U.S. patent application number 13/477847 was filed with the patent office on 2013-11-28 for offset error compensation systems and methods in sensors.
The applicant listed for this patent is Udo Ausserlechner, Mario Motz. Invention is credited to Udo Ausserlechner, Mario Motz.
Application Number | 20130314075 13/477847 |
Document ID | / |
Family ID | 49621105 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130314075 |
Kind Code |
A1 |
Ausserlechner; Udo ; et
al. |
November 28, 2013 |
OFFSET ERROR COMPENSATION SYSTEMS AND METHODS IN SENSORS
Abstract
Embodiments relate to reducing offset error in sensor systems.
In embodiments, the sensitivity and offset of a sensor depend
differently on some parameter, e.g. voltage, such that operating
the sensor at two different values of the parameter can cancel the
offset error. Embodiments can have applicability to stress sensors,
Hall plates, vertical Hall devices, magnetoresistive sensors and
others.
Inventors: |
Ausserlechner; Udo;
(Villach, AT) ; Motz; Mario; (Wernberg,
AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ausserlechner; Udo
Motz; Mario |
Villach
Wernberg |
|
AT
AT |
|
|
Family ID: |
49621105 |
Appl. No.: |
13/477847 |
Filed: |
May 22, 2012 |
Current U.S.
Class: |
324/202 ;
73/1.01 |
Current CPC
Class: |
G01D 3/036 20130101;
G01R 33/07 20130101; G01R 33/072 20130101; G01D 5/145 20130101;
G01L 1/12 20130101; G01R 33/091 20130101; G01R 35/00 20130101 |
Class at
Publication: |
324/202 ;
73/1.01 |
International
Class: |
G01R 35/00 20060101
G01R035/00; G01N 3/62 20060101 G01N003/62 |
Claims
1. A sensor configured to sense a physical characteristic
comprising: at least one sensor element having an output, wherein
an output signal comprises an offset error in an absence of the
physical characteristic; an input quantity other than the physical
characteristic that affects the offset error, wherein in a first
phase of operation of the sensor a first input quantity produces a
first output signal having a first offset error, and wherein in a
second phase of operation of the sensor a second input quantity
different from the first input quantity produces a second output
signal having a second offset error; and offset correction
circuitry coupled to the output and configured to provide a sensor
output signal comprising a sum of the first output signal and a
product of the second output signal and a correction factor chosen
to offset a difference between the first offset error and the
second offset error.
2. The sensor of claim 1, wherein the input quantity is selected
from the group consisting of a voltage, a current or a bias
magnetic field.
3. The sensor of claim 1, wherein the at least one sensor is
selected from the group consisting of a Hall-effect sensor, a
vertical Hall sensor, a spinning current vertical Hall sensor, a
magnetoresistive sensor, and a stress sensor.
4. The sensor of claim 1, wherein the at least one sensor element
comprises a plurality of sensor elements coupled in a sensor
bridge.
5. The sensor of claim 1, wherein the offset correction circuitry
comprises a shift register coupled to the output, and wherein the
first and second output signals are stored by the shift
register.
6. The sensor of claim 1, wherein the correction factor comprises
-1*(the first offset error)/(the second offset error).
7. A method comprising: operating a sensor in a first operating
phase having a first sensor input quantity to obtain a first sensor
output signal and a first sensor offset error; operating the sensor
in a second operating phase having a second sensor input quantity
to obtain a second sensor output signal and a second sensor offset
error; and providing a total sensor output signal comprising the
sum of the first and second sensor output signals adjusted by a
offset correction factor related to the first and second sensor
offset errors.
8. The method of claim 7, further comprising determining the offset
correction factor by dividing the first sensor offset error by the
second sensor offset error and multiplying the result by -1.
9. The method of claim 7, wherein providing a total output signal
further comprises summing the first sensor output signal with a
product of the offset correction factor and the second sensor
output signal.
10. The method of claim 7, wherein the offset correction factor is
chosen to cancel an offset error of the sensor.
11. The method of claim 7, wherein the first and second sensor
input quantities are selected from the group consisting of a
voltage, a current and a magnetic field.
12. The method of claim 7, wherein the sensor has an offset error
in the absence of a physical quantity to be sensed by the
sensor.
13. The method of claim 12, wherein the physical quantity comprises
a magnetic field, a voltage, a current, or a temperature.
