U.S. patent number RE42,159 [Application Number 11/320,559] was granted by the patent office on 2011-02-22 for circuit for compensation for time variation of temperature in an inductive sensor.
This patent grant is currently assigned to Microstrain, Inc.. Invention is credited to Steven W. Arms, Christopher P. Townsend.
United States Patent |
RE42,159 |
Townsend , et al. |
February 22, 2011 |
Circuit for compensation for time variation of temperature in an
inductive sensor
Abstract
A sensing device includes a circuit that compensates for time
and spatial changes in temperature. The circuit includes elements
to correct for variation in permeability of a highly permeable core
of a differential variable reluctance transducer as temperature
changes. The circuit also provides correction for temperature
gradients across coils of the transducer.
Inventors: |
Townsend; Christopher P.
(Shelburne, VT), Arms; Steven W. (Williston, VT) |
Assignee: |
Microstrain, Inc. (Williston,
VT)
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Family
ID: |
43597259 |
Appl.
No.: |
11/320,559 |
Filed: |
December 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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60220364 |
Jul 24, 2000 |
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Reissue of: |
09911959 |
Jul 24, 2001 |
06828779 |
Dec 7, 2004 |
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Current U.S.
Class: |
324/207.12;
324/207.24; 324/225 |
Current CPC
Class: |
G01D
5/2013 (20130101); G01D 3/036 (20130101) |
Current International
Class: |
G01B
7/14 (20060101) |
Field of
Search: |
;324/207.12,217.15-207.16,207.19,207.24,225,234,236,239,669-670 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: LeDynh; Bot L
Attorney, Agent or Firm: Leas; James Marc
Parent Case Text
RELATED APPLICATION
This application is a continuation-in-part of Provisional U.S.
Patent Application 60/220,364, filed Jul. 24, 2000.
Claims
What is claimed is:
1. An electronic device, comprising a sensor sensitive to position
of .Iadd.at least one from the group consisting of .Iaddend.a
conductive .[.or.]. .Iadd.material and a .Iaddend.ferrous material,
said sensor comprising a single coil inductance transducer, a
temperature measurement circuit for providing a temperature output
derived from said sensor, a position measuring circuit for
measuring position of said .[.conductive or ferrous.]. material,
and a voltage controlled gain adjusting device, wherein said
temperature measurement circuit provides a voltage proportional to
temperature to said voltage controlled gain adjusting device to
adjust output voltage of said position measuring circuit to provide
temperature compensated sensor data, wherein said temperature
measurement circuit uses a signal derived from resistance of said
single coil inductance transducer to provide said voltage
proportional to temperature.
2. The electronic device as recited in claim 1, wherein said
.[.conductive or ferrous.]. material comprises a magnetically
permeable member, wherein said magnetically permeable member is
moveable.
3. The electronic device as recited in claim 2, wherein said
moveable magnetically permeable member is located within said
single coil inductance transducer.
4. The electronic device as recited in claim 3, wherein said
magnetically permeable member has a member length and said single
coil has a coil length, wherein said member length is about equal
to said coil length.
5. The electronic device as recited in claim 1, wherein said sensor
is a displacement sensor.
6. The electronic device as recited in claim 1, wherein said sensor
comprises input pads for receiving a first signal and a second
signal, said first signal having a higher frequency than said
second signal.
7. The electronic device as recited in claim 1, wherein said
voltage controlled gain adjusting device comprises a variable gain
amplifier.[.or a microprocessor.]. .
8. The electronic device as recited in claim .[.1.].
.Iadd.2.Iaddend., wherein said magnetically permeable member
comprises a highly permeable material.
9. The electronic device as recited in claim 8, wherein said highly
permeable material comprises permalloy.[., ferrite, and 400 series
stainless steel.]. .
10. The electronic device as recited in claim 1, wherein said
magnetically permeable member comprises magnetoelastic
characteristics.
11. The electronic device as recited in claim 10, wherein said
magnetoelastic characteristics are modulated by .Iadd.at least one
from the group consisting of .Iaddend.strain, stress, .[.or.].
.Iadd.and .Iaddend.torque.
12. The electronic device as recited in claim 1, wherein said
sensor is to detect .[.the.]. .Iadd.at least one from the group
consisting of .Iaddend.position .[.or.]. .Iadd.and
.Iaddend.presence of .Iadd.at least one from the group consisting
of .Iaddend.a conductive .[.or.]. .Iadd.target and a
.Iaddend.ferrous target.
