Note: Descriptions are shown in the official language in which they were submitted.
1~31
This invention is concerned with devices used to measure a torque applied
to an object, such as a shaft, which may be either static or moving. A
device to measur applied torque is described which uses photoelastic
material to detect the amount of torque induced strain at a preselected
locus on the surface of the object. By observing the changes in the
optical behaviour of the photoelastic material the applied torque can be
measured. Further, the data obtained can also be used at least to
identify other stress-induced strains, for example, a mis-alignment of the
bearings for a shaft.
In the past in order to measure accurately the torque applied to an object
some form of contact with the object has been necessary, for example, the
commonly used variable resistance electrical strain gauges. Static tests
were used to measure strain between selected points, such as over the
length of the object. These tests would allow for accurate analysis of
the behaviour of an object under static conditions and permit
determination of a maximum safe load. They do not permit evaluation of
the object either under dynamic test conditions, or in its conditions of
use. Electronic measurement of strain can also be made, in both static
and dynamic situations, but these devices require some form of electrical
connection with the object being tested. Whilst such connections can be
easily made for static testing, the need for reliable electrical contacts
with the object results in difficulties when trying to measure torque
induced strain either under dynamic conditions or within a normal working
environment. The use of slip ring contacts seems to have overcome some of
these problems, but such systems are bulky, and present the designer with
added complications and maintenance considerations.
There are at present several methods (discussed in a paper entitled
"Rotorshaft Torquemeter" by R. B. Bossler, Presented at The 35th Annual
National Forum of the American Helicopter Society, May 1979) of measuring
torque to within 2% involving limited or no contact. OnP of these, phase
displacement, is obtrusive to the shaft being tested. Telemetric
transmitters are problematic due to noise. Mercury transmitters are
problematic in their bulk and weight. Though capacitative transmitters
are smaller than rotational transmitters both remain obtrusive. Even
though telemetric devices are currently used in some helicopters, it would
greatly simplify design if a less obtrusive torque meter could be used.
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202~2
In 1959 F. Zandman proposed (Product Engineering, March 2, 1959, Vol 30p.43) a torque meter using the photoelastic properties of some materials.
The device as described requires two different photoelastic patch areas on
a rotating shaft each of which must be illuminated. One of the patches is
connected to the shaft in such a way as to be unstrained, and is used as a
base comparison. The other is connected to the shaft in such a way as to
be strained under a torque condition. The light beams reflected off each
patch area are combined in order to form an interference pattern which is
observed. A mechanical screw is used to perform the opto-mechanical
compensation, This device is slow and is obtrusive. It would also appear
that the device as described cannot be used in an environment of
operation.
One main disadvantage of the Zandman system, is ehat it bases its valuefor torque on the readings taken at one point on the surface of an object.
This system only works under conditions of pure torque, but when a bending
component is introduced, the accuracy of Zandman's system is greatly
reduced. This limits the potential uses of the system to carefully
monitored environments. It is therefore desirable to measure torque in
such a fashion so as to allow the value for pure torque to be separated
from the other forces affecting the object.
It is also a disadvantage of the Zandman system that two photoelastic
patches are required, together with a coherent split light beam system
involving a mechanical splitter in order to provide one value. It is
therefore desirable to be able to measure torque photoelastically by a
technique not requiring these complications.
It is therefore desirable to be able to measure torque in a rotating ormoving object, such as a shaft, without any physica] connection having to
be made to the object itself. Furthermore, it is desirable to be able to
measure torque in such a rotating or moving object without having to make
any modifications at all to the object: that is, by a non-obtrusive
method. In addition to eliminating the difficulties of maintaining
adequate electrical contacts, such a method also allows for shapes other
than simple cylinders and the like better to be accommodated. It is also
desirable to be able to measure the applied torque in a moving or rotating
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202fl~3~
object either under predefined test conditions or during use in its
intended environment.
It is also desirable to be able to measure torque in a rotating shaft
without having to attach to the shaft any significant amount of weight or
bulk.