14. A sensor comprising: at least one sensor element configured to
sense a characteristic and having an input and an output; and
offset compensation circuitry coupled to the output and configured
to cancel an offset error of the sensor by correcting an output
signal of the at least one sensor by a correction factor related to
an offset error of the sensor when operated in a first phase and an
offset error of the sensor when operated in a second phase.
15. The sensor of claim 14, wherein a signal at the input of the at
least one sensor element is different in the first phase and the
second phase.
16. The sensor of claim 15, wherein a signal at the output of the
at least one sensor element is different in the first phase and the
second phase.
17. The sensor of claim 14, wherein the correction factor comprises
a result of dividing the offset error in the first phase by the
offset error in the second phase and multiplying by -1.
18. The sensor of claim 14, wherein the sensor comprises a magnetic
field sensor, a stress sensor, or a current sensor.
19. The sensor of claim 14, wherein a total output of the sensor
comprises the sum of outputs of the sensor in the first and second
phases adjusted by the correction factor.
20. The sensor of claim 19, wherein the total output of the sensor
comprises the sum of the output of the sensor in the first phase
and a product of the output of the sensor in the second phase and
the correction factor.
Description
TECHNICAL FIELD
[0001] The invention relates generally to sensors and more
particularly to compensating for offset errors in sensors.
BACKGROUND
[0002] Sensors often are used as sensor bridges, for example with
four identical sensor elements coupled in a Wheatstone bridge
configuration. Bridge circuits are supplied by a voltage or current
and provide a differential output voltage. Examples include stress
sensors, magnetoresistive sensors, and Hall plates and vertical
Hall devices, among others.
[0003] A common problem with sensor bridges, however is offset
error. Offest is the output signal in the absence of the physical
quantity which the sensor should detect. For example, for Hall
plates the offset is the output signal at zero applied magnetic
field, and for stress sensors it is the output signal at zero
mechanical stress. The origin of offset error typically is a slight
mismatch between the sensor elements of the bridge. In other words,
the "identical" sensor elements are not exactly identical. A
typical mismatch is on the order of about 0.1% to about 1%, which
means that although the four sensor elements have identical
resistances they actually differ by about 0.1-1%.
[0004] Conventional approaches include, for Hall sensors, using the
spinning current principle which uses different sensor elements in
multiple clock phases to cancel any offset and enhance magnetic
field proportional terms in the signals. This technique can be
extended to more than two clock phases by reversing supply
polarities and using more than four sensor elements. This
technique, however, still results in a small offset error, referred
to as the residual offset. The residual offset typically is on the
order of about 30 micro-Tesla for Hall plates and about 0.5-1 mT
for vertical Hall devices.
[0005] Therefore, there is a need for improved offset error
compensation for sensors.
SUMMARY
[0006] Embodiments relate to offset error compensation in sensors.
In an embodiment, a sensor configured to sense a physical
characteristic comprises at least one sensor element having an
output, wherein an output signal comprises an offset error in an
absence of the physical characteristic; an input quantity other
than the physical characteristic that affects the offset error,
wherein in a first phase of operation of the sensor a first input
quantity produces a first output signal having a first offset
error, and wherein in a second phase of operation of the sensor a
second input quantity different from the first input quantity
produces a second output signal having a second offset error; and
offset correction circuitry coupled to the output and configured to
provide a sensor output signal comprising a sum of the first output
signal and a product of the second output signal and a correction
factor chosen to offset a difference between the first offset error
and the second offset error.
[0007] In an embodiment, a method comprises operating a sensor in a
first operating phase having a first sensor input quantity to
obtain a first sensor output signal and a first sensor offset
error; operating the sensor in a second operating phase having a
second sensor input quantity to obtain a second sensor output
signal and a second sensor offset error; and providing a total
sensor output signal comprising the sum of the first and second
sensor output signals adjusted by a offset correction factor
related to the first and second sensor offset errors.
[0008] In an embodiment, a sensor comprises at least one sensor
element configured to sense a characteristic and having an input
and an output; and offset compensation circuitry coupled to the
output and configured to cancel an offset error of the sensor by
correcting an output signal of the at least one sensor by a
correction factor related to an offset error of the sensor when
operated in a first phase and an offset error of the sensor when
operated in a second phase.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0010] FIG. 1 is a diagram of a stress sensor circuit according to
an embodiment.