13. The electronic device as recited in claim 12, wherein said
single coil and said target are non-contacting and wherein
.[.relative.]. .Iadd.said .Iaddend.position .Iadd.measuring circuit
measures relative position .Iaddend.of said single coil and said
target.[.are measured.]. .
14. The electronic device as recited in claim 12, wherein said
.[.target.]. .Iadd.material .Iaddend.has magnetoelastic
characteristics.
15. The electronic device as recited in claim 1, wherein said
sensor .[.comprises.]. .Iadd.includes at least one from the group
consisting of .Iaddend.a displacement sensor, a force sensor, an
acceleration sensor, a pressure sensor, .[.or.]. .Iadd.and
.Iaddend.a torque sensor.
16. The electronic device as recited in claim 1, wherein said
sensor further comprises a flexure element.
17. An electronic device, comprising a single coil inductance
transducer having a single coil and a magnetically permeable member
that extends in said single coil, said device further comprising a
temperature measurement circuit, a position measuring circuit, and
a voltage controlled gain adjusting device, wherein said
temperature measurement circuit provides a voltage proportional to
temperature to said voltage controlled gain adjusting device to
adjust output voltage of said position measuring circuit to
compensate for a change in temperature in said single coil and in
said member.
18. The electronic device as recited in claim 17, wherein said
magnetically permeable member is movable with respect to said
single coil.
19. The electronic device as recited in claim 17, wherein said
circuit uses resistance of said single coil to compensate for
change in temperature of said single coil and in said member.
20. The electronic device as recited in claim 17, wherein said
single coil inductance transducer comprises a displacement
sensor.
21. The electronic device as recited in claim 17, wherein said
transducer comprises input pads for receiving a first signal and a
second signal, said first signal having a higher frequency than
said second signal.
22. The electronic device as recited in claim 17, wherein said
voltage controlled gain adjusting device comprises a variable gain
amplifier.[.or a microprocessor.]. .
23. The electronic device as recited in claim 17, wherein said
magnetically permeable member comprises a highly permeable
material.
24. The electronic device as recited in claim 23, wherein said
highly permeable material comprises permalloy.[., ferrite, and 400
series stainless steel.]. .
25. The electronic device as recited in claim 17, wherein said
magnetically permeable member comprises magnetoelastic
characteristics.
26. The electronic device as recited in claim 25, wherein said
magnetoelastic characteristics are modulated by .[.stain.].
.Iadd.at least one from the group consisting of strain.Iaddend.,
stress, .[.or.]. .Iadd.and .Iaddend.torque.
27. The electronic device as recited in claim 17, wherein said
magnetically permeable member has a member length and said single
coil has a .Iadd.single .Iaddend.coil length, wherein said member
length is about equal to said .Iadd.single .Iaddend.coil
length.
28. An electronic .[.deuce.]. .Iadd.device.Iaddend., comprising a
single inductor, a .[.conductive or magnetically permeable.].
member .[.coupled to said single inductor,.]. a temperature
measurement circuit, an inductance measuring circuit, and a voltage
controlled gain adjusting device, .Iadd.wherein said member
includes at least one from the group consisting of a conductive
material and a magnetically permeable material, wherein said member
is coupled to said single inductor, .Iaddend.wherein said
temperature measurement circuit provides a voltage proportional to
temperature to said voltage controlled gain adjusting device to
adjust output voltage of said inductance measuring circuit to
provide an adjusted output voltage independent of temperature of
said single inductor and .Iadd.independent of .Iaddend.temperature
of said .[.conductive or magnetically permeable.]. member.
29. The electronic device as recited in claim 28, wherein said
.[.magnetically permeable.]. member is moveable with respect to
said inductor.
30. The electronic device as recited in claim 28, wherein said
circuit uses resistance of said single inductor to compensate for
change in temperature of said single inductor and in said
member.
31. The electronic device as recited in claim 28, wherein said
single inductor, .Iadd.said .Iaddend.member.Iadd., .Iaddend.and
.Iadd.said .Iaddend.circuit comprise a sensor.
32. The electronic device as recited in claim 31, wherein said
single inductor, .Iadd.said .Iaddend.member.Iadd., .Iaddend.and
.Iadd.said .Iaddend.circuit comprise a displacement sensor.
33. The electronic device as recited in claim 32, wherein said
sensor comprises input pads for receiving a first signal and a
second signal, said first signal having a higher frequency than
said second signal.
