JPH0439894B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a differential pressure conversion device that converts differential pressure into an electrical signal by capacitance, and particularly relates to a differential pressure converter that converts differential pressure into an electrical signal by capacitance, and particularly relates to a differential pressure converter that converts differential pressure into an electric signal by capacitance, and in particular, converts differential pressure by capacitance into an electrical signal. This invention relates to a conversion device.

<Prior Art> FIG. 4 is a configuration diagram for explaining the concept of compensating for zero point fluctuations and span fluctuations due to fluctuations in temperature and static pressure in a conventional differential pressure converter. 1 shows a cross section of the main body of a differential pressure converter having a one-chamber structure, and diaphragms 2 and 3 that receive the pressures P _H and P _L to be measured are placed on both cross sections, with their peripheral edges welded to this main body. A sealing liquid 5 such as silicone oil is filled in a hollow chamber surrounded by the through hole 4 formed in the diaphragm and the diaphragm. An expanded electrode chamber is formed in the center of the hollow chamber, and within this electrode chamber there is a movable electrode 7 supported on one side by an insulator 6 fitted to the main body, and a capacitance C ₁ , C opposite to the movable electrode 7 . Fixed electrodes 8 and 9 for forming ₂ are arranged. Reference numeral 10 denotes a rod that connects the hollow parts of both diaphragms 2 and 3 via a hollow chamber, and its 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. , capacitance C ₁ , C ₂
Differentially change. The capacitances C ₁ and C ₂ are led to the arithmetic circuit 11, where they are subjected to the calculation (C ₂ −C ₁ )/(C ₂ +C ₁ ) and converted into a DC output signal e _O. This signal e _O is led to the output circuit 12 and converted into an output current I _O with a span of 4 to 20 mA for the series circuit of the remote load R _L and the power supply E _B. 13 is a temperature sensor for measuring the temperature T of the main body 1 or the sealing liquid 5; 14;
are pressure sensors that measure the pressure of the sealing liquid 5, that is, the static pressure P _S , and the outputs of these sensors are led to compensation voltage generation circuits 15 and 16 to generate a temperature signal for zero point compensation.
e _T is converted into a static pressure signal e _P for zero point compensation, and added to or subtracted from the output signal e _O of the arithmetic circuit 11 at summing points 17 and 18 to compensate for zero point fluctuations due to temperature fluctuations or static pressure fluctuations. Ru. 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 circuits 15 and 16 generate temperature signals and static pressure signals for span fluctuation compensation shown by dotted lines. e _T ′ and e _P ′ are generated, and the voltage-to-current conversion gain of the output circuit 12 is changed to compensate for span fluctuations.

<Problems to be Solved by the Invention> However, such conventional capacitive conversion devices require a pressure sensor to compensate for static pressure in addition to the differential capacitance sensor, which is an obstacle to miniaturization and is low cost. This also poses an obstacle to the realization of an analog capacitive converter.

<Means for Solving the Problems> In consideration of the above points, the present invention has the following features in order to enable static pressure compensation for differential pressure in an analog form.
A first electrode and a second electrode are provided to face the moving electrode, and a sealing liquid is filled between these electrodes, and the first and second electrostatic charges change differentially in accordance with the differential pressure to be detected. A differential capacitance sensor that forms a capacitance, a temperature sensor that detects the temperature of a sealing liquid, and an amplification means in which a fixed capacitor is connected in negative feedback between an input end and an output end, and a moving electrode is connected to the input end. , a detection gate means that detects the output voltage of the amplification means at a predetermined threshold value and changes the output level; a first or second selection gate means for selectively applying the signal to the electrode; a resistance means for feeding back an inverted signal of the output level of the detection gate means to the input end of the amplification means; and counting the period of change in the output of the detection gate means. a counting means that inverts the output level every predetermined value of the count value and selects the first or second selection gate means based on the output level; and the other end of the biasing end of the first or second selection output means. an operating means for applying an output voltage corresponding to the output level of the counting means; a static pressure calculating means for calculating a static pressure signal to which the change period signal and the temperature signal of the temperature sensor are input; The configuration includes a correction calculation means for performing correction calculations based on the pressure signal and the temperature signal and outputting a differential pressure signal.

<Example> Hereinafter, an example of the present invention will be described based on the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention. Note that parts having the same functions as those of the conventional capacitive converter shown in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted as appropriate.

The differential capacitance sensor 19 has fixed electrodes 8 and 9 arranged opposite to the movable electrode 7 to form capacitances C ₁ and C ₂ respectively, and further includes a temperature sensor 13 incorporated therein.

