JP4272267B2 - Capacitance type sensor circuit - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a capacitance sensor circuit for outputting a capacitance change of a capacitance sensor as a voltage change, and in particular, a capacitance sensor capable of easily correcting variations in manufacturing of the sensor itself. Regarding the circuit.
[0002]
[Prior art]
Capacitance sensors are widely used as acceleration sensors because of their high sensitivity and resistance to impacts. When a mechanical external force is applied, the capacitance changes, and the change in capacitance changes the voltage. It is comprised so that it can take out as. In such a capacitance type sensor, a change in capacitance with respect to an external pressure (capacity / external pressure characteristics) is not constant due to variations at the time of manufacture or the like, and it is attempted to calibrate the variations by external mechanical and electronic means. ing.
[0003]
FIG. 3 shows a conventional example in which a capacitive 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 the silicon cantilever 11 by a silicon microfabrication technique, and the movable electrode 12 is disposed between a pair of fixed electrodes 13 and 14. The movable electrode 12 tends to be displaced depending on the magnitude and direction of the acceleration G, but these electrodes are set so that the distance d between the movable electrode 12 and one fixed electrode is constant by feedback control. An electrostatic force is applied between them, 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 the pulse width modulation method using the electrostatic servo circuit 15 or controlled to include a bias electrostatic component.
[0004]
[Problems to be solved by the invention]
However, the conventional example of FIG. 3 requires an electrostatic servo mechanism or the like that performs pulse width modulation in order to correct variations in characteristics of the sensor element itself, and therefore the sensor circuit is relatively large-scale, And complex. If the electrostatic servo mechanism as in the conventional example described above is not used to reduce the circuit scale, there is a problem that the output error increases due to variations in sensor elements.
The object of the present invention is to solve such problems of the conventional example, and to change the capacitance of the capacitance type sensor element with a simple circuit and high accuracy output without depending on manufacturing variations. It is an object to provide a capacitive sensor circuit that can be extracted as a signal.
[0005]
[Means for Solving the Problems]
In order to achieve the above object, in the capacitive sensor circuit of the present invention, (a) an input voltage generating means for generating an input voltage, and (b) the generated input voltage is supplied to the first terminal. A capacitive sensor element, (c) an amplifier having an input terminal connected to the second terminal of the sensor element, and (d) a feedback connected between the input terminal and the output terminal of the amplifier. A capacitor, (e) an adjustment voltage generating means for generating an adjustment voltage, and (f) the generated adjustment voltage is supplied to the first terminal, and the second terminal of the sensor element is connected to the second terminal. An adjustment capacitor is provided, and the variation of the sensor element can be calibrated by adjusting the adjustment voltage.
[0006]
The present invention further includes (g) a first switch connected between the first terminal of the sensor element and the input voltage generating means, and (h) a first terminal of the sensor element. And a reference potential, and a second switch that is turned on / off complementarily with the first switch, and (i) 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 adjustment capacitor and the adjustment voltage generating means, and turned on / off in synchronization with the first switch; (K) including a fifth switch connected between the first terminal of the adjustment capacitor and the reference potential and turned on / off in synchronization with the second switch; Turn on the 3rd and 5th switches for the first time to start the capacitive sensor circuit. However, then, these switches are turned on the first and fourth switches off vital, so that it can be operated as a sensor circuit. As a result, since the voltage across the capacitor of the circuit can be initialized to zero before operation, more accurate measurement can be performed.
In the capacitive sensor circuit of the present invention, it is preferable to use an operational amplifier as an amplifier.
[0007]
Embodiment
FIG. 1 shows a capacitive sensor circuit according to an embodiment of the present invention, in which C _s is a capacitive sensor element (sensor capacitor), and the sensor capacitor C _s the first terminal, 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 and the complementarily oN / ground via a second switch S ₂ is turned off, i.e. is connected to a reference potential. The inverting input terminal of the operational amplifier 2 is connected to the second terminal of the sensor capacitor C _s , and is turned on / off in synchronization with the switch S ₂ between the inverting input terminal and the output terminal of the operational amplifier 2. parallel circuit of a switch S ₃ and the capacitor C _f is connected that. The sensor output is taken _out from the output terminal of the operational amplifier 2 as the output voltage _Vout .
