Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MAGNETIC DISPLACEMENT SENSOR
BACKGROUND OF THE INVENTION
The present invention is a magnetic displacement sensor having a
magnetic circuit with a reluctance of the magnetic circuit varying with
displacement. More specifically, the present invention is a valve position
sensor employing a magnetic circuit having a magnetic source and a magnetic
flux sensor mounted in fixed relation to a valve and a target on a valve
operator forming a flux conduction path having a reluctance that varies with
displacement of the valve operator.
Magnetic displacement sensors provide electrical signals indicative of
displacement. Generally, such sensors are arranged to provide a magnetic field
or magnetic flux that varies with displacement, and a pickup device that
detects
a change in the magnetic field or magnetic flux. While the displacement can
be of any form, typically the displacement will be rotational or linear. Since
a magnetic displacement sensor detects displacement of a first member with
respect to a second member, a first portion of the sensor must be attached to
the first member and a second portion of the sensor must be attached to the
second member.
Riggs et al., U.S. Patent No. 5,359,288,discloses a linear position sensor
for use in an automotive suspension system. The sensor comprises a
telescopically movable portion and a stationary portion. The movable portion
includes permanent magnets arranged such that the intensity of the magnetic
field produced by the magnets varies lengthwise along the movable portion.
The stationary portion includes a magnetic sensor (such as a Hall effect
sensor)
that detects the magnetic field produced by the magnets on the movable
portion. Accordingly, as the displacement of the movable portion changes with
respect to the stationary portion, the magnetic field detected by the sensor
changes and the sensor produces a signal representative of displacement.
Baba et al., U.S. Patent No. 3,777,273, discloses a rotational
displacement sensor that uses two magnetic circuit paths. A stationary portion
of the sensor includes two magnetic flux sensors, one in each circuit path.
The
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movable portion of the sensor includes an asymmetrical portion that changes
the reluctance of the circuit paths based on the rotary position of the
movable
part. The rotational displacement of the movable portion with respect to the
stationary portion is measured on the basis of the ratio of flux passing
through
each flux sensor.
While Baba et al. and Riggs et al. each disclose magnetic displacement
sensors capable of accurately measuring displacement, they, like most magnetic
displacement sensors known in the art, deploy significant portions of the
sensor
on both the stationary and movable members. Therefore, replacement of the
sensor often entails replacing both the stationary and movable portions. In
addition, many magnetic sensors (such as Baba et al.) require that the
displacement to be measured be mechanically linked into the sensor.
Displacement sensors of various types are used in distributed control
systems employing field devices, such as valves, that are operated by remote
systems. Microprocessors are often employed to monitor and control valve
positions. For example, one type of valve is operated by a pressure-responsive
diaphragm. A valve operator, such as a shaft, is connected between the
diaphragm and the valve and moves linearly to open and close the valve. A
sensor, such as a potentiometer, is coupled to the shaft and provides a signal
to the microprocessor representative of linear displacement of the shaft.
Another type of valve is operated by a rotary actuator. A valve
operator, such as a shaft, is connected between the rotary actuator and the
valve. The operator or shaft rotates with the rotary actuator to open and
close
the valve. A sensor is coupled to the shaft to provide a signal to the
microprocessor representative of rotational displacement of the shaft.
Magnetic displacement sensors have also been used to sense
displacement of linear and rotary valve operators or shafts. These have
typically employed a flux sensor mounted to a stationary member and in the
flux path of a magnetic source mounted to the operator or shaft. In an
example of a displacement sensor for a rotary shaft, the shaft carrying the
magnetic source surrounds an eccentrically mounted flux sensor so that the
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reluctance of the magnetic circuit through the flux sensor varies with
rotational
displacement. In an example of a displacement sensor for a linearly displaced
shaft, the area of the magnetic source confronting the flux sensor varies with
linear displacement.
A common problem of the field devices described above resides in the
fact that displacement sensors deteriorate over time and need to be replaced.
However, replacement of portions of the displacement sensor that are mounted
to the movable member is not easily accomplished.
