JPS5977311A - Capacitor type converter - Google Patents

Abstract

PURPOSE:To compensate for nonlinearity effectively, by using a pair of variable capacitors, whose capacity is differentially varied in correspondence with the quantity to be measured, and a reference capacitor, whose capacity is constant. CONSTITUTION:A pair of variable capacitors C1 and C2 are constituted by a movable electrode 10, which is displaced in correspondence with the quantity to be measured such as pressure, and fixed electrodes 11 and 12. The capacity of a reference capacitor C3 is constant. A capacity pulse width converting circuit C/P detects the change in capacity of the capacitors and outputs a pulse width signal PW1, whose duty ratio corresponds to the difference in capacities of the capacitors C2 and C1 and a pulse width signal PW1, whose duty ratio corresponds to the difference in capacities of the capacitors C2 and C3. A switch SW2 is driven by the signal PW2 and turns ON and OFF a preset voltage Vr. An integrating circuit IC adds and integrates a voltage, which is obtained by turning ON and OFF the voltage Vr by the switch SW2, an output voltage Vo, and a reference voltage VS1, and obtains the voltage Vr. A filter circuit FC smoothes the voltage, which is turned ON and OFF by a switch SW1 and obtains the output voltage Vo.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a capacitive transducer using a pair of variable capacitors whose movable electrodes are displaced and whose capacitance differentially changes depending on a measured quantity such as pressure or differential pressure.

In general, capacitive converters have the disadvantage of poor linearity due to the influence of stray capacitance that exists in parallel with the variable capacitor. In addition, in the case of metal diaphragms such as pressure transducers in which the movable electrode is displaced according to the measured quantity, the relationship between the measured quantity and the capacitance is often expressed as a hyperbola, and the measured quantity and output are often expressed as a hyperbola. It was non-linear.

The present invention uses a pair of variable capacitors whose capacitance differentially changes depending on the measured quantity and a reference capacitor whose capacitance is constant, and the first pulse width of the duty ratio is determined according to the difference in capacitance of the pair of variable capacitors. A second pulse width signal with a duty ratio corresponding to the signal and the difference between the capacitance of one of the pair of variable capacitors and the capacitance of the reference capacitor is obtained;
After turning the set voltage on and off using the pulse width signal of By setting the value as the set voltage, a capacitive converter is realized that can effectively compensate for the nonlinearity as described above.

FIG. 1 is a connection diagram showing one embodiment of the converter of the present invention.
In diagram J, C□ and C2 are a pair of variable capacitors,
Movable that moves according to the measured quantity such as pressure! a pole 10 and a fixed electrode 1.1 arranged opposite to this movable electrode 10.
．． It consists of 12. C3 is a reference capacitor with constant capacitance. C/P is a capacitive pulse width conversion circuit, which is connected to a pair of variable capacitors C1 and C2 as a reference.

An example of a specific configuration of the C/P is shown in FIG. Fig. 2 shows the output, 5W11.5W12.5W13 (4 parts), a switch such as a field effect transistor connected in parallel to C2 and C3, PGti, which outputs a pulse P1 with a constant period T and time width t. Pulse generator, B
A, BA2. BA3 is a buffer amplifier, cp
l, cp2°C20 are comparators, G1. G2
．． G3. G4. G5 is a Noah gate, and G5 is an inverter. 5W11 rSWSWl "W13 is driven by the same RK with a pulse P1 of a constant period T and a constant pulse width t as shown in the third p (a) from the PG, and is turned on for a period of t. When turned on, C□.

When the charges stored in C2 and C3 are discharged and turned off, the resistors R□□, R□2. C□, C2, and C3 are charged with the current of DC power supply Va through R□3. The result C
□, C2° C3 charging voltage VC□, vO2, VC3 is shown in Fig. 3. Comparator Cp1. C
p2. Cp3 monitors vcl, vO2, vO3 given through BAl, BA2゜BA3, and vcl, vO
2. When vO3 reaches a certain value vb, Cpl, C20, C
The output of 20 is inverted as shown in FIGS.

The time required for VC□, vO2, and vO3 to reach vb is 1
1.12°t3, Noah Gate G4. At the output terminal of G5, as shown in FIGS.
is live T T . And t□, t2. t3 is resistance R□ITR□2
．． If the value of R□3 is R□1-R□2 ” R□3=R, then t1=k C1(1) t2=kC2(2) t3-kC3(5) However, k=-Re n (1 -Wb/Va), the duty ratios of pwl and pw2 correspond to the difference in capacitance between C2 and 01 and the difference in capacitance between C2 and 03, respectively.

Again in FIG. 1, SW□ and 5W2U are each driven by a switch, and SW2 is driven by the pulse width signal PW1.
are driven by the pulse width signal pW2 to set the respective set voltages V
It turns on and off r. In addition, switch SW1゜S
As W2, an electronic switch such as a field effect transistor is suitable. The IC is an integrator circuit using an operational amplifier Op2.
The set voltage V is obtained by adding and integrating the output voltage Vo applied through the resistor R3 and the reference voltage VS1 applied through the resistor R3.
This is to obtain r. FC is a filter circuit using an operational amplifier OP1, which smoothes the voltage turned on and off by SW1 to obtain an output voltage Vo.

In the converter of the present invention configured as described above, the period T of pW2 is sufficiently short compared to the time constant C R1 of the integrating circuit IC, and the DC component of the output voltage of the integrating circuit IC when the steady state is reached. When Vr is the average input current of the integrating circuit IC, the average input current of the integrating circuit IC is zero. Therefore, when the DC component of the output voltage of the filter circuit FC is Vo, the following relationship holds true.

