U.S. patent application number 16/282284 was filed with the patent office on 2020-08-27 for planar linear inductive position sensor having edge effect compensation.
This patent application is currently assigned to Microsemi Corporation. The applicant listed for this patent is Microsemi Corporation. Invention is credited to Bala Sundaram Nauduri, Sudheer Puttapudi, Ganesh Shaga.
Application Number | 20200271480 16/282284 |
Document ID | / |
Family ID | 1000003894861 |
Filed Date | 2020-08-27 |
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United States Patent
Application |
20200271480 |
Kind Code |
A1 |
Shaga; Ganesh ; et
al. |
August 27, 2020 |
PLANAR LINEAR INDUCTIVE POSITION SENSOR HAVING EDGE EFFECT
COMPENSATION
Abstract
A planar linear inductive position sensor is formed on a
substrate and includes at least one oscillating coil, a first
sensing coil having opposing edges extending beyond opposing edges
of the oscillating coil along a linear axis along which a linear
position of a conductive object is to be sensed, and a second
sensing coil having opposing edges extending beyond opposing edges
of the oscillating coil along the linear axis. The first and second
sensing coils have geometries selected such that equal opposing
magnetic fields are induced in the first and second sensing coils
in the presence of a magnetic field generated by the oscillating
coil when no conductive target is proximate to the first and second
sensing coils and unequal opposing magnetic fields are induced in
the first and second sensing coils when the conductive target is
proximate to the first and second sensing coils, a difference in
the unequal opposing magnetic fields induced in the first and
second sensing coils correlated to the position of the conductive
target.
Inventors: |
Shaga; Ganesh; (Warangal,
IN) ; Nauduri; Bala Sundaram; (Hyderabad, IN)
; Puttapudi; Sudheer; (Hyderabad, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Microsemi Corporation |
Chandler |
AZ |
US |
|
|
Assignee: |
Microsemi Corporation
Chandler
AZ
|
Family ID: |
1000003894861 |
Appl. No.: |
16/282284 |
Filed: |
February 21, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01D 5/2053 20130101;
G01D 5/2073 20130101; G01D 5/204 20130101 |
International
Class: |
G01D 5/20 20060101
G01D005/20 |
Claims
1. A planar linear inductive position sensor comprising: a
substrate; at least one oscillating coil; a first sensing coil
having opposing edges extending beyond opposing edges of the
oscillating coil along a linear axis along which a linear position
of a conductive object is to be sensed; a second sensing coil
having opposing edges extending beyond opposing edges of the
oscillating coil along the linear axis; the first and second
sensing coils having geometries selected such that equal opposing
magnetic fields are induced in the first and second sensing coils
in the presence of a magnetic field generated by the oscillating
coil when no conductive target is proximate to the first and second
sensing coils and unequal opposing magnetic fields are induced in
the first and second sensing coils when the conductive target is
proximate to the first and second sensing coils, a difference in
the unequal opposing magnetic fields induced in the first and
second sensing coils correlated to the position of the conductive
target.
2. The planar linear inductive position sensor of claim 1 wherein
the substrate is a printed circuit board.
3. The planar linear inductive position sensor of claim 1 wherein
the oscillator coil is shaped as a rectangle, a long side of the
rectangle aligned in the direction of travel of the conductive
target to be sensed.
4. The planar linear inductive position sensor of claim 1 wherein:
the first sensing coil is formed as a first segment having the
shape of a 360.degree. cycle of a sine function sin x starting at
0.degree. and a second segment having the shape of a 360.degree.
cycle of a sine function -sin x starting at 0.degree., both the
first and second segments sharing a common axis, first ends of the
first and second segments meeting at and electrically connected to
each other at a first location along the common axis, and second
ends of the first and second segments meeting at and electrically
connected to each other at a second location along the common axis;
the second sensing coil is formed as a first segment having the
shape of a 360.degree. cycle of a cosine function cos x starting at
0.degree. and an opposing second segment having the shape of a
360.degree. cycle of a cosine function cos x starting at 0.degree.,
both the first and second segments of the second sensing coil
sharing the common axis, first ends of the first and second
segments of the second sensing coil electrically connected to each
other, and second ends of the first and second segments of the
second sensing coil electrically connected to each other; and
wherein a linear position of the target is sensed as positions
along the common axis.
5. The planar linear inductive position sensor of claim 1 wherein
the at least one oscillating coil is formed as a center-tapped coil
having two segments.
6. The planar linear inductive position sensor of claim 1 wherein
the opposing edges of the first and second sensing coils each
extend beyond opposing edges of the oscillating coil along the
linear axis by an amount selected to cancel a no-target-present
offset voltage induced at the opposing edges of the second sensing
coil.
