JPH1123608A - Capacitive sensor circuit - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To calibrate a variation in manufacturing a capacitance type sensor element. A turns on the switch S _1, S ₄ turns off the other switches, each of the input voltage V _in and regulated voltage V _m, the sensor element C _s, and supplies the tuning capacitors C _m. A feedback capacitor _Cf is connected between the input and output terminals of the operational amplifier 2, and the output voltage from the output of the operational amplifier is
V _out = − (C _s / C _f ) (V _in −V _f ) − (C _m / C _f )
Represented by _{(V m -V f) + V} f. Therefore, by adjusting the adjustment voltage V _m, even if there are variations in the capacitance value C _s of the sensor element, upon the supply of a predetermined input voltage V _in, it is possible to obtain a constant output voltage V _out.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitance type sensor circuit for outputting a capacitance change of a capacitance type sensor as a voltage change, and more particularly, to a method of easily correcting a variation in manufacturing of the sensor itself. The present invention relates to a capacitive sensor circuit that can be used.

[0002]

2. Description of the Related Art Capacitance sensors are widely used as acceleration sensors and the like because of their high sensitivity and resistance to impact. When a mechanical external force is applied, the capacitance changes, and the capacitance of the capacitance sensor changes. It is configured so that the change can be extracted as a change in voltage. In such a capacitance type sensor, a change in capacitance with respect to an external pressure (capacity / external pressure characteristic) is not constant due to a variation at the time of manufacture or the like, and it is attempted to calibrate the variation by external mechanical and electronic means. ing.

FIG. 3 shows a conventional example in which a capacitance type sensor element to which such a calibration means is added is used as an acceleration sensor. In the conventional example, a movable electrode 12 is provided at the tip of a silicon cantilever 11 by a silicon micromachining technique, and the movable electrode 12 is arranged between a pair of fixed electrodes 13 and 14. Then, the movable electrode 12 tries to be displaced depending on the magnitude and direction of the acceleration G. However, the movable electrode 12
So that the distance d between the first electrode and the one fixed electrode is constant,
An electrostatic force is applied between these electrodes, and the acceleration applied to the cantilever is detected from the magnitude of the applied electrostatic force. In the conventional example of FIG. 3, the electrostatic force is controlled by a pulse width modulation method using the electrostatic servo circuit 15 or controlled to include a bias electrostatic component.

[0004]

However, the prior art shown in FIG. 3 requires an electrostatic servo mechanism or the like for executing pulse width modulation in order to correct variations in the characteristics of the sensor element itself. The circuit is relatively large and complex. When the electrostatic servo mechanism as in the above-described conventional example is not used in order to reduce the circuit scale, there is a problem that an output error increases due to variations in sensor elements and the like. An object of the present invention is to solve such a problem of the conventional example and to provide a simple circuit and a high-precision output which does not depend on manufacturing variations. An object of the present invention is to provide a capacitance type sensor circuit which can be extracted as a signal.

[0005]

In order to achieve the above-mentioned object, a capacitive sensor circuit according to the present invention comprises:
(A) input voltage generating means for generating an input voltage; (b)
A capacitive sensor element to which the generated input voltage is supplied to a first terminal, (c) an amplifier having an input terminal connected to a second terminal of the sensor element, and (d) an input of the amplifier. A feedback capacitor connected between the terminal and the output terminal;
(E) adjusting voltage generating means for generating an adjusting voltage;
The generated adjustment voltage is supplied to a first terminal, and a second terminal is provided with an adjustment capacitor connected to a second terminal of the sensor element. Is characterized in that it can be calibrated.

The present invention further provides, in addition to the above-described configuration,
(G) a first switch connected between the first terminal of the sensor element and the input voltage generating means, and (h) a first switch connected between the first terminal of the sensor element and the reference potential. A second switch which is turned on / off complementarily to the switch of
(I) a third switch connected in parallel to the feedback capacitor and turned on / off in synchronization with the second switch; (j)
A fourth switch connected between the first terminal of the adjusting capacitor and the adjusting voltage generating means and turned on / off in synchronization with the first switch; and (k) a first switch of the adjusting capacitor.
And a fifth switch, which is connected between the second switch and the reference potential, and is turned on / off in synchronization with the second switch, wherein the second, third, and fifth switches are The sensor circuit is turned on to initialize the capacitance type sensor circuit, and then these switches are turned off and the first and fourth switches are turned on so that the sensor circuit can be operated. As a result, the operation can be performed after the voltage between both ends of the capacitor of the circuit is initialized to zero, so that more accurate measurement can be performed. In the capacitance type sensor circuit of the present invention, it is preferable to use an operational amplifier as an amplifier.