[0008]
The second terminal of the sensor capacitor C _s is further supplied from the adjustment voltage generation circuit 3 via the switch S ₄ that is turned on / off in synchronization with the switch S ₁ and the adjustment capacitor C _{m. m} is supplied, also via a switch S ₅ is complementarily turned on / off switch S _4, is connected to a reference potential.
As is clear from the above, the switches S ₂ , S ₃ , S ₅ are turned on / off synchronously, and the switches S ₁ , S ₄ are turned on / off synchronously. These two switch groups are complementarily turned on / off.
[0009]
The operation of the circuit shown in FIG. 1 will be described.
First, in order to initialize the circuit, the switches S ₁ and S ₄ are turned off, and the switches S ₂ , S ₃ and S ₅ are turned on. 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 , the feedback capacitor C _f , and the adjustment capacitor C _m becomes zero. Note that the operational amplifier 2 operates so that the potentials of the inverting input terminal and the non-inverting input terminal are equal (that is, the inverting input terminal and the non-inverting input terminal are in the short-circuit state). The terminal potential V _{f is} also the reference potential (V _f = 0), and therefore the voltage across the sensor capacitor C _s and the adjustment capacitor C _m is zero. After such initialization, when the on / off state of the switch is inverted, the switches S ₂ , S ₃ , S ₅ are turned off and the switches S ₁ , S ₄ are turned on, the first sensor capacitor C _s is turned on. input voltage V _in the terminals is applied, also the adjustment voltage V _m to the first terminal of the adjusting capacitor C _m is applied.
[0010]
The input / output transfer characteristics of the circuit when the switches S ₁ and S ₄ are on are expressed by the following equation (1). In the following equation, the capacitance value of each capacitor is represented using the corresponding capacitor sign.
[Expression 1]
V _out
= − (C _s / C _f ) (V _in −V _f ) − (C _m / C _f ) (V _m −V _f ) + V _f (1)
In order to simplify the description, when C _m and C _f which are fixed values are set to C _m = C _f , Expression (1) is expressed as follows.
[Expression 2]
V _out
= − (C _s / C _f ) (V _in −V _f ) − (V _m −V _f ) + V _f (2)
[0011]
_If the value of C _s / C _f is used as a parameter and Expression (2) is represented in a graph, it is represented by straight lines L ₀ , L ₁ , and L _{2 as shown} in FIG. However, in these straight lines L _{0 to} L ₂ , V _m is set to a fixed value of V _m = V _{m (0)} . The straight line L ₀ represents the case of C _s = C _f , and the straight line L ₀ is set by setting the value of C _f to coincide with the standard capacitance value C _{s (0)} of the standard sensor capacitor. ₀ indicates the input / output transfer characteristics when a standard sensor capacitor is used.
The straight line L ₁ shows an example when C _s = C _{s (1)} > C _f , and the straight line L ₂ shows an example when C _s = C _{s (2)} <C _f. The input / output transfer characteristics when the capacitance value is larger and smaller than the standard value are shown.
[0012]
When the sensor capacitor has a standard capacitance value (C _s = C _f = C _{s (0)} ), the output voltage V _{out (0)} is output while a certain input voltage V _{in (0)} is supplied. The However, when C _{s (1)} > C _f , the output voltage corresponding to the input voltage V _{in (0)} is smaller than V _{out (0)} , as is apparent from the straight line L _1.
V _{out (1)} = V _{out (0)} -△ V (3)
It becomes. 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) as} when a sensor capacitor having a standard capacitance value C _{s (0)} is used can be obtained.
[0013]
It is clear from the equation (2) and the graph that the adjustment voltage V _m may be changed from V _{m (0)} in order to translate the straight line L ₁ into the straight line L ₁ ′ in this way. The adjusted voltage V _{m (1)} after the change can be calculated from the following equation (4).