[0011] FIG. 2 is a diagram of a Hall plate circuit according to an
embodiment.
[0012] FIG. 3 is a diagram of a vertical Hall circuit according to
an embodiment.
[0013] FIG. 4 is a diagram of an offset correction circuit
according to an embodiment.
[0014] While the invention is amenable to various modifications and
alternative forms, specifics thereof have been shown by way of
example in the drawings and will be described in detail. It should
be understood, however, that the intention is not to limit the
invention to the particular embodiments described. On the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION
[0015] Embodiments relate to reducing offset error in sensor
systems. In embodiments, the sensitivity and offset of a sensor
depend differently on some parameter, e.g. voltage, such that
operating the sensor at two different values of the parameter can
cancel the offset error. Embodiments can have applicability to
stress sensors (FIG. 1), Hall plates (FIG. 2), vertical Hall
devices (FIG. 3), magnetoresistive sensors and others.
[0016] The sensitivity S and offset Off of a sensor depend
differently on certain parameters, such as the supply voltage. In
embodiments, operating the sensor at a first supply voltage Usup1
and a second supply voltage Usup2 provides two different output
signals Ua1 and Ua2 that each depend on two unknowns: the physical
quantity Q to be measured by the sensor and the Offset Off. In
other words:
Ua1=S1Q+Off1
Ua2=S2Q+Off2
Some linear combination of the signals can be found which will
cancel the offsets:
Ua1+kUa2=(S1+kS2)Q+(Off1+kOff1)
where k=-Off1/Off2. Thus, the offset, or zero point error, is
removed:
Ua,total=Ua1+kUa2=(S1+kS2)Q
[0017] Embodiments typically will not be applicable to perfectly
linear sensors, those for which both sensitivity and offset depend
in the same way on the parameter, such voltage. In practice,
however, sensors are rarely perfectly linear, and all semiconductor
sensors are nonlinear due to junction field effects. Therefore,
embodiments are generally and widely applicable.
[0018] A first example embodiment relates to the stress sensor 100
of FIG. 1, of which the individual resistors R.sub.n are diffused
resistors in silicon and the resistors R.sub.n are aligned in two
different directions, .phi. and .phi.+.alpha.. In an embodiment, a
1V supply voltage is applied to bridge circuit 100, and the output
voltage Ua1 is measured. Then, a 2V supply voltage is applied to
bridge circuit 100, and the output voltage again is measured. The
1V and 2V supply voltage values are merely exemplary for purposes
of this example and can vary in embodiments. Typically the
sensitivity of bridge 100 with respect to mechanical stress is
directly proportional to the supply voltage, yet the resistance and
thus the offset voltage is not purely directly proportional to the
supply voltage but contains some quadratic terms. Therefore, the
offset error of bridge 100 at 2V is more than twice the offset
voltage at 1V. For this example discussion, it is assumed that this
factor is 2.1, which will vary in embodiments. Next, the output
voltage at the second voltage Ua2 is divided by the factor 2.1 and
then the output voltage at the first supply Ua1 is subtracted from
the result. This removes the offset error. Mathematically:
Ua1(Usup=1V)=S(Usup=1V)*STRESS+Off(Usup=1V)
Ua2(Usup=2V)=S(Usup=2V)*STRESS+Off(Usup=2V)
with
S(Usup=2V)=2*S(Usup=1V)
and
Off(Usup=2V)=2.1*Off(Usup=1V).
Then the following is determined:
Ua2(Usup=2V)/2.1-Ua1(Usup=1V)
which is identical to
(S(Usup=2V)/2.1-S(Usup=1V))*STRESS
because the terms
Off(Usup=2V)/2.1-Off(Usup=1V)=0.
[0019] Referring to FIGS. 2 and 3, other examples relate to Hall
effect devices, such as ordinary Hall plates 200 or vertical Hall
devices 300. In embodiments, the aforementioned spinning current
technique is used, in which the Hall device has several contacts,
some of which are used as supply terminals and others as sense
terminals in a first clock phase, and in other clock phases the
roles of supply and sense terminals are exchanged and the signs of
voltage or current supplies inverted. The sensed signals are then
added with proper signs. After adding all of the signals with
proper signs, the sensor has a first output signal Ua1 with a first
residual zero point offset Off1. Next, the sensor system operates
Hall device 200 or 300 at a different supply voltage or supply
current, which provides a different magnetic sensitivity, a
different second output signal Ua2 and a different second residual
zero point error Off2. Finally the total output can be
calculated:
Ua_total=Ua1+k*Ua2
where k=-Off1/Off2.
