and 3. On the silicon substrate 4, four beams 5 formed in a cross shape by micromachining and weights 6 supported by the beams and formed around the beams 5 are formed. The movable electrodes 7a and 7b are formed on both surfaces of the weight 6. The surfaces of the conductive silicon substrate 4 also serve as the movable electrodes 7a and 7b. The wiring for the movable electrodes 7a and 7b also serves as the silicon substrate 4.

Movable electrodes 7 formed on both sides of weight 6
a and 7b are upper fixed electrodes 9a formed on the upper glass substrate 2 with the minute gaps 8a and 8b interposed therebetween, respectively.
It faces the lower fixed electrode 9b formed on the lower glass substrate 3. A first capacitance C1 is formed between the movable electrode 7a and the upper fixed electrode 9a.
, And a second capacitance C2 is formed.
42a and 42b are movable electrodes 7a and 7b, respectively.
Is an insulating stopper for preventing connection to the upper fixed electrode 9a and the lower fixed electrode 9b.

The upper fixed electrode 9a and the lower fixed electrode 9b are respectively connected to pads 10a and 10b formed on the upper surface 4a of the silicon substrate 4 not covered with the upper glass substrate 2 and insulated from the silicon substrate 4. It is connected.
The movable electrodes 7 a and 7 b are electrically connected to each other via the silicon substrate 4 and are electrically connected to pads 11 formed on the upper surface 4 a of the silicon substrate 4 which is not covered with the upper glass substrate 2.

The circuit shown in FIG. 3 is connected to these pads 10a, 10b and 11. The circuit shown in FIG. 3 is an IC
Has been In FIG. 3, C1 and C2 correspond to the first
And the second capacitance. A first switch 21, a second switch 22, and a second switch 22 are provided between the voltage source V _DD and the ground.
A series connection circuit of the third switch 23 and the fourth switch 24 is provided. These switches are constituted by analog gates using transistors. The connection point a between the first switch and the second switch is connected to the upper fixed electrode 9a of the first capacitance C1, and the connection point c between the third switch and the fourth switch is connected to the second static switch. It is connected to the lower fixed electrode 9b of the capacitance C2.

The second switch 22 and the third switch 23
Is connected to the non-inverting input terminal of the operational amplifier 31, and the reference potential V _DD / 2 is applied to the inverting input terminal of the operational amplifier 31 via a resistor. This reference potential V _DD /
2 is substantially equal to the potential of the connection point b when no acceleration is applied to the weight 6. The output of the operational amplifier 31 is connected to a non-inverting input terminal of the operational amplifier 32 via an integrating circuit 33, and a reference potential V ₀ is applied to the inverting input terminal of the operational amplifier 32 via a resistor. The output of the operational amplifier 32 is connected to the movable electrodes 7a and 7b of the weight 6. The amplification factor of the operational amplifier 32 is designed to be large.

The first switch 21 to the fourth switch 24
Is higher frequency f by the control circuit 40 having the oscillator 41.
Is turned on and off. In this control, the first switch 21 and the fourth switch 24 are turned on and the second switch 22
When the third switch 23 is off, the first capacitance C1 and the second capacitance C2 are charged from the voltage source _VDD . or,
When the first switch 21 and the fourth switch 24 are off and the second switch 22 and the third switch 23 are on, the first capacitance C1 and the second capacitance C2 are discharged.

A first capacitance C1 and a second capacitance C2
Is equal to the reference potential V _DD
/ 2, and when the first capacitance C1 is larger than the second capacitance C2, the potential V at the connection point b becomes larger than the reference potential V _DD / 2, The capacity C1 is the second
Is smaller than the capacitance C2, the potential V at the connection point b becomes smaller than the reference potential V _DD / 2. In this circuit, a deviation between the potential V at the connection point b and the reference potential V _DD / 2 is amplified, and the amplified value is a potential V which is a deviation from the reference potential V _0.
S is applied to the movable electrodes 7a and 7b. Movable electrodes 7a This potential V _S, 7b is charged, a first capacitance C1 and the second capacitance C2 and is equal direction to the movable electrode 7a, 7b are subjected to electrostatic forces. When the first capacitance C1 and the second capacitance C2 try to be equal, the potential V at the connection point b approaches the reference potential V _DD / 2, and the movable electrode 7
a, the potential V _S applied to 7b also close to the reference potential V _0, the movable electrodes 7a by electrostatic force, centripetal force experienced by the 7b is also reduced. Note that the reference potential V ₀ is set to a potential necessary to hold the movable electrodes 7a and 7b at the center when the acceleration is 0.

