JPH0326322B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a differential pressure transmitter comprising a differential pressure/current converter using a differential capacitance type differential pressure sensor and an electric circuit for amplifying and converting the output of the converter. In particular, the present invention relates to means for reducing performance degradation due to the influence of ambient temperature of a differential pressure transmitter.

As shown in the cross-sectional view of FIG. 1, the differential pressure sensor of the differential capacitance type is placed in a measuring chamber 7, inside a container whose both sides are sealed with liquid-contact diaphragms 4 and 5.
A diaphragm 1 that is separated into two parts (8) is fixed to the periphery of the measurement chamber.The diaphragm 1 is used as a movable electrode, and fixed electrodes 2 and 3 are arranged on both sides of the diaphragm 1, which are fixed to the inner wall surface of the measurement chamber facing each other. Three electrodes form a differential capacitance. The chamber is filled with a sealed liquid 6. In this differential pressure sensor, when high pressure P _H and low pressure P _L are applied from the outside to the wetted diaphragms 4 and 5 on both sides of the container, both pressures are reduced to the filled liquid 6.
is transmitted to both sides of the movable electrode via the movable electrode 1.
The differential pressure between P _H and P _L acts on the movable electrode 1 , and the movable electrode 1 is deviated in the direction approaching the low-pressure side fixed electrode 2 . The following equation holds between the center deviation x of the movable electrode 1 and the differential pressure at this time.

x=K(P _H −P _L ) (1) Here, K is a constant under the condition that the ambient temperature of the differential pressure sensor is constant.

In the differential pressure/current converter PD configured using this differential capacitance type differential pressure sensor, a capacitor _C and 2nd by _C.L.
The electric bridge circuit shown in the figure is formed. (Capacitors C _H and _CL are named branch capacitances on the high voltage side and low voltage side.) In this bridge circuit, by controlling the excitation voltage e of the bridge, the currents i _H and i flowing through the capacitors C _H and _CL are The sum of _L is always kept at a constant value I _C. Furthermore, if the initial gap between the movable electrode 1 and each of the fixed electrodes 2 and 3 is d _p , then the differential pressure (P _H − P _L ) and (i _L
−i _H ), the following formula holds true. (Note that i _L and i _H are named the branch currents on the low voltage side and high voltage side, respectively.) i _L −i _H = x/d _p I _C = K _{I C} /d _p (P _H − P _L ) (2) Top The equation shows that (i _L −i _H ) is proportional to (P _H −P _L ) when K is constant. In addition, in the actual differential pressure/current converter PD, (i _L −i _H ) and (i _L +
i _H ) are converted into corresponding DC signals I _M1 and I _M2 , respectively. The DC signal I _M1 is applied as an output current representing the input differential pressure to an electric circuit connected in series to the converter PD, is amplified by the circuit, is converted into a normalized current output I _p of, for example, 4 to 20 mA, and is sent to the receiver. Ru.
The normalized current I _p is used to measure differential pressure. Also,
The DC signal I _M2 is used within the converter PD to keep I _C supplied to the bridge circuit, that is, (i _L +i _H ) constant.

Equation (2) above is a conversion characteristic equation that holds true under the condition that the ambient temperature of the converter is constant, but if the ambient temperature changes, the output current I _M1
is no longer proportional to the input differential pressure. Various improvements have been made to the structure of this type of converter in order to reduce the effects of changes in ambient temperature on its conversion characteristics, but a converter that is completely unaffected by temperature changes has not yet been realized. Not yet. Therefore, if there is a temperature change in the location where the differential pressure/current converter is installed, the converter output current I _M1 will be affected by that, and the transmitter output I _p , which amplifies and converts it, will also be affected by it, and this will be I. This appears as zero point error and span error in the receiver's instructions for measuring _p . As a result, there is a drawback that the accuracy of differential pressure measurement decreases.

The main object of the present invention is to realize a differential pressure transmitter including a differential pressure transducer with means for reducing the effect of ambient temperature on the output of the differential pressure transducer included in the differential pressure transmitter.

Note that the zero point error and span error that occur in the measured values of the receiver on the receiving side of the differential pressure transmission system are mainly caused by changes in the converter output caused by changes in ambient temperature in the converter on the transmitter side.
Therefore, in the following description, the former, that is, the zero point error and span error that occur in the measured values of the receiving receiver, and the latter, that is, the fluctuations in the output of the transmitting side converter, are referred to as the zero point error and the span error, respectively.

