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BACKGROUND OF THE INVENTION
The present invention relates to apparatus for sensing the angular
displacement, the torsional displacement and speed of rotation of a shaft
or other object and, more particularly, to sensing the angular and/or
torsional displacement of a shaft under torque loading as an indication of
the applied torque, position and the speed of rotation of the shaft.
Various devices and methods are known for quantitatively determining the
angular and torsional displacement, or twist, of a shaft under load as an
indication of the applied torque. When torque is applied to a non-rotating
shaft, the torsional displacement can be simply measured as a function of
the relative angular displacement, or twist, of one end of the shaft
relative to the other. The measurement of angular and torsional
displacement is more complicated in dynamic situations where the shaft is
rotating at relatively high speeds. Dynamic situations requiring
calculation of the torsional displacement include, for example,
turbo-machines, such as aircraft gas turbines and other turbo-shaft
engines. In these devices, a compressor is located at the forward end of
the engine and is connected through a main shaft to a turbine at the rear
end of the engine. The torsional displacement of the main shaft is sensed
as an incident to measuring engine torque, and the quantitative result is
then available as an indication of engine power.
In traditional torque sensing systems for shafts, magnetic sensors are
positioned at opposite ends of the engine main shaft and respond to
respective toothed wheels secured to the shaft with each sensor providing
an electrical pulse output at a pulse repetition rate that varies with the
speed of rotation of the shaft. As the shaft is subjected to varying
levels of torque, one end of the shaft is torsionally displaced, or
twisted, relative to the other end. This causes a change in the phase
relationship between the pulse trains from the magnetic sensors.
Evaluation of the change in the phase relationship between the pulse
trains allows an accurate determination of applied torque and the pulse
repetition rate also allows an accurate determination of shaft speed.
In turbo-shaft engine applications, main shaft deflection and speed are
determine by a monopole torque sensor of the type disclosed in commonly
assigned Eichenlaub U.S. Pat. No. 4,602,515 and Parkinson U.S. Pat. No.
4,488,443. In the disclosed structure, two toothed wheels are positioned
adjacent one another on the engine shaft with one of the wheels secured to
the shaft and the other of the wheels secured to the end of a hollow
reference sleeve. The opposite end of the reference sleeve is connected to
the shaft so that torsional twisting of the shaft will cause a relative
rotational displacement between the two wheels. A single magnetic pick-up
provides an output signal representative of the relative position of the
wheels.
While magnetic sensing systems have been developed to a relatively reliable
state, they represent a comparatively expensive instrumentation system.
Since the sensing system operates in temperature ranges that vary from
ambient temperature at engine start-up to 1500.degree. F. or more, the
sensors must be designed with heat-resistant materials, and the electrical
response characteristics as a function of temperature must be known to
provide an accurate output for all operating temperatures. Additionally,
magnetic sensing systems are susceptible to electro-magnetic interference
(EMI) which can interfere with the correct output of the sensors. While
EMI can be reduced with shielding, this solution adds considerable weight
to the system and is particularly disadvantageous in airborne
applications.
In common with the above torsion and speed sensing apparatus, devices for
determining the angular displacement of a rotatably mounted shaft as an
incident to the torsional, speed, and angular position, are known as
shown, for example, in Emmaninegal U.S. Pat. No. 3,602,719 which discloses
an apparatus for measuring the angular position of an object relative to a
radiation source. A plane-parallel glass plate is mounted for rotation
along an axis parallel to the incident radiation. As the glass plate
rotates, the incident radiation beam is refracted in proportion to the
angle at which it strikes the glass plate to provide an indication of the
angular position of the glass plate and an object to which it is attached.
As can be appreciated from the above, various arrangements have addressed
the need to determine angular position, torsional displacement, and speed
in the context of a rotatably mounted object, such as a shaft, although
such arrangements have not fully utilized optical techniques to provide a
lightweight and cost effective system.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention, among
others, to provide a system for sensing the torsional displacement of a
shaft under dynamic loading that is more efficient, simpler, and less
expensive than prior systems.
It is another object of the present invention to provide a system for
sensing the torsional displacement of a shaft under dynamic conditions as
an incident to measuring the applied torque.
It is still another object of the present invention to provide a system for
sensing both torsional displacement of a shaft under dynamic loading as
well as shaft rotation speed.
It is a further object of the present invention to provide a system for
measuring the torsional displacement of a shaft that operates through a
wide temperature range and is relatively immune to electromagnetic
interference.
It is a further object of the present invention to provide a system for
measuring the angular displacement of an article, such as a shaft mounted
controller or the like.
