JP3157907B2 - Acceleration detector - 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 detecting device using a differential impedance element whose impedance changes differentially in response to an external acceleration. More specifically, the present invention relates to a drive circuit configuration for operating a differential impedance element.

[0002]

2. Description of the Related Art In general, it is easy to detect a relatively high acceleration. For example, an acceleration sensor using a piezoelectric element,
2. Description of the Related Art An acceleration sensor of a type that electrically detects a distortion generated in a semiconductor is widely used. However, detection of low acceleration is relatively difficult, and the structure of the acceleration sensor is complicated. For low acceleration detection, a differential impedance type, for example, a differential capacitance type acceleration sensor has a relatively simple structure and is generally used.

FIG. 6 shows a conventional differential capacitive acceleration sensor. The structure shown is, for example, U.S. Pat.
No. 87. As illustrated, the acceleration sensor has a laminated structure including five plate-like members. On the inner surfaces of the pair of substrates 101 and 102, metal patterns similar to each other are formed. For example, the lower substrate 10
1 has a frame-shaped metal pattern 10 along its outer edge.
3 and a rectangular metal pattern 104 arranged at the center. Both metal patterns are electrically insulated from each other by the separator. Further, spacers 105 and 106 are arranged so as to overlap with the substrates 101 and 102, respectively. Spacers 105 and 106 each have a central opening. An intermediate substrate 107 is sandwiched between the spacers 105 and 106. The intermediate substrate 107 has a central segment 109 supported by a spring member 108. The spring member 108 is formed by a slit-shaped pattern extending in the surface direction of the intermediate substrate 107 and located around the central segment 109. With this configuration, the center segment 109 is flexibly supported by the frame 110. The five plate members described above are stacked on each other, and the center segment 109 and the spring member 108 are located at the center openings formed in the spacers 105 and 106. The center segment 109 can be displaced relative to a rectangular metal pattern formed on the substrates 101 and 102. The pair of rectangular metal patterns and the central segment disposed therebetween constitute variable capacitors C1 and C2 connected in series to obtain a so-called differential capacitor. In response to externally applied acceleration, the central segment 109
Is displaced along the direction perpendicular to the metal pattern, causing a change in impedance or a change in capacitance. The direction and magnitude of the acceleration are measured by electrically detecting the change in capacitance.

[0004]

The output of the above-described differential capacitance element is linear in principle with respect to acceleration. If the dielectric medium forming the capacitor is air, there is almost no temperature dependency. In addition, since the capacitance of the capacitor is increased by reducing the electrode interval, the detection accuracy is improved. Further, the Q value of the resonance is reduced due to the damper effect of air, so that good frequency response can be obtained.

[0005] When an acceleration sensor is incorporated in an automobile control system or the like, several G in a band of several tens Hz or less.
It is necessary to accurately detect the following low acceleration. Therefore, the gap between the electrodes is set to several μm to several tens μm.
At this time, the amount of displacement of the movable electrode in response to the external acceleration is about one tenth of the gap between the electrodes, and the required accuracy is 0.1.
It is on the order of 01 μm to 0.1 μm. Therefore, no matter how precisely the differential capacitance element is assembled, the movable electrode is displaced from the midpoint position, and the output in the no-acceleration state includes an offset. Further, there is a variation in sensitivity due to a subtle difference in a mechanical configuration of each impedance element. For this reason, the midpoint adjustment and the sensitivity adjustment have conventionally been required, and the adjustment has been performed mainly on the drive circuit side that operates the differential impedance element. However, in the conventional adjustment method, the midpoint adjustment and the sensitivity adjustment cannot be performed independently, and a large amount of work time is required for the adjustment operation, which is a problem.

[0006]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, an object of the present invention is to provide a drive circuit configuration capable of independently and independently adjusting the midpoint and sensitivity of a differential impedance element. And The following measures were taken to achieve this purpose. That is, the acceleration detection device according to the present invention includes a differential impedance element whose impedance changes differentially in response to an external acceleration, and a drive circuit that operates the same. The driving circuit includes an oscillation circuit for supplying a constant alternating input signal to the differential impedance element and obtaining a primary alternating output signal having an amplitude corresponding to the impedance change, and only an AC component of the primary alternating output signal. An AC amplifier circuit for amplifying the amplitude, an adding circuit for adding a predetermined DC component to the amplified AC component to generate a secondary alternating output signal, and sample-hold and rectify the secondary alternating output signal And a sample and hold circuit for generating a corresponding DC signal, and a DC amplifier circuit for amplifying the DC signal to obtain an acceleration detection signal.