Another object of the present invention is to provide a differential pressure converter having a temperature error compensation circuit as a component, which generates a zero point error compensation current and a span error compensation current for compensating the zero point error and span error of the output of the differential pressure converter. To provide pressure transmitters.

Another object of the present invention is to provide a differential pressure transmitter equipped with a differential pressure converter capable of independently compensating for zero point error and span error of the differential pressure converter.

Another object of the present invention is to provide a transmitter having a differential pressure converter equipped with the temperature error compensation circuit that generates a zero point error compensation current and a span error compensation current only when the ambient temperature is different from a specific reference temperature. It is on offer.

Another object of the present invention is to provide a differential pressure converter equipped with a temperature error compensation circuit capable of compensating for the zero point error and span error of the differential pressure converter in response to constant ambient temperature changes in a single circuit. To provide electric transmitters with

Hereinafter, the present invention will be described in detail with reference to embodiments of the present invention.

First, a two-wire differential pressure transmission system using a conventional differential pressure converter will be described in order to clarify the difference between the configuration of the present invention and the configuration of a conventional differential pressure transmitter. FIG. 3 is a diagram showing its configuration. In the figure, the differential pressure transmitter P/I _p is a two-wire transmitter, and the differential pressure/current converter PD is a differential pressure conversion output representative of the input differential pressure.
Operating current I _C supplied to I _M1 and the bridge circuit = (i _L + i _H )
The electric bridge circuit section _PD1 outputs a DC signal I _M2 corresponding to the DC current I _M2 and the reference current I _R , and the operating current I _C of the electric bridge circuit is adjusted so that both currents are equal.
It consists of an operating current control circuit section PD ₂ that controls the current. Hereinafter, in order to distinguish between the output current I _M1 and the output current I _M2 output from PD ₁ , I _M1 will be referred to as a first output current, and I _M2 will be referred to as a second output current. The electrical circuit cascaded to the differential pressure/current converter PD includes an I/V converter that converts the first output current I _M1 into a voltage E ₂ , a conversion amplifier A, an output transistor Q ₁ and a feedback resistor Rf ₂ . The transmitter P/I _p is installed at the site where differential pressure is measured, and the operating power of each arithmetic circuit included in this transmitter P/I _p is
It is supplied through the transmission line 12 from the power source E _I of the PM. The first output current I _M1 of the electric bridge circuit portion PD ₁ of the converter PD is converted into a normalized output current I _p of 4 to 20 mA DC by a series of signal conversion circuits such as I/V and V/I, and then connected to the transmission line 12. is transmitted to the receiving PM via.
Also, the second part of the electric bridge circuit part PD ₁ of the differential pressure converter PD
The output current I _M2 is compared to the reference current I _R and the difference is applied to the control amplifier U ₁ . The output of the control amplifier U ₁ is connected to the bridge circuit (not shown) included in the converter PD.
The output amplitude e of the AC oscillator OSC is controlled so that the operating current (i _L +i _H ) always corresponds to I _R and becomes a constant current I _C . Current input/output terminals 18, 1 of transmitter P/I _p
The voltage V ₁ of 9 is converted into a constant voltage V ₂ by a constant current circuit J and a Zener diode D _Z. V ₂ is used as a power source to drive each operational amplifier of the transmitter P/I _p .

FIG. 4 shows a schematic configuration of a circuit example in which the present invention can be implemented in a differential pressure transmitter P/I _p employing the two-wire transmission system illustrated in FIG. In FIG. 3, the parts with the same symbols as those given to each part are the same parts as in FIG. 3. Below, in each figure, E
represents a voltage source and its voltage value, and R represents a resistor and its resistance value. In the figure, the differential pressure transmitter P/I _p shown in Fig. 3 and the corresponding fourth
The only difference from the transmitter P/I _p shown in the figure is the converter PD of both. The latter converter PD is equipped with a temperature error compensation circuit TC, whose output current ±I _Z has an appropriate polarity with the first output current I _M1 of PD ₁ at the current addition point S ₁ of the PD output circuit. The other output current ±I _S is the second output current I _M2 of PD ₁
At the addition point _S2 of the output circuit, it is added to (I _M2 - I _R ) with an appropriate polarity. ±I _Z is the zero point error compensation current,
± _IS is named the span error compensation current.

