U.S. patent application number 16/258809 was filed with the patent office on 2019-05-23 for magnetic field generator and position sensing assembly.
The applicant listed for this patent is ROTA ENGINEERING LIMITED. Invention is credited to Bruce Fletcher.
Application Number | 20190154466 16/258809 |
Document ID | / |
Family ID | 51869421 |
Filed Date | 2019-05-23 |
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United States Patent
Application |
20190154466 |
Kind Code |
A1 |
Fletcher; Bruce |
May 23, 2019 |
MAGNETIC FIELD GENERATOR AND POSITION SENSING ASSEMBLY
Abstract
A magnetic field generator having at least one magnet extending
along a longitudinal axis, wherein the magnetic material of the at
least one magnet is arranged such that the at least one magnet
produces a magnetic field with a magnetic flux density that changes
substantially continuously in magnitude in the axial direction
substantially along the length of the at least one magnet in the
axial direction and a position sensing assembly comprising the
magnetic field generator.
Inventors: |
Fletcher; Bruce; (Rochdale,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ROTA ENGINEERING LIMITED |
Bury |
|
GB |
|
|
Family ID: |
51869421 |
Appl. No.: |
16/258809 |
Filed: |
January 28, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15513787 |
Mar 23, 2017 |
10222238 |
|
|
PCT/GB2015/052742 |
Sep 22, 2015 |
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16258809 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01D 5/145 20130101;
G01F 23/74 20130101 |
International
Class: |
G01D 5/14 20060101
G01D005/14; G01F 23/74 20060101 G01F023/74 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 24, 2014 |
GB |
1416870.2 |
Claims
1. A magnetic field generator comprising a magnet extending along a
longitudinal axis, wherein the magnetic material of the magnet is
arranged such that the magnet produces a magnetic field with a
magnetic flux density that changes substantially continuously in
magnitude in the axial direction substantially along a length of
the magnetic field generator; wherein the magnet extends in the
axial direction from a first end, that forms a first pole, to a
second end, that forms a second pole, wherein the outer diameter of
the magnet changes substantially continuously from the first and/or
second ends of the magnet towards the midpoint between the first
and second ends such that the amount of magnetic material in a
plane substantially perpendicular to the axis changes substantially
continuously from the first and/or second ends of the magnet
towards the midpoint between the first and second ends.
2. A magnetic field generator according to claim 1 wherein the
amount of magnetic material is varied by at least one continuous
change in the amount of magnetic material in said plane as the
axial position of the plane varies, along at least a section of the
entire length of the magnet in the axial direction.
3. A magnetic field generator according to claim 1 wherein the
magnet is disposed within a wall of a housing of magnetically
insulating material.
4. A magnetic field generator according to claim 1 wherein the
magnet is received within a bore in the wall of the housing.
5. A magnetic field generator according to claim 1 wherein the
magnet is substantially enclosed by the magnetically insulating
material.
6. A magnetic field generator according to claim 1 wherein the
magnetic field generator comprises one or more sets of magnets,
each set comprising at least a pair of said magnets.
7. A magnetic field generator according to claim 1, wherein the
amount of magnetic material in the plane increases linearly from
the first and/or second ends towards the midpoint.
8. A magnetic field generator according to claim 1, wherein the
amount of magnetic material in the plane increases non-linearly
from the first and/or second ends towards the midpoint.
9. A magnetic field generator according to claim 1, wherein the
amount of magnetic material in the plane increases from the first
and/or second ends to the midpoint.
10. A magnetic field generator according to claim 1, wherein the
magnet is solid.
11. A magnetic field generator according to claim 1, wherein a
diameter of the magnet is greatest at the midpoint.
12. A magnetic field generator according to claim 11, wherein the
diameter is greatest in a flat central section comprising the
midpoint.
13. A position sensing assembly comprising a movable member
arranged to move along an axis, wherein a magnetic field generator
according to claim 1 is coupled to the movable member, to move with
the movable member, and a magnetic sensor arrangement comprising at
least one magnetic sensor arranged to determine the axial position
of the magnetic field generator.
14. A position sensing assembly according to claim 13, wherein the
axis of the moveable member is parallel to, and offset from, an
axis of the magnetic sensor.
Description
[0001] The present application is a continuation of co-pending U.S.
Non-Provisional patent application Ser. No. 15/513,787, filed Mar.
23, 2017, which is a 371 U.S. National Phase filing of and claims
the priority of PCT International Patent Application No.
PCT/GB2015/052742, filed Sep. 22, 2015, which claims the priority
benefit of British Patent Application No. 1416870.2, filed Sep. 24,
2014, all of which are incorporated by reference herein in their
entirety.
[0002] The present invention relates to a magnetic field generator
and position sensing assembly for detecting the position of a
magnetic field generator.
[0003] In many applications it is desirable to detect the position
of a movable member. For example, where hydraulic or pneumatic
cylinder actuators are used to control the movement or positioning
of an object it is often desirable to determine the displacement of
the actuator.
[0004] A typical hydraulic or pneumatic piston actuator comprises a
cylinder that houses a slidable piston and piston rod assembly
arranged for reciprocal movement in the axial direction. The piston
is sealed to the inside surface of the cylinder so as to divide the
cylinder into two chambers and is moveable, under the influence of
hydraulic or pneumatic fluid introduced under pressure into one or
other of the chambers, between a retracted stroke position in which
the piston rod is substantially wholly received within the housing
and an extended stroke position in which the length of the rod
projects out of the housing. The movement of the piston is
typically effected by using one or more control valves to introduce
the fluid into the chambers. In order to ensure accurate
positioning it is desirable to operate the control valves in
response to a feedback signal representing the position of the
piston or piston rod relative to the cylinder in which case it is
necessary to have the ability to sense the stroke position of the
piston or piston rod in an accurate manner.
[0005] In one approach, a series of Hall-effect sensors or Hall
effect elements are arranged in a linear array in a tube along the
bore in the piston rod and a permanent magnet fitted to the piston
rod slides relative to the tube thus activating each of the sensors
in turn.
[0006] On longer hydraulic cylinders it is economically
advantageous to use fewer Hall effect devices in the electronic
position transducer. This can be achieved by increasing the
strength of the magnetic field so that the length of the magnetic
field in the axial direction is increased. This provides a wider
Hall output curve width which enables the number of Hall effect
devices to be reduced.
[0007] One way this can be achieved is by increasing the diameter
of the magnet. However, this is undesirable as it requires drilling
a larger diameter bore in the piston assembly, in order to
accommodate the larger diameter magnet, which weakens the piston
assembly. This is especially a problem in small diameter cylinders.
In addition, increasing the diameter of the magnet increases the
cost of the magnet significantly.
[0008] Another way this can be achieved is by increasing the length
of the magnet. However this suffers from the problem that the
magnetic field produced by the magnet has a central section,
between the magnetic poles of the magnet, that has substantially no
change in magnetic flux density, i.e. the magnetic flux density in
this region is substantially constant. Therefore, when the piston
is in a certain range of axial positions, the Hall voltage produced
on one or more Hall-effect sensors in this section of magnetic
field, is substantially constant. Therefore, the resolution of the
sensed axial positions of the piston/piston rod by the Hall-effect
sensors is significantly reduced.
[0009] Similar problems exist where the magnetic field generator is
used in other applications, such as in a float with a magnetic
sensor arranged to determine the level of the float. Larger magnets
increase the float diameter, subsequently increasing the diameter
of the access port required in the top of the tank for
installation.
