JPH0377938B2 - - Google Patents

Description

Detailed Description of the Invention <Industrial Application Field> The present invention addresses temperature or static pressure fluctuations that are a problem in differential pressure/pressure transmitters using pressure receiving elements such as bellows or diaphragms used in process control devices. This invention relates to a circuit that compensates for zero point fluctuations caused by.

<Prior Art> FIG. 1 is a configuration diagram for explaining the concept of compensating for zero point fluctuations and span fluctuations due to temperature and static pressure fluctuations in a conventional differential pressure transmitter. Here, static pressure fluctuation refers to a phenomenon in which the output changes when the same pressure is applied to both the high pressure P _H side and the low pressure P _L side, although the prepressure ΔP is zero. 1 shows the cross section of the main body of a differential pressure transmitter with a one-chamber structure, and the pressure to be measured is shown on both end faces.
Diaphragms 2 and 3 that receive P _H and P _L are arranged with their peripheral edges welded to this main body, and the through hole 4 formed in the main body and the hollow chamber surrounded by these diaphragms are sealed with silicone oil or the like. It is filled with liquid 5. An electrode chamber is formed in the center of the hollow chamber,
In this electrode chamber, a movable electrode 7 whose one side is supported by an insulating material 6 fitted to the main body, and fixed electrodes 8 and 9 opposite to this for forming electrostatic capacitances C ₁ and C ₂ are arranged. There is. Reference numeral 10 denotes a rod that connects the central parts of both diaphragms 2 and 3 via a hollow chamber, and the central part is fixed to the movable electrode 7 in the electrode chamber, and transmits the displacement of the diaphragm in response to the differential pressure to the movable electrode. , the capacitances C ₁ and C ₂ are changed differentially. The capacitances C ₁ and C ₂ are guided to the calculation circuit 11 and subjected to the calculation of (C ₁ −C ₁ )/(C ₁ +C ₂ ),
It is converted into a DC output signal _eO . This signal _eO is led to the output circuit 12, where it connects the load RL at the remote point and the power supply EB.
Output current of 4~20mA span for series circuit of
Converted to _IO . 13 is a temperature sensor that measures the temperature T of the main body 1, and 14 is a pressure sensor that measures the pressure of the sealing liquid 5, that is, P. The output of these sensors is
The signals are guided to compensation signal generation circuits 15 and 16, converted into a temperature signal e _T for zero point compensation and a static pressure signal e _P for zero point compensation, and added to the output signal e _O of the arithmetic circuit 11 at addition points 17 and 18. Subtracted to compensate for zero point variations due to temperature or static pressure variations. If span fluctuations caused by changes in the spring constants of diaphragms 2 and 3 due to temperature or static pressure fluctuations become a problem,
Compensation voltage generation circuits 15 and 16 generate span fluctuation compensation temperature signals and static pressure signals e _T ′ and e _P ′ shown by dotted lines, and change the voltage-current conversion gain of the output circuit 12 to compensate for span fluctuations. do.

When such compensation means are used, it is necessary to provide the temperature sensor 13 and the pressure sensor 14 inside the main body.

In order to compensate for both temperature and static pressure, it is necessary to provide two sensors within the main body. Particularly, the configuration in which the pressure sensor is provided within the main body has the drawback that the structure of the transmitter is complicated and expensive.

<Structure of the Invention> The present invention has been made in view of the above-mentioned problems of the prior art. A calculation means for outputting a pulse signal whose duty ratio changes in response to displacement, which relates to a pressure/differential pressure transmitter having a sealing liquid sealed therein;
A constant pulse signal generating means for generating a constant pulse signal of a constant time width in synchronization with the pulse signal, and a signal related to a change in the dielectric constant of the sealing liquid is calculated using the pulse signal and the constant pulse signal. a compensation signal generation circuit that outputs a zero point compensation signal; a temperature compensation signal generation means that receives a temperature signal corresponding to the temperature of the sealing liquid and outputs it as a temperature compensation signal;
The device is equipped with an output means for compensating for a change in the zero point using a zero point compensation signal and a temperature compensation signal for a smoothed signal obtained by smoothing a pulse signal, and outputting a signal corresponding to pressure or differential pressure. It is something.

