U.S. patent application number 12/134689 was filed with the patent office on 2009-12-10 for direct shaft power measurements for rotating machinery.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Klaus Franz Otto Raum, Victor Donald Samper, Simon Schramm, Christof Martin Sihler.
Application Number | 20090301223 12/134689 |
Document ID | / |
Family ID | 40862860 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090301223 |
Kind Code |
A1 |
Sihler; Christof Martin ; et
al. |
December 10, 2009 |
DIRECT SHAFT POWER MEASUREMENTS FOR ROTATING MACHINERY
Abstract
Direct shaft power measurements of rotating machinery, including
a magnetic encoding system for the shaft, having at least one
conducting member having a first end and a second end which is
disposed proximate the shaft with a gap between the member and the
shaft. There is a pair of electrodes proximate each end of said
conducting member, wherein the electrodes are electrically coupled
to the shaft. One of the electrodes is electrically coupled to the
second end of the conductor member. An encoding source is
electrically coupled to the first end of the conducting member and
electrically coupled to the other electrode, wherein unipolar
current pulses from said encoding source are applied to the
electrodes and the conducting member, thereby creating sectional
encoded polarized magnetic regions in the shaft.
Inventors: |
Sihler; Christof Martin;
(Hallbergmoos, DE) ; Samper; Victor Donald;
(Hallbergmoos, DE) ; Raum; Klaus Franz Otto;
(Bavaria, DE) ; Schramm; Simon; (Munchen,
DE) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
Schenectady
NY
|
Family ID: |
40862860 |
Appl. No.: |
12/134689 |
Filed: |
June 6, 2008 |
Current U.S.
Class: |
73/862.333 |
Current CPC
Class: |
G01L 3/102 20130101;
G01L 3/103 20130101; Y02B 10/30 20130101; H01F 13/00 20130101 |
Class at
Publication: |
73/862.333 |
International
Class: |
G01L 3/10 20060101
G01L003/10 |
Claims
1. A magnetic encoding system of a shaft, comprising: at least one
conducting member having a first end and a second end which is
disposed proximate said shaft with a gap between said member and
said shaft; a pair of electrodes proximate each end of said
conducting member and electrically coupled to said shaft, wherein
one of said electrodes is electrically coupled to said second end
of the conductor member; and an encoding source electrically
coupled to said first end of the conducting member and electrically
coupled to the other of said electrodes; wherein unipolar current
pulses from said encoding source are applied to said electrodes and
said conducting member, thereby creating sectional encoded regions
in the shaft.
2. The system according to claim 1, further comprising a
non-conductive encoding assembly disposed about at least a section
of said shaft, wherein said conducting member is disposed within
said encoding assembly.
3. The system according to claim 1, wherein said conducting member
is oriented axially along a section of said shaft.
4. The system according to claim 1, wherein said conducting member
is oriented circumferentially along a section of said shaft.
5. The system according to claim 1, wherein said conducting member
is oriented diagonally along a section of said shaft.
6. The system according to claim 1, wherein said electrodes are
temporarily coupled to said shaft.
7. The system according to claim 1, wherein at least two of said
conducting members are positioned adjacent to each other about said
shaft for creating sectional encoded regions of opposing
polarities.
8. The system according to claim 7, further comprising magnetic
domain boundaries formed between said adjacent sectional encoded
regions.
9. The system according to claim 8, wherein said adjacent
conducting members are positioned in close proximity to each
other.
10. The system according to claim 1, said at least one conducting
member comprises a plurality of conducting members oriented about
at least a section of said shaft.
11. The system according to claim 1, wherein said shaft is made of
a ferromagnetic material.
12. The system according to claim 11, wherein said ferromagnetic
material is steel.
13. The system according to claim 1, wherein said shaft has a layer
of ferromagnetic material applied about a surface of said
shaft.
14. The system according to claim 1, wherein said conducting
members are substantially linear or circular and extend
longitudinally or circumferentially along a section of said
shaft.
15. A system for measuring properties of a rotating shaft,
comprising: a plurality of previously encoded polarized magnetic
regions disposed about said shaft, wherein adjacent magnetic
regions have opposing polarities and wherein there are domain
boundaries between said adjacent magnetic regions; at least one
sensor disposed proximate said encoded polarized magnetic regions
with a gap therebetween; and a processing section coupled to said
sensor for processing a field component thereby measuring
properties of said rotating shaft based on a magnetostrictive
effect.
16. The system according to claim 15, wherein said previously
encoded polarized magnetic regions provide for generation of
alternating magnetic fields as the shaft rotates, wherein said
alternating fields measure at least one of shaft power, torque,
speed, and bending moments.
17. The system according to claim 15, wherein said sensor is at
least one of an air core induction coil or a magnetic field sensor
(MFS).
18. The system according to claim 17, wherein said magnetic field
sensors are at least one of Hall Effect sensors, giant
magneto-resistive (GMR) sensors, fluxgate sensors, or
magnetoimpedance sensors.
19. The system according to claim 15, wherein said properties
include a non-contact measurement of at least one of shaft power,
torque, speed or bending moments based on sensing magnetic field
components in the shaft.
20. A method for encoding a shaft, comprising: disposing at least
one conducting member in close proximity about a section of said
shaft, said conducting member having a first end and a second end;
disposing electrodes onto said shaft proximate the first end and
the second end, wherein said second end electrode is coupled to
said second end of said conducting member; electrically coupling
said first end electrode to a current source and coupling said
current source to said first end of said conducting member; and
applying unipolar current pulses to said conducting members thereby
inducing sectional encoding regions.
21. The method according to claim 20, wherein at least two of said
conducting members are oriented adjacent to each other, wherein
said encoding is with opposing polarities such that the resulting
polarized magnetic channels have domain boundaries.
Description
BACKGROUND OF THE INVENTION
[0001] There are numerous applications and industries that are
based on rotating shafts to accomplish some form of work or energy
conversion. Early examples of rotating shaft functionality include
the watermills and windmills thousands of years ago to grind
grains. Rotating shafts are still used on current windmills and
hydroelectric plants, however they incorporate advanced technology
and processing. While small rotating shafts are used in electronic
equipment such as computer disk drives, media recorders/players,
and household appliances, these shafts are generally of a smaller
length and width such that the torque is relatively small. Larger
rotating shafts experience larger torque and are deployed in
applications including locomotives, airplanes, ships, and energy
conversion to name just a few examples. The modern usage of
equipment utilizing larger rotating shafts typically incorporates
sensing and processing capabilities to achieve safe and efficient
operation.
[0002] One of the ways of addressing the design and operation of
equipment using rotating shafts is via measuring the stress or
strain at the shaft surface that can be used to measure torque,
bending and twisting due to externally applied forces. Conventional
technologies employ a number of different systems of sensing or
measuring the torque such as strain gauge systems, encoder/tooth
systems, acoustic wave systems, elastic systems, magnetostrictive
systems, and magnetoelastic systems. Each of these systems has
certain characteristics and applications.
[0003] Strain gauges provide for local strain measurements of the
shaft and typically require some form of coupling to the rotating
shaft that can be via a physical connection (e.g.: slip rings) or
telemetry. The gauges generally suffer from low stability, have
limitations in the bandwidth and tend to have calibration and
environmental correction requirements. The limited operating
temperature range of strain gauges limits their use in a harsh
environment.
[0004] The encoder/tooth-wheel pickup style of torque sensing
usually has at least some partial attachment to the rotating shaft
such as by a magnetic tooth-wheel. The tooth-wheel design tends to
be costly and impractical for many implementations. Such a design
is not practical for higher speed applications and although stable,
lacks high resolution and can cause reliability issues in harsh
environment
[0005] The acoustic wave systems utilize sensors such as surface
acoustic wave (SAW) and bulk acoustic wave (BAW) devices that use
acoustic waves to detect strain-induced changes to the shaft via
telemetry with transducers connected on the shaft. The application
of acoustic wave technology to torque sensing is relatively new and
the present systems are being used for smaller shafts that have
high manufacturing tolerances.