7. A planar linear inductive position sensor system comprising: a
substrate; an oscillating coil; a first sensing coil having
opposing edges extending beyond opposing edges of the oscillating
coil along a linear axis along which a linear position of a
conductive object is to be sensed; a second sensing coil having
opposing edges extending beyond opposing edges of the oscillating
coil along the linear axis; the first and second sensing coils
having geometries selected such that equal opposing magnetic fields
are induced in the first and second sensing coils in the presence
of a magnetic field generated by the oscillating coil when no
conductive target is proximate to the first and second sensing
coils and unequal opposing magnetic fields are induced in the first
and second sensing coils when the conductive target is proximate to
the first and second sensing coils, a difference in the unequal
opposing magnetic fields induced in the first and second sensing
coils correlated to the position of the conductive target; a
capacitor coupled to the oscillating coil to form a resonant LC
circuit at a resonant frequency; a signal generator generating a
signal at the resonant frequency coupled to the oscillator coil;
and sensor circuits coupled to the first and second sensing
coils.
8. The planar linear inductive position sensor system of claim 7
wherein the substrate is a printed circuit board.
9. The planar linear inductive position sensor system of claim 7
wherein: the first sensing coil is formed as a first segment having
the shape of a 360.degree. cycle of a sine function sin x starting
at 0.degree. and an opposing second segment having the shape of a
360.degree. cycle of a sine function sin x starting at 0.degree.,
both the first and second segments sharing a common axis, first
ends of the first and second segments of the first sensing coil
meeting at and electrically connected to each another, and second
ends of the first and second segments of the first sensing coil
meeting at and electrically connected to each another; the second
sensing coil is formed as a first segment having the shape of a
360.degree. cycle of a cosine function cos x starting at 0.degree.
and an opposing second segment having the shape of a 360.degree.
cycle of a cosine function -cos x starting at 0.degree., both the
first and second segments of the second sensing coil sharing the
common axis, first ends of the first and second segments of the
second sensing coil electrically connected to each other, and
second ends of the first and second segments of the second sensing
coil are electrically connected to each other; and wherein linear
position of the target is sensed as positions along the common
axis.
10. The planar linear inductive position sensor system of claim 7
wherein the oscillating coil is formed as a center-tapped coil.
11. The planar linear inductive position sensor system of claim 10
wherein the capacitor coupled to the oscillating coil to form a
resonant LC circuit at a resonant frequency comprises: a first
capacitor coupled to a first end of the center-tapped coil; and a
second capacitor coupled to a second end of the center-tapped
coil.
12. The planar linear inductive position sensor system of claim 7
wherein the opposing edges of the first and second sensing coils
each extend beyond opposing edges of the oscillating coil along the
linear axis by an amount selected to cancel a no-target-present
offset voltage induced at the opposing edges of the second sensing
coil.
13. A method for detecting the linear position of a conductive
target with respect to a substrate comprising: providing an
oscillating coil on the substrate; providing first and second
sensing coils on the substrate, the first and second sensing coils
having geometries selected such that equal opposing magnetic fields
are induced in the first and second sensing coils in the presence
of a magnetic field generated by the oscillating coil when no
conductive target is proximate to the first and second sensing
coils and unequal opposing magnetic fields are induced in the first
and second sensing coils when the conductive target is proximate to
the first and second sensing coils, a difference in the unequal
opposing magnetic fields induced in the first and second sensing
coils correlated to the position of the conductive target, the
provided first and second sensing coils having opposing edges
extending beyond opposing edges of the oscillating coil along a
linear axis along which the linear position of the conductive
object is to be sensed; exciting the oscillating coil with a
generated oscillator signal; measuring voltages induced in the
first and second sensing coils from the generated oscillator
signal; and determining presence and position of the conductive
target as a function of a difference in the voltages induced in the
first and second sensing coils.
14. The method of claim 13 wherein providing an oscillating coil on
the substrate comprises providing a resonant circuit including the
oscillating coil and a capacitor, the resonant circuit having a
resonant frequency; and exciting the oscillating coil with a
generated oscillator signal comprises exciting the oscillating coil
with a generated oscillator signal at the resonant frequency.
15. The method of claim 13 wherein providing first and second
sensing coils on the substrate comprises: providing the first
sensing coil formed as a first segment having the shape of a
360.degree. cycle of a sine function sin x starting at 0.degree.
and an opposing second segment having the shape of a 360.degree.
cycle of a sine function sin x starting at 0.degree., both the
first and second segments sharing a common axis, first ends of the
first and second segments of the first sensing coil meeting at and
electrically connected to each other, and second ends of the first
and second segments of the first sensing coil meeting at and
electrically connected to each other; and providing the second
sensing coil formed as a first segment having the shape of a
360.degree. cycle of a cosine function cos x starting at 0.degree.
and an opposing second segment having the shape of a 360.degree.
cycle of a cosine function cos x starting at 0.degree., both the
first and second segments of the second sensing coil sharing the
common axis, first ends of the first and second segments of the
second sensing coil electrically connected to each other, and
second ends of the first and second segments electrically connected
to each other.