Figure 5 shows a temperature error compensation circuit TC included in the differential pressure converter PD in the embodiment of the present invention illustrated in Figure 4.
A more detailed configuration of the parts is shown. This circuit TC consists of a temperature difference detection circuit TB and a voltage/current conversion circuit E/I that converts the output voltage E of TB into two currents: a zero point error compensation current ±I _Z and a span error compensation current ± _IS .

The temperature difference detection circuit TB is a kind of temperature difference detection bridge, which produces an output voltage E that is proportional to the difference (t-t _O ) between a specific reference temperature T _p within the operating temperature and the ambient temperature t. The TB illustrated here has a transistor Q ₂ and a diode D ₁ as a sensor for ambient temperature,
D ₂ and the operation power supply V ₂ of the operational amplifier included in the differential pressure transmitter is used as the bridge power supply. Generally, the forward voltage drop V _BE between the base and emitter of a silicon transistor has a temperature coefficient of about -2 mV/°C in the normal temperature range of -50 to 200°C where the transistor is used. The forward voltage drop of a silicon diode also has a similar temperature dependence.

TB is a bridge circuit that utilizes this temperature characteristic. As shown in the figure, the branch between - is the base-emitter circuit of _Q2 , diodes _D1 , _D2 , and resistor.
Consists of a series circuit of R ₁ and variable resistor R ₂ ,
A constant resistance R ₃ is connected to the branch line between -. −
The potential point G in between is a point at the same potential as the operational reference point G of the operational amplifier included in the transmitter, and the reference voltage V ₂
is maintained at a specific potential between . The resistors R _A and R _B indicated by dotted lines in the figure are not actually used here. The calculation reference point G is set to the reference voltage V ₂ in the transmitter.
This is an equivalent representation of a voltage dividing element that is set at a desired potential between . For TB, the upper limit temperature t ₊ of the ambient temperature at which the transmitter is used is set to, for example, 80°C, the lower limit temperature t _- is set to, for example, −20°C, and the intermediate temperature, for example, 30°C, is set as the reference temperature t _p , and the variable resistor is _R2 is adjusted to balance the voltage E generated between the voltage detection terminal and the voltage detection terminal to zero at a reference temperature _tp , and thereafter the adjustment position of _R2 is fixed at that position for use. In a circuit adjusted to this state, a voltage E approximately proportional to the difference (t- _tp ) between the ambient temperature t and the reference temperature _tp is generated between the voltage detection terminals. In this case, the temperature coefficient of the voltage generated between the terminals of the temperature sensing elements Q ₂ , D ₁ , and D ₂ due to the forward current flowing through them is negative, so in the region where t is higher than t _p , the voltage E is positive. , is negative in the region where t is lower than t _p . E corresponding to the upper limit temperature t+ has a positive maximum value and decreases as t decreases.
= t becomes zero at _p , and if t decreases, E changes to negative and reaches a negative minimum value at the lower limit temperature t-.

Although the temperature difference detection circuit TB shown in FIG. 5 uses a combination of transistor Q ₂ and diodes D ₁ and D ₂ as a temperature sensor, the temperature sensor is not limited to the combination of these elements. For example, only diodes may be used, and any number of diodes may be used. Alternatively, a thermistor or a metal wire with a relatively large temperature coefficient of resistance may be used. Here, elements used as temperature sensors are collectively called temperature sensing elements.

The conversion circuit E/I is a voltage follower U ₃ (hereinafter the voltage follower is represented by U ₃ ), which is composed of an operational amplifier U ₃ .
an inverting circuit U ₄ (hereinafter this circuit is represented by U ₄ and referred to as U ₄ ), which is composed of two resistors R of the same value and an operational amplifier U ₄ , and _an output terminal of U ₃ . and two sliding wire resistors bridging the output end of U ₄
RV ₁ and RV ₂ and slider of each sliding line resistance RV ₁ and RV ₂
It consists of output resistors R _Z and R _S connected to d ₁ and d ₂ , respectively.

