JPH0412466Y2 - - Google Patents

Description

[Detailed description of the invention] <Technical field to which the invention pertains> The invention is a pair of variable capacitors in which a movable electrode is displaced in accordance with a measured quantity such as pressure or differential pressure, and the capacitance between the electrodes is differentially changed. The present invention relates to a capacitive conversion device using a detector having the following characteristics.

<Prior Art> A detector having a pair of variable capacitors is configured as shown in the cross-sectional view of FIG. 1, for example.
Measuring diaphragm 1 moves measuring chambers 7 and 8 into the container, which is closed on both sides by sealing diaphragms 4 and 5.
The measurement chambers 7 and 8 are filled with the liquid 6. The measurement diaphragm 1 is used as a movable electrode, and fixed electrodes 2 and 3 are arranged on the inner wall surfaces of the measurement chamber facing both sides of the movable electrode, forming a pair of variable capacitors C ₁ and C ₂ . In this detector, seal diaphragms 4, 5 on both sides of the container
When high pressure PH and low pressure PL are applied from the outside to , both pressures are transmitted to the measuring diaphragm 1 through the sealed liquid 6 and displace the measuring diaphragm, that is, the movable electrode 1 . This results in a pair of variable capacitors C ₁ , C ₂
The capacitance changes differentially depending on the measured quantity (PH-PL). In a capacitive conversion device using such a detector, an oscillation output is generally applied from an oscillator to a pair of variable capacitors, and an electrical signal corresponding to the sum of the capacitances that differentially changes depending on the measured quantity is generated. The amount to be measured is converted into an electrical signal by controlling the oscillator so that the capacitance remains constant and outputting an electrical signal corresponding to the differentially varying capacitance difference. However, the capacitive converter configured in this manner has poor linearity due to the influence of stray capacitance existing between the electrodes of a pair of variable capacitors.

For this reason, linearity has conventionally been improved by using a fixed capacitor for compensation, an example of which is shown in FIG. In Figure 2, a pair of variable capacitors
Since the oscillation output of the oscillator OSC is applied to C ₁ , C ₂ and the compensation fixed capacitor C ₃ via the transformer T, alternating currents i ₁ , i ₂ , i ₃ flow according to their respective capacities. . The alternating currents i ₁ , i ₂ , i ₃ are rectified and smoothed by rectifying diodes D ₁ , D ₂ , D ₃ , D ₄ , D ₅ , D ₆ and smoothing capacitors Cf ₀ , Cf ₁ , Cf ₂ . Therefore, the current voltage converter CON1
The input of is the difference between alternating currents i ₁ and i ₂ , that is, the capacitance C ₁ ,
An average current Io corresponding to the difference in C ₂ is given.
CON1 consists of an operational amplifier _OP1 and a resistor Ro connected to its feedback circuit, and converts the average current Io into an output voltage Eo (=-Io·Ro). The alternating current i ₂ , that is, the average value current I ₂ corresponding to the capacitance C ₂ is applied to the input of the current-voltage converter CON2. CON2 consists of an operational amplifier OP ₂ and a resistor R ₁ connected to its feedback circuit, and converts the average current I 2 according to the capacitance of C ₂ applied to its input into a voltage E ₂ equal to R ₂ and I _{2 .} do. The alternating current i ₁ , that is, the average value current I ₁ according to the capacitance C ₁ is given to the input of the integrator INT. INT consists of an operational amplifier OP ₃ and a capacitor CI connected to its feedback circuit, and its input includes an average current I ₁ corresponding to the capacitance of C ₁ as well as an output voltage E ₂ of CON 2.
is given through resistor R ₂ and the negative reference voltage
Er is given through resistor _R3 . Furthermore, the compensation current I ₄ obtained by dividing the AC current i 3, that is, the average current according to the capacity of C ₃ with _the resistor R ₄ , is also the average value current I ₁
and is given to the input of INT with the opposite polarity. Note that the compensation current I ₄ can be adjusted by moving the brush position α of the resistor R ₄ . The integrator INT calculates the average value current I ₁ according to the capacity of C ₁ and the output voltage E ₂ of CON 2.
A current I ₃ of E ₂ /R ₂ corresponding to the reference voltage Er and a reference current Ir and a compensation current I ₄ of Er/R ₃ corresponding to the reference voltage Er are added and integrated. If the values of the resistors R ₁ and R ₂ are chosen to be equal, the current I ₃ becomes a current that is the inversion of I ₂ and has the same polarity as I ₁ and the opposite polarity to the reference current Ir as shown in the figure. In other words, the integrator INT controls the oscillator OSC so that the sum of the average value current (I ₁ + I ₂ ) corresponding to the sum of the capacitances of C ₁ and C ₂ , the reference current Ir, and the compensation current I ₄ becomes zero.
Control the oscillation output of current-voltage converter CON1
An output voltage Eo as shown in the following equation is obtained at the output terminal of .