Description
[0001] The present invention relates to position sensing technology
and to inductive position sensing technology. More particularly,
the present invention relates to a planar linear inductive position
sensor having edge effect compensation.
BACKGROUND
[0002] Position sensors are a common element in automotive,
industrial, and aerospace applications. Whenever safety is a
concern, it is an absolute must that highly robust and reliable
position sensors are required. Potentiometers are used as position
sensors. They are contact-type sensors and lead to wear and noise.
To overcome these drawbacks, non-contact type sensors are used.
These sensors are based on inductive, capacitive, optical, and Hall
Effect principles. Optical encoders provide good resolution but
lead to higher cost and reliability related issues in the
harsh/contaminated environment. Hall Effect sensors are sensitive
to temperature and external magnetic fields. Capacitive sensors are
very sensitive to extreme environmental changes.
[0003] Inductive sensors are used to convert a linear displacement
or an angular motion of a conductive target into a proportional
electrical signal using currents generated by a magnetic field
induced in one or more sensing coils. Some inductive position
sensors include at least one primary coil that sustains an
oscillating signal producing a magnetic field and one or more
secondary coils that receive the position information as currents
induced by the magnetic field as a function of position of the
conductive target.
[0004] Conventional inductive position sensors are expensive, and
they occupy more space as the oscillator and sensor coils are
radially wound on a core. Planar inductive position sensors are
cost-effective as coils are laid out in a plane on a printed
circuit board.
[0005] Planar inductive sensors consist of one or more oscillator
coils, two sensing coils and one movable conductive target which
influences the strength of magnetic coupling between the one or
more oscillator coils and the sensing coils. An alternating current
is induced through the one or more oscillator coils. The magnitude
and phase of this alternating current depend on the position of the
target. The eddy currents through the conductive target cause a
difference in sense coil currents and voltages. Attempts are made
to configure the sensing coils such that in a uniform magnetic
field not influenced by a conductive target the voltages and
currents induced in the sensing coils cancel each other out.
[0006] A few examples of prior-art planar inductive position
sensors include United States Patent Publication US 20050253576,
U.S. Pat. Nos. 4,507,638, 6,522,128, 7,196,604, and WO2002097374. A
planar inductive sensor having two sensing coils and a pair of
oscillator coils wrapped around the sensing coils is disclosed in
Application Note AN-S1412 "Inductive Sensor Coil Design Using
LX3301A" (2017) from Microsemi Corporation of Chandler Ariz.
[0007] One drawback of available linear inductive planar position
sensors is that there is always a non-uniform magnetic field that
exists at the edges of the sensor. This non-uniform magnetic field
at the edges of the sensor causes an offset voltage to be induced
in the sensing coils, which negatively effects the accuracy of
sensor.
[0008] Generally, edge effects in linear sensors can be minimized
by placing the sensing coils far away from oscillator coil edges,
but this technique is not feasible in space-constrained
applications because it increases the sensor printed circuit board
size.
[0009] Referring first of all to FIG. 1, a diagram shows an example
of a prior-art planar inductive linear position sensor 8 formed on
a substrate 10, including an oscillator coil having a pair of
oscillator coil segments 12 and 14 driven from a center tap, a sine
sensing coil formed from two 360.degree. segments 16a and 16b, the
segment 16a starting from the left side of FIG. 1 at 0.degree. (sin
0.degree.=0) and the segment 16b starting from the left side of
FIG. 1 at 0.degree. (-sin 0.degree.=0) and joined at their end
points 42, 44, a cosine sensing coil formed from two segments of
opposing phase shown in dashed lines at reference numerals 18a and
18b, the segment 18a starting from the left side of FIG. 1 at
0.degree. (cos 0.degree.=1) and the segment 18b starting from the
left side of FIG. 1 at 0.degree. (-cos 0.degree.=-1) and joined at
their ends by segments 46, 48, a movable conductive target 20. The
movable target 20 is formed from a conductive material and is
preferably formed from a material having a relatively high
electrical conductivity, such as copper or aluminum. The sine
sensing coil 16a and 16b is shown having leads 22 and 24 and the
cosine sensing coil 18a and 18b is shown having leads 26 and
28.