34. The electronic device as recited in claim 28, wherein said
voltage controlled gain adjusting device comprises a variable gain
amplifier.[.or a microprocessor.]. .
35. The electronic device as recited in claim 28, wherein said
.[.magnetically permeable.]. member comprises a highly
.Iadd.magnetically .Iaddend.permeable material.
36. The electronic device as recited in claim 35, wherein said
highly .Iadd.magnetically .Iaddend.permeable material comprises
permalloy.[., ferrite, and 400 series stainless steel.]. .
37. The electronic device as recited in claim 28, wherein said
.[.magnetically permeable.]. member comprises magnetoelastic
characteristics.
38. The electronic device as recited in claim 37, wherein said
magnetoelastic characteristics are modulated by .Iadd.at least one
from the group consisting of .Iaddend.strain, stress, .[.or.].
.Iadd.and .Iaddend.torque.
39. The electronic device as recited in claim 28, wherein said
.[.magnetically permeable.]. member has a member length and said
single inductor has .[.an.]. .Iadd.a single .Iaddend.inductor
length, wherein said member length is about equal to said
.Iadd.single .Iaddend.inductor length.
40. .[.A.]. .Iadd.An electronic .Iaddend.device comprising a single
component, a temperature measurement circuit, a first parameter
measuring circuit for measuring a value of said single component,
and a voltage controlled gain adjusting device, wherein said
temperature measurement circuit provides a voltage proportional to
temperature to said voltage controlled gain adjusting device to
adjust output voltage of said first parameter measuring circuit to
make adjusted output voltage of said first parameter measuring
circuit independent of change in temperature with time.
41. .[.A circuit.]. .Iadd.The electronic device .Iaddend.as recited
in claim 40, wherein said single component comprises a single
inductor.
42. .[.A circuit.]. .Iadd.The electronic device .Iaddend.as recited
in claim 41, wherein said single inductor has a magnetically
permeable core.
43. The electronic device as recited in claim 42, wherein said
magnetically permeable core has a core length and said single
inductor has a single inductor length, wherein said core length is
about equal to said inductor length.
44. The electronic device as recited in claim 40, wherein said
voltage controlled gain adjusting device comprises a variable gain
amplifier or a microprocessor.
45. The electronic device as recited in claim 40, further
comprising a lower frequency power supply and a higher frequency
power supply connected to provide a lower frequency and a higher
frequency signal to said single component.
46. The electronic device as recited in claim 45, wherein said
lower frequency power supply provides direct current.
47. The electronic device as recited in claim 40, further
comprising a low pass filter and a high pass filter, each connected
to receive an output of said single component.
48. The electronic device as recited in claim 40, further
comprising a demodulator positioned after said high pass
filter.
49. The electronic device as recited in claim 40, further
comprising a difference amplifier connected to receive said low
frequency signal output from said coil, wherein said difference
amplifier provides a voltage proportional to a temperature of said
coil.
50. The electronic device as recited in claim 49, wherein said
difference amplifier comprises an instrumentation amplifier.
51. The electronic device as recited in claim 40, further
comprising a span adjustment circuit.
52. The electronic device as recited in claim 51, wherein said span
adjustment circuit comprises a variable gain amplifier.
53. The electronic device as recited in claim 51, wherein said span
adjustment circuit comprises a microprocessor.
.Iadd.54. The electronic device as recited in claim 28, wherein
said member comprises a magnetoelastic material..Iaddend.
.Iadd.55. The electronic device as recited in claim 28, wherein
said member comprises a target..Iaddend.
.Iadd.56. The electronic device as recited in claim 55, wherein
said single inductor and said target are parts of a non-contacting
position sensor..Iaddend.
.Iadd.57. The electronic device as recited in claim 55, wherein
said target exhibits magnetoelastic characteristics..Iaddend.
.Iadd.58. The electronic device as recited in claim 57, wherein
said single inductor and said target are parts of at least one from
the group consisting of a non-contacting strain sensor, a
non-contacting stress sensor, and a non-contacting torque
sensor..Iaddend.
.Iadd.59. The electronic device as recited in claim 41, further
comprising a conductive material, wherein said single inductor is
coupled to said conductive material..Iaddend.
.Iadd.60. The electronic device as recited in claim 41, further
comprising a magnetoelastic material, wherein said single inductor
is coupled to said magnetoelastic material..Iaddend.
.Iadd.61. The electronic device as recited in claim 41, further
comprising a target..Iaddend.