[0010] It is an object of the present invention, amongst others, to
obviate or mitigate the aforementioned disadvantages. It is also an
object to provide for an improved magnetic field generator. It is
also an object of the invention to provide an improved position
sensing assembly.
[0011] According to a first aspect of the present invention there
is provided a magnetic field generator comprising at least one
magnet extending along a longitudinal axis, wherein the magnetic
material of the at least one magnet is arranged such that the at
least one magnet produces a magnetic field with a magnetic flux
density that changes substantially continuously in magnitude in the
axial direction substantially along the length of the at least one
magnet in the axial direction.
[0012] This allows the magnetic field produced by the magnetic
field generator to be relatively long, without the magnetic field
having a section of substantially no change in magnitude of
magnetic flux density, for example at a central area between the
magnetic poles of the at least one magnet. This is advantageous
where a magnetic sensor arrangement is arranged to determine the
axial position of the magnetic field generator. In this regard,
because the magnetic flux density changes substantially
continuously in magnitude in the axial direction, substantially
along the axial length of the magnetic field generator, the
resolution of the sensed axial positions of the magnetic field
generator by the magnetic sensor arrangement is significantly
increased (relative to if the magnetic field did not vary
substantially continuously in the axial direction). This allows the
magnetic sensor arrangement to use relatively few magnetic sensors,
which saves cost.
[0013] In addition, this allows the diameter of the magnetic field
generator to be relatively small, as compared to if an axially
uniform magnet was used and the diameter of the axially uniform
magnet increased so as to provide a magnetic field of a similar
length. Accordingly, when the magnetic field generator is housed
within a bore in a piston, for example of a linear actuator, this
allows the bore to be of a relatively small diameter, which
maintains the structural integrity of the piston. Similar
advantages arise where the magnetic field generator is used in
other applications, such as in a float with a magnetic sensor
arranged to determine the level of the float. For example, the
relatively small diameter of the magnetic field generator allows
the float to be of a relatively small diameter. This advantageously
allows an access port in a chamber containing a fluid, that the
float is arranged to measure the level of, to be of a relatively
small diameter.
[0014] Optionally, the amount of magnetic material of the at least
one magnet in a plane substantially perpendicular to the axis
varies with the axial position of the plane such that the at least
one magnet produces a magnetic field with a magnetic flux density
that changes substantially continuously in magnitude in the axial
direction substantially along the length of the at least one magnet
in the axial direction.
[0015] The amount of magnetic material may be varied by increasing
and/or decreasing the amount of magnetic material in said plane as
the axial position of the plane varies.
[0016] The amount of magnetic material may be varied by at least
one discrete change in the amount of magnetic material in said
plane as the axial position of the plane varies.
[0017] Alternatively, or additionally, the amount of magnetic
material may be varied by at least one continuous change in the
amount of magnetic material in said plane as the axial position of
the plane varies, along at least a section of the length of the at
least one magnet in the axial direction. The amount of magnetic
material may be varied by a continuous change in the amount of
magnetic material in said plane as the axial position of the plane
varies, substantially along the length of the at least one magnet
in the axial direction.
[0018] The variation in the amount of magnetic material in said
plane, with the axial position of the plane, may be linear.
Alternatively, or additionally, the variation may be
non-linear.
[0019] The at least one magnet may extend in the axial direction
from a first end, that forms a first pole, to a second end, that
forms a second pole, wherein the amount of magnetic material in
said plane increases from the first and/or second ends of the
magnet towards the midpoint between the first and second ends.
[0020] This is advantageous in that it increases the magnetic flux
density of the magnetic field produced by the at least one magnet
in the central section between the magnetic poles of the magnet.
This prevents the magnetic field, in this central section, having
substantially no change in magnetic flux density.
[0021] The at least one magnet may comprise a wall of magnetic
material having a thickness that extends from a radially inner
surface to a radially outer surface, wherein the thickness of the
wall of magnetic material varies with axial position so as to
provide said variation in magnetic material with axial
position.
[0022] The variation in thickness may be substantially continuous
with axial position. The thickness may vary as one or more step
changes with axial position. The variation in thickness with axial
position may be linear or non-linear.
[0023] The radius of the radially inner and/or outer surface may
vary with axial position so as to provide said variation of
magnetic material in the axial direction.
[0024] The at least one magnet may be arranged such that as the
radially inner surface extends from the first and/or second ends of
the at least one magnet towards the midpoint between the first and
second ends, the radial distance between the radially inner surface
and the axis decreases. In this respect, as the radially inner
surface extends from the first and/or second ends of the at least
one magnet towards the midpoint, the distance in the radial
direction between diametrically opposite points on the radially
inner surface decreases. The decrease in radial distance maybe
linear or non-linear. In this case, the radius of the radially
outer surface may be substantially constant with axial
position.
[0025] The at least one magnet may be arranged such that as the
radially outer surface extends from the first and/or second ends of
the at least one magnet towards the midpoint between the first and
second ends, the radial distance between the radially outer surface
and the axis increases. In this respect, as the radially outer
surface extends from the first and/or second ends of the at least
one magnet towards the midpoint, the distance in the radial
direction between diametrically opposite points on the radially
outer surface increases. In this case, the radius of the radially
inner surface may be substantially constant with axial position.
The increase in radial distance maybe linear or non-linear.
[0026] The at least one magnet may be substantially elongate, with
its length extending along said axis. The at least one magnet may
have a substantially circular cross-sectional shape about said
axis. The at least one magnet may have different cross-sectional
shapes about said axis.
[0027] The at least one magnet may comprise a plurality of magnets
that have different lengths in the axial direction and overlap in
the axial direction such that the amount of magnetic material of
the at least one magnet in a plane substantially perpendicular to
the axis varies with the axial position of the plane such that the
at least one magnet produces a magnetic field with a magnetic flux
density that changes substantially continuously in magnitude in the
axial direction substantially along the length of the at least one
magnet in the axial direction.
[0028] The overlapping arrangement maybe such that amount of
magnetic material in a plane substantially perpendicular to the
axis varies with the axial position of the plane by at least one
discrete change in the amount of magnetic material in said plane as
the axial position of the plane varies.
[0029] Optionally the magnets are distributed radially in a nested
arrangement.
[0030] In this regard, a first of the magnets is disposed radially
inwardly of, and received within, a second of the magnets. The
second of the magnets may be radially inwardly of, and received
within, a third of the magnets. The third of the magnets may be
radially inwardly of, and received within, a fourth of the magnets.
The nested arrangement may comprise more, or fewer, magnets.
[0031] The magnets may be substantially concentrically aligned with
each other.
[0032] Each magnet may be substantially annular, extending in a
circumferential direction about its longitudinal axis. Each magnet
may be a substantially hollow cylinder.
[0033] Radially adjacent magnets may contact each other along at
least a section of their overlapping lengths in the axial
direction.
[0034] Optionally the magnets are distributed circumferentially
about the longitudinal axis and are located at substantially the
same radius from the longitudinal axis.
[0035] Optionally the magnets are disposed within a wall of a
housing of magnetically insulating material. The magnets may be
separated from each other by the magnetically insulating material.
Alternatively the magnets may be in contact with each other. Each
magnet may be received within a bore in a wall of the housing. Each
magnet may be substantially enclosed by the magnetically insulating
material.
[0036] Optionally the magnets are distributed in the
circumferential and/or radial direction, relative to the
longitudinal axis.