Furthermore, in many applications it is desirable to be able to measurethe torque applied to an object in its environment of use on an on-going
real-time basis. Such measurements would allow temporarily exceeding a
safe loading factor, for example9 in an emergency situation.
Alternatively, such data would allow prediction of approaching catastrophe
and the control of the power being applied to the object to produce the
observed torque to limit that observed torque to within acceptable values.
The invention herein described broadly utilises the photoelasticproperties of certain materials in order to facilitate the non-invasive
measurement of torque induced strain in an object such as a shaft. A
small patch or band of birefringent photoelastic material with a
reflective backing is attached to a pre-chosen locus on the surface of the
object at the point the torque induced strain is to be measured. As the
object deflects under torque induced strain, the birefringent photoelastic
plastic material similarly deforms thus causing changes in its optical
properties. By observing these changes in optical properties the applied
torque can be readily derived. As the only "contact" required with the
object is the ability to illuminate the patch of strained birefringent
photoelastic plasic material, and to capture at least some of the light
reflected from it, no physical connection with the object is needed. The
use of fiber optics allows both the source of light, and the analyzer
means used to observe changes in optical properties in the birefringent
photoelastic plastic material to be remote from the object itself.
Further~ the distance between the ends of the fiber optics cables and the
patch also often can be kept quite short, thus facilitating measurements
in a condition of use environment. The system may be used under static or
dynamic test conditions, but is especially useful as it permits
measurements in many cases in the environment of use.
~2~
In the case of bending components taking several readings about an object
will allow these to be isolated. In the case of a uniform force such as
compression or tension, a load cell placed on another object providing or
supporting the specific forc~J such as the load bearingssupporting a
rotating shaft, can be used for isolation purposes. In this way it is
possible to measure torque, total force on the component, tension or
compression, bending, or any combination of these elements.
In a first broad aspect this invention seeks to provide a method for
measuring forces including a torsional force applied to an object, such as
a shaft transmitting a power load, contained by a housing which comprises
in combination,
(i) fixedly applying to the object at least one patch of a
birefringent photelastic material of known photoelastic properties at
at least one preselected locus on the object;
(ii) applying the forces to the object thereby causing strain in both
the object and the at least one patch;
(iii)illuminating at least a portion of the strained at least one
patch with a plurality of separately located single beam illumination
means;
(iv) observing changes related to torque induced strain in at least
some of the light reflected from the at least one strained patch
derived from more than one of the separately located illumination
means which has passed through the affixed birefringent photoelastic
material;
(v) correlating the observed changes in at least some of the light
reflected from the at least one patch with the known properties of
the birefringent photoelastic material by means of an optical
analyzer means together with a fringe counter means thereby providing
an electronic signal;
(vi) obtaining an electronic slgnal corresponding to at least the
applied torque; and
(vii)processing the obtained signal to provide a value for the
applied substantially torsional force.
In a more detailed embodiment, this invention seeks to provide a methodfor measuring forces including a torsional force applied to an object,
2a~$3~
such as a shaft transmitting a power load, contained by a housing which
comprises in comblnation,
(i) fixedly applying to the object at least one first patch of a
birefringent photoelastic material of known photoelastic properties;
(ii) fixedly applying to the housing at least one second patch of a
birefringent photoelastic material of known photoelastic properties;
(iii)applying forces including a torsional force to the object;
(iv) illuminating at least a portion of the strained at least one
first patch with a plurality of separately located suitable single
beam illumination means;
(v) illuminating at least a portion of the strained at least one
second patch with at least one suitable single beam illumination
means;
(vi) observing changes related to strains induced by the applied
forces in at least some of the light reflected from at least one
first patch and at least one second patch which has passed through
the affixed birefringent photoelastic material;
(vii)correlating the observed changes in at least some of the light
reflected with the known properties of the birefringent photoelastic
material by means of an optical analyæer means together with a fringe
counter means to provide an electronic signal corresponding to the
applied forces induced strains; and
(viii)processing the obtained signal to provide a value for the
applied forces.
