JP6372288B2 - Capacitance type pressure measuring apparatus and linearity correction method thereof - Google Patents

Description

  In the present invention, a displacement amount corresponding to the magnitude of the differential pressure between the first pressure and the second pressure is detected as a change in differential capacitance by the first and second sensor capacitors, and current is output. The present invention relates to a capacitance type pressure measuring device and a linearity correction method thereof.

  2. Description of the Related Art Conventionally, there is a capacitance type pressure measuring device that detects a differential pressure in an industrial process as a mechanical minute displacement proportional to the differential pressure and outputs it as an electrical signal.

  For example, in Patent Document 1, the amount of displacement corresponding to the magnitude of the differential pressure between the first pressure and the second pressure is detected as a change in differential capacitance by the first and second sensor capacitors. A displacement transducer that outputs current is described. And in patent document 1, in order to correct the linearity with respect to the differential pressure | voltage of the detected displacement amount, the 1st and 2nd linearity correction capacitor is provided. The first and second linearity correcting capacitors eliminate the influence of the stray capacitance of the first and second sensor capacitors, respectively, and are straight lines for the differential pressures of the displacement amounts detected by the first and second sensor capacitors. Each capacitance is adjusted to improve the performance.

JP-A-2-120614

  However, in order to achieve good linearity by adjusting the capacitances of the first and second linearity correction capacitors, the adjustment work of the capacitances of the first and second linearity correction capacitors There is a problem that it is necessary to repeatedly perform the output current check operation, which is a change in the differential capacitance by the first and second sensor capacitors, by this adjustment operation.

  The present invention has been made in view of the above, and is a capacitance type pressure that can easily adjust the linearity correction by adjusting the capacitance of the first and second linearity correction capacitors. It is an object of the present invention to provide a measuring apparatus and a linearity correction method thereof.

  In order to solve the above-described problems and achieve the object, the capacitance-type pressure measuring device according to the present invention has a displacement amount corresponding to the magnitude of the differential pressure between the first pressure and the second pressure. A capacitance-type pressure measuring device that outputs a current as a change in differential capacitance caused by a first sensor capacitor and a second sensor capacitor, wherein the linearity of the displacement amount with respect to the differential pressure is corrected. First and second linearity correction capacitors, an oscillation circuit, and an oscillation voltage output from the oscillation circuit to rectify the first sensor capacitor, the second sensor capacitor, and the first linearity correction The capacitor for charging and the second linearity correcting capacitor are charged by applying an alternating voltage of the same magnitude and then repeatedly charged / discharged, and the charge / discharge current rectification for the first sensor capacitor is repeated. From the first current obtained by subtracting the rectified value of the charge / discharge current for the first linearity correction capacitor from the rectified value of the charge / discharge current for the second sensor capacitor, the charge / discharge current for the second linearity correction capacitor is charged. A second current obtained by subtracting a rectified value of the discharge current; a charge / discharge circuit that generates a sum current of the first current and the second current; an oscillation frequency of the oscillation circuit so that the sum current is constant; And / or an oscillation circuit control circuit that controls the amplitude of the oscillation voltage, an output control circuit that outputs an output current corresponding to the difference between the first current and the second current, and the oscillation frequency and oscillation voltage of the oscillation circuit Corresponds to the amplitude, the first voltage corresponding to the first current and the second current, the second voltage corresponding to the sum of the first current and the second current, the difference between the first current and the second current. Detect third voltage The first sensor based on the detection unit, the oscillation frequency in a state where a plurality of pressure differences are applied from the outside, the amplitude of the oscillation voltage, the first voltage, the second voltage, and the third voltage A stray capacitance calculation unit that calculates a first stray capacitance of a capacitor and a second stray capacitance of the second sensor capacitor, and the first stray capacitance calculated by the stray capacitance calculation unit The capacitance of the linearity correction capacitor is adjusted, and the capacitance of the second linearity correction capacitor is adjusted based on the second floating capacitance calculated by the floating capacitance calculation unit. .

  Further, the capacitance type pressure measuring device according to the present invention is the above-described invention, wherein a shut-off switch for shutting off a current flowing through the first and second linearity correcting capacitors is provided, and the shut-off switch is shut off. In the state, the capacitance of the first linearity correction capacitor is adjusted based on the first stray capacitance calculated by the stray capacitance calculation unit, and the second stray capacitance calculated by the stray capacitance calculation unit is also obtained. And adjusting the capacitance of the second linearity correcting capacitor.

  In the capacitance-type pressure measuring device according to the present invention, in the above invention, the first and second linearity correcting capacitors have fixed capacitances, and the first and second The first and second variable series resistors and the first and second rectifying capacitors are connected to the respective linearity correcting capacitors, and the first stray capacitance calculated by the stray capacitance calculating unit is used as the first stray capacitance. The effective series capacitance of the first linearity correction capacitor is adjusted, and the second variable series resistance is adjusted based on the second stray capacitance calculated by the stray capacitance calculation unit. By adjusting, the effective capacitance of the second linearity correcting capacitor is adjusted.

  Further, the linearity correction method for the capacitance type pressure measuring device according to the present invention includes the first and second sensor capacitors, the oscillation circuit, and the first voltage obtained by rectifying the oscillation voltage output from the oscillation circuit. Charge and discharge to be applied to the sensor capacitor, the second sensor capacitor, the first linearity correction capacitor, and the second linearity correction capacitor by applying an alternating voltage of equal magnitude, and then discharging. The first current obtained by subtracting the rectified value of the charge / discharge current for the first linearity correction capacitor from the rectified value of the charge / discharge current for the first sensor capacitor and the charge / discharge for the second sensor capacitor are repeated. A second current obtained by subtracting a rectified value of charge / discharge current for the second linearity correction capacitor from a rectified value of current, and a sum current of the first current and the second current A difference between the first current and the second current; a charge / discharge circuit to be configured; an oscillation circuit control circuit that controls an oscillation frequency and / or an amplitude of an oscillation voltage of the oscillation circuit so that the sum current is constant; And an output control circuit for outputting an output current corresponding to the first sensor capacitor and the second sensor, and a displacement amount corresponding to a differential pressure between the first pressure and the second pressure. A method for correcting the linearity of a capacitance-type pressure measuring device that outputs current as a change in differential capacitance due to a sensor capacitor, the oscillation frequency of the oscillation circuit in a state where a plurality of pressure differences are applied from the outside The amplitude of the oscillation voltage, the first voltage corresponding to the first current and the second current, the second voltage corresponding to the sum of the first current and the second current, the first current and the second current Detection to detect the third voltage corresponding to the difference A first stray capacitance of the first sensor capacitor and the second sensor based on the oscillation frequency, the amplitude of the oscillation voltage, the first voltage, the second voltage, and the third voltage. A stray capacitance calculation step for calculating a second stray capacitance of the capacitor, and adjusting a capacitance of the first linearity correction capacitor based on the first stray capacitance calculated by the stray capacitance calculation step. The capacitance of the second linearity correction capacitor is adjusted based on the second stray capacitance calculated in the stray capacitance calculation step.