[0010] Persons of ordinary skill in the art will appreciate that
while the particular prior-art configuration shown in FIG. 1
employs a pair of oscillator coils 12 and 14 that may be
conveniently formed from a single coil center tapped at reference
numeral 30 from which it is driven by a signal Vin and having end
leads 32 and 34, other prior art configurations employ a single
oscillator coil driven by a suitable signal generator.
[0011] As is known in the art, the pair of oscillator coils 12 and
14, the sine sensing coil segments 16a and 16b, and the cosine
sensing coil segments 18a and 18b can be formed as separate layers
on multilayer substrate 10 using conventional printed circuit board
fabrication techniques.
[0012] The oscillator signals may be generated by and the sensed
signals may be received and processed by sensor interface
circuitry, for example, a single sensor interface integrated
circuit 36 such as a LX3301A Inductive Sensor Interface integrated
circuit, available from Microsemi Corporation of Chandler, Ariz.
Such a sensor interface circuit can include a signal generator
section 36a used to generate the oscillator signal that is injected
into the one or more oscillator coils, and sensing circuits 36b and
36c for sensing signals from the sine and cosine sensing coils,
respectively. Capacitors 38 and 40 are coupled, respectively,
between the end leads 32 and 34 of the oscillator coils 12 and 14
and ground to form LC resonant circuits. A typical value for
capacitors 38 and 40 can be about 1.2 nF. The oscillator signal
shown as Vin injected into the oscillator coil segments 12 and 14,
ends of which are shown in FIG. 1 connected to the signal generator
section 36a of the inductive sensor interface integrated circuit 36
at connections Osc. 1 and Osc. 2, respectively, is preferably a
sine wave and the frequency of the oscillator signals injected into
the oscillator coils depends only on the inductance of the
oscillator coils 12, 14 and the respective capacitance values of
capacitors 38 and 40. A typical, non-limiting oscillator frequency
range can be between about 1 MHz and about 6 MHz.
[0013] The voltage induced by the movable conductive target 20 in
one of the sine or cosine sensing coils is a time-dependent
derivative of the magnetic flow from Maxwell's equation
E = - d .PHI. B dt ##EQU00001##
[0014] Faraday's law of induction makes use of the magnetic flux
.PHI.B through a region of space enclosed by a wire loop. The
magnetic flux is defined by a surface integral
O.sub.B=.intg.BdA
[0015] An alternating current Io(t) is applied to the oscillator
coil segments 12, 14 that creates an alternating magnetic field
Bt(t). The alternating magnetic field Bo(t) induces in the movable
conductive target 20, which in a simplified form is a closed
conductive loop, a current It(t) that, in turn, creates an
alternating magnetic field Bt(t) that opposes the exciting
alternating magnetic field Bt(t).
[0016] A voltage is induced in each of the sine and cosine sensing
coils from the overlapping alternating magnetic fields Bo(t)+Bt(t)
according to the relationship
E = - d .0. dt = - d .intg. .intg. ( ( B 0 ( t , x , y ) - B t ( t
, x , y ) dA dt = - d dt .intg. .intg. ( B 0 ( t , x , y ) dA + d
dt .intg. .intg. ( B t ( t , x , y ) ) d A ##EQU00002##
[0017] with A representing a surface area of the respective sensing
coil.
[0018] It should be noted that each of the sensing coils have two
surfaces, a positive lobe and a negative lobe, directed in opposite
directions. The voltage induced in the positive lobe is Ep and the
voltage induced in the negative lobe is En. The result is that,
with a sine sensing coil having a symmetrical coil geometry as seen
easily in FIG. 2, the portion of the induced voltage that is caused
by the oscillator coil segments 12, 14 is zero; that is
d dt .intg. .intg. B 0 ( t , x , y ) ) dA = 0 ##EQU00003## i . e .
, E p - E n = 0 ##EQU00003.2##
[0019] When a current carrying conductor is placed in parallel with
another conductor, there will be a magnetic coupling between the
two conductors, resulting in additional induced voltage. Because
the ends of the sine sensing coil segments 16a and 16b closest to
the end portions of the oscillator coil segments 12, 14 are joined
together at points 42 and 44 they present very little, if any,
conductors having significant lengths that are close enough to form
parallel conductors that will magnetically couple with the
conductors forming the end portions of the oscillator coils 12 and
14.
