JPS6098328A - Temperature and static pressure compensating method of pressure and differential pressure transmitter - Google Patents

Abstract

PURPOSE:To compensate fluctuations of the zero point and the span with an inexpensive and simple constitution by compensating these fluctuations due to fluctuations of temperature and static pressure on a basis of the fluctuation of a dielectric constant of a sealed liquid. CONSTITUTION:Signals related to electrostatic capacities C1 and C2 obtained through an operating circuit 11 are led to a dielectric constant operating circuit 19, and a zero point fluctuation compensating signal proportional to the dielectric constant is calculated. This signal is added to the output signal of the operating circuit 11 at an addition point 20 to compensate the fluctuation of the zero point due to fluctuations of temperature and static pressure. The dielectric constant may be operated on a basis of the electrostatic capacity of a reference electrostatic capacity means 21 provided in a sealed liquid 5.

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 or pressure transmitters using pressure receiving elements such as bellows or diaphragms used in process control devices. The present invention relates to a method for compensating for zero point fluctuations or span fluctuations caused by.

<Prior Art> FIG. 1 is a block 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. 1 shows a cross section of the main body of a differential pressure transmitter with a one-chamber structure,
Diamond 72 receives pressure PH+PL to be measured on both end faces
The diaphragms 2 and 3 are arranged with their peripheral edges welded to the main body, and a through hole 4 formed in the main body and a hollow chamber surrounded by these diaphragms are filled with a sealing liquid 5 such as silicone oil. There is.

An enlarged electrode chamber is formed in the center of the hollow chamber, and in this electrode chamber, a movable electrode 7 supported on one side by an insulating material 6 fitted to the main body and an electrostatic capacitor icm, C2 are formed opposite to this. Further fixed electrodes 8 and 9 are arranged. 10 is a rod that connects the central parts of both diaphragms 2.3 through an enlarged hollow chamber, and the central part is connected to the movable electrode 7 in the electrode chamber.
The displacement of the diaphragm in response to the differential pressure is calculated on the moving electrode, and the DC output signal e. K-transformed. This signal e. is led to the output circuit 12 and connected to the series circuit of the load PL at the remote point and the power supply EB at a distance of 4 to 20 m.
A span output current. is converted to 13 is main body 1
14 is a pressure sensor that measures the pressure of the sealing liquid 5, that is, the static pressure P8, and the outputs of these sensors are led to compensation voltage generation circuits 15 and 16 for zero point compensation. The temperature signal GTH is converted into the static pressure signal ep for zero point compensation, and the output signal e of the arithmetic circuit 11 at the addition point i7.18. is added to or subtracted from to compensate for zero point fluctuations due to temperature fluctuations or static pressure fluctuations. If span fluctuations caused by changes in the spring constants of the diaphragms 2 and 3 due to temperature or static pressure fluctuations become a problem, the compensation voltage generation circuit 15.
1.6 The temperature signal and static pressure signal ° ゛ for compensating for span fluctuations shown by dotted lines in 1.6 are generated, and the voltage-current conversion gain of the output circuit 12 is changed to compensate for the fluctuation in span.

If such a compensation method is used, it is necessary to provide the temperature sensor 13 and the pressure sensor 14 inside the main body 1. Conventional technology that installed a temperature sensor inside the main body is, for example, Utility Model Application No. 55-1
3317, and a conventional technique in which a pressure sensor is provided inside the main body is disclosed in, for example, Japanese Patent Laid-Open No. 54-67480.

This configuration in which one temperature sensor and one pressure sensor are specially provided has the disadvantage that the structure of the transmitter is complicated and expensive, especially when the pressure sensor is provided inside the main body.

<Configuration of the present invention> The present invention overcomes the drawbacks of the prior art described above.

This method provides a static pressure compensation method, and its feature is that it uses the fact that the rate of contact of the sealing liquid changes with temperature or static pressure, detects changes in electrical resistance, and compensates for temperature fluctuations. There are two ways to compensate for both or one of static pressure fluctuations and static pressure fluctuations.

Prior to explaining the embodiments, the causes of zero point fluctuation and the principle of the compensation method of the present invention will be explained using FIGS. 2 to 4. FIG. 2 is a horizontal view of the one-chamber structure differential pressure transmitter shown in FIG.
It consists of J liquid 5 and rod 1o.

AH+AL is the effective area of diaphragms 2 and 3, FH2
FL indicates the force applied to the diaphragms 2 and 3 and the υ rod 10, ■ indicates the volume of the sealing liquid 5, p□ indicates the internal pressure of the sealing liquid 5, and pS indicates the static pressure. - Most of the causes of zero point fluctuation in a differential pressure transmitter with a chamber structure are the effective area AH of the diaphragms 2 and 3.
This is due to slight differences in L during the manufacturing process.