  Moreover, the linearity correction method of the capacitance-type pressure measuring device according to the present invention includes a current interruption step of interrupting a current flowing through the first and second linearity correction capacitors in the above invention, In the state where the current is interrupted in the current interrupting step, the capacitance of the first linearity correction capacitor is adjusted based on the first stray capacitance calculated in the stray capacitance calculating step, and the stray capacitance calculation is performed. The capacitance of the second linearity correction capacitor is adjusted based on the second stray capacitance calculated in the step.

  In the linearity correction method for a capacitance-type pressure measuring device according to the present invention, the capacitance of the first and second linearity correction capacitors is fixed in the above invention, and the first The first and second variable series resistors and the first and second rectifying capacitors are connected to the second linearity correcting capacitors, and the first stray capacitance calculated by the stray capacitance calculating step is used. Based on the second stray capacitance calculated by the stray capacitance calculation step, the first variable series resistance is adjusted to adjust the effective capacitance of the first linearity correction capacitor. The effective series capacitance of the second linearity correcting capacitor is adjusted by adjusting the variable series resistance.

  According to the present invention, the detection unit calculates the rectification value of the charge / discharge current for the first linearity correction capacitor from the oscillation frequency of the oscillation circuit, the amplitude of the oscillation voltage, and the rectification value of the charge / discharge current for the first sensor capacitor. A first voltage corresponding to a second current obtained by subtracting a rectified value of the charge / discharge current for the second linearity correction capacitor from a rectified value of the charge / discharge current for the second sensor capacitor and the subtracted first current; The second voltage corresponding to the sum of the current and the second current and the third voltage corresponding to the difference between the first current and the second current are detected, and the stray capacitance calculation unit applies a plurality of pressure differences from the outside. Based on the oscillation frequency, the amplitude of the oscillation voltage, the first voltage, the second voltage, and the third voltage, the first stray capacitance of the first sensor capacitor and the second sensor capacitor Second of The free capacitance is calculated, the capacitance of the first linearity correction capacitor is adjusted based on the first stray capacitance calculated by the stray capacitance calculation unit, and the second stray capacitance calculated by the stray capacitance calculation unit is calculated. The capacitance of the second linearity correcting capacitor is adjusted based on the capacitance. As a result, it is possible to easily adjust the linearity correction without repeating the adjustment work of the capacitances of the first and second linearity correction capacitors and the operation of checking the output current by the adjustment work. Can do.

FIG. 1 is a block diagram schematically showing a schematic configuration of a capacitance-type pressure measuring apparatus according to Embodiment 1 of the present invention. FIG. 2 is a circuit diagram showing a detailed configuration of the capacitance type pressure measuring apparatus shown in FIG. FIG. 3 is a flowchart showing the linearity correction processing procedure of the capacitive pressure measuring device shown in FIG. FIG. 4 is a circuit diagram showing a detailed configuration of the capacitance-type pressure measuring device according to the second embodiment of the present invention. FIG. 5 is a flowchart showing a linearity correction processing procedure of the capacitive pressure measuring device shown in FIG. FIG. 6 is a circuit diagram showing a detailed configuration of the capacitance-type pressure measuring device according to the third embodiment of the present invention. FIG. 7 is a flowchart showing the linearity correction processing procedure of the capacitance-type pressure measuring device shown in FIG.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings.

(Embodiment 1)
[Overview configuration]
FIG. 1 is a block diagram schematically showing a schematic configuration of a capacitance-type pressure measuring apparatus according to Embodiment 1 of the present invention. The capacitance-type pressure measuring device shown in FIG. 1 is configured so that a displacement amount corresponding to a differential pressure between the first pressure and the second pressure, for example, the differential pressure p0, is compared with the first sensor capacitor 1 and the first pressure. Current is output as a change in differential capacitance due to the two sensor capacitors 2.

  As shown in FIG. 1, the capacitance-type pressure measuring device includes first and second sensor capacitors 1 and 2, first and second linearity correcting capacitors 43 and 44, an oscillation circuit 3, and a charge / discharge circuit. 4, an oscillation circuit control circuit 5, an output control circuit 6, a detection unit 7, and a stray capacitance calculation unit 8.

  The first and second sensor capacitors 1 and 2 have, for example, two fixed electrodes sandwiching a movable electrode such as a diaphragm, and the first sensor capacitor 1 is formed by one fixed electrode and the movable electrode. The second sensor capacitor 2 is formed of the other fixed electrode and movable electrode. Then, pressure is applied to the gap of the first sensor capacitor 1 and the gap of the second sensor capacitor 2, and the movable electrode is displaced by the pressure difference between the gaps. In accordance with this displacement, the capacitances C1 and C2 of the first and second sensor capacitors 1 and 2 change differentially.

  The first and second linearity correction capacitors 43 and 44 correct the linearity of the differential displacement amounts of the capacitances C1 and C2 with respect to the pressure difference.