The voltage follower _U3 is a buffer amplifier for electrically separating the output terminal of TB and the output terminal of _U3 , and obtaining a voltage source of the same voltage E as that of the TB output terminal at a point. If the ambient temperature t is in the range between t+ and _tp , +E is applied to one end of the parallel circuit of RV ₁ and RV ₂ , and -E is applied to the other end. Also, if it is in the range of t _p to t-, then -
+E is applied to E. The variable voltage ±E _Z generated on the slider d ₁ of RV ₁ is converted into a variable current ±I _Z by the output resistor R _Z , and ±I _Z is used as the zero point error compensation current at the summing point S ₁ of the differential pressure converter PD. sent to. RV ₂
The variable voltage ±E _S generated on the slider _d2 is the output resistance
R _S converts the span error compensation current ±I _S to PD
is sent to the summing point _S2 .

FIG _{. 6A is an explanatory diagram of the configuration of one sliding line resistance, for example RV 1 ,} of the converter E/I, and _FIG . ₁ is a characteristic diagram showing the correspondence relationship with E _Z picked up in No. 1. FIG. In the figure (1), the ambient temperature is the upper limit temperature t
In the case of +, (2) is the characteristic line in the case of the lower limit temperature t-. As is clear from the figure, in the case of t+, if the voltage applied to the end is +E, the slider d ₁ will move on the sliding line by 1
As the slider moves from the end to
The voltage E _Z of d ₁ continuously decreases from +E, reaches zero at the center point, becomes negative after passing through it, and becomes -E when it reaches the other end. Therefore, the zero point error compensation current I _Z flowing through the resistor R _Z having a specific value and flowing into the output circuit also decreases continuously in response to the change in E _Z. Further, when the ambient temperature is t-, the change in E _Z is opposite to that when the ambient temperature is t+.
When the ambient temperature t is an arbitrary temperature t, E _Z follows a change corresponding to the temperature t. In the figure, a characteristic line for an arbitrary temperature t is illustrated in (3).

Figure 7 shows 2 in the E/I circuit included in the TC.
This is an equivalent circuit showing the configuration of a circuit portion that outputs two compensation currents ±I _Z and ±I _S. RV ₁ is the sliding line resistance for obtaining the zero point error compensation current ±I _Z , and RV ₂ is the sliding line resistance for obtaining the span error compensation current ± _IS . To change a TC circuit that outputs a single compensation current to a circuit that outputs two compensation currents, use +E, -
It is only necessary to change the number of sliding wire resistances bridging to the output end that outputs E from one to two.

Next, the operation of the circuit that compensates for the zero point error and span error using the output currents ±I _Z and ±I _S of the temperature error compensation circuit TC will be explained.

The following points are experimentally known regarding the performance of many differential pressure converters currently in use. Now, the conversion coefficient between input and output ΔI _M1 /ΔP _i (where ΔP _i is the input signal
For similar converters that are guaranteed to have the same small change in P _i (ΔI _M1 is the small change in output current I _M1 with respect to ΔP _i) , (i) the conversion coefficient of the differential pressure converter PD is determined by the measurement range of the converter. can be regarded as constant for the input P _i . Therefore, the full scale value P _i of input P _i
Full scale value of output I _M1 for (f・s)
The ratio of I _M1 (f·s) can also be considered constant.

(ii) The zero point error and span error at any temperature t within the operating temperature range t+ to t- of the converter PD are the difference between temperature t and a specific reference temperature t _p (t-t _p )
is proportional to.

(iii) In a large number of differential pressure converters PD of the same type, the zero point error and span error of each converter are within several percent of the minimum span, but the magnitude and polarity of the error in each converter vary.

In the present invention, the zero point error and span error due to the temperature of such converters of the same type are compensated by the zero point error compensation current ±I _Z and the span error compensation current I _S of the temperature error compensation circuit TC having the same configuration as described above. That is. Moreover, the compensation circuit of the present invention
The error compensation currents I _Z and I _S of the TC are converted into the output current of the converter PD.
I _M1 and a TC that does not affect the input/output characteristics with respect to the reference temperature t _p even when added to the PD output current I _M2
This is what we aim to achieve.