Eo=C ₁ −C ₂ /C ₁ +C ₂ −αC ₃ Ir・Ro (1) Then, the capacitance of the pair of variable capacitors C ₁ and C ₂ is calculated as follows: When the initial capacitance is Co, movable electrode 1 and fixed electrodes 2 and 3
Letting d be the reference interval between the electrodes (the interval when X=0) and Cs be the stray capacitance existing in parallel between the electrodes, they are given by the following equations.

C ₁ = Cod/d−x+Cs (2) C ₂ = Cod/d+x+Cs (3) From equations (1), (2), and (3), the output voltage Eo is: Eo=2x/2d+(2Cs−αC ₃ ) AIr・Ro (4) However, A=(d ² −x ² )/Co・d. Here, if the shunt ratio α is adjusted to satisfy α=2Cs/C ₃ (5), the output voltage Eo becomes Eo=x/dIr・Ro (6), and the influence of the stray capacitance Cs can be removed. Linearity can be improved.

By the way, since the stray capacitance Cs of the pair of variable capacitors C ₁ and C ₂ has a temperature coefficient, it is necessary to match the temperature coefficient of the capacitance of the fixed capacitor C ₃ for compensation. However, fixed capacitors have only a limited temperature coefficient, making it difficult to match the temperature coefficient of a pair of variable capacitors, and there are problems in that they are affected by changes in ambient temperature. Furthermore, since the current flowing through the compensation capacitor _C3 is used, if there is stray capacitance in parallel with _C3 , it will be affected.

<Purpose of the present invention> The purpose of the present invention is to realize a capacitive converter that can compensate for linearity without being affected by changes in ambient temperature or stray capacitance of a compensation circuit.

<Configuration of the present invention> The present invention has a linear compensation circuit that rectifies the oscillation output of an oscillator and supplies it to a compensation resistor to obtain a compensation current. This realizes a conversion device.

<Embodiment> FIG. 3 is a connection diagram showing an embodiment of the device of the present invention. In Fig. 3, the difference from Fig. 2 is that the voltage generated in the winding n ₄ of the transformer T is rectified and smoothed by a diode D ₅ and a smoothing capacitor Cf ₃ and applied to a compensation resistor Rs, and the direct current flowing through the resistor Rs is The point is that the current is divided by resistor _R4 to obtain the compensation current, and no compensation capacitor is used.

In the present invention configured in this manner, the output voltage Eo is given by the following equation.

Eo=2x/2d+(2Cs-α/2fRs)A Ir・Ro (7) Therefore, if α is adjusted to satisfy α=4Cs・f・Rs (8), the output voltage Eo
is the same as equation (6), the influence of stray capacitance Cs can be removed, and linearity can be improved. Furthermore, there are resistors with various temperature coefficients, and it is easy to select one that matches the temperature coefficient of the stray capacitance of the pair of variable capacitors, so that it is less susceptible to changes in ambient temperature. Furthermore, since the direct current is rectified near the winding of the transformer T, it is not affected by the stray capacitance of the compensation circuit.

In addition, although the case where the voltage obtained by rectifying and smoothing the oscillation output is applied to the compensation resistor Rs is illustrated above, the fourth
As shown in the figure, the smoothing capacitor Cf ₃ may be omitted to provide only rectification. In this case, noise resistance can be improved by connecting a diode D ₆ instead of Cf ₃ as shown in FIG.

<Effects of the present invention> In the present invention, linearity is improved using a compensation resistor, so a capacitive conversion device is realized that is less susceptible to the effects of changes in ambient temperature and stray capacitance of the compensation circuit. can.

[Brief explanation of drawings]

Fig. 1 is a sectional view showing an example of a detector using a pair of variable capacitors, Fig. 2 is a connection diagram showing an example of a conventional capacitive conversion device, and Fig. 3 is an example of an embodiment of the device of the present invention. 4 and 5 are connection diagrams showing other embodiments of the device of the present invention. C ₁ , C ₂ ... variable capacitor, C ₃ ... fixed capacitor, Rs ... compensation resistor, OSC ... oscillator,
CON1, CON2...Current voltage converter, D ₁ ~ _{D 6} ...
... Rectifier diode, T...Transformer, OP ₁ ~
OP ₃ ...Operation amplifier, INT...Integrator.

Claims (1)

[Scope of utility model registration request] a detector having a pair of variable capacitors whose capacitance differentially changes depending on the measured quantity; an oscillator applying an oscillation output to the pair of variable capacitors;
A compensation resistor to which a voltage obtained by rectifying the oscillation output of this oscillator is applied, a circuit that divides the current flowing based on this compensation resistor to obtain a compensation current, and an average corresponding to the sum of the capacitances of the pair of variable capacitors. A circuit for controlling the oscillator so that the value current is equal to the sum of a reference current and a compensation current, and a circuit for obtaining an output signal related to an average value current corresponding to a difference in capacitance of the pair of variable capacitors. Capacitive conversion device.