U.S. patent application number 09/361457 was filed with the patent office on 2001-11-01 for bridge circuit for detector.
Invention is credited to FURUKAWA, MASANAO.
Application Number | 20010035758 09/361457 |
Document ID | / |
Family ID | 17302543 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010035758 |
Kind Code |
A1 |
FURUKAWA, MASANAO |
November 1, 2001 |
BRIDGE CIRCUIT FOR DETECTOR
Abstract
A bridge circuit serving as a detector of a physical variable
such as pressure has four resistors forming a wheatstone bridge,
one of the resistors changing resistance according to variations in
the physical variable to be measured. Voltage difference between a
mutually opposite pair of junctions of the four resistors is
measured by one circuit and that between the other mutually
opposite pair of junctions is measured by another circuit. The
effect of temperature variation is calculated from these measured
values to reduce the drift in the value of pressure calculated from
the voltage imbalance of the wheatstone bridge.
Inventors: |
FURUKAWA, MASANAO; (KYOTO,
JP) |
Correspondence
Address: |
COUDERT BROTHERS
600 Beach Street
SAN FRANCISCO
CA
94109
US
|
Family ID: |
17302543 |
Appl. No.: |
09/361457 |
Filed: |
July 27, 1999 |
Current U.S.
Class: |
324/721 |
Current CPC
Class: |
G01R 17/105
20130101 |
Class at
Publication: |
324/721 |
International
Class: |
G01R 027/08 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 10, 1998 |
JP |
10-257160 |
Claims
What is claimed is:
1. A bridge circuit comprising: a wheatstone bridge consisting of a
first resistor group and a second resistor group connected in
parallel between a first junction point and a second junction
point, said first resistor group consisting of a first resistor and
a second resistor connected in series with a third junction point
therebetween, said second resistor group consisting of a third
resistor and a fourth resistor connected in series with a fourth
junction point therebetween; a first voltage measuring means for
measuring and outputting a voltage difference between said first
junction point and said second junction point; a second voltage
measuring means for measuring and outputting a voltage value at
said third junction point; and a calculating means for calculating
a correction on said voltage difference outputted by said first
voltage measuring means by using said voltage value outputted from
said second voltage measuring means.
2. The bridge circuit of claim 1 wherein said first resistor, said
second resistor, said third resistor and said fourth resistor all
have same resistance under a normal condition.
3. The bridge circuit of claim 1 wherein said calculating means
stores a preliminarily calculated proportionality constant between
said correction and said voltage value at said third junction
point.
4. The bridge circuit of claim Herein said proportionality constant
is calculated from values of said correction and said voltage value
at said third junction point at two different temperatures.
5. The bridge circuit of claim 1 further comprising a constant
current circuit connected to said third junction point for causing
a constant current to pass through said Wheatstone bridge.
6. The bridge circuit of claim 1 wherein said first resistor
changes resistance according to environmental temperature.
7. The bridge circuit of claim 1 wherein said first resistor
changes resistance according to tensile force thereon.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a bridge circuit serving as a part
of a detector. More particularly, the invention relates to a bridge
circuit with reduced drift due to variations in the environmental
temperature and detectors of various kinds incorporating such a
bridge circuit.
[0002] It has been known to make use of the changes in the
resistance of a resistor according to variations in various
measurable physical quantities such as pressure, temperature and
degree of vacuum in forming a bridge circuit by using such a
resistor, as well as to provide detectors of various kinds, such as
pressure sensors and detectors of thermal conductivity for a gas
chromatograph, a strain gauge, a vacuum gauge or a temperature
detector, for detecting physical variables by using such a
circuit.
[0003] FIG. 1 shows the structure of a pressure sensor of a gas
chromatograph, as an example of detector using such a bridge
circuit including a pressure-sensitive resistor 1 enclosed inside a
sealed vessel into which a gas can be introduced to have its
pressure measured. This pressure-sensitive resistor 1 is connected
with three other resistors 2, 3 and 4 to together form a
quadrangle. A constant current circuit 5 is connected to the
junction between the resistors 1 and 4, an amplifier circuit 6 has
one of its input terminals connected to the junction indicated by
numeral 9 between the resistors 1 and 2 and the other of its input
terminals connected to the junction indicated by numeral 8 between
the resistors 3 and 4, and a voltage measuring circuit 7 is
connected to the output terminal of the amplifier circuit 6. The
constant current circuit 5 is for passing a constant current of
several mA through the resistors 1, 2, 3 and 4 forming a Wheatstone
bridge and the voltage difference between the two voltage
measurement points 8 and 9 is amplified by the amplifier circuit 6
and detected by the voltage measuring circuit 7.