In general, the dielectric constant ε of silicone oil is the dielectric _constant ε in the standard state (temperature T = 20℃, static pressure P = 0Kg/cm ² )
α, β for T, P change ΔT, ΔP
It is expressed as ε=ε _S (1−αΔT+βΔP) (1) where is a constant. Therefore, if the temperature T of the sealing liquid is detected separately and the change in ε due to the temperature change ΔT is compensated for, (1)
Since the equation is a function only of the static pressure change ΔP, the pressure sensor can be eliminated by using a signal proportional to ε as the static pressure compensation signal e _P and a signal proportional to temperature as the temperature compensation signal e _T. Temperature and static pressure compensation can be realized.

FIG. 2 is a block diagram showing the basic configuration of the present invention based on such a principle. The difference from FIG. 1 is that the input of the compensation signal generation circuit 16 that generates the static pressure compensation signal _eP is It is not based on a pressure sensor that measures static pressure, but on the output of a calculation circuit 11 of capacitances C ₁ and C ₂ , thereby calculating a static pressure compensation signal e _P that is proportional or inversely proportional to dielectric constant ε. On the other hand, the temperature _T of the sealing liquid measured by the temperature sensor 13 is the same as that shown in _FIG . It simultaneously compensates for the temperature characteristics of _P and the temperature fluctuations at the zero point of the converter itself. The same is true for span fluctuations, e _P ′,
This can be realized by changing the voltage-current conversion gain of the output circuit 12 according to e _T '.

Fig. 3 is an example of a circuit configuration when Fig. 2 is realized, and the capacitance C ₁ , which changes in relation to the differential pressure,
C ₂ is led to the arithmetic circuit 11, and the duty is C ₁ ,
After being converted into a pulse signal related to _C2 , it is smoothed and converted into a DC output signal _eO . The components within 11 include an amplifier G ₁ , which forms a comparator;
G ₂ , gates G ₃ to _{G 5} forming the changeover switch, counter CT, inverters G ₆ , G ₇ and _{bidirectional} constant current circuit CC are combined. When the counter CT counts n times, it switches to the oscillation related to _C2 ,
When n pulses are counted in the same way, the operation to return to the original state is repeated, and the on time becomes _C1 and the off time becomes the output of counter CT or the output of inverter _G7 .
A duty cycle signal is obtained which is related to C ₂ (or vice versa) and whose amplitude is the reference voltage V _Z (details of this arithmetic circuit are explained in Japanese Patent Laid-Open No. 14714/1983).

FIG. 4 shows the output waveform of the counter CT, in which the on-time T ₁ is related to C ₁ and the off-time T ₂ is related to C ₂ . is the output of inverter _G7 , and is a signal with the opposite phase. This signal is smoothed by a filter with a resistor R ₁ and a capacitor C ₃ and converted into a DC output signal e _O , which is supplied to the non-inverting input terminal Y point of the amplifier A ₁ of the output circuit 12 via the summing resistor R ₂ . be done. VR ₁ is a zero point adjustment means, the output of which is connected to the Y point via an addition resistor R ₃ . VR ₂ is a span adjustment means provided in the feedback circuit of the amplifier A ₁ , and is biased to 1/2 of the reference voltage V _Z by the amplifier A ₂ . Therefore, the input e _O of amplifier A ₁ is V _Z /2 as shown in FIG.
It is a smooth signal based on (C ₁ − C ₂ )/(C ₁
+C ₂ ). The output of amplifier A ₁ is the output of amplifier A ₃
The feedback resistor R _F is guided by the output current I _O
It is compared and amplified with the feedback voltage e _F generated in the output transistor TR, and controls the output current I _O by driving the output transistor TR.

