JPS6236088Y2 - - Google Patents

Description

[Detailed description of the invention] This invention uses a pair of variable capacitors whose capacitance changes differentially depending on the amount of displacement corresponding to the quantity to be measured, such as pressure or differential pressure, to convert the quantity to be measured into an electrical signal. The present invention relates to a capacitive conversion device that performs conversion.

In general, capacitive converters are, for example,
As shown in Publication No. 15398, the oscillator is controlled so that the DC current corresponding to the sum of capacitances that differentially changes depending on the amount of displacement is constant, and the DC current corresponding to the difference in capacitance is By outputting it, the measured quantity is converted into an electrical signal. In such a configuration, the DC current corresponding to the sum of capacitances is controlled to be constant, so the conversion circuit part is not affected by the temperature coefficient of the rectifier diode. Since the detector portion including the capacitance has a temperature coefficient, the signal current corresponding to the difference in capacitance has temperature dependence. Moreover, the temperature coefficient of the detector part is different between the differential pressure transducer and the pressure transducer, even when, for example, differential pressure or pressure is converted into displacement using a metal diaphragm to differentially change the capacitance of a pair of variable capacitors. There is. For this reason, conventionally, a temperature compensation resistance network including a thermistor was provided in the conversion circuit section to compensate for the temperature dependence of the signal current, but a special temperature compensation resistance network had to be provided. The disadvantage was that it was complicated.

By the way, in this type of capacitive converter,
In order to compensate for the deterioration of linearity due to the influence of stray capacitance that exists in parallel with the variable capacitor, and for the nonlinearity when converting the measured quantity into displacement using a metal diaphragm, such as in differential pressure transducers and pressure transducers, A compensation capacitor is provided, a voltage equal to the voltage applied to the pair of variable capacitors is applied from an oscillator to obtain a compensation current according to the capacitance, and by adding this compensation current to the reference current, the pair of variable capacitors is There is a device that controls an oscillator so that the difference between the current corresponding to the sum of the capacitances and the compensation current is constant, and adjusts the value of the compensation current to perform linearization.

The present invention realizes a capacitive converter that can compensate for temperature dependence without using a special temperature compensation circuit by effectively utilizing the linearization circuit described above.

FIG. 1 is a connection diagram showing one embodiment of the device of the present invention. In the figure, C ₁ and C ₂ are a pair of variable capacitors whose capacitance changes differentially depending on the amount of displacement.
The figure shows a movable electrode 10 that is displaced according to the amount to be measured.
and fixed electrodes 11 and 12 which are arranged opposite to each other with the movable electrode 10 in between. C ₃
is a fixed capacitor for compensation, A is a high-gain differential amplifier, O is an oscillator, T is a transformer, and the primary coil
It has n ₁ and three secondary coils n ₂ , n ₃ , n ₄ , and the number of turns of n ₄ can be adjusted.
Vr is a stabilized DC power supply, D ₁ to _{D 6} are each a rectifying diode, R ₀ to _{R 5} are each a resistor, C _f0 to C _f2 are each a smoothing capacitor, and C ₄ is a DC blocking capacitor. .

In the device of the present invention configured as described above, since the oscillation output of the oscillator O is given to the pair of variable capacitors C ₁ and C ₂ via the transformer T, alternating currents i ₁ and i corresponding to their capacitances are generated respectively. ₂ flows. The alternating currents i ₁ and i ₂ are rectified and smoothed by diodes D ₁ to D ₄ and smoothing capacitors C _f0 to C _f2 . Therefore, an average current Io corresponding to the difference between the currents i ₁ and i ₂ flows through the resistor R ₀ , and the output voltage Eo generated across the resistor R 0 is equal to the number of turns of the primary winding n ₁ of the transformer T.
N ₁ , the number of turns of the secondary windings n ₂ and n ₃ is N ₂ , the half amplitude of the oscillation output of the oscillator O is Ea, the frequency is variable, and the forward voltage drop of the rectifier diodes D ₁ to D ₄ is variable as E _d1 Letting Cs ₁ and Cs ₂ be the stray capacitances existing in parallel with capacitors C ₁ and C ₂ , it can be approximately expressed by the following equation.