.Iadd.62. The electronic device as recited in claim 61, wherein
said single inductor and said target are parts of a non-contacting
position sensor..Iaddend.
.Iadd.63. The electronic device as recited in claim 61, wherein
said target includes a material that exhibits magnetoelastic
characteristics..Iaddend.
.Iadd.64. The electronic device as recited in claim 63, wherein
said single inductor and said target are parts of at least one from
the group consisting of a non-contacting strain sensor, a
non-contacting stress sensor, and a non-contacting torque
sensor..Iaddend.
.Iadd.65. The electronic device as recited in claim 1, wherein said
voltage controlled gain adjusting device comprises a
microprocessor..Iaddend.
.Iadd.66. The electronic device as recited in claim 8, wherein said
highly permeable material comprises ferrite..Iaddend.
.Iadd.67. The electronic device as recited in claim 8, wherein said
highly permeable material comprises 400 series stainless
steel..Iaddend.
.Iadd.68. The electronic device as recited in claim 17, wherein
said voltage controlled gain adjusting device comprises a
microprocessor..Iaddend.
.Iadd.69. The electronic device as recited in claim 23, wherein
said highly permeable material comprises ferrite..Iaddend.
.Iadd.70. The electronic device as recited in claim 23, wherein
said highly permeable material comprises 400 series stainless
steel..Iaddend.
.Iadd.71. The electronic device as recited in claim 28, wherein
said voltage controlled gain adjusting device comprises a
microprocessor..Iaddend.
.Iadd.72. The electronic device as recited in claim 35, wherein
said highly permeable material comprises ferrite..Iaddend.
.Iadd.73. The electronic device as recited in claim 35, wherein
said highly permeable material comprises 400 series stainless
steel..Iaddend.
Description
FIELD OF THE INVENTION
This invention generally relates to sensors. More particularly, it
relates to variable reluctance transducers. Even more particularly,
it relates to an improved system for greater accuracy in
determining position or displacement in view of changes in
temperature.
BACKGROUND OF THE INVENTION
Sensors are being developed for use in roads, bridges, dams,
buildings, towers, and vehicles that may provide many information,
including displacement, strain, speed, acceleration, temperature,
pressure, and force.
Temperature changes can effect components within a sensor, and one
challenge has been that sensor output has varied with temperature
of sensor components. The effect of temperature on inductive
transducers limits their overall absolute accuracy. To reduce the
temperature effect, inductive transducers, such as a differential
variable reluctance transformer (DVRT), have been designed with a
differential pair of coils. A DVRT includes a ferrite core that
moves within two coils that are arranged in a bridge configuration.
Inductance and impedance of the coils changes with dis- placement
of the ferrite core within them, and the displacement of the core
is accurately determined by the change in inductance or impedance
of the coils. Since the output signal of a differential pair is the
difference between the output of two coils, temperature changes
that both coils experience equally are theoretically subtracted
out. However, if one coil experiences a different temperature
environment than the other coil, a signal proportional to the
temperature gradient between the two coils will appear at the
circuit output, significantly reducing absolute accuracy.
Commonly assigned U.S. Pat. No. 5,914,593 ("the '593 patent"),
incorporated herein by reference, provides a circuit for a DVRT
that compensates for a temperature gradient across a sensor. The
'593 patent recognizes that a temperature gradient across the coils
can provide a change in the resistance of the wire forming one of
the coils more than it changes the resistance in the other coil,
and this difference in resistance can change the differential
impedance of the coils, essentially mimicking a change in position,
resulting in an error in measured displacement. Situations where
one coil may be hotter than the other coil are common in
applications such as automotive. In the '593 patent a circuit is
provided to adjust the output compensating for the difference in
resistance in the two coils introduced by a temperature gradient or
a temperature difference between the coils.
However, the present inventors found that not all temperature
effects were compensated by the circuit of the '593 patent. Thus, a
better system for acquiring data is needed that provides improved
temperature compensation, and this solution is provided by the
following invention.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
circuit that improves compensation for temperature changes in a
sensor;
It is a further object of the present invention to provide a
circuit that corrects for uniform temperature change over time and
for spacial gradients in temperature;
It is a feature of the present invention to provide an ac signal
and a dc signal to a differential sensor in a Wheatstone bridge
configuration and to correct for time and spacial temperature
differences based on the ac and dc output signals;
It is a feature of the present invention to provide a time varying
temperature compensating circuit in combination with a temperature
gradient compensating circuit;
It is an advantage of the present invention that more accurate data
can be collected from a DVRT that is independent of variations in
temperature either in time or space.