SUMMARY OF THE INVENTION
The present invention is a magnetic displacement sensor that forms a
magnetic circuit through stationary and movable portions. The stationary
portion of the displacement sensor includes a magnetic source and a magnetic
flux sensor. The magnetic source provides a magnetic field and is the source
for magnetic flux, and the magnetic flux sensor is positioned in a lossy flux
path
having a fixed reluctance. The movable portion of the sensor comprises a
target that completes a flux conduction path across a gap between the target
and the magnetic source, the flux conduction path having a magnetic reluctance
that varies with displacement. The variable reluctance path is arranged in
parallel to the fixed reluctance path, so that as the reluctance of the
variable
path changes, flux in the fixed reluctance path through the flux sensor also
changes.
The magnetic displacement sensor is used in connection with rotary and
sliding operators. lVIore particularly, the target is attached to the moving
valve
shaft and shaft displacement is sensed by the stationary flux sensor sensing
lossy flux from the stationary magnetic source.
One aspect of the present invention resides in a magnetic flux generator
~ and pickup apparatus for a magnetic displacement sensor that senses relative
displacement between first and second members, where the displacement
sensor has a target mounted to the second member to form a flux path normal
to the direction of displacement and has a reluctance that varies with
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displacement between the first and second members. The flux generator and
pickup apparatus includes a housing to be mounted to the first member.
Magnet means is mounted to the housing to generate a magnetic flux. The _
magnet means has first and second pole tips arranged to confront the target
along the flux path when the housing is mounted to the first member. A flux
sensor is mounted to the housing between the first and second pole tips.
Another aspect of the present invention resides in a displacement sensor
for sensing relative displacement between first and second members. The
sensor includes a housing to be mounted to the first member. Magnet means
having first and second pole tips is mounted to the housing. A flux sensor is
mounted to the housing between the first and second pole tips. A target to be
mounted to the second member forms a flux path between the first and second
pole tips having a reluctance that varies with displacement between the first
and second members.
Another aspect of the present invention resides in a displacement sensor
having magnet means to be mounted to the first member, the magnet means
having first and second pole tips. A magnetic circuit provides a first or
primary
flux path between the first and second pole tips that contains a target to be
mounted to the second member. The magnetic circuit also provides a
secondary flux path of fixed reluctance between the first and second pole tips
that contains a magnetic flux sensor. The second flux path has a reluctance
that varies with displacement between the first and second members.
In one embodiment, the target has a surface whose height varies with
displacement to thereby change the gap length between the target and the
magnet means. In another embodiment, the target has a surface whose edge
is at an acute angle to the direction of displacement to thereby change the
gap
length between the target and the magnet means. In yet another embodiment,
the target is configured sot that a surface area of the target confronting the
magnet means changes with displacement.
The magnetic displacement sensor of the present invention may also be
used differentially to compensate for unwanted variability in the gap between
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the stationary portion and the movable portion. In the differential
k
embodiment, the stationary and movable portions are arranged to form two
magnetic circuits, each having a target path and a Iossy path. The target path
of one magnetic circuit has an increasing reluctance with displacement in a
given direction while the target path of the other magnetic circuit has a
decreasing reluctance. By calculating displacement based on the reluctances
of the two magnetic circuits, the effects of unwanted variations in gap space
are
canceled.
In another embodiment, the target includes an eccentric cam or cams
arranged to provide a variable height gap between a radially-oriented
stationary portion and the cam surface of the target.
ERIEF DESCRIPTION QF THE DRAWINGS
Figure 1 is a perspective view of a magnetic displacement sensor of the
present invention.
Figure 2 is a perspective view of a stationary portion for a displacement
sensor of the present invention.
Figure 3 is a front view of the stationary portion shown in Figure 2.
Figure 4 is a side view of the stationary portion shown in Figure 2.
Figure 5 is a bottom view of the stationary portion shown in Figure 2.
Figures 6 and 7 are diagrams illustrating the magnetic circuit of the
displacement sensor of Figure i .
Figure 8 is a diagram of an equivalent magnetic circuit illustrated in
Figures 6 and 7.
Figure 9 is a perspective view, as in Figure 1, of a magnetic
displacement sensor in accordance with a modification of the present
invention.
Figure 10 is a diagram illustrating the magnetic circuit of the
displacement sensor of Figure 9.
Figure 11 is a diagram, as in Figure I0, illustrating the magnetic circuit
of a displacement sensor in accordance with a modification of the present
invention.