Now, when the effective area AH>AL, the volume V of the sealing liquid increases by ΔV due to the temperature change ΔT, and as a result, the internal pressure Pi of the sealing liquid increases by ΔV.
When P1□ rises (it becomes the state of P□>P), the changes in FF ΔF and ΔFL□ are respectively H' L H1 ΔFH1=AH×Δp□□ (occurs in the left direction) Δ・FL
□=AL×ΔP□□ (occurs in the right direction). Here, since AH>AL, ΔF>ΔF, and a leftward zero point eclipse HI Ll movement occurs.

Next, as the static pressure P increases by Δp, the volume S of the sealing liquid decreases by ΔV (even incompressible liquid actually has slight compressive properties), and the internal pressure decreases by Δpi2 (p <
p state) and changes in FH2FL ΔF, ΔFL
2 are respectively 2 ΔFH2=AH×ΔP (occurs to the right) 2 ΔFL2 ”” AL XΔPi2 (occurs to the left)
becomes. Here, since AH>AL, ΔF > ΔF
Then, a zero-pointing H2L2 movement to the right occurs.

When the relationship between the effective areas is reversed, that is, when AH<AL, the direction of variation due to temperature and static pressure variation is opposite to that described above.

In other words, the direction of zero point fluctuation due to the difference in effective area is
It can be seen that the increase in temperature and the increase in static pressure are in opposite directions. Figure 3 (4).

(B) illustrates these relationships; if the temperature error is in the negative direction as in (4), the static pressure error is in the positive direction as in (). In (4) and (B), the error shown by the dotted line is the error caused by the difference in effective area, and by compensating for this, the zero point fluctuation can be significantly reduced.

The above-mentioned zero point fluctuation occurs due to an internal pressure change based on a volume change (density change) of the sealing liquid. Here, looking at the relationship between the rate of change of the sealing liquid with respect to temperature changes and static pressure changes, and the rate of change of the sealing liquid's electrical resistance ε with respect to temperature changes and static pressure changes,
For example, with common silicone oil, 4EL--upper,! L8 (2) It is expressed as V Δp ′ ε ΔP 1.30, and it can be seen that the rate of change in dielectric constant ε due to temperature or static pressure is almost equal to the rate of change in volume due to temperature or static pressure of the sealing liquid. This indicates that it is possible to detect changes in volume due to temperature or static pressure of the sealing liquid by detecting changes in dielectric constant 6.

Since zero fluctuation occurs when there is a difference in effective area due to a change in the volume of the sealing liquid, if a change in the composition of the n liquid can be detected by a change in the U electric rate, the zero point fluctuation can be calculated by using this detection signal. compensation is possible.

Here, the dielectric constant ε is generally in the reference state CT=2oC.

T, Po for the dielectric constant 6 of p = Okg/am
For p conversion ΔT and ΔP, a=js(
1 - αΔT + βΔP ) (3) It has a characteristic that the direction of change with respect to temperature and the direction of change with respect to static pressure are opposite.

On the other hand, as shown in Figure 3, the zero point error occurs with opposite polarity at temperature and static pressure, so an appropriate coefficient (
(including polarity) to produce the output signal e in FIG. By adding K to If the cause of this is due to a difference in the effective area of the diamond 72, the zero point fluctuation can be effectively compensated for without providing a W liquid sensor or a pressure sensor in the main body as in the prior art.

A specific means for detecting the dielectric constant ε may be a method of providing a reference capacitance in the sealing liquid 5 and detecting a change in this capacitance, but the capacitance 1kc changes in response to the differential pressure.

It can be easily calculated based on C2, and a configuration that does not require a special sensor is possible.

Figure 4 is a diagram showing the principle of a differential pressure/pressure transmitter to which the compensation method of the present invention is applied, and elements 1 corresponding to Figure 1.
This is indicated by the same signal. C□ obtained through the arithmetic circuit 11,
The signal related to C2 is guided to the dielectric constant calculation circuit 19, where a zero point fluctuation compensation signal eTP proportional to the U electric constant 6 is calculated, and the output signal e of the calculation circuit 11 is obtained at the addition point 2o. is added to compensate for zero point fluctuations due to temperature and static pressure fluctuations. It is a reference capacitance means provided in the sealing liquid 5 of 21 companies, and K is used to calculate the dielectric constant 5 based on this capacitance CsK.
You may. eTpI is a span variation compensation signal calculated by the dielectric constant calculation circuit 19; Compensation can be done by adding to the amount. Compensation for span fluctuations will be specifically explained in later embodiments.