[0007]

According to the present invention, first, only the AC component of the primary alternating output signal is amplitude-amplified by the AC amplifier circuit. The amplitude value of this AC component is proportional to the external acceleration. Subsequently, a predetermined alternating current component is added to the amplified alternating current component by an adding circuit to generate a secondary alternating output signal. Since the level of the DC component is set so as to cancel the offset, the midpoint adjustment of the differential impedance element is performed in a circuit. Depending on the direction of the offset, a DC component of a negative polarity may be added. At this time, the DC component is substantially subtracted. Therefore, the addition processing here is a concept including the subtraction processing. Subsequently, the secondary alternating output signal subjected to the offset compensation is converted into a DC signal by a sample and hold circuit. The DC level of this DC signal is proportional to the external acceleration. This DC signal is amplified by a DC amplifier circuit to obtain a final acceleration detection signal. The gain of the DC amplifier circuit is set to a desired value, and the variation in sensitivity of each differential impedance element can be adjusted in a circuit. As described above, according to the present invention, the midpoint adjustment and the sensitivity adjustment of each differential impedance element can be performed separately and independently using circuit means.

[0008] As the differential impedance element, for example, a differential capacitance element comprising a pair of fixed electrodes and a common movable electrode interposed in the gap therebetween can be used. However, it is needless to say that the present invention is not limited to this, and can be applied to other differential impedance elements such as a differential inductance element and a differential resistance element.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an example of the acceleration detection device according to the present invention. As shown in the figure, the acceleration detecting device is composed of a differential capacitive element 1 and a driving circuit 2 thereof. The differential capacitance element 1 is composed of a pair of variable capacitances C1 and C2 connected in series to each other. The fixed electrode forming the capacitance C1 is connected to the input terminal 3. The fixed electrode forming the other capacitor C2 is connected to another input terminal 4. A movable electrode interposed between the fixed electrodes facing each other is connected to the output terminal 5.

On the other hand, the drive circuit 2 has an oscillation circuit 6. The oscillating circuit 6 supplies constant alternating input signals VP and VN to a pair of input terminals 3 and 4. In response to this, the differential capacitive element 1 operates and a primary alternating output signal A having an amplitude corresponding to the impedance change appears at the output terminal 5. An AC amplifier circuit 7 is connected to the output terminal 5,
Only the AC component of the primary alternating output signal A is amplitude-amplified. An addition circuit 8 is connected to the AC amplification circuit 7, and adds a predetermined DC component to the amplified AC component B to generate a secondary alternating output signal C. The level of the DC component added by the adding circuit 8 is set by the resistance element R1.

A sample and hold circuit 9 is connected to the addition circuit 8 and the oscillation circuit 6. The sample and hold circuit 9 samples and holds and rectifies the secondary alternating output signal C in response to the clock signal G synchronized with the cycle of the aforementioned alternating input signal, and generates a corresponding DC signal D. A DC amplification circuit 10 is connected to an output terminal of the sample hold circuit 9, and amplifies the DC signal D to obtain a final acceleration detection signal. The gain of the DC amplifier 10 is set by the resistance element R2. Note that the output signal E of the DC amplifier 10 becomes the acceleration detection signal F via the output circuit 11 at the last stage.

As described above, the differential capacitance element 1 is composed of a pair of capacitors C1 and C2. Assuming that the electrode area of one capacitor C1 is S1, the distance between the electrodes is d1, and the dielectric constant of the space between the two is ε, the capacitance is C1 =
εS1 / d1. As for the other capacitor C2, assuming that the electrode area is S2 and the distance between the electrodes is d2,
Its capacity is given by C2 = εS2 / d2. Generally, the electrode areas S1 and S2 are set equal. If the intermediate movable electrode is correctly held at the midpoint position, the distances d1 and d2 between the electrodes become equal. In this case, in the no-acceleration state, the capacitances C1 and C2 of the capacitors connected in series are equal. Here, when an external acceleration α is applied, mα is applied to the movable electrode (m is the mass of the movable electrode).
Is applied, and is displaced by Δd until the equilibrium position is reached. As a result, the initial distances d1 and d2 between the electrodes are d1 +
Δd, d2-Δd. Therefore, the capacitor capacity C
1 and C2 change differentially and appear as a change in capacitance. For example, when an alternating input signal VP composed of a rectangular wave pulse is applied to one input terminal 3 and an alternating input signal VN composed of a rectangular wave pulse having a 180 ° phase difference is applied to the other input terminal 4 as shown in FIG. In this state, since the capacitances of the pair of capacitors are equal, a zero-level primary alternating output signal A appears at the output terminal 5. When an external acceleration is applied, the movable electrode is displaced according to the direction and magnitude of the acceleration, and the balance of the capacitance of the capacitor is broken. As a result, a primary alternating output signal A having a phase according to the displacement direction of the movable electrode and an amplitude according to the displacement amount appears at the output terminal 5. The drive circuit 2 can detect the direction and magnitude of the acceleration by electrically processing the phase and amplitude of the primary alternating output signal A.

