U.S. patent application number 12/774503 was filed with the patent office on 2010-08-26 for method and apparatus for detecting rotational movement of a piston rod.
This patent application is currently assigned to SRI INTERNATIONAL. Invention is credited to C. Bruce Clark, Joseph S. Eckerle, Thomas P. Low, Ronald E. Pelrine, Chris Smith, Glovanni Zangarl.
Application Number | 20100213928 12/774503 |
Document ID | / |
Family ID | 33435186 |
Filed Date | 2010-08-26 |
United States Patent
Application |
20100213928 |
Kind Code |
A1 |
Low; Thomas P. ; et
al. |
August 26, 2010 |
Method and Apparatus for Detecting Rotational Movement of a Piston
Rod
Abstract
A piston rod position sensing system includes a cylinder and a
piston rod arranged in the cylinder for movement with respect
thereto. A magnetically hard layer is formed on the piston rod to
provide a recording medium. A magnetic pattern is recorded in the
magnetically hard layer. The magnetic pattern includes tracks
recorded in the magnetically hard layer lengthwise of the piston
rod. Each track comprises magnetically written regions used to
identify a current position of the piston rod. The sensing system
also includes a plurality of magnetic field sensors in greater
number than the plurality of tracks. Each magnetic field sensor
senses the magnetically written regions of one or more of the
tracks while the piston rod is moving with respect to the cylinder
and generates signals used for detecting rotation of the piston rod
in response to the sensed magnetically written regions.
Inventors: |
Low; Thomas P.; (Belmont,
CA) ; Clark; C. Bruce; (Los Altos, CA) ;
Pelrine; Ronald E.; (Longmont, CO) ; Eckerle; Joseph
S.; (Redwood City, CA) ; Smith; Chris; (San
Carlos, CA) ; Zangarl; Glovanni; (Charlottersville,
VA) |
Correspondence
Address: |
GUERIN & RODRIGUEZ, LLP
5 MOUNT ROYAL AVENUE, MOUNT ROYAL OFFICE PARK
MARLBOROUGH
MA
01752
US
|
Assignee: |
SRI INTERNATIONAL
Menlo Park
CA
|
Family ID: |
33435186 |
Appl. No.: |
12/774503 |
Filed: |
May 5, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11953070 |
Dec 9, 2007 |
7737685 |
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12774503 |
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11409576 |
Apr 24, 2006 |
7307418 |
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11953070 |
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|
11258308 |
Oct 25, 2005 |
7034527 |
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11409576 |
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10840781 |
May 6, 2004 |
6989669 |
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11258308 |
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60468507 |
May 6, 2003 |
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Current U.S.
Class: |
324/207.11 |
Current CPC
Class: |
G01D 5/145 20130101;
F15B 15/2861 20130101; G11B 5/667 20130101; G11B 5/7368 20190501;
G11B 5/656 20130101; F15B 15/2846 20130101; H01F 10/16 20130101;
F16F 9/3221 20130101; G11B 5/7325 20130101; F16F 9/3292
20130101 |
Class at
Publication: |
324/207.11 |
International
Class: |
G01B 7/14 20060101
G01B007/14 |
Claims
1. A piston rod position sensing system, comprising: a cylinder; a
piston rod arranged in the cylinder for movement with respect
thereto; a magnetically hard layer formed on the piston rod to
provide a recording medium, a magnetic pattern being recorded in
the magnetically hard layer, wherein the magnetic pattern comprises
a plurality of tracks recorded in the magnetically hard layer
lengthwise of the piston rod, each track comprising magnetically
written regions that are used to identify a current position of the
piston rod; and a plurality of magnetic field sensors in greater
number than the plurality of tracks, each magnetic field sensor
sensing the magnetically written regions of one or more of the
tracks while the piston rod is moving with respect to the cylinder
and generating signals used for detecting rotation of the piston
rod in response to the sensed magnetically written regions.
Description
RELATED APPLICATIONS
[0001] This application is a continuation application claiming the
benefit of co-pending U.S. patent application Ser. No. 11/953,070,
filed Dec. 9, 2007, titled "Compositions for a Magnetically Hard
Layer of a Piston Rod", which is a continuation application of U.S.
patent application Ser. No. 11/409,576, filed on Apr. 24, 2006, now
U.S. Pat. No. 7,307,418, issued Dec. 11, 2007, titled "Systems for
Recording Position Information in a Magnetic Layer on a Piston
Rod," which is a continuation application of U.S. patent
application Ser. No. 11/258,308, filed Oct. 25, 2005, now U.S. Pat.