[0007]

[Aspect of an] FIG. 1 is shown the capacitive sensor circuit of an embodiment of the present invention is, in figure, C _s is the capacitance type sensor element (sensor capacitors), the sensor · the first terminal of the capacitor C _s, the input voltage V _in from the input voltage generating circuit 1 is supplied via the switch S _1, the terminal of said first, first switch S ₁ It is connected to ground, ie, a reference potential, via a _second switch S2 which is turned on / off complementarily. The second terminal of the sensor capacitor C _s is connected to the inverting input terminal of the operational amplifier 2, between the output terminal of the inverting input terminal and an operational amplifier 2 is turned on / off in synchronization with the switch S ₂ parallel circuit of a switch S ₃ and the capacitor C _f is connected that. From the output terminal of the operational amplifier 2, a sensor output is taken _out as an output voltage _Vout .

The second terminal of the sensor capacitor C _s is further connected from the adjustment voltage generation circuit 3 via a switch S ₄ which is turned on / off in synchronization with the switch S ₁ and an adjustment capacitor C _m . adjustment voltage V _m is supplied, also via a switch S ₅ is complementarily turned on / off switch S _4, it is connected to a reference potential. As is clear from the above,
The switches S ₂ , S ₃ and S ₅ are turned on / off synchronously, and the switches S ₁ and S ₄ are turned on / off synchronously. Then, these two groups of switches are turned on / off complementarily.

The operation of the circuit shown in FIG. 1 will be described. First, the switches S ₁ and S ₄ are turned off and the switches S ₂ , S ₃ and S ₅ are turned on to initialize the circuit. This on / off state is opposite to the on / off state shown in FIG.
As a result, the voltage across each of the sensor capacitor C _s , feedback capacitor C _f , and adjustment capacitor C _m becomes zero. The operational amplifier 2 acts to equalize the potentials of the inverting input terminal and the non-inverting input terminal (ie, the inverting input terminal and the non-inverting input terminal are in an imminent short state). potential V _f also the reference potential (V _f = 0) next to the terminals, therefore, the sensor capacitor C _s and capacitor C _m voltage both ends for adjustment is zero. After such initialization, the on / off states of the switches are reversed to switch S ₂ , S ₃ ,
Off the S _5, when turning on the switch S _1, S _4, the sensor input voltage V _in to the first terminal of the capacitor C _s is applied, also the adjustment voltage to the first terminal of the adjusting capacitor C _{m Vm} is applied.

The input / output transfer characteristics of the circuit when the switches S ₁ and S ₄ are on are represented by the following equation (1). In the following equation, the capacitance value of each capacitor is represented using the sign of the corresponding capacitor.

V _out = − (C _s / C _f ) (V _in −V _f ) − (C _m / C _f ) (V _m −V _f ) + V _f (1) Fixed for simplicity of explanation. The values C _m and C _f are represented by C _m =
When set to C _f , equation (1) is expressed as follows.

V _out = − (C _s / C _f ) (V _in −V _f ) − (V _m −V _f ) + V _f (2)

_If the value of C _s / C _f is used as a parameter and the equation (2) is represented by a graph, it is represented by straight lines L ₀ , L ₁ and L _{2 as shown} in FIG. However, these straight lines L _{0 to} L
_{In 2} , _Vm is set to a fixed value such that _Vm = _{Vm (0)} . The straight line L ₀ represents the case where C _s = C _f , and C _f
Is the standard sensor capacitor value C
By setting to match the _{s (0),} the straight line L ₀ is capable of showing the input-output transfer characteristics when the standard sensor capacitor. Also, the straight line L ₁ is C _s = C _{s (1)} >
An example of a case of C _f, the straight line L ₂ is _{C s = C s (2)} < shows an example of a case of C _f, input when the capacitance value of the sensor capacitor is greater than the standard value and smaller 4 shows output transfer characteristics.

When the sensor capacitor has a standard capacitance value (C _s = C _f = C _{s (0)} ), the output voltage V _{out (0)} is supplied while a certain predetermined input voltage V _{in (0)} is supplied. Is output. However, when C _{s (1)} > C _f , as is clear from the straight line L ₁ , the output voltage corresponding to the input voltage Vin ₍₀₎ is V _{out (0)}
Less than

V _{out (1)} = V _{out (0)} − △ V (3) Therefore, if a shift to the straight line L ₁ 'the straight line L ₁ so that the output voltage V _out increases from V _{out (1)} △ V only, the sensor of the _{C s = C s (1)} > C f becomes capacitance value It can be seen that even when a capacitor is used, the same output voltage _{Vout (0)} is obtained as when a sensor capacitor having a standard capacitance value Cs ₍₀₎ is used.

[0013] To this way lines L ₁ lines L ₁ is moved parallel 'to the regulated voltage V _m may be changed from V _{m (0)} is clear from equation (2) and a graph The adjusted voltage V _{m (1)} after the change is expressed by the following equation (4).
Can be calculated from

Equation 4] _{V m (1) = V m} (0) - (C s (1) -C f) (V in (0) -V f) / C f (4) In the above, the sensor capacitor C _Although the output calibration in the case where the capacitance value of _s is larger than the standard value has been described, in the case of the capacitance value C _{s (2)} smaller than the standard value, the adjustment voltage V _{m (2)} becomes

Equation 5] _{V m (2) = V m} (0) - represented by _{(C s (2) -C f} ) (V in (0) -V f) / C f (5).

