JP2000258272A - Capacitive pressure sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a general purpose capacitive pressure sensor in which the effect of parasitic capacitance can be corrected with high accuracy at low cost. SOLUTION: A constant capacitor Cc having a reference capacitance is provided and the capacitance is detected by switching the constant capacitor and capacitors C1-Cn for pressure detection. Effect of parasitic capacitance is then corrected based on the detection results of the constant capacitor Cc. More than one reference voltage Vref having different temperature coefficient and a set voltage Vth are provided and combined arbitrarily such that temperature dependence of the capacitors C1-Cn is substantially nullified at the pressure sensor section 1. Pressure can be measured with high accuracy by detecting the temperature while increasing temperature dependence of the constant capacitor Cc. When two constant capacitors Cc1, Cc2 are employed, correction can be made with higher accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitance type pressure sensor for determining the magnitude of pressure based on a change in capacitance due to applied pressure.

[0002]

2. Description of the Related Art A capacitance type pressure sensor which detects a pressure or a differential pressure as a change in a diaphragm and directly outputs a displacement as a current value, detects a displacement of the diaphragm caused by a pressure or a pressure difference applied to the diaphragm. The pressure is determined by detecting the change in the capacitance value of a capacitor formed with the counter electrode.

In a capacitance type pressure sensor (hereinafter referred to as a pressure sensor) for detecting a displacement of a distance between the diaphragm and a counter electrode as a capacitance value, the capacitance of a capacitor caused by a pressure to be measured or a pressure difference is measured. Since the change in the capacitance value is a small value of about 10 pF to about 100 pF, there is a problem that the influence of the stray capacitance is large and the output characteristic becomes non-linear or a measurement error occurs due to the influence.

To solve these problems, for example, there is a method disclosed in Japanese Patent Application Laid-Open No. 64-71211. The method disclosed in the above publication is to measure by combining two capacitors, and to eliminate the influence of the parasitic capacitance based on the measurement results of each.

FIG. 9 shows a circuit according to this method. FIG. 9 shows a CR transmission circuit. C1 and C2 in FIG. 9 correspond to the capacitors in the pressure sensor, and the pressure or the differential pressure is obtained by obtaining the capacitance values of C1 and C2. 9 is determined by the resistance value R505 (fixed) and the value of the capacitance C.

[0006] Each of the capacitors C1 and C2 in the figure can be switched between ground and non-ground by switches 503 and 504 by transistors, whereby the capacitors can be changed from active / inactive to a transmission circuit. By operating the switches 503 and 504, connection and disconnection of the two capacitors C1 and C2 are selected, and the cycle of each output is measured.

The relationship between this cycle and the capacitance value of the capacitor can be expressed by the following equation. Select only C1: T1 = 1 / f1 = (C1 + Cp) · k ·
R505 Select only C2: T2 = 1 / f2 = (C2 + Cp) .k.
R505 Not select both: T3 = 1 / f3 = Cp · k · R505 Select both: T4 = 1 / f4 = (C1 + C2 + Cp) · k
R505 In the above equation, Cp indicates a parasitic capacitance, and k indicates a coefficient unique to each circuit. Using the periods T1 to T4 of the outputs measured in these four equations, the parasitic capacitance Cp and the coefficient k are deleted, and the capacitance value C
Find the ratio of 1 to C2.

[0008]

However, in the above method, in order to eliminate the influence of the parasitic capacitance, the value of the parasitic capacitance Cp in the above four equations obtained by selecting the two capacitors C1 and C2 is required. They must all be the same value.
In other words, the difference between the respective parasitic capacitances between the electrodes C1 and C2 must be zero.

However, it is theoretically impossible to make the parasitic capacitance generated between the electrodes C1 and C2 zero, and even if the adjustment can be performed so that the parasitic capacitance disappears at a certain specific temperature, the value of C1 is not increased. , C2 have different temperature characteristics of the parasitic capacitance, so that an output error occurs when the temperature changes.

The method disclosed in Japanese Patent Application Laid-Open No. 64-71211 is a method in which current supplied to two capacitors is adjusted by a resistor, and active / inactive of the two capacitors is switched by a transistor switch. The currents supplied to the capacitors C1 and C2 differ due to the difference in the on-resistance of each transistor switch and the difference in the temperature dependency, and an output error occurs.

A method of canceling the parasitic capacitance by using a fixed capacitor equivalent to the parasitic capacitance is generally known. However, since this fixed capacitor is usually arranged at a place away from the pressure sensor, such as on a Pt substrate, a temperature difference occurs from the pressure sensor, and a correction error occurs due to a difference in thermal gradient between the fixed capacitor and the pressure sensor. There was a problem.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a low-cost and versatile capacitance-type pressure sensor capable of removing the effect of temperature-induced parasitic capacitance with high accuracy. And

[0013]

According to a first aspect of the present invention, there is provided a capacitance type pressure sensor in which one or more capacitors are formed by electrodes and a diaphragm. , Pressure or differential pressure as a change in capacitance of the capacitor, and includes a constant current unit, a fixed capacitor, a switch unit, a charge / discharge switching unit, and a voltage comparison unit. The constant current means outputs a constant current. The fixed capacitor is a capacitor having a fixed capacitance. The switch means selects at least one of the capacitor or the fixed capacitor. The charge / discharge switching means switches between discharging of the capacitor selected by the switching means and charging with the constant current. The voltage comparison means instructs the switch means to charge if the potential of the capacitor selected by the switch means has not reached the set voltage, and to discharge if the potential of the capacitor has reached the set voltage.