Eo=2R ₀ f {(C ₁ +Cs ₁ )−(C ₂ +Cs ₂ )} (N ₂ /N ₁ Ea−E _d1 ) (1) Also, the resistors R ₁ and R ₂ have currents i ₁ and i ₂ , respectively. Average value currents I ₁ and I ₂ corresponding to the currents flow with the polarities shown. Furthermore, resistors R ₁ and R ₂ are connected to the DC power supply along with resistors R ₄ and R ₅ .
It is connected in series with Vr, and a constant DC current Ir is supplied in the opposite direction to the average value currents I ₁ and I ₂ . On the other hand, the fixed capacitor _C3 for compensation also has a transformer T.
Since the oscillation output of the oscillator O is applied through the oscillator O, an alternating current _i3 corresponding to its capacity flows. This alternating current _i3 is rectified by diodes _D5 and _D6 . A current corresponding to the capacitance of the compensation capacitor _C3 is divided into a compensation current _I3 and a bypass current _I4 by the volume resistor _R3 . Therefore, the compensation current I ₃ is calculated using the voltage dividing ratio α of the volume resistor R ₃ and 22 of the transformer T.
If the number of turns of the next winding n ₄ is N ₃ and the forward voltage drop of diodes D ₅ and D ₆ is E _d2 , then I ₃ = 2α (N ₃ /N ₁ Ea - E _d2 ) fC ₃ (2) Given. This compensation current I ₃ is supplied to the series circuit of resistors R ₁ and R ₂ in the opposite direction to the average value currents I ₁ and I ₂ . Therefore, a feedback voltage E _f as shown in the following equation is generated across the series circuit of resistors R ₁ and R ₂ .

E _f =2R {Ir+2αfC ₃ (N ₃ /N ₁ Ea−E _d2 ) −f(C ₁ +C ₂ +Cs ₁ +Cs ₂ )(N ₃ /N ₁ Ea−E _d2 )}(3
) However, R = R ₁ = R ₂ This feedback voltage E _f is applied to the differential amplifier A,
For example, the power supply voltage of the oscillator O is controlled so that E _f =0, so the amplitude Ea of the oscillation output is given by the following equation.

Therefore, the output voltage Eo is given by the following equation from equations (1) and (4).

And the forward voltage drops of the diodes E _d1 , E _d2
has a negative temperature coefficient and can be approximated by the following equation.

E _d1 = mE _d2 = E _d0 (1-βt) (6) Here, m is D ₁ to D ₄ and D ₅ as shown,
If each of D ₆ is configured with one diode element, it is 1, and if only D ₁ to D ₄ sides are each configured with two diode elements, m = 2.
becomes. Substituting equation (6) into equation (5), the output voltage Eo
teeth, By selecting the winding ratio N ₃ /N ₂ of the secondary windings n ₂ (n ₃ ) and n ₄ of the transformer T, the temperature coefficient of Eo can be adjusted to be positive or negative. That is, the temperature coefficient is m
If N ₃ /N ₂ >1 is chosen, it will be negative, and if mN ₃ /N ₂ <1 is chosen, it will be positive. Note that if mN ₃ /N ₂ =1 is selected, the temperature coefficient becomes zero. Therefore, temperature dependence can be effectively compensated by selecting the winding ratio according to the temperature coefficient of the detector portion.

On the other hand, the pair of variable capacitors C ₁ and C ₂ are made symmetrically, and the stray capacitance can be considered as Cs ₁ = Cs ₂ = Cs, so Cs = αC ₃ N ₃ /N ₂ (8) _{If the} voltage division ratio _{α of the volume resistor} R _{3 is adjusted} as shown in FIG. βt)} (9) Therefore, the influence of stray capacitance Cs can be removed and linearity can be improved. Moreover, if α is adjusted so that α<Cs/C ₃・N ₃ /N ₂ , the increase rate can be increased as C ₁ −C ₂ /C ₁ +C ₂ becomes larger, and α>Cs/C ₃・N ₃ / _N2 , the increase rate can be reduced as _C1 - _C2 / _C1 + _C2 becomes larger.
This can be corrected by appropriately selecting the partial pressure ratio of R ₃ . Note that when the capacitance of capacitor C ₃ is made adjustable and the winding ratio N ₃ /N ₂ is changed, N ₃ /N ₂ C ₃
By making it constant, it is possible to eliminate the influence of the temperature coefficient compensation operation on the linear ratio.