[0006] The elastic torque systems measure the twisting of the shaft
by using markers across a length of the shaft and measuring the
angular displacement. This system has accuracy issues when applied
to large diameter shafts, and there are practical implementation
problems.
[0007] In a magnetoelastic system, stress induces an `easy axis` of
magnetization through the strain it produces in the material. This
effect is typically used in circularly magnetizing a shaft region,
and using a magnetic field sensor to pick up the resulting field.
If there is no torsional stress, all of the magnetic flux is
contained within the sensing region and there is no external field
detected. If there is torsional stress, the magnetic domains are
re-aligned and the external field is changed with a polarity and
strength that corresponds to the direction and magnitude of torque
on the shaft.
[0008] Magnetostrictive measurement methods make use of the
phenomenon that material changes dimensions upon being magnetized.
Magnetostrictive sensors are used with ferromagnetic shafts, such
as industrial steel, as well as sections of ferromagnetic material
applied to the shafts. With such materials the magnetostrictive
effect is very small. Typical magnetostrictive coefficients
.DELTA.l/l are in the order of 1 to 25.times.10.sup.-6. Making
direct use of the magnetostrictive effect for measuring torque in
ferromagnetic material requires complex sensor arrangements,
difficult calibration procedures and typically results in limited
accuracy.
[0009] One conventional magnetostrictive torque sensor design
employs a primary coil in the center of the measuring head and
measurement coils disposed about the periphery in a specific
orientation. The sensor generates a constant high frequency
magnetic field via the primary coil wherein the resulting field is
measured by measurement coils that measure the magnetic flux. When
the resulting magnetic field from all the measurement coils is
equal to zero, there is no torque on the shaft and likewise, any
resulting magnetic field that is non-zero indicates some torque is
present.
[0010] This approach does not require any encoding or other
modifications to the shaft and does have long-term stability.
However, the accuracy is limited, the installation process is
cumbersome and calibration tends to be difficult. Furthermore,
there are generally tight tolerance requirements for keeping a
small gap between the shaft and the sensor that is difficult to
achieve with temperature varying environments. The accuracy of the
measurements is acceptable in certain requirements such as
monitoring dynamic torque components and torsional vibrations
however subsequent signal processing is required to minimize shaft
run-out issues.
[0011] Improving the accuracy of magnetostrictive measurement
systems can be achieved in combining the magnetostrictive effect
with a magnetic encoding of the shaft or the encoding section
applied to the shaft. In such sensor designs the alignment of the
magnetic domains in the ferromagnetic material imparts some change
of the material dimensions along the magnetic axis. The inverse
effect is the change of magnetization of a ferromagnetic material
due to mechanical stress. The magnetic encoding essentially turns
the shaft into a component of the sensing system. When a mechanical
torque is applied to the shaft, a torque-dependent magnetic field
is measurable close to the encoded region of the shaft.
[0012] A typical magnetostrictive torque sensor design employs
total shaft encoding and the magnetization occurs by current
flowing in the axial direction of the shaft. For illustrative
purposes, this conventionally uses current pulses to create a
"magnetized ring" about the shaft. The encoding is
circumferentially uniform as the magnetic encoding requires the
entire cross-section to be magnetized and therefore becomes
difficult and costly for larger diameter shafts. In addition, there
are limitations to this approach with respect to variations of
currents due to inhomogeneity of electrical and magnetic properties
of the shaft. As torque is applied, a torque dependent magnetic
field can be measured externally, such as by fluxgate sensors.
[0013] There are known implementations for magnetostrictive
encoding which can be used, for example, in the automotive
industry. The shaft is typically axially encoded along axial
sections of the shaft establishing magnetically encoded regions.
Magnetic field sensors are deployed externally to the magnetically
encoded region, and the measured responses are subsequently
processed for the torque. While there are a number of various other
embodiments, the descriptions herein illustrate the basic
operation.
[0014] Referring to FIG. 1a, a shaft 5 is composed of a
ferromagnetic material. In order to encode the shaft 5, electrodes
10, 15 are disposed along the circumference of the shaft such that
an encoded region 20 can be formed. In this example, the electrodes
consist of a pair of outer rings 10, 15 and are spaced apart to
provide satisfactory uniform magnetic flux density during encoding
that depends upon several factors such as the shaft diameter.
[0015] The encoding process typically involves sending a current
pulse 25 to a first ring 15 that establishes a current flow 30
along the longitudinal axis of the shaft 5 that is discharged at a
corresponding second ring 10 with an output return current signal
35. The current 30 flowing in the encoded region 20 through the
shaft 5 induces a magnetic flow. Various embodiments implement this
basic concept including using multiple electrodes and with various
encoding techniques.
[0016] In operation, a sensor is used to measure an output magnetic
field signal that reflects the torque applied to the shaft. With no
stress applied, there is no relevant magnetic field detected
however as torque is applied to the shaft, the change in magnetic
field emerging from the encoded region is measured by the sensor.
The sensor is typically coupled to some processing electronics.
[0017] Referring to FIG. 1a-1b, the current 30 flowing in the
encoded region 20 through the shaft 5 creates a magnetic flow 40
that is a magnetic field at a center of the shaft 5.
[0018] One embodiment for illustrative purposes uses a single
current pulse for creating a "magnetized ring" within the shaft
such that there is an associated discharge curve reflecting the
characteristics of the shaft at a particular instance of time.
Alternatively, consecutive pulses with different polarities and
different time constants can be utilized such that two magnetized
rings can be encoded. There can be multiple encoding electrodes in
addition to the encoding pulse.
[0019] This conventional system employs uniform encoding of the
surface of the shaft and the magnetization occurs by current
flowing in the axial direction of the shaft. The magnetic encoding
is circumferentially uniform and requires the entire cross-section
of the shaft to be magnetized. Such encoding makes it difficult to
achieve a uniform current distribution in circumferential
direction, especially with large diameter shafts.
[0020] With no stress applied, there is essentially no magnetic
field detected however as torque is applied to the shaft the
magnetic field emitted by the encoded region is measured by the
sensor 45. In operation, as torque is applied, the magnetic field
is measured externally, such as by sensor coils 45. The sensor 45
is typically coupled to some processing electronics (not shown) and
is typically used to measure the output magnetic field signal that
reflects the torque applied to the shaft 5. An example of a sensor
45 for torque sensing is a fluxgate sensor that is installed close
to the shaft surface.
[0021] The conventional magnetic shaft encoding described herein
generally applies to small diameter shafts with an encoding that is
based on a uniform (constant) flux density in the circumferential
direction. It is not practical for larger shafts as the encoding
currents increase with the shaft diameter and large amperage would
be required in order to get sufficient flux densities in the large
diameter shafts.
[0022] In order to attempt to alleviate these large diameter shaft
concerns, one conventional method uses multi-channel electrical
connections as shown in FIG. 1c. In this example, a pair of rings
50, 55 are disposed proximate the shaft 5 with multiple electrical
connections 60 electrically coupling to the shaft 5 such that the
input current signals 65 travel along a magnetic encoding section
length 80 of the shaft 5 with the return output signal 70 such that
the encoding defines a magnetized region 75 in the shaft.
[0023] This complex encoding arrangement requires the spacing
between the individual circumferentially placed current entry
points be small in relation to the shaft diameter. Otherwise a
sufficiently uniform magnetization in a circumferential direction
is not achievable. Larger spacing requires the section length 80 be
larger which causes implementation problems in many applications.
In addition, the individual currents applied to the electrical
connections must be controlled to all have the same amplitudes
which becomes costly for larger diameter shafts.