FIG. 8 is an explanatory diagram of the configuration of a circuit portion related to zero point error compensation in the converter PD. In the figure
Equivalently, the power supply that outputs the first output current I _M1 of the PD is
Denote by E _i and its output resistance by the equivalent resistance R _i . In addition, the output circuit portion of the zero point error compensation current I _Z in the temperature error compensation circuit TC is expressed as TC (Z), and this is expressed as the variable voltage ±E _Z generated on the slider d ₁ of the sliding wire resistance RV ₁ and the output resistance R _Z Indicated by Compensation current ±I _Z is
It is introduced at the summing point S ₁ between the output terminal 11 of the PD and the input terminal 13 of the I/V converter. The variable resistor R _V0 to which a voltage of ±V is applied is a zero adjustment resistor for I _M1 .

FIG. 9 shows the circuit configuration of a portion related to span error compensation in the differential pressure converter PD. differential pressure converter
A span error compensation current ±I _S is introduced from TC(S) to the addition point S ₂ of the second output current I _M2 of the bridge circuit portion PD ₁ of the PD and the reference current i _R to be compared with it. ± _ES is
It represents the variable voltage present on the slider d ₂ of the second sliding wire resistance RV ₂ of the TC circuit.

The zero point error compensation action will be explained with reference to FIGS. 6 and 8.

First, adjust the variable resistor for zero point adjustment at the reference temperature t _p to adjust the output current when the input differential pressure P _i is zero.
I Set _M1 to zero. The zero point error at the upper limit temperature t+ differs for each converter as mentioned above, but the maximum error value at t+ is I _M1 (MAX)
is known. In such cases, temperature error compensation circuit
The slider d ₁ of the slip wire resistance RV ₁ of the E/I circuit at TC is placed at or on the end of the slip wire and the absolute value of the compensation current I _Z flowing out through d ₁ at t+
so that the value is greater than the absolute value of I _M1 (MAX)
Determine the resistance value of the resistor R _Z of the I _Z output circuit. For each converter, the magnitude of the zero point error at any temperature t within the operating temperature range t+ to t- is t
Since it is less than the absolute value of I _M1 (MAX) at +,
If the output resistance R _Z is set as described above, even if the ambient temperature t is unknown, by sliding the slider d ₁ between - on the sliding line, the zero point error, that is, the output current I _M1 will be zero. We can position d ₁ at the point where d 1 becomes . Note that the positioning of slider _d1 is based on the reference temperature.
This is done with reference to the measured value of the normalized output I _p of the transmitter corresponding to the zero of the input P _i being calibrated at t _p .

Next, the span error compensation effect will be explained with reference to FIGS. 7, 9, and 10.

FIG. 10 is a diagram showing the input/output characteristics between the input differential pressure P _i and the output current I _M1 of the differential pressure converter PD.
In the figure, characteristic line (1) shows the characteristic line at the reference temperature _tp . The diagram shown here indicates that the operating current (i _L + i _H ) of the electric bridge circuit (see Figure 2) by the differential pressure sensor of the converter is kept at a constant value corresponding to a constant reference current I _R. This is the characteristic line under the following conditions. Characteristic line (2) exemplifies the input/output characteristic line under the above conditions, for example, when the ambient temperature is at its upper limit temperature t+. Note that line 2 is a characteristic line for which the zero point error at t+ has been compensated. Now, in the converter illustrated in Fig. 9, if the output current I _M1 at the upper limit temperature t+ for any value p of the input P _i is expressed as I _(t+)p , and I _M1 at the reference temperature t _p is expressed as I _(tp)p , then The span error ε _S(t+) with respect to the input differential pressure p at the upper limit temperature t+ is defined by the following formula.

ε _S(t+) = I _(t+)p /p−I(to)p/p (4) For convenience of explanation, the output current _I at t+ and t _p with respect to the maximum value p (MAX) of input P i is expressed as I _{(t+ )(fs)} and I _(tp)(fs) , ε _S(t+) can be expressed by the following formula.

ε _S(t+) = I _(t+)(fs) −I(to)(fs)/P(MAX) (5) However, the output current at the upper limit temperature t+ is (see Figure 2) is proportional to the operating current I _M2 = (i _L + i _H ) and is therefore proportional to the reference current I _R which is compared with I _M2 at the addition point _S2 of the PD. Therefore, equation (5) ε _S(t+) becomes the following equation.

ε _s(t+) =K _(t) I _R・P(MAX)−I _(tp)(fs) P(MAX)(6
) However, K _(t) is a function of the ambient temperature t and is constant at the upper limit temperature t+.