[0020] In the case of the cosine sensing coil segments 18a and 18b
there is always an edge effect resulting from its end segments 46
and 48 which are provided to connect the cosine segments 18a and
18b together at their ends, as can be seen in FIG. 3. These end
segments 46 and 48 are oriented in parallel with the conductors
that form end portions of the oscillator coils 12 and 14, and will
be magnetically coupled to the conductors that form end portions of
the oscillator coils 12 and 14. This coupling results in additional
induced voltage (Ep+.DELTA.Ep) being coupled into the positive
lobes of the cosine coils 18a and 18b from the oscillator coils 12
and 14 compared to voltage (En) induced in the negative lobe of the
cosine sensing coil 18a and 18b. Because of this always there is an
offset voltage .DELTA.Ep induced in cosine sensing coil, which
means
d dt .intg. .intg. B 0 ( t , x , y ) dA .noteq. 0 ##EQU00004## i .
e . , ( E p + .DELTA. E p ) - E n .noteq. 0 ##EQU00004.2##
[0021] The asymmetrically produced voltage on the cosine sensing
coil adds the offset voltage to the voltage coupled in from the
movable target. This creates difficulties for further processing
the signal and introduces substantial measuring errors.
[0022] FIG. 4 is an amplitude vs. position plot of demodulated
waveforms from the planar inductive linear position sensor 8 of
FIG. 1, showing the waveform 50 sensed from the sine sensing coil
16a and 16b and the waveform 52 sensed from the cosine sensing coil
18a and 18b of the sensor of FIG. 1. As can be seen from FIG. 4,
there is an induced offset voltage indicated at reference numeral
54 in the demodulated cosine waveform 52 of FIG. 4.
BRIEF DESCRIPTION
[0023] According to the present invention a linear inductive planar
position sensor provides high accuracy linear position by
compensating for edge effects within optimal space structure. Edge
effects are cancelled by configuring the length of the oscillator
coils to lie inside the lengthwise edges of the sensing coils.
Minimal space structure has the benefit of reduced cost because of
smaller printed circuit board size. The linear inductive planar
position sensor of the present invention can also be implemented in
multi-layered printed circuit boards.
[0024] The linear inductive planar position sensor of the present
invention is robust because it has no moving electrical contacts,
has good performance with respect to temperature, and has a high
immunity to dust. The linear inductive planar position sensor of
the present invention is an absolute position sensing device, which
mean that it does not require motion at power-up to determine
position.
[0025] The linear inductive planar position sensor of the present
invention provides better accuracy and space utilization when
compared to traditional planar position sensors. The linear
inductive planar position sensor of the present invention fits in
space-constrained and price-sensitive applications. Applications
include brake pedals, level sensing and actuators, etc.
[0026] The linear inductive planar position sensor of the present
invention provides improved accuracy within a given space by
cancellation of edge effects.
[0027] As with the prior-art linear inductive planar position
sensors, high-frequency AC carrier signals are injected into the
one or more oscillator coils.
[0028] According to an aspect of the invention, a planar linear
inductive position sensor is formed on a substrate and includes at
least one oscillating coil. A first sensing coil has opposing edges
extending beyond opposing edges of the oscillating coil along a
linear axis along which a linear position of a conductive object is
to be sensed. A second sensing coil has opposing edges extending
beyond opposing edges of the oscillating coil along the linear
axis. The first and second sensing coils have geometries selected
such that equal opposing magnetic fields are induced in the first
and second sensing coils in the presence of a magnetic field
generated by the oscillating coil when no conductive target is
proximate to the first and second sensing coils and unequal
opposing magnetic fields are induced in the first and second
sensing coils when the conductive target is proximate to the first
and second sensing coils, a difference in the unequal opposing
magnetic fields induced in the first and second sensing coils
correlated to the position of the conductive target.
[0029] According to an aspect of the invention, the substrate is a
printed circuit board.
[0030] According to an aspect of the invention, the oscillator coil
is shaped as a rectangle, a long side of the rectangle aligned in
the direction of travel of the conductive target to be sensed.
[0031] According to an aspect of the invention, the first sensing
coil is formed as a first segment having the shape of a 360.degree.
cycle of a sine function sin x starting at 0.degree. and a second
segment having the shape of a 360.degree. cycle of a sine function
-sin x starting at 0.degree., both the first and second segments
sharing a common axis, first ends of the first and second segments
of the first sensing coil meet at and are electrically connected to
each other at a first location along the common axis, and second
ends of the first and second segments meet at and are electrically
connected to each other at a second location along the common axis,
the second sensing coil is formed as a first segment having the
shape of a 360.degree. cycle of a cosine function cos x starting at
0.degree. and an opposing second segment having the shape of a
360.degree. cycle of a cosine function cos x starting at 0.degree.,
both the first and second segments of the second sensing coil
sharing the common axis. First ends of the first and second
segments of the second sensing coil are electrically connected to
each other and second ends of the first and second segments of the
second sensing coil are electrically connected to each other. A
linear position of the target is sensed as positions along the
common axis.