[0024] Referring to FIG. 1d and FIG. 1e, the conventional
magnetoelastic sensing of torque 90 is illustrated in which there
are polarized rings 92, 93 that are coupled about the shaft 94 such
that the rings 92, 93 magnetically divide opposing polarization
regions. In this example, a domain wall 98 separates the polarized
rings 92, 93. A magnetic field sensor 95 is located proximate the
rings 92, 93 and senses the magnetic flux density 96. The results
from the sensor 95 are processed such that the stresses in the
rings 92, 93 correspond to torque imparted upon the shaft 94.
Typically the results are transmitted to a computing device 99 such
as a computer for the post processing. Once again, applying this
type of sensing it typically inefficient for large diameter
shafts.
[0025] The conventional sensing systems such as shown in FIGS. 1a-e
generally operate on the principles referred to as the inverse
Joule effect, the Matteucci effect, the Wertheim effect, the
Villari effect and the inverse Wiedemann effect such as detailed in
"A Study of the Inverse Wiedemann Effect on Circular Remanence" by
I. J. Garshelis and J. Ivan, IEEE Transactions on Magnetics, Vol.
10, No. 2, June 1974. These effects are related to magnetostriction
that explains changes in volume of magnetized material when torque
is applied, thus explaining the connection between mechanics and
magnetics.
[0026] Various processes and systems have been used to provide
accurate and reliable measuring capabilities for rotating shaft,
however continued improvements are needed especially with respect
to larger diameter shafts and enhancements in operational
efficiency.
SUMMARY OF THE INVENTION
[0027] The system and methods described herein relate generally to
encoding and measurement methods and systems, and more particularly
to measurement of torque, power, and bending moment in a
ferromagnetic shaft with magnetized shaft sections.
[0028] One embodiment is a magnetic encoding system of a shaft,
comprising at least one conducting member having a first end and a
second end which is disposed proximate the shaft with a gap between
the member and the shaft. There are a pair of electrodes proximate
each end of the conducting member and electrically coupled to the
shaft, wherein one of the electrodes is electrically coupled to the
second end of the conductor member. An encoding source is
electrically coupled to the first end of the conducting member and
electrically coupled to the other electrode. The encoding source
uses unipolar current pulses that are applied to the electrodes and
the conducting member, thereby creating sectional encoded regions
in the shaft.
[0029] The system in one example includes a non-conductive encoding
assembly disposed about at least a section of the shaft, wherein
the conducting member is disposed within the encoding assembly. The
conducting member in one aspect includes orienting the conducting
members axially, diagonally or circumferentially along at least a
section of the shaft.
[0030] One further feature includes having the electrodes
temporarily coupled to the shaft during the encoding and removed
during rotating operations.
[0031] According to one embodiment, at least two of the conducting
members are positioned adjacent to each other about the shaft with
sectional encoded regions of opposing polarities. An additional
feature includes domain boundaries formed between adjacent
sectional encoded regions. In one example, the adjacent conducting
members are positioned in close proximity to each other. According
to one feature there are a plurality of conducting members oriented
about at least a section of the shaft.
[0032] The shaft in one aspect is made of a ferromagnetic material,
and in one example the ferromagnetic material is steel. According
to another embodiment, the shaft has a layer of ferromagnetic
material applied about a surface of the shaft.
[0033] The conducting members according to one aspect are
substantially linear or circular and extend longitudinally or
circumferentially along a section of the shaft.
[0034] One embodiment is a system for measuring properties of a
rotating shaft, having a plurality of previously encoded magnetic
sections disposed about the shaft, wherein adjacent magnetic
sections have opposing polarities and domain boundaries between the
adjacent magnetic sections. There is at least one sensor disposed
proximate the encoded magnetic sections with a gap therebetween. A
processing section is coupled to the sensor for processing an AC
field component developed during rotation of the shaft thereby
measuring properties of the rotating shaft based on a
magnetostrictive effect.
[0035] The sensor in one aspect is at least one of an air core
induction coil or a magnetic field sensor (MFS). The magnetic field
sensors may include at least one of Hall Effect sensors, giant
magneto-resistive (GMR) sensors, fluxgate sensors, or
magnetoimpedance sensors.
[0036] One aspect of the properties include a non-contact
measurement of at least one of shaft power, torque, speed or
bending moments based on sensing magnetic field components in the
shaft.
[0037] One embodiment is a method for encoding a shaft, including
disposing at least one conducting member in close proximity about a
section of the shaft, with the conducting member having a first end
and a second end. This includes disposing electrodes to the shaft
proximate the first end and the second end, wherein the second end
electrode is coupled to the second end of the conducting member,
electrically coupling the first end electrode to a current source
and coupling the current source to the first end of the conducting
member, and applying unipolar current pulses to the conducting
members thereby creating sectional encoded regions.
[0038] According to one aspect, at least two of the conducting
members are oriented adjacent to each other, wherein the encoding
is with opposing polarities such that the resulting polarized
magnetic channels have domain boundaries.
[0039] One technical effect of the present systems and methods is
that the magnetic encoding essentially turns the shaft into a
component of the sensing system such that as a mechanical torque is
applied to the shaft, a torque-dependent magnetic flux component
appears in the direct environment of the encoded region of the
shaft.
[0040] The features and advantages described herein are not
all-inclusive and, in particular, many additional features and
advantages will be apparent to one of ordinary skill in the art in
view of the drawings, specification, and claims. Moreover, it
should be noted that the language used in the specification has
been principally selected for readability and instructional
purposes, and not to limit the scope of the inventive subject
matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIGS. 1a-c are prior art illustrations of magnetic encoding
arrangements for shaft torque sensing.
[0042] FIGS. 1d-e are prior art presentations of magnetoelastic
encoded shafts.
[0043] FIG. 2a is a basic depiction an encoding system configured
in accordance with one embodiment.
[0044] FIG. 2b is a basic depiction of an encoding system
configured in accordance with another embodiment.
[0045] FIG. 3 is a cross sectional illustration of the magnetic
flux density in the shaft associated with one embodiment.
[0046] FIG. 4 is a flowchart for the encoding of magnetic sections
configured in accordance with one embodiment.
[0047] FIG. 5 shows a sensing system configured in accordance with
one embodiment.
[0048] FIG. 6 illustrates encoding of a shaft to generate
magnetically polarized regions along sections of a shaft and the
orientation of the magnetic field sensors according to one
embodiment
[0049] FIG. 7 shows encoding of a shaft along the circular
circumference according to another embodiment.
[0050] FIG. 8 shows one measurement system according to one
embodiment.
[0051] FIG. 9 is an integral sensing system for sectional encoded
shafts according to one embodiment.
[0052] FIG. 10a-f is another integral sensing system for sectional
encoded shafts according to one embodiment.
DETAILED DESCRIPTION
[0053] According to one embodiment, enhanced encoding systems for
shafts and measuring properties thereof is achieved by sectional
encoding where encoded zones or magnetic channels are generated in
axial or circumferential directions of the shaft. For large
diameter shafts, it is beneficial to employ this magnetic encoding
where relevant flux densities can be achieved with lower encoding
currents.
[0054] Referring to FIG. 2, encoding of magnetic polarized regions
or channels according to one encoding embodiment is described. As
previously noted, the shaft 205 can be a ferromagnetic material or
have disposed upon it at least a section of ferromagnetic material
affixed to the shaft. The encoding can be accomplished in a number
of manners and one manner of operation disposes conducting members
215, 217 such as cables or metallic bars that are arranged about
the shaft 205. As depicted, the conducting members 215, 217 extend
along the shaft 205 longitudinally, although they may also extend
circumferentially or diagonally.
[0055] In more particular detail of this embodiment, an encoding
structure 210 is disposed about at least a section of the shaft
205. The encoding structure 210 in this example is a unit that is
coupled proximate the shaft during encoding such that the
conducting members 215, 217 extend along at least a portion of the
shaft 205. The conducting members 215, 217 in this embodiment are
arranged such that there are positive encoding conducting members
215 and negative encoding conducting members 217. The frame 212 of
the encoding structure 210 is typically a non-conductive material
such that the conducting members 215, 217 are isolated.