No. 7,034,527, titled "Systems of Recording Piston Rod Position
Information in a Magnetic Layer on a Piston Rod," which is a
divisional application of U.S. patent application Ser. No.
10/840,781, filed May 6, 2004, now U.S. Pat. No. 6,989,669, titled
"Systems of Recording Piston Rod Position Information in a Magnetic
Layer on a Piston Rod," which claims the benefit of the filing date
of U.S. Provisional Application No. 60/468,507, filed May 6, 2003,
titled "A System for Magnetic Encoding of Cylinder Rod Position and
Movement and Methods of Use," the entireties of which U.S. patent
application, patents, and provisional application are incorporated
by reference herein.
FIELD OF THE INVENTION
[0002] The invention relates generally to methods and systems for
measuring the absolute position of a piston rod moving relative to
a cylinder. More particularly, the invention relates to systems and
methods of determining position, speed, and direction of motion of
a piston rod from position information encoded in a magnetic film
formed on the piston rod.
BACKGROUND
[0003] Various industrial and mobile applications use hydraulic
cylinders to control the movement and position of machinery. In
general, these cylinders include a cylinder barrel within which a
piston is arranged for reciprocating motion along an axis. A piston
rod is secured at one end to the piston. The piston rod extends out
of one end of the cylinder barrel along the axis of motion. The end
of the piston rod that is external to the cylinder barrel is
coupled directly or indirectly to a machine component. The piston
divides the cylinder barrel into separate chambers. Fluid entering
one of the chambers causes the piston and, thus, the piston rod to
move relative to the housing. This movement of the piston rod
drives the movement of the machine component.
[0004] Precise control of the position of the piston is generally
fundamental to controlling the operation of the machinery.
Measuring the absolute position or velocity of the piston relative
to the cylinder is often needed to achieve such control using
conventional feedback control techniques. Accordingly, industry has
produced various mechanical, magnetic, acoustic, and optical
techniques for detecting the instantaneous position of the moving
piston or piston rod.
[0005] Many position detection systems are expensive, cumbersome,
or difficult to mount on the cylinder. Further, position detection
systems for hydraulic cylinders often operate in harsh environments
caused by internal conditions, such as pressurized fluid that
drives the motion of the piston, and external conditions, such as
dust and debris. Some types of position detection systems, such as
Linear Variable Differential Transformers (LVDTs) and linear
scales, can be unreliable or easily damaged in a harsh
environment.
[0006] Some techniques entail encoding piston rod positions on the
position rod itself, and reading the encoded positions as the
piston rod moves past a reference point, using a reading technique,
e.g., optical, magnetic, mechanical, suited to the particular type
of encoding. Some known techniques cut grooves, etch recesses, or
marks in the rod. Such modifications, however, can adversely affect
the rod's strength. Another known technique, described in the UK
Patent Application No. GB 2 096 421, is to encode the position
information magnetically in the rod material of the piston rod.
Here, the piston rod is constructed of steel and can be magnetized.
However, this rod material is magnetically "soft." Magnetically
soft material has low coercivity, which is a measure of difficulty
for magnetically encoding and erasing information in that material.
Thus, the position information encoded in rod material with low
coercivity is subject to accidental erasure or alteration.
SUMMARY
[0007] In one aspect, the invention features a piston rod position
sensing system including a cylinder and a piston rod arranged in
the cylinder for movement with respect thereto. A magnetically hard
layer is formed on the piston rod to provide a recording medium. A
magnetic pattern is recorded in the magnetically hard layer,
wherein the magnetic pattern comprises a plurality of tracks
recorded in the magnetically hard layer lengthwise of the piston
rod. Each track comprises magnetically written regions that are
used to identify a current position of the piston rod. The sensing
system also includes a plurality of magnetic field sensors in
greater number than the plurality of tracks. Each magnetic field
sensor senses the magnetically written regions of one or more of
the tracks while the piston rod is moving with respect to the
cylinder and generates signals used for detecting rotation of the
piston rod in response to the sensed magnetically written
regions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The above and further advantages of this invention may be
better understood by referring to the following description in
conjunction with the accompanying drawings, in which like numerals
indicate like structural elements and features in various figures.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0009] FIG. 1 is a side view of an embodiment of a cylinder,
including a piston and a piston rod constructed in accordance with
the invention.
