Displacement converting device and method for measuring pressure differences using same

A displacement converter in which a pressure difference is determined by detecting capacitances C1 and C2 between fixed electrodes 3,4 and a diaphragm 1, which receives pressure differences on both of its sides and can be displaced. The pressure difference is output as a process-unified signal via a V/I converter 209. This eliminates the need for tedious adjustments involving hardware such as a compensation capacitor in order to compensate the floating capacitance in capacitances C1, C2, which is unrelated to diaphragm displacement. Instead, calibration is simplified without decreasing measurement precision.

BACKGROUND OF THE INVENTION 
The present invention relates to methods for detecting pressure differences 
and a devices for converting displacements in order to perform process 
control. More specifically, the present invention relates to a method for 
detecting pressure differences and a device for converting displacements 
which detects extremely small displacements in a diaphragm caused by 
pressure differences as differential changes in capacitance and converts 
these changes into a unified signal in order to perform process control. 
Numerous difficulties are among the longstanding problems with using 
displacement converters to correct floating capacitances which have been 
addressed by the devices of the prior art. For example, in parallel flat 
plate types of sensors, use of known methods makes it possible to 
determine small diaphragm displacement values, and thus determine pressure 
differences. 
However, conventional conversion characteristics must be measured and 
re-checked, repeatedly, in order to be confirmed. In practical terms, 
making high precision adjustments using this method requires numerous 
trial-and-error attempts. Thus, much time and effort is required to make 
adjustments. 
Additionally, with conventional methods and devices, (such as parallel flat 
plate types of sensors) linearity is decreased because of changes in 
floating capacity caused by changes in temperature. 
Finally, with regard to temperature characteristics for zero and span, 
corrections have to be made with combinations of temperature-sensitive 
resistors, thermistors and the like. However, precise corrections are not 
possible, requiring onerous trial-and-error attempts too numerous to be 
efficient. 
According to known displacement converters of the parallel flat plate type 
of sensor type, (such as the present applicant's Japanese patent 
application no. 63-273120 entitled, DISPLACEMENT CONVERTER WITH IMPROVED 
LINEARITY) two additional capacitors were employed for compensation of 
floating capacitance. 
The capacitances (or the equivalent capacitances from combinations with 
resistors and the like) C.sub.C1 and C.sub.C2 were adjusted so that 
C.sub.C1 =C.sub.S1 and C.sub.C1 =C.sub.S1. A voltage having a prescribed 
potential and prescribed frequency was applied to capacitances C1, C2, CC1 
and CC2 in order to determine (C1-C.sub.C1) and (C2-C.sub.C2) from the 
charge current. By dividing the difference of these two by the sum, the 
following operation was performed: ( For this application "*" is used as a 
multiplication symbol "x") 
##EQU1## 
This equation makes it possible to determine very small displacement delta 
d of the diaphragm, and thus determine pressure difference P of the two 
sides of diaphragm 1. 
However, as discussed above making high precision adjustments using this 
method requires numerous trial-and-error attempts, linearity is decreased 
because of changes in floating capacity caused by changes in temperature, 
and corrections have to be made with combinations of temperature-sensitive 
resistors, thermistors and the like. However, precise corrections have not 
been possible prior to the advent of the present invention. 
In sum, among the prior art, hardware methods have been used in 
displacement converters to correct the floating capacitance contained in 
capacitances C1, C2 of the sensor capacitors. In the present invention, it 
is possible to perform linear, zero and span adjustments of a displacement 
converter easily and accurately. This is done by using capacitances C1(P), 
C2(P) measured beforehand for a plurality of measurement points with known 
pressure difference P, in order to calculate constants alpha and beta, 
which relate to the floating capacitance appearing at prescribed 
coefficient value f(P). 
In contradistinction to known methods, these constants are used to 
determine function f based on the capacitance for the pressure difference, 
and the pressure difference is calculated and output. Thus, the floating 
capacitance is corrected using a software method. 
The present invention performs the above corrections for predetermined 
temperature points beforehand, stores constants for each of these 
temperature points, measures the temperature as well as the sensor 
capacitor capacitance values when the pressure difference is measured, and 
uses the temperature-corrected constant to calculate and output the 
pressure difference. This provides a displacement converter having good 
linear, zero and span temperature properties. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide a method for 
measuring pressure differences which overcomes the drawbacks of the prior 
art. 
It is a further object of the invention to provide a device for converting 
displacements which overcomes the drawbacks of the prior art. 
It is a still further object of the invention to provide the above objects 
in a software method for measuring pressure differences and a device for 
converting displacements which detects extremely small displacements in a 
diaphragm caused by pressure differences as differential changes in 
capacitance and converts these changes into a unified signal in order to 
perform process control. 
It is yet a still further object of the invention to provide a displacement 
converter in which a pressure difference is determined by detecting 
capacitances between fixed electrodes and a diaphragm which receives 
pressure differences on both of its side and can be displaced. 
Briefly stated, there is provided a displacement converter in which a 
pressure difference is determined by detecting capacitances C1 and C2 
between fixed electrodes 3,4 and a diaphragm 1, which receives pressure 
differences on both of its sides and can be displaced. The pressure 
difference is output as a process-unified signal via a V/I converter 209. 