[0056] The encoding can be done during manufacture of the shaft or
post installation and is permanent when applied to the right type
of material and created with high current densities. The encoding
structure in this example is depicted as encircling the shaft 205
and may include additional frame elements (not shown) to maintain
its orientation and position about the shaft. This may include
frame supports (not shown) to ensure the conducting members 215,
217 are arranged properly and sufficient for the encoding
operation.
[0057] While the depicted unit 210 goes around the shaft 205, there
is no requirement for the unit to encircle the shaft with
conducting members. In a further embodiment, the encoding structure
210 is located proximate to a portion of the shaft 205 and there
can be multiple encoding structures arranged about the shaft such
that each of the encoding structures generates the magnetic
polarized regions.
[0058] The conducting members 215, 217 are disposed proximate the
shaft 205 with a gap between the member 215, 217 and the exterior
surface of the shaft 205. According to one example, the conducting
member 215, 217 are reinforced isolated copper bars, although other
suitable conductors are within the scope of the system. The
conducting members 215, 217 can be bars with a shape that can be
round, oval, square or rectangular, for example. The length can
vary depending upon the design criteria. Longer conducting members
215, 217 can provide greater surface area for sensing. The diameter
of the conducting members 215, 217 should have sufficient rigidity
and provide for the required current pulses. In one example, the
bars can be less than one inch or several inches long and have a
diameter ranging from 1/16inch to 1/2 inch.
[0059] The conducting members 215, 217 have a first end and a
second end and for illustrative purposes the first end shall be
coupled to the encoding source 250 and the second end shall be
coupled to the shaft 205. In order to electrically couple the
conducting members 215, 217 to the encoding source 250, electrical
connections 240, 260, 280, 290 are provided at the first end of the
conducting members 215, 217. There are first end electrodes 247
that are used to establish electrical connections from the shaft
205 to the encoding source 250. There are second end electrodes 242
that are used to establish electrical connections from the second
end of the conducting members to the shaft 205. The first and
second end electrodes 247, 242 refer to the electrical coupling to
the shaft 205 and in one embodiment the first end electrodes 247
are conductive elements coupled about the non-conductive frame 212
that contact the shaft 205. The second end electrodes 242 in one
embodiment refers to a conductive element that extends from the
second end of the conducting members 215, 217 to the shaft 205. The
electrodes can also be points of contact with jumpers or wires that
connect to the shaft 205.
[0060] For the positive conducting members 215, the encoding source
250 positive terminal is coupled to the first end of the positive
conducting members 215 along the positive electrical connector 240.
The encoding source 250 negative terminal is coupled to the
electrode 247 and the shaft via an electrical connector 260.
[0061] For the negative conducting members 217, the encoding source
250 negative terminal is coupled to the first end of the negative
conducting member 217 along electrical connection 290. The encoding
source 250 positive terminal is connected to the electrode 247 and
the shaft via electrical connector 280.
[0062] In one embodiment, electrical signals 245 travel through the
shaft 205 such that magnetized regions are generated on the shaft
205. One of the features of this encoding system is the ability to
magnetically encode channels or magnetic polarization regions in
the shaft. In particular, steel shafts have a high relative
permeability and the electric currents that travel through the
steel shaft create distinct encoded channels.
[0063] One example of the sectional magnetic encoding uses four
conducting members uniformly distributed approximately ninety
degrees apart. The positive polarity current pulse 240 is coupled
to the conducting member 215 and the current pulse travels along
the conducting member to an electrode 242 that contacts the shaft
205 about the second end. The current discharged by the electrode
242 travels back along the shaft 205 to the first end electrode 247
and the negative connection of the encoding source via the
electrical connector 260. The current 245 flowing along the shaft
205 creates a polarized magnetic channel on the shaft. Each of the
adjacent conducting members in the structure 210 would have
alternating polarities and the pulse encoding may simultaneously
encode the conducting members all at one time, grouped, or
individually. For example, the first set of positive conducting
members 215 can be encoded simultaneously followed by the negative
set of conducting members 217.
[0064] The encoding source typically can generate unipolar current
pulses from a few hundred A to a few kA with a pulse length
typically 1-100 ms. One example of an encoding source is based on a
capacitor bank or a power electronic device generating the desired
unipolar current waveforms. Another example of an encoding source
is a pulse generator. In one embodiment the current pulses are
short and can be characterized by a high frequency content.
[0065] The conducting members in one example include rigid or
semi-rigid bars that define a relatively straight path for the
current flow in a longitudinal direction, circumferentially or
diagonally along the shaft. According to one embodiment a cage
assembly is utilized to position the conducting members about the
shaft in a secure manner for the encoding process. In one
embodiment, the cage with the conducting members is affixed about
the shaft such that the shaft and the cage are in a fixed
relationship to each other until encoding is completed.
[0066] The current penetration, namely the depth of the current
density in the shaft, is controlled by the duration of the current
pulse in one embodiment. The current pulses are unipolar in this
example, and are positive current pulses without the negative
half-wave, or without the positive half-wave if negative current
pulses are applied. In one embodiment, the current pulses are
generated by discharging a capacitor bank wherein the size of the
discharge resistor determines the discharge time constant and
therefore the depth of the current penetration. By way of
illustration, the sectional encoding method in one example uses
five consecutive 500 A current pulses with a pulse length of about
5 ms each to generate permanent magnetic flux densities of about 5
Gauss used to encode an industrial steel shaft with a diameter of
60 mm.
[0067] According to a simple encoding approach, a magnetized
section is encoded one circuit at a time. For example, a positive
polarity current pulse can be applied to encode a first encoded
section followed by another section magnetized by applying a second
circuit with a negative polarity. Subsequent sections are encoded
using alternating polarity current pulses.
[0068] Such a sequential encoding process with alternating polarity
current pulses creates multiple almost identical encoded sections.
If only one current pulse is applied to each section to be
magnetized, the sections are generally not identical because
magnetizing the second section also affects the first magnetized
section. This undesired interaction is higher in the middle of the
encoding tool than at the beginning and end of it, where the
electrodes contact the shaft. Almost identical encoded sections can
be achieved in performing sequential current pulses, alternating
the sections while magnetizing and by performing the magnetic field
measurements close to the regions where the electrodes contact the
shaft. Another example for sequentially creating magnetized zones
in the shaft measures the field strength created in each segment or
zone and adapts the amplitude of the current pulses for the
subsequent encoding steps.
[0069] To avoid that the influence of sequential magnetization of
one section by the next magnetization, another encoding embodiment
is to apply the same current amplitude to all the conducting
members and encoding all the sections at once. In one embodiment,
the conducting members would use separate or split encoding sources
to accommodate the multiple conducting members. In one example,
separate capacitor banks would be used for each conducting
member.
[0070] For example, if there are four segments, one encoding source
can be used to apply the same current pulses to each of the four
current encoding sources with alternating polarities. In another
example, there are four separate encoding sources thereby avoiding
short circuits between the different encoding currents during the
encoding process. In another example, a switching scheme can be
employed to apply the current pulse signals with alternating
polarities.
[0071] While the conventional techniques rely upon total
circumferential shaft magnetization, one embodiment of the system
herein encodes magnetic channels in the shaft using the return
currents. The sectional magnetic encoding takes advantage of the
asymmetrical skin effect and the fact that a current always takes
the path of least impedance. The impedance is dominated by
inductance if the frequency of the current is high enough. In the
case of a short current pulse the return current flowing in the
shaft will be more localized than in the case of a longer pulse,
enabling polarized and well defined/narrow magnetic patterns. This
effect is used to magnetize sections of a shaft with more localized
channels that lead to faster changes in the magnetic field during
sensing. Therefore the pulse length during encoding affects the
signal frequencies observed during sensing application.