According to the present invention, in addition to the capacitance of the capacitor for detecting pressure or differential pressure, a measurement result by a fixed capacitor having a fixed capacitance can be obtained. In the capacitance type pressure sensor according to claim 2 of the present invention, the electrode and the diaphragm form at least one set of capacitors whose capacitance is relatively changed by the displacement of the diaphragm. Is detected as a change in the capacitance, and includes a constant current unit, a fixed capacitor, a switch unit, a charge / discharge switching unit, and a voltage comparison unit. The constant current means outputs a constant current. The fixed capacitor is a capacitor having a fixed capacitance. The switch means selects at least one of the capacitor or the fixed capacitor. The charge / discharge switching means is
The switching means switches between discharging of the selected capacitor and charging with the constant current. The voltage comparison means instructs the switch means to charge if the potential of the capacitor selected by the switch means has not reached the set voltage, and to discharge if the potential of the capacitor has reached the set voltage.

The invention according to claim 2 is characterized in that a capacitor is provided as a pair, and in addition to the capacitance of the capacitor for detecting pressure or differential pressure, a measurement result by a fixed capacitor having a fixed capacitance is obtained. can get.

According to a third aspect of the present invention, in the capacitance type pressure sensor, one or more capacitors are formed by the electrodes and the diaphragm, and the pressure or the differential pressure is detected as a change in the capacitance of the capacitor. And a switch means for selecting at least one of the constant current means, the plurality of fixed capacitors, the capacitor or each of the fixed capacitors, the charge / discharge switching means, and the voltage comparison means.

According to a third aspect of the present invention, a plurality of fixed capacitors having a fixed capacitance are provided in addition to the capacitance of the capacitor for detecting pressure or differential pressure. Is obtained.

In the capacitance type pressure sensor according to a fourth aspect of the present invention, the electrode and the diaphragm form at least one set of capacitors whose capacitance is relatively changed by displacement of the diaphragm. Alternatively, the differential pressure is detected as a change in the capacitance, and the constant current means, the plurality of fixed capacitors, a switch means for selecting at least one of the capacitors or the fixed capacitors, and the charge / discharge switching. Means and the voltage comparison means.

According to a fourth aspect of the present invention, a plurality of capacitors are provided in pairs, and a plurality of fixed capacitors are provided. Can be obtained by a plurality of fixed capacitors.

According to a fifth aspect of the present invention, in the capacitance type pressure sensor according to any one of the first to fourth aspects, the switch is selected as the switch means. An operation control means for instructing a capacitor and correcting the parasitic capacitance by using a cycle of an instruction to the switch means by the voltage comparison means for each of the selected capacitors is further provided.

According to the fifth aspect of the present invention, the measurement result when the switch means selects the capacitor can be corrected for the parasitic capacitance by the arithmetic control means using the measurement result when the fixed capacitor is selected. .

According to a sixth aspect of the present invention, there is provided a capacitance type pressure sensor according to any one of the first to fifth aspects, wherein the constant current means comprises:
Outputting the constant current of a magnitude based on the set input voltage; and when the switch means selects one of the capacitors, the input voltage and the input voltage having a temperature coefficient that cancels the temperature coefficient of the capacitor. A set voltage is provided to the constant current means and the voltage comparison means. According to the present invention, when the pressure detecting capacitor is selected by the switch means, the temperature coefficient of the selected capacitor is canceled.

According to a seventh aspect of the present invention, there is provided a capacitance type pressure sensor according to any one of the first to fifth aspects.
The input voltage having a temperature coefficient smaller than the temperature coefficient of the fixed capacitor when the constant current is output based on the set input voltage and the switch means selects one of the capacitors. And the set voltage are supplied to the constant current means and the voltage comparison means. According to the seventh aspect of the present invention, when the pressure detecting capacitor is selected by the switch means, the temperature sensitivity of the signal proportional to the fixed capacitor can be increased.

According to another aspect of the present invention, there is provided a capacitance type pressure sensor according to any one of claims 1 to 7, wherein the constant current means comprises:
The constant current having a magnitude based on the set input voltage is output, and when the switch means selects the fixed capacitor, the input voltage having a temperature coefficient larger than the temperature coefficient of the fixed capacitor and the setting are set. A voltage is applied to the constant current means and the voltage comparison means.

According to the eighth aspect of the invention, it is possible to increase the temperature sensitivity of a signal proportional to the fixed capacitor. According to a ninth aspect of the present invention, in the capacitance type pressure sensor according to any one of the first to seventh aspects, the fixed capacitor is provided under the same environment as the capacitor. It is characterized by being arranged in.

According to the ninth aspect of the present invention, the fixed capacitor and the capacitor can maintain the same condition even when the environment changes. According to a tenth aspect of the present invention, an electrode and a diaphragm form one or more capacitors, and the pressure or the differential pressure is detected from a pressure sensor that detects a change in the capacitance of the capacitor. And outputs a change in the period as a change in the cycle, and comprises a constant current unit, a switch unit, a charge / discharge switching unit, a voltage comparison unit, and an output unit. The switch means selects at least one of a capacitor in the capacitance type pressure sensor or one or more fixed capacitors having a fixed capacitance. The charge / discharge switching means switches between discharging of the capacitor selected by the switching means and charging with the constant current. The voltage comparing means instructs the charging means if the potential of the capacitor selected by the switching means has not reached the set voltage, and the discharging means if the potential of the capacitor has reached the set voltage. Is output as a detection signal.