In a second more detailed embodiment this invention seeks to provide a
method for measuring forces including a torsional force applied to an
object, such as a shaft transmitting a power load, contained by a housing
which comprises in combination,
(i) fixedly applying to the object at least one patch of a
birefringent photelastic material of known photoelastic properties at
at least one preselected locus on the object comprising a strip
around a preselected locus of the object;
(ii) applying the forces to the object thereby causing strain in both
the object and the at least one patch;
(iii)illuminating at least a portion of the strained at least one
patch with a plurality of separately located single beam illumination
means;
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~2~32
(iv) observing changes related to torque induced strain in at least
some of the light reflected from the at least one strained patch
derived from more than one of the separately located illumination
means which has passed through the affixed birefringent photoelastic
material;
(v) correlating the observed changes in at least some of the light
reflected from the at least one patch with the known properties of
the birefringent photoelastic material by means of an optical
analyzer means together with a fringe counter means thereby providing
an electronic signal,
(vi) obtaining an electronic signal corresponding to at least the
applied torque; and
(vii~processing the obtained signal to provide a value for the
applied substantially torsional force.
In a third more detailed embodiment, this invention seeks to provide a
method for measuring forces including a torsional force applied to an
object, such as a shaft transmitting a power load, contained by a housing
which comprises in combination,
(i) fixedly applying to the ob~ect at least one first patch of a
birefringent photoelastic material of known photoelastic properties
comprising a strip around a preselected locus of the object;
(ii) fixedly applying to the housing at least one second patch of a
birefringent photoelastic material of known photoelastic properties,
(iii)applying forces including a torsional force to the object,
(iv) illuminating at least a portion of the strained at least one
first patch with a plurality of separately spaced suitable single
beam illumination means;
(v) illuminating at least a portion of the strained at least one
second patch with at least one suitable single beam illumination
means,
(vi) observing changes related to strains induced by the applied
forces in at least some of the light reflected from at least one
first patch and at least one second patch which has passed through
the affixed birefringent photoelastic material;
(vii)correlating the observed changes related to torque induced
strain in at least some of the light reflected with the known
properties of the birefringent photoelastic material by means of an
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2~2~3~
optical analyzer means together with a fringe counter means to
provide an electronic signal corresponding to at least the torque
induced strai~; and
(viii)processing the obtained signal to provide a value for the
applied forces.
Preferably, the suitable illumination means provides circularly polarized
light.
Preferably the birefringent photoelastic material is a plastic to
facilitate molding and affixing.
Preferably, the birefringent photoelastic plastic material is fixedly
applied to the object by means of a reflective glue or cement such as an
epoxy glue.
Preferably, the method is used in an environment of dynamic use under
dynamic load.
Preferably~ the method is used in an environment of statlc use under
static load.
Preferably the method is used in test environments providing both static
and dynamic testing.
Preferably, the method is used to obtain values for both the applied
torque and other applied non-torsional forces.