Next, the compensation signal generation circuit 16 will be explained.
M ₁ is a monostable circuit that receives the duty cycle signal shown in Figure 4 and is triggered at the rising edge of the signal to output T ₁
Generates an output pulse of T _O for a certain period of time, such as shorter than +T ₂ . R ₄ and C ₄ are time constant circuits that determine T _{O. SW1} is a switch that opens and closes a signal, and _R5 and _C5 are filters that smooth the output signal of switch _SW1 .

As mentioned above, the output of the counter CT generates a voltage T1=C1K1 (however, _K1 is a constant) whose on time is proportional to the capacitance C1, but this capacitance _C1
is the capacitance formed between the moving electrode 7 and the fixed electrode 8, and it changes depending on the differential pressure ΔP between the pressures P _H and P _L. Similarly, T ₂ becomes T ₂ =C ₂ ·K ₁ .

Generally, the capacitance is calculated by dividing the value (Sε) obtained by multiplying the opposing area S between the moving electrode 7 and the fixed electrode 8 by the dielectric constant ε by the distance d between the moving electrode 7 and the fixed electrode 8 (Sε)/d. is proportional to.

Now, assuming that the movable electrode 7 is placed at the center of the fixed electrodes 8 and 9, the distance between each fixed electrode 8 and 9 with respect to the movable electrode 7 is d ₀ , and the rate at which the movable electrode 7 is displaced due to the differential pressure ΔP is If K ₂ is assumed to be constant, the distance d becomes d=d ₀ −K ₂ ·ΔP. Therefore, C ₁ becomes C ₁ =K ₃ ·εS/d =K ₃ ·εS/(d ₀ −K ₂ ·ΔP) =εC/(1−K·ΔP) with K ₃ as a proportionality constant. However, C and K are constants, and C=K3S/
d ₀ , K=K ₂ /d ₀ . If this is calculated similarly for C ₂ , it becomes C ₂ =εC/(1+K·ΔP). The reason why the sign is + is that the gap between C ₁ and C ₂ changes in the opposite direction with respect to the differential pressure ΔP of the moving electrode.

Here, the counter CT is supplied with the reference voltage V _Z as a power supply, and as shown in Figure 4, a pulse voltage that repeats at a period of (T ₁ + T ₂ ) is output at the output terminal, making it a monostable. Circuit _M1 is counter CT
A predetermined time width T ₀ (fourth
Since the switch SW ₁ is opened and closed by the pulse output from the filter (see figure), the voltage V _C generated at the output terminal of the filter consisting of C ₅ and R ₅ is: V _C = V _Z・T ₁ /T ₀ ＝V _Z・C ₁・K ₁ ／T ₀ ＝V _Z・K ₁ εC／T ₀ (1-K・ΔP) …(2) A voltage is generated.

On the other hand, switch SW2 opens and closes voltage V _C using a pulse voltage obtained by inverting the output of counter CT with inverter G ₇ , so the output terminal of the filter composed of C ₆ and R ₆ has a voltage V 〓 expressed by the following equation. Occur.

V = V _C・T ₂ / (T ₁ + T ₂ ) = V _C・C ₂ / (C ₁ + C ₂ ) Here, C ₂ / (C ₁ + C ₂ ) = [εC/(1+K・ΔP)] /[εC/2] = (1-K・ΔP)/2, so V〓=[V _Z・K ₁ εC/T ₀ (1-K・ΔP)] ・[(1-K・ΔP) /2] =V _Z・K ₁ εC/2T ₀ =A・ε (3), and V〓 is proportional to the dielectric constant ε. However, A is a constant.

From equations (1) and (3), V〓 becomes V〓=Aε _S (1−αΔT+βΔP) =Aε _S −Aε _S αΔT+Aε _S βΔP (4).