[0032] According to an aspect of the invention, the at least one
oscillating coil is formed as a center-tapped coil having two
segments.
[0033] According to an aspect of the invention, the opposing edges
of the first and second sensing coils each extend beyond opposing
edges of the oscillating coil along the linear axis by an amount
selected to cancel a no-target-present offset voltage induced at
the opposing edges of the second sensing coil.
[0034] According to an aspect of the invention, a planar linear
inductive position sensor system is formed on a substrate and
includes at least one oscillating coil. A first sensing coil has
opposing edges extending beyond opposing edges of the oscillating
coil along a linear axis along which a linear position of a
conductive object is to be sensed. A second sensing coil has
opposing edges extending beyond opposing edges of the oscillating
coil along the linear axis. The first and second sensing coils have
geometries selected such that equal opposing magnetic fields are
induced in the first and second sensing coils in the presence of a
magnetic field generated by the oscillating coil when no conductive
target is proximate to the first and second sensing coils and
unequal opposing magnetic fields are induced in the first and
second sensing coils when the conductive target is proximate to the
first and second sensing coils, a difference in the unequal
opposing magnetic fields induced in the first and second sensing
coils correlated to the position of the conductive target. A
capacitor is coupled to the oscillating coil to form a resonant LC
circuit at a resonant frequency. A signal generator generates a
signal at the resonant frequency coupled to the oscillator coil,
and sensor circuits are coupled to the first and second sensing
coils.
[0035] According to an aspect of the invention, the first sensing
coil is formed as a first segment having the shape of a 360.degree.
cycle of a sine function sin x starting at 0.degree. and an
opposing second segment having the shape of a 360.degree. cycle of
a sine function sin x starting at 0.degree., both the first and
second segments sharing a common axis, first ends of the first and
second segments of the first sensing coil meet at and are
electrically connected to each other, and second ends of the first
and second segments meet at and are electrically connected each
other. The second sensing coil is formed as a first segment having
the shape of a 360.degree. cycle of a cosine function cos x
starting at 0.degree. and an opposing second segment having the
shape of a 360.degree. cycle of a cosine function -cos x starting
at 0.degree.. Both the first and second segments of the second
sensing coil share the common axis. First ends of the first and
second segments of the second sensing coil are electrically
connected to each other, second ends of the first and second
segments of the second sensing coil are electrically connected to
each other. The linear position of the target is sensed as
positions along the common axis.
[0036] According to an aspect of the invention, a method for
detecting the linear position of a conductive target with respect
to a substrate includes providing an oscillating coil on the
substrate, providing first and second sensing coils on the
substrate, the first and second sensing coils having geometries
selected such that equal opposing magnetic fields are induced in
the first and second sensing coils in the presence of a magnetic
field generated by the oscillating coil when no conductive target
is proximate to the first and second sensing coils and unequal
opposing magnetic fields are induced in the first and second
sensing coils when the conductive target is proximate to the first
and second sensing coils, a difference in the unequal opposing
magnetic fields induced in the first and second sensing coils
correlated to the position of the conductive target, the provided
first and second sensing coils having opposing edges extending
beyond opposing edges of the oscillating coil along a linear axis
along which the linear position of the conductive object is to be
sensed, exciting the oscillating coil with a generated oscillator
signal, measuring voltages induced in the first and second sensing
coils from the generated oscillator signal, and determining
presence and position of the conductive target as a function of a
difference in the voltages induced in the first and second sensing
coils.
[0037] According to an aspect of the invention, providing an
oscillating coil on the substrate includes providing a resonant
circuit including the oscillating coil and a capacitor, the
resonant circuit having a resonant frequency, and exciting the
oscillating coil with a generated oscillator signal includes
exciting the oscillating coil with a generated oscillator signal at
the resonant frequency.
[0038] According to an aspect of the invention, providing first and
second sensing coils on the substrate includes providing the first
sensing coil formed as a first segment having the shape of a
360.degree. cycle of a sine function sin x starting at 0.degree.
and an opposing second segment having the shape of a 360.degree.
cycle of a sine function sin x starting at 0.degree., both the
first and second segments sharing a common axis. First ends of the
first and second segments meet at and are electrically connected to
each other at a first location along the common axis, and second
ends of the first and second segments meet at and are electrically
connected at a second location along the common axis. Providing the
second sensing coil formed as a first segment having the shape of a
360.degree. cycle of a cosine function cos x starting at 0.degree.
and an opposing second segment having the shape of a 360.degree.
cycle of a cosine function cos x starting at 0.degree., both the
first and second segments of the second sensing coil sharing the
common axis, first ends of the first and second segments of the
second sensing coil are electrically connected to each other, and
second ends of the first and second segments are electrically
connected to each other.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0039] The invention will be explained in more detail in the
following with reference to embodiments and to the drawing in which
are shown:
[0040] FIG. 1 is a drawing depicting a prior-art linear inductive
planar position sensor;
[0041] FIG. 2 is a drawing depicting the sine sensing coil portion
of the prior-art linear inductive planar position sensor of FIG.