Similarly, the pulse width signal PW10 period T is the filter circuit FC.
Since the time constants CF and R5 are sufficiently short compared to the time constants CF and R5, the DC component vO of the output voltage of the filter circuit FC when the steady state is reached is as follows. From equations (4) and (5), vO is.

becomes. In this way, vO is the pulse width of pWl (12-1
1), the output point (zero point) when 12=1□ is the stray capacitance of switch SW2,
Resistor R□l R21R31R41R5 is not affected by temperature changes and has good zero point stability. Further, even if the period T of the pulse p□ changes, it is not affected by the change. In equation (6), (
Ri ceremony.

By substituting equations (2) and (3), Vo can be expressed by the following equation.

On the other hand, the capacitance of the variable capacitor ClIC2 is determined by setting the initial capacitance to Co for the displacement amount
) If k is d (when x=O) and stray capacity is Cs, then C1= Co -7,-+ Cs
(8n C2 “Co-H near + Cs
It changes due to the relationship (9). Therefore, the output voltage Vo is as follows. Here, the reference capacitor C3
The capacity of C3=Cs
Of), the output voltage vO is as follows. Therefore, if the resistance value of variable resistor R3 is adjusted to satisfy the following,
The output voltage Vo is as follows. Since R2, RA, d, and Vsl are constant values, the output voltage vO corresponds accurately to the amount of displacement X.

Then, the rate of increase of the output voltage vO is as follows:
As the amount of displacement X increases, the rate of increase decreases,
The input/output relationship can be made non-linear. Moreover, the magnitude of the nonlinearity can be set by changing the value of the resistance value R3. Therefore, when converting differential pressure or pressure into displacement using a metal diaphragm, for example, the nonlinearity between the amount to be measured and the amount of displacement X of the movable electrode 10 can be corrected. In other words, linearization is possible.

In addition, in the above, variable resistor R3! Although the case where linearization is performed by adjusting the output voltage Vo is shown as an example, if the output voltage Vo is amplified by the amplifier A and then applied to the integrating circuit ICE via the resistor R3 as shown in FIG. 4, the output voltage Vo becomes Amplifier A
If the gain of is kXC3=C8, then linearization can be achieved by adjusting the gain k of the amplified good person. In addition, in FIG. 4, a non-inverting type amplifier A consisting of an operational amplifier Op3 and an operational resistor R6°R7 is shown, and its gain is (n6+R7)/ne.
, and the calculation resistor R6 is driven by the width signal PW2, but if the amplifier A is made inverted as shown in FIG. 5, it can also be driven by the variable pulse width signal PW3. good.

The output voltage vO in this case is C3-C6, and by adjusting the gain k of the amplifier A, the resistance R3 can be linearly raised. In addition, Figures 4 and 5
In the figure, switch sw1. The resistors r□ and r2 connected between one end of sW2 and the reference point are used to eliminate the influence of stray capacitance of the elements and wiring of SW□ and SW2. Note that instead of the resistor r□+r2, a switch that turns on and off in the opposite phase to SW□+SW2 may be used, or SWl.

SW2't) May be used as a transfer switch. In addition, in the above, the capacitance of the reference capacitor C3 is adjusted so that C3=C
Although the case where swo is satisfied is illustrated, the resistor R13 in FIG.
The same effect can be achieved by adjusting R□3 as a variable resistor to satisfy C3-CsR/It13@. However, in this case, n-n□□=R1□.

As explained above, in the present invention, a pair of variable capacitors whose capacitance differentially changes depending on the measured quantity and a reference capacitor whose capacitance is constant are used, and the date ratio is set according to the difference in capacitance of the pair of variable capacitors. A first pulse width signal and a second pulse width signal with a duty ratio corresponding to the difference between the capacitance of one of the pair of variable capacitors and the reference capacitor are obtained, and the set voltage is set using the first pulse width signal. It turns on and off to obtain the output voltage, and the set voltage is the sum of the output voltage and the reference voltage multiplied by the reciprocal of the duty ratio of the second pulse width signal, so it is effective with a simple configuration. A container-type transducer that can correct nonlinearity is obtained.

[Brief explanation of drawings]

Fig. 1 is a connection diagram showing an embodiment of the converter of the present invention, Fig. 2 is a connection diagram showing an example of a specific configuration of a capacitive pulse width conversion circuit used in the converter of the present invention, and Fig. 3 is its operation. 4 and 5 are connection diagrams showing other embodiments of the converter of the present invention. C1, C2... A pair of variable capacitors, C3... Reference capacitor, CIP... Capacitance pulse width conversion circuit, S
Wl, SW2...Switch, IC...Integrator circuit, F
C... Filter circuit, Opl, O20, O20...
Operational amplifier, Vs□...Reference voltage, A...Amplifier.

Claims (1)

[Claims] a pair of variable capacitors whose capacitance differentially changes depending on the quantity to be measured; a reference capacitor whose capacitance is constant; and a first pulse width signal with a duty ratio corresponding to the difference in capacitance of the pair of variable capacitors. a circuit for obtaining a second pulse width signal with a duty ratio corresponding to the difference between the capacitance of either one of the variable capacitors and the capacitance of the reference capacitor, and a circuit that turns on and off a set voltage using the first pulse width signal and then smoothes and outputs the signal. A capacitive converter comprising: means for obtaining a voltage; and means for setting the set voltage to a value obtained by multiplying the sum of the output voltage and the reference voltage by the reciprocal of the duty ratio of the second pulse width signal.