This eliminates the need for tedious adjustments involving hardware such 
as a compensation capacitor in order to compensate the floating 
capacitance in capacitances C1, C2, which is unrelated to diaphragm 
displacement. Instead, calibration is simplified without decreasing 
measurement precision. 
In accordance with these and other objects of the invention, there is 
provided a displacement converting device, which comprises, a pair of 
capacitors formed by a diaphragm and a pair of fixed electrodes arranged 
and facing either side of a diaphragm, means for measuring capacitance 
measuring capacitances C1, C2 of said pair of capacitors, first means for 
calculating constants calculating constants .alpha., .beta., based on the 
floating capacitances within said capacitances C1, C2, wherein f(P) of a 
first operation (1) is linear to a plurality of known pressure differences 
P in both the negative and positive ranges of said pressure difference P, 
said device effective for using capacitances C1, C2 of said pair of 
capacitors measured by said capacitance measuring means, using 
capacitances C1, C2 of said pair of capacitors measured by said 
capacitance measuring means during preliminary calibration based on C1(P), 
C2(P) of said first operation (1) for known pressure differences P, second 
means for calculating constants calculating f(P) of said first operation 
(1) during said preliminary calibration for each of said known plurality 
of pressure differences P, said device effective for using constants 
.alpha., .beta. calculated by said first means for calculating constants, 
and capacitances C1(P), C2(P) measured by said means for measuring 
capacitances for said plurality of known pressure differences P 
calculating constant f(0) corresponding to f(P) when pressure difference P 
is 0 based on a second operation (2), which defines the linearity of the 
two, using values for f(P) and said known pressure differences P, said 
device effective for calculating a proportional constant KP for a positive 
range of pressure difference P or (and) a proportion constant KP for a 
negative range of pressure difference P, means for measuring pressure 
difference calculating f(P) of operation (1) during pressure difference 
measurement using constants alpha, beta calculated by first means for 
calculating constants, and using capacitances C1(P), C2(P) measured by 
means for measuring capacitance for pressure difference P, and said device 
effective for calculating pressure difference P from the relationship in 
operation (2) using said f(P) and using constant f(0) and proportional 
constant KP calculated by said second means for calculating constants. 
According to a further feature of this invention, there is provided a 
method for measuring pressure difference detecting displacements of a 
diaphragm caused by pressure difference expressed as a change in 
capacitance in a pair of capacitors formed by a diaphragm and a pair of 
fixed electrodes arranged and facing either side of said diaphragm, which 
comprises measuring capacitances C1, C2 of said pair of capacitors, 
assuming f(P) of a first operation (1) is linear to pressure difference P, 
said first operation (1) being defined according to the following formula, 
f(P)={C1(P)-C2(P)-.alpha.}/{C1(P)+C2(P)-.beta.}, according to operation 
(2), said second operation (2) being defined according to the following 
formula, f(P)=K.sub.P *P+f(0), containing f(0) corresponding to f(P) when 
pressure difference P=0 and proportional constants KP corresponding to 
positive and negative ranges of pressure difference P, calculating 
constants .alpha., .beta., f(0), KP from operations (1) (2) using 
capacitances C1(P), C2(P) calculated during preliminary calibration for a 
plurality of known pressure differences P in a positive or (and) negative 
range of pressure difference P, and calculating pressure difference P from 
operations (1) (2) using constants calculated during preliminary 
calibration and using capacitances C1(P), C2(P) at pressure difference P 
measured during pressure difference measurement by said means for 
measuring capacitance. 
According to still a further feature of the invention there is provided, 
method for measuring pressure difference, by detecting capacitances C1 and 
C2 between fixed electrodes 3,4 and a diaphragm 1, which receives pressure 
differences on both of its sides and can be displaced, comprising, 
measuring capacitances C1, C2 of a pair of capacitors, assuming f(P) of a 
first operation (1) is linear to pressure difference P, said first 
operation (1) being defined according to the following formula: 
f(P)={C1(P)-C2(P)-.alpha.}/{C1(P)+C2(P)-.beta.} according to operation 
(2), said second operation (2) being defined according to the following 
formula: f(P)=K.sub.P *P+f(0), containing f(0) corresponding to f(P) when 
pressure difference P=0 and proportional constants KP corresponding to 
positive and negative ranges of pressure difference P, calculating 
constants .alpha., .beta., f(0), KP from operations (1) (2) using 
capacitances C1(P), C2(P) calculated during preliminary calibration for a 
plurality of known pressure differences P in a positive or (and) negative 
range of pressure difference P and calculating pressure difference P from 
operations (1) (2) using constants calculated during preliminary 
calibration and using capacitances C1(P), C2(P) at pressure difference P 
measured during pressure difference measurement by said means for 
measuring capacitance wherein each means except said first means for 
calculating constants is assembled as an integral device, said device 
setting constants .alpha., .beta. calculated by first memos for 
calculating constants; and pressure difference P is output as a 
process-unified signal via a V/I converter 209. 