According to the tenth aspect of the present invention, it is possible to obtain detection results for the capacitance from the pressure sensor and the capacitance from the fixed capacitor. According to an eleventh aspect of the present invention, in the electrostatic capacitance detecting device according to the tenth aspect, the constant current means, the switch means, the charge / discharge switching means, the voltage comparison means, the output means, and the fixed capacitor are provided. It is characterized by being configured on the same IC.

According to the eleventh aspect of the present invention, each of the means and the fixed condenser can be placed in the same environment as the pressure sensor.

[0029]

FIG. 1 is a block diagram showing the principle of operation of a capacitance type pressure sensor according to the present invention. The capacitance type pressure sensor of FIG. 1 includes a pressure sensor unit 1, a detection unit 2, a counting unit 3, and a calculation control unit 4.

The pressure sensor unit 1 has a structure in which one to three or more capacitors C1 to Cn are provided inside, and the diaphragm is displaced by pressure applied to a sealed fluid.
The pressure is obtained by a change in the capacitance of the capacitor based on the displacement amount.

The detecting unit 2 converts a change in the capacitance of the capacitor in the pressure sensor unit 1 into a periodic output pulse.
A switch unit 21 for switching the capacitors C1 to Cn in the pressure sensor, a capacitance-frequency conversion unit 22 for converting the capacitance of the capacitor of the selected pressure sensor into a frequency signal, a fixed capacitor Cc having a fixed capacitance, and the like. . Among them, the fixed capacitor Cc serves as a reference capacitance whose capacitance does not change due to pressure, and the effect of the parasitic capacitance is corrected using this fixed capacitor Cc. Further, a configuration is adopted in which a reference capacitor corresponding to the fixed capacitor Cc is filled in the pressure sensor unit 1 with the same sealing liquid as the capacitors C1 to Cn, and the capacitance does not change with pressure. You can also. However, in the case of this configuration, in the case of silicon oil, which is generally used as a sealing liquid for the pressure sensor, since the relative dielectric constant has temperature and electrostatic dependence, a sensor that detects only static pressure is required. Increase costs.

The counting section 3 counts the output pulse of the detecting section 2 by a counter circuit or the like and converts it into a digital value. The operation control unit 4 performs output operation correction based on the counting result of the counting unit 3 and controls the switch unit 201.

In the capacitance type pressure sensor shown in FIG. 1, the switch switching unit 11 of the detection unit 2 switches and selects the capacitors C1 to Cn and the capacitor Cc in the pressure sensor in response to the pressure applied to the pressure sensor unit 1. The capacitance value is converted into a cycle of a rectangular wave and output. The output to each capacitor is counted by the counting unit 3 and based on the result,
The calculation control unit 4 performs output calculation correction and control of the switch unit 201.

FIG. 2 is a sectional view showing the structure of the pressure sensor unit 1 when a pair of capacitors is formed. In the capacitance type pressure sensor of the present invention, the pressure detection capacitor is 1
In the following description, a configuration including a pair of capacitors whose capacitance is relatively changed via a diaphragm, which is generally used in a capacitance type pressure sensor, will be described. .

In FIG. 1, a silicon substrate 10 is used as a conductive substrate, and has a suitable electrical conductivity, for example, a P-type silicon wafer having a [100] plane having a crystal plane orientation of about 0.1 Ωcm. On top of the gaps 103a and 103
A circular recess corresponding to 3b and a ring groove for forming a ring-shaped thin portion 102 on the outside thereof are formed by plasma etching twice or more on each surface. A flat portion 1 thicker than the ring-shaped thin portion 102
01 is formed, and the flat portion 101 and the thin portion 102 constitute a diaphragm that is displaced by a pressure difference. This diaphragm becomes a movable electrode of the pressure sensor unit 1. Note that the thicknesses of the silicon substrate 10 and the thin portion 102 are appropriately determined depending on the pressure to be measured or the magnitude of the pressure difference.

On both sides of the silicon substrate 10, insulating substrates 11a and 11b made of a mullite-cordierite ceramic are joined by electrostatic joining. On the insulating substrates 11a and 11b, fixed electrodes 12a and 12b and electrodes 13a and 13b are provided, respectively.
It is formed by mask sputtering of three layers of chromium, platinum and gold. The fixed electrodes 12a and 12b and the electrodes 13a and 13b are connected to a pressure guiding port 14a through which pressure is introduced.
And 14b are connected through the inner wall.

The two fixed electrodes 12a and 12b,
With the flat part 101 of the silicon substrate 11 that becomes a movable electrode,
A pair of capacitors C1, whose capacitance values change relatively,
C2 is formed.

A 1.6 μm thick Pyrex glass sputter film (not shown) is formed on each of the electrostatic bonding surfaces of the insulating substrates 11a and 11b. It becomes possible. The reason for using mullite-cordierite ceramics for the insulating substrates 11a and 11b is that ceramics having a thermal expansion coefficient very close to that of silicon and having high rigidity can be obtained. .

On the lower surface of the insulating substrate 11a, an insulating member 17 made of a mullite-cordierite ceramic having a through hole 171 in the center is provided with the through hole 171 and the pressure-guiding port 1.
4a is positioned so as to conduct, and is joined by gold-tin eutectic solder. On the upper and lower surfaces of the insulating member 17, three sputtered films of chromium, platinum, and gold for soldering are formed.

The lower surface of the insulating member 17 has a through hole 1 at the center.
The base 18 made of stainless steel having the through hole 81
81 and the pressure-guiding port 14a and the through-hole 171 are positioned so as to conduct, and are joined by gold-tin eutectic solder. A three-layer sputtered film of chromium, platinum, and gold is formed on a portion of the base 18 to be joined to the insulating member 17 for soldering.

