Device for measuring variable with automatic compensation for offset

A measurement device for measuring a physical property such as temperature difference, displacement, pressure, fluid flow rate, etc. comprises: (a) an amplifying means which receives and amplifies a voltage signal corresponding to a detected physical property; (b) a reference voltage generator which generates and applies a reference voltage to the amplifying means at a predetermined time; (c) a memory which stores the output voltage signal from the amplifying means when the reference voltage generator applies the reference voltage to the amplifying means; and (d) a calculating means which outputs at least one command signal to the reference voltage generator to apply the reference voltage to the amplifying means and to the memory to store the output voltage of the amplifying means into a specified memory location thereof at the predetermined time and calculates the value of the desired physical property from the current output voltage of the amplifying means with reference to the stored voltage value in the memory, whereby an accurate physical property measurement can be made by compensating for individual differences and deterioration of the characteristics of the measurement device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a measurement device for 
measuring a variable, e.g., temperature difference, with an automatic 
offset error compensation, and more specifically to a measurement device 
which automatically compensates for discrepancies in operating 
characteristics due to aging and/or random effects of dispersed circuit 
parameters and variations during manufacture in an amplifying section 
which amplifies a detector voltage signal according to changes in the 
variable to be measured. 
2. Description of the Prior Art 
In the case where a variable such as displacement, pressure, flow rate, and 
temperature, etc., (hereinafter referred simply to as a parameter) is 
detected in the form of a voltage, the detected voltage is amplified by 
the measurement device and the amplified voltage is processed by a 
subsequent calculating circuit so as to obtain a measurement value. 
An automotive vehicle engine requires a measurement of the temperature 
difference (.DELTA.T=T.sub.2 -T.sub.1) between the temperature (T.sub.1) 
of intake air from outside of the engine and the temperature (T.sub.2) of 
the intake air intermixed with exhaust gas. 
The temperature difference described above can be measured by means of a 
thermocouple utilizing the Seebeck effect. 
If two junctions of two different kinds of metals are disposed at higher 
and lower temperature sections, respectively, a thermoelectromotive force 
is generated according to the temperature difference .DELTA.T and 
consequently a voltage (V.sub.2 -V.sub.1) is produced according to the 
detected temperature difference and is outputted to an amplifying circuit. 
The amplified voltage is processed by a calculating circuit to convert a 
desired temperature difference, as is shown in FIG. 1. 
The amplifying circuit described above comprises the following items, as 
shown in FIG. 2: 
(a) a first operational amplifier having a noninverting input terminal 
connected to one terminal of the thermocouple to receive a lower voltage 
V.sub.1 indicative of the thermo-electromotive force generated by the 
thermocouple on the lower temperature side and connected to a constant 
voltage supply V.sub.0 via first resistor Ro, an inverting input terminal 
connected to the constant voltage supply V.sub.0 via a second resistor 
R.sub.1, and an output terminal thereof connected to the inverting input 
terminal via a third resistor R.sub.2 ; (b) a second operational amplifier 
having a noninverting input terminal connected to the other terminal of 
the thermocouple to receive a higher voltage V.sub.2 indicative of the 
thermo-electromotive force generated by the thermocouple on the higher 
temperature side, an inverting input terminal connected to the output 
terminal of the first operational amplifier via a fourth resistor R.sub.3, 
and an output terminal thereof connected to the inverting input terminal 
of the second operational amplifier via a fifth resistor R.sub.4 ; and (c) 
a third operational amplifier having a noninverting input terminal 
connected to the output terminal of the second operational amplifier, an 
inverting input terminal connected to ground via a sixth resistor R.sub.5, 
and an output terminal connected to its inverting input terminal via a 
seventh resistor R.sub.6. In the processing circuit described above, if 
the ratio of the resistances of the resistors are expressed as 
.alpha.=(R1/R2)=(R4/R3), .beta.=(R6/R5), and w.sub.1, w.sub.2, and w.sub.3 
are respective offset voltages of the first, second, and third operational 
amplifiers, the output voltage u.sub.1 of the first operational amplifier 
and output voltage u.sub.2 of the second operational amplifier can be 
expressed, respectively, in the following equations: 
EQU u.sub.1 =(1/.alpha.)(V.sub.1 -V.sub.0 +w.sub.1)+V.sub.1 +w.sub.1 ( 1) 
EQU u.sub.2 =.alpha.(V.sub.2 -u.sub.1 +w.sub.2)+V.sub.2 +w.sub.2 ( 2) 
in addition, the output voltage E of the processing circuit can be 
expressed by the equation: 
EQU E=(1+.beta.)(u.sub.2 +w.sub.3) (3) 
From these three equations (1), (2), and (3), the output voltage E can be 
rewritten as: 
EQU E=(1+.alpha.)(1+.beta.)(V.sub.2 -V.sub.1)+(1+.beta.){(1+.alpha.)(w.sub.2 
-w.sub.1)+w.sub.3 +V.sub.0 } (4) 
, wherein (1+.alpha.) and (1+.beta.) are amplification factors of the 
associated operational amplifiers. 