[0020] Another example embodiment relates to magnetoresistive
sensor bridges, such as giant magnetoresistive (GMR) sensors. In
this embodiment, instead of applying two different voltages, two
different magnetic fields are applied. These fields are referred to
as secondary fields to distinguish them from the primary field from
an external source to be detected by the sensor. The sensor system
has control over the secondary magnetic field but not the primary.
In embodiments, therefore, an electromagnet, coil, wire or other
source is arranged proximate the
[0021] GMR sensor bridge in order to generate the secondary
magnetic field when the system injects some current through it. In
particular, the secondary magnetic field can be orthogonal to the
primary magnetic field, and the GMR can be constructed in such a
way so as to respond mainly on the primary magnetic fields and only
with much lower sensitivity to the secondary magnetic fields.
[0022] In operation, the sensor system can apply a first secondary
magnetic field (e.g. zero) to the GMRs and sample the output signal
Ua1:
Ua1=S1*Bx+Off(By1)
where S1 is the magnetic sensitivity of the sensor bridge during
this first operating phase, Bx is the primary magnetic field to be
detected by the system and Off is the offset error of the bridge,
which is assumed to be a function of the secondary magnetic field
By1, where Bx and By1 are perpendicular to one another. Next, a
second secondary magnetic field is applied to the GMRs, and the
output signal Ua2 is sampled:
Ua2=S2*Bx+Off(By2)
[0023] Finally, the total output is determined:
Ua_total=Ua1+k*Ua2
where k=-Off(By1)/Off(By2). Thus:
Ua_total=(S1+k*S2)*Bx
which no longer has offset error.
[0024] While examples comprising bridge configurations have been
given, bridges need not be used. The GMR embodiment, for example,
does not rely on any bridge property.
[0025] In view of the above-discussed embodiments, and referring to
FIG. 4, an example offset correction circuit 400 according to
embodiments is depicted. Circuit 400 comprises at least one sensor
402, such as any of the sensors discussed herein above. Sensor 402
is supplied by two supplies, U1 and U2, sequentially, via switches
S1 and S2 and clocked by a master clock oscillator 404 in an
embodiment. An output signal of sensor 402 can be amplified in
embodiments by an analog-to-digital converter (not shown in FIG. 4)
and fed to a shift register 406 synchronously with master clock
404. The n-th value in shift register 406 is delayed by n clock
cycles and is multiplied by a suitable chosen constant k and added
to the (n+1)-th value in shift register 406. The result is sampled
in a track and hold circuit 408 and is the offset compensated
output.
[0026] Various embodiments of systems, devices and methods have
been described herein. These embodiments are given only by way of
example and are not intended to limit the scope of the invention.
It should be appreciated, moreover, that the various features of
the embodiments that have been described may be combined in various
ways to produce numerous additional embodiments. Moreover, while
various materials, dimensions, shapes, configurations and
locations, etc. have been described for use with disclosed
embodiments, others besides those disclosed may be utilized without
exceeding the scope of the invention.
[0027] Persons of ordinary skill in the relevant arts will
recognize that the invention may comprise fewer features than
illustrated in any individual embodiment described above. The
embodiments described herein are not meant to be an exhaustive
presentation of the ways in which the various features of the
invention may be combined. Accordingly, the embodiments are not
mutually exclusive combinations of features; rather, the invention
may comprise a combination of different individual features
selected from different individual embodiments, as understood by
persons of ordinary skill in the art.
[0028] Any incorporation by reference of documents above is limited
such that no subject matter is incorporated that is contrary to the
explicit disclosure herein. Any incorporation by reference of
documents above is further limited such that no claims included in
the documents are incorporated by reference herein. Any
incorporation by reference of documents above is yet further
limited such that any definitions provided in the documents are not
incorporated by reference herein unless expressly included
herein.
[0029] For purposes of interpreting the claims for the present
invention, it is expressly intended that the provisions of Section
112, sixth paragraph of 35 U.S.C. are not to be invoked unless the
specific terms "means for" or "step for" are recited in a
claim.
* * * * *