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Figure 12 is a diagram illustrating a differential magnetic displacement
sensor in accordance with a modification of the present invention using plural
magnetic displacement sensors of Figure 9.
Figure 13 is a diagram of the target of the differential magnetic
displacement sensor of Figure 12, useful in explaining the operation of the
differential displacement sensor.
Figure 14 is a diagram, as in Figure 12, illustrating a differential
magnetic displacement sensor in accordance with a modification of the present
invention using plural magnetic displacement sensors of Figure 1.
Figure 15 is a diagram of a stationary portion for a differential magnetic
displacement sensor according another modification of the present invention.
Figure 16 is a top view of a target for a differential magnetic
displacement sensor in accordance with another mod~cation of the present
invention.
Figures 17 and 18 are frontal and side views, respectively, of a magnetic
displacement sensor in accordance with another modification of the present
invention.
Figure 19 is a side view of a differential magnetic displacement sensor
in accordance with another modification of the present invention using plural
magnetic displacement sensors of Figures I7 and 18.
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DETAILED DESCRIPTION OF THE PREFERRED E~1V~ODIMENTS
Figure 1 is a perspective view of a magnetic displacement
sensor in
accordance with the presently preferred embodiment of the
present invention.
The magnetic displacement sensor comprises a stationary
portion 10 (shown
in greater detail in Figures 2-5) and a target 20. Stationary
portion 10 is
mounted to a stationary member, such as a valve housing.
Conveniently, the
valve housing includes a mounting arm 12 which is fastened
to portion i0
through mounting hole 14. Target 20 is mounted to the shaft
or stem 22 of the
valve. The embodiment illustrated in Figure I is arranged
for linear or
reciprocal movement of shaft 22 in the direction of arrow
24 along axis 26.
Shaft 22 is a linear valve operator mounted between a valve
actuator, such as
a pressure-responsive diaphragm (not shown), and the valve
(not shown) under
control in a manner well known in the art. Shaft 22 is
arranged to reciprocate
in the direction of arrow 24 to open and close the valve.
IS As shown particularly in Figures 2-5, stationary portion
10 comprises a
housing 30 supporting permanent magnets 32 and 34 at the
sides of the
housing. Magnets 32 and 34 are arranged so that one of
the magnets, such as
magnetic 32, has its north pole orientated towards the
top of housing 30 and
its south pole (pole tip 33) orientated toward the bottom
of housing 30. The
other magnet, such as magnet 34, is oriented opposite to
magnet 32 with its
north pole (pole tip 35) at the bottom of housing 30 and
its south pole at the
top of housing 30. Magnets 32 and 34 are preferably Alnico
V magnets. A
permanent magnet is preferred because it can be easily
incorporated into the
sensor and does not require a separate power source. A
magnetic field sensor
36, such as an Allegro 3507LU Hall effect sensor, is mounted
in slot 38 in the
bottom surface 48 of housing 30 between the pole tip 33
of magnet 32 and
pole tip 35 of magnet 34. The conductors of sensor 36 are
connected to
. separate terminals on conductor pad 40 for connection to the control
equipment, such as a microprocessor. Pole piece 42 is mounted to the top of
housing 30 and abuts the north pole of magnet 32 and the south pole of
magnet 34. Pole piece 42 is preferably formed of sheet metal or iron to
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provide a low reluctance path between the north pole of magnet 32 and the
a
south pole of magnet 34. Pole piece 42 is fastened to housing 30 and magnets
32 and 34 by a suitable fastening technique, such as adhesive. It will be
appreciated that magnets 32 and 34 and pole piece 44 form a magnetic source
forming a constant flux source.
In one form of the invention, housing 30 is formed of a suitable non-
magnetic material such as rigid platstic. Non-magnetic fasteners, such as
screws
44 and 46 mount magnets 32 and 34 to the housing. Sensor 36 is mounted
within slot 38 by a suitable adhesive or other fastening mechanism.