FIG. 5 shows a specific example of a circuit of a differential pressure transmitter in which zero point fluctuation is compensated by the method of the present invention. The duty is converted into a pulse signal related to C□, C2, and then smoothed to form a DC output signal e. K-transformed.

11 includes an amplifier G and G2 forming a comparator, gates 03 to G5 forming a changeover switch, a counter CT1, an inverter G6 . This is a self-oscillation circuit that combines C7 and bidirectional constant current circuit cc1. When n oscillation pulses with a period related to C are counted by counter CT□, the oscillation is switched to related to C2, and this pulse In the same way, n pieces are counted to obtain a duty cycle signal whose interval is related to 02 (or vice versa) and whose amplitude is the reference voltage V. ).

Figure 6 ■ is the output waveform of the counter CT□, and the on time T□
is associated with 01, and off time T2 is associated with 02. 2 is the output of the inverter G7, and is a signal with the opposite phase to 2.

This signal is smoothed by a filter consisting of a resistor R1 and a capacitor c3 and converted into a DC output (fi No. e.
The signal is supplied to the non-inverting input terminal Y of the amplifier A□ via the adding resistor R2. VRl is Zee AI! The output is connected to the Y point via the summing resistor It3. V
H2 is a span adjustment means provided in the feedback circuit of the amplifier A□, and is biased by 1/2 K of the reference voltage V by the amplifier A2. Therefore, the input e of amplifier A□. is width gauge A3
Guided by the output current engineer. is compared and amplified with the feedback voltage eF generated in the feedback resistor RL given to it, and drives the output transistor TR to generate an output current. control.

Next, a dielectric constant calculation circuit 19 to which the method of the present invention is applied, which is added to such a configuration, will be explained.

M□ is a monostable circuit which receives the data cycle signal shown in Fig. 6 (■) and is triggered at the rising edge of the signal, and the circuit operates for a certain period of time T (2), which is shorter than T□+T2. generates an output pulse of RC is a time constant circuit that determines T and has 4p 4 0. SW□ is a switch that opens and closes the signal with the signal ■, R
5 and C5 are filters that smooth the output signal of the switch SW□. Therefore, the voltage of the output signal O of this filter is 1-KJp (K' constant 7P, differential pressure C: constant). This voltage is further increased by the switch S, which opens and closes with the signal ■.
Filters R6 and C6 are charged via W2.

SW3 is a switch that similarly opens and closes in the opposite phase to 8w2 with the signal ■, and discharges the charge of the cono filter. With this configuration, the voltage Vθ of the output (■) of the filters R6, C6 becomes °vz = c - ■, ε"A, ε (A: constant) (5) Toz, and vθ is proportional to the dielectric constant ε. The voltage is converted into positive and negative voltages v0+ and 'Vθ- by an inverting amplifier A54' which receives the output of the buffer amplifier A, and a zero point fluctuation compensation signal with an appropriate coefficient and polarity is generated by a potentiometer vR3 for polarity selection and coefficient setting. It is converted to TP, guided to point Y via the addition resistor R6, and added to the output signal eo to compensate for zero point fluctuations due to temperature and static pressure fluctuations.vR4 supplies an appropriate bias to the signal eTp. It is a means of adjustment.

FIG. 7 is an example of an arithmetic circuit for obtaining a zero point fluctuation compensation signal eTp proportional to -M2. II (3 is a monostable circuit, the time constant is T□, constant time T is shorter than the shortest period of T2. They are the same, M2 is the signal ■, and M3 is the signal ■
The output is opened and closed by switches sw, sw5 driven by signals ① and ②, and then smoothed by a filter. The voltage of output No. 8 [F] and ◎ of each filter is the signal ■
The voltage ■θ is VB = (p) 10 = (1+, JP +', Kal
P)'ro-v72vz, ICTo ε -B・-(B: constant) (6), which is proportional to the soil. The subsequent circuit configuration is the same as that shown in FIG. The configuration for calculating 7 is ε
This has the advantage of being simpler with fewer switches than a configuration that calculates . Since the variation range of 6 is small, - also varies linearly with respect to the temperature and static pressure, and even if this signal is used as a compensation signal, the compensation effect is the same as in the case of FIG. 5.