[0037] The magnetic field generator may comprise one or more sets
of magnets, each set comprising at least a pair of magnets of
different lengths disposed within the wall of the housing. The
magnets of a set may be substantially aligned circumferentially or
radially within the wall. Where the magnets of a set are aligned
radially, the sets may be distributed in the circumferential
direction. Where the magnets of a set are aligned
circumferentially, the sets of magnets may be aligned with each
other in the circumferential direction. In this regard, the magnets
of the sets may be located at substantially the same radius from
the longitudinal axis. Alternatively, the magnets of a set may be
distributed circumferentially such that in a circumferential
direction, the radius of the magnets from the longitudinal axis
increases. The magnets of a set may be located at different radial
and/or circumferential locations.
[0038] Each magnet may be substantially solid. The at least one
magnet may be a bar, rod or cylinder magnet. Alternatively, the
magnets may have different cross-sectional shapes, for example
square, triangular, etc.
[0039] Alternatively, or additionally, the distance between the
magnetic material of the at least one magnet and the axis, at each
axial position, may be varied with axial position such that the at
least one magnet produces a magnetic field with a magnetic flux
density that changes substantially continuously in magnitude in the
axial direction substantially along the length of the at least one
magnet in the axial direction.
[0040] The distance between the magnetic material of the at least
one magnet and the axis, at each axial position, may be varied
linearly or non-linearly with axial position.
[0041] The distance between a radially inner surface of the at
least one magnet and the axis may be varied with axial position to
produce said change in magnetic flux density.
[0042] The at least one magnet may extend in the axial direction
from a first end, that forms a first pole, to a second end, that
forms a second pole, wherein as the radially inner surface extends
from the first and/or second ends of the at least one magnet
towards the midpoint between the first and second ends, the radial
distance between the radially inner surface and the axis decreases.
In this respect, as the radially inner surface extends from the
first and/or second ends of the at least one magnet towards the
midpoint, the distance in the radial direction between
diametrically opposite points on the radially inner surface
decreases.
[0043] The amount of magnetic material of the at least one magnet
in a plane substantially perpendicular to the axis may be
substantially constant with the axial position of the plane. In
this respect, the thickness of the at least one magnet may be
substantially constant with the axial position. The radially outer
surface may be substantially parallel to the radially inner
surface.
[0044] Alternatively, or additionally, the density of the magnetic
material of the at least one magnet may be varied with axial
position such that the at least one magnet produces a magnetic
field with a magnetic flux density that changes substantially
continuously in magnitude in the axial direction substantially
along the length of the at least one magnet in the axial
direction.
[0045] Alternatively, or additionally, the strength of the magnetic
material of the at least one magnet may be varied with axial
position such that the at least one magnet produces a magnetic
field with a magnetic flux density that changes substantially
continuously in magnitude in the axial direction substantially
along the length of the at least one magnet in the axial
direction.
[0046] The strength of the magnetic material of the at least one
magnet may be varied with axial position by varying the chemical
composition of the magnetic material with axial position.
[0047] The at least one magnet may comprise at least one axially
extending strip of magnetic material that extends in the
circumferential direction partly about the longitudinal axis.
[0048] The at least one magnet may comprise a plurality of said
strips circumferentially spaced about the longitudinal axis.
[0049] The magnetic field generator may comprise a housing of a
magnetically insulating material, wherein the at least one strip is
fixedly attached to the housing. The at least one strip may be
fixedly attached to a radially inner surface of the housing.
[0050] According to a second aspect of the present invention there
is provided a position sensing assembly comprising a movable member
arranged to move along an axis, wherein a magnetic field generator
according to the first aspect of the invention is coupled to the
movable member, to move with the movable member, and a magnetic
sensor arrangement comprising at least one magnetic sensor arranged
to determine the axial position of the magnetic field
generator.
[0051] The magnetic field generator may be mounted internally or
externally of the movable member. The magnetic field generator may
be mounted in a bore in the movable member. The magnetic field
generator may be directly attached to the movable member.
Alternatively, the magnetic field generator may be coupled to the
movable member by a coupling member that moves the magnetic field
generator in dependence on the axial position of the movable
member.
[0052] The movable member may be arranged to move axially relative
to a magnetic sensor housing, within which the magnetic sensor
arrangement is provided. The magnetic sensor housing may be
disposed radially inwardly of the magnetic field generator.
Alternatively, the magnetic sensor housing may be disposed radially
outwardly of the magnetic field generator. The magnetic sensor
housing may extend in the axial direction, with the movable member
constrained to move in the axial direction by the magnetic sensor
housing. The magnetic sensor housing may be tubular.
[0053] The movable member may be made of a non-magnetic material.
For example, the movable member may be made of a magnetically
insulating material, for example Austenitic Stainless Steel,
Aluminium or Nylon.
[0054] In this case, the at least one magnet may contact the
movable member.
[0055] The movable member may be made of a ferromagnetic material,
for example magnetic carbon, steel or iron. In this case, the at
least one magnet may be arranged such that it does not contact the
movable member.
[0056] In this respect, a spacer of magnetically insulating
material may be provided between the at least one magnet of the
magnetic field generator and the movable member. The at least one
magnet of the magnetic field generator may be disposed in a housing
of magnetically insulating material. This is advantageous in that
it acts to increase the axial length of the magnetic field.
[0057] Where the at least one magnet comprises said at least one
strip of magnetic material, the at least one strip may be coupled
to the movable member such that it is axially fixed relative to the
movable member. The at least one strip may be directly attached to
the movable member. Where the at least one strip is fixedly
attached to a housing of magnetically insulating material, the
housing of magnetically insulating material may be fixedly attached
to the movable member.
[0058] The at least one strip may be coupled to the movable member
at a circumferential location that produces a maximum sensed signal
from the at least one magnetic sensor. In this regard, the at least
one strip may be coupled to the movable member at a location that
is circumferentially aligned with the at least one magnetic
sensor.
[0059] The at least one strip may be fixedly attached to the
movable member by any suitable means of fastening, including by
pinning.
[0060] Optionally the at least one magnetic sensor is a Hall-effect
sensor. The at least one magnetic sensor may be any suitable type
of magnetic sensor, for example a magneto-resistive element or GMR
(giant magneto-resistive) technology.
[0061] Optionally the at least one magnet comprises a plurality of
magnets.
[0062] Optionally the at least one magnetic sensor comprises a
plurality of magnetic sensors distributed in the axial
direction.
[0063] The movable member may be arranged to move along an axis
that is substantially straight. Alternatively the axis may be
curved or part-curved.
[0064] The movable member may be a piston disposed in a housing for
reciprocal movement along an axis, the housing having a wall with
an internal surface, the piston having first and second axially
spaced end surfaces, at least a first chamber defined between one
of the first and second end surfaces and the internal surface of
the wall for receipt of actuating fluid, the magnetic field
generator being coupled to the piston so as to move with the piston
along said axis, and the magnetic sensor arrangement being arranged
to determine the axial position of the magnetic field generator
relative to the housing. In this case, the position sensing
assembly may be a linear actuator.
[0065] The movable member may also comprise a piston rod coupled to
the piston such that it moves with the piston. The magnetic field
generator may be coupled to the piston rod (and therefore to the
piston) so as to move axially with the piston rod (and therefore
the piston).
[0066] The movable member may be a float arranged such that, in
use, it is movable in the axial direction in dependence on the
level of a fluid in which the float is located. The axial direction
may be substantially vertical. The position sensing assembly may
comprise a fluid housing for containing the fluid. The fluid
housing may comprise an access port, through which the magnetic
sensor housing passes.