FIG. 2 is a connection diagram showing another embodiment of the device of the present invention, which differs from the embodiment of FIG. 1 in that it is configured in a two-wire system. In Figure 2, R ₀ is a variable resistor for span adjustment, A ₁ is a high gain differential amplifier,
The sum of the potential E _{1 at the connection point of the resistors R 1 and} R ₂ and the voltage Eo across the resistor R ₀ is applied to its input terminal (+) via the resistor R ₆ , and the voltage E across the feedback resistor R _f is applied to the input terminal (+). ₂ is applied via resistor _R7 , and the other input terminal (-) is connected to a voltage dividing resistor for zero point adjustment with a constant voltage.
A voltage E ₃ divided by R ₁₀ is applied via a voltage dividing circuit including resistors R ₈ and R ₉ . Note that the resistors R ₆ to _{R 9} are selected to be sufficiently large compared to the signal source resistance. Q is an output transistor driven by the output of the amplifier _A1 , and supplies an output current Io to a load R _L placed on the receiving side via a feedback resistor R _f and a pair of transmission lines l. The voltage drop produced across the resistor R _f by the output current Io becomes the voltage E ₂ . CC
is a constant current circuit, which is made up of, for example, a transistor and a field effect transistor as shown in the figure, and supplies a constant current to a Zener diode D _Z to obtain a stabilized DC voltage Vr across it. This stabilized DC voltage Vr is applied to the DC circuit of resistors R ₄ , R ₁ , R ₂ , R ₅ and the voltage dividing resistor R ₁₀ for zero point adjustment, and is applied to the amplifiers A and A ₁ via transistors. Added to power terminal. E is a DC power supply, which is the only power supply in this device, and is connected in series to the load R _L and placed on the receiving side.

In the device of the present invention configured in this way, the output current Io transmitted to the load R _L on the receiving side via the pair of transmission lines l is Io = 1/R _f {Eo + (R ₅ Ir - γVr)} (10) However, R ₆ = R ₇ = R ₈ = R ₉ γ = voltage dividing ratio of the voltage dividing resistor R ₁₀ , which corresponds to the signal voltage Eo. (Ten)
As is clear from the equation, by adjusting the voltage dividing ratio β of the voltage dividing resistor _R10 , the value of the output current Io when Eo is 0% can be set to a desired value (for example, 4 mA), and the zero point can be adjusted. Also, by adjusting the value of resistance R ₀ , the amount of displacement x
Set the output current Io value at 100% to the desired value (e.g.
20mA) and the span can be adjusted. Furthermore, changing the value of the variable resistor R ₀ does not affect the zero point, and changing the voltage dividing ratio β of the voltage dividing resistor R ₁₀ does not affect the span. That is, in a two-wire converter, zero point adjustment and span adjustment can be performed independently of each other.

In addition, in this example, the compensation capacitor is
Use two C ₃₁ and C ₃₂ , one end of C ₃₁ is connected to point P ₁ where number of turns N 31 of secondary winding n ₄ of transformer T is N ₃₁ , and one end of C ₃₂ is connected to point P ₂ where number of turns N ₃₂ of transformer T is n ₄ . The other ends of C ₃₁ and C ₃₂ are connected to each other by a switching means J, and their capacities are selected so as to satisfy N ₃₁ C ₃₁ =N ₃₂ C ₃₂ . If the value of N ₃₁ is determined according to the temperature coefficient of the differential pressure converter and the value of N ₃₂ is determined according to the temperature coefficient of the pressure converter, the C ₃₁ side or the C ₃₂ side can be selected by the switching means J. Temperature dependence can be compensated for only by

In the above example, the voltage applied from the oscillator O to the compensation capacitor is applied by changing the number of turns of the secondary winding _n4 of the transformer T, but it is also possible to divide the voltage of the secondary winding _n4 with a voltage dividing resistor. You may also change it accordingly.

As explained above, according to the present invention, since the linearization circuit is effectively used, a capacitive type that can compensate for temperature dependence with a simple configuration does not require a special circuit for temperature compensation. A conversion device is obtained.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram showing an embodiment of the device of the present invention;
FIG. 2 is a connection diagram showing another embodiment of the device of the present invention. C ₁ , C ₂ ... A pair of variable capacitors, C ₃ ... Compensation capacitor, A ... High gain differential amplifier, O
...oscillator, T...transformer.

Claims (1)

[Scope of utility model registration request] a pair of variable capacitors whose capacitance differentially changes depending on the measured quantity; a fixed capacitor for compensation; an oscillator that applies an oscillation output to the pair of variable capacitors and the fixed capacitor via a transformer; a resistor circuit through which a difference between a current corresponding to the sum of the capacitances of the capacitors and a reference current flows; a means for adjusting a compensation current flowing according to the capacitance of the compensation capacitor; and a means for adjusting the compensation current to the sum of the capacitances. means for causing the current to flow in the opposite direction to the resistor circuit; means for controlling the oscillator so that the voltage across the resistor circuit becomes zero; and a current corresponding to the difference in capacitance of the pair of variable capacitors. In a capacitive conversion device, the voltage applied from the oscillator to the compensation capacitor can be adjusted, and the temperature coefficient can be adjusted by changing the ratio of the voltage applied to the pair of variable capacitors. A capacitive conversion device characterized by being able to adjust.