In this manner, feedback control is performed by the above-described circuit so that the movable electrodes 7a and 7b are always kept at the center position. In this state, the deviation [Delta] V _S with respect to the reference potential V ₀ potential V _S applied movable electrode 7a, and 7b is proportional to the electrostatic force commensurate with the force exerted on the weight 6 by the acceleration, the deviation [Delta] V _S is measured Will be proportional to the acceleration.

As described above, in the present invention, the capacitance is measured by the four series-connected switch circuits, so that the circuit configuration is extremely simple. Also, since the acceleration is measured while the weight 6 is always held at the center position regardless of the magnitude of the applied acceleration, the measurement sensitivity and linearity are improved, and the dynamic range is expanded.

[Brief description of the drawings]

FIG. 1 is a side sectional view showing a configuration of a capacitive acceleration sensor according to a specific embodiment of the present invention.

FIG. 2 is an exemplary plan view showing the configuration of the capacitive acceleration sensor according to the embodiment;

FIG. 3 is a circuit diagram showing a circuit configuration for measuring the acceleration of the capacitive acceleration sensor according to the embodiment.

[Explanation of symbols]

 C1: first capacitance C2: second capacitance 2: upper glass substrate 3: lower glass substrate 4: silicon substrate 5: beam 6: weight 7a, 7b: movable electrode 8a, 8b: minute gap 9a, 9b ... fixed electrode 21 ... 1st switch (capacitance detection circuit) 22 ... 2nd switch (capacitance detection circuit) 23 ... 3rd switch (capacity detection circuit) 24 ... 4th switch (capacitance detection circuit) 40 ... control circuit (capacitance) Detection circuit) 31, 32 ... operational amplifier (feedback circuit) 32 ... integration circuit (feedback circuit)

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI G01R 15/00 G01R 15/10 F 15/08 (72) Inventor Sakurai Tosuijo 1-1-1, Asahimachi, Kariya-shi, Aichi Pref. (72) Inventor Masayoshi Esashi 1-11-11, Yagiyama-Minami, Taihaku-ku, Sendai, Miyagi Prefecture 9 (56) References JP-A-2-110383 (JP, A) JP-A-4-337468 (JP, A) JP-A-5-340958 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01P 15/125 G01D 3/02 G01D 3/06 G01D 5/24 G01H 11/06 G01R 15/00 G01R 15/08

Claims (1)

(57) [Claims] 1. A weight formed by fine processing of a semiconductor substrate and displaceable according to acceleration, a movable electrode formed on both sides of the weight, and a small gap from the movable electrode. A pair of fixed electrodes provided, a capacitance detection circuit for detecting a change in a pair of capacitances connected in series formed by the movable electrode and the pair of fixed electrodes,
A feedback circuit that applies a feedback voltage between the movable electrode and the fixed electrode so that a change in the capacitance detected by the capacitance detection circuit with respect to the applied acceleration is suppressed, and the feedback voltage is detected. A capacitive acceleration sensor that outputs a signal corresponding to the acceleration to be applied, wherein the capacitance detection circuit includes a first switch, a second switch, a third switch, and a fourth switch connected in series to a power supply; A control circuit that alternately turns on and off a set of one switch and the fourth switch, and a set of the second switch and the third switch, wherein one of the fixed electrodes includes the first switch and the second switch; And the other of the fixed electrodes is connected to the third
The first switch is connected to a connection point between a switch and a fourth switch, and the first switch and the fourth switch are turned on, and the second switch and the third switch are turned off to charge the capacitance. And turn off the fourth switch,
The capacitance is discharged by turning on the second switch and the third switch, and a capacitance change of the capacitance is detected by a change in potential at a connection point between the second switch and the third switch. Wherein the feedback circuit is a circuit that applies a feedback voltage to the movable electrode in accordance with an amount of change in a potential at a connection point between the second switch and the third switch with respect to a reference potential. Capacitive acceleration sensor.