As shown in Figure 1, target 20 includes a surface 50 that slopes with
respect to axis 26. Target 20 is mounted to non-ferromagnetic base or carrier
21 which in turn is mounted to shaft or stem 22 in spaced relation to portion
10 so that target 20 has a uniform thickness. Target 20 has a surface 50 that
confronts bottom surface 48 of portion 10 to form gap 52 between target 20
and the pole tip 33 of magnet 32 and gap 54 between target 20 and the pole
tip 35 of magnet 34. As shaft 22 reciprocates along its axis, the heights of
gaps
52 and 54 change, based upon the lineal position of shaft 22. Target 20 is
constructed of a suitable low reluctance material, such as sheet metal or
iron.
Target 20 provides a flux path between pole tips 33 and 35 normal to the
direction of displacement. Since air has a significantly higher reluctance
than
the material forming target 20, changes in the reluctance of the flux path
through target 20 are due primarily to the changes in the heights of gaps 52
and 54 as the lineal positions of shaft 20 changes. It may also be desired to
provide a slight curve to surface 50 of target 20 (see Figure 14) to
compensate
for any non-linearity of the change of reluctance with the change of gap
height.
Figures 6-8 illustrate the magnetic circuit of the magnetic displacement ,
sensor illustrated in Figure 1. As shown in Figures 6 and 7, magnet 32, pole
piece 42, magnet 34 provides a magnetic field that follows two primary flux ,
paths. One path is through gap 54, target 20 and gap 52. A secondary path
56 of lossy flux, in the form of leakage and fringing flux, extends through
sensor 36 between pole tip 33 of magnet 32 and pole tip 35 of magnet 34. The
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reluctance of lossy flux path 56 is established by the construction of housing
30
and sensor 36, and is therefore known. On the other hand, the reluctance of
the path through gaps 52 and 54 is dependant, at least in part, by the heights
of those gaps.
Target 20, together with gaps 52 and 54, form a shunt with respect to
Iossy flux path 56. Any change in the reluctance of the path through target 20
due to changes in the height of gaps 52 and 54 will change the shunt. Since
the magnets provide a constant flux source, any change in the reluctance of
the
path through target 20 will change the flux through that path, thereby
changing
the amount of lossy flux in path 56. Consequently, the amount of flux passing
through sensor 36 is dependant on the height of gaps 52 and 54 and hence the
lineal position of shaft 22 with respect to stationary member 10. Figure 8
illustrates an equivalent magnetic circuit, showing the circuit path formed by
reluctances ~2 and ~4 and target 20 in parallel with the fixed reluctance
and sensor 36. As shown in Figure 8, microprocessor 49 is connected to sensor
36, such as through the terminals on pad 40 (Figures 1-5). Microprocessor 49
is connected to the valve actuator to control valve position in a manner well
known in the art.
Although the stationary portion 10 of the magnetic displacement sensor
is arranged in Figure 1 so that the magnets 32 and 34 are disposed normal to
axis 26 so that the flux path 56 and the flux path through target 20 are
normal
to axis 26, it may be more convenient in some cases to arrange the stationary
portion 10 so that the magnets, flux path 56 and the flux path through target
20 are aligned with, or even at some other angle to, axis 26. Such a
modification would simply orient stationary portion 10 at the convenient
angle,
such as 90° from that shown in Figure 1. Additionally, although Figure
1
illustrates surface 48 parallel to axis 26 of shaft 22, it may actually be
more
convenient to align surface 48 to be parallel to confronting surface 50 of
target
20. This can be accomplished, for example, by aligning the axis of mounting
arm 12 parallel to the slope of surface 50. Since the permeability of the
magnetic target 20 is several thousand times that of air, the majority of the
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change in reluctance through target 20 is due to the changes in heights of
gaps
52 and 54.
Figures 9 and 10 illustrate a magnetic displacement sensor in
accordance with a mod~cataon of the present invention. Like the embodiment
illustrated in Figure 1, the embodiment illustrated in Figures 9 and 10 is
arranged for linear or reciprocal movement of shaft 22 in the direction of
arrow 24 along axis 26. The magnetic displacement sensor comprises a
stationary portion 10 (which is the same stationary portion shown in Figures
2-5) and a low reluctance target 70. Stationary portion 10 is mounted to a
stationary member, such as a valve housing, by mounting arm I2 fastened
through mounting hole I4 in member I0. Target 70 is mounted to a base or
Garner 77 formed of low permeability material which in turn is mounted to the
shaft or stem 22 of the valve. Carrier ?7 compensates for the curvature, if
any,
of shaft or stem 22 so that low reluctance target 70 has a uniform thickness.