[0010] FIG. 2 is a simplistic diagrammatic view of two techniques
for magnetically recording information on the piston rod.
[0011] FIG. 3 is a cross-sectional view of the various layers and
their relative thicknesses deposited on the piston rod, the layers
including a magnetic film that provides a recording medium for
storing encoded rod position information.
[0012] FIG. 4 is a two-dimensional view of a cylindrical piston rod
showing a plurality of tracks in which bits of information are
stored, the particular arrangement and values of these bits in the
tracks providing one embodiment of an encoded pattern from which
the absolute position, velocity, and direction of motion of the
piston rod can be determined.
[0013] FIG. 5 is an end view of the cylindrical piston rod and a
plurality of read heads positioned near the rod to read the bits of
stored information from each of the tracks.
[0014] FIG. 6 is a two-dimensional view of a cylindrical piston rod
showing a plurality of tracks in which bits of information are
stored, the particular arrangement and values of these bits in the
tracks providing another embodiment of an encoded pattern from
which the absolute position, velocity, and direction of motion of
the piston rod can be determined.
[0015] FIG. 7 is a two-dimensional view of a cylindrical piston rod
showing an embodiment of a pair of magnetized lines extending along
a length of the piston rod and illustrating another embodiment of
an encoded pattern from which the absolute position, velocity, and
direction of motion of the piston rod can be determined.
[0016] FIG. 8 is a two-dimensional view of a cylindrical piston rod
showing an embodiment of a magnetized region extending along a
length of the piston rod and illustrating another embodiment of an
encoded pattern from which the absolute position, velocity, and
direction of motion of the piston rod can be determined.
DETAILED DESCRIPTION
[0017] The present invention features methods and systems for
detecting an absolute position, velocity, and direction of motion
of a piston rod (or cylinder rod) while the piston rod moves
relative to an actuating cylinder. In accordance with the
principles of the invention, a physically and magnetically hard
material coats the piston rod. Using standard magnetic recording
techniques, a magnetic pattern or code is recorded in this coating
layer or film of magnetically hard material. As used herein,
magnetically "hard" material is material with high coercivity.
Magnetic material of high coercivity requires considerable energy
to magnetize, i.e., record information, but also to demagnetize
recorded information. One or more flux-sensitive magnetic read
heads mounted on an end cap of the cylinder read the magnetic
pattern. Circuitry in communication with the read heads can process
the information obtained from the magnetic pattern to determine the
instantaneous incremental position of the piston rod, its velocity,
and direction of motion relative to the cylinder.
[0018] Various techniques can be used to encode absolute positions
of the piston rod in the magnetically hard layer that coats the
piston rod. Some techniques record binary codes. Each binary code
is uniquely associated with a particular piston rod position.
Another technique measures the lateral, spatial distance between
two diverging magnetized lines extending along a length of the
piston rod. Yet another technique magnetizes an area bounded on two
sides by two such diverging lines, extends a sensor (or sensor
array) across this bounded area, and correlates the extent of the
magnetized area detected by the sensor to a piston rod position.
These techniques are illustrative of the many, diverse ways in
which position information can be recorded on the piston rod. Other
magnetic patterns or encodings can be recorded without departing
from the principles of the invention.
[0019] FIG. 1 shows a side cross-sectional view of an embodiment of
a cylinder 2 including a cylinder barrel 3, a cylinder end cap 4
(also called a "packing gland"), and a housing 6. A piston 8 is
arranged within the cylinder barrel 3 for reciprocating motion
along an axis. The piston 8 partitions the cylinder barrel 3 into
two chambers 10a and 10b.