In the equation (4) expressed above, the first item on the right side of 
the equation indicates a pure amplified voltage corresponding to the 
thermoelectromotive force on the basis of the difference in temperature 
and the second item on the right side of the equation represents a voltage 
including the offset voltage of each of the operational amplifiers. It 
should be noted that because of irregularities in 
thermo-electromotive-force characteristics of each thermocouple, the first 
item inherently includes another offset voltage factor such that the 
difference between the higher and lower thermoelectromotive forces 
(V.sub.2 -V.sub.1) will probably not be zero when the ambient temperature 
is at 0.degree. C., i.e., the temperatures at both junctions is 0.degree. 
C. 
Hoever, there are drawbacks in such a conventional measurement device. When 
many such measurement devices are massproduced, the circuit parameters of 
the resistors R.sub.0 through R.sub.6 and constant voltage V.sub.0 all 
need to be adjusted to make the amplification factor (1+.beta.)(1+.alpha.) 
and the offset voltage (1+.beta.){(1+.alpha.)(w.sub.2 -w.sub.1)+w.sub.3 
+V.sub.0 } of each amplifying circuit equal to those of other measurement 
devices. This adjustment requires a great amount of labor in manufacturing 
of the measurement devices. Furthermore, after the circuit parameters and 
offset voltage of each measurement device are adjusted, the 
characteristics of the thermocouple used in the measurement device may 
change due to heat when the thermocouple is soldered into the measurement 
device and the operating characteristics of the measurement device itself 
may change due to the effects of aging. 
Therefore, errors may occur in the output voltage E of the amplifying 
circuit and measurement errors may occur in the output value of the 
calculating circuit of the measurement device. 
SUMMARY OF THE INVENTION 
With the above-described drawbacks in mind, it is an object of the present 
invention to provide a measurement device which automatically compensates 
for discrepancies between the characteristics of individual measurement 
devices and for deterioration of the characteristics of an element 
converting a measured variable into a voltage signal and of the 
measurement device itself, wherein in addition to the conventional 
amplifying and calculating circuits there are provided a reference voltage 
generator which generates and applies a reference voltage across the input 
terminals of the amplifying circuit in response to a command signal from 
the calculating circuit and a memory which stores the output voltage of 
the amplifying circuit when the reference voltage is applied across the 
input terminals of the amplifying circuit, whereby the calculating circuit 
receives the current output voltage of the amplifying circuit when a 
varable is measured and calculates the measured variable on the basis of 
the stored value within the memory to obtain an accurate value of the 
measured variable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will be made to the drawings and first to FIG. 1 which shows a 
conventional measurement device applied to the measurement of a 
temperature difference. 
In FIG. 1, numeral 1 denotes an intake air passage of an automotive vehicle 
engine, wherein intake air from outside is intermixed with part of the 
engine exhaust (exhaust gas recirculation) and the intermixed air is sent 
into the engine cylinders. To control exhaust gas recirculation, it is 
necessary to measure the difference (.DELTA.T=T.sub.2 -T.sub.1) between 
the lower temperature T.sub.1 of the atmospheric intake air and the higher 
temperature T.sub.2 of the air/exhaust mixture. 
The temperature difference between the higher temperature section of the 
air intake passage and lower temperature section can be detected by 
utilizing the Seebeck effect. 
If two different kinds of metals 2 and 3 are disposed within the intake air 
passage 1 in such a way that two junctions thereof are located at the 
higher and lower temperature sections respectively, a thermoelectromotive 
force according to the temperature difference .DELTA.T is generated and a 
voltage (V.sub.2 -V.sub.1) indicative thereof can be obtained as shown in 
FIG. 1. In conventional measurement devices, the voltage (V.sub.2 
-V.sub.1) is amplified by means of an amplifying circuit 4 to output a 
voltage signal E. The output voltage signal E is processed by means of a 
calculating circuit 5 wherein the output voltage signal E is converted 
into a measurement result, i.e., the desired temperature difference 
.DELTA.T. 
FIG. 2 shows the internal construction of the amplifying circuit 4. 