[0067] All of the features described herein may be combined with
any of the above aspects, in any combination.
[0068] A specific embodiment of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings in which:
[0069] FIG. 1 shows a cutaway perspective view of a linear actuator
according to a first embodiment of the second aspect of the present
invention, with a longitudinal half of the linear actuator omitted
for illustrative purposes;
[0070] FIG. 2 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to a first
embodiment of the first aspect of the present invention;
[0071] FIG. 2A shows an end view of the magnetic field generator
shown in FIG. 2;
[0072] FIG. 2B shows a cross-sectional view of the magnetic field
generator of FIG. 2A, taken along the line B-B in FIG. 2A;
[0073] FIG. 3 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to a second
embodiment of the first aspect of the present invention;
[0074] FIG. 3A shows an end view of the magnetic field generator
shown in FIG. 3;
[0075] FIG. 3B shows a cross-sectional view of the magnetic field
generator of FIG. 3A, taken along the line B-B in FIG. 3A;
[0076] FIG. 4 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to a third
embodiment of the first aspect of the present invention;
[0077] FIG. 4A shows an end view of the magnetic field generator
shown in FIG. 4;
[0078] FIG. 4B shows a cross-sectional view of the magnetic field
generator of FIG. 4A, taken along the line B-B in FIG. 4A;
[0079] FIG. 5 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to a fourth
embodiment of the first aspect of the present invention;
[0080] FIG. 5A shows an end view of the magnetic field generator
shown in FIG. 5;
[0081] FIG. 5B shows a cross-sectional view of the magnetic field
generator of FIG. 5A, taken along the line B-B in FIG. 5A;
[0082] FIG. 6 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to a fifth
embodiment of the first aspect of the present invention;
[0083] FIG. 6A shows an end view of the magnetic field generator
shown in FIG. 6;
[0084] FIG. 6B shows a cross-sectional view of the magnetic field
generator of FIG. 6A, taken along the line B-B in FIG. 6A;
[0085] FIG. 7 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to a sixth
embodiment of the first aspect of the present invention;
[0086] FIG. 7A shows an end view of the magnetic field generator
shown in FIG. 7;
[0087] FIG. 7B shows a cross-sectional view of the magnetic field
generator of FIG. 7A, taken along the line B-B in FIG. 7A;
[0088] FIG. 8 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to a seventh
embodiment of the first aspect of the present invention;
[0089] FIG. 8A shows an end view of the magnetic field generator
shown in FIG. 8;
[0090] FIG. 8B shows a cross-sectional view of the magnetic field
generator of FIG. 8A, taken along the line B-B in FIG. 8A;
[0091] FIG. 8C shows a schematic view illustrating the different
lengths of the magnets used in the magnetic field generator shown
in FIG. 8;
[0092] FIG. 9 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to an eighth
embodiment of the first aspect of the present invention;
[0093] FIG. 9A shows an end view of the magnetic field generator
shown in FIG. 9;
[0094] FIG. 9B shows a cross-sectional view of the magnetic field
generator of FIG. 9A, taken along the line B-B in FIG. 9A;
[0095] FIG. 10 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to a ninth
embodiment of the first aspect of the present invention;
[0096] FIG. 10A shows an end view of the magnetic field generator
shown in FIG. 10;
[0097] FIG. 10B shows a cross-sectional view of the magnetic field
generator of FIG. 10A, taken along the line B-B in FIG. 10A;
[0098] FIG. 11 shows a perspective view of a magnetic field
generator of the linear actuator of FIG. 1, according to a tenth
embodiment of the first aspect of the present invention;
[0099] FIG. 11A shows an end view of the magnetic field generator
shown in FIG. 11;
[0100] FIG. 11B shows a cross-sectional view of the magnetic field
generator of FIG. 11A, taken along the line B-B in FIG. 11A;
[0101] FIG. 12A shows a perspective view a magnetic field generator
of the linear actuator of FIG. 1, according to an eleventh
embodiment of the first aspect of the present invention;
[0102] FIG. 12B shows a perspective view a magnetic field generator
of the linear actuator of FIG. 1, according to a twelfth embodiment
of the first aspect of the present invention;
[0103] FIG. 12C shows a perspective view a magnetic field generator
of the linear actuator of FIG. 1, according to a thirteenth
embodiment of the first aspect of the present invention;
[0104] FIG. 13 shows a representative graph of the variation of the
magnetic flux density (.beta.) of the magnetic field generated by
the magnetic field generator shown in the embodiments of FIGS. 2A
to 12C with axial position (x) across the magnetic field;
[0105] FIG. 14 shows a representative graph of the variation of
Hall Effect voltage (V.sub.H) generated, by the magnetic field
generator shown in the embodiments of FIGS. 2A to 12C, across a
Hall-effect sensor of the magnetic sensor arrangement of the linear
actuator of FIG. 1, with axial position (x) across the magnetic
field, and
[0106] FIG. 15 shows a position sensing assembly in the form of a
fluid level sensor assembly according to a second embodiment of the
second aspect of the present invention.
[0107] Referring to FIG. 1, there is shown a position sensing
assembly in the form of hydraulic linear actuator comprising a
housing in the form of a cylinder 1 and a reciprocal piston 2. The
cylinder 1 extends along a longitudinal axis 100. The cylinder 1
has first and second end fittings 4a, 4b so as to define an
internal chamber 5 in which the piston 2 is slidably disposed to
reciprocate along the longitudinal axis 100.
[0108] The piston 2 is cylindrical with first and second end
surfaces 6, 7 penetrated by a central bore 8. The piston 2 is made
of a ferromagnetic material, for example magnetic carbon, steel or
iron.
[0109] The piston 2 is concentrically mounted on a piston rod 9
towards a first end of the rod 9 and is fixed axially relative to
the rod 9. A first end 13 of the piston rod 9 is secured to the
piston 2. A second end 14 of the piston rod 9 projects outside the
cylinder though a bore in the second end fitting 4b and terminates
in an eyelet 14b for connection to a first component. The first end
fitting 4a has an eyelet 15 for connection to a second component,
the first and second components designed to be movable relative to
one another by the actuator.
[0110] The piston 2 serves to divide the chamber 5 into two
variable volume sections 5a, 5b, defined between the first and
second end surfaces 6, 7 of the piston 2 and an inner surface of
the wall 3 of the cylinder 1 for receipt of hydraulic fluid. Ports
16, 17 penetrate the wall 3 axially inboard of each end fitting 4a,
4b and allow hydraulic fluid to be delivered or removed so as to
alter the fluid pressure within the respective chamber sections 5a,
5b and effect movement of the piston 2 within the cylinder 1.
[0111] A cylindrical bore 23 is provided in the piston rod 9 that
extends in the direction of the longitudinal axis 100. The bore 23
extends from the first end 13 of the piston rod 9 towards the
second end 14 of the piston rod 9 along the longitudinal axis 100,
terminating axially inboard of the second end 14 of the piston rod
9.
[0112] A magnetic sensor arrangement in the form of a plurality of
Hall-effect sensors 105 are arranged in a linear array in a
magnetic sensor housing in the form of a tube 24 provided in the
bore 23 in the piston rod 9. The tube 24 is cylindrical and extends
along the longitudinal axis 100 of the cylinder 1. The tube 24 is
fixed axially relative to the cylinder 1. In order to fix the tube
24 to the cylinder 1, the first end of the tube 24 is fixedly
attached to the first end fitting 4a. The Hall-effect sensors 105
are distributed along the length of the tube 24 in the axial
direction 100.