[0020] One end of a piston rod 12 is secured to the piston 8 and
extends along the axis of motion. The other end of piston rod 12
extends out of the housing 6 through the end cap 4, and may be
coupled directly or indirectly to a machine component. Typically,
the piston 8 and piston rod 12 are constructed of steel (i.e., a
ferromagnetic material). In the embodiment shown, the piston rod 12
is cylindrical; other piston rod shapes can be employed without
departing from the principles of the invention. In accordance with
the invention, a magnetically hard film or layer 14 coats the
piston rod 12 to provide a recording medium. This coating can be
continuous or discontinuous on the piston rod 12 and cover a
portion or all of the piston rod 12. For example, typically the
magnetically hard layer 14 is not formed on the end of the piston
rod 12. A pattern or code is magnetically recorded in the
magnetically hard layer 14 along a length of the piston rod 12, as
described in more detail below.
[0021] The end cap 4 has a channel 16 for the passage of fluid
(e.g., oil, water, steam, gas) into and out of the chamber 10b, for
moving the piston 8. A fluid passageway to the other chamber 10a is
not shown. Seals 18 within the end cap 4 are arranged to lie flush
with a surface of the piston rod 12 and thus prevent fluid from
leaving the chamber 10b.
[0022] The housing 6 encloses a plurality of flux-sensitive
magnetic read heads 20 and read-head electronics 22. Only one read
head is shown in FIG. 1 to simplify the illustration. Read heads 20
can be Hall-effect devices or magnetoresistive sensors. The
location of the read head 20 within the housing 6 provides
protection from the environment and permits ready access for easy
replacement (i.e., the housing 6 can be removed without removing
the end cap 4 from the cylinder 2). The read heads 20 are mounted
in the housing 6 within proximity of the piston rod's surface to
permit reading of the encoded position information in the magnetic
pattern recorded in the magnetically hard layer 14. The housing 6
also includes a rod wiper 24 for wiping away small magnetizable
particles that may adhere to the piston rod 12. In another
embodiment, the end cap 4 houses the read heads 20 and read head
electronics 22. In such an embodiment, the housing 6 is optional
because the end cap 4 can protect the read heads 20 from the harsh
operating environment.
[0023] In brief overview, fluid within the chambers 10a, 10b at
time-varying, differential pressures causes the piston 8 and thus
the piston rod 12 to move in and out relative to the read heads 20.
The read heads 20 read the recorded magnetic pattern on the piston
rod 12 and produce a corresponding analog or digital signal. From
the combined instantaneous readings of the read heads 20, the
read-head electronics 22 can determine the actual piston rod
position, velocity, and direction, or any combination thereof.
[0024] FIG. 2A and FIG. 2B are diagrammatic views of two standard
magnetic recording techniques for magnetically recording binary
information in the magnetically hard layer 14 of FIG. 1. A write
transducer (i.e., a pole write head) can magnetize the magnetically
hard layer 14 with an identifiable magnetic pattern in one of two
standard ways: 1) longitudinal; and 2) perpendicular. When a
current is applied to the write transducer, an external field is
generated, thereby aligning the magnetic domains within the
magnetically hard layer 14. Write transducers are currently able to
record on the order of tens of megabits per square inch.
[0025] In longitudinal media, the magnetization lies in the plane
of the magnetically hard layer 14, which is shown in FIG. 2A as
left and right arrows. The magnetic pattern of longitudinal
recording media consists of "transitions," i.e., head-to-head
reversals of the in-plane magnetization from one polarity to the
other. Such a reversal is marked by the existence of magnetic poles
whose stray flux is sensed by the read head 20 located above the
medium. In perpendicular media, the magnetization is perpendicular
to the plane, shown as up and down arrows in FIG. 2B. Here, the
magnetic marking occurs by creating transitions between regions
magnetized "up" and "down."
[0026] Longitudinal and perpendicular recording media can both be
produced by electrochemical methods (e.g., electroless,
electroplating, chemical vapor deposition, and electrochemical
deposition (sputtering)). For longitudinal and perpendicular
recording media, the materials used are often cobalt-based alloys.
Pure cobalt (Co) can be used to produce a magnetic film of high
coercivity, but alloying elements are typically used to tailor the
magnetic properties of the recording media and to increase its
coercivity. Examples of alloying elements include group VA (P, As,
Sb, Bi) and VIB (Cr, Mo, W) elements and the noble elements Pt and
Pd. For longitudinal media, example alloys include Co--P,
Co--Ni--P, Co--W, and Co--Mo. For perpendicular media, example
alloys include Co--P based, Co--W, and Co--Cr. Approximate high
coercivity values obtained from using such Co-based alloys range
from 1-2 kOe.