According to yet a still further feature of the invention there is provided 
a method for measuring pressure difference, by detecting capacitances C1 
and C2 between fixed electrodes 3,4 and a diaphragm 1, which receives 
pressure differences on both of its sides and can be displaced, 
comprising, measuring capacitances C1, C2 of a pair of capacitors, 
assuming f(P) of a first operation (1) is linear to pressure difference P, 
said first operation (1) being defined according to the following formula: 
f(P)={C1(P)-C2(P)-.alpha.}/{C1(P)+C2(P)-.beta.}, according to operation 
(2), said second operation (2) being defined according to the following 
formula: f(P)=K.sub.P *P+f(0), containing f(0) corresponding to f(P) when 
pressure difference P=0 and proportional constants KP corresponding to 
positive and negative ranges of pressure difference P calculating 
constants .alpha., .beta., f(0), KP from operations (1) (2) using 
capacitances C1(P), C2(P) calculated during preliminary calibration for a 
plurality of known pressure differences P in a positive or (and) negative 
range of pressure difference P, and calculating pressure difference P from 
operations (1) (2) using constants calculated during preliminary 
calibration and using capacitances C1(P), C2(P) at pressure difference P 
measured during pressure difference measurement by said means for 
measuring capacitance wherein each means except said first means for 
calculating constants is assembled as an integral device, said device 
setting constants .alpha., .beta. calculated by first means for 
calculating constants; and pressure difference P is output as a 
process-unified signal via a V/I converter 209.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The above objects are achieved in accordance with the teachings of the 
invention by providing a method and device including an initial 
calibration mode, in which microprocessor 205 takes a plurality of known 
pressure differences and measures capacitances C1, C2 using time constant 
measuring unit 202, A/D converter 203 and timer counter 206. 
This data, which is needed to determine the constants needed to perform 
correction on floating capacitance and detection of pressure, is stored in 
memory 204. In measurement mode, pressure difference is calculated by 
using the measured capacitances C1, C2 and the constants stored in memory 
204. The results are output by the converter via D/A converter 207 and V/I 
converter 209. 
Referring to FIG. 1, a diagrammatic representation describes a parallel 
flat plate type of sensor. The sensor includes; a movable electrode which 
comprises a circular plate-shaped diaphragm that is displaced in a 
parallel direction by a distance of delta d (which is proportional to a 
pressure difference P (=PH-PL) between the two surfaces), and two fixed 
electrodes arranged on either side of the diaphragm so that they are 
parallel to and face the diaphragm. 
The two fixed electrodes and the movable electrode form a pair of 
capacitors. FIG. 1(A) shows the arrangement of the electrodes and FIG. 
1(B) shows the identical circuit. 
In FIG. 1, diaphragm 1 (1A, 1B) indicates a diaphragm (movable electrode) 
at different displacement positions. Fixed electrodes 3, and 4 are 
arranged on either side of diaphragm 1 so that they are parallel to the 
surface of diaphragm 1. PL and PH indicate the negative and positive 
pressure applied to the left and right surfaces of diaphragm 1 via small 
holes 3a and 4a arranged on fixed electrodes 3, and 4. 
Distance 2.sub.d indicates the distance between fixed electrodes 3, and 4. 
The area of electrodes 1, 3 and 4 are all equal. 
Position 1A indicates the position of diaphragm 1 when the pressures 
applied to diaphragm 1 are PH=PL (i.e. when pressure difference P=0). 
Distances d.sub.1 and d.sub.2 indicate the gaps between diaphragm 1 and 
fixed electrodes 3 and 4 when PH=PL. Similarly, .delta. indicates a 
displacement of diaphragm 1 from the center point between fixed electrodes 
3 and 4. 
Position 1B indicates a position of diaphragm 1 when the applied pressure 
difference between the diaphragm surfaces is P=PH-PL&gt;0. Delta d is the 
displacement of diaphragm 1. 
Referring to FIG. 1(B), capacitance CA is the part of total capacitance C1 
between diaphragm 1 and fixed electrode 3 that changes according to the 
displacement of diaphragm 1. Similarly, floating capacitance C.sub.S1 is 
the part of capacitance C1 that does not change according to the 
displacement of diaphragm 1. 
Capacitance CB is the part of total capacitance C2 between diaphragm 1 and 
fixed electrode 4 that changes according to the displacement of diaphragm 
1. Floating capacitance C.sub.S2 is the part of capacitance C2 that does 
not change according to the displacement of diaphragm 1. 
In a sensor as in FIG. 1, in which diaphragm 1 is displaced in a parallel 
direction to fixed electrodes 3 and 4, the capacitances can be expressed 
by the following equations: 
EQU C1=CA+C.sub.S1 =.epsilon.*A/(d1-.DELTA.d)+C.sub.S1 ={C.sub.OO 
/(1-.DELTA.d/d-.delta./d)}+C.sub.S1 (3) 
EQU C2=CB+C.sub.S2 =.epsilon.*A/(d2+.DELTA.d)+C.sub.S2 ={C.sub.OO 
/(1-.DELTA.d/d-.delta./d)}+C.sub.S2 (4) 
where: 
EQU C.sub.OO =.epsilon.*A/d, d=(d1+d2)/2, .delta.=(d2-d1)/2 (5) 
and 
d1,d2: gaps between electrodes (when pressure difference P=0) 
.DELTA.d: displacement of diaphragm (proportional to pressure difference P) 
.epsilon.: dielectric constant of dielectric between electrodes 
A: electrode area 
C.sub.S1, C.sub.S2 : floating capacitance 
Referring now to FIG. 2 through FIG. 7, the following is a description of a 
preferred embodiment of the present invention according to these figures. 