Here, in order to compensate for the error, the span error compensation current I _S is added to I _R at the addition point S ₂ , but as shown in Fig. 10, when I _(t+)(fs) > I _(tp)(fs) In order to reduce the error ε _S(t+) to zero, it must be supplied to the I _S addition point S ₂ that satisfies the following equation, where the right side of equation (6) is set to zero.

K _(t) (I _R − I _S ) P (max) − I _(tp)(fs) = 0 (7) On the other hand, at the upper limit temperature t+, the maximum value of the span error, that is, the maximum value of I _(t+)(fs) If I _(t+)MAX is known, as in the case of zero point error compensation, the span error compensation current of the E/I circuit in the TC circuit ±I _S The slider d of the power supply sliding wire resistance RV ₂ of the output circuit ₂ on the end of the sliding wire and the resistance R of the output circuit of ± _{I S} so that the absolute value of the span error compensation current ±I _S flowing out through 2 is greater than the absolute value of I _(t+)MAX. Determine the value of _S. In this way, since the output signal I _(t)(fs) of t+ in each converter is less than I _(t+)MAX , by sliding the slider d ₂ between - on the sliding line, (7) d ₂ can be positioned at a point that satisfies Eq. In other words, the characteristic line (2) can be matched with the characteristic line (1) at the reference temperature _tp .

Also, as shown in characteristic line (4), I _(t+)(fs)2 is I _(tp)(fs)
In the following cases, the characteristic line (4) can be made to match the characteristic line (1) as long as the polarity of I _S added to the reference current I _R is within the positive range. Furthermore, even if t is unknown, the span error at any temperature t within the operating temperature range t+ to t- can be determined by simply sliding the slider d ₂ between RV ₂ and -. The output current I _(t) for the value p can be matched with I _(tp) at the reference temperature.

According to the present invention, (i) zero point error and span error can be compensated for without mutual interference. (ii) At any temperature t, by operating only the zero point error compensation circuit and the span error compensation circuit, a differential pressure converter with characteristics matching the input/output characteristics during calibration at the reference temperature t _p can be realized. (iii) Zero point error compensation current according to the present invention ±
Since I _Z and the span error compensation current ±I _S become zero at the reference temperature t _p , adding them to the converter will not cause any change in the input/output characteristics at the reference temperature. (iv) Since the transmitter is configured so that zero point error compensation can be performed before the I/V converter that changes the measurement range, changing the measurement range will not cause excess or deficiency in the zero point error compensation current.

[Brief explanation of drawings]

FIG. 1 shows a cross-sectional view of a differential capacitance type differential pressure sensor that is part of the differential pressure converter of the present invention. FIG. 2 shows a principle diagram of an electric bridge circuit section constructed using the differential pressure sensor shown in FIG. 1. FIG. 3 is a block diagram of a conventionally known two-wire differential pressure transmission system. FIG. 4 shows a schematic configuration of a two-wire differential pressure transmitter in which the present invention can be implemented. FIG. 5 shows a detailed configuration of the temperature error compensation circuit included in the differential pressure converter in the embodiment of the present invention shown in FIG. FIG. 6A is an explanatory diagram of the configuration of one sliding wire resistance of a voltage/current converter which is a part of the temperature error compensation circuit of a differential pressure converter in which the present invention can be implemented, and FIG. 6B
represents the correspondence between the contact position of the slider sliding on the sliding wire resistance and the voltage on the slider. FIG. 7 is an equivalent circuit showing the configuration of the output circuit portion of the voltage/current converter included in the temperature error compensation circuit. FIG. 8 shows the configuration of a circuit section related to zero point error compensation in a differential pressure converter section in which the present invention can be implemented. FIG. 9 shows a circuit configuration of a portion related to span error compensation in a differential pressure converter portion in which the present invention can be implemented. FIG. 10 is a diagram showing the input/output characteristics of the differential pressure converter. In FIG. 4, PD...differential pressure converter, _PD1 ...bridge circuit part of the differential pressure converter, _PD2 ...operating current control circuit part of the differential pressure converter, TC...temperature error compensation circuit,
I/V...Current voltage conversion circuit, _S1 ...Summing point, _S2 ...Summing point, I _R ...Reference current, _RV1 ...First sliding line resistance, _d1
...First slider, RV ₂ ...Second sliding line resistance, d ₂ ...Second
Slider, R _Z ...first output resistance, R _S ...second output resistance.