[0027] Embodiments of the magnetically hard layer 14 can be
synthesized with such materials to produce a magnetic layer with
high coercivity. Magnetic layers or films of high coercivity, such
as the magnetically hard layer 14, can maintain recorded
information under external stray fields and under the
demagnetization fields deriving from the imposed magnetic
transitions. This magnetically hard layer 14 may also provide good
mechanical and corrosion resistance. However, considering the harsh
operational environment of the cylinder 2, the magnetically hard
layer 14 can be coated to insure sufficient resistance to
mechanical wear and corrosion. One example of such a protective
layer can be hard chrome, i.e., a Cr layer.
[0028] FIG. 3 shows a cross-sectional view of one embodiment of a
portion of the piston rod 12, including a substrate 50, an optional
intermediate layer 52, the magnetically hard layer 14, and an
optional protective layer 54. Generally, the substrate 50 can be
magnetic or non-magnetic, that is, although typically ferromagnetic
(e.g., a steel rod), the piston rod 12 can be constructed of
non-magnetic material (e.g., plastic, aluminum, ceramic, or glass)
without departing from the principles of the invention. The layers
14, 52, and 54 can cover all or a portion of the piston rod 12. For
example, the magnetically hard layer 14 (and the optional layers
53, 54) are not typically applied to the end of the piston rod 12,
in particular, to those one or more portions of the piston rod 12
that do not pass near enough the read heads 20 for sensing.
[0029] The composition of the intermediate layer 52 depends upon
the type of the recording media and of the substrate material. For
longitudinal media, for instance, a ferromagnetic substrate can
pose a problem for information retention. Being magnetically
permeable, the ferromagnetic substrate provides a low reluctance
path for the flux, which, in effect, shunts and reduces the flux
available to be sensed. For embodiments in which the magnetically
hard layer 14 is produced as longitudinal media, the intermediate
layer 52 can be a non-magnetic amorphous layer, such as Ni--P, to
obstruct the low reluctance path. Because non-magnetic substrates
lack the permeability of ferromagnetic substrates, use of such an
intermediate layer 52 is optional for non-magnetic substrates.
[0030] For perpendicular media, the permeability of a ferromagnetic
substrate serves an advantage. The return path in a ferromagnetic
substrate between adjacent oppositely magnetized regions does not
affect the stray flux in the region above the magnetically hard
layer 14, and assists in the write process and in the retention of
the written information. Lacking such permeability, non-magnetic
substrates lack such beneficial properties for information
retention. To improve the information retention of perpendicular
media on non-magnetic substrates, the intermediate layer 52 can be
as a magnetically soft layer (e.g., permalloy or Ni--Fe). The
permeability of a ferromagnetic substrate 50, however, makes use of
the magnetically soft intermediate layer 52 optional; although use
of the magnetically soft intermediate layer 52 can mask unreliable
or non-uniform permeability of the ferromagnetic substrate 50, and
therefore its presence can be beneficial.
[0031] FIG. 3 also shows the relative thicknesses of the layers 14,
52, and 54 on the magnetic substrate 50 of this embodiment of the
piston rod 12. In an embodiment employing a perpendicular recording
medium, the magnetically hard layer 14 is approximately 5 um thick,
the protective layer 54 is approximately 25 um thick, and the
intermediate layer 52, here, a magnetically soft layer (e.g.,
permalloy), is approximately 1-2 um thick. The thickness of the
protective layer 54 affects the resolution of the piston rod
position sensing system by limiting how near the read heads 20 can
be to the magnetically hard layer 14. For example, with a 25 um
thick protective layer 54, bits may need to be spaced apart by at
least 25 um (approximately) for the read heads 20 to be able to
distinguish between them. In embodiments without the protective
layer 54, the bits can be located more closely together because the
read heads 20 can directly contact the encoded magnetically hard
layer 14. The particular thicknesses shown in FIG. 3 provide an
illustrative example; other thicknesses for the layers 14, 52, and
54 can be used to practice the invention.