These and other objects, features, and advantages of the invention
are accomplished by an electronic device that includes a sensor
having a magnetically permeable member and a circuit. The circuit
adjusts sensor output to provide sensor output data independent of
temperature of the magnetically permeable member. The circuit uses
a signal derived from resistance of the sensor to correct for
temperature.
Another aspect of the invention is accomplished by an electronic
device that includes a coil, a magnetically permeable member that
extends in the coil, and a circuit. The circuit adjusts output
voltage of the coil to compensate for a change in temperature in
the coil and in the member.
Another aspect of the invention is accomplished by an electronic
device that includes an inductor, a magnetically permeable member
coupled to the inductor, and a circuit. The circuit adjusts a
voltage output of the inductor to provide a voltage independent of
temperature of the inductor and temperature of the magnetically
permeable member.
Another aspect of the invention is accomplished by an electronic
device for sensing at least one parameter that includes a first
circuit element comprising a reactance and a resistance, the first
circuit element comprising input terminals and output terminals.
The input terminals are for providing a first input signal and a
second input signal different from the first signal to the first
circuit element. The output terminals are for providing a first
output signal and a second output signal from the first circuit
element. A second circuit element is connected to the output
terminals to use the first output signal and the second output
signal, wherein the second circuit element generates a first
parameter that depends exclusively on the resistance and a second
parameter that depends exclusively on the reactance. A third
circuit element is connected to the second circuit element wherein
the third circuit element compensates the second parameter for
changes in the first parameter.
Another aspect of the invention is accomplished by a sensor
comprising a component and a circuit. The component is used by the
circuit both for sensing a first parameter and for sensing
temperature. The temperature is used in the circuit for correcting
the first parameter to make output of the sensor independent of
change in temperature with lime.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features, and advantages of the
invention will be apparent from the following detailed description
of the invention, as illustrated in the accompanying drawings, in
which:
FIG. 1 is a block diagram of a circuit to correct a DVRT for time
and spatial variations in temperature;
FIG. 2a is cross sectional view of a DVRT in the null position;
FIG. 2b is cross sectional view of the DVRT of FIG. 2a in the fully
displaced position;
FIG. 3 is a block diagram showing measurement of resistance of one
coil of a DVRT to correct a DVRT for time and spatial variations in
temperature;
FIG. 4a is a cross sectional view of a single coil DVRT in the null
position;
FIG. 4b is cross sectional view of the DVRT of FIG. 4a in the fully
displaced position;
FIG. 5 is a flow chart showing the algorithm of FIG. 1 to
compensate for change in temperature in the permeable member;
and
FIG. 6 is a graph of test data comparing a device of the present
invention to a device without the circuit element for compensating
for temperature changes in the highly permeable member.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors recognized that a DVRT sensor that includes
the circuit of '593 patent, incorporated herein by reference,
provides substantial advantage in temperature compensation, but
does not include all the temperature correcting needed. They
recognized that a bridge circuit, such as that included in a DVRT,
cancels the effects of uniform changes in temperature in the wiring
of the coils of the DVRT, a substantial advantage for such
differential sensors. They also recognized that the circuit of the
'593 patent adequately corrects for the different coil wire
resistances introduced by temperature gradients across the coils,
an advantage in applications, such as automotive where such
temperature gradients are common. The inventors then recognized an
additional mechanism by which a change in temperature can cause
errors: the magnetic permeability of the ferrite core varies with
temperature. This change in permeability with temperature
introduces a change in the inductance and reactance of the coils.
They also recognized that uniform temperature change of the core is
not canceled out by the bridge configuration of a DVRT and that no
correction was provided by the gradient of the temperature
correcting circuit of the '593 patent that corrected only for
spacial variation in temperature.
The present invention provides a circuit that corrects for
variation in permeability of the ferrite core as a result of a
spatially uniform change in temperature. The present invention also
integrates the new circuit with the circuit of the '593 application
to compensate both for temperature changes in the ferrite core and
for temperature gradients across the coils so as to provide very
precise displacement sensing that is independent of both changes in
temperature over time in the windings of the coils or in the
ferrite core and for temperature gradients across the coils. In
addition, the present invention enables using a single coil
inductance transducer which until now has not been practical in
part because of their higher sensitivity to temperature variation.
The sensitivity to temperature variation is eliminated by this
invention.