[0072] Referring to FIG. 2b, another encoding configuration is
illustrated wherein there are paired conducting members 294, 295
proximate the shaft 205. In this embodiment, the paired conducting
members 294, 295 are coupled about a non-conducting frame 292
wherein several paired conducting members can be positioned about
the frame 292. The conducting members 294, 295 are located in
proximity to each other approximately parallel and separated by a
gap 296.
[0073] Similar to the description of FIG. 2a, the conducting
members are coupled to an encoding source such that the members are
coupled with opposing polarity. There are connections that
electrically couple one end of the conducting member to the
encoding source while the other end is electrically coupled to the
shaft 205. The encoding is similar in functionality to that
detailed for FIG. 2a however the resulting sectional magnetic
encoding sections are paired and establish isolated domain
boundaries with respect to each other as further detailed herein
such as in FIG. 6.
[0074] Referring to FIG. 3, a cross sectional view of one example
of an encoded shaft is depicted. According to one embodiment, the
encoding involves conducting members proximate the shaft that apply
encoding pulses and uses the return current flowing through the
shaft to encode the shaft.
[0075] In this example, there are four conducting members 310, 312,
314, 316 that are spaced approximately 90 degrees about the shaft
350. The gap 375 between conducting members 310, 312, 314, 316 and
the shaft 350 is typically small, as the closer the conducting
member is to the shaft surface, the stronger the generated field or
the less energy required. In one example the gap 375 is less than 1
mm and may include an isolating sheet (not shown) between the
conducting member and the shaft surface. Tolerance is generally not
problematic as this is only used during the encoding process and
not during the shaft operation.
[0076] The conducting members 310, 312, 314, 316 illustrate the
alternating opposing polarities used during the encoding process by
an encoding source (not shown) such that there are positive
polarity members 312, 316 and negative polarity members 310, 314.
The encoding generates the sectional polarized magnetic regions
330, 332, 334, 336. For better illustration, the magnetic field
lines shown in FIG. 3 belong to a magnetization with DC currents.
When performing the magnetic encoding with DC current pulses, the
skin effect prevents the magnetic field lines from penetrating the
whole shaft cross section. Instead, they are concentrated near the
shaft surface. The shorter the DC current pulse length, the higher
the current and flux density near the shaft surface. This is
advantageous for measuring torque based on magnetic field
measurements because the highest magnetic flux densities are
created close to the shaft surface, in only a few millimeters
radial distance to the magnetic field sensors. One of the unique
attributes of this encoding is the establishment of the magnetic
polarized regions 330, 332, 334, 336 with domain boundaries 380.
When torque is applied to the shaft, the greatest change in the
magnetic flux density occurs at the domain boundaries 380.
[0077] Thus, the magnetic polarized regions 330, 332, 334, 336 show
the magnetic fields are well-defined polarized magnetic regions
respectively channels when encoded with short current pulses and
define the domain boundaries 380 with respect to each other. The
boundaries 380 tend to be the regions that demonstrate the highest
magnetic flux densities measurable near the shaft surface when the
shaft is subject to torsional movement. These domain boundaries 380
thus represent the optimal location for sensing.
[0078] Referring to FIG. 4, one embodiment for processing the
encoded sections with magnetically polarized regions with domain
boundaries is detailed. The shaft can be encoded during the
manufacturing process, post manufacture, or post installation
depending upon the requirements. As previously noted, the shaft
should be of a ferromagnetic material or have a ferromagnetic
material affixed to the shaft for the encoding.
[0079] Some form of encoding assembly is installed or positioned
about the section of the shaft 410, wherein the assembly helps to
maintain the conducting members in proper relation to each other
and also to the shaft. The conducting members are typically
arranged in a spatial relationship to each other and the assembly
helps to maintain that relationship. The installed assembly can be
of the various types including clamshell design and two-piece
assembly such as known in the art. The positioned assembly can be a
more integral housing that is positioned about a portion of the
shaft.
[0080] In one embodiment the assembly has two or more degrees of
freedom with respect to the conducting members and allows the
conducting members to establish a spatial relationship to each
other such as a phase spacing based on the shaft diameter.
[0081] There can be multiple assemblies coupled to the shaft during
the encoding process or a single assembly that is used multiple
times at different sectional locations along the shaft.
[0082] The conducting members are electrically coupled to an
encoding source and also to the shaft 415. In one embodiment,
electrodes couple the conducting members and encoding source with
the shaft and electrical leads or jumpers can also be used to
couple to provide electrical coupling. The electrodes can be placed
approximately perpendicular proximate the end of the conductor
member so that the electrodes are aligned roughly directly below
the end of the conducting members.
[0083] The electrical connectivity can be done in a variety of ways
depending upon the design criteria. For example, electrodes on one
end of the conducting members can be connected to the encoding
source by electrical jumpers or wire leads of sufficient thickness
to accommodate the encoding signals. In one example, conducting
members are coupled to the encoding source so that the alternating
conducting members are coupled with alternating polarity.
[0084] The encoding source introduces current pulses to the
conducting members 420. Typically more than one current pulse is
applied to each conducting member, and several current pulses are
typically applied. In one embodiment, the current pulses are
applied sequentially to each of the conducting members. This can
occur by applying the current pulses to each of the positive
polarity conducting members first (with the corresponding
electrodes coupled to the negative polarity) or by applying the
current pulses sequentially to each conducting member and
alternating the polarities as needed. One disadvantage of
connecting conducting members in parallel is the reduced current
amplitude in each conductor and difficulty in achieving equal
current distribution among the paralleled conductors.
[0085] By way of example of a sequential encoding, a first current
pulse is applied to the first conducting member and several current
pulses with identical polarity are applied. As the cage is
typically deployed about a stationary shaft, the location of the
encoded segments is known. This makes it simple to apply encoding
pulses to additional segments by rotating the shaft a fixed amount
or rotating the assembly housing the conducting members a fixed
amount.
[0086] In one embodiment the current pulse is generated by
discharging a capacitor bank, wherein the pulse length of the
current pulse can be modified by a discharge resistor. In one
example, the fall time is approximately 10 ms. Alternatively, the
duration of the current pulse can by adjusted by a switching device
that can be switched from non-conducting state to the conducting
state and after a certain time back to non-conducting state.
[0087] A short current pulse is typically utilized as higher
frequencies decrease the depth penetration of the shaft. The
current pulse in this example is a unipolar pulse and is
sufficiently short to penetrate only the surface region and
generate the same sectional magnetic polarized regions 330, 332,
334, 336 close to the shaft surface, as illustrated in FIG. 3 for a
DC current encoding.
[0088] The depth of the current density is characterized by the
skin depth .delta. which can be calculated according to the
following relation:
.delta. = 1 .pi. .times. f .times. .mu. 0 .times. .mu. r .times.
.sigma. ##EQU00001##
where
[0089] .sigma.--electrical conductivity of the shaft
[0090] f--frequency
[0091] .mu..sub.0--permeability of the vacuum
[0092] .mu..sub.r--relative permeability of the shaft material
[0093] Assuming a current pulse generated by a capacitor discharge,
where the rise time of the current is much smaller than the current
decay time (fall time), the frequency in the above relation is
related to the fall time of the current pulse applied during
encoding. In general, the frequency in the above relation is
related to the fundamental frequency (first harmonic) of the
applied current pulse. Due to the high relative permeability of the
ferromagnetic material being encoded, the skin depth gets
relatively small even at moderate frequencies.
[0094] Regardless of how the current pulses are applied, the
current pulses induce magnetic fields in the shaft 430 that
generate magnetic polarized regions. It should be understood that
the application of the current pulse can be simultaneous to all
conducting members, sequentially to each member, or upon some
grouping of members.