  The charge / discharge circuit 4 rectifies the oscillation voltage output from the oscillation circuit 3 to rectify the first sensor capacitor 1, the second sensor capacitor 2, the first linearity correction capacitor 43, and the second linearity correction. The capacitor 44 is charged as an alternating voltage having the same magnitude, and then charged and discharged repeatedly. The charge / discharge circuit 4 also subtracts the charge / discharge current rectified value Ic1 for the first linearity correction capacitor 43 from the charge / discharge current rectified value I1 for the first sensor capacitor 1 to obtain a first current Δ1 (= I1). −Ic1) and a second current Δ2 (= I2−Ic2) obtained by subtracting the rectified value Ic2 of the charge / discharge current for the second linearity correction capacitor 44 from the rectified value I2 of the charge / discharge current for the second sensor capacitor 2. Then, a sum current Ik of the first current Δ1 and the second current Δ2 is generated.

  The oscillation circuit control circuit 5 controls the oscillation frequency f of the oscillation circuit 3 and / or the amplitude Vm of the oscillation voltage so that the sum current Ik is constant.

  The output control circuit 6 outputs an output current Io corresponding to the difference between the first current Δ1 and the second current Δ2.

  The detection unit 7 corresponds to the sum of the oscillation frequency f of the oscillation circuit 3, the amplitude Vm of the oscillation voltage, the first voltage V1 corresponding to the first current Δ1 and the second current Δ2, the first current Δ1 and the second current Δ2. A second voltage V3 corresponding to the difference between the second voltage Vc, the first current Δ1, and the second current Δ2 is detected.

  The stray capacitance calculation unit 8 includes an oscillation frequency f, an oscillation voltage amplitude Vm, a first voltage V1 (= V2), and a second voltage when a plurality of pressure differences are applied from the outside, that is, differential pressures p0, p1, and p2. Based on Vc and the third voltage V3, the first stray capacitance Cs1 of the first sensor capacitor 1 and the second stray capacitance Cs2 of the second sensor capacitor 2 are calculated.

  Then, based on the first stray capacitance Cs1 calculated by the stray capacitance calculation unit 8, the capacitance C43 of the first linearity correction capacitor 43 is adjusted to be the first stray capacitance Cs1. Further, based on the second stray capacitance Cs2 calculated by the stray capacitance calculation unit 8, the capacitance C44 of the second linearity correction capacitor 44 is adjusted to become the second stray capacitance Cs2. By simply adjusting the capacitances C43 and C44 of the first and second linearity correction capacitors 43 and 44 to the first and second stray capacitances Cs1 and Cs2, respectively, good linearity correction can be performed. it can.

[Detailed configuration and pressure measurement operation]
FIG. 2 is a circuit diagram showing a detailed configuration of the capacitance type pressure measuring apparatus shown in FIG. In addition, the same code | symbol is attached | subjected to the component corresponding to FIG. In FIG. 1, a resistor 9 is an external load resistor. Further, the power source V is an external DC power source that supplies power to the capacitance type pressure measuring device. The capacitance type pressure measuring device excluding the detection unit 7 and the stray capacitance calculation unit 8 is a direct current signal proportional to a minute differential pressure (minute displacement) detected by the first and second sensor capacitors 1 and 2. The output current Io is output to the resistor 9. This output current Io is, for example, 4 to 20 mA.

  Reference numerals 31 to 34 constitute the constant voltage circuit 101. Reference numerals 71 to 78 form an oscillation circuit 3 that oscillates a sine wave signal. Reference numerals 80 to 82 constitute the reference voltage source 102. Reference numerals 83 to 86 constitute the oscillation circuit control circuit 5. The oscillation circuit control circuit 5 controls the amplitude of the oscillation voltage of the oscillation circuit 3. The windings 10, 11, and 12 in the charge / discharge circuit 4 are secondary windings that are integrally wound around the winding 72 that is the primary winding in the oscillation circuit 3, and have the same number of turns. The winding 99 is also a secondary winding integrally wound around the winding 72, but the number of turns is not necessarily equal.

  In the charging / discharging circuit 4, the AC voltage induced in the windings 10, 11, 12 is applied to the first and second sensor capacitors 1, 2, and charging / discharging is repeated. The charging route for the first (second) sensor capacitor 1 (2) is as follows: winding 12, diode 15 (16), first (second) sensor capacitor 1 (2), resistor 39, winding 12 in this order. It becomes. On the other hand, the discharge route includes the first (second) sensor capacitor 1 (2), the diode 13 (14), the winding 10 (11), the resistor 35 (36, 37), and the first (second) sensor capacitor. The order is 1 (2). In this case, the charging current and the discharging current are equal to each other.

  In the charging / discharging circuit 4, the AC voltage induced in the windings 10, 11, 12 is also applied to the first and second linearity correcting capacitors 43, 44, and charging / discharging is repeated. The discharge route of the first (second) linearity correction capacitor 43 (44) is the first linearity correction capacitor 43 (44), the diode 47 (48), the output of the operational amplifier 40, and the negative power supply of the operational amplifier 40. The order is the terminal, the winding 12, and the first (second) linearity correcting capacitor 43 (44). On the other hand, the charging route is in the order of the winding 12, the resistor 39, the resistor 35 (36, 37), the diode 45 (46), the first (second) linearity correcting capacitor 43 (44), and the winding 12. is there. In this case, the charging current and the discharging current are equal to each other.

The charging current and discharging current described above are smoothed by the capacitors 20-22. When the forward voltages of the diodes 13 to 16 and 45 to 48 are ignored, the charge / discharge currents I1 and I2 for the first and second sensor capacitors 1 and 2 and the first and second linearity correction capacitors 43 are ignored. , 44 are represented by the following equations (1) to (4).
I1 = f (2Vm-V1-Vc) C1 (1)
I2 = f (2Vm-V2-Vc) C2 (2)
Ic1 = f {2Vm− (1 + R41 / R42) (V1 + Vc) + (V1 + Vc)} C43 (3)
Ic2 = f {2Vm- (1 + R41 / R42) (V1 + Vc) + (V1 + Vc)} C44 (4)
Note that f is an oscillation frequency. Vm is the amplitude of the oscillation voltage. V <b> 1 is a voltage between the terminals of the resistor 35. V2 is the voltage across the resistors 36-37. Vc is a voltage between the terminals of the resistor 39.