The through holes 171 and 181 and the pressure guiding port 1
4a, the enclosed pressure transmitting fluid 19
Is transmitted to the lower surface of the diaphragm. On the other hand, a through hole is formed at one corner of a joint portion with the silicon substrate 10 on the insulating substrate 11b. After joining both substrates, the electrode 1 is formed in the through hole by aluminum mask sputtering.
6 are formed. This electrode 16 serves as an extraction electrode from the silicon substrate 10.

The silicon substrate 10 of the insulating substrate 11b
On the surface opposite to the bonding surface with the silicon substrate 10, a detection unit 2 of a bare chip is formed on a region corresponding to the bonding surface with the silicon substrate 10. The area where the detection section 2 is placed is the area having the highest rigidity in the pressure sensor section 1. Further, since the distance between the pair of capacitors C1 and C2 is short, the connection wiring can be shortened, and the floating due to the wiring can be achieved. This is an area where the capacity is small. The detection unit 2 includes fixed electrodes 12a and 12a.
b and 16 are connected by an aluminum wire. The fixed capacitor Cc may be built in the bare chip of the detection unit 2 or may be arranged near the bare chip so that the temperature environment is the same as that of the capacitors C1 and C2.

In the vicinity of the pressure sensor unit 1 and the upper surface of the diaphragm, a pressure P2 with respect to the pressure P1 guided through a through hole provided in the base 18 or the like is transmitted to the upper surface of the diaphragm, not shown. A pressure transmitting fluid is enclosed.

The thin portion 102 is deformed by the difference between the pressure P1 and the pressure P2 applied to both surfaces of the diaphragm, and the flat portion 1 is deformed.
01 is displaced perpendicular to the fixed electrodes 12a and 12b. The capacitance values C1 and C2 relatively change with the change in the distance between the flat portion 101, which is the movable electrode, and the fixed electrodes 12a and 12b due to the displacement. By measuring this value, the difference between the pressures P1 and P2 is detected.

In the pressure sensor 1, the pressure P1
If one of P2 and P2 is set to the atmospheric pressure, it becomes a gauge pressure sensor, and if one is set to a vacuum, it becomes an absolute pressure sensor. In the configuration as shown in FIG. 2, the magnitude relationship between the introduction pressures P1 and P2 is arbitrary, but in the following description, P1 <
The description will be made assuming that P2 is reached.

When a pressure difference P1 <P2 is applied, the diaphragm is displaced in the direction of the arrow in FIG. The capacitances C1 and C2 based on the displacement of the diaphragm are detected by the detecting unit 2 and the counting unit 3, respectively, and the results are used to calculate the following equation (1) by the calculation control unit 4 to calculate the displacement of the diaphragm. Is required.

[C1−C2− (Cs1−Cs2)] / [C1 + C2− (Cs1 + Cs2)] = △ d1 / d1 (1) (1) where d1 represents the diaphragm 21 and the fixed electrode 31a or 32b. , Cs1 is the parasitic capacitance of the capacitor C1, Cs2 is the parasitic capacitance of the capacitor C2, △
d1 indicates the amount of displacement of the diaphragm 21 due to the applied pressure.

Here, the detection signal Tref and the coefficients A and B when the fixed capacitor Cc is selected in the operating temperature range are determined so that the following equations (2), (3) and (4) are satisfied.
By substituting Equations (4) into (1), the following Equation (5) is obtained.

(Cs1−Cs2) = A / C · f (Tref) (2) (Cs1 + Cs2) = B / C · f (Tref) (3) T1 = C · Cs1, T2 = C Cs2 (4) (T1-T2-Af (Tref)) / (T1 + T2-Bf (Tref)) = Δd1 / d1 (5) T1: The capacitance of the capacitor C1 T2: cycle of the output signal according to the capacitance of the capacitor C2 f (Tref): function of the output signal according to the capacitance of the fixed capacitor Cc A, B, C: coefficient ), The displacement of the diaphragm is obtained by using the measurement result when the switch unit 210 selects the fixed capacitor Cc.

FIG. 3 is a configuration diagram of the capacitance type pressure sensor according to the first embodiment. The pressure sensor unit 1 in FIG.
Is simplified here except for the capacitors C1 and C2.

In the figure, a detection unit 2 includes a switch unit 21, a fixed capacitor Cc having a constant capacitance, a constant current circuit unit 23 corresponding to the capacitance-frequency conversion unit 22, a current mirror circuit 24, a comparator unit. 25 and a mono-multi unit 26.

The switch section 21 switches between the capacitors C1 and C2 and the fixed capacitor Cc in accordance with an instruction from the arithmetic and control section 4, and includes, for example, MOS transistor switches 211 to 213 and a switch selection circuit 214.

The constant current circuit section 23 includes the input voltage generation circuit 2
31, operational amplifier 232, field effect transistor 23
3. It is composed of a resistor R234. The constant current circuit section 23 has a voltage V
The operational amplifier 232 controls the field effect transistor 233 so that ref is equal to the voltage (the voltage at point a in the figure) generated by the current IL flowing through the resistor 234. As a result, the current IL becomes a constant value, Vref / R234.

The current mirror circuit 24 stably outputs a current Ic equal to the current IL generated by the constant current circuit section 23, and is composed of a MOS transistor. The comparator unit 25 includes a set voltage generation circuit 251 and a comparator 252, and the input voltage is set to the set voltage generation circuit 2
When the voltage reaches the set voltage Vth set by the comparator 51 in the comparator 252, the output is changed from “L” level to “H” level.