Target 70 includes a surface 72 parallel to axis 26 of shaft 22. Unlike target
in Figure 1, target 70 is configured so that its edges taper at an acute angle
to the direction of movement from an apex 74 to a base 76 at different axial
locations along shaft 22. Surface 48 of stationary portion IO confronts
surface
72 of target 70 to form gaps 78 and 80 between the edges of target 70 and
20 magnets 32 and 34. In Figure 10, the motion of shaft 22 is into and out of
the
page. While the distance between surface 72 and surface 48 remains
substantially constant with movement of shaft 22, the width W of target 70
changes. Accordingly, the length of air gap 78 between target 70 and magnet
32 arid the length of air gap 80 between target 70 and magnet 34 change.
Therefore, the reluctance of the circuit through target 70 changes with motion
into and out of the page and magnetic sensor 36 produces a signal
representative of displacement.
The reluctance of the flux path through target 70 illustrated in Figures
9 and 10 is based on the distance d between pole tips 33 and 35 of magnets 32
and 34 through target 70. The distance d is equal to the sum of lengths of
gaps
78 and 80 and the width W of target 70 normal to the direction of
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displacement at the region of confrontation with stationary portion 10. Since
the permeability of the ferromagnetic target 70 is several thousand times that
of air, the majority of the change in reluctance is due to the changes in the
lengths of gaps 78 and 80. A small portion of the change is also due to the
change in the width W of target 70.
Figure 11 is a view similar to Figure 10 illustrating another mod~cation
of the target according to the present invention. Target 71 is configured with
edges tapered at acute angles to the direction of movement in a manner
similar to that shown in Figure 9, except that target surface 73 of target 71
confronts tips 33 and 35 of magnets 32 and 34 at all axial displacements of
target 71 with respect to stationary portion 10. Hence, the gaps 75 between
surface 73 and pole tips 33 and 35 are constant, but the width W of target
material varies with displacement. As shown in Figure 11, most of the flux
lines between target 71 and magnets 32 and 34 will be concentrated in the
region of pole tips 33 and 35 confronting surface 73, with only a small
fringing
flux extending from the extremities of the pole tips to the edges of the
target.
As a result, the distance d between pole tips 33 and 35 through target 71 is
equal to the sum of gaps 75 (which is constant) and the width W of target 71
normal to the direction of displacement at the region of confrontation with
stationary portion 10..
The reluctance of a magnetic circuit using target 70 of Figures 9 and 10
or target 71 of Figure 11 may tend to vary somewhat more non-linearly with
displacement than the reluctance of a magnetic circuit using target 20 of
Figure
1, due to non-linearity of the reluctance of the target materials. However,
the
effect of any non-linearity of reluctance can be compensated by altering the
taper defining the width of target 70 or 7I in a non-linear fashion based on
empirical observations and/or by employing an empirically-derived look-up
table for displacement data in microprocessor 49. The effects of non-linearity
of target 70 may be also be compensated by adding chamfer edges 82 and 84
to the edges of target 70 confronting magnets 32 and 34.
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As described above, the displacement sensor of the present invention
a
has a gap between stationary portion 10 and the target 20, 70. This gap must
be precisely controlled for the sensor to accurately measure displacement.
However, wear to mechanical parts introduces play into the displacement
sensor, thereby affecting the accuracy of the sensor. In addition, vibration
and
other environmental factors may make it difficult to precisely control the gap
between stationary part 13 and movable part 15.
To overcome the difficulties associated with gap control, two
displacement sensors are differentially arranged to correct for gap error.
Figures 12 and 13 illustrate a differential sensor comprising a pair of
displacement sensors, as in Figure 9, having respective stationary portions
l0a
and lOb coupled by linkage 81 (Figure 13) and confronting respective targets
70a and 70b. Targets 70a and 70b are arranged so that they taper in opposite
directions along the axis of the movable member to which they are attached.
Thus, the apex of target 70a is arlaxiged axially adjacent the base of target
70b,
and the base of target 70a is axially adjacent the apex of target 70b.