Here, the operational amplifier 63 and the transistor 67 of the output control circuit 6 supply the feedback current If to the resistor 37 so that the potentials at the points a and b are equal. As a result, V1 = V2. Further, it is assumed that R41 = R42. Expressions (1) to (4) in this case become the following expressions (5 to 8).
I1 = f (2Vm-V1-Vc) C1 (5)
I2 = f (2Vm-V1-Vc) C2 (6)
Ic1 = f (2Vm-V1-Vc) C43 (7)
Ic2 = f (2Vm-V1-Vc) C44 (8)
Therefore, it can be seen that the same voltage is applied to the first and second sensor capacitors 1 and 2 and the first and second linearity correction capacitors 43 and 44.

On the other hand, the oscillation circuit control circuit 5 controls the amplitude Vm of the oscillation voltage so that the Zener voltage Vz of the Zener diode 82 in the reference voltage source 102 is equal to the terminal voltage Vc of the resistor 39. Therefore,
Vc = R39 (I1-Ic1 + I2-Ic2)
Vz = R39 (I1-Ic1 + I2-Ic2) (9)

Substituting equations (5) to (8) into equation (9),
f (2Vm-V1-Vc) = Vz / R39 / (C1-C43 + C2-C44)
(10)
Therefore, when using the equations (5) to (8) and (10),
I1-Ic1 = (C1-C43) / (C1-C43 + C2-C44) .Vz / R39
... (11)
I2-Ic2 = (C2-C44) / (C1-C43 + C2-C44) .Vz / R39
(12)

here,
R59 · If1 + Vps = R60 · If3 (13)
If3 = If + If′−If1 (14)
From equations (13) and (14),
If1 = {R60 (If + If ′) − Vps} / (R59 + R60) (15)
If3 = {R59 (If + If ′) + Vps} / (R59 + R60) (16)

Also,
R69 · If2 = V4 + R59 · If1 (17)
From equation (17)
If2 = (V4 + R59 · If1) / R69 (18)

on the other hand,
V1 = R35 (I1-Ic1) = (R36 + R37) (I2-Ic2) + R37 · If
... (19)
If R35 = R36 + R37, then from equation (19), If = R35 / R37. {(I1-Ic1)-(I2-Ic2)} (20)

Here, the second voltage Vc becomes a constant voltage by the oscillation circuit control circuit 5. That is, the current Ik flowing through the resistor 39 is a constant current. This current Ik can be expressed by the following equation (21).
Ik = I1-Ic1 + I2-Ic2 (21)
From Expressions (20) and (21) I2-Ic2 = (1/2) Ik-R37 / (2.R35) If (22)
Also,
V3 = R37 (I2-Ic2 + If)
= R37 (R35 + R36) / (2.R35) If + (R37 / 2) Ik
... (23)
further,
R69 · If2 = R59 · If1 + R61 · If ′ (24)
From Expressions (15) and (24), If2 = (R59 / R69) If1 + (R61 / R69) If ′
= (R59 / R69) {R60 (If + If ′) − Vps} / (R59 + R60) + (R61 / R69) If ′
= (R59 / R69) · R60 / (R59 + R60) If + {(R59 / R69) · R60 / (R59 + R60) + (R61 / R69)} If ′ − (R59 / R69) / (R59 + R60) Vps (25)
Also,
R61 · If ′ = R62 · If + V3 (26)
From formulas (23) and (26), If ′ = (R62 / R61) If + (R37 / R61) (R35 + R36) / (2.R35) If + (R37 / R61 / 2) Ik
= {(R62 / R61) + (R37 / R61) (R35 + R36) / (2.R35)} If + (R37 / R61 / 2) Ik (27)

here,
Io = If2 + If ′ + If (28)
From Expressions (25), (27), and (28), Io = (R59 / R69) · R60 / (R59 + R60) If + {(R59 / R69) · R60 / (R59 + R60) + (R61 / R69)} If ′ − ( R59 / R69) / (R59 + R60) Vps + If ′ + If
= {(R59 / R69) · R60 / (R59 + R60) +1} If + {(R59 / R69) · R60 / (R59 + R60) + (R61 / R69) +1} If ′ − (R59 / R69) / (R59 + R60) Vps
= [{(R59 / R69) .R60 / (R59 + R60) +1} + {(R59 / R69) .R60 / (R59 + R60) + (R61 / R69) +1} {(R62 / R61) + (R37 / R61) ( R35 + R36) / (2.R35)}] If + {(R59 / R69) .R60 / (R59 + R60) + (R61 / R69) +1} (R37 / R61 / 2) Ik− (R59 / R69) / (R59 + R60) Vps ... (29)

From the equations (11), (12), (20), (29), the output current Io is
Io = [{(R59 / R69) · R60 / (R59 + R60) +1} + {(R59 / R69) · R60 / (R59 + R60) + (R61 / R69) +1} {(R62 / R61) + (R37 / R61) (R35 + R36) / (2.R35)}] R35 / R37. {(I1-Ic1)-(I2-Ic2)} + {(R59 / R69) .R60 / (R59 + R60) + (R61 / R69) +1} (R37 / R61 / 2) Ik- (R59 / R69) / (R59 + R60) Vps
= [{(R59 / R69) .R60 / (R59 + R60) +1} + {(R59 / R69) .R60 / (R59 + R60) + (R61 / R69) +1} {(R62 / R61) + (R37 / R61) ( R35 + R36) / (2.R35)}]. R35 / R37. {C1-C43- (C2-C44)} / (C1-C43 + C2-C44) .Vz / R39 + {(R59 / R69) .R60 / (R59 + R60) + (R61 / R69) +1} (R37 / R61 / 2) Ik- (R59 / R69) / (R59 + R60) Vps
... (30)
It becomes. Here, since Ik and Vps are constant, if C43 and C44 are adjusted so that C43 = Cs1, C44 = Cs2, stray capacitances Cs1 and Cs2 included in C1 and C2 are removed, respectively, An output current Io that is a direct current signal proportional to the minute displacement of the second sensor capacitors 1 and 2 can be output to the resistor 9 that is a load resistance.