The mono-multi unit 26 includes a comparator 25
ON / OF of the transistor switch 29 depending on the output
F. When the output of the comparator unit 25 becomes “H” level, the transistor switch 29 is set to “O”.
When the output goes to "L" level at "N", it is turned "OFF".

In the capacitance type pressure sensor of FIG. 3, the switch selection circuit 214 operates the switches 211 to 213 on / off based on the instruction from the arithmetic and control unit 4 to switch the capacitor C1 in the pressure sensor unit 1. And C2, and a fixed-capacitance capacitor Cc. First, it is assumed that the capacitor C1 is selected.

As the output current Ic of the current mirror circuit 24 charges the capacitor selected by the switch section 21, the potential of the capacitor (potential at point b in the figure) increases accordingly. Then, when the voltage at the point b reaches the set voltage Vth of the comparator section 25, the output of the comparator 252 turns on from "L" level to "H" level.

Assuming that the charging time of the capacitor C1 until the potential reaches the set voltage Vth when the capacitor C1 is selected is tc1, tc1 is expressed by the following equation. tc1 = C1 · Vth / Ic = C1 · R234 · V
th / Vref (6) where Vth = 2 · V
ref, tc1 is the capacitance value C1 as shown in equation (7).
And a resistance value R234 only.

Tc1 = 2 · C1 · R234 (7) The mono-multi unit 26 switches the transistor switch 29 at the moment when the output of the comparator unit 25 becomes “H” level.
To “ON”. Then, the point b is grounded to Vss, the capacitor C1 is discharged, and the potential at the point b becomes zero.
Then, the comparator unit 25 sets the input voltage to the set voltage Vt.
h, the output is turned off to the “L” level. In response, the mono-multi unit 26 turns the transistor switch 29 “OFF”.

As a result, the capacitor C1 returns to the current Ic.
When the point b reaches the set voltage Vth, the transistor switch 29 is turned "ON" again, and the capacitor C1 is discharged. Thereafter, such a charging / discharging operation is repeated, and the detecting unit 2 outputs a square wave signal having a constant cycle in proportion to the capacitance C1. The period t of this output signal
1 is represented by the following equation.

T1 = tc1 + tr = 2 · C1 · R234 + tr (8) (8) In the equation, tr is a value after the mono-multi unit 26 sets the transistor switch 29 to “ON” to start discharging the capacitor. This is a reset time until the switch 29 is turned “OFF”.

When the detection of the cycle of the output signal is completed, the arithmetic and control unit 4 operates the switches 211 to 213 according to the instruction from the switch selection circuit 21 to sequentially change the capacitors to be selected. As a result, an output signal having a period proportional to the capacitor C2 and the fixed capacitor Cc is obtained from the same operation as that of the capacitor C1. The cycle t2 of the output signal when the capacitor C2 is selected and the cycle tref when the fixed capacitor Cc is selected can be expressed by the following equations (9) and (10).

T2 = tc2 + tr = 2 · C2 · R234 + tr (9) tref = tcref + tr = 2 · Cc · R234 + tr (10) Note that the mono-multi unit 26 in the equations (8) to (10) is Switch 16
Since the reset time tr from turning "ON" to turning "OFF" is about several 100 nsec, there is almost no effect on the linearity and resolution of the output signal.

The rectangular wave output signal of the detecting section 2 is input to the counting section 3 and counted by the counting section 3. The counting result is represented by the following equations (11), (12), and (13). T1 = n · t1 · fclk (11) T2 = n · t2 · fclk (12) Tref = n · tref · fclk (13) n: coefficient fclk: counting in the counting unit 3 Clock frequency On the basis of the counting result of the counting section 3, the calculation is performed by the calculation control section 4 using the above equations (1) to (4), and the displacement of the diaphragm corrected for the parasitic capacitance is obtained. As a result, it is possible to obtain an output that is not affected by the parasitic capacitance and is proportional to the diaphragm displacement of the capacitance type sensor.

As described above, in the configuration of FIG. 3, the transistor switches 29 and 2
Since the ON resistances 11 to 213 are not involved, even if the ON resistance of each transistor switch changes with temperature, the output signal is not affected at all.

Next, a second embodiment will be described. In the case of the method according to the first embodiment, the capacitance C1,
When the temperature coefficient of the resistor R234 is reduced in order to reduce the temperature dependence of the periods T1 and T2 of the output signal proportional to C2, the temperature coefficient of the fixed capacitor Cc is generally smaller than that of the resistor. The temperature sensitivity of the signal Tref proportional to Cc also decreases. The method according to the second embodiment takes this point into consideration.

FIG. 4 is a configuration diagram of a capacitance type pressure sensor according to the second embodiment. 4, the same components as those in the first embodiment in FIG. 3 are denoted by the same reference numerals. In the capacitance type pressure sensor of FIG.
At least two or more input voltage generation circuits 271-1 and 271-2
71-n. These have different voltage values, and among them, the voltage value selectively connected by operating the switch 275 by the switch selection circuit 214 is applied to the operational amplifier 272 as the set voltage Vref. With this configuration, the constant current circuit unit 27 can selectively output the current IL from two or more different current values according to an instruction from the arithmetic control unit 4.

The comparator section 28 also has at least two or more set voltage generating circuits 281 having different output voltages.
-1 to 281-n. Then, based on an instruction from the arithmetic control unit 4, the switch selection circuit 214 selects these by switching the switch 283,
The output value can be used as the set voltage Vth for the comparator 282. With this configuration, an appropriate input voltage Vref and set voltage Vth can be set for each capacitor.