[0113] A magnetic field generator 50 is received within the bore 24
in the piston rod 9 towards the first end of the piston rod 9. In
this respect, the magnetic field generator 50 is also located
within the bore 8 in the piston 2.
[0114] The magnetic field generator 50 is coupled to the piston 2
so as to move with the piston 2 along the longitudinal axis 100. In
this respect, the magnetic field generator 50 is housed between a
radially inner surface of the piston rod 9 and a radially outer
surface of the tube 24. A radially outer surface of the magnetic
field generator 50 is fixedly attached to the radially inner
surface of the piston rod 9 such that it moves axially with the
piston rod 9, and therefore the piston 2, with a radially inner
surface of the magnetic field generator 50 sliding over the
radially outer surface of the tube 24.
[0115] The Hall-effect sensors 105 and the magnetic field generator
50 are arranged such that as the magnetic field generator 50 slides
with the piston 2, the magnetic field produced by the magnetic
field generator 50 is sensed by each of the Hall-effect sensors in
turn, so as to sense the position of the magnetic field generator
50, and therefore of the piston 2, relative to the cylinder 1,
along the longitudinal axis 100.
[0116] In this respect, a connecting cable 41 passes from the tube
24 through a bore in the first end fitting 4a to a suitable
electronic circuit arranged to output the axial position of the
piston 2 and/or piston rod 9 from the voltages produced across the
Hall-effect sensors 105. This output axial position may then be
used by a suitable control system.
[0117] Referring to FIGS. 2, 2A and 2B, there is shown a first
embodiment of the magnetic field generator 50. The magnetic field
generator 50 comprises a single magnet 51, formed by an annular
wall 104 of magnetic material.
[0118] The annular wall 104 is generally elongate extending from a
first end 52 to a second end 53 along a longitudinal axis 101. The
first end 52 forms a North pole and the second end 53 forms a South
pole (as shown in FIG. 2B).
[0119] The annular wall 104 extends in a thickness direction from a
radially outer surface 54 to a radially inner surface 55. The
annular wall 104 extends circumferentially substantially around the
longitudinal axis 101.
[0120] The radially outer surface 54 has a substantially circular
cross sectional shape that is substantially centred on the
longitudinal axis 101 of the magnet 51 and has a substantially
constant radius across the axial length of the magnet 51.
[0121] The radially inner surface 55, has a substantially circular
cross-sectional shape about the axis 101 (which varies in radius
with axial position--see below).
[0122] The wall of magnetic material 104 linearly increases in
thickness from its first and second ends 52, 53, to the midpoint 56
along its length between its first and second ends 52, 53. In this
respect, as the radially inner surface 55 extends from the first
and second ends 52, 53 of the magnet 51 to the midpoint 56 along
its length between its first and second ends 52, 53, the radial
distance between the radially inner surface 55 and the axis 101
decreases linearly. Accordingly, the distance in the radial
direction between diametrically opposite points on the radially
inner surface 55 decreases.
[0123] As stated above, the radially outer surface 54 of the
annular wall 104 is fixedly attached to the radially inner surface
of the piston rod 9 such that it moves axially with the piston rod
9, and therefore the piston 2, with the radially inner surface 55
of the annular wall 104 sliding over the radially outer surface of
the tube 24.
[0124] The longitudinal axis 101 of the magnet 51 is coincident and
substantially parallel with the longitudinal axis 100 of the
cylinder 1.
[0125] In this arrangement, the amount of magnetic material of the
magnet 51 in a plane 102 substantially perpendicular to the axis
100, 101 varies with the axial position of the plane such that the
magnet 51 produces a magnetic field with a magnetic flux density
that changes substantially continuously in magnitude in the axial
direction substantially along the length of magnet 51 in the axial
direction 100, 101 (as shown represented in FIG. 13).
[0126] In this regard, the amount of magnetic material of the
magnet 51 in a plane 102 substantially perpendicular to the axis
100, 101 varies with the axial position of the plane 102 by a
continuous change in the amount of magnetic material in said plane
as the axial position of the plane varies, substantially along the
length of the magnet 51 in the axial direction.
[0127] This allows the magnetic field produced by the magnetic
field generator 50 to be relatively long, without the magnetic
field having a section of substantially no change in magnitude of
magnetic flux density, for example at a central area between the
magnetic poles of the magnet 51.
[0128] In this respect, because the magnetic flux density changes
substantially continuously in magnitude in the axial direction 101,
substantially along the axial length of the magnetic field, the
Hall-effect voltage produced on the Hall-effect sensors also
changes substantially continuously in magnitude in the axial
direction 101 substantially along the length of the magnetic field
in the axial direction 101 (as shown represented in FIG. 14).
Accordingly, the resolution of the sensed axial positions of the
magnetic field generator 50, and therefore of the piston 2, is
significantly increased (relative to if the magnetic field did not
vary substantially continuously in the axial direction). This
allows the magnetic sensor arrangement to use relatively few
Hall-effect sensors, which saves cost.
[0129] In addition, this allows the diameter of the magnetic field
generator 50 to be relatively small, as compared to if an axially
uniform magnet was used and the diameter of the axially uniform
magnet increased so as to provide a magnetic field of a similar
length. Accordingly, this allows the bore 8 in the piston 2 to be
of a relatively small diameter, thereby maintaining the structural
integrity of the piston 2.
[0130] FIGS. 3 to 12C show different embodiments of the magnetic
field generator 50. In each of the following embodiments, the same
reference numerals will be used for features in common with the
first embodiment.
[0131] Referring now to FIGS. 3, 3A and 3B, there is shown a second
embodiment of the magnetic field generator 50. The magnetic field
generator 50 of the second embodiment is identical to that of the
first embodiment, except in that the wall of magnetic material 104
non-linearly increases in thickness from its first and second ends
52, 53, to the midpoint 56 along its length between its first and
second ends 52, 53. In this regard, the radially outer surface 54
has a substantially constant radius across the axial length of the
wall 104 and the radially inner surface 55 non-linearly varies in
radius with axial position to produce said variation in
thickness.
[0132] Referring now to FIGS. 4, 4A and 4B, there is shown a third
embodiment of the magnetic field generator 50. As with the first
embodiment, the annular wall 104 linearly increases in thickness
from its first and second ends 52, 53 of the magnet 51 to the
midpoint 56 along its length between its first and second ends 52,
53. The magnetic field generator 50 of this embodiment is identical
to that of the first embodiment except in that the arrangement of
the radially inner and outer surfaces 55, 54 is reversed, i.e. the
radially inner surface 55 has a substantially constant radius
across the axial length of the wall 104 and the radially outer
surface 54 varies in radius with axial position to produce said
variation in thickness.
[0133] Referring now to FIGS. 5, 5A and 5B, there is shown a fourth
embodiment of the magnetic field generator 50. As with the second
embodiment, the annular wall 104 non-linearly increases in
thickness from its first and second ends 52, 53 to the midpoint 56.
The magnetic field generator 50 of this embodiment is identical to
that of the second embodiment except in that the arrangement of the
radially inner and outer surfaces 55, 54 is reversed, i.e. the
radially inner surface 55 has a substantially constant radius
across the axial length of the wall 104 and the radially outer
surface 54 varies in radius with axial position to produce said
variation in thickness.
[0134] Referring now to FIGS. 6, 6A and 6B, there is shown a fifth
embodiment of the magnetic field generator 50. In this embodiment,
the magnetic field generator 50 comprises a plurality of magnets.