20 and 21 are selection gate circuits, which include CMOS transistors Q ₁ , Q ₂ , Q ₃ , Q ₄ and NAND gates.
Each consists of G ₁ and G ₂ . fixed electrode 8,
9 is connected to the output terminals of the selection gate circuits 20 and 21, respectively. The power supply voltage +E is applied to one end of the biasing end of the selection gate circuit 20, and the operating voltage V is applied to the other end.
are applied, respectively, and the power supply voltage -E is applied to one end of the biasing end of the selection gate circuit 21, and the operating voltage V is applied to the other end. The gates of CMOS transistors Q ₁ and Q ₂ are connected to the output terminal of number gate G ₁ , and the gates of CMOS transistors Q ₃ and Q ₄ are connected to the output terminal of NAND gate G ₂ .

The amplifier circuit 22 is composed of a fixed capacitor C _k and a differential amplifier _Q5 , and the inverting input terminal (-) of the differential amplifier _Q5 is connected to the moving electrode 7, and the non-inverting input terminal (+) is connected to the common potential point COM. each connected. A fixed capacitor C _k is connected between the inverting input terminal (−) output terminal and the differential amplifier Q ₅ .

Q ₆ is a comparator as a detection gate means,
Its non-inverting input terminal (+) is connected to the common potential point COM,
The inverting input terminal (-) is connected to the output terminal of the differential amplifier _Q5 , respectively.

G ₃ is an inverter whose input terminal is a comparator Q ₆
Its output terminal is connected to the inverting input terminal (-) of the differential amplifier _Q5 via a resistor R, and the output terminal of n
Connected to bit counter _Q7 .

The output terminal of comparator Q ₆ and the output terminal of counter Q 7 are connected to the input terminal of inverter G ₁ , respectively, and the output terminal of comparator Q ₆ and the output terminal of counter Q ₇ are connected to the input terminal of inverter G ₂ via inverter G ₄ . The output ends of Q ₇ are connected respectively.

The output end of the inverter _G4 is connected to the input end of an integrator 23 as an operating means, and an operating voltage V is obtained at its output end.

On the other hand, the static pressure calculation circuit 24 inputs the temperature T from the temperature sensor 13 and the change period t _H (=t _L ) of the counter Q ₇ , calculates the static pressure applied to the sealing liquid 5, and generates a static pressure signal.
Output as P _s .

The correction calculation circuit 25 receives the operating voltage V, temperature T, and static pressure signal _Ps corresponding to the differential pressure ΔP, performs static pressure correction on the differential pressure Δp, and outputs the corrected differential pressure Δp _c at the output end 26. do.

Next, the operation of the embodiment configured as described above will be explained with reference to the waveform diagram shown in FIG.

The output terminal Q _o of counter Q ₇ is at high level “H” (=
+E) In the state shown in FIG. 2F, the selection gate circuit 20 side is selected and the NAND gate _G1 is opened and closed in response to the level change at the output terminal of the comparator _Q6 (FIG. 2D). Now, when the output terminal of comparator _Q6 is at high level "H" (Fig. 2 D), the transistor
The gates of Q ₁ and Q ₂ become low level "L", transistor Q ₁ is turned on and transistor Q ₂ is turned off, and a voltage of +E is applied to the fixed electrode 8 (FIG. 2A). Therefore, the input terminal of the differential amplifier Q ₅ tends to rise positively, but the input terminal of the differential amplifier Q ₅ through the fixed capacitance C _k
Due to the feedback action of , the voltage at the output end of the differential amplifier _Q5 rapidly drops by a predetermined value _eH (Fig. 2C). This voltage drop is detected by comparator _Q6 and inverts the voltage level at its output to positive (FIG. 2D). This level change inverts the output of inverter G ₃ (Fig. 2 E), discharges the charge of capacitance C ₁ through resistor R, and gradually lowers its potential, so that differential amplifier Q ₅ It operates to neutralize the charge through the fixed capacitor C _k and raises the potential at its output terminal as shown in FIG. 2C. common potential
When the potential of COM is exceeded, the potential of the output terminal of comparator _Q6 rapidly turns negative (Fig. 2 D). If the period until this inversion is t _H and the current flowing through the resistor R is i, then the following equation holds true taking into account the charge fluctuation in the capacitance C ₁ .