Inductive displacement sensors utilize a coil and a permeable core
that can be moved relative to the coil. When the core is moved into
the coil additional permeable material moves in, and this increases
the inductance of the coil. A corresponding reduction in inductance
occurs when the core moves out of the coil. In most systems a
single core is used with two coils. The core leaves one coil as it
enters more deeply into the other. The two coils are wired to
provide a differential measurement, and this differential
measurement lowers sensitivity to temperature change in the wiring
since an increase or decrease in resistance in both two coils is
cancelled by the differential measurement.
However the differential measurement is inadequate to correct for
changes in permeability of the ferrite core except when an equal
amount of the core is in both coils. In this unique case the error
due to change in permeability of the core with temperature is zero.
As soon as the core is displaced from the null position, however,
so more is in one coil than the other, the differential measurement
amplifies error, and the amount of error increases with the
displacement from the null position.
In view of the temperature dependence of core permeability,
accuracy of inductive sensors was limited to about 0.05% gain
error/degree C. at the fully displaced core position. Thus, for a
100 degree temperature swing the error is 5%, which may be
unacceptable large for some applications. A unique solution to
reduce the error from temperature variation is provided by the
present invention, and this solution can be applied in both single
coil and dual coil transducers. By overcoming the error introduced
by temperature variation the accuracy of the single coil sensor is
substantially improved. However, dual coil transducers continue to
have advantage since they also cancel out error from change in
winding resistance with change in temperature.
Differential pair of coils 20a, 20b for displacement sensor 22 are
arranged in full Wheatstone bridge circuit 24a, as shown in FIG. 1
and FIGS. 2a, 2b. A single coil in a half Wheatstone bridge circuit
can also be used. Both an AC sinusoidal voltage and a DC voltage
are applied across bridge inputs 26a, 26b. Typically AC voltage is
about 5 volts peak to peak at about 70 kHz and DC voltage is about
1 volt. Of course various other voltage amplitudes and frequencies
can be used for each. For example, instead of a DC voltage a low
frequency AC voltage can be used, such as 1 volt at 100 Hz. This
would enable the use of an AC synchronous demodulator, as shown in
FIG. 1, which improves signal to noise ratio in a high noise
environment.
Capacitors 28a, 28b and inductors 30a, 30b are used to isolate AC
and DC drive voltages from each other. Capacitors 28a, 28b can have
a value of about 1 microfarad and inductors 30a, 30b a value of
about 1000 microhenries. Various other values can be used depending
on the two frequencies of the two applied voltages.
The DC resistance of each coil 20a, 20b of Wheatstone bridge 24a is
proportional to temperature. This DC resistance of wiring in coils
20a, 20b is independent of the position of core 41 within coils
20a, 20b, varying only with temperature, so a change in DC voltage
across the coils, which is proportional to their resistance,
provides a measurement of a change in temperature. Coils 20a, 20b
are fabricated of wire having a dimension of about 48 gauge and
typically has a resistance of about 20 ohms for each coil.
Because the DC resistance of each coil is very low greater accuracy
is obtained by summing DC voltages across both coils to provide an
indication of the temperature of the coils. This DC voltage is
obtained by passing output signals from output terminals 32a, 32b
of coils 20a, 20b through low pass filter 34 and summing the
voltage drops across each coil 20a, 20b individually in summing
amplifier 40. Typically the voltage sum is about 100 mV at room
temperature and this will vary as temperature changes.
Voltage across single coil 20a of pair of coils 20a, 20b can also
be used to provide a measure of temperature, as shown in FIG. 3.
Difference amplifier 40' is used instead of summing amplifier 40
and the voltage provided to difference amplifier 40' is the voltage
across one of the coils 20a or 20b. Sensor 22' having single coil
20 can also be used, as shown in FIGS. 4a, 4b, and correction for
temperature variation equally provided. This measured temperature
can be used to correct for variation in the wiring of the single
coil and variation in the permeability of core 41. Advantage in
having a single coil is that the sensor can be smaller since half
the length is needed to measure the same displacement with a single
coil.