Although the embodiments shown in FIGS. 5 and 7 are applied only to zero point variation compensation, it is also possible to easily obtain a span variation compensation signal by superimposing it on the zero point variation compensation signal. Figure 8 is an example in which a signal proportional to 6 is obtained, a zero point fluctuation compensation signal and a span fluctuation compensation signal at the same time, and the configuration is until the output of the filter R6''6 obtains V in equation (5) above. is the same as in Fig. 5.This signal v is further connected to a switch sw6.s
The voltage ■θ' obtained by multiplying w7 by the signal ■ and smoothing it with the filter of R8''7 becomes the voltage ■θ', where the value is a constant.This voltage is applied to the non-inverting input terminal of 〇 to the operational amplifier, and the output voltage of A6 is When V. and Vθ are distributed to the ratio of (1-α):α by potentiometer vR5 and applied to the inverting input terminal of 80, the output voltage ■ is ΔP2α.
-1(8) = εA' 2. + a A (2a). (8
) The first term in the equation represents the span fluctuation compensation signal component, and the second term represents the zero point fluctuation compensation signal component, and the ratio of both components can be changed by adjusting α. The configuration after Vo summer is the same as that shown in FIG.

Furthermore, it is possible to simultaneously obtain a zero point fluctuation compensation signal and a span fluctuation compensation signal using the same calculation for the output (2) in FIG. 8, which obtains a voltage proportional to -.

This method of span variation compensation is different from the method of changing the voltage-to-current conversion gain in the output circuit 12 explained in FIG. The configuration can be basically the same as the point fluctuation compensation method, and as shown in FIG. 9, both compensation signals can be handled in a superimposed manner using one common circuit means, simplifying the configuration of the compensation circuit. Is possible.

FIG. 9 shows the reference capacitance means 2 explained in FIG.
An example of the configuration of a dielectric constant calculation circuit in the case where a uL ratio of 6 is provided and a uL ratio of 6 is separately detected is shown. C8 is a comparator amplifier that receives one end of 21 at its input, and a bidirectional constant current circuit CC2 is connected between the input and output, and its output is connected to the other end of 21 via inverter G9, and the output is proportional to the reference capacitance Cs. A self-oscillation circuit is formed that generates a pulse signal with a period Ts□.

The period T8□ of this pulse is given by Ta2'']C8, where the constant current of CC2 is 11 and the power supply voltage is V.The period T of the pulse obtained by counting n pulses with the counter CT2 is: z T2mon T1 child n + TC8 = AC8 (A: constant) (9)
becomes. The voltage Vθ obtained by smoothing the signal with a constant pulse width To by triggering the monostable circuit M with this signal by the switch SW6 filter is, assuming C8-6C, =B・ε (B: constant) αQ and However, it is proportional to the dielectric constant. This signal is supplied to the addition point 2o via an appropriate coefficient setting means 22 to compensate for zero point fluctuation.

In this way, the present invention can also be realized by providing a separate means for detecting the dielectric constant ε, and it is possible to realize the present invention by separately providing a means for detecting the dielectric constant ε. The present invention can also be applied to a transmitter that does not use capacitance change, such as a transmitter.

FIG. 10 shows capacitance c1 for differential pressure detection. The amplitude of the present invention is applied to a transmitter that excites c2 with an AC oscillator and detects the differential pressure or pressure from the difference in current flowing through each capacitor.The amplitude is controlled by the amplifier A7. cp is a reference capacitance installed at the same -m border as C□ and C2, excited by e, the current flowing through it is rectified by diode D□, and is filtered by a filter circuit of resistor R9 and capacitance C8. This voltage is converted into a DC voltage, and this voltage and a constant voltage V8 are compared and amplified by an amplifier A7, and the output e of O20 is controlled. Therefore, ■S'''ejm R9CP ('1).The capacitances i C, and C2 are also excited by ej, and the currents flowing through them are rectified by diodes D2 and D3, respectively, and the resistor R and capacitance Cc: ,,Resistance R work 0.Capacitance C work.

The filter circuits each have a voltage V1. Converted to V2.

With this configuration, Vl"CI RIO'ejO'I V2':C2R11(11ej).Here, if +19"'Rlo-R□□, then 01~(b) formula and C1=" 1- to Δp・C2=“
``1+'' and ``1+'' are obtained from the door-.These voltages are subjected to the calculation of ``2'' by the arithmetic circuit 23, and an output signal e is generated.

Means for calculating the dielectric constant 6 are implemented in blocks 24-26. 24 compares the voltage V□ associated with C□. As a result, the data cycle D1 of the output of A8
, I say. This data signal further opens and closes the switch SW8, and the constant voltage V is sampled and smoothed by a filter, resulting in the output v3.

The block 25 has the same configuration as the block 24, receives the voltage v2 related to C2, and generates the voltage V expressed as α force -1·-vF 2v2 . These voltages V3. v4 is added by the force U calculation circuit 26, and its output Vθ is ve = V3
+V4-2, V3HVF (v〒-+v, ).