Claims (1)

[Claims] 1. An output current corresponding to the difference in capacitance of a pair formed between a movable electrode that is displaced by differential pressure and a pair of fixed electrodes placed opposite to each other is output to a summing point. a differential pressure/current converter, a conversion circuit that inputs the signal output from this addition point, amplifies it, converts it into a predetermined current signal corresponding to the differential pressure, and outputs it; one end connected to a reference potential point. a first voltage source that is connected to the reference potential point at one end and generates a first output voltage proportional to the difference between the ambient temperature and a reference temperature at the other end; a second voltage source that generates an output voltage, the first voltage source and the second voltage source;
It has a sliding wire resistor connected between the other ends of each of the voltage sources, and an output resistor having one end connected to the slider of the sliding wire resistor and having a predetermined resistance value, and the other end of the output resistor. and a temperature error compensation circuit connected to the addition point as an output terminal, selectively setting a contact position on the sliding wire resistance of the slider to supply a zero point error compensation current to the addition point. A differential pressure transmitter comprising: compensating for a zero point error current generated in the differential pressure/current converter. 2. A differential pressure/current converter that outputs an output current corresponding to the difference in capacitance between a pair of fixed electrodes arranged opposite to a movable electrode that is displaced by differential pressure; A conversion circuit receives an output current, amplifies it, converts it to a predetermined current signal corresponding to the differential pressure, and outputs it, and a converter circuit that connects one end to a reference potential point and the other end to the difference between the ambient temperature and the reference temperature. a first voltage source that generates a proportional first output voltage; a second voltage source that has one end connected to the reference potential point and that generates an output voltage of opposite polarity to the first output voltage at the other end; voltage source and said second
It has a sliding wire resistor connected between the other ends of each of the voltage sources, and an output resistor having one end connected to the slider of the sliding wire resistor and having a predetermined resistance value, and the other end of the output resistor. a temperature error compensation circuit that outputs a span error compensation current from the differential pressure/current converter; and a temperature error compensation circuit that applies the span error compensation current to a summation point of a reference current and a direct current corresponding to the sum of the pair of capacitances in the differential pressure/current converter. and control means for amplifying the results of algebraic calculation of each current at the addition point to control the direct current to become the direct current corresponding to the reference current, The contact position is selectively set to determine the difference between the output current of the differential pressure/current converter for the differential pressure at a given ambient temperature and the output current of the differential pressure/current converter for the differential pressure at a reference temperature. A differential pressure transmitter characterized in that the pressure is reduced to substantially zero. 3 Differential pressure/current that outputs an output current corresponding to the difference in capacitance of a pair formed between a movable electrode that is displaced by differential pressure and a pair of fixed electrodes placed opposite to each other to the first summing point. a converter; a conversion circuit that inputs the signal output from the first summing point, amplifies it, converts it into a predetermined current signal corresponding to the differential pressure, and outputs the signal; one end of which is connected to a reference potential point; A first voltage source that generates a first output voltage proportional to the difference between the ambient temperature and a reference temperature at the other end, and an output voltage having one end connected to the reference potential point and having the opposite polarity to the first output voltage at the other end. a second voltage source that generates a
A first sliding line resistor is connected between the other ends of the voltage source, and one end is connected to the first slider of the first sliding line resistor, and the zero point error compensation current is applied from the other end to the first summing point. a first output resistor having a predetermined resistance value to be output to, a second slip line resistance connected in parallel with the first slip line resistance, and a second output resistance of the second slip line resistance.
a temperature error compensation circuit comprising a second output resistor having one end connected to the slider and having a predetermined resistance value that outputs a span error compensation current from the other end to a second addition point, the first slider; selectively setting a contact position on the sliding line resistance of the second slider to compensate for a zero point error current occurring in the differential pressure/current converter, and selecting a contact position of the second slider on the second sliding line resistance. setting the difference between the output current of the differential pressure/current converter for the differential pressure at a given ambient temperature and the output current of the differential pressure/current converter for the differential pressure at a reference temperature to be substantially zero. A differential pressure transmitter characterized in that the pressure is reduced to .