Specifically, the magnetic field generator 50 comprises first,
second, third and fourth magnets 71, 72, 73, 74.
[0135] The magnetic field generator 50 extends from a first end 52
to a second end 53 along a longitudinal axis 103. Each magnet 71,
72, 73, 74 is a substantially hollow cylindrical magnet. Each
magnet 71, 72, 73, 74 extends from a first end to a second end
about the longitudinal axis 103 and has a radially outer surface 54
and a radially inner surface 55 that have a substantially circular
cross-sectional shape that is substantially centred on the
longitudinal axis 103.
[0136] The magnets 71, 72, 73, 74 are nested concentrically
together. In this respect, the first magnet 71 is received within
the second magnet 72 which is itself received within the third
magnet 73, which is received within the fourth magnet 74. The
radially outer surface of the first magnet 71 is in contact with
the radially inner surface of the second magnet 72, the radially
outer surface of the second magnet 72 is in contact with the
radially inner surface of the third magnet 73 and the radially
outer surface of the third magnet 73 is in contact with the
radially inner surface of the fourth magnet 74.
[0137] The magnets 71, 72, 73, 74 have different lengths in said
axial direction 103. Specifically, the first magnet 71 is shorter
than the second magnet 72, in the axial direction 103, the second
magnet 72 is shorter in length, in the axial direction 103, than
third magnet 73 and the third magnet 73 is shorter in length, in
the axial direction 103, than fourth magnet 74.
[0138] The first, second and third magnets 71, 72, 73 are axially
located such that the midpoints along their axial lengths are
substantially aligned with the midpoint 56 along the axial length
of the fourth magnet 74. The first and second ends 52, 53 of the
magnetic field generator 50 are defined by the first and second
ends of the fourth magnet 74.
[0139] The magnets 71, 72, 73, 74 together define a radially inner
stepped surface 55 and a radially outer surface 54.
[0140] The radially outer surface 54 is a radially outer surface of
the fourth magnet 74 and has a substantially circular cross
sectional shape that is substantially centred on the longitudinal
axis 103. The radially outer surface 54 has a substantially
constant radius substantially along the axial length of the
magnetic field generator 50.
[0141] The stepped radially inner surface 55 has a substantially
circular cross-sectional shape about the axis 103. As the radially
inner surface 55 extends from the first and second ends 52, 53 of
the of the magnetic field generator 50 to the midpoint 56 between
the first and second ends 52, 53, the radial distance between the
radially inner surface 55 and the axis 103 decreases in a step-wise
manner.
[0142] Referring now to FIGS. 7, 7A and 7B, there is shown a sixth
embodiment of the magnetic field generator 50. This embodiment is
identical to the fifth embodiment except in the ordering of the
lengths of the magnets 71, 72, 73, 74. In this regard, the fourth
magnet 74 is shorter than the third magnet 73, in the axial
direction 103, the third magnet 73 is shorter in length, in the
axial direction 103, than second magnet 72 and the second magnet 72
is shorter in length, in the axial direction 103, than the first
magnet 71.
[0143] As with the preceding embodiments, in each of the
embodiments shown in FIGS. 6 to 7b the amount of magnetic material
in a plane 102 substantially perpendicular to the axis 103 varies
with the axial position of the plane such that the magnetic field
generator 50 produces a magnetic field with a magnetic flux density
that changes substantially continuously in magnitude in the axial
direction substantially along the length of magnet 51 in the axial
direction 100, 101 (as shown represented in FIG. 13).
[0144] In this regard, the amount of magnetic material of the
magnet 51 in a plane 102 substantially perpendicular to the axis
103 varies with the axial position of the plane 102 by a plurality
of discrete changes in the amount of magnetic material in said
plane as the axial position of the plane varies.
[0145] This provides the advantages described above in relation to
the preceding embodiments.
[0146] Referring now to FIGS. 8 to 8C, there is shown a seventh
embodiment of the magnetic field generator 50. In this embodiment,
the magnetic field generator 50 comprises a holder 90 of
magnetically insulating material. The holder 90 may be made from
any suitable magnetically insulating material, such as a suitable
plastics material, aluminium, brass, nylon or the like.
[0147] The holder 90 comprises a wall 133 of magnetically
insulating material that has the general shape of a hollow
cylinder, extending from a first end 91 to a second end 92 about a
longitudinal axis 110 (see FIG. 8b). The wall 133 has a circular
cross-sectional shape that is substantially centred on the
longitudinal axis 110. The wall 133 extends in a thickness
direction from a radially inner surface 134 to a radially outer
surface 135.
[0148] The radially outer surface 135 of the wall 133 is fixedly
attached to the radially inner surface of the piston rod 9, so as
to move axially with the piston rod 9 (and therefore the piston
2).
[0149] The magnetic field generator 50 comprises a plurality of
sets 111 of magnets 95, 96, 97, 98. Each set 111 of magnets is
substantially identical. Each set 111 of magnets is disposed within
the thickness of the wall 133, with the sets 111 being distributed
circumferentially about the longitudinal axis 110.
[0150] In the described embodiment each set of magnets consists of
four magnets 95, 96, 97, 98.
[0151] Each of the magnets 95, 96, 97, 98 is a substantially solid
cylindrical magnet, extending along a longitudinal axis that is
substantially parallel to the longitudinal axis 110. In each set
111, a first magnet 98 of the set is shorter than a second magnet
97 of the set, the second magnet 97 of the set is shorter than a
third magnet 96 of the set and the third magnet 96 of the set is
shorter than a fourth magnet 95 of the set.
[0152] Each magnet is received within a respective cylindrical bore
93 in the wall 133 that extends along a longitudinal axis that is
substantially parallel to the longitudinal axis 110 and is spaced
radially from said axis 110. The magnets 95, 96, 97, 98 in each set
are distributed in the circumferential direction. The magnets 95,
96, 97, 98 in each set are located at substantially the same radius
from the axis 110. Each set of magnets are located at substantially
the same radius from the axis 110. Accordingly, each magnet of the
magnetic field generator is located at substantially the same
radius from the axis 110.
[0153] Each magnet 95, 96, 97, 98 is separated from each other
magnet 95, 96, 97, 98 by the surrounding wall 133 of magnetically
insulating material. In addition, the longitudinal ends of each
magnet 95, 96, 97, 98 are covered by respective ends of the wall
133 of magnetically insulating material. Each magnet 95, 96, 97, 98
is substantially enclosed by the magnetically insulating
material.
[0154] In each set 111, the first magnet 98 is centred along the
axial length of the second magnet 97, the second magnet 97 is
centred along the axial length of the third magnet 96 and the third
magnet is centred along the axial length of the fourth magnet
95.
[0155] The circumferential order of the magnets 95, 96, 97, 98, of
each set, in the clockwise direction, when viewed in the direction
of FIG. 8a, is the first magnet 98, followed by the second magnet
97, followed by the third magnet 96, followed by the fourth magnet
95. Accordingly, the first magnet 98 of each set is
circumferentially adjacent to the fourth magnet 95 of an adjacent
set.
[0156] Due to the arrangement of said magnets of differing lengths
within the holder 90, the amount of magnetic material in a plane
substantially perpendicular to the axis 110 varies with the axial
position of the plane such that a magnetic field is produced that
has a magnetic flux density that changes substantially continuously
in magnitude in the axial direction substantially along the length
of the magnetic field generator 50.
[0157] As with the preceding embodiments, this reduces the number
of Hall-effect sensors required, and allows the diameter of the
magnetic field generator to be relatively small.