Displacement is measured by measuring the change in AC voltage
across the coil and adjusting this voltage to correct for the
temperature. The AC voltage difference signal, or a signal
conditioned to correct for a temperature gradient, is provided to
voltage controlled variable gain amplifier (VGA) 42, as shown in
FIG. 1. The sum of the DC voltages from summing amplifier 40 is fed
to modulate the gain of VGA 42 to correct the AC difference signal
from the coils for changes in temperature. The gain of VGA 42 is
modulated by the DC voltage signal in the opposite direction of the
gain error caused by the temperature sensitive permeability of the
core, providing a corrected output voltage that is now more nearly
independent of temperature. This temperature output voltage is now
used to determine the displacement value. Wiring of the device
itself is used to correct readings for changes in temperature in
another part of the device, the core. Alternatively, a programmable
device, such as a microprocessor, can be used instead of a VGA to
apply the compensation by using appropriate scale factors for
temperature stored in a memory associated with the microprocessor.
The microprocessor can also be used to provide the excitation
signals.
The present invention is easily combined with the technique
described in the '593 patent, and the combination is shown in FIG.
1, providing a complete temperature compensation method for both
time and spacial temperature variation. Several components are used
by both elements, so only VGA 42 need be added to the circuit of
the '593 patent. Voltage controlled amplifier 42 is connected to
receive gradient compensated output from difference amplifier 50
and temperature information from summing amplifier 40, and its
output provides the additional correction for temperature change in
ferrite core 41.
As more fully described in the '593 patent, incorporated herein by
reference, temperature gradient correcting circuit 52 provides a DC
voltage level corresponding to amplitude of displacement of core 41
corrected for spatial variation in temperature across coils 20a,
20b. The AC signal between output terminals 32a, 32b of coils 20a,
20b is first analyzed by stripping off DC and low frequency signal
at high pass filter 44. The AC signal is now converted to a DC
level in AC synchronous demodulator 54. The DC level gives an
uncompensated indication of the magnitude of displacement of core
41 in coils 32a, 32b. A first compensation step is now provided to
the signal from demodulator 54 with gradient of temperature
information from DC signal conditioner 56 in difference amplifier
50. DC signal conditioner 56 provides an output that is
proportional to the difference in temperature between the two coils
to give the temperature gradient information. The output of
difference amplifier 50 is a signal proportional to the
displacement of core 41 corrected for gradient of temperature
across coils 20a, 20b. The output of VGA 42 is a signal
proportional to the displacement of core 41 corrected for both
gradient of temperature across coils 20a, 20b and for a change of
temperature with time across both coils 20a, 20b and core 41.
In one alternative, a low frequency signal can be used instead of a
DC signal. In this case AC synchronous demodulator 58 would be used
in place of signal conditioner 56, and similarly connected as shown
by dotted line 59 of FIG. 1.
To summarize, summing amplifier 40 receives low frequency inputs
from both coils after the signal passes through low pass filter 34.
Summing the low frequency outputs gives a voltage that tracks with
temperature. AC synchronous demodulator 56 takes the difference in
voltages across the coils on the two sides of the bridge and
converts to a DC voltage to be used for compensating for the
gradient in temperature. The DC difference voltage represents the
temperature gradient across the two coils arranged in a bridge
configuration. No correction is needed for a uniform temperature
change with time same the bridge circuit automatically cancels that
out. Some temperature differences do not equally effect both sides
of the bridge, such as a change in permeability of the core 41 with
temperature. In this case, there will be no cancellation, and
output of the sensor will vary with temperature. This variation can
be corrected by determining the temperature by measuring resistance
of the windings with a DC or low frequency signal and using that
result to correct the high frequency signal. The low frequency
signal can also be used to correct for gradient of temperature.
The use of a voltage controlled gain amplifier modulated with a
voltage containing temperature information can be used in any
system using a coil to measure a physical parameter. These coils
may be wound on a bobbin or coil form, may be free standing, or can
be etched or patterned onto a substrate. The substrate can be a
circuit board, and can also include signal conditioning
electronics. These coils may be used for other purposes than
displacement measurement, such as to detect the position or
presence of conductive and ferrous targets. The coil and the target
may be used in non-contacting position sensors and in plunger type
position sensors, where the relative position of the coil and the
target are measured. These types of devices include displacement
sensors, but can be combined with the appropriate flexure element
to construct force, acceleration, pressure, and torque sensors.
Targets may also be a gear tooth or may be patterned onto a
substrate such that the output of the sensing coil produces a
waveform which can be used to measure linear or angular position.
Furthermore, the target material may exhibit magnetoelastic
characteristics, such as permalloy ribbons and maraging steels and
these magnetoelastic targets may be combined with sensing coils to
produce non-contact strain, stress, and torque sensors.