C ₁ (E-V) = C _k e _H (1) t _H i = C _k e _H (2) The level inversion at the output terminal of comparator Q ₆ turns off transistor Q ₁ via NAND gate G ₁ Q ₂ is turned on and an operating voltage V is applied to the fixed electrode 8 (FIG. 2A). For this reason, the fixed electrode 8 causes a sudden potential drop of (E-V), and in order to neutralize this potential drop, the differential amplifier Q ₅ rapidly increases the voltage by a predetermined value e _H via the fixed capacitor C _k . (Figure 2 C). On the other hand, the potential increase at the output terminal of the differential amplifier _Q5 attempts to charge the capacitance _C1 via the comparator _Q6 , inverter _G3 , and resistor R and increase its potential.
_Q5 gradually lowers the potential at its output terminal (second
Figure c) Neutralize the charge on capacitance _C1 . When the potential at the common potential point COM is exceeded, the potential at the output end of the comparator _Q6 rapidly reverses to positive and returns to the initial state. The period t _H ′ until this reversal is equal to the period t _H (1)(2)
The formula holds true. Repeat this from now on.

When these steps are repeated a predetermined number of times, the level of the output terminal _Qo of the counter _Q7 is inverted (FIG. 2), and this time the selection gate circuit 21 side is selected (FIG. 2, G).
It operates in the same way as the selection gate circuit 20 side. In this case, e _L for a given value e _H and t _L for a period t _H
Then, the following formula holds true.

C ₂ (V-(-E)) = C _k e _L (3) t _L i = C _k e _L (4) Next, the output terminal of counter Q ₇ is at high level “H”
If the period during which the level is low is T _H and the period during which the low level is “L” is T _L , then as shown in the second figure, for example, T _H > T _L
When , the average output voltage of inverter _G4 is (T _H
It is given by (-E)+T _L E/( _TH + T _L ) and becomes a negative voltage. This negative voltage increases the output of the integrator 23, that is, the operating voltage V. As the operating voltage V increases, the amplitude of the voltages (A and B in FIG. 2) of the fixed electrodes 8 and 9 decreases and increases. As a result, the periods t _H and t _L decrease,
increases, and finally reaches equilibrium at t _H =t _L , that is, T _H =T _L . In this case, the following equation holds true from equations (1) to (4).

V=E・C ₁ −C ₂ /C ₁ +C ₂ (5) Also, from t _H , t _L , and equations (1) to (4), t _H = t _L = 2E/i・C ₁ C ₂ /C We get ₁ +C ₂ (6).

The operating voltage V depends on the power supply voltage E, and the period t _H (=
t _L ) depends on the power supply voltage E and current i, but since the current i is given by i = ±E/R, if the power supply voltage E is stabilized, the operating voltage or period T _H will be the capacitance C ₁ , C ₂ is obtained as a value proportional to the difference in the sums or the product of the sums.

Note that the distributed capacitance C _s formed between the moving electrode 7 and the common potential point COM is affected by the influence of the distributed capacitance C _s because the potential of this part is always held at the common potential point COM potential by the differential amplifier Q ₅ . It becomes difficult to accept.

Next, the relationship between the capacitances C ₁ and C ₂ and the differential pressure Δp before correction will be explained. If the value of each capacitance C ₁ and C ₂ when the differential pressure ΔP is zero is C ₀ and the spring constant of the moving electrode 7 is K, then the capacitance C ₁ and C ₂ are as follows: C ₁ =C ₀ Express it as 1/1−KΔP (7) C ₂ =C ₀ 1/1+KΔP (8). From these equations, the differential pressure ΔP becomes ΔP=1/K(C ₁ −C ₂ /C ₁ +C ₂ )(9). Therefore, using equation (5), we obtain V=KE・ΔP (10). Furthermore, if the dielectric constant of the sealing liquid ₅ is ε and the capacitance in vacuum is C _v , the capacitance C 0 is C ₀ = εC _v , so using equations (7) and (8), ε= 2/C _v (C ₁ C ₂ /C ₁ C ₂ ) (11). Substituting equation (6) into this equation, we obtain t _H = t _L = E/iC _v ·ε (12).

By the way, as the temperature T increases, the dielectric constant ε decreases, and as the static pressure P _S increases, the dielectric constant ε increases, so if the dielectric constant ε ₀ at the reference temperature, a, and b are constants, the dielectric constant ε is It is shown by the following formula.

ε=ε ₀ (1−aT+bT _S ) (13) Modifying this, the static pressure P _S is P _S =1/b・ε−ε ₀ /ε ₀ +a/bT(
14) becomes. Let the rate of change of the dielectric constant ε be Δε, α=1/b,
If β=−a/b, equation (14) becomes P _S =α·Δε−βT (15). α and β are correction coefficients for Δε and T, respectively.