[0032] Piston rod position information can be recorded in the
magnetically hard layer 14 of the piston rod 12 in a multitude of
ways. Some techniques explicitly record the identities of the
absolute piston rod positions on the piston rod 12 (e.g., using
binary code), other techniques magnetize shapes in or regions of
the magnetically hard layer 14 from which piston rod positions can
be computed. Herein, a magnetic pattern means generally any type of
magnetically recorded that directly or indirectly identifies a
piston rod position.
[0033] Binary code representing the particular absolute positions
can appear on the piston rod 12 in at least two general directions:
1) around the circumference of the piston rod 12 (or laterally);
and 2) along a length of the piston rod 12. In the first instance,
the binary code representing a particular piston rod position is
read concurrently by multiple read heads. The combined concurrent
readings of the read heads produce that particular position. In the
second instance, a single read head reads the binary code
representing a particular piston rod position.
[0034] FIG. 4 shows an embodiment in which piston rod positions are
encoded circumferentially around the piston rod 12. This embodiment
is merely exemplary of circumferential magnetic patterns for
representing piston rod positions. Others can be used without
departing from the principles of the invention. Shown in two
dimensions, the cylindrical piston rod 12 is partitioned into a
plurality of tracks 80 into which bits of information are
magnetically recorded. The tracks 80 extend lengthwise along the
piston rod 12 along the direction of the reciprocating motion of
the piston 6. The width of each track 80 spans a particular degree
range of the cylindrical piston rod 12. Each track is read by one
read head 20 (FIG. 1). For example, for a piston rod 12 with twelve
tracks 80, each track 80 spans an arc of 30 degrees, and twelve
read heads 20 each read the bits recorded in one of the tracks 80.
FIG. 5 shows a cross-sectional view of the cylindrical piston rod
12 and a plurality of read heads 20 positioned near a surface of
the rod to read the bits of stored information from each of the
tracks 80.
[0035] Returning to FIG. 4, position identifying binary code is
magnetically written onto these twelve tracks. Each identifiable
piston rod position 82 wraps around the circumference of the piston
rod 12. The binary information recorded in the tracks 80 for each
position 82, when read together, uniquely identifies that piston
rod position. In this example, each unique binary code for a given
position is twelve tracks wide. To ensure that a given read head 20
is reading bit information from the desired track 80, in one
embodiment the piston rod 12 is not be permitted to rotate more
than the width of a single track. In another embodiment, the
precise location of each read head 20 is used to detect rotational
movement of the piston rod 12. Error detection code can also be
used so that misread code does not cause an error in positioning.
An advantage of this arrangement is that resolution of known
absolution piston rod positions can be almost as small as the
physical size of one bit. For magnetic encodings, the size of each
bit is between 0.001 and 0.002 inches.
[0036] The desired spatial resolution between identifiable
magnetically recorded bits and the length of the piston rod 12 are
factors in determining the number of unique binary codes needed to
identify each piston rod position uniquely. For example, consider a
55-inch piston rod for which 0.04 inch resolution is desired. Such
a position detection system requires 1375 unique binary codes to
identify uniquely each of the 1375 positions on the piston rod 12
(55/0.04). Accordingly, at least eleven bits are needed to
represent each piston rod position. Eleven tracks 80 and eleven
read heads 20 are used to read the eleven bits. Additional bits,
tracks and read heads may be used in this example for purposes
other than uniquely identifying piston rod position, such as for
detecting piston rod rotation and for performing error code
correction.
[0037] In FIG. 4, a simplistic example is shown of a binary code
that can be used to identify incremental piston rod positions. For
this example, shaded regions signify regions of the magnetically
hard layer 14 that have a recorded bit value of 1. Non-shaded
regions signify recorded bit values of 0. Starting from the bottom
of FIG.4, with the rightmost bit being the least significant bit,
the binary coded piston rod positions 82 that are shown are
identified by code values 1 through 7.
[0038] A position sensing system of the invention determines the
absolute position of the piston rod 12 whenever the read heads 20
read the present encoding. The read head electronics 22 can compute
the velocity of the piston rod 12 from multiple readings of the
instantaneous absolute position. From the multiple absolute
position readings, the electronics 22 can compute the distance
traveled by the piston rod 12 and divide that distance by the time
between readings. A comparison of absolute positions also enables a
determination of the direction in which the piston rod 12 is
moving.