[Expression 4]
V _{m (1)
= V m (0) - (} C s (1) -C f) (V in (0) -V f) / C f (4)
In the above description, the output calibration when the capacitance value of the sensor capacitor C _s is larger than the standard value has been described. However, when the capacitance value C _{s (2)} is smaller than the standard value, the adjustment voltage V _{m (2 )}
[Equation 5]
V _{m (2)
= V m (0) - (} C s (2) -C f) (V in (0) -V f) / C f (5)
It is represented by
[0014]
As described above, in the circuit of FIG. 1, by adjusting the adjustment voltage V _m , the output voltage V _out may vary even if the capacitance value C _s of the sensor element used varies. Absent. Therefore, in the present invention, when a sensor capacitor having a capacitance value different from the standard value is used, adjustment is performed by substituting C _{s (1)} or C _{s (2)} into Equation (4) or Equation (5). The voltage V _{m (1)} or 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 the memory in advance. The adjustment voltage V _{m of the} circuit may be variably controlled by feedback control so as to coincide with the input / output transfer characteristics.
[0015]
In addition, any input / output transfer characteristic can be obtained by appropriately controlling the adjustment voltage V _m without being based on the equations (4) and (5).
Furthermore, although the equations (4) and (5) are equations obtained based on the premise that C _m = C _f , even when C _m and C _f vary and C _m ≠ C _f , Since C _m / C _f in equation (1) only translates the straight line representing the input / output transfer characteristics, variation in output voltage due to variation in C _m and C _f is reduced by adjusting adjustment voltage V _m. Can be made.
Furthermore, as apparent from the equations (1) and (2), by changing V _in , the output voltage on the straight line L ₍₁₎ or L ₍₂₎ in FIG. 2 can be increased or decreased. . Therefore, even if the input voltage and the reference voltage are adjusted instead of controlling the adjustment voltage, the variation of the capacitive sensor element can be calibrated.
[0016]
In the embodiment of FIG. 1, although the operational amplifier 2 is connected to the second terminal of the sensor capacitor C _s , it goes without saying that any other linear amplifier can be used instead of the operational amplifier.
It will be apparent to those skilled in the art that the switches S _{1 to} S ₅ can be any switches, 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 it may be turned on with the switch S _3. If necessary, a resistor for limiting the discharge current may be connected to each of the switches connected in parallel.
[0017]
As described above, in the present invention, the capacitance value of the capacitive sensor can be detected with high accuracy without using a servo control mechanism for feedback control of electrostatic force as in the conventional example, Even if there is a variation in the capacitance value of the sensor element and the capacitance value of the capacitor in the circuit, it can be adjusted to reduce the variation in 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 capacitive sensor circuit according to an embodiment of the present invention.
2 is a graph for explaining an adjustment principle of an output voltage based on input / output transfer characteristics of the circuit shown in FIG. 1; FIG.
FIG. 3 is a cross-sectional view showing a structure of a conventional acceleration sensor.

Claims (1)

In a capacitance type sensor circuit that outputs a change in capacitance as an electrical signal,
An input voltage generating means for generating an input voltage;
A capacitive sensor element to which the generated input voltage is supplied to the first terminal;
An operational amplifier having a first input terminal connected to a second terminal of the capacitive sensor element;
A feedback capacitor connected between a first input terminal and an output terminal of the operational amplifier;
An adjustment voltage generation means for generating an adjustment voltage that can be variably controlled, wherein V _{m (0) is} an adjustment voltage to be applied when the capacitance of the capacitance type sensor element is a reference capacitance value, and the capacitance type sensor When the capacitance of the element is C _s , the capacitance of the feedback capacitor is C _f , the input voltage is V _in , and the voltage at the second input terminal of the operational amplifier is V _f ,
_{V m = V m (0)} - (C s -C f) (V in -V f) / C f
Adjusting voltage generating means for generating the adjusting voltage V _m represented by:
The generated adjustment voltage is supplied to the first terminal, the second terminal of the capacitive sensor element is connected to the second terminal, and the adjustment capacitor having the same capacitance value as the feedback capacitor;
A first switch connected between the first terminal of the capacitive sensor element and the input voltage generating means;
A second switch connected between the first terminal of the capacitive sensor element and the reference potential and turned on / off in a complementary manner with the first switch;
A third switch connected in parallel to the feedback capacitor and turned on / off in synchronization with the second switch;
A fourth switch connected between the first terminal of the adjustment capacitor and the adjustment voltage generating means and turned on / off in synchronization with the first switch;
It is connected between the first terminal and a reference potential of the control capacitor, by synchronously with the second switch and a fifth switch which is turned on / off, adjusting the regulated voltage, the electrostatic capacitance Capacitance type sensor circuit characterized in that the variation of the type sensor element can be calibrated.

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