[Adjustment of linearity correction capacitor]
Next, adjustment of the first and second linearity correcting capacitors 43 and 44 described above will be described with reference to FIGS. FIG. 3 is a flowchart showing the linearity correction processing procedure of the capacitance-type pressure measuring device shown in FIG. First, the detection unit 7 is connected to the voltage measurement terminals 90 to 94. In the state where the differential pressures p0, p1, and p2 are applied to the first and second sensor capacitors 1 and 2, the detection unit 7 determines the voltage between the voltage measurement terminals 90 and 91 as the first voltage V1 (V1 (p0 ), V1 (p1), V1 (p2)), the voltage between the voltage measuring terminals 90, 92 is detected as the second voltage Vc, and the voltage between the voltage measuring terminals 90, 94 is the third voltage V3. (V3 (p0), V3 (p1), V3 (p2)), the voltage between the voltage measuring terminals 92 and 93 is observed, and the oscillation frequency f and the amplitude Vm (Vm (p0), Vm (P1), Vm (p2)) are detected (step S101).

Thereafter, the stray capacitance calculator 8 detects the first voltage V1 (V1 (p0), V1 (p1), V1 (p2)), the second voltage Vc, and the third voltage V3 (V3 (V3 ( p0), V3 (p1), V3 (p2)), oscillation frequency f, oscillation voltage amplitude Vm (Vm (p0), Vm (p1), Vm (p2)), as well as known resistances 35 to 37 The stray capacitances Cs1 and Cs2 are calculated using the resistance values R35 to R37, the known capacitances C43 and C44 of the first and second linearity correction capacitors 43 and 44, and the known ratio r (step S102). . Where the ratio r is
r = (p1-p0) / (p2-p1) (31)
It is.

  Thereafter, by using the stray capacitances Cs1 and Cs2 calculated in step S102, the first and second linearity correction capacitors 43 and 44 are adjusted so that the electrostatic capacitances C43 and C44 become the stray capacitances Cs1 and Cs2, respectively. (Step S103). In step S103, since the stray capacitances Cs1 and Cs2 to be adjusted are already known, the capacitances C43 and C44 can be easily adjusted.

[Calculation principle of stray capacitances Cs1, Cs2]
Here, the first voltage V1 (V1 (p0), V1 (p1), V1 (p2)), the second voltage Vc, the third voltage V3 (V3 (p0), V3 (p1)) detected by the detection unit 7, The principle by which the stray capacitances Cs1 and Cs2 can be calculated using V3 (p2)), the oscillation frequency f, and the oscillation voltage amplitude Vm (Vm (p0), Vm (p1), Vm (p2)) will be described.

First, from equations (5) and (7),
I1-Ic1 = f (2Vm-V1-Vc) (C1-C43) (32)
From equations (6) and (8),
I2-Ic2 = f (2Vm-V1-Vc) (C2-C44) (33)
It becomes.
on the other hand,
V1 = R35 (I1-Ic1) (34)
V1 = R36 (I2-Ic2) + V3 (35)
It is. Therefore, from the equations (32) to (35),
C1-C43- = V1 / R35 / {f (2Vm-V1-Vc)} (36)
C2-C44 = (V1-V3) / R36 / {f (2Vm-V1-Vc)}
... (37)
Is obtained.

Here, when the differential pressure applied to the first and second sensor capacitors 1 and 2 is p, from the equations (36) and (37),
C1 (p) = V1 (p) / R35 / {f (2Vm (p) −V1 (p) −Vc)} + C43− (38)
C2 (p) = {V1 (p) −V3 (p)} / R36 / {f (2Vm (p) −V1 (p) −Vc)} + C44 (39)
Can be expressed as

on the other hand,
C1 (p) = ε · A / {d−δ−Δ (p)} + Cs1 (40)
C2 (p) = ε · A / {d + δ + Δ (p)} + Cs2 (41)
It is. Here, ε is a dielectric constant between the electrodes of the first and second sensor capacitors 1 and 2. A is the electrode area of the first and second sensor capacitors 1 and 2. Further, d is a value that is ½ of the distance between the fixed electrodes of the first and second sensor capacitors 1 and 2. Further, δ is a positional deviation amount when there is no differential pressure (p = 0) of a movable electrode such as a diaphragm shared by the first and second sensor capacitors 1 and 2. Δ (p) is the displacement of the movable electrode such as a diaphragm shared by the first and second sensor capacitors 1 and 2 at the time of the differential pressure p.
From these equations (40) and (41),
1 / {C1 (p) −Cs1} = {d−δ−Δ (p)} / (ε · A) (42)
1 / {C2 (p) −Cs2} = {d + δ + Δ (p)} / (ε · A) (43)
Is obtained.

Here, the different differential pressures p0, p1, and p2 are
p0 <p1 <p2 (44)
Have a relationship.
Moreover, since the displacement Δ (p) is proportional to the differential pressure p, if the proportionality coefficient is k,
Δ (p) = k · p (45)
It can be expressed as.

With respect to the electrostatic capacitance C1 (p) and the stray capacitance Cs1 of the first sensor capacitor 1, the following equation holds for the differential pressure p = p0, p1 from the equations (42), (44), and (45).
1 / {C1 (p0) -Cs1} -1 / {C1 (p1) -Cs1}
-= {D-δ-Δ (p0)} / (ε · A)-{d-δ-Δ (p1)} / (ε · A)
= {Δ (p1) −Δ (p0)} / (ε · A)
= K (p1-p0) / (ε · A) (46)
When formula (46) is transformed,
[1 / {C1 (p0) −Cs1} −1 / {C1 (p1) −Cs1}] / (p1−p0) = k / (ε · A) (47)
It becomes.
Similarly, the following equation holds for the differential pressure p = p1, p2 for the capacitance C1 (p) and the stray capacitance Cs1 of the first sensor capacitor 1.
[1 / {C1 (p1) −Cs1} −1 / {C1 (p2) −Cs1}] / (p2−p1) = k / (ε · A) (48)