This voltage is converted into positive and negative voltages V ⁺ 〓, V ^- 〓 by an inverting amplifier A ₅ which receives the output of the buffer amplifier A ₄ , and is set to an appropriate coefficient and polarity zero by a potentiometer VR ₃ for polarity selection and coefficient setting. Point fluctuation compensation signal
It is converted into _eP , guided to point Y via the addition resistor _R7 , and added to the output signal _eO , thereby compensating for zero point fluctuations due to temperature and static pressure fluctuations. VR ₄ is an adjustment means for supplying a suitable bias to the signal e _P.

Next, the temperature compensation signal generation circuit 15 will be explained.

A current is applied from a constant voltage V _Z to a series circuit of diodes D ₁ and D ₂ and a resistor R ₈ forming the temperature sensor 13, and the voltage drop across R ₈ is divided appropriately using a potentiometer VR ₅ to obtain a point. The resulting voltage V _T is V _T =(V _Z −V _dp )/B+C _cd ·Δt/B (5). Here, B is a constant, V _dp is the sum of forward voltages of the diodes D ₁ and D ₂ in the reference state, and C is the temperature sensitivity coefficient of the diode expressed as about -2.3 mV/°C. This voltage V _T is tapped off via a buffer amplifier A ₆ and multiplied by a suitable coefficient in a potentiometer VR ₆ to obtain a temperature compensated signal e _T .
_A7 is the output circuit 12 for this temperature compensation signal _eT.
This is a means for biasing the operating reference potential V _Z /2 of. After setting the output of equation (5) in the reference state to the reference potential V _Z /2 by adjusting potentiometer VR ₅ , set the appropriate coefficient to the second term related to temperature by adjusting VR ₆ . It is multiplied by k and led to point Y via the summing resistor _R9 .

Figure 5 shows an arithmetic circuit for obtaining a compensation signal e _P proportional to 1/ε, M ₂ and M ₃ are monostable circuits, and their time constants are constant and shorter than the shortest period of T ₁ and T ₂ . They are assumed to be the same at period T ₀ .

The monostable circuit M ₂ is the output pulse of the counter CT,
The monostable circuit _M3 is triggered by an inverted pulse that is the inversion of the output pulse of the counter CT.These outputs are filtered via switches _SW4 and _SW5 , which are opened and closed by the output pulse of the counter CT and its inverted pulse. is output to.

The output voltages V _H and V _I of these filters are T ₁ >
Considering T ₀ , T ₂ > T ₀ and in the same way as when formula (3) was derived, V _H =T ₀・V _Z /T ₁ =T ₀・V _Z /C ₁ K ₁ =T ₀・V _Z (1-K・ΔP)/K ₁ εC V _I =T ₀・V _Z ／T ₂ =T ₀・V _Z ／C ₂ K ₁ =T ₀・V _Z (1+K・ΔP)/ K ₁ εC.

These voltages are added via resistors R ₁₀ and R ₁₁ , and the added voltage V〓 becomes V〓=2T ₀・V _Z /εK ₁ C =B/ε...(6), and 1/ The voltage is proportional to ε. However, B
is a constant.

The configuration that calculates 1/ε has the advantage of being simpler and has fewer switches than the configuration that calculates ε. Since the range of change in ε is small, 1/ε is also a function of temperature,
It changes linearly with static pressure, and even if this signal is used as a compensation signal, the effect remains the same.

Next, the adjustment means will be explained.

(1) Compensation for static pressure fluctuations: Adjust VR ₄ and VR ₅ so that the voltage between the terminals of potentiometers VR ₃ and VR ₆ becomes zero at reference temperature and static pressure = 0. By this adjustment, the output I _O in the reference state becomes unaffected by the adjustment of VR ₃ and VR ₆ . Static pressure is applied here, and the variation in output current I _O that occurs is adjusted and compensated for by potentiometer VR ₃ .