[0004] The pressure-sensitive resistor 1 and the other resistors 2,
3 and 4 are adjusted such that they all have the same resistance
when the pressure-sensitive resistor 1 is under a standard
condition not subjected to a pressure to be measured. In other
words, the voltage difference between the two voltage measurement
points 8 and 9 is nearly zero under such a normal condition and
hence the voltage measured by the voltage measuring circuit 7 is
also zero.
[0005] When the pressure-sensitive resistor 1 is subjected to a
pressure, it undergoes a strain due to the applied pressure. The
pressure-sensitive resistor 1 is made of a material with the
property of changing its resistance when strained. As the
resistance of the pressure-sensitive resistor 1 changes due to a
change in pressure, a non-zero voltage difference appears between
the measurement points 8 and 9 and this voltage difference is
inputted through the amplifier circuit 6 to the measurement circuit
7 and is thereby measured.
[0006] Although a pressure-sensitive resistor is connected with
three other resistors as described above to form a pressure sensor,
the resistance of a resistor is affected not only by pressure but
also by other physical variables such as temperature and tension.
Thus, if a resistor is used such that such other physical variables
can be measured, it is possible to form detectors of such other
physical variables such as a temperature detector and a strain
gauge. Japanese Patent Publication Tokkai 9-236592 disclosed a
resistor placed inside a sealed container used as a detector of
thermal conductivity for a gas chromatograph by heating it,
introducing a sample-containing gas into the container and making
use of the fact that the temperature of the resistor changes
according to the thermal conductivity of the gas.
[0007] One of the problems of prior art pressure sensors of the
type described above was that the measured pressure varied,
depending on the environmental temperature. In the pressure sensor
described above with reference to FIG. 1, for example, the
pressure-sensitive resistor 1 was used for detecting changes in a
target gas but if the temperature of the target gas changes as well
as its pressure, the change in resistance due to the change in
pressure and that due to the change in temperature are superposed
and hence the change in pressure could not be measured accurately.
Moreover, the resistance of the pressure-sensitive resistor changes
due to changes in temperature even if the pressure remains
constant, giving rise to the phenomenon of drift in the output.
SUMMARY OF THE INVENTION
[0008] It is therefore an object of this invention to provide an
improved bridge circuit and a sensor using such an improved bridge
circuit with which the effect of drift due to changes in
temperature can be eliminated and the changes in the physical
variable originally intended to be measured can be accurately
measured.
[0009] A bridge circuit embodying this invention, with which the
above and other objects can be accomplished, may be characterize
not only as comprising four resistors forming a Wheatstone bridge,
one of the resistors being set at a position for measuring a
desired physical variable and the bridge being adapted to output a
voltage imbalance due to a change in the physical variable, but
also as being provided with voltage measuring means for measuring
the voltage differences between the two pairs of mutually opposite
junctions of the resistors forming the Wheatstone bridge and a
means for calculating the effects of temperature from these
measured voltage differences to make a correction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The accompanying drawings, which are incorporated in and
form a part of this specification, illustrate an embodiment of the
invention and, together with the description, serve to explain the
principles of the invention. In the drawings:
[0011] FIG. 1 is a block diagram of a prior art bridge circuit;
and
[0012] FIG. 2 is a block diagram of a bridge circuit embodying this
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] FIG. 2 shows an example of bridge circuit embodying this
invention serving as a part of a pressure sensor. Components which
are like or equivalent to those described above with reference to
FIG. 1 are indicated by the same numerals for convenience. Thus,
numeral 1 indicates a pressure-sensitive resistor, numerals 2, 3
and 4 are resistors which form a bridge circuit together with the
pressure-sensitive resistor 1, numeral 5 indicates a constant
current circuit and numeral 6 indicates an amplifier circuit 6.
Numeral 7 indicates a voltage measuring circuit herein referred to
as the "first voltage measuring circuit" because numeral 11
indicates what is herein referred to as the "second voltage
measuring circuit". For reference, the junction between the
resistors 3 and 4 is indicated by numeral 8, the junction between
the resistors 1 and 2 is indicated by numeral 9, and the junction
between the resistors I and 4, at which the second voltage
measuring circuit 11 is connected to the bridge circuit, is
indicated by numeral 10. Numeral 12 indicates a microcomputer
connected to both the first and second voltage measuring circuits 7
and 11. In summary, the circuit shown in FIG. 2 is different from
the prior art circuit shown in FIG. 1 only wherein the second
voltage measuring circuit 11 for measuring the voltage at the
junction 10 and the microcomputer 12 for carrying out various
calculations, to be described below, are additionally provided.