[0039] FIG. 6 shows an embodiment in which piston rod positions are
encoded lengthwise on the piston rod 12. Shown in two dimensions,
the cylindrical piston rod 12 is partitioned into a plurality of
tracks 80' (here, three tracks) into which bits of information are
magnetically recorded. Each track is read by one read head 20. An
additional read head can be used to detect rotation of the piston
rod 12. The tracks 80' extend lengthwise along the piston rod 12
along the direction of the reciprocating motion of the piston rod
12. The width of each track 80' spans a particular degree range of
the cylindrical piston rod 12. For example, for a piston rod 12
with three tracks 80', each track 80' spans an arc of 120
degrees.
[0040] Magnetically written onto each of these tracks 80' are words
82'. Each word 82' includes a magnetic pattern of binary
information that uniquely identifies a particular piston rod
position. For example, 12 bits of information can uniquely
identify, with 0.04-inch resolution, the 1375 piston rod positions
in the exemplary 55-inch piston rod 12 described above. In an
embodiment in which the magnetically hard layer 14 is protected by
a hard chrome or Cr-layer having 0.001-inch thickness, a 12-bit
word can be magnetically recorded in a linear space of
approximately 0.012 inches.
[0041] In one of the tracks 80a', the words 82' identifying the
absolute piston rod positions are incrementally recorded along the
length of the piston rod 12. This track 80a' includes a word 82'
for each desired absolute position (e.g., 1375 words in 0.04-in
increments for the exemplary 55-inch piston rod described
above).
[0042] The other two tracks 80b' and 80c' are partitioned into
regions 90. The regions 90 within the track 80c' are staggered with
respect to the regions 90 in the track 80b'. The identities of
positions represented by the words 82' are also staggered:
even-numbered positions are coded in track 80b' and odd-numbered
positions are coded in track 80c'. Staggering the words 82' in this
fashion uses less length of the piston rod 12 to represent 1375
unique positions than the incremental technique employed in track
80a'.
[0043] One implication of lengthwise words is that in the event of
a loss of power, the piston rod 12 needs to move a certain distance
before the position sensing system can know its current position.
In this respect, this embodiment of a position-sensing system is
pseudo-absolute. This distance corresponds to the absolute
resolution of a word (i.e., the physical word length). In the
example described above having 12-bit words, this distance is 0.012
inches. To eliminate any need for initial movement before being
able to detect a piston rod position, a battery backup can be
included in the system to maintain the last known piston position
and movement direction when the power was lost.
[0044] The embodiment shown in FIG. 6 is merely exemplary of
lengthwise magnetic patterns for representing piston rod positions.
Other examples include, but are not limited to, a single track
having each possible piston rod position, such as track 80a' or
just the two tracks 80b' and 80c'.
[0045] FIG. 7 shows an embodiment of a magnetic pattern 100
recorded in the magnetically hard layer 14 and from which piston
rod position, velocity, and direction of motion can be determined.
Here illustrated in two dimensions, the magnetic pattern 100
includes a pair of magnetized lines 120, 122 extending along a
length of the piston rod 12. The lines 120, 122 are recorded in the
magnetically hard layer 14. The first line 120 extends in the
direction of motion of the piston rod 12 and parallel to the axis
of the piston rod 12. The second line 122 extends away from the
first straight line 120, diagonally for a planar embodiment,
helically for a cylindrical embodiment. In the helical embodiment,
the second line 122 does not make one full revolution around the
circumference of the piston rod 12 so as to avoid intersecting the
first straight line 120.
[0046] An array of sensors 20' (diagonal-shaded box) mounted in the
housing 6 (FIG. 1) magnetically senses the two magnetized lines
120, 122. Sensors can be standard Hall-effect sensors or
constructed from magnetoresistive material (i.e., permalloy)
deposited on a copper cladding and selectively etched (the cladding
and permalloy) to leave and array of individual sensors.