In FIG. 2, the thermocouple comprising two different kinds of metals 2 and 
3 is connected to the amplifying circuit 4. A thermoelectromotive force is 
generated between two junctions if there is a temperature difference 
.DELTA.T between two bimetallic junctions, i.e., the Seebeck effect. The 
lower-temperature thermoelectromotive force V.sub.1 is supplied to the 
amplifying circuit 4 at the noninverting input terminal of a first 
operational amplifier OP.sub.1. The higher-temperature thermoelectromotive 
force V.sub.2 is supplied to the amplifying circuit 4 at a noninverting 
input terminal of a second operational amplifier OP.sub.2. Therefore, the 
resultant thermoelectromotive force (V.sub.2 -V.sub.1) is applied across 
the noninverting input terminal of the first operational amplifier 
OP.sub.1 and the noninverting input terminal of the second operational 
amplifier OP.sub.2. A constant voltage V.sub.0 is also supplied to the 
inverting input terminal of the first operational amplifier OP.sub.1 via a 
second resistor R.sub.1 and the noninverting input terminal thereof via a 
first resistor Ro. A third resistor R.sub.2 connects the inverting input 
terminal to the output terminal of the first operational amplifier 
OP.sub.1. The first operational amplifier OP.sub.1 acts as a differential 
amplifier. The inverting input terminal of the second amplifier OP.sub.2 
is connected to the output terminal of the first operational amplifier 
OP.sub.1 via a fourth resistor R.sub.3 and the output terminal thereof via 
a fifth resistor R.sub.4. The second operational amplifier OP.sub.2 also 
acts as a differential amplifier. The output terminal of the second 
operational amplifier OP.sub.2 is connected to a noninverting input 
terminal of a third operational amplifier OP.sub.3. 
The inverting input terminal of the third operational amplifier OP.sub.3 is 
grounded via sixth resistor R.sub.5 and connected to the output terminal 
thereof via a seventh resistor R.sub.6. The third operational amplifier 
OP.sub.3 outputs a voltage E corresponding to the measured temperature 
difference by means of the thermocouple. 
The third operational amplifier OP.sub.3 acts as a noninverting amplifier. 
The first, second, and third operational amplifiers OP.sub.1, OP.sub.2, 
and OP.sub.3 have their own offset voltages w.sub.1, w.sub.2, and w.sub.3, 
respectively. In the construction of the amplifying circuit 4 described 
above, the output voltage u.sub.1 of the first operational amplifier 
OP.sub.1 can be expressed by the following first equation: 
##EQU1## 
, wherein .alpha.=R1/R2. 
An output voltage u.sub.2 of the second operational amplifier OP.sub.2 can 
be expressed by a second equation: 
EQU u.sub.2 =.alpha.(V.sub.2 -u.sub.1 +w.sub.2)+V.sub.2 +w.sub.2 (2) 
, wherein .alpha.=R4/R3. The output voltage E of the amplifying circuit 4 
can be expressed by a third equation: 
EQU E=(1+.beta.)(u.sub.2 +w.sub.3) (3) 
, wherein .beta.=R6/R5. 
Consequently, the third equation (3) can be rearranged by substituting the 
first and second equations (1) and (2) thereinto to obtain the following 
fourth equation: 
EQU E=(1+.alpha.)(1+.beta.)(V.sub.2 -V.sub.1)+(1+.beta.){(1+.alpha.)(w.sub.2 
-w.sub.1)+w.sub.3 +V.sub.0 } (4) 
In the fourth equation (4) described above, the first item on the right 
side of the equation corresponds to the measured difference in temperature 
and the second item corresponds to an offset voltage. The conventional 
measurement device uses the fourth equation (4). It should be noted that 
the first item of the right side of the fourth equation (4) also includes 
an inherent offset voltage which means that the difference between the 
higher- and lower-temperature thermo-electromotive forces (V.sub.2 
-V.sub.1) will probably not equal zero even when the ambient temperature 
of the thermocouple junctions is at 0.degree. C., that is, when both 
T.sub.1 and T.sub.2 are 0.degree. C., due to irregularity of a 
thermoelectromotive-force characteristics of the thermocouple junctions 
used therein. 
FIG. 3 shows a first preferred embodiment of a measurement device according 
to the present invention. 
In FIG. 3, the amplifying and calculating circuits are the same as those 
shown in FIG. 1 and FIG. 2. A memory 10 is connected to the amplifying 
circuit 4 via a first switching means 9. The memory 10 is also connected 
to the calculating circuit 5. The first switching means 9 is opened or 
closed depending on a command signal from the calculating circuit 5. It 
should be noted that the first switching means 9 comprises a single 
independent switching element or a switching function included in the 
calculating circuit 5 or memory 10. It should also be noted that either a 
microcomputer or individual IC capable of performing arithmetic 
calculations may be used for the calculating circuit 5 and that the memory 
10 needs to be nonvolatile. 