(2) Compensation for temperature fluctuations... Next, change the temperature from the reference state (static pressure is arbitrary) and compensate for the fluctuations in the output current that occur by adjusting the potentiometer VR ₆ . Output fluctuations due to temperature changes are the sum of output fluctuations due to dielectric constant fluctuations, that is, fluctuations in the static pressure compensation signal _eP , and temperature fluctuations of the converter itself, but both of these fluctuations are compensated for together by this adjustment. Ru.

In the present invention, the circuit 16 that calculates the static pressure fluctuation compensation signal e _P is similar to the above embodiment that calculates the dielectric constant of the sealing liquid based on the capacitances C ₁ and C ₂ that are the own sensors of the converter. In some cases, there is no need for a special dielectric constant sensor, which has the advantage of simplifying the overall configuration. However, the present invention can be carried out by providing a pair of fixed electrodes arranged opposite each other in the sealing liquid and calculating the dielectric constant based on the change in capacitance of the electrodes. Even in that case, the structure of the converter is simpler than the conventional configuration in which a special pressure sensor is provided, and the effects of the present invention can be fully exhibited.

Further, the present invention can be applied not only to a converter having a one-chamber structure as in the embodiment, but also to a converter having a two-chamber structure by a range spring provided in the center of the electrode chamber.

<Effects> As specifically explained above along with the examples,
According to the present invention, when a pulse signal whose duty ratio changes in accordance with displacement is calculated by the calculation means, and this pulse signal is smoothed and output as a smoothed signal corresponding to the displacement, synchronization is made with this pulse signal. Calculate the constant pulse signal that has been calculated, use this constant pulse signal and the pulse signal to calculate the zero point compensation signal caused by static pressure, etc., and use this to calculate the zero point compensation signal caused by static pressure etc. for the smooth signal. Since the zero point error that occurs is compensated for, it can be compensated by calculation without the need to separately prepare a pressure sensor inside the main body for static pressure correction, resulting in a simple structure and low cost. Moreover, high precision pressure
A differential pressure transmitter can be realized.

[Brief explanation of drawings]

Fig. 1 is a block diagram illustrating zero point fluctuation compensation and span variation compensation of a conventional pressure/differential pressure transmitter, Fig. 2 is a basic block diagram showing an embodiment of the present invention, and Fig. 3 is a detailed diagram of the same. FIG. 4 is an explanatory diagram of its operation, and FIG. 5 is a circuit diagram showing another embodiment of the main part of the present invention. C ₁ , C ₂ ... Capacitance, 1 ... Main body, 2, 3 ...
...Diaphragm, 5...Sealing liquid, 11...Arithmetic circuit, 12...Output circuit, 15...Temperature compensation signal generation circuit, 16...Static pressure compensation signal generation circuit, _eP ...
Static pressure compensation signal, e _T ...Temperature compensation signal.

Claims (1)

[Claims] 1. A pressure/differential pressure transmitter having a pressure receiving element fixed to a main body and displacing in response to the pressure or differential pressure to be measured, and a sealing liquid surrounded by the pressure receiving element and sealed in the main body, which corresponds to the displacement. a calculation means for outputting a pulse signal whose duty ratio changes, a constant pulse signal generation means for generating a constant pulse signal of a fixed time width in synchronization with the pulse signal output from the calculation means, and an output from the calculation means. generating a compensation signal for calculating a signal related to the change in dielectric constant of the sealing liquid using the pulse signal generated by the pulse signal and the constant pulse signal output from the constant pulse signal generating means, and outputting the signal as a zero point compensation signal; a circuit, a temperature compensation signal generating means for receiving a temperature signal corresponding to the temperature of the sealing liquid and outputting it as a temperature compensation signal, and compensating the smoothed signal obtained by smoothing the pulse signal output from the calculation means. Compensating for the change in the zero point using a zero point compensation signal output from the signal generation circuit and a temperature compensation signal output from the temperature compensation signal generation means, and outputting a signal corresponding to the pressure or differential pressure. A pressure/differential pressure transmitter comprising: an output means.