Figure 13 illustrates linear displacement of the targets along line 82 over
a range between DMIN ~d DMAx ~ Tm'get 70a is arranged to provide
increasing reluctance from DMr~, to DMA, and decreasing reluctance from
DMIN to D~q~ . As target 70a moves from DMIIV to DMA , the reluctance of
the magnetic circuit formed by stationary portion l0a and target 70a varies
from MIN t0 ~M~lx . At intermediate displacement Dl, the reluctance is
~MEASa ~ ~ewise, as target 70b moves between DMI1V to DMA , the
reluctance of the magnetic circuit formed by stationary part 92 and target 90
varies from ~?~q~ to ~tMIN. At intermediate displacement D1, the reluctance
iS ~MEASb
If an error is introduced into the gap between the stationary portions
and the targets, such as by wear or misalignment, one sensor will overestimate
actual displacement while the other sensor will underestimate actual
displacement. Moreover, each sensor will have the same error. As a result,
the average of the measured displacement will equal the actual displacement.
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Hence misalignment due to manufacturing tolerance, wear, materials or
environmental conditions affects the stationary parts and targets uniformly.
To measure displacement employing the displacement sensor illustrated
in Figures 12 and 13, output signals from magnetic flux sensors 36 in each of
stationary portions 10a and lOb provide outputs signals Va and Vb
respectively.
One common differential output of the two flux sensors is based on the
relationship
ya-Vb
Vatff- Va+vb
but other well-known relationships may also be employed. Hence, by
differentially measuring the output signals from flux sensors 36 of stationary
portions l0a and lOb, common mode errors are eliminated. Hence the
differential signal is an accurate representation of the displacement of the
valve operator.
Figure 14 illustrates a differential sensor comprising a pair of
displacement sensors, as in Figure 1, having respective stationary portions
l0a
and lOb confronting respective targets 20a and 20b. Targets 20a and 20b are
arranged so that they slope in opposite directions along the axis of the
movable
member to which they are attached. Thus, surface SOa of target 20a slopes
toward axis 26 as surface SOb of target 20b slopes away from axis 26. The
differential sensor shown in Figure 14 operates similarly to that illustrated
in
Figures 12 and 13.
While the embodiments employing differential sensors described in
connection with Figures I2-14 may be utilized with two displacement sensors
as described above, Figure 15 illustrates a differential displacement sensor
90
employing three magnets 92, 94, and 96, pole piece 98 and magnetic field
sensors 100 and 102. The sensor operates in the manner described above,
- except that magnet 94 is common to the two magnetic circuits of the
differential pair. Hence, magnet 94 is oriented oppositely from magnets 92
and 96.
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It is also possible to employ a single target for the differential sensor.
Figure 16 is a top view of differential sensor 90 of Figure 15, arranged
adjacent
a parallelogram-shaped target 104. Edges 106 and 108 are each at an acute
angle to the direction of motion I10, as linear motion occurs in the direction
shown by arrow 110, the reluctance of one magnetic circuit increases while the
reluctance of the other magnetic circuit decreases thereby providing a
differential displacement measurement as described above. Target 104 may be
used with the differential sensor 90 as shown, or with two separate sensors as
described in conjunction with Figures 12-14.
Figures 17 and 18 illustrate a magnetic displacement sensor for use with
a rotary actuator. In this case a rotary actuator 120 is arranged to rotate
about
axis 122 in the direction of arrow 124 (Figure 17). Target 126 is a
cylindrical
cam eccentrically mounted to the shaft of actuator 120. Cam 126 is
constructed of a high permeable material, such as iron. The surface 128 of
cam I26 confronts surface 48 of stationary portion i0 across gap 130.
Stationary portion 10 is described above in connection with Figures 1-5.
In operation of the sensor illustrated in Figures 17 and 18, as actuator
shaft 120 is rotated about axis 122, cam 126 is rotated, causing surface I28
to
move closer or farther from surface 48 of stationary portion 10. The flux path
through cam 126 is parallel to axis 122 and normal to the plane of rotation of
the shaft. Hence, gap 130 between surface 128 and the magnets of stationary
portion 10 changes in length, based on the radial position of shaft 120.