In FIG. 3, if a temperature difference .DELTA.T is measured as shown in 
FIG. 1, the first switching means 9 is closed and a first reference 
voltage, the zero-difference reference voltage (0 V) is applied across the 
input terminals of the amplifying circuit 4. When the first reference 
voltage is applied as described above, the value of output voltage E.sub.0 
of the amplifying circuit 4 is stored within the memory 10. Subsequently, 
when for example, a second reference voltage (4.1 mV) corresponding to the 
temperature difference at 100.degree. C. is applied across the input 
terminals of the amplifying circuit 4, the value of an output voltage 
E.sub.100 is similarly stored within the memory 10. These values of output 
voltages E.sub.0 and E.sub.100 are stored at different memory locations 
within the memory 10. If previous values of the output voltages E.sub.0 
and E.sub.100 are stored, the previous values are corrected on the basis 
of the current values. 
When the current temperature difference .DELTA.T.sub.c is measured, the 
switching means is opened so as to apply the voltage signal corresponding 
to the current temperature difference across the input terminals of the 
amplifying circuit 4. At this time, the voltage value E.sub.c outputted 
from the amplifying circuit 4 is sent into the calculating circuit 5. In 
the calculating circuit 5, the calculating operation listed below is 
performed using the stored values E.sub.0 and E.sub.100 from the memory 
10: 
##EQU2## 
Thus, the current output voltage E.sub.c of the amplifying circuit is 
converted into the desired temperature difference value .DELTA.T. 
When such a measurement device as described above is used, the error of the 
measurement device due to the normal slight discrepancies between 
individual measurement devices, changes in the characteristic of the 
element thermocouple) during installation on the measurement device and 
deterioration of the characteristic of the element and the measurement 
device itself due to aging can automatically be cancelled by using the 
stored value within the memory 10 at the time of actual measurement, the 
stored value being derived from the output voltage of the amplifying 
circuit when the reference voltage is applied across the input terminals 
of the amplifying circuit. 
This automatic compensation operation is performed at an interval of time 
predetermined by the calculating circuit 5 by updating the previously 
stored value. 
It is preferable to store the values of output voltages E.sub.0 and 
E.sub.100 that are determined when the two different reference voltages 
are applied across the input terminals of the amplifying circuit, if the 
adjustment of the device's amplification factor and the device's offset 
voltage in the amplifying circuit 4 is difficult. If it is easy to adjust 
either characteristic of the measurement device due to the particular 
circuit construction of the amplifying circuit 4 or individual disparities 
and aging deterioration in a characteristic are not too great, one 
reference voltage can be applied to the amplifying circuit so as to update 
respective the stored voltage value E.sub.0 or E.sub.100 with the other 
output voltage value remaining stored as a constant value. 
FIG. 4 shows another conventional measurement device. This type of 
measurement device is applicable for measuring two temperature differences 
.DELTA.T.sub.21 =T.sub.2 -T.sub.1 and .DELTA.T.sub.32 =T.sub.3 -T.sub.2 as 
seen in FIG. 4 in the case where exhaust gas is recirculated into the 
intake manifold 1, wherein T.sub.1 denotes the temperature of intake air 
introduced from outside the engine, T.sub.3 denotes the temperature of 
engine exhaust, and T.sub.2 denotes the temperature of intake air 
intermixed with exhaust. Consequently, the ratio of two temperature 
differences R can be expressed by the following equation: 
EQU {R=.DELTA.T.sub.21 /.DELTA.T.sub.32 =(T.sub.2 -T.sub.1)/(T.sub.3 -T.sub.2)} 
The junctions of two different kinds of metals 2 and 3 are disposed at a 
lower temperature section, intermediate temperature section, and higher 
temperature section, respectively. 
A thermoelectromotive force is generated according to the temperature 
difference .DELTA.T.sub.21 and .DELTA.T.sub.32 and two voltages (V.sub.2 
-V.sub.1) and (V.sub.3 -V.sub.2) representing these forces are inputted to 
the input terminals of parallel amplifying circuits 4a and 4b. The 
internal circuit construction of the two amplifying circuits is the same 
as that shown in FIG. 2. The output voltage values Ea and Eb of the two 
amplifying circuits 4a and 4b can be obtained in the same way as expressed 
in the fourth equation (4). The value of the ratio R includes error due to 
random differences between the characteristics of the two each amplifying 
circuits and aging deterioration of the characteristic of the element and 
measurement device itself in the same way as in the conventional 
measurement device shown in FIG. 1. 