[0158] Furthermore, because each magnet 95, 96, 97, 98 is
substantially enclosed by the magnetically insulating material, the
magnetic field generator 50 has a magnetic flux density
distribution that is substantially symmetrical in the axial
direction, and increases the length of the magnetic field.
[0159] Referring now to FIGS. 9 to 9B, there is shown an eighth
embodiment of the magnetic field generator 50. The magnetic field
generator 50 of this embodiment is identical to the magnetic field
generator of the previous embodiment (shown in FIG. 8, 8A to 8C)
except in that the magnets 95, 96, 97, 98 of each set 111 are
substantially aligned in the radial direction and increase in
length with increasing radial distance from the longitudinal axis
110. The sets 111 are distributed in the circumferential
direction.
[0160] In this respect, in each set 111, the first magnet 98 is
provided radially inwardly of the second magnet 97, the second
magnet 97 is provided radially inwardly of the third magnet 96 and
the third magnet 96 is provided radially inwardly of the fourth
magnet 95.
[0161] Alternatively the magnets 95, 96, 97, 98 may be arranged
such that they decrease in length with increasing radial distance
from the longitudinal axis 110.
[0162] Referring now to FIGS. 10 to 10B, there is shown a ninth
embodiment of the magnetic field generator 50. The magnetic field
generator 50 comprises a single magnet 51. The magnet 51 is
generally elongate and comprises an annular wall 104 of magnetic
material that extends from a first end 52 to a second end 53 along
a longitudinal axis 101. The first end 52 forms a North pole and
the second end 53 forms a South pole (as shown in FIG. 10B).
[0163] The annular wall 104 extends in a thickness direction from a
radially outer surface 54 to a radially inner surface 55. The
annular wall 104 has a substantially constant thickness across its
axial length.
[0164] The radially inner surface 55 has a substantially circular
cross-sectional shape that is substantially centred on the axis 101
(although the diameter of the circular cross-section varies with
axial position). As the radially inner surface 55 extends from the
first and second ends 52, 53 to the midpoint 56, the radial
distance between the radially inner surface 55 and the axis 101
decreases linearly, i.e. the distance in the radial direction
between diametrically opposite points on the radially inner surface
55 decreases linearly.
[0165] The radially outer surface 54 is substantially parallel to
the radially inner surface 55.
[0166] The spacing of the annular wall 104 of magnetic material and
the longitudinal axis 101 is varied with axial position such that
the magnet 51 produces a magnetic field with a magnetic flux
density that changes substantially continuously in magnitude in the
axial direction 101 substantially along the length of the magnet 51
in the axial direction (as shown represented in FIG. 13).
[0167] In this arrangement, the amount of magnetic material in a
plane substantially perpendicular to the longitudinal axis 101 is
substantially constant with the axial position of the plane.
[0168] Referring now to FIGS. 11 to 11B, there is shown a tenth
embodiment of the magnetic field generator 50. This embodiment is
identical to the ninth embodiment except in that as the radially
inner surface 55 extends from the first and second ends 52, 53 to
the midpoint 56, the radial distance between the radially inner
surface 55 and the axis 101 decreases non-linearly, i.e. the
distance in the radial direction between diametrically opposite
points on the radially inner surface 55 decreases non-linearly.
[0169] Referring to FIGS. 12a to 12c there is shown an eleventh,
twelfth and thirteenth embodiment of the magnetic field generator
50. In these embodiments, the magnetic field generator 50 comprises
a magnet formed by an elongate strip 104 of magnetic material. The
strip 104 is a circumferential section of the magnet of the second,
first and tenth embodiments respectively. Alternatively, the strip
104 may be a circumferential section of the magnet of any of the
other above described embodiments.
[0170] In the described embodiment the strip 104 is pinned to the
radially inner surface of the piston rod 9. However, it will be
appreciated that any suitable means of attachment may be used.
[0171] The strip 104 is attached to the radially inner surface of
the piston rod 9 at a circumferential location that produces a
maximum Hall output voltage from the Hall sensors 105.
[0172] Alternatively, the magnetic field generator 50 may comprise
a magnet housing of a magnetically insulating material, which is
fixedly attached to the radially inner surface of the piston rod 9,
where the strip 104 is fixedly attached to a radially inner surface
of the housing of magnetically insulating material.
[0173] As with the embodiments shown in FIGS. 2 to 9b, in the
embodiments shown in FIGS. 12a and 12b, the amount of magnetic
material of the strip 104 in a plane substantially perpendicular to
the axis 101 varies with the axial position of the plane such that
the magnet produces a magnetic field with a magnetic flux density
that changes substantially continuously in magnitude in the axial
direction substantially along the length of magnet in the axial
direction 101.
[0174] As with the embodiment shown in FIGS. 10 to 11b, in the
embodiment shown in FIG. 12c the spacing of the strip 104 and the
longitudinal axis 101 is varied with axial position such that the
magnet 51 produces a magnetic field with a magnetic flux density
that changes substantially continuously in magnitude in the axial
direction 101 substantially along the length of the magnet 51 in
the axial direction (as shown represented in FIG. 13).
[0175] In this arrangement, the amount of magnetic material in a
plane substantially perpendicular to the longitudinal axis 101 is
substantially constant with the axial position of the plane.
[0176] For each of the embodiments shown in FIGS. 12a, 12b and 12c,
the magnetic field generator 50 may comprise a plurality of said
strips 104 distributed circumferentially about the longitudinal
axis 101.
[0177] In view of the above, it can be seen that the magnetic field
generator of each of the above embodiments, allows the number of
Hall-effect sensors 105 required by the magnetic sensor arrangement
to be reduced, thereby saving cost. The magnetic field generator 50
is also of a relatively small diameter, thereby allowing the
diameter of the bore 8 in the piston 2 to be relatively small,
which maintains the structural integrity of the piston 2.
[0178] It will be appreciated that any of the magnetic field
generators 50 of the above described embodiments may be used in the
linear actuator of FIG. 1, in place of the magnetic field generator
50 shown in FIG. 1.
[0179] Referring to FIG. 15, there is shown a position sensing
assembly in the form of a fluid level sensor assembly 80 located in
a fluid 81 contained in a fluid housing 82. The fluid housing 82
defines a chamber for containing the fluid. The chamber extends
along a longitudinal axis that is substantially vertical. The fluid
housing is substantially closed at its upper end by a substantially
horizontal upper wall 88. The upper wall 88 is provided with an
access port 89. The access port 89 extends through the thickness of
the upper wall 88, from an inner surface to an outer surface of the
upper wall 88. The access port 89 extends along a longitudinal axis
and has a substantially circular cross-sectional shape about the
longitudinal axis.
[0180] The fluid level sensor assembly 80 comprises a magnetic
sensor housing in the form of an elongate cylindrical tube 83 which
extends along a longitudinal axis 180. In the described embodiment
the tube 83 is oriented such that its longitudinal axis 180 is
substantially vertical. An upper end of the tube 83 passes through
the access port 89.
[0181] A magnetic sensor arrangement in the form of a series of
Hall-effect sensors 105 are arranged in a linear array in the tube
83. The tube 83 is fixed axially relative to the fluid housing 82
by any suitable means of attachment. The Hall-effect sensors are
distributed along the length of the tube 83 in the axial direction
180.