[0095] The magnetic shaft encoding according to one embodiment
introduces regular magnetization patterns in circumferential
direction that can be arranged at sufficient circumferential
spacing. In this example, the magnetic field resulting from the
rotating magnetized shaft is an AC field, and the frequency of the
AC field is coupled to the rotational frequency of the shaft.
Moderate frequencies enable the use of standard magnetic field
sensors, such as Hall effect or fluxgate sensors for measuring the
resulting magnetic field caused by the rotating shaft. In another
embodiment high temperature resistant air core induction coils are
used for field detection. A sufficiently high number of encoded
sections can be implemented in the shaft, such that even at a high
rotational speed a sufficiently high frequency signals are
detectable.
[0096] In accordance with one embodiment, a simple encoding
arrangement uses at least one isolated conductor attached proximate
the shaft surface having a first end and a second end. There may be
electrodes on the shaft near the input side and end of the
conducting member in order to establish for the current flow and
return paths.
[0097] A further embodiment uses four or more conducting members
having alternating polarities disposed about at least one section
of a shaft such that the alternating polarity of the conducting
members is negative, positive, negative and positive and so forth.
In a further embodiment the conducting members are paired and in
close proximity to each other with alternate polarities, and there
can be several paired conducting members about the shaft.
[0098] In one aspect, signals are transmitted on the conducting
members such that a first set of encodings are instilled upon the
section of the shaft. One additional aspect is that the first set
of encodings is simultaneously encoded such that all channels are
encoded at the same time. The conducting members can be supplied
from different sources to avoid short circuits between return
currents flowing in the shaft. In a further aspect the encodings
are performed in one conducting member at a time.
[0099] An additional set of encodings can be disposed proximate a
set of local flux density minimums located about the section of the
shaft. The local minimums are disposed at locations offset from the
first set of encodings, and centered between the conducting members
with different polarities, as illustrated in FIG. 3.
[0100] By way of illustration, the encoding system can be
implemented by a housing structure surrounding a section of the
shaft. The housing typically contains a number of conducting
members proximate the shaft but not in direct contact with the
shaft. The conducting members can be isolated conductors oriented
about the shaft having only the required number of members such as
four or eight members for the first encoding and four or eight
members for the second encoding. Alternatively, there can be
additional members such that only some of the members are utilized
depending upon the applications. Such an implementation provides
redundancy and also permits a greater number of conductors for
other applications.
[0101] In certain cases, the shaft diameter is a factor in
determining whether the sequential encoding is acceptable as there
should be sufficient separation between the encoding members so
that the individual encoding does not disturb other encodings.
[0102] In one aspect the system creates a circumferentially
changing magnetic field nearby the shaft, and as the shaft rotates
this results in AC field components. The resulting AC field
components are measurable by search coils. Such a system is
insensitive to DC field disturbances and applicable to higher
temperature ranges for other sensors such as flux gate sensors. In
another aspect, involving large diameter shafts rotating at low or
moderate speed, the magnetic field sensing can be performed by
magnetic field sensors (MFS). Magnetic field sensors are generally
used to measure the magnetic flux and/or the strength and direction
of a magnetic field and can be based on several types of sensors
including magnetoresistive devices, flux gate or coil sensors, Hall
Effect sensors, magnetoinductive sensors. Selecting the specific
magnetic field sensor depends upon the design criteria and includes
certain aspects such as flux density, resolution, accuracy, and
number of axes. With respect to the shaft, the sensors can be
located to maximize the measured response, such that the sensors
can be oriented for example at 0 degrees, 45 degrees and/or 90
degrees. Different types of sensors can be utilized to obtain
various data from the shaft.
[0103] The sectional encoding can be performed in several ways
including encoding only one or more shaft sections in a
circumferential direction and multiple encodings in the
circumferential direction in order to generate periodically
alternating flux components so that inductive sensors can be used
for sensing the magnetic flux density. Depending upon the intended
application requirements, the multiple encodings allow for more
data to be extracted during the rotation of the shaft that can
provide more accurate and timely processing. By way of example, a
coil with 400 turns, 1000 .OMEGA. resistance and 0.9 mm.sup.2 area
has a noise level in the range of 2 .mu.T.times. {square root over
(Hz)} for a sinusoidal AC magnetic field. In the case of a rotating
shaft with 1000 rpm and a measurement bandwidth of 50 Hz this
results in a noise level of 0.8 .mu.T. Therefore magnetic fields in
the range of tens or hundreds of microteslas can be accurately
measured by small air coils.
[0104] Referring to FIG. 5, another encoding embodiment is
depicted. In this example, the magnetically polarized sections are
formed at one or more sections about the shaft. Such an
implementation is particularly useful for sectional magnetic
encoding for large diameter shafts wherein encoding the entire
circumference is extremely costly and time consuming.
[0105] The resulting field lines after the encoding process are
shown in FIG. 5. In this example, there are four encoded sections
510, 520, 530, 540 each having respective magnetic fields 515, 525,
535, 545 proximate to the shaft skin with the magnetic flux density
having properties based on the encoding. The penetration depth for
the field lines depends on factors such as the length of the
current pulses used in the encoding process. In one embodiment,
well-defined domain boundaries 580 between encoded sections are
established by encoding pulses with different polarities and
lengths that are applied at each magnetically polarized section.
Thus, domain boundaries 580 between magnetic polarized sections
with different polarities can be created close to the shaft
surface.
[0106] The orientation of the magnetic domain boundaries 580 helps
to determine the optimum orientation of the magnetic field sensors
560 for measuring torque dependent field changes. The non-contact
sensors 560 are positioned to detect the magnetic fields of the
rotating shaft that can be processed to provide certain properties
about the shaft.
[0107] The sensing frame assembly 550 can be a portion of the
circumference of the shaft or disposed about the entire
circumference. In certain embodiments, the sensors 560 are
integrated into existing shaft housings such that a separate sensor
assembly is not required and the integrated structure performs the
functionality of the sensor assembly. The housing can provide a
plurality of sensor slots 570 such that any number of sensors 560
can be deployed. One advantage of metallic housings is that it
provides screening against external magnetic AC field
components.
[0108] High power applications require reliable torque monitoring.
According to one embodiment, the system deploys more sensing coils
and therefore obtains greater frequency of readings and greater
reliability. The sensing coils are relatively inexpensive and
multiple coils can be easily deployed in a sensor holder. In one
example, there are multiple sensors 560 disposed within the sensor
assembly. According to one embodiment the plurality of sensors 560
are used to provide greater reliability by allowing more frequent
measurements. The multiple sensors 560 can also be used to provide
redundancy so that the sensing functions are operable even with
some sensor failure. In another example, different types of sensors
are deployed such that different types of data can be measured. The
multiple sensor types can take advantage of the sensing properties
of the particular sensor or otherwise allow for enhanced sensing
functionality. In a further embodiment, the sectional encoding
process includes different encoding sections having different
encoding properties such that the sensors can obtain multiple forms
of data.
[0109] By way of example, the use of four encoding sections as
shown in FIG. 3 can be used for shafts such as 60 mm. For larger
diameter shafts or for more precise measurements, additional
encodings can be employed to add further magnetic polarization
regions and corresponding boundaries. To minimize the time effort
and cost for the magnetic encoding of large shafts in applications
where no high sampling rate for the torque is required, the shaft
can be only partially magnetized in circumferential direction, such
as illustrated in FIG. 6.
[0110] Generation of the magnetically encoded polarized regions and
neutral zones therebetween enables generation of a torque dependent
AC field component in the surroundings of a rotating shaft, so that
the shaft power can be measured directly by measuring the induced
voltage in a coil placed nearby the shaft.
[0111] Direct measurement of the shaft power (or speed and torque)
using induced voltage in a search coil can be applied at
temperatures of more than 500.degree. C. and is only limited by the
coil material properties. The system is also not sensitive to
constant magnetic fields originating from the surrounding as only
AC field components are measured.