Selection of the output current IL of the constant current circuit section 27 and the set voltage Vth of the comparator section 28 is determined according to the capacitor selected by the switch selection circuit 214.

Each input voltage generating circuit 271-1 to 271
-N and the voltages output from the set voltage generation circuits 281-1 to 281-n vary depending on the temperature even with the output from the same circuit. Therefore, assuming that these are functions of temperature, selection of the input voltage generation circuits 271-1 to 271-n and the setting voltage generation circuits 281-1 to 281-n accurately determines the temperature coefficient of the input voltage Vref and the setting voltage Vth. Means to choose. The temperature dependency of this voltage can be easily controlled by using the temperature characteristics of the base-emitter voltage of the transistor.

FIG. 5 is a table showing the relationship between the temperature coefficient of the input voltage Vref in the constant current circuit unit 27 and the temperature coefficient of the set voltage Vth of the comparator unit 28 for each selected capacitor.

As shown in FIG. 5, when the capacitor C1 or the capacitor C2 is selected, the arithmetic control unit 4 sets the input voltage Vt so as to cancel the temperature coefficient of the capacitor C1 or C2.
h and the set voltage Vref are set so that the input voltage generation circuits 271-1 to 271-n and the set voltage generation circuit 28 are set.
The selection is made from 1-1 to 281-n, and the switch selection circuit 214 is instructed. When the capacitor Cc is selected, a large temperature coefficient is set according to the detection resolution.

As a result, the input voltage Vth or the set voltage Vref set for the capacitors C1 and C2 becomes
A temperature coefficient for canceling the temperature coefficient of the capacitance or a voltage having a temperature coefficient of almost zero is selected, and the voltages Vref and Vth set for the fixed capacitor Cc for temperature detection are selected.
, A voltage having a large temperature coefficient is selected and set. Therefore, the temperature dependency can be reduced to almost zero with respect to the change in the capacitance of the pressure detecting capacitors C1 and C2, and the temperature dependency can be increased with respect to the change in the capacitance of the fixed capacitor Cc. Periods T1 and T2 of output signals from unit 2
For Tref and Tref, highly accurate temperature correction for parasitic capacitance can be realized. Next, a third embodiment will be described.

In the equations (3) and (4), the coefficients A and B for correcting the parasitic capacitance are usually determined at a plurality of arbitrary temperatures, and the coefficients A and B between the arbitrary correction points are obtained by approximation. And an error occurs due to approximation.

Since the approximation error can be reduced as the temperature difference between the correction points is smaller, it is necessary to increase the number of correction points to further improve the measurement accuracy.
There is a problem of cost increase.

The method according to the third embodiment takes this point into consideration. FIG. 6 is a configuration diagram of a capacitance type pressure sensor according to the third embodiment. The configuration of FIG.
It is based on the configuration of the first embodiment in FIG. 3, and the same components as those in FIG. 3 are given the same numbers.

In the capacitance type pressure sensor shown in FIG. 10, two fixed capacitors Cc1 and Cc2 are provided in the switch unit 30. Then, in accordance with an instruction from the arithmetic control unit 4, the switch selection circuit 305
By operating ON / OFF of .about.304, the capacitors C1 and C2 in the pressure sensor unit 1 and the capacitors Cc1 and Cc2 of the fixed capacity are sequentially selected to switch the connection.

In the capacitance type pressure sensor shown in FIG. 6, the fixed capacitors Cc1 and Cc1 corresponding to the equations (10) and (13) are used.
Cycles tref1, tref2 and coefficient section 3 when c2 is selected
The coefficient results Tref1 and Tref2 by (14) and (1
5) and (16), (17).

Tref1 = tc1ref + tr = 2 · Cc1 · R234 + tr (14) tref2 = tc2ref + tr = 2 · Cc2 · R234 + tr (15) Tref1 = n · tref1 · fclk (16) Tref2 = n tref2 · fclk (17) In addition, the signals proportional to the capacitances C1 and C2 expressed by the equations (8) and (9) are strictly expressed in terms of (18) and (19) in consideration of the measurement error.
It looks like an expression.

T11 = tc1 + terr = 2 · (C1 + Cerr) · R234 + tr · (18) t12 = tc2 + terr = 2 · (C2 + Cerr) · R234 + tr · (19) Of these, ter is expressed by equation (20).

Ter = ts + tr (20) Further, ts in the above equation can be expressed by equation (21). ts = 2 · Cs · R234 (21) In the third embodiment, a measured value of a signal proportional to Cc2 is used for the fixed capacitor Cc1 to correct the error Cerr.

The fixed capacitor Cc shown in the equations (14) and (15)
Considering Cerr in a signal proportional to 1, Cc2,
Equations (22) and (23) are obtained. tref11 = 2 · (Cc1 + Cerr) · R234 (22) tref12 = 2 · (Cc2 + Cerr) · R234 (23) Since the capacitance values of the fixed capacitors Cc1 and Cc2 are known, Cc2 is represented by the formula (24). Can be represented by

Cc2 = M · Cc1 (24) From the equations (22), (23) and (24), Cerr becomes the equation (25). Cerr = (M · tref11−tref22) / 2 / R234 / (M−1) (25) By applying the Cerr of the equation (25) to the equations (18) and (19), strict static Since a signal proportional to the capacitances C1 and C2 can be obtained, the arithmetic and control 4
By performing the output and correction calculations of the above equations (2) to (5), it is possible to obtain a more accurate output proportional to the diaphragm displacement of the capacitive pressure sensor without the influence of the parasitic capacitance. Becomes

Next, a fourth embodiment will be described. In the system of the third embodiment, as in the case of the system of the first embodiment, when the temperature coefficient of the resistor R234 is reduced to reduce the temperature dependence of the signal periods T1 and T2, the fixed capacitor Cc1 Signal Tref proportional to Cc2
1. The temperature sensitivity of Tref2 also decreases. The method according to the fourth embodiment takes this point into consideration.