Change
in the length of gap 130 changes the reluctance of the magnetic circuit
through
cam 126, thereby changing the amount of flux along the lossy path between the
magnets and through the magnetic flux sensor as heretofore described.
Figure 19 illustrates a differential magnetic displacement sensor
employing a pair of rotary sensor illustrated in Figures 17 and 18. In the
case
of the sensor shown is Figure 19, cams 126a and 126b are eccentrically .
mounted to shaft 120, as described in connection with Figures 17 and 18,
except that the eccentric mounting of cams 126a and 126b are offset 180
° so
that as the surface of one of cams 126a and 126b moves closer to its
respective
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stationary portion l0a or IOb, the surface of the other of cams 126a and 126b
moves away from its respective stationary portion l0a or lOb. Thus, as the
surface of cam 126 a moves closer to stationary portion 10a, the surface of
cam
I26b moves away from stationary portion 10b, and vice versa. The magnetic
flux sensors of stationary portions IOa and lOb provides signals to cancel
common mode errors in the manner described in connection with Figures 12-
16.
It will be appreciated to those skilled in the art that the target for the
rotary displacement sensor may be configured as a section of changing axial
width by "wrapping" the target 70 shown in Figure 9 or the target 104 shown
in Figure 16 about the axis of the shaft for radial position sensing. Those
skilled in the art will also recognize other types of targets may be used to
achieve the same functionality. For example, a target formed of a material
having a magnetic permeability that changes with displacement would be
suitable for use with displacement sensors of the present invention. In any
case, the target need not be constructed of special materials, such as
Carpenter
49 or HyMu 80 material used in previous sensors. Instead, low grade through
magnetic materials may be employed for both the target as well as pole piece
42.
Additionally, the present invention may be accomplished through the
use of cast Alnico V magnets, rather than more expensive rare earth magnets
or other magnetic sources of high cost. Therefore, the sensor of the present
invention may be achieved relatively inexpensively and highly effectively.
The present invention also provides a magnetic displacement sensor
having interchangeable parts. More particularly, stationary member IO may be
employed in both rotary and linear displacement sensors, so that manufacturing
and inventory concerns dealing with several models and classes of sensors may
be minimized by employing the same model of stationary portion IO in several
types or classes of sensors.
It will be appreciated to those skilled in the art that while targets 20 and
70 are illustrated as mounted to high reluctance bases or carriers so that the
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target thickness is substantially constant over the displacement range, the
carrier may be eliminated where the valve stem itself provides a flat surface
at the correct slope for the target (parallel to the direction of displacement
in
the case of target 70 or acute to the direction of displacement in the case of
target 20). Alternatively, the carrier may simply be an integral part of the
target, in which case the target may have a variable thickness along the
direction of displacement, adding to non-linearity of the sensor and the need
to compensate for non-linearity by adjusting the shape of the target based on
empirical observations or by using look-up tables in the microprocessor based
on empirical data as herein described. Likewise, linearity issues of the
rotary
sensors illustrated in Figures 17-19 can be addressed by empirically adjusting
the cam offset from the axis of rotation of the actuator, and the relative
radial
positions of the cams in the case of the differential sensor of Figure 19.
The present invention provides a noncontacting magnetic displacement
sensor that minimizes that number of components required to be attached to
the object whose displacement is being measured. In many applications, the
target can be formed into the object whose displacement is being measured.
For example, a metal piston can be formed with a wedge-shaped portion or an
eccentric cam that can be used to form the magnetic circuit with the
stationary
part of the sensor described above.
Since the moving object requires a minimal number of components,
replacing or servicing the sensor merely entails replacing the stationary
part.
This feature is particularly advantageous because many prior sensors employed
permanent magnets on the movable portion of the sensor, and a magnetic field
sensor on the stationary part of the sensor. Since permanent magnet field
strength tends to weaken over time, and the magnetic field sensor may
malfunction, servicing prior sensors often required replacing both the movable
part and the stationary part. By incorporating both the permanent magnets .
and the field sensor in a single part, repair and replacement costs are
minimized.
CA 02237303 1998-OS-11
WO 97/18441 PCT/IJS96/18434
-17-
Although the present invention has been described with reference to
preferred embodiments, workers skiiled in the art will recognize that changes
may be made in form and detail without departing from the spirit and scope
of the invention.