[0014] As explained above with reference to FIG. 1, the constant
current circuit 5 serves to pass a constant current of several mA
through the bridge formed by the resistors 1, 2, 3 and 4, the
voltage difference between the junctions 8 and 9 is amplified by
the amplifier circuit 6 and detected by the first voltage measuring
circuit 7. The pressure-sensitive resistor 1 and the other
resistors 2, 3 and 4 are designed such that they all have the same
resistance when the pressure-sensitive resistor 1 is not subjected
to any pressure to be measured but there may be differences due to
variations at the time of their production. Let us consider a
situation, for example, where the resistance of resistors 1, 3 and
4 is R but that of resistor 2 is R+r. If the total current passed
by the constant current circuit 5 is I, the current which flows
through the branch with resistors 1 and 2 is i.sub.1 and that flows
through the other branch with resistors 3 and 4 is i.sub.2, the
following equations hold: I=i.sub.1+i.sub.2 and
i.sub.1(R+R+r)=i.sub.2(R+R), while the voltage difference .DELTA.V
between the junctions 8 and 9 is given by
.DELTA.V=i.sub.1(R+r)-i.sub.2R. Eliminating i.sub.1 and i.sub.2
from these three equations, one obtains:
.DELTA.V=RrI/(4R+r) (1)
[0015] Suppose that the environmental temperature has changed such
that the resistance of each of the bridge resistors 1, 2, 3 and 4
has increased by a same factor k, or that they each became (1+k)
times as large as the original resistance value. Under this
condition, the voltage difference .DELTA.V.sub.1 between junctions
8 and 9 is be given by:
.DELTA.V.sub.1=(1+k)RrI/(4R+r) (2)
[0016] Thus, the change in the voltage difference before and after
the rise in the temperature is given as follows:
.DELTA..DELTA.V=.DELTA.V.sub.1-.DELTA.V=kRrI/(4R+r) (3)
[0017] In other words, the drift in the output due to the change in
environmental temperature is .DELTA..DELTA.V=kRrI/(4R+r).
[0018] According to the present invention, the voltage V .sub.10 at
junction 10 (or the voltage difference between the junction 10 and
the grounded junction between the resistors 2 and 3) is measured by
the second voltage measuring circuit 11. It is easy to calculate
that this voltage V .sub.10 is given by the following
expression:
V .sub.10=2IR(2R+r)/(4R+r) (4)
[0019] Thus, one can easily ascertain from (2) and (4) that V
.sub.10 and .DELTA..DELTA.V are in a mutually proportional
relationship. In other words, if the voltage V .sub.10 at junction
10 is measured, the value of .DELTA..DELTA.V can be predicted by
means of the microcomputer 12. The output voltage difference
.DELTA.V .sub.1 measured by the first voltage measuring circuit 7
can thus be corrected to obtain the value of .DELTA.V.
[0020] In practice, the values of V .sub.10 and .DELTA..DELTA.V are
preliminarily measured at two different temperatures and the
proportionality constant therebetween obtained by the measurements
is stored in the microcomputer 12. In subsequent occasions, this
stored proportionality constant is retrieved to correct the effects
of temperature changes.
[0021] Although the invention was described above by way of an
example wherein a bridge circuit embodying the invention is used as
a pressure sensor, it goes without saying that an application can
be made equally well to detectors of different kinds such as
detectors of thermal conductivity, strain gauges and vacuum
gauge.
[0022] A bridge circuit embodying this invention may be more
generally described as comprising a Wheatstone bridge circuit with
a first series connection of a first resistor and a second resistor
and a second series connection of a third resistor and a fourth
resistor, where the first series connection and the second series
connection are connected in parallel, a constant current source for
passing a constant current between the junctions of this parallel
connection, a first voltage measuring circuit for measuring the
voltage difference between the junction between the first and
second resistors and the junction between the third and fourth
resistors, a second voltage measuring circuit for measuring the
voltage at one of the junctions of the parallel connection and a
means for correcting the voltage difference measured by the first
voltage measuring circuit by the voltage value measured by the
second voltage measuring circuit.
[0023] According to a preferred embodiment of the invention where
the bridge circuit as described above is used as a detector, one of
the resistors forming a Wheatstone bridge may be provided with a
means for attaching to an object of measurement such that specified
physical characteristics of the target object can be detected
properly. When the bridge circuit is used to measure a gas
pressure, one of the resistors is enclosed inside a sealed
container, as described above, with a means for introducing the
target gas into such a sealed container.
[0024] In summary, a bridge circuit according to this invention is
advantageous in that a stable output with little drift can be
obtained in spite of variations in the environmental
temperature.
* * * * *