In a second broad aspect this invention seeks to provide an apparatus for
measuring forces including a torsional force applied to an object, such as
a shaft transmitting a power load, within a housing comprising in
combination,
(i) at least one patch of birefringent photoelastic material with
known photoelastic properties adaptPd to be afEixed to the surface of
the object at a preselected locus;
(ii) an affixing means adapted to secure the patch to the object and
provide a reflective surface between the patch and the sur~ace of the
ob;ect;
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202~32
(iii)a plurality of separately located illumination means adapted to
illuminate at least some of the patch with suitable light;
(iv)a plurality of observation means adapted to receive a portion of
the light reflected through the birefringent phctoelastic material
derived from morP than one of the separately located illumination
means and pass it to the analyzer means;
(v) at least one analyzer means adapted to receive the at least some
light passed from the observation means and convert it into an input
signal adapted to act as the input into a processor means;
(vi) a processor means adapted to receive the input signal and
provide an appropriate output signal to the output means; and
(vii)an output means adapted to provide in a readable form an output
related to the observed value for the torqueO
In a more detailed embodiment this invention seeks to provide an apparatus
for measuring forces including a torsional force applied to an object,
such as a shaft transmitting a power load, within a housing comprising in
combination,
(i) at least one first patch of birefringent photoelastic material
with known photoelastic properties adapted to be affixed to the
surface of the object at a preselected locus;
(ii) at least one second patch of birefringent photoelastic material
with known photoelastic properties adapted to be affixed to the
surface of the housing at a preselected locus;
(iii)an affixing means adapted to secure the patches to the object
and provide a reflective surface between the first patch and the
suface of the object and between the second patch and the surface of
the housing;
(iv)a plurality of separately located illumination means adapted to
illuminate at least some of the at least one first patch w~th
suitable light;
(v)at least one illumination means adapted to illum{nate at least
some of the at least one second patch with suitable light;
(vi) a plurality of observation means adapted to receive a portion of
the light reflected through at least one first patch and at least one
second patch of birefringent photoelastic material derived from more
than one of the il~umination means and pass it to the analyzer means;
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2~2~32
(vii)at least one analyzer means adapted to receive the at least some
llght passed from the observation means and convert it into at least
one input signal adapted to act as the at least one input into a
processor means and relating to observed induced strain;
(viii)a processor means adapted to receive the at least one input
signal process said at least one signal in order to provide at least
one appropriate output signal relating to at least the torque induced
strain to the output means; and
(ix) an output means adapted to provide in a readable form an output
related to the observed value for the applied forces.
In a second more detailed embodiment this invention seeks to provide anapparatus for measuring forces including a torsional force applied to an
object, such as a shaft transmitting a power load, within a housing
comprising in combination~
(i~ at least one patch of birefringent photoelastic material with
known photoelastic properties generally in the form of a continuous
strip or ring adapted to be affixed to the surface periphery of the
object at a preselected locus;
(ii) an affixing means adapted to secure the patch to the objece and
provide a reflective surface between the patch and the suface of the
object;
(iii)a plurality of separately located illumination means adapted to
illuminate at least some of the patch with suitable light;
(iv)a plurality of observation means adapted to receive a portion of
the light reflected through the birefringent photoelastic material
derived from more than one of the separately located illumination
means and pass it to the analyzer means;
(v) at least one analyzer means adapted to receive the at least some
light passed from the observation means and convert it into an input
signal adapted to act as the input into a processor means;
~vi) a processor means adapted to receive the input signal and
provide an appropriate output signal to the output means; and
(vii)an output means adapted to provide in a readable form an output
related to the observed value for the torque.
In a third more detailed embodiment this invention seeks to provide an
apparatus for measuring forces including a torsional force applied to an
_ 9 _
20~32
object, such as a shaft transmitting a power load, within a housing
comprising in combination,
(i) at least one first patch of birefringent photoelastic material
with known photoelastic properties generally in ~he form of a
continuous strip or ring adapted to be affixed to the surface
periphery of the object at a preselected locus;
(ii) at least one second patch of birefringent photoelastic material
with known photoelastic properties adapted to be affixed to the
surface of the housing at a preselected locus;
~iii)an affixing means adapted to secure the patches to the object
and to the housing and provide a reflective surface between the first
patch and the suface of the object and the second patch and the
surface of the housing;
(iv)a plurality of separately located illumination means adapted to
illuminate at least some of the at least one first patch with
suitable light;
(v)at least one illumination means adapted to illuminate at least
some of the at least one second patch with suitable light;
(vi) a plurality of observation means adapted to receive a portion of
the light reflected through at least one first patch and at least one
second patch of birefringent photoelastic material derived from more
than one of the separately located illumination means and pass it to
the analyzer means;
(vii)at least one analyzer means adapted to receive the at least some
light passed from the observation means and convert it into at least
one input signal adapted to act as the at least one input into a
processor means and relating to observed induced strain;
(viii)a processor means adapted to receive the at least one input
signal process said at least one signal in order to provide at least
one appropriate output signal relating to at lea.st the torque induced
strain to the output means; and
(ix) an output means adapted to provide in a readable form an output
related to the observed value for the applied forces.