Since the left sides of equations (47) and (48) are equal,
[1 / {C1 (p0) -Cs1} -1 / {C1 (p1) -Cs1}] / (p1-p0)
= [1 / {C1 (p1) -Cs1} -1 / {C1 (p2) -Cs1}] / (p2-p1)
... (49)
Is obtained. Using equation (31), equation (49) becomes
[1 / {C1 (p0) -Cs1} -1 / {C1 (p1) -Cs1}]-r [1 / {C1 (p1) -Cs1} -1 / {C1 (p2) -Cs1}] = 0 ... (50)
It becomes.
Multiplying both sides of equation (50) by {C1 (p0) −Cs1} {C1 (p1) −Cs1} {C1 (p2) −Cs1}
{C1 (p1) -Cs1} {C1 (p2) -Cs1}-{C1 (p0) -Cs1} {C1 (p2) -Cs1} -r {C1 (p0) -Cs1} {C1 (p2) -Cs1 } + R {C1 (p0) -Cs1} {C1 (p1) -Cs1}
= C1 (p1) .C1 (p2) -C1 (p1) .Cs1-C1 (p2) .Cs1 + Cs1 ²
-C1 (p0) .C1 (p2) + C1 (p0) .Cs1 + C1 (p2) .Cs1-Cs1 ²
-R.C1 (p0) .C1 (p2) + r.C1 (p0) .Cs1 + r.C1 (p2) .Cs1-r.Cs1 ²
+ R * C1 (p0) * C1 (p1) -r * C1 (p0) * Cs1-r * C1 (p1) * Cs1 + r * Cs1 ²
= C1 (p1) .C1 (p2)-(1 + r) C1 (p0) .C1 (p2) + r.C1 (p0) .C1 (p1) + [C1 (p0)-(1 + r) C1 (p1) + r. C1 (p2)] Cs1
= 0 (51)
It becomes. As a result,
Cs1 = − [C1 (p1) · C1 (p2) − (1 + r) C1 (p0) · C1 (p2) + r · C1 (p0) · C1 (p1)] / [C1 (p0) − (1 + r) C1 ( p1) + r · C1 (p2)] (52)
It becomes. That is, the stray capacitance Cs1 can be calculated.

Similarly, by giving differential pressures p = p0, p1 and p = p1, p2 with respect to the capacitance C2 (p) of the second sensor capacitor 2, an equation for obtaining the stray capacitance Cs2 similarly to the stray capacitance Cs1. (53) is obtained.
Cs2 =-[C2 (p1) .C2 (p2)-(1 + r) C2 (p0) .C2 (p2) + r.C2 (p0) .C2 (p1)] / [C2 (p0)-(1 + r) C2 ( p1) + r · C2 (p2)] (53)
That is, the stray capacitance Cs2 can be calculated.

Therefore, assuming that the capacitance after adjustment of the first and second linearity correcting capacitors 43 and 44 is C43 ′ and C44 ′, the calculated Cs1 and Cs2 are used.
C43 ′ = Cs1 (54)
C44 '-= Cs2 (55)
Adjustment may be performed. As a result, the adjusted capacitances of the first and second linearity correction capacitors 43 and 44 become the stray capacitances Cs1 and Cs2.

(Embodiment 2)
Next, the second embodiment will be described. In the second embodiment, as shown in FIG. 4, a cutoff switch 111 is provided that cuts off the current flowing through the first and second linearity correction capacitors 43 and 44. The cutoff switch 111 is provided between the diodes 47 and 48 and the output terminal of the operational amplifier 40. Other configurations are the same as those shown in FIG. 2, and the same components are denoted by the same reference numerals.

[Adjustment of linearity correction capacitor]
Next, adjustment of the first and second linearity correcting capacitors 43 and 44 according to the second embodiment will be described with reference to FIGS. FIG. 5 is a flowchart showing the linearity correction processing procedure of the capacitance-type pressure measuring device shown in FIG. First, the detection unit 7 is connected to the voltage measurement terminals 90 to 94. The detection unit 7 turns off the cutoff switch 111 (step S201), and cuts off the current flowing through the first and second linearity correction capacitors 43 and 44. Subsequently, in a state where the differential pressures p0, p1, and p2 are applied to the first and second sensor capacitors, the voltage between the voltage measurement terminals 90 and 91 is set to the first voltage V1 (V1 (p0), V1 (p1 ), V1 (p2)), the voltage between the voltage measuring terminals 90, 92 is detected as the second voltage Vc, and the voltage between the voltage measuring terminals 90, 94 is detected as the third voltage V3 (V3 (p0)). , V3 (p1), V3 (p2)), the voltage between the voltage measuring terminals 92, 93 is observed, and the oscillation frequency f and the amplitude of the oscillation voltage Vm (Vm (p0), Vm (p1), Vm) are detected. (P2)) is detected (step S202).

  Thereafter, the stray capacitance calculator 8 detects the first voltage V1 (V1 (p0), V1 (p1), V1 (p2)), the second voltage Vc, and the third voltage V3 (V3 (V3 ( p0), V3 (p1), V3 (p2)), oscillation frequency f, oscillation voltage amplitude Vm (Vm (p0), Vm (p1), Vm (p2)), as well as known resistances 35 to 37 The stray capacitances Cs1 and Cs2 are calculated using the resistance values R35 to R37 and the known ratio r (step S203). At this time, the capacitances C43 and C44 of the first and second linearity correction capacitors 43 and 44 are C43 = C44 = 0.

  Thereafter, by using the stray capacitances Cs1 and Cs2 calculated in step S203, the first and second linearity correction capacitors 43 and 44 are adjusted so that the electrostatic capacitances C43 and C44 become the stray capacitances Cs1 and Cs2, respectively. (Step S204). In step S204, since the stray capacitances Cs1 and Cs2 to be adjusted are already known, the capacitances C43 and C44 can be easily adjusted.

  Since the current flowing through the first and second linearity correction capacitors 43 and 44 is interrupted by the cutoff switch 111, the linearity correction by the first and second linearity correction capacitors 43 and 44 is invalid. Become. In this state, the first and second linearity correcting capacitors 43 and 44 may be directly adjusted to the stray capacitances Cs1 and Cs2.

That is, assuming that the capacitance after adjustment of the first and second linearity correction capacitors 43 and 44 is C43 ′ and C44 ′, the calculated Cs1 and Cs2 are used.
C43 ′ = Cs1 (56)
C44 '-= Cs2 (57)
Adjustment may be performed. As a result, the adjusted capacitances of the first and second linearity correction capacitors 43 and 44 become the stray capacitances Cs1 and Cs2.