FIG. 7 is a configuration diagram of a capacitance type pressure sensor according to the fourth embodiment. In FIG. 7, the same components as those in the second embodiment in FIG. 4 and the third embodiment in FIG. 6 are denoted by the same reference numerals.

In FIG. 7, based on the configuration of the third embodiment of FIG. 6, at least two or more input voltage generating circuits 2 of FIG. 4 are used instead of the constant current circuit section 23 and the comparator section 25.
A constant current circuit unit 27 having 71-1 to 271-n and a comparator unit 28 having at least two or more set voltage generation circuits 281-1 to 281-n having different output voltages are provided.

The constant current circuit section 27 operates the switch 275 in accordance with an instruction from the arithmetic control section 4 to operate the operational amplifier 27.
2 is changed, and the current IL flowing through the resistor R274 is selected from a plurality of different current values. In addition, the comparator unit 28 also selects the setting voltage Vth for the comparator 282 from a plurality of voltage values and changes the setting to an optimum voltage based on an instruction from the arithmetic control unit 4. With this configuration, in the capacitance type pressure sensor of FIG. 7, an appropriate input voltage Vref and set voltage Vth for the selected capacitor can be selected and set.

Each input voltage generation circuit 271-1 to 271-
n and the voltages output from the setting voltage generation circuits 281-1 to 281-n as a function of temperature, the input voltage generation circuits 271-1 to 271-n and the setting voltage generation circuit 28
Selection of 1-1 to 281-n means selecting the temperature coefficient of the input voltage Vref and the set voltage Vth. The temperature dependency of this voltage can be easily controlled by using the temperature characteristics of the base-emitter voltage of the transistor.

FIG. 8 is a table showing the relationship between the temperature coefficient of the input voltage Vref in the constant current circuit section 27 and the temperature coefficient of the set voltage Vth of the comparator section 28 for each selected capacitor.

As shown in FIG. 8, when the capacitor C1 or the capacitor C2 is selected, the arithmetic control unit 4 sets the input voltage Vt to cancel the temperature coefficient of the capacitor C1 or C2.
h and the set voltage Vref are set so that the input voltage generation circuits 271-1 to 271-n and the set voltage generation circuit 28 are set.
The selection is made from 1-1 to 281-n, and the switch selection circuit 214 is instructed. When the capacitor Cc is selected, a large temperature coefficient is set according to the detection resolution.

Thus, the input voltage Vth or the set voltage Vref set for the capacitors C1 and C2 is
A temperature coefficient for canceling the temperature coefficient of the capacitance or a voltage having a temperature coefficient of almost zero is selected, and a voltage Vref set for the fixed capacitors Cc1 and Cc2 for temperature detection.
And Vth, a voltage having a large temperature coefficient is selected and set. Therefore, it is possible to select a capacitor having almost zero temperature dependency with respect to a change in the capacitance of the pressure detecting capacitors C1 and C2, and a large temperature dependency with respect to a change in the capacitance of the fixed capacitor Cc. Therefore, it is possible to realize highly accurate temperature correction for the parasitic capacitance with respect to the periods T1, T2, Tref1, and Tref2 of the output signal from the detection unit 2.

When the output signal is obtained by the combination shown in FIG. 8, the error Cerr of the capacitance becomes an error factor. Therefore, by applying the equation (25) in the arithmetic and control unit 4, more accurate correction can be performed. It becomes possible.

[0093]

According to the capacitance type pressure sensor according to the present invention, it is possible to realize a capacitance type sensor which enables a high correction for a parasitic capacitance and enables a highly accurate pressure measurement.

Further, the capacitance type pressure sensor of the present invention performs the correction for the parasitic capacitance in consideration of the capacitance change due to the temperature change, so that accurate pressure detection over a wide temperature range can be realized with a simple configuration. it can.

Further, since the measured value of the capacitance is not affected by the on-resistance of the transistor switch, and no thermal noise occurs when a diffused resistor is used, a highly accurate pressure measurement is performed. I can do it.

Since the fixed capacitor and the pressure detecting capacitor can be placed in the same environment, the influence of the parasitic capacitance on changes in the environment such as temperature can be canceled with high accuracy.

[0097] Since it can be realized with a simpler configuration, pressure measurement can be performed at low cost and with high accuracy. Further, in the case where a configuration including a plurality of fixed capacitors is employed, more accurate correction including an approximation error can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram of a capacitance type sensor according to the present invention.

FIG. 2 is a sectional view showing a structure of a pressure sensor when a pair of capacitors is formed.

FIG. 3 is a configuration diagram of a capacitance type pressure sensor according to the first embodiment.

FIG. 4 is a configuration diagram of a capacitance type pressure sensor according to a second embodiment.

FIG. 5 is a table showing a relationship between a temperature coefficient of an input voltage and a temperature coefficient of a set voltage for a selected capacitor.

FIG. 6 is a configuration diagram of a capacitance type pressure sensor according to a third embodiment.

FIG. 7 is a configuration diagram of a capacitance type pressure sensor according to a fourth embodiment.

FIG. 8 is a table showing a relationship between a temperature coefficient of an input voltage and a temperature coefficient of a set voltage with respect to a selected capacitor in the fourth embodiment.