Like reference designators are used, as possible, to designate the same 
elements. 
Referring to FIG. 2, this embodiment has a displacement converter 200, a 
sensor 201 comprising a diaphragm 1 and fixed electrode 3, 4 (as described 
in FIG. 1). A microprocessor 205, serving as an operation control means 
for controlling this displacement converter, time constant measuring unit 
202 for measuring capacitances C1, C2 of the sensor capacitors arranged 
between diaphragm 1 and fixed electrodes 3 and 4 respectively. 
A/D converter 203 performs an A/D conversion of the time constant measured 
by time constant measuring unit 202 and sends the result to microprocessor 
205. Time counter 206 is used for microprocessor 205, in timing operations 
and the like. Memory 204 serves as memory for microprocessor 205 and 
stores various constants such as capacitance values. D/A converter 207 
converts the measured pressure difference into an analog voltage signal. 
V/I converter 207 converts a voltage signal into a current signal in a 
range such as 4-20 mA. Modem 208 produces a modulating signal when the 
displacement converter sends out digital data externally. 
External DC power supply 210, is located outside of displacement converter 
200, and serves as the power supply for generating the current signal 
noted above. External load resistor 211 is for converting the current 
signal to a voltage signal (for example, in order to convert a 4-20 mA 
signal to 1-5 V, a 250 ohm resistor would be used). 
External communicator 212, is for when displacement converter 200 transmits 
data externally. External pressure measuring unit 213, serves to measure 
pressure in cases such as when a known pressure or the like is being 
applied from outside to sensor 201. Temperature detector 214 is arranged 
on displacement converter 200 to perform temperature correction and the 
like for displacement converter 200. 
FIG. 3 shows the operations flow of microprocessor 205 when this embodiment 
is outputting a linear signal (i.e. in measurement mode with pressure 
difference P). Steps 301-310 represent steps in this flow. At step 302, 
microprocessor 205 controls time constant measuring unit 202, A/D 
converter 203 and time counter 206 in order to determine times T.sub.1 and 
T.sub.2 which are in proportion to capacitances C1 and C2 of the sensor 
capacitors. 
Times T.sub.1 and T.sub.2 can be determined by using, for example, the 
method shown in a previous application by the present applicant (Japanese 
laid-open publication no. 4-257430). According to this method, a sensor 
capacitor is charged by a prescribed voltage from a power source via a 
prescribed resistance, and the time it takes for the capacitor to be 
charged to a prescribed threshold level is measured. 
In another possible method, disclosed by the present applicant in Japanese 
laid-open publication no. 5-66168, instead of determining times T.sub.1 
and T.sub.2, one of the following are determined to obtain T.sub.1 and 
T.sub.2 : (T.sub.1 -T.sub.2) and (T.sub.1 +T.sub.2); (T.sub.1 +T.sub.2) 
and T.sub.1 or T.sub.2 ; (T.sub.1 -T.sub.2) and T.sub.1 or T.sub.2. 
In the next step, step 303, the reference operation noted in operation (17) 
(as shown and described below) is performed using time constants T.sub.d 
and T.sub.a in memory 204. 
EQU f=(T.sub.1 -T.sub.2 -T.sub.d)/(T.sub.1 +T.sub.2 -T.sub.a) (17) 
In the next step, step 304, constant f(0) (f when pressure difference is 0 
percent) in memory 204 is used to determine PN, the difference between f 
and f(0). In the next step, step 305, K.sub.S (the span coefficient) and 
K.sub.Z (the zero coefficient) in memory 204 are used in operation (18) to 
perform the operations for the output signal for the process handling the 
pressure difference measurement. In step 306, the result from this, 
converter output P.sub.out, is sent to D/A converter 207. 
EQU P.sub.out =K.sub.S *PN+K.sub.Z (18) 
The calculation in this described operation (18) provides an output signal 
P.sub.out that is linear to pressure difference P. 
For example, referring to FIG. 2, assuming the current signal from V/I 
converter 209 corresponding to a pressure difference P of 0-100 percent is 
4-20 mA, when P=0% (i.e. f=f(0) and PN=0), zero coefficient ZK is the 
signal element sent to D/A converter 207 so that the current signal from 
V/I converter 209 is 4 ma. Span coefficient K.sub.S is the signal element 
sent to D/A converter 207 so that the difference in the current signal 
from V/I converter 209 from when P=100% and P=0% is 16 mA. 
In the next step, step 307, if there is a read/write request for memory 204 
from external communicator 212 (e.g. reading T.sub.1, T.sub.2, writing 
T.sub.d, T.sub.a, and the like), the read/write operation is performed on 
memory at step 308. 