[0112] Referring to FIG. 6, one embodiment for sectional magnetic
encoding is depicted. In this example pairs of conducting members
610, 612; 620, 622; 630, 632; and 640, 642 are located about the
shaft 605. Each pair is used to create magnetically polarized
regions with domain boundaries for the paired conducting members of
the corresponding magnetic fields 614, 616; 624, 626; 634, 636; and
644, 646. Each pair of conducting members 610, 612; 620, 622; 630,
632; 640, 642 can be coupled to an encoding source (not shown) that
provides different polarities to the members in each pair in order
to generate the alternating polarized magnetic fields.
[0113] For example, the conducting member 612 employs a positive
polarity encoding while the paired conducting member 610 uses a
negative polarity encoding. The polarized magnetic regions 616, 614
resulting from the encoding have a domain boundary region 650 that
represents the location of optimal response from torsional movement
of the shaft 605. Unlike the domain boundaries of FIG. 3, the
paired polarized magnetic regions are far apart and there is a void
space 660 between each respective region.
[0114] In the illustrated example of FIG. 6, there are eight
encoded regions 614, 616; 624, 626; 634, 636; and 644, 646 on the
shaft 605 generated by the four pairs of conducting members 610,
612; 620, 622; 630, 632; 640, 642. Thus, well-defined magnetic
polarized regions with four magnetic domain boundaries 650, 652,
654, 656 can be achieved without having to magnetize the whole
shaft. The magnetic field sensors used in this example can be
oriented in a circumferential direction to measure magnetic field
components that are tangential to the shaft surface and
perpendicular to the magnetic domain boundaries. This is the
optimum orientation for the magnetic field sensors under ideal
conditions. If the field sensors are installed in proximity to the
locations where the encoding tool electrically contacted the shaft
surface, other orientations of the magnetic field sensors may be
favorable.
[0115] Referring to FIG. 7, a further embodiment of encoding is
depicted for magnetically encoding and creating magnetic domain
boundaries on the shaft along the radial circumference of the shaft
705. In this example, there are arc segments 720, 740 positioned
about a segment of the shaft 705. One conducting arc segment 720 is
coupled to a positive polarity encoding source (not shown) on a
positive end 710 such that the encoding currents travel along from
the positive end and along the arc 720. In this example, the other
end of the conducting arc segment 720 is coupled to the shaft 705
by an electrode 725 in contact with the shaft 705. The encoding
current pulse thus travels along the conducting member 720 with the
return currents traveling along the shaft 705 to the return
electrode at the return end 715 that is electrically coupled to the
encoding source (not shown).
[0116] The other conducting arc segment 740 is coupled on a return
end 730 to the encoding source (not shown). The encoding signals
travel from the encoding source (not shown) to the positive end 735
via an electrode in contact with the shaft 705 along the surface of
the shaft and through the electrode 745. The encoding currents
travel along the arc segment 740 and return via the return end 730
to the encoding source (not shown). Once again, this encoding
generates sectional magnetic regions about the circumference of the
shaft 705. The combination of the pair of conducting arc segments
720, 740 that create the polarized magnetic regions also creates
the domain boundary 750 therebetween that is the optimal sensing
region.
[0117] In this example, there are two polarized regions orientated
in an axial direction to the shaft. The magnetic field measurement
is simpler than other embodiments because the shaft rotates
radially and there is a greater length of sensing area in the
circumferential direction.
[0118] It should be readily apparent that while depicted as an arc
segment of about a semi-circle, the arc segments can be a small
portion of the shaft or larger portions of the circular
circumference. Furthermore, while shown as being circumferential,
the encoded channels can be along any direction of the shaft such
as diagonal for the magnetic polarized channels.
[0119] Referring to FIG. 8, one embodiment of a simplified
measurement system is depicted. In this example, the magnetically
encoded shaft 805 includes a plurality of polarized magnetized
regions 810 with magnetic domain boundaries therebetween caused by
encoding pulses with alternating polarities in each of these
sections. The number of magnetically encoded polarized regions
depends upon the encoding and the design criteria, such as the
diameter of the shaft. It should be appreciated that the shape of
the polarized magnetic channels are shown as being linear for
convenience. In this example, sensors 820, 825, 830, 835 are used
to measure aspects of the rotating shaft during operation.
[0120] Referring again to FIG. 8, when the shaft 805 is not subject
to torque or bending, the magnetic flux paths mainly close within
the shaft material. Upon the application of some torque or bending
moments of the shaft 805, discontinuities of flux components at the
magnetic domain boundaries create an additional magnetic field in
space (outside the shaft material) that is measurable by one or
more of the sensors 820, 825, 830, 835.
[0121] In this example, sensor units 820, 825, 830, 835 reside at
some small distance from the shaft 805 and are oriented to detect
the magnetic fields. The sensor 820, 825, 830, 835 can be the same
or similar type sensors and oriented in the same or similar plane
with respect to the shaft. In another embodiment, the sensors are
oriented in different manners such as different angles to be able
to detect different measurements. While in yet another embodiment,
the sensors 820, 825, 830, 835 are of varying types configured to
detect different properties. The measured data is typically subject
to subsequent processing to determine the characteristics
associated with the shaft based on the measured magnetic field.
[0122] High temperature operation of the conventional
magnetostrictive sensor systems is limited by the sensors that have
to be capable of sensing DC magnetic fields. In the case of flux
gate sensors, for example, high temperature operation is limited by
the Curie temperature of the core material used in the fluxgate
sensor. Sectional magnetic encoding provides the ability to measure
induced currents due to the AC magnetic field caused by the
rotating sections of the shaft. In one example, the sensor is a
sensor coil with an air-core. However, other sensors are possible
including fluxgate, Hall effect, anisotropic magnetostrictive, and
giant magneto-resistive (GMR).
[0123] In one embodiment, it is desirable that the sensor operate
in pairs 820, 825 and 830, 835 and work in a differential mode of
operation, since this makes them more robust against common mode
types of disturbances, such as the ambient temperature or external
magnetic fields and provides symmetrical responses to torques of
opposite directions. In one example, the first sensor pair 820, 825
is oriented with respect to the shaft at a different angle that the
second sensor pair 830, 835.
[0124] One embodiment of the system provides a magnetic encoding
system and method for magnetostrictive measurements. In contrast to
conventional schemes, the shaft has magnetic polarized regions
encoded to generate alternating flux components so that inductive
sensors can be used for sensing the magnetic flux density that is
directly proportional to the applied torque.
[0125] In certain applications, when applying sectional magnetic
encoding, it is difficult to accurately achieve the same flux
distribution and/or magnetization in all shaft sections. There are
several causes for the lack of consistency between sections, such
as the magnetic encoding tools applied subsequently to different
sections, material inhomogeneities, and differing magnetizing
current densities applied to the shaft sections. This is
particularly relevant when the magnetizing tools cannot be
connected simultaneously and in series with the sections. Due to
the differences in the magnetic properties between the encoded
channels, sensing the same torque or power with identical sensors
over different magnetically encoded shaft sections might lead to
inaccurate results.
[0126] Therefore, the magnetic field sensing in one embodiment is
based on a method and system where the measurement values are based
on an integration method. The integration typically involves
several factors for the integration processing. The integration
typically includes at least one full revolution of the shaft
[0127] In FIG. 9, one measurement system example is provided for
illustrative purposes to explain the operations, wherein the
encoded shaft 905 has a number of polarized magnetic regions 907 on
the shaft. A first pair of sensors 910 labeled A and A' are
disposed proximate the shaft 905 with a small gap therebetween. The
sensors 910 measure the properties of the shaft based on the
measured changes to the magnetic fields 907. As one example to
illustrate the processing, the sensors A and A' are orientated to
measure the tangential magnetic field components perpendicular to
the axis of the shaft. For better understanding, it is assumed that
the sensors measure ideal sine wave signals as the shaft rotates.