Substituting the α◆ and θQ equations into this, we get, which is proportional to -.

Therefore, by converting this signal into a zero point fluctuation compensation signal GPT via the coefficient setting means 22 and applying it to the addition point 20, zero point fluctuation compensation due to temperature and static pressure can be performed simultaneously. Similarly, compensation for span temperature and static pressure fluctuations can be easily realized by means of the Caloric circuit shown in FIG.

Capacitance c1. One of C2 has a fixed capacity, e.g. C2
=co (fixed), 64. O') v□, v2 of formula
can be determined by calculating ΔP, and the dielectric constant ε can be easily calculated by using the signal of v2.

In all of the above embodiments, the basic structure of the main body for measuring differential pressure or pressure is a so-called -chamber structure in which two measurement diaphragms are connected by a rod.The present invention is used to compensate for zero point fluctuations and static pressure fluctuations. This is an example of application: - A so-called two-chamber structure transmission in which the pressure to be measured is received on both sides of a single measuring diaphragm via a pressure receiving diaphragm, and the differential pressure or pressure is measured depending on the displacement of this measuring diaphragm. Also in the case of vessels,
By calculating the dielectric constant change of the sealing liquid or using a device for detecting the dielectric constant change, it is possible to compensate for zero point deviation and span fluctuation based on temperature and static pressure fluctuations. In the case of a two-chamber structure,
Due to its structural characteristics, the spring constant of the measuring diaphragm changes due to static pressure fluctuations, which tends to cause span fluctuations. Therefore, if only static pressure fluctuations are to be compensated precisely, a signal obtained by subtracting a component related to temperature fluctuations from a signal related to dielectric constant may be used as the compensation signal. In order to subtract the temperature component, it is necessary to separately install a temperature sensor, but compared to a pressure sensor that measures static pressure fluctuations, the temperature sensor can be installed inside the main body more easily, so the overall structure is simpler. Even in the case of a configuration like this, which does not become complicated, the effect of the present invention is sufficiently maintained in that a pressure sensor is not required.

Effects> As explained above, the present invention can be applied to any type of differential pressure/pressure transmitter that has a sealing liquid, by detecting changes in the dielectric constant of the sealing liquid, and thereby suppressing temperature fluctuations or static electricity. Zero point fluctuations or span fluctuations caused by one or both of pressure fluctuations can be easily realized without incorporating a complicated structure such as a pressure sensor into the main body.

- In the case of a differential pressure/pressure transmitter with a chamber structure, a large proportion of fluctuations are caused by the effective area difference between the two measurement diaphragms based on changes in the volume (density change) of the sealing liquid. According to the compensation method of the invention, zero point deviation and span fluctuation due to temperature fluctuations and static pressure fluctuations can be realized with an extremely simple circuit configuration in which compensation signals are supplied to - number of addition points.

~Moreover, if the displacement is measured using capacitance changes, compensation can be performed without using any special permittivity detection sensor, making it possible to produce high-precision, highly reliable products at low cost. This can be achieved with

[Brief explanation of drawings]

Figure 1 shows zero point fluctuation compensation for a conventional differential pressure transmitter. 2 and 3 are wedge diagrams and characteristic diagrams illustrating the mechanism of error generation due to fluctuations in temperature and static pressure. FIG. FIG. 5 is a block diagram showing the basic configuration when applied, FIG. 5 is a circuit configuration diagram embodying FIG. 4, FIG. 6 is an explanatory diagram of its operation, and FIG. 8 is a circuit diagram showing an embodiment in which span fluctuation compensation is also performed at the same time, and FIG. 9 is a circuit diagram showing a case where the present invention is applied to a configuration in which a sensor for detecting changes in dielectric constant is separately provided. FIG. 10 shows a circuit diagram in which the present invention is applied to a differential pressure/pressure transmitter that utilizes alternating current excited capacitance changes. C□, C2...Capacitance, 1...Body, 2,3...
・Diaphragm, 5...Sealing liquid, 11...Arithmetic circuit,
12... Output circuit, 19... Dielectric constant calculation circuit, 21
...Reference capacitance means, 6...Dielectric constant, eTp...
- Zero point fluctuation compensation signal, eTp'...Span fluctuation compensation signal.

Claims (1)

[Claims] In a pressure/differential pressure transmitter that has a pressure receiving element and a sealing liquid that are displaced in response to the pressure or differential pressure to be measured, the dielectric constant of the sealing liquid is detected, and the temperature and/or static pressure is determined based on the change in the dielectric constant. A temperature/static pressure compensation method for a pressure/differential pressure transmitter, characterized by compensating for zero point fluctuations and/or span fluctuations caused by fluctuations.