FIG. 4 is a flow chart of steps 401-413 indicating the sequence of 
operations of microprocessor 205 during output adjustment (calibration) of 
displacement converter 200. In steps 402-405, data required for the 
aforementioned constants T.sub.d, T.sub.a, which are necessary for linear 
correction, are retrieved. At step 403, applied pressure difference Px 
(where X is the parameter representing the number of the measurement 
point) is sent to sensor 201. At step 403 and step 404, detected time 
values T.sub.1 (P.sub.x), T.sub.2 (P.sub.x), which are proportional to 
capacitances C1, C2 of the sensor capacitors, are read from time measuring 
unit 202. This operation is repeated for X=0-n. 
Examples of the types of measurement points include: 1) five points, where 
pressure difference Px is -100, -50, 0, 50, 100%; 2) four points with 0, 
25, 50, 100%; 3) three measurement points for both positive and negative 
pressure differences, as noted previously (a total of 6 points). 
In step 404, referred to above, if displacement conversion takes place by 
determining the sum and difference of capacitance C1, C2 of the sensor 
capacitors, a read of T.sub.1 (P.sub.x)+T.sub.2 (P.sub.x) and T.sub.1 
(P.sub.x)-T.sub.2 (P.sub.x) is performed. 
If the displacement conversion takes place by determining capacitance C1 or 
C2 and their sum, or capacitance C1 or C2 and their difference, a read of 
T.sub.1 (P.sub.x) or T.sub.2 (P.sub.x) and T.sub.1 (P.sub.x)-T.sub.2 
(P.sub.x) is performed. At step 406, constants Ta, Td noted above are 
calculated, and at step 407, the values for Ta and Td are written to 
memory 204. 
In the calculation at step 406, if 5 measurement points (-100, -50, 0, 50, 
100%) are used for pressure difference P.sub.x as noted in (1) above, 
T.sub.a and T.sub.d are determined so that they satisfy the following 
equations: 
EQU f(+100)-f(+50)=f(+50)-f(0) (19) 
EQU f(-100)-f(-50)=f(-50)-f(0) (20). 
If 4 measurement points (0, 25, 50, 100%) are used for pressure difference 
P.sub.x as noted in (2) above, T.sub.a and T.sub.d are determined so that 
they satisfy the following equations: 
EQU f(100)-f(50)=f(50)-f(0) (21) 
EQU f(50)-f(25)=f(25)-f(0) (22) 
If six measurement points are used, as noted in (3) above, operations (13) 
and (16) would be used. 
In step 406, it would also be possible to perform the calculations of 
constants T.sub.a, T.sub.d outside of displacement converter 200 instead 
of having microprocessor 205 perform them. Then at step 407 microprocessor 
205 would read in the results of the calculations as T.sub.a and T.sub.d, 
and would write these results to memory 204. 
Next, steps 408-410 perform zero-adjustments. At step 408, differential 
pressure 0% is input. At step 409, the detected time values, T.sub.1 (0), 
T.sub.2 (0) and constants T.sub.a, T.sub.d stored in memory 204 are used 
in operation (17) to determine function f(0). This value is written to 
memory 204 as a constant. As a result, with pressure difference P=0%, 
operation (18) shows that PN=f-f(0)=0. Therefore P.sub.out =K.sub.Z. At 
step 410, zero coefficient K.sub.Z is set so that converter output 
P.sub.out is set at a desired value (e.g. 4 mA), and K.sub.Z is written to 
memory 204. 
Next, in steps 411 and 412, span adjustment is performed. At step 411, a 
differential pressure of 100% is entered. The detected time values, 
T.sub.1 (100), T.sub.2 (100) and constants T.sub.a, T.sub.d stored in 
memory 204 are used in operation (17) to determine f(100). From this can 
be obtained PN=f(100)-f(0). Using this and the aforementioned zero 
coefficient KZ, span coefficient KS is determined so that converter output 
P.sub.out =K.sub.X *PN+K.sub.Z can be a determined value (e.g. 20 mA). At 
step 412, the coefficient is written to memory 204. 
FIG. 5 is a flowchart indicating the operations of microprocessor 205 when 
displacement converter 200 is outputting linear converter output 
P.sub.out, which has been temperature-corrected. Steps 501-514 indicate 
this process. 
At step 502, microprocessor 205 controls time constant measuring unit 202, 
A/D converter 203 and timer counter 206. Also, time values T.sub.1, 
T.sub.2 proportional to capacitances C1 and C2 of the sensor 201 
capacitors are determined. At step 503, temperature T.sub.T is measured 
with temperature detector 214. At step 504, constants T.sub.d, T.sub.a 
that correspond to the current temperature T.sub.T are determined using a 
data table stored in memory 204 beforehand. This data table contains 
constants T.sub.di and T.sub.ai for temperatures T.sub.Ti (the "i" in 
T.sub.Ti, T.sub.di and T.sub.ai is a parameter indicating the temperature 
range of T.sub.T, T.sub.d and T.sub.a). 
FIG. 7 shows an example of the operations procedure for temperature 
correction. Steps 701-706 perform this procedure. In this example, it is 
assumed that the data table in memory 204 contains constants (T.sub.d1, 
T.sub.d2, T.sub.d3 and T.sub.a1, T.sub.a2, T.sub.a3) for the three 
temperatures for parameters i=1, 2, 3 (T.sub.T1, T.sub.T2, T.sub.T2, where 
T.sub.T1 &lt;T.sub.T2 &lt;T.sub.T3). 