FIG. 9 is a circuit diagram showing a basic configuration of a conventional capacitance type pressure sensor.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 pressure sensor unit 2 detection unit 3 counting unit 4 arithmetic control unit 21, 30 switch unit 22 capacitance-frequency conversion unit 23, 27 constant current circuit unit 24 current mirror unit 25, 28 comparator unit 26 mono-multi unit C1, C2 Capacitor Cc, Cc1, Cc2 Fixed capacitor

Claims (11)

[Claims] A constant current means for outputting a constant current in a capacitance type pressure sensor for forming one or more capacitors by electrodes and a diaphragm and detecting a pressure or a differential pressure as a change in capacitance of the capacitor. A fixed capacitor having a fixed capacitance; switch means for selecting at least one of the capacitor and the fixed capacitor; charging for switching between discharging of the capacitor selected by the switch means and charging with the constant current. Discharge switching means, and voltage comparison means for instructing the switch means to charge until the potential of the capacitor selected by the switch means reaches a set voltage, and to discharge the switch means when the potential reaches the set voltage. Capacitive pressure sensor. 2. An electrostatic capacitance type wherein at least one set of capacitors whose capacitance is relatively changed by displacement of the diaphragm is formed by an electrode and a diaphragm, and a pressure or a differential pressure is detected as a change in the capacitance. In the pressure sensor, a constant current unit that outputs a constant current, a fixed capacitor having a fixed capacitance, a switch unit that selects at least one of the capacitor or the fixed capacitor, and a capacitor that is selected by the switch unit. Charge and discharge switching means for switching between discharging and charging with the constant current; and instructing the switch means to charge until the potential of the capacitor selected by the switch means reaches a set voltage, and to discharge when the potential reaches the set voltage. And a voltage comparing means. 3. A constant current means for outputting a constant current in an electrostatic capacitance type pressure sensor in which one or more capacitors are formed by electrodes and a diaphragm, and a pressure or a differential pressure is detected as a change in capacitance of the capacitor. A plurality of fixed capacitors having a fixed capacitance; switch means for selecting at least one of the capacitors or the fixed capacitors; discharging the capacitor selected by the switch means; and charging by the constant current. Charge / discharge switching means for switching between the two, and voltage comparing means for instructing the switch means to charge until the potential of the capacitor selected by the switch means reaches a set voltage, and to instruct the switch means to discharge when the potential reaches the set voltage. A capacitance type pressure sensor characterized by the above-mentioned. 4. An electrostatic capacitance type wherein at least one set of capacitors whose electrostatic capacitance relatively changes by displacement of the diaphragm is formed by the electrode and the diaphragm, and a pressure or a differential pressure is detected as a change in the electrostatic capacitance. In the pressure sensor, a constant current unit that outputs a constant current; a plurality of fixed capacitors having a fixed capacitance; a switch unit that selects at least one of the capacitors or each of the fixed capacitors; Charge / discharge switching means for switching between discharging of the obtained capacitor and charging with the constant current, charging until the potential of the capacitor selected by the switching means reaches a set voltage, and discharging when the potential reaches the set voltage. And a voltage comparison means for instructing the means. 5. An arithmetic and control unit for instructing the switch means to select a capacitor to be selected and for correcting a parasitic capacitance using a cycle of an instruction to the switch means by the voltage comparison means for each of the selected capacitors. The capacitance type pressure sensor according to any one of claims 1 to 4, wherein: 6. The constant current means outputs the constant current having a magnitude based on a set input voltage, and when the switch means selects one of the capacitors, a temperature coefficient of the capacitor is provided. The capacitance type pressure sensor according to any one of claims 1 to 5, wherein the input voltage and the set voltage having a temperature coefficient for canceling are provided to the constant current unit and the voltage comparison unit. 7. The constant current means outputs the constant current having a magnitude based on a set input voltage, and when the switch means selects one of the capacitors, the temperature of the fixed capacitor increases. The capacitance type pressure sensor according to any one of claims 1 to 5, wherein the input voltage and the set voltage having a temperature coefficient smaller than the coefficient are supplied to the constant current unit and the voltage comparison unit. . 8. The constant current means outputs the constant current having a magnitude based on a set input voltage, and is larger than a temperature coefficient of the fixed capacitor when the switch means selects the fixed capacitor. The capacitance type pressure sensor according to any one of claims 1 to 7, wherein the input voltage having a temperature coefficient and a set voltage are provided to the constant current unit and the voltage comparison unit. 9. The fixed capacitor according to claim 1, wherein the fixed capacitor is arranged in the same environment as the capacitor.
The capacitance type pressure sensor according to any one of the above items. 10. One or more capacitors are formed by electrodes and a diaphragm, and a change in the capacitance is output as a change in the period from a pressure sensor that detects a pressure or a differential pressure as a change in the capacitance of the capacitor. An electrostatic capacitance detection device, comprising: a constant current unit that outputs a constant current; and at least one of a capacitor or one or more fixed capacitors having a fixed capacitance in the capacitance type pressure sensor. Switch means; charge / discharge switching means for switching between discharging of the capacitor selected by the switch means and charging with the constant current; charging if the potential of the capacitor selected by the switch means has not reached a set voltage; A voltage comparison unit that instructs the switch unit to perform discharge when the set voltage is reached; and an instruction to the switch unit by the voltage comparison unit. The electrostatic capacitance detection device, characterized in that it comprises an output means for outputting as output signal. 11. The capacitance detecting device according to claim 10, wherein said constant current means, switch means, charge / discharge switching means, voltage comparison means, and fixed capacitor are configured on the same IC.