[0047] Signals generated by the array of sensors 20' enable the
read head electronics 22 (FIG. 1) to compute the current distance
between the lines 120, 122. Because the lines 120, 122 diverge from
each other, the measured distance is uniquely associated with a
particular absolute piston rod position (a lookup table can store
these associations). The distance between any two sensors in the
sensor array 124 is sufficient to establish the position of the
piston rod 12. There is no need for the lines 120, 122 to be
oriented relative to a single sensor or for the sensor array to be
uniquely positioned, initially or during operation. Accordingly,
the piston rod 12 can rotate during operation without affecting the
sensing of position, velocity, or direction of motion.
[0048] Consider, for example, a piston rod having a 3.5-inch
diameter and a 55-inch length and a specified resolution of 0.04
inches: a 0.04-inch lengthwise movement of the piston rod 12
corresponds to a 0.08-in increase in the distance between the two
lines 120, 122. This lengthwise movement of 0.08 inch, when spread
over the circumference of the piston rod 12, corresponds to 1375
unique piston rod positions. An array of 1375 sensors can be used
to sense the two lines 120, 122. The use of shorter piston rods
(than 55 inches) enables the use of fewer sensors. Alternatively,
another magnetized line in parallel with one of the other lines
120, 122, enables doubling the length of the piston rod without
increasing the number of sensors. Also, if the length of the piston
rod 12 is such that a single helical revolution of the line 122
does not provide sufficient positional sensitivity, a second
helical revolution about the piston rod 12 can be made. In another
embodiment, the magnetized lines 120, 122 both extend helically
around the circumference of the piston rod 12, but in opposite
rotational directions of each other.
[0049] From multiple readings of the absolute position of the
piston rod 12, the read head electronics 22 can compute the
velocity of the piston rod 12. For example, the read head
electronics 22 can compute the distance traveled by the piston rod
12 from a first absolute position to a second absolute position,
and divide that distance by the time between position readings. A
comparison of absolute positions also enables a determination of
the direction in which the piston rod 12 is moving. For example, an
increase or decrease in the measured distance from one position
reading to a subsequent position reading can be used to identify
the movement direction.
[0050] FIG. 8 shows, in two dimensions, another embodiment of a
magnetic pattern 100' recorded in the magnetically hard layer 14
and from which piston rod position, velocity, and direction of
motion can be determined. The magnetic pattern 100'' includes a
pair of boundary lines 140, 142 extending along a length of the
piston rod 12. The first straight boundary line 140 extends in the
direction of motion of the piston rod 12 and parallel to the axis
of the piston rod 12. The second boundary line 142 extends away
from the first straight line 120, diagonally for a planar
embodiment, helically for a cylindrical embodiment. In the helical
embodiment, the second boundary line 142 does not make one full
revolution around the circumference of the piston rod 12 so as to
avoid intersecting the first boundary line 140. In this embodiment,
the boundary lines 140, 142, and the region 144 (shown shaded)
bounded by the boundary lines 140, 142 are magnetized.
[0051] A sensor 20'' (diagonal-shaded box) is mounted in the
housing 6 (FIG. 1) to extend across the magnetized region 144. As
the piston rod 12 moves, the extent of the magnetized region 144
covered by the sensor 20'' changes because the lines 140, 142
diverge. The sensor 20'' produces a signal (e.g., analog or
digital) that depends upon the detected extent of coverage and
serves, directly or indirectly, as a measurement of this coverage.
The readings of the sensor 20'' are then used to compute the
position of the piston rod 12. Each absolute piston rod position is
uniquely associated with a particular measure of coverage (a lookup
table can store these associations). Velocity can be computed from
multiple readings of the absolute position as a measure of the
distance traveled by the piston rod 12 from a first measurement to
a second measurement divided by the time between measurements. A
comparison of measurements also enables a determination of the
direction in which the piston rod 12 is moving. For example, an
increase or decrease in the measured amount of magnetized region 44
covered by the sensor 20'' from one measurement to a subsequent
measurement can be used to identify the movement direction.
[0052] Although the invention has been shown and described with
reference to specific preferred embodiments, it should be
understood by those skilled in the art that various changes in form
and detail may be made therein without departing from the spirit
and scope of the invention as defined by the following claims. For
example, although the embodiments described above relate primarily
to sensing piston rod position for linear movement, the principles
of the invention can be used to determine position, velocity, and
movement direction for objects that rotate with respect to each
other.
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