Temperature correction values T.sub.d ' and T.sub.a ', which approximate 
constants T.sub.d and T.sub.a, are determined by performing linear 
approximations between temperatures T.sub.T3 -T.sub.T2 or temperatures 
T.sub.T2 -T.sub.T1 (steps 703, 704) depending on whether measured 
temperature T.sub.T is greater or less than measured temperature T.sub.T2 
(step 702). 
Returning momentarily to FIG. 5, at step 505, detected time values T.sub.1, 
T.sub.2 and constants T.sub.d ' and T.sub.a ', obtained from step 504, are 
used to determine function f. 
In the next step, step 507, constant f(0) (the f value when differential 
pressure is 0%), stored in memory beforehand by temperature, is used to 
determine f(0)' as a value for constant f(0) corresponding to the current 
measured temperature T.sub.T. 
In the next step, step 507, temperature correction for the zero point is 
performed by setting PN to the difference between f and f'(0). In the next 
step, step 508, operation (23) below is used to perform temperature 
correction on the span corresponding to the PN value. 
##EQU2## 
PN100(T.sub.T1) and PN100(T.sub.T2) are values of PN when input is 100% at 
temperatures T.sub.T1 and T.sub.T2, which were set beforehand. Temperature 
T.sub.T1 is the temperature for which adjustments to zero coefficient KZ 
and span coefficient KS are performed (this is called the reference 
temperature). The above equation is the equation for when T.sub.T 
&lt;=T.sub.2. If T.sub.T &gt;T.sub.2, then T.sub.T1 and T.sub.T2 in operation 
(23) are reversed. 
In the next step, step 509, temperature-corrected converter output 
P.sub.out is determined using operation (24) below. At step 510, the 
resulting P.sub.out is sent to D/A converter 207. 
EQU P.sub.out =K.sub.S *PN'+K.sub.Z (24) 
If, at step 511, a memory read/write is requested by external communicator 
212, step 512 performs a read/write operation (e.g. a read of T.sub.1, 
T.sub.2, a write of T.sub.d, T.sub.a). 
FIG. 6 is a flowchart indicating an embodiment of the operations performed 
by microprocessor 205 when the output from displacement converter 200 is 
calibrated so that temperature correction is possible. Steps 601-620 
perform this operation. In this case, temperature T.sub.Ti is changed to a 
number of preset temperature points within a certain range (the "i" in 
T.sub.Ti is a parameter indicating the number of the point). For each case 
(i.e. for each temperature point), a constant is determined according to 
the procedure in FIG. 6 and stored. 
In FIG. 6, assuming that the temperature is in one of the above temperature 
points, steps 602-605 collects the data necessary for calculating the 
linear correction constants T.sub.di and T.sub.ai. Next, at step 606, the 
linear correction constants Tdi and Tai for that temperature is 
calculated. The operation in steps 602-606 above are identical to steps 
402-406 in FIG. 4. 
Next, at step 607, current temperature data T.sub.Ti is measured using 
temperature detector 214. Next, at step 608, T.sub.di, T.sub.ai and 
T.sub.Ti are written to memory 204. At step 609, assuming input pressure 
difference to be 0%, f(0) is measured and is written to memory 204 at step 
610. 
Next, at step 612, zero coefficient K.sub.Z is written to memory 204 only 
if the temperature is the reference temperature (step 611, branch Y). 
Next, at step 614, the input pressure difference is set to 100%, If the 
temperature is the reference temperature (step 615, branch Y), span 
coefficient K.sub.S is written to memory 204 at step 616. Meanwhile, if 
the temperature is not the reference temperature (step 615, branch N), 
PN100.sub.i is calculated as the PN value in this case at step 617. At 
step 618, PN100i is written to memory 204. 
In prior art, hardware methods have been used in displacement converters to 
correct the floating capacitance contained in capacitances C1, C2 of the 
sensor capacitors. In the present invention, it is possible to perform 
linear, zero and span adjustments of a displacement converter easily and 
accurately. This is done by using capacitances C1(P), C2(P) measured 
beforehand for a plurality of measurement points with known pressure 
difference P, in order to calculate constants .alpha. and .beta., which 
relate to the floating capacitance appearing at prescribed coefficient 
value f(P). 
Then, these constants are used to determine function f based on the 
capacitance for the pressure difference, and the pressure difference is 
calculated and output. Thus, the floating capacitance is corrected using a 
software method. 
The present invention performs the above corrections for predetermined 
temperature points beforehand, stores constants for each of these 
temperature points, measures the temperature as well as the sensor 
capacitor capacitance values when the pressure difference is measured, and 
uses the temperature-corrected constant to calculate and output the 
pressure difference. This provides a displacement converter having good 
linear, zero and span temperature properties. 
The reason the results obtained from the method for measuring pressure 
difference is operational as set forth in claim 1 is further explained as 
follows. 
The method for measuring pressure difference detecting an extremely small 
movement, delta d, of a diaphragm (e.g. diaphragm 1) is caused by a 
pressure difference P, as a change in capacitance in a pair of capacitors 
formed by a diaphragm and two fixed electrodes (e.g. fixed electrodes 3, 
4) on either side of and facing the diaphragm. 