The circuit of the present invention is a physical embodiment of a
linear equation linking displacement of core 41 and voltage across
coils: D=M(T)V.sub.AC(.omega.L.sub.1-.omega.L.sub.2)G(T)+C (1)
where D is the displacement of core 41. At any particular
temperature, M(T) is a constant indicating the slope of the linear
relationship between displacement and measured AC voltage V. M(T)
depends on the permeability of core 41, on the magnitude of
V.sub.AC and the voltage gain of amplifiers in AC synchronous
demodulator 54. Since the permeability of core 41 depends on
temperature M(T) will be a function of temperature too. More
specifically, equation (1) links displacement D of core 41 to
voltage V across coils 20a, 20b at output pads 32a, 32b. .omega. is
the frequency of the AC signal applied across pads 26a, 2b. L.sub.1
is the inductance of coil 20a and L.sub.2 is the inductance of coil
20b including the effect of core 41 within each coil. G(T) is a
function that has an inverse relationship to temperature to that of
M(T) to compensate for the change in permeability of core 41 with
temperature so that product of M(T) and G(T) is constant at all
temperatures. C is the y intercept of the linear relationship and
is a constant that depends on the definition of the initial
position of core 41core 41. Voltage
V(.omega.L.sub.1-.omega.L.sub.2) is determined from voltages
proportional to resistance and reactance in coils 32a, 32b of
sensor 22. For each coil, Z=.omega.L+R (2) where Z is the impedance
of that coil including both impedance from coil resistance and
impedance from coil inductance. The voltage associated with the
impedance Z is what is measured at pads 32a, 32b. Coil inductance L
includes the effect of turning the wire in a coil and the effect of
core 41 within that wiring. Thus, L for each coil is a function of
displacement of core 41. L is the total inductance of that coil
with core 41 as it is located within the coil for a particular
displacement of core 41. R is the resistance of the wiring making
up that particular coil. R varies with temperature of the wiring
and may be determined from the DC signal applied to pads 26a, 26b
as described herein above. However, in the circuit of the invention
the voltage associated with this resistance is actually determined
and subtracted to obtain the voltage from the inductive reactance
alone. Thus, for the first coil:
V.sub.AC(.omega.L.sub.1)=V.sub.AC(Z.sub.1)-V.sub.DC(R.sub.1) (3)
where V.sub.AC(.omega.L.sub.1) is the voltage associated with the
inductive reactance of coil 1. V.sub.AC(Z.sub.1) is the voltage
associated with the measured impedance of coil 1 and
V.sub.DC(R.sub.1) is the voltage associated with the resistance of
wiring within coil 1. A similar equation applies to coil 2.
difference amplifier 50 subtracts the sum of voltages associated
with wire resistance of both coils from the voltage associated with
the overall impedance of the coils to obtain the voltage associated
with just the inductive reactance of the coils. This voltage is now
corrected according to G(T) for temperature change in core 41 based
on the same resistance used in equations (2) and (3).
The invention follows the steps shown in the flow chart in FIG. 5.
AC voltage difference measured across pads 32a, 32b provides a
measure of impedance (Z.sub.1-Z.sub.2) of coils 20a, 20b, as shown
in step 60. DC voltage across coil 20a plus DC voltage across coil
20b provides a measure of the sum of the resistances of coils 20a,
20b, as shown in step 62. Temperature can be calculated from the
resistances as shown in step 64 using the equation T=KR+B (4)
The voltage associated with the resistance is actually used by
difference amplifier 50 to calculate the voltage difference
associated just with the difference in inductive reactance of the
two coils, as shown in step 66. Temperature correction of voltage
is now provided for change in temperature of core 41. Displacement
is now calculated as shown in step 68.
Test results for a device of the present invention are shown in
FIG. 6 in comparison with a standard uncompensated DVRT. Both
devices have the gradient of temperature compensation provided by
the '593 patent. The bottom trace shows substantial variation in
output voltage as temperature changes from 25 C. to 100 C. and then
back to 40 C. The top trace shows no change in output voltage over
these same temperature changes.
While several embodiments of the invention, together with
modifications thereof, have been described in detail herein and
illustrated in the accompanying drawings, it will be evident that
various further modifications are possible without departing from
the scope of the invention. Temperature corrections for the output
of a DVRT have been described but the same concept of separating
resistive from reactive components of an output signal and using
the resistive part to provide temperature correction for the
reactive part can be used for other types of sensors, devices, and
circuits as well. Nothing in the above specification is intended to
limit the invention more narrowly than the appended claims. The
examples given are intended only to be illustrative rather than
exclusive.
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