1;
[0042] FIG. 3 is a drawing depicting the cosine sensing coil
portion of the prior-art linear inductive planar position sensor of
FIG. 1; and
[0043] FIG. 4 is an amplitude vs. position plot of demodulated
waveforms from the planar inductive linear position sensor of FIG.
1, showing the waveforms sensed from the sine and cosine sensing
coils;
[0044] FIG. 5 is a drawing depicting a linear inductive planar
position sensor in accordance with the present invention;
[0045] FIG. 6 is an amplitude vs. position plot of demodulated
waveforms from the planar inductive linear position sensor of FIG.
5 with offset compensation in accordance with the present
invention, showing the waveforms sensed from the sine and cosine
sensing coils; and
[0046] FIG. 7 is a flow diagram showing an illustrative method in
accordance with the present invention.
DETAILED DESCRIPTION
[0047] Persons of ordinary skill in the art will realize that the
following description is illustrative only and not in any way
limiting. Other embodiments will readily suggest themselves to such
skilled persons.
[0048] Referring now to FIG. 5, a diagram shows a linear inductive
planar position sensor 60 in accordance with the present invention.
The planar inductive linear position sensor 60 of FIG. 5 includes
elements present in the prior-art planar inductive linear position
sensor 8 of FIG. 1. These elements will be referred to in FIG. 5
using the same reference numerals used to identify these elements
in FIG. 1 although the relative geometries of these elements differ
in accordance with the principles of the present invention.
[0049] The planar inductive linear position sensor 60 of FIG. 5
includes a center-tapped oscillator coil having a pair of
oscillator coil segments 12 and 14. The oscillator coil is
preferably rectangularly shaped, a long side of the rectangle
aligned in the direction of travel of a movable conductive target
20 to be sensed by the planar inductive linear position sensor 60.
A sine sensing coil is formed from two complementary 360.degree.
sine function segments (sin x and -sin x both starting at
0.degree., where sin 0.degree.=-sin 0.degree.=0) 16a and 16b, and a
cosine sensing coil is formed from two complementary 360.degree.
cosine function segments cos x and -cos x both starting at
0.degree., where cos 0.degree.=1 and -cos 0.degree.=-1) shown in
dashed lines at reference numerals 18a and 18b. The movable target
20 is formed from a conductive material and is preferably formed
from a material having a relatively high electrical conductivity,
such as copper or aluminum. As is known in the art, the pair of
oscillator coils 12 and 14, the sine sensing coil segments 16a and
16b, and the cosine sensing coil segments 18a and 18b can be formed
as separate layers on a multilayer substrate 10 using conventional
printed circuit board fabrication techniques.
[0050] The sine sensing coil is shown having leads 22 and 24 and
the cosine sensing coil is shown having leads 26 and 28. The pair
of oscillator coils 12 and 14 may be formed from a single coil
having a center tap at reference numeral 30 from which it is driven
and end leads 32 and 34. The oscillator signals may be generated by
and the sensed signals may be received and processed by sensor
interface circuitry, for example, a single sensor interface
integrated circuit 36 such as a LX3301A Inductive Sensor Interface
integrated circuit, available from Microsemi Corporation of
Chandler, Ariz. Such a sensor interface circuit 36 can include a
signal generator section 36a used to generate the oscillator signal
that is injected into the one or more oscillator coils, and sensing
circuits 36b and 36c for sensing signals from the sine and cosine
sensing coils, respectively. Capacitors 38 and 40 are coupled,
respectively, between the end leads 32 and 34 of the oscillator
coils 12 and 14 and ground to form LC resonant circuits. A typical
value for capacitors 38 and 40 can be about 1.2 nF. The oscillator
signal injected into the oscillator coils 12 and 14 is preferably a
sine wave and the frequency of the oscillator signals injected into
the oscillator coils depends only on the inductance of the
oscillator coils and the respective capacitance values of
capacitors 38 and 40. A typical, non-limiting oscillator frequency
range can be between about 1 MHz and about 6 MHz.
[0051] As in the prior-art planar inductive linear position sensor
8 of FIG. 1, the ends of the sine segments 16a and 16b closest to
the end portions of the oscillator coils in the planar inductive
linear position sensor 60 of FIG. 5 are joined together at points
42 and 44 Similarly, the ends of the cosine segments 18a and 18b
are joined together at end segments 46 and 48.