The sensor A' receives the sinusoidal signal 920 that is combined
with the signal 930 from sensor A in the subtractor 925 to produce
the resulting signal 935. This signal 935 is integrated by the
integrator 940 producing the first integral signal 945.
[0128] The second pair of sensors 915 labeled B and B' are also
disposed proximate the shaft 905 with a small gap therebetween. The
sensors 915 also measure the properties of the shaft based on the
measured changes to the magnetic fields 907. The sensor B receives
the sinusoidal signal 950 that is combined with the signal 955 from
sensor B' in the subtractor 960 to produce the resulting signal
965. This signal 965 is integrated by the integrator 970 producing
the second integral signal 975. The first integral signal 945 is
combined with the second integral signal 975 in the combined signal
980 at the summer 985 to produce the combined output signals
990.
[0129] In one embodiment, there are eight encoded sections and
eight induction coils, wherein eight measurement values are
achieved during one revolution. For example, one new sample is
taken every 2.5 ms for shafts rotating at 3000 rpm, which is
typically sufficient for most high power applications such as large
shafts. If this sampling rate is not sufficient, the number of
encoded sections or the number of sensing coils can be
increased.
[0130] In cases where a higher sampling rate of the measured torque
is required, more advanced signal processing algorithms can be
utilized enabling a realtime output of the measurement signal
without integration.
[0131] Referring to FIG. 10a-10f, a sensing embodiment with
advanced signal processing for a sectional encoded shaft 1005 is
detailed and includes two measuring scenarios, namely ramp torque
changes (FIG. 10c-d) and step-function torque changes (FIG. 10e-f).
This embodiment can be used for higher sampling rates of the
measured torque using sine functions, wherein the sine functions
are typically employed on the encoding and decoding. The higher
sampling rate enables a realtime output of the measurement signal
since no integration is needed.
[0132] Referring to FIG. 10a and 10b, there are four encoded
sections 1015 having respective domain boundaries and six sensor
units 1010. The respective outputs of two of the sensors 1010 that
are 180.degree. apart are coupled in series at the receiving coils.
The three resulting outputs are then combined to form a three phase
signal 1020 and in one example phase one 1022 consists of the
output from sensors 1 and 4; phase two 1024 includes the output
from sensors 3 and 6; and the last phase 1026 includes sensor
outputs 5 and 2. At some time the shaft is subject to some torque
and the measurements are processed.
[0133] FIGS. 10c and 10d graphically illustrate the processing for
a ramp function for the torque change that undergoes dq
transformation. The ramp function 1030 includes phase `a` 1032,
phase `b` 1034 and phase `c` 1036 and shows how the output ramps
downward in a linear fashion over time. The dq transformation
output 1040 shows the d output 1042 and the q output 1044.
[0134] FIG. 10e and 10f graphically illustrate the processing for a
step function for the torque change that undergoes the dq
transformation. The step function graph 1050 shows phase `a` 1052,
phase `b` 1054 and phase `c` 1056 and includes the difference
between the outputs 1058 as a result of some shaft changes.
[0135] The transformed output is graphically depicted 1060 showing
the d output 1062 and the q output 1064 wherein the torque is
represented as the change in levels 1066.
[0136] In one embodiment there is a dq0 transformation of the
sensed signals as follows:
V d = 2 3 ( V a sin ( .omega. t ) + V b sin ( .omega. t - 2 .pi. /
3 ) + V c sin ( .omega. t + 2 .pi. / 3 ) ) ##EQU00002## V q = 2 3 (
V a cos ( .omega. t ) + V b cos ( .omega. t - 2 .pi. / 3 ) + V c
cos ( .omega. t + 2 .pi. / 3 ) ) ##EQU00002.2## V 0 = 1 3 ( V a + V
b + V c ) ##EQU00002.3##
[0137] The encoding method and system according to one embodiment
enables highly accurate measurements of direct power, torque,
and/or bending moment for rotating machinery. Depending on the
number of sensing coils and the sensing coil processing, the
electrical signal is directly proportional to the shaft power,
torque or bending moments. Ease of sensor integration is yet
another feature of this system achieved by the use of small coils.
It is also possible to locate the sensor electronics at some
distance from the sensors and up to several meters away from the
sensor installation to enable measurement in harsh
environments.
[0138] One of the features of the system detailed herein is the
non-contact measurement of shaft power, torque and/or speed based
on sensing AC field components with respect to the shaft. High
sampling rates can simply be achieved in shafts rotating at high
speed, for example, high-speed electrical machines or jet engines.
The measurement of shaft power, torque, and speed can be based on
air-core induction coils which are smaller, low cost and enables
operation in environments involving temperature ranges above 220
deg C.
[0139] This system also provides a non-contact measurement system
as nothing is attached to the rotating shaft during operation. This
non-contact system enables direct monitoring of the shaft power
that is extremely relevant for detecting efficiency decreases in
different sections of a shaft system such as large turbine
trains.
[0140] One application example that might benefit from the
measurement system detailed herein relates to a drive train of a
wind turbine. The frame with the magnetizing members can be
installed during the manufacturing process quickly, thereby
satisfying the magnetic encoding requirements for the main shaft.
High interest in a wind turbine main shaft is the monitoring of
bending moments and the reduction of such moments in applying
asymmetrical load control (ALC), based on the bending moment
measurement. An example design for a magnetic encoding enabling
bending moment measurements is for the large diameter shaft of a
wind turbine. If a torque is applied to this shaft, all sensors
measure the same magnetic field change. In case of bending moments
in vertical or horizontal direction, the two opposite field sensors
measure different field changes. In one example, sensor pairs are
applied at the four sensor locations to enable differential
measurement. The accuracy and reliability of the sensing system can
be increased in adding more sensor pairs in circumferential
direction.
[0141] Since wind turbine main shafts are rotating slowly and
typically have slip rings enabling to easily extract signals from
the rotating shaft, it would be possible to install the magnetic
field sensors directly on the shaft surface, in proximity to the
magnetic domain boundaries, such that the sensor system would be
rotating with the main shaft, enabling very accurate torque and
bending moment output signals because the sensors produce the same
magnetic field output at all rotor positions. Thus, such secondary
sensors can be permanently installed in the main drive shaft with
the magnetic field or bending moment signals transmitted via
existing slip rings or wireless communications.
[0142] Shaft power measurements with high resolution and
repeatability also enable to detect vibrations or technical issues
in the rotor shaft system at an early stage, for an advanced
maintenance planning or for fatigue diagnostics. High accuracy
monitoring of the shaft power under all operating conditions and
over the lifetime of a shaft system enables to achieve higher
turbine or jet engine efficiency.
[0143] The ability to accurately measure shaft torque inbetween
individual stages of steam and gas turbines or compressors is an
important tool for measuring and optimizing the performance. In
conventional turbine and compressor applications there is no direct
shaft torque or power measurement available between individual
stages after commissioning of the system.
[0144] Torque sensing systems based on permanent magnetic encoded
steel sections enable to measure the shaft power output and bending
at different shaft sections with high long term stability. This is
of high relevance for wind turbines, but also for steam turbines,
gas turbines or large compressors where small decreases in the
power output between individual turbine stages can easily be
detected, enabling to perform preventive maintenance and keep the
energy efficiency high at all times during the lifetime of a
turbine or compressor.
[0145] Another aspect relates to the inclusion of multiple sensing
coils for redundancy. The low costs of the sensing elements enable
practical use of multiple sensing coils such that the measurement
system is operational even if one or more individual sensor
elements fail.
[0146] The foregoing description of the embodiments of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Many modifications and
variations are possible in light of this disclosure. It is intended
that the scope of the invention be limited not by this detailed
description, but rather by the claims appended hereto.
* * * * *