However, the mechanical structure of each differential capacitance element varies, and the movable electrode is not always accurately held at the midpoint position in a non-acceleration state. Therefore, usually C
1 = Neutral condition of C2 is not satisfied, and circuit-neutral compensation is required. For the same reason, the sensitivity of each differential capacitance element varies, and it is necessary to perform circuit-like sensitivity compensation. In view of this point, the drive circuit 2 according to the present invention performs the midpoint adjustment and the sensitivity adjustment independently, and will be described in detail below. FIG. 2 is a waveform diagram for explaining the midpoint adjustment operation. This midpoint adjustment is performed without acceleration. In general, since the condition of C1 = C2 is not satisfied, the primary alternating output signal A appearing at the output terminal 5 contains a small amplitude change, for example, with a reference voltage set at 2.5 V as a center. Vibrating. The primary alternating output signal A is amplified by the AC amplifier circuit 7 to remove a DC component, and then an AC component B centered on a reference voltage is obtained. Here, since C1 and C2 are not equal at the acceleration of 0 as described above, an offset is apparently generated in an alternating current. In order to subtract this offset, a DC component of a predetermined level is added by an adder circuit 8, and an apparent offset is set to 0.
Adjust to Secondary alternating output signal C obtained as a result of adjustment
Has its peak level equal to the reference voltage level. Note that this adjustment may be performed with a trimmer resistor, but here, one or both of the split resistors connected between the power supply voltage VCC and the ground potential GND are selected and performed. By using the power supply voltage VCC, the final acceleration detection signal can be changed in proportion to the power supply voltage. Now, the magnitude of the acceleration can be detected from the peak level of the secondary alternating output signal C thus obtained. Further, it is necessary to perform phase detection to know the direction or polarity of the acceleration. For this reason, the sample-and-hold circuit 9 performs sample-and-hold in synchronization with the clock signal G, and then rectifies to generate the DC signal D. The level of the DC signal D is proportional to the acceleration, and the polarity indicates the direction of the acceleration. In the no-acceleration state, the resistor R1 is set so that the DC signal D matches the reference voltage.

Next, the sensitivity adjusting operation will be described with reference to FIG. This sensitivity adjustment is performed under a known acceleration state, for example. First, a primary alternating output signal A having an amplitude and a phase corresponding to the magnitude and polarity of the acceleration is obtained. By adding a preset DC component to a waveform B obtained by amplitude-amplifying only the AC component, a secondary alternating output signal C from which the offset has been removed is obtained. This is sampled and held in synchronization with the clock signal G and rectified to obtain a DC signal D. The DC signal D is separated from the reference voltage (2.5 V) by a predetermined level according to the magnitude and polarity of the acceleration. This level varies with each differential capacitance element and is not constant. Therefore, the DC amplifier 10 further amplifies the DC, and at the same time, adjusts the gain of the DC amplifier 10 by setting the resistor R2 to adjust the sensitivity to a standard value. The direct-current signal E thus gain-adjusted is subjected to impedance conversion by the output circuit 11, and is output as a final acceleration detection signal F.

FIG. 4 shows a specific configuration example of the acceleration detecting device according to the present invention. This example has a structure in which the differential capacitance element 1 and the drive circuit 2 are integrated as a package. The differential capacitance element 1 is assembled on a base 21. The base 21 is connected to the case 2 via the post 22.
3 is attached. A damper base 24 is fixed on the base 21 and a plurality of terminal pins 25 are inserted therethrough. The spindle 2 planted in the center of the damper stand 24
6 is provided with a differential capacitance element 1 having a laminated structure. This differential capacitive element 1 is hermetically sealed using a cap 27. The differential capacitive element 1 includes a pair of upper and lower fixed substrates 28 and 29. Fixed electrodes are respectively formed on opposing inner surfaces of each substrate. This fixed electrode is electrically connected to the terminal pin 25 via the flexible substrate 30. A movable electrode piece 31 is interposed between the pair of substrates 28 and 29 via a spacer. The movable electrode piece 31 is also connected to the terminal pin 25 via a conductive spacer, at least one fixed substrate, and the flexible substrate 30. These laminated plate materials are washers 32
And are fixed to each other by a nut 34 via a spring washer 33.

On the other hand, the drive circuit 2 is mounted on a circuit board 35. This circuit board 35 is arranged facing the base 21. The wiring pattern formed on the surface of the circuit board 35 is connected to the terminal pins 25 by soldering. A connector pin 36 penetrates through the circuit board 35 and is connected to the outside. After covering the drive circuit 2 with the electromagnetic shield 36, the cover 37 is engaged with the case 23 to complete the package of the acceleration detecting device.