  After this adjustment is completed, the cutoff switch 111 is turned on (step S205), and this process is terminated.

(Embodiment 3)
Next, the second embodiment will be described. In the second embodiment, as shown in FIG. 6, the capacitances C43 and C44 of the first and second linearity correction capacitors 43 and 44 are not variable but fixed. Also, first and second variable resistors 49 and 50 and first and second rectifying capacitors 51 and 52 are provided at the position of the cutoff switch 111 shown in FIG. That is, the variable resistor 49 is connected in series to the diode 47, and the variable resistor 50 is connected in series to the diode 48. Furthermore, rectifying capacitors 51 and 52 are connected in parallel to variable resistors 49 and 50, respectively. Other configurations are the same as those shown in FIG. 2, and the same components are denoted by the same reference numerals.

[Adjustment of linearity correction capacitor]
Next, adjustment of the first and second linearity correcting capacitors 43 and 44 according to the third embodiment will be described with reference to FIGS. FIG. 7 is a flowchart showing the linearity correction processing procedure of the capacitive pressure measuring device shown in FIG. First, the detection unit 7 is connected to the voltage measurement terminals 90 to 94. In the state where the differential pressures p0, p1, and p2 are applied to the first and second sensor capacitors, the detection unit 7 determines the voltage between the voltage measurement terminals 90 and 91 as the first voltage V1 (V1 (p0), V1). (P1), V1 (p2)), the voltage between the voltage measuring terminals 90, 92 is detected as the second voltage Vc, and the voltage between the voltage measuring terminals 90, 94 is detected as the third voltage V3 (V3 (V3 ( p0), V3 (p1), V3 (p2)), and the voltage between the voltage measuring terminals 92 and 93 is observed to determine the oscillation frequency f and the amplitude of the oscillation voltage Vm (Vm (p0), Vm (p1)). , Vm (p2)) is detected (step S301).

  Thereafter, the stray capacitance calculator 8 detects the first voltage V1 (V1 (p0), V1 (p1), V1 (p2)), the second voltage Vc, and the third voltage V3 (V3 (V3 ( p0), V3 (p1), V3 (p2)), oscillation frequency f, oscillation voltage amplitude Vm (Vm (p0), Vm (p1), Vm (p2)), as well as known resistances 35 to 37 The stray capacitances Cs1 and Cs2 are calculated using the resistance values R35 to R37, the known capacitances C43 and C44 of the first and second linearity correction capacitors 43 and 44, and the known ratio r (step S302). .

The calculation formulas of C1 (p) and C2 (p) at this time are the following formulas (58) and (59) corresponding to formulas (38) and (39).
C1 (p) = V1 (p) / R35 / {f (2Vm (p) −V1 (p) −Vc)}
+ C43 / (1 + f · R49 · C43) (58)
C2 (p) = {V1 (p) -V3 (p)} / R36 / {f (2Vm (p) -V1 (p)
−Vc)} + C44 / (1 + f · R50 · C44) (59)

Thereafter, using the stray capacitances Cs1 and Cs2 calculated in step S302, the effective capacitance C43 / (1 + f · R49 ·) is obtained with respect to the capacitances C43 and C44 of the first and second linearity correction capacitors 43 and 44. The variable resistors 49 and 50 are adjusted so that C43) and C44 / (1 + f · R50 · C44) become the stray capacitances Cs1 and Cs2, respectively (step S303). In other words, the resistance values R49 and R50 of the variable resistors 49 and 50 are adjusted to satisfy the following expressions (60) and (61).
R49 = (C43−CS1) / (f · C43 · Cs1) (60)
R50 = (C44−CS2) / (f · C44 · Cs2) (61)

DESCRIPTION OF SYMBOLS 1 1st sensor capacitor 2 2nd sensor capacitor 3 Oscillation circuit 4 Charging / discharging circuit 5 Oscillation circuit control circuit 6 Output control circuit 7 Detection part 8 Stray capacitance calculation part 9,35-37,39,41,42,59- 62 Resistor 10, 11, 12, 72, 99 Winding 13-16, 45-48 Diode 20-22, 51, 52 Rectifier capacitor 40, 63 Operational amplifier 43 First linearity correcting capacitor 44 Second linearity Correction capacitor 49, 50 Variable resistance 67 Transistor 82 Zener diode 90-94 Voltage measurement terminal 101 Constant voltage circuit 102 Reference voltage source 111 Cutoff switch C1, C2, C43, C44, C43 ', C44' Capacitance Cs1, Cs2 Floating capacitance f Oscillation frequency I1, I2, Ic1, Ic2 Charge / discharge current Δ1 First current (= I -Ic1)
Δ2 Second current (= I2−Ic2)
Ik Sum current (= Δ1 + Δ2)
If feedback current If 'current flowing through resistor 61 If1 current flowing through resistor 59 If2 current flowing through resistor 69 If3 current flowing through resistor 60 If4 total current of current If1 and current If2 Io output current p, p0, p1, p2 differential pressure V Power supply V1, V2 First voltage Vc Second voltage V3 Third voltage Vm Oscillation voltage amplitude V4 Voltage generated across resistor 61 Vz Zener voltage of Zener diode 82 Vps Circuit voltage of constant voltage circuit 101 R35 to R37, R39, R41, R42, R49, R50, R59 to R62,
R69 Resistance value ε Dielectric constant between the electrodes of the first and second sensor capacitors 1 and 2 A Electrode area of the first and second sensor capacitors 1 and 2 d Fixing of the first and second sensor capacitors 1 and 2 ½ value of distance between electrodes δ Amount of displacement Δ (p) when there is no differential pressure of a movable electrode such as a diaphragm shared by the first and second sensor capacitors 1 and 2 (p = 0) Displacement of a movable electrode such as a diaphragm shared by the second sensor capacitors 1 and 2 at the time of differential pressure p k Proportional coefficient of displacement Δ (p) with respect to differential pressure p r Difference between differential pressures p1 and p0 and p2 and p1 Ratio (= (p1-p0) / (p2-p1))

Claims (6)