A means for measuring capacity (time constant measuring unit 202, A/D 
converter 203, time counter 206, and the like) is arranged to measure 
capacitances C1, C2 of the pair of capacitors. 
At pressure difference P, f(P), as shown in operation (1) above, is based 
on C1(P) and C2(P), the capacitances of the pair of capacitors measured by 
means for measuring capacity, and constants .alpha. and .beta., from the 
floating capacities in the two capacitances C1 and C2. 
F(P) is assumed to fulfill linear conditions in relation to pressure 
difference P according to operation (2), which contains constant f(0) 
corresponding to f(P) when pressure difference P=0 and proportional 
constant KP which corresponds to the positive and negative ranges of 
pressure difference P. 
During preliminary calibration, capacitances C1(P) and C2(P) for a 
plurality of known pressure difference P measurement points in the 
positive range or (and) negative range of pressure difference P are used 
to calculate constants .alpha. and .beta., f(0) and KP from operations (1) 
and (2). 
During pressure difference measurement, when measured pressure difference 
is P, capacitances C1(P) and C2(P) and the constant calculated during the 
preliminary calibration above are used in operations (1) and (2) to 
calculate measured pressure difference P. 
The device for converting displacement described below is defined by the 
inventors as follows, it includes a system for converting displacement by 
determining small displacement, .DELTA.d of a diaphragm (e.g. diaphragm 1) 
caused by pressure difference P from the differential change in 
capacitance in the pair of capacitors formed by the diaphragm and the two 
fixed electrodes (e.g. electrodes 3, 4) arranged on either side. 
The system further includes means for measuring capacitance (time constant 
measuring unit 202, A/D converter 203, timer counter 206, and the like) 
measuring capacitances C1 and C2 of the pair of capacitors noted above, 
first means for calculating constants (microprocessor 205, external 
communicator 212 or the like) calculating constants alpha, beta, based on 
the floating capacitance of capacitances C1, C2 where f(P) of operation 
(1) for a plurality of pressure differences measurement points P in the 
negative or (and) positive range of P fulfill linear conditions to 
pressure difference P. 
The calculation uses capacitances C1, C2 of the pair of capacitors measured 
during preliminary calibration by means for measuring capacitance for 
known pressure difference P in C1(P), C2(P) of operation (1). 
Also included are second means for calculating constants (microprocessor 
205 or the like) calculating constants KP and f(0). F(P) of operation (1) 
is derived for each pressure difference P using constants alpha, beta 
calculated using first means for calculating constants, and C1(P), C2(P), 
measured during preliminary calibration by means for measuring capacitance 
for a known plurality of pressure differences P. Based on f(P) and the 
known pressure differences P, second means for calculating constants 
calculates the following two elements in operation (2) that determines 
linearity--proportional constant KP for the positive range or (and) the 
negative range of pressure difference P, as well as constant f(0) 
corresponding to function f(P) when pressure difference P is 0. 
During pressure difference measurement, constants .alpha., .beta. 
calculated by first means for calculating constants and capacitances 
C1(P), C2(P) measured at pressure difference P by means for measuring 
capacitance are used to determine f(P) of operation (1). The system 
further includes means for measuring pressure difference (microprocessor 
205 or the like) derives pressure difference P from the relationship in 
operation (2) using f(P) as well as constant f(0) and proportional 
constant KP calculated by said second means for calculating constants. 
The system for converting displacement of further includes the device for 
converting displacement described above wherein the means for measuring 
capacitance determines the capacitances of the pair of capacitors by 
measuring the difference and the sum of the capacitances. 
The system for converting displacement also includes the device for 
converting displacement described above, wherein the difference of sum of 
the two capacitances as well as one of the capacitances is measured and 
the capacitance of the other capacitors is determined. 
The system for converting displacement further includes the device for 
converting displacement further comprising means for detecting temperature 
(means for detecting temperature 214 or the like) wherein means for 
calculating (microprocessor 205 or the like) calculates constants alpha, 
beta corresponding to the temperature detected by means for detecting 
temperature during pressure difference measurement using constants alpha, 
beta calculated by said first means for calculating constants using the 
calibration by temperature for a plurality of temperatures detected by 
means for detecting temperature. The resulting constants are used by means 
for measuring pressure difference to calculate f(P). 
The device for converting displacement further comprises means for 
detecting temperature (means for detecting temperature 214 or the like) 
and means for calculating constants f(0), KP (microprocessor 205 or the 
like) corresponding to the temperature detected by means for measuring 
pressure difference during pressure difference measurement using constants 
f(0), KP calculated by second means for calculating constants based on the 
calibration by temperature for a plurality of temperatures detected by 
means for detecting temperature. The resulting constants are used by means 
for measuring pressure difference for calculating pressure difference P. 
The device for converting displacement is also described in that all means 
except first means for calculating constants can be assembled as an 
integral device (displacement converter 200 or the like); and the device 
uses constants calculated by first means for calculating constants. 