In the above-described embodiment, a differential capacitance element is used as a differential impedance element. However, the present invention is not limited to this, and can be applied to other types of differential impedance elements. For example, FIG. 5 shows an acceleration sensor called a differential inductance method or a differential transformer method. One excitation coil 42 and two detection coils 43 and 44 are wound around the coil bobbin 41. An iron ball 45 is mounted in the hollow portion of the coil bobbin 41. The iron ball 45 is adjusted in advance so as to be positioned at the center of the coil bobbin 41 in a state of no acceleration by the coercive force of the magnet. However, in practice, the coercive force of the magnet varies and includes some offset, and it is necessary to adjust the midpoint in a circuit. Therefore, the present invention is effective. An alternating current is applied to the exciting coil 42, and an AC magnetic field passing through the center of the coil bobbin 41 is generated. Therefore, 2 wound on the same coil bobbin
An electromagnetic induction current is generated in the detection coils 43 and 44. In the state of zero acceleration, the two detection coils 43 and 44
Since the iron ball 45 is located at the boundary between the two detection coils 4
3, 44 are placed under approximately equal conditions to the alternating magnetic field. However, a slight output difference occurs because a slight offset is included. Next, when acceleration occurs in the axial direction of the coil bobbin 41, the iron ball 45 is displaced to a position where the coercive force of the magnet and the force due to the acceleration applied to the iron ball are balanced, and one of the two detection coils 43, 44 To one side. Therefore, the inductance of the pair of detection coils changes, and a difference occurs in the current induced in the detection coils. By processing this current difference using the drive circuit according to the present invention, acceleration information can be obtained. The offset of the iron ball 45 can be compensated in a circuit. Also, variations in the coercive force and the like of the iron ball 45 can be compensated in a circuit by performing sensitivity adjustment.

[0018]

As described above, according to the present invention, only the AC component of the primary alternating output signal obtained from the differential impedance element is amplitude-amplified and then a predetermined DC component is added thereto to add the secondary alternating output signal. Has been generated. With such a configuration, there is an effect that the circuit of the offset of the differential impedance element can be easily compensated. Further, the secondary alternating output signal is sampled and held and rectified to generate a corresponding DC signal, and the DC signal is amplified with a predetermined gain to obtain a final acceleration detection signal. With such a configuration, there is an effect that a variation in sensitivity of each differential impedance element can be easily compensated for by a circuit. In particular, according to the present invention, the midpoint compensation and the sensitivity compensation can be performed independently, and there is an effect that the adjustment work is facilitated.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an electrical configuration of an acceleration detection device according to the present invention.

FIG. 2 is a waveform chart for explaining the operation of the acceleration detection device shown in FIG.

FIG. 3 is a waveform chart for explaining the operation.

FIG. 4 is a schematic cross-sectional view showing a mechanical configuration of the acceleration detection device according to the present invention.

FIG. 5 is a schematic diagram showing another example of the differential impedance element to which the present invention is applied.

FIG. 6 is an exploded perspective view showing a conventional example of a differential capacitive acceleration sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Differential capacitance element 2 Drive circuit 3 Input terminal 4 Input terminal 5 Output terminal 6 Oscillation circuit 7 AC amplification circuit 8 Addition circuit 9 Sample hold circuit 10 DC amplification circuit 11 Output circuit

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-62-121312 (JP, A) JP-A-5-172845 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01P 15/11-15/125 G01P 21/00 G01R 27/26 G01D 3/00

Claims (4)

(57) [Claims] 1. A differential impedance element whose impedance changes differentially in response to an external acceleration, and a primary alternating element having a constant alternating input signal supplied to the differential impedance element and having an amplitude corresponding to the impedance change. An oscillation circuit for obtaining an output signal; an AC amplification circuit for amplifying only the AC component of the primary alternating output signal; and a secondary alternating output signal by adding a predetermined DC component to the amplified AC component. An addition circuit for generating, a sample-and-hold circuit for sample-holding and rectifying the secondary alternating output signal in synchronization with the alternating input signal to generate a corresponding DC signal, and amplifying the DC signal. D to obtain the acceleration detection signal
An acceleration detection device comprising a C amplifier circuit. 2. The acceleration detection device according to claim 1, wherein the addition circuit adds a DC component set for adjusting a midpoint of the differential impedance element. 3. The acceleration detection device according to claim 1, wherein the DC amplification circuit amplifies the DC signal with a gain set for adjusting the sensitivity of the differential impedance element. 4. The acceleration detecting device according to claim 1, wherein the differential impedance element is a differential capacitance element including a pair of fixed electrodes and a common movable electrode interposed in a gap between the fixed electrodes.