An electrostatic current that outputs a displacement amount corresponding to the magnitude of a differential pressure between the first pressure and the second pressure as a change in differential capacitance between the first sensor capacitor and the second sensor capacitor. A capacitive pressure measuring device comprising:
First and second linearity correcting capacitors for correcting linearity of the displacement with respect to the differential pressure;
An oscillation circuit;
The oscillating voltage output from the oscillation circuit is rectified and is sized to the first sensor capacitor, the second sensor capacitor, the first linearity correction capacitor, and the second linearity correction capacitor. The charging and discharging to be charged and then discharged are repeated, and the charge / discharge current rectified to the first linearity correction capacitor is determined from the rectified value of the charge / discharge current to the first sensor capacitor. A first current obtained by subtracting a value; a second current obtained by subtracting a rectified value of charge / discharge current for the second linearity correcting capacitor from a rectified value of charge / discharge current for the second sensor capacitor; A charge / discharge circuit for generating a sum of current and the second current;
An oscillation circuit control circuit for controlling the oscillation frequency and / or the amplitude of the oscillation voltage of the oscillation circuit so that the sum current is constant;
An output control circuit that outputs an output current corresponding to a difference between the first current and the second current;
The oscillation frequency of the oscillation circuit, the amplitude of the oscillation voltage, the first voltage corresponding to the first current and the second current, the second voltage corresponding to the sum of the first current and the second current, the first current And a detection unit for detecting a third voltage corresponding to the difference between the second currents;
Based on the oscillation frequency, the amplitude of the oscillation voltage, the first voltage, the second voltage, and the third voltage when a plurality of pressure differences are applied from the outside, the first sensor capacitor first A stray capacitance calculator for calculating stray capacitance and a second stray capacitance of the second sensor capacitor;
With
The capacitance of the first linearity correction capacitor is adjusted based on the first stray capacitance calculated by the stray capacitance calculation unit, and the second stray capacitance calculated by the stray capacitance calculation unit is used to adjust the capacitance. A capacitance-type pressure measuring device that adjusts the capacitance of the second linearity correcting capacitor.   Provided is a cut-off switch that cuts off the current flowing through the first and second linearity correction capacitors, and the cut-off switch is cut off, based on the first stray capacitance calculated by the stray capacitance calculation unit. Adjusting the capacitance of the first linearity correction capacitor and adjusting the capacitance of the second linearity correction capacitor based on the second floating capacitance calculated by the floating capacitance calculation unit; The capacitive pressure measuring device according to claim 1, wherein Capacitances of the first and second linearity correcting capacitors are fixed,
Connecting the first and second variable series resistors and the first and second rectifying capacitors to each of the first and second linearity correcting capacitors;
The first variable series resistance is adjusted based on the first stray capacitance calculated by the stray capacitance calculation unit, the effective capacitance of the first linearity correction capacitor is adjusted, and the stray capacitance calculation unit 2. The effective capacitance of the second linearity correction capacitor is adjusted by adjusting a second variable series resistance based on the second stray capacitance calculated by the method of claim 1. Capacitance type pressure measuring device. First and second sensor capacitors;
An oscillation circuit;
The oscillation voltage output from the oscillation circuit is rectified to have the same magnitude as the first sensor capacitor, the second sensor capacitor, the first linearity correction capacitor, and the second linearity correction capacitor. The charging / discharging is repeated by applying and charging as an alternating voltage, and the rectification value of the charging / discharging current for the first linearity correction capacitor is determined from the rectification value of the charging / discharging current for the first sensor capacitor. A first current obtained by subtracting, a second current obtained by subtracting a rectified value of charge / discharge current for the second linearity correcting capacitor from a rectified value of charge / discharge current for the second sensor capacitor, and the first current, A charge / discharge circuit for generating a sum current with the second current;
An oscillation circuit control circuit for controlling the oscillation frequency and / or the amplitude of the oscillation voltage of the oscillation circuit so that the sum current is constant;
An output control circuit that outputs an output current corresponding to a difference between the first current and the second current;
With
A displacement amount corresponding to the magnitude of the differential pressure between the first pressure and the second pressure is output as a change in differential capacitance caused by the first sensor capacitor and the second sensor capacitor. A linearity correction method for a capacitance-type pressure measuring device,
The oscillation frequency of the oscillation circuit, the oscillation voltage amplitude, the first voltage corresponding to the first current and the second current, the first current and the second current in a state where a plurality of pressure differences are applied from the outside. A detection step of detecting a second voltage corresponding to a sum, a third voltage corresponding to a difference between the first current and the second current;
Based on the oscillation frequency, the amplitude of the oscillation voltage, the first voltage, the second voltage, and the third voltage, the first stray capacitance of the first sensor capacitor and the second of the second sensor capacitor 2 stray capacitance calculation step for calculating stray capacitance;
Including
The capacitance of the first linearity correction capacitor is adjusted based on the first stray capacitance calculated in the stray capacitance calculation step, and the second stray capacitance calculated in the stray capacitance calculation step is used to adjust the capacitance. A method for correcting linearity of a capacitance-type pressure measuring device, comprising adjusting the capacitance of a second linearity correcting capacitor. A current interruption step of interrupting a current flowing through the first and second linearity correction capacitors;
In the state where the current is interrupted in the current interrupting step, the capacitance of the first linearity correction capacitor is adjusted based on the first stray capacitance calculated in the stray capacitance calculating step, and the stray capacitance calculation is performed. 5. The linearity correction of the capacitance type pressure measuring device according to claim 4, wherein the capacitance of the second linearity correction capacitor is adjusted based on the second stray capacitance calculated in the step. Method. Capacitances of the first and second linearity correcting capacitors are fixed,
The first and second variable series resistors and the first and second rectifying capacitors are connected to the first and second linearity correcting capacitors, respectively.
The first variable series resistance is adjusted based on the first stray capacitance calculated in the stray capacitance calculation step, the effective capacitance of the first linearity correction capacitor is adjusted, and the stray capacitance calculation step 5. The effective capacitance of the second linearity correction capacitor is adjusted by adjusting a second variable series resistance based on the second stray capacitance calculated by the method of claim 4. Linearity correction method for capacitance pressure measuring device.

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