[0052] As in the prior-art planar inductive linear position sensor
8 of FIG. 1, the ends of the sine sensing coil segments 16a and 16b
closest to the end portions of the oscillator coils are joined
together at points 42 and 44 and they present very little, if any,
conductors having significant lengths that are close enough to form
parallel conductors that will magnetically couple with the
conductors forming the end portions of the oscillator coils 12 and
14.
[0053] In the planar inductive linear position sensor 60 of FIG. 5,
the edge effect of the cosine sensing coil segments 18a and 18b
resulting from its end segments 46 and 48 which are provided to
connect the opposing cosine segments 18a and 18b together at their
ends also induces a voltage .DELTA.Ep/2 at each end of the sensor.
These voltages induced by the presence of the end segments 46 and
48 is compensated by canceling the additional induced voltage
(Ep+.DELTA.Ep) coupled into the positive lobes of the cosine coils
18a and 18b from the oscillator coils 12 and 14. This is
accomplished by shortening the oscillator coils 12 and 14 by a
distance d as shown in FIG. 5 so that their outer edges lie inside
the end segments 46 and 48 of the cosine sensing coil segments 18a
and 18b. It is to be noted that d is defined in relation to end
segments 46, 48 and not in relation to the placement of the prior
art. This shortening induces an additional opposing offset voltage
.DELTA.En/2 at each edge of the sensor. At the distance d, the
magnitude of the additional opposing offset voltage .DELTA.En/2 is
equal to and opposite from the offset voltage .DELTA.Ep/2 induced
at the edges of the sensor when the movable target is not present.
A zero-sum voltage results at output taps of the cosine sensing
coils.
d dt .intg. .intg. B 0 ( t , x , y ) dA = 0 ##EQU00005## i . e . ,
( E p + .DELTA. E p ) - ( E n + .DELTA. E n ) = 0
##EQU00005.2##
[0054] The exact shortening of the oscillator coils in accordance
with the present invention depends on the geometry of the
individual sensor being compensated and must be determined for each
individual case. The distance d by which the oscillator coils are
displaced inwardly from the edges of the cosine coils depends on
many factors including coil geometry and target size. The distance
d should be tuned for each case to compensate the offset because
the sensor is a very complex electromagnetic structure there is no
direct mathematic relationship between d and the geometry of the
sensor. As an example, in a sensor with cosine coils having a width
of 55 mm using a target having a width of 10.5 mm, a distance d of
1.24 mm compensates for the offset error.
[0055] FIG. 6 is an amplitude vs. position plot of demodulated
waveforms from the planar inductive linear position sensor 60 of
FIG. 5 with offset compensation in accordance with the present
invention, showing the waveforms sensed from the sine and cosine
sensing coils. The waveform 62 is sensed from the sine sensing coil
16a and 16b and the waveform 64 is sensed from the cosine sensing
coil 18a and 18b of the planar inductive linear position sensor 60
of FIG. 5. As can be seen from FIG. 5, there is no induced offset
voltage in the demodulated cosine waveform 64 of FIG. 5.
[0056] Referring now to FIG. 7, a flow diagram shows an
illustrative method in accordance with the present invention. The
method begins at reference numeral 70. At reference numeral 72, a
substrate is provided. At reference numeral 74, an oscillating coil
is provided on the substrate. At reference numerals 76 and 78,
first and second sensing coils are provided on the substrate, the
first and second sensing coils having geometries selected such that
equal opposing magnetic fields are induced in the first and second
sensing coils in the presence of a magnetic field generated by the
oscillating coil when no conductive target is proximate to the
first and second sensing coils and unequal opposing magnetic fields
are induced in the first and second sensing coils when the
conductive target is proximate to the first and second sensing
coils, a difference in the unequal opposing magnetic fields induced
in the first and second sensing coils correlated to the position of
the conductive target, the first and second sensing coils having
opposing edges extending beyond opposing edges of the oscillating
coil along a linear axis along which the linear position of the
conductive object is to be sensed. At reference numeral 80, the
oscillating coil is excited with a generated oscillator signal. At
reference numeral 82, voltages induced in the first and second
sensing coils from the generated oscillator signal are measured. At
reference numeral 84, the presence and position of the conductive
target are determined as a function of a difference in the voltages
induced in the first and second sensing coils.
[0057] The method ends at reference numeral 86.
[0058] While embodiments and applications of this invention have
been shown and described, it would be apparent to those skilled in
the art that many more modifications than mentioned above are
possible without departing from the inventive concepts herein. The
invention, therefore, is not to be restricted except in the spirit
of the appended claims.
* * * * *