[0182] A float 84 is slidably mounted to the tube 83, for movement
along the axis 180 of the tube. The float 84 has a buoyancy,
relative to the density of the fluid 81, such that as the level of
the fluid 81 rises and falls, the float 84 rises and falls. In this
respect, the buoyancy of the float is such that it floats on the
surface of the fluid 81.
[0183] The magnetic field generator 50 shown in FIGS. 2 to 2B is
received within the float 84 and moves with the float 84 in the
axial direction 180.
[0184] The Hall-effect sensors and the magnetic field generator 50
are arranged such that as the magnetic field generator 50 slides
with the float 84, the magnetic field produced by the magnetic
field generator 50 is sensed by each of the Hall-effect sensors in
turn, so as to sense the position of the magnetic field generator
50, and therefore of the float 84, relative to the tube 83, along
the longitudinal axis 180. This sensed position may be used to
calculate the depth of the fluid 81.
[0185] In this respect, a connecting cable 85 passes from the upper
end of the tube 83, through the access port 89 to a suitable
electronic circuit arranged to output the axial position of the
float 84 from the voltages produced across the Hall-effect
sensors.
[0186] Since the magnetic field generator 50 is of a relatively
small diameter, this allows the float 84 to be of a relatively
small diameter. This advantageously allows the access port 89 to be
of a relatively small diameter.
[0187] It will be appreciated that any of the magnetic field
generators of the above described embodiments may be used in this
arrangement, in place of the magnetic field generator 50 shown in
FIG. 15.
[0188] Furthermore, it will be appreciated that the magnetic field
generators of the above described embodiments may be used in any
other suitable position sensing assembly.
[0189] The described and illustrated embodiments are to be
considered as illustrative and not restrictive in character, it
being understood that only the preferred embodiments have been
shown and described and that all changes and modifications that
come within the scope of the inventions as defined in the claims
are desired to be protected.
[0190] For example, in the described embodiments, the linear
actuator is a hydraulic linear actuator. Alternatively the
hydraulic actuator may be a pneumatic actuator, or any other type
of linear actuator.
[0191] The piston 2 may be made of a non-ferromagnetic
material.
[0192] In the first to the fourth, and the ninth and tenth
embodiments of the magnetic field generator, the magnet 51 is a
single magnet. Alternatively, the magnet 51 may be formed by a
plurality of separate magnets arranged to form the shape of the
magnet 51.
[0193] In the fifth to the eighth embodiments of the magnetic field
generator 50, the magnetic field generator 50 comprises a plurality
of magnets. Alternatively, the magnetic field generator 50 may
comprise a single magnet forming the shape of the plurality of
magnets.
[0194] In the first to the sixth embodiments the magnet 51 has a
radially outer surface 54 and a radially inner surface 55 that have
a substantially circular cross-sectional shape. Alternatively, the
radially outer surface 54 and/or the radially inner surface 55 may
have a different cross-sectional shape, for example rectangular,
square, triangular, etc.
[0195] In the seventh and eighth embodiments the magnets 95-98 of
each set 111 are a substantially solid cylindrical shape.
Alternatively, the magnets may have different cross-sectional
shapes, for example square, triangular, etc. The magnets within a
set 111 may or may not have the same cross-sectional shape. The
magnets within a set 111 may or may not be the same diameter.
[0196] The magnetic field generator may comprise one or more sets
111 of magnets. Each set 111 of magnets may comprise at least a
pair of said magnets of different lengths positioned anywhere
within the wall 133. The magnets of a set 111 may be aligned
circumferentially or radially within the wall 133. The magnets of a
set 111 may be in contact with each other or may be separated from
each other by the magnetically insulating material.
[0197] In the fifth and sixth embodiments the magnetic field
generator 50 comprises a plurality of sets of magnets 71, 72, 73,
74 of different lengths. Each magnet 71, 72, 73, 74 is formed by a
single magnet. Alternatively, each magnet 71, 72, 73, 74 may be
formed by a plurality of magnet sections joined end to end. This
allows shorter magnets to be used, which is advantageous in terms
of ease of manufacture.
[0198] In this case, the magnets may be arranged such that for
radially adjacent joints between magnet sections, the joints are
axially spaced from each other, to produce a magnetic field with a
magnetic flux density that changes substantially continuously in
magnitude in the axial direction substantially along the length of
the at least one magnet in the axial direction. In this respect,
radially adjacent joints between magnet sections are overlapped
axially by magnets of an adjacent layer such that no joints between
magnet sections in radially adjacent layers are axially aligned.
The lengths and overlapping arrangements of the magnets (and magnet
sections) may be varied.
[0199] In the seventh and eighth embodiments the magnetic field
generator 50 comprises a plurality of sets 111 of magnets 95, 96,
97, 98 of different lengths. Each magnet 95, 96, 97, 98 is formed
by a single magnet. Alternatively, each magnet 95, 96, 97, 98 may
be formed by a plurality of magnet sections joined end to end. This
allows shorter magnets to be used, which is advantageous in terms
of ease of manufacture.
[0200] In this case, the magnets may be arranged such that for
radially adjacent joints between magnet sections, the joints are
axially spaced from each other, to produce a magnetic field with a
magnetic flux density that changes substantially continuously in
magnitude in the axial direction substantially along the length of
the at least one magnet in the axial direction. In this respect,
radially adjacent joints between magnet sections are overlapped
axially by magnets of an adjacent layer such that no joints between
magnet sections in radially adjacent layers are axially aligned.
The lengths and overlapping arrangements of the magnets (and magnet
sections) may be varied.
[0201] The number of magnets used in any of the described
embodiments may be varied.
[0202] In the described embodiments the magnetic sensors are
Hall-effect sensors. However, the magnetic sensors may be any
suitable type of magnetic sensor, for example, magneto-resistive
elements or GMR (giant magneto-resistive) technology.
[0203] In the described embodiments the magnetic material of the
magnetic field generator 50 is arranged such that it produces a
magnetic field with a magnetic flux density that changes
substantially continuously in magnitude in the axial direction
substantially along the length of magnetic field generator in the
axial direction. This is achieved by varying the amount of magnetic
material in a plane substantially perpendicular to the axis with
the axial position of the plane and/or by varying the distance
between the magnetic material and the axis, at each axial position,
with axial position.
[0204] Alternatively, or additionally, the density of the magnetic
material may be varied with axial position such that it produces a
magnetic field with a magnetic flux density that changes
substantially continuously in magnitude in the axial direction
substantially along the length of the magnetic field generator in
the axial direction.
[0205] Alternatively, or additionally, the strength of the magnetic
material may be varied with axial position such that it produces a
magnetic field with a magnetic flux density that changes
substantially continuously in magnitude in the axial direction
substantially along the length of the magnetic field in the axial
direction.
[0206] The strength of the magnetic material may be varied with
axial position by varying the chemical composition of the magnetic
material with axial position such that it produces a magnetic field
with a magnetic flux density that changes substantially
continuously in magnitude in the axial direction substantially
along the length of the magnetic field generator in the axial
direction.
[0207] It should be understood that while the use of words such as
"preferable", "preferably", "preferred" or "more preferred" in the
description suggest that a feature so described may be desirable,
it may nevertheless not be necessary and embodiments lacking such a
feature may be contemplated as within the scope of the invention as
defined in the appended claims. In relation to the claims, it is
intended that when words such as "a," "an," "at least one," or "at
least one portion" are used to preface a feature there is no
intention to limit the claim to only one such feature unless
specifically stated to the contrary in the claim. When the language
"at least a portion" and/or "a portion" is used the item can
include a portion and/or the entire item unless specifically stated
to the contrary.
* * * * *