FIG. 5 shows a second preferred embodiment of a measurement device 
according to the present invention. 
As shown in FIG. 5, the memory 10a is connected to the two amplifying 
circuits 4a and 4b via two separate but similar switching means 9a and 9b 
and is also connected to the calculating circuit 5a. The reference voltage 
generator 11 is connected to the input terminals of the two amplifying 
circuits 4a and 4b via a second switching means 12. These switching means 
9a, 9b, and 12 are closed according to a command signal a from the 
calculating circuit 5a. The command signal a is also inputted into the 
reference voltage generator 11 to control the output of the reference 
voltage. 
In the measurement device shown in FIG. 5, the command signal a is 
outputted by the calculating circuit 5a at a predetermined time (e.g., 
when the engine is started). At this time, the reference voltage is 
applied to the amplifying circuits 4a and 4b with the first switching 
means 9a and 9b closed simultaneously and two pairs of memory locations 
within the memory 10a are reserved for the respective output values Ea and 
Eb of the amplifying circuits 4a and 4b. 
At first, when the first reference voltage 0 V corresponding to a 
temperature difference of zero is applied to the amplifying circuits 4a 
and 4b, the output values Ea.sub.0 and Eb.sub.0 of the respective 
amplifying circuits 4a and 4b are stored in the first pair of reserved 
locations of the memory 10a. Subsequently, a second reference voltage 
corresponding to a temperature difference of 200.degree. C. is applied to 
the amplifying circuits 4a and 4b. The output voltages Ea.sub.200 and 
Eb.sub.200 of the amplifying circuits 4a and 4b are stored in the other 
pair of reserved memory locations of the memory 10a. If previous output 
voltage values Ea.sub.200 and Eb.sub.200, Ea.sub.0, and Eb.sub.0 are 
stored in the respective locations stored values of the output voltages 
are updated. 
It should be noted that the first and second switching means 9a, 9b, and 12 
are ordinarily open when measurments of the ratio R are made. 
The voltage signal (V.sub.2 -V.sub.1).sub.c representing the temperature 
difference T.sub.2 -T.sub.1 =.DELTA.T.sub.21c is applied to the associated 
amplifying circuit 4a and voltage signal (V.sub.3 -V.sub.2).sub.c 
representing the temperature difference T.sub.3 -T.sub.2 =.DELTA.T.sub.32c 
is applied to the associated amplifying circuit 4b. From the output values 
Ea.sub.c and Eb.sub.c, the calculating circuit 5a performs a series of 
arithmetic operations expressed in the equations given below by using the 
stored values Ea.sub.0, Eb.sub.0, Ea.sub.200, and Eb.sub.200 : 
##EQU3## 
Consequently, the output voltage values Ea.sub.c and Eb.sub.c are 
converted to the respective tempeature differences .DELTA.T.sub.21c and 
.DELTA.T.sub.32c to obtain the desired ratio of R. 
The equation (6) expressed above is shown in FIG. 6 and FIG. 7(A) and 
equation (7) is shown in FIG. 6 and FIG. 7(B). 
In the second preferred embodiment described hereinabove, the errors in the 
measurement device due to the differences between indivdiual measurement 
devices, accidental damage to the elements during installation, and aging 
deterioration of the measuring characteristics are compensated with the 
degree of compensation being adjusted for at predetermined intervals of 
time. 
In these preferred embodiments, the measurement of temperature difference 
was described merely as an example of the variable to be measured, as 
explained previously. 
Since the measurement device according to the present invention can measure 
variables by converting the detected voltage into a desired variable by 
means of the calculating circuit, the measurement device can apply equally 
well to the measurement of various measurable variables such as 
displacement, pressure, fluid flow rate, etc. 
As described before, since the measurement device according to the present 
invention stores an output voltage value of the amplifying circuit in 
response to a reference voltage applied automatically to the amplifying 
circuit at a time predetermined by the calculating circuit and calculates 
the desired variable by performing a calculation operation using the 
stored value, the labor required for the adjustment of circuit parameters 
in the measurement device can be avoided and normal individual variations 
in the characteristics of the measurement devices at the time of 
manufacture can automatically be compensated for. Therefore, there arises 
no special problem due to indivdiual variations or deterioration of the 
characteristics of the measurement device. Consequently, the measurement 
device can perform accurate measurements with improved reliability and 
service life. 
It will be fully understood by those skilled in the art that the foregoing 
description is made in terms of the preferred embodiments and 
modifications may be made without departing the scope and spirit of the 
present invention which is to be defined by the appended claims.