[0014] As described above, in the circuit of Figure 1, by adjusting the adjustment voltage V _m, even if there are variations in the capacitance value C _s of the sensor element used, a variation in the output voltage V _out Will not occur. Therefore, in the present invention, when a sensor capacitor having a capacitance value different from the standard value is used, the adjustment is made by substituting Cs ₍₁₎ or Cs ₍₂₎ into Equation (4) or Equation (5). Voltage V _{m (1)}
Alternatively, V _{m (2)} may be calculated to determine the adjustment voltage V _m output from the adjustment voltage generation circuit 3. If the capacitance value C _s of the sensor capacitor used is unknown, the input / output transfer characteristics of the circuit obtained using the standard capacitance value C _{s (0)} are stored in a memory in advance, to match the said input output transfer characteristic, the adjustment voltage V _m of the circuit may be variably controlled by a feedback control.

Also, if the adjustment voltage _Vm is appropriately controlled without being based on the equations (4) and (5), an arbitrary input / output transfer characteristic can be obtained. Further, Equations (4) and (5) are equations obtained based on the premise of C _m = C _{f. Even} when C _m and C _f vary and C _m ≠ C _f , Since C _m / C _f in the equation (1) merely translates the straight line representing the input / output transfer characteristic, the variation of the output voltage due to the variation of C _m and C _f is reduced by adjusting the adjustment voltage V _m. Can be done. Furthermore, as it is clear from equation (1) and (2), by varying the V _in, it is possible to increase or decrease the output voltage of the linear FIG 2 L ₍₁₎ or L ₍₂₎ . Therefore, even if the input voltage and the reference voltage are adjusted instead of controlling the adjustment voltage, it is possible to calibrate the variation of the capacitance type sensor element.

[0016] In the embodiment of FIG. 1, connects the operational amplifier 2 to the second terminal of the sensor capacitor C _s, in place of the operational amplifier, it is of course possible to use any other linear amplifier . It will be apparent to those skilled in the art that any switches can be used as the switches S _{1 to} S ₅ , whether they are electronic switches or mechanical switches. Further, instead of using a switch _{S 1, S 2, S 4} , S 5, the sensor capacitor C _s, and the switch to both ends of the adjusting Condesa C _m are connected in parallel, these switches during initialization only may be turned on with the switch S _3. If necessary, a resistor for limiting a discharge current may be connected to each of the switches connected in parallel.

As described above, in the present invention, the capacitance value of the capacitance type sensor can be detected with high accuracy without using a servo control mechanism for performing feedback control of electrostatic force as in the conventional example. Besides, even if the capacitance value of the sensor element and the capacitance value of the capacitor in the circuit vary, adjustment can be made to reduce the variation of the output voltage. In addition, since the capacitance detection unit and the output adjustment unit can be configured by the same circuit, the circuit can be simplified and downsized.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a capacitance type sensor circuit according to one embodiment of the present invention.

2 is a graph for explaining a principle of adjusting an output voltage based on input / output transfer characteristics of the circuit shown in FIG. 1;

FIG. 3 is a cross-sectional view showing the structure of a conventional acceleration sensor.

Claims (3)

[Claims] 1. An electrostatic capacitance type sensor circuit for outputting a change in capacitance as an electric signal, comprising: an input voltage generating means for generating an input voltage; and a static electricity supply means for supplying the generated input voltage to a first terminal. A capacitive sensor element, an amplifier having an input terminal connected to the second terminal of the sensor element, a feedback capacitor connected between an input terminal and an output terminal of the amplifier, and an adjustment for generating an adjustment voltage. A voltage generating means, and a regulating capacitor to which the generated regulating voltage is supplied to the first terminal and the second terminal of which is connected to the second terminal of the sensor element. A capacitance sensor circuit wherein variations in sensor elements can be calibrated. 2. The capacitive sensor circuit according to claim 1, further comprising: a first switch connected between a first terminal of the sensor element and input voltage generating means; Is connected between the first terminal of
A second switch that is turned on / off complementarily to the first switch; a third switch that is connected in parallel with the feedback capacitor and that is turned on / off in synchronization with the second switch; A fourth switch that is connected between the first terminal and the adjustment voltage generating means and that is turned on / off in synchronization with the first switch; and that is connected between the first terminal of the adjustment capacitor and the reference potential. And turned on / off in synchronization with the second switch.
A capacitance type sensor circuit comprising: 3. The capacitance-type sensor circuit according to claim 1, wherein the amplifier is an operational amplifier.