As described above, the present inventors have discovered that the present 
invention yields the above described advantages. Further, the following 
means are used in the present invention: 
1) microprocessor 205 serving as means for calculating and controlling 
2) time constant measuring unit 202 serving as means for measuring 
capacitances C1, C2 of the sensor capacitor 
3) time constant 206 performing A/D conversion of the time constant 
obtained from 2) 
4) memory 204 storing the determined capacitance 
5) memory 204 storing linear correction constants alpha and beta 
6) means for performing read/write operations on memory (microprocessor 
205) 
The following means are also used to prevent decreases in linearity due to 
changes in floating capacitance caused by variations in temperature: 
7) means for detecting temperature (temperature detector 214) 
8) memory 204 for storing temperature correction coefficients for alpha and 
beta 
The following means are also used to correct temperature characteristics 
for zero and span. 
9) memory 204 storing temperature correction coefficients for zero and span 
Pressure difference is measured according to the following method. Instead 
of using a hardware method for compensating the floating capacitances 
contained in capacitances C1 and C2 from the sensor capacitor, the 
floating capacitances are determined by performing an initial calibration 
in which sensor capacitor capacitances C1(P), C2(P) are measured for a 
plurality of known pressure differences P. This is then used to perform 
compensation on floating capacitance (using software methods) when 
pressure differences are to be measured. * * * 
In other words in operation (1) above, 
EQU .alpha.=C.sub.S1 -C.sub.S2, .beta.=C.sub.S1 +C.sub.S2 (7) 
and f(P) of operation (1) becomes equivalent to when C.sub.C1 =C.sub.S1, 
C.sub.C2 =C.sub.S2 in operation (6) above, and can be expressed as 
EQU f(P)=.DELTA.d/d+.delta./d (8) 
In this equation, diaphragm displacement delta d is proportional to applied 
pressure difference P, so if proportional constant is set to KP, then 
EQU .DELTA.d/d=KP*P (9) 
(However, this proportional constant will generally be different for the 
positive and negative range of pressure difference P because of the margin 
of error in the assembly of the diaphragm.) 
.delta./d is equivalent to f(0) when, displacement delta d=0 (i.e. when 
pressure difference P=0), so 
EQU .delta./d=f(0) (10) 
Therefore, f(P) fulfills the linear condition of operation (2). 
EQU f(P)=K.sub.P *P+f(0) (2) 
Let us assume that during calibration of the displacement converter, the 
sensor capacitances C1(P), C2(P) were measured for 3 known separate 
pressure differences P in the positive range (P.sub.0, P.sub.1, P.sub.2) 
and for 3 known separate pressure differences P in the negative range 
(P.sub.3, P.sub.4, P.sub.5). 
By taking the difference of the function f for the two pressure difference 
values P.sub.0 and P.sub.1, operation (2) shows that 
EQU f(P.sub.1)-f(P.sub.0)=K.sub.P *(P.sub.1 -P.sub.0) (11) 
Likewise, by taking the difference of function f for pressure differences 
P.sub.1 and P.sub.2, 
EQU f(P.sub.2)-f(P.sub.1)=K.sub.P *(P.sub.2 -P.sub.1) (12) 
The following operation (13) results from operations (11) and (12). 
EQU f(P.sub.2)-f(P.sub.1)={(P.sub.2 -P.sub.1)/(P.sub.1 
-P.sub.0)}{f(P.sub.1)-f(P.sub.0)} (13) 
Likewise, for pressure differences P.sub.3, P.sub.4, P.sub.5, 
EQU f(P.sub.4)-f(P.sub.3)=K.sub.P *(P.sub.4 -P.sub.3) (14) 
EQU f(P.sub.5)-f(P.sub.4)=K.sub.P *(P.sub.5 -P.sub.4) (15) 
Operations (14) and (15) show that: 
EQU f(P.sub.5)-f(P.sub.4)={(P.sub.5 -P.sub.4)/(P.sub.4 
-P.sub.3)}{f(P.sub.4)-f(P.sub.3)} (16) 
Therefore, the differences and sums of sensor capacitor capacitance values 
C1, C2 for pressure differences P.sub.0 -P.sub.5 can be obtained, the 
equations in operations (13) and (16) can be solved, and the unknown 
constants alpha and beta satisfying operation (8) (and therefore operation 
(2)) can be determined. Then a pressure difference can be determined 
linearly by performing operation (1) using constants alpha and beta and 
capacitances C1 and C2, measured at that pressure difference. 
Instead of directly measuring sensor capacitor capacitances C1 and C2, the 
embodiment below measures the charging times T.sub.1 and T.sub.2 of the 
capacitors, which are proportional to the capacitances, under prescribed 
circuit conditions. Then, instead of the reference operation in operation 
(1), the following operation is performed. 
EQU f=(T.sub.1 -T.sub.2 -T.sub.d)/(T.sub.1 +T.sub.2 -T.sub.a) (17) 
In this operation, T.sub.d and T.sub.a are constants corresponding to 
.alpha. and .beta.. 
Having described preferred embodiments of the invention with reference to 
the accompanying drawings, it is to be understood that the invention is 
not limited to those precise embodiments, and that various changes and 
modifications may be effected therein by one skilled in the art without 
departing from the scope or spirit of the invention as defined in the 
appended claims.