Preferably, the illumination means consists of circularly polarized light
source.
Preferably, the photoelastic material is a plastic.
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2~2'~,~3~
Preferably, the observation means includes fiber optic cables and suitable
optics.
Preferably, the processor means processes the input signal into a valuefor the applied torque.
Preferably, the analyzer means includes an optical analyzer/detector and a
fringe counter means.
Alternately the analyzer means includes a spectral analyzer means.
Preferably the output means includes a display means, a storage means, a
display/storage means, or both a display and a storage means, in which the
storage and display functions can be separate (such as in a personal
computer memory and a video display) or combined (such as a chart recorder
trace or a data logger).
The invention will now be described by way of reference to the attachedfigures in which:
FIGURE 1 represents a complete system overview in the form of a block
diagram;
FIGURE 2 represents a preferred embodiment of the invention.
FIGURE 3 represents an end on view of the shaft in the preferred
embodiment.
The invention in its broadest embodiment comprises at least one
illumination means 1, at least one patch of birefringen~ photoelastic
material 5 affixedly attached to the object to which the applied torque is
being measured, at least one observation means 3, at least one analyzer
means 7, at least one processor means 9, and at least one output means 10.
The light emitted by the light source 17 is polarized by a polarizer 2
before it reaches the birefringent photoelastic plastic material 5. Once
it has passed through the coating 5 it is reflected off the affixing means
6 back through the coating 5. An observation means 3 receives the
reflected light waves which contain a phase shift. It is analyzed by an
optical analyzer 4 and a fringe pattern is produced. The analyzer means
17 comprises a fringe counting analyzer means 7 and an analog to digital
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2~2'~3 ~
converter 8. The fringe counting analyzer means 7 calculates the fringe
order, and the offset from the last fringe boundary, and the analog to
digital converter 8 converts the results into an electronic signal which
will act as the input to the processor means 9.
In the processor, it is converted into a value for applied torque based on
the known properties of the test object and of the birefringent
photoelastic plastic material which can be passed to the output means 10.
The output means 10 may comprise a display means 11, a storage means 12,
and a feedback means 13.
A display means 11 can comprise a digital display, an LED device, a light,
a bell, a printer, a chart recorder, a meter, a buzzer, a television
display, or any other audio/visual display means.
A storage means 12 can comprise a printer, a chart recorder, a magneticdisk, an optical disk, a random access memory, an electronic memory, a
magnetic tape, a videotape, and any other suitable storage means.
A feedback means 13 may comprise any suitable control means having as at
least one input thereto a signal from or generated by or as a result of a
signal from or generated by the processor means 9.
The processor means 9 may comprise markings on a metering device, a
comparison means, a microprocessor devise, or any other means of rendering
the input signal readable.
In the preferred embodiment, shown as FIGURES 2 and 3, the invention
comprises a constant light source 17 and a perimeter of birefringent
photoelastic material 5 applied to the ob;ect, such as a shaft, with
reflective epoxy. Several fiber optic cables 14 are used to transmit the
polarized light from the one light source to the object allowing for the
other components to remain at a remote location. The observation means 3
comprises several other fiber optic cables 34 which transmit the reflected
light back to the remote location where it is analyzed by the optical
analyzer 4. The fringe patterns which result pass to the analyzer means
17. The values calculated at the fringe counting analyzer means 7 are
digitized by an A/D converter 8 in the analyzer means 17. A load cell 15
- 12 -
2 ~ 2 ~ 2
located on the bearlngs 16 provides a value for the applied tension ~o the
processing means 9. At least one processor means 9 receives all the data
and calculates the applied torque as well as any information desired by
the user or necessary for feedback. This can be accomplished through the
use of the several fiber optic cables illuminating the patch at specific
points such that each cable is of a pair a~ 90 degrees ~o each oth~r. The
desired information is transmitted to at least one output means 10.
13 -