The static pressure calculation circuit 24 inputs the temperature T from the temperature sensor 13 and the period T _H (=T _L ) signal related to the dielectric constant ε, calculates the rate of change in the dielectric constant Δε, and calculates the rate of change in the dielectric constant Δε, which is expressed by equation (15). After the calculation, static pressure P _S is calculated.

On the other hand, the differential pressure ΔP is expressed by equation (9), which is expressed in the ideal case, that is, when the fixed electrodes 8 and 9 and the movable electrode 7 are parallel to each other, and the spring constant K is also constant. This is an equation that holds true for the following cases. However, in reality, when the static pressure P _S or the temperature T changes, the main body 1 deforms and the differential pressure ΔP obtained by equation (9) changes. Therefore, it is necessary to correct the differential pressure ΔP.

This correction is executed by the correction calculation circuit 25. The correction calculation circuit 25 generates an operating voltage V proportional to the differential pressure ΔP.
is input, but in response to this, the static pressure calculation circuit 24
The calculation shown in the following formula is performed using the static pressure P _S from the temperature sensor 13 and the temperature T from the temperature sensor 13, and the corrected differential pressure ΔP _C is outputted to the output terminal 26.

Assuming that the correction coefficient for static pressure P _S is k ₁ and the correction coefficient for temperature is k ₂ , the corrected differential pressure ΔP _C is as follows.

ΔP _C =ΔP1+k ₁ P _S +k ₂ T) (16) FIG. 3 is a partial block diagram showing another embodiment of the present invention. When a cable is used to connect the movable electrode 8 and the amplifier circuit 22, a distributed capacitance _Cs is created between the common potential COM and the inverting input terminal (-) of the differential amplifier _Q5 in the embodiment shown in FIG. Since the potential of is held at the potential of the common potential point COM, there is a guard effect. However, although this is true when the open loop gain of the differential amplifier _Q5 is sufficiently high, it has an attenuating effect on the signal transmitted to the differential amplifier _Q5 via the capacitance _C1 or _C2 . cannot be avoided. On the other hand, when the potential of the output end of the differential amplifier _Q5 is applied to the guard GD of the cable as shown in Fig. 3, the guard capacitance is formed in parallel with the fixed capacitance _Ck , and the guard capacitance is formed in parallel with the fixed capacitance Ck. The effect of distributed capacitance C _s on COM is removed. This can also be seen from the fact that the fixed capacitance C _k is eliminated in the derivation of equation (5).

Conversely, it is also possible to replace the fixed capacitor C _k with a guard capacitor and eliminate the fixed capacitor C _k as a component.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, the static pressure signal is obtained using the change period obtained at the output end of the counter, and the differential pressure obtained at the output of the integrator 23 is Since the signal was corrected to obtain an analog differential pressure signal with static pressure correction,
A compact, static pressure compensated, highly accurate capacitive conversion device can be realized.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention;
Fig. 2 is a waveform diagram explaining the operation of the embodiment shown in Fig. 1, Fig. 3 is a partial block diagram showing another embodiment of the present invention, and Fig. 4 shows the configuration of a conventional capacitive conversion device. It is a block diagram. 5... Sealing liquid, 7... Moving electrode, 8, 9... Fixed electrode, 13... Temperature sensor, 19... Differential capacitance sensor, 20, 21... Selection gate circuit, 22...
Amplifier circuit, 24...Static pressure calculation circuit, 25...Correction calculation circuit, C _k ...Fixed capacitance, _C1 , _C2 ...Capacitance, V...Operating voltage, P _S ...Static pressure.

Claims (1)

[Claims] 1 A first electrode and a second electrode are provided facing the moving electrode, and a sealing liquid is filled between the first and second static electrodes, and the first and second static electrodes change differentially according to the differential pressure to be detected. A differential capacitance sensor forming a capacitance, a temperature sensor detecting the temperature of the sealing liquid, a fixed capacitor connected in negative feedback between an input end and an output end, and the movable electrode connected to the input end. amplification means, a detection gate means for detecting the output voltage of the amplification means at a predetermined threshold value and changing the output level; first or second selection gate means for selectively applying the signal to the first or second electrode; resistance means for feeding back an inverted signal of the output level of the detection gate means to the input end of the amplification means; and the detection gate. counting means for counting the change period of the output of the means, inverting the output level every predetermined value of the counted value, and selecting the first or second selection gate means based on the output level; operating means for applying an output voltage corresponding to the output level of the counting means to the other end of the biasing end of the selection output means; A capacitive conversion device comprising a pressure calculation means and a correction calculation means for performing a correction calculation on the output of the operation means using the static pressure signal and the temperature signal and outputting a differential pressure signal.