In view of these objects, and others, the present invention provides, in
one form, for a retardation plate mounted to an axial face of the shaft
and irradiated by polarized light. Angular position, torsional deflection,
applied torque, and rotation speed are determined, in part, by the changes
in the retardation of the linearly polarized light, i.e., the zero degree
polarization component and the 90 degree polarization component, as a
function of torsional displacement of the shaft.
In one embodiment of the invention, a quarter-wave retardation plate and
underlying mirror are mounted to an axially aligned interior end wall of a
hollow shaft. An optical source, such as a light emitting or laser diode,
and cooperating polarizer provide linearly polarized light that is
directed to the retardation plate and mirror to irradiate the fast axis of
the quarter-wave retardation plate at a predefined angle .theta. under
no-load conditions. The light reflected from the mirror, having made two
passes through the quarter-wave retardation plate, is accordingly
retarded, i.e., the light includes both a zero degree and a 90 degree
polarization component compared to the initial, linearly polarized light.
When torsional displacement occurs under load conditions, the angle
.theta. of the linearly polarized light beam irradiates the quarter-wave
retardation plate changes and the magnitude of the polarization components
of the retarded light beam also change. Comparison and use of the
magnitudes of the no-load polarization components, viz., the zero degree
and the 90 degree polarization component, with the polarization component
values under load conditions allows calculation of the torsional
displacement and shaft rotation speed.
In another embodiment of the present invention, a first axially aligned end
wall of the shaft is provided with alternating reflective and
non-reflective areas and is irradiated by polarized light to produce
reflected light pulses. A polarizer is mounted to an axially aligned
interior end wall of the shaft at an end opposite the first end and is
irradiated by polarized light directed along the center of the shaft.
Torsional displacement of the shaft under load conditions produces
reflected light pulses when the transmission axis of the various
polarizers are parallel. Comparison of the periods between pulses obtained
from the front shaft end with the rear shaft end allows the calculation of
both the shaft speed and torsional displacement.
In still another embodiment of the present invention, a retardation plate
is attached to an article for which angular position information is
required. The retardation plate is mounted so that it is rotatable about
its fast axis and is positioned between two stationary linear polarizers
that have their transmission axis at a 45.degree. angle to the fast axis
of the retardation plate. Broadband light is irradiated through the first
polarizer, the retardation plate, and the second polarizer, and is then
detected. The angle at which the polarized light strikes the rotatable
retardation plate affects the wavelength of the light that is affected
most by the slow axis of the retardation plate. Comparison of the
wavelength having minimum intensity that passes through the second
polarizer allows the calculation of the angular displacement of the
retardation plate and its attached object.
The present invention advantageously provides a system for measuring
angular displacement of an article or the torsional displacement and speed
of rotation of a shaft using lightweight optical components compared to
prior systems.
Other objects and further scope of applicability of the present invention
will become apparent from the detailed description to follow, taken in
conjunction with the accompanying drawings, in which like parts are
designated by like reference characters.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 illustrates the first embodiment of the present invention;
FIG. 2 illustrates a second embodiment of the system presented in FIG. 1;
FIG. 3 illustrates a third embodiment of the present invention;
FIG. 4 illustrates a fourth embodiment of the present invention;
FIG. 5 illustrates a fifth embodiment of the present invention;
FIG. 5A is a front end view of a shaft presented in FIG. 5;
FIG. 6 illustrates a block diagram of the circuitry used to detect the
torsional displacement and speed of the rotating shaft in the third
embodiment of the present invention;
FIG. 7 illustrates a sixth embodiment of the present invention; and
FIG. 8 illustrate the output signal obtained from the sixth embodiment of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in FIG. 1, a preferred embodiment of a shaft torsional deflection
and speed sensing system in accordance with the present invention is
designated generally by the reference character 10. The system 10 is
designed to measure the torsional deflection and rotation speed w of a
shaft 28 mounted for rotation about a longitudinal axis A.sub.x. The shaft
28 is defined as a hollow cylinder about the axis A.sub.x and includes end
walls 32 and 35 at the forward and rearward ends, respectively, of the
shaft 28. In the various figures, the shaft 28 of portions thereof has
been shown in phantom outline to reveal interior components. The
cylindrical wall surface 29 of the shaft 28 is provided with four
equi-spaced windows 30 into which light is introduced and from which light
exits to provide the desired torsional deflection and rotational speed
information w as discussed more fully below.
A pyramidal-shaped, four-fold mirror 34 is secured to the forward end wall
32 along the axis of rotation A.sub.x so that light entering from the
windows 30 along a path 24 will be reflected by the corresponding inclined
reflecting surface of the mirror 34 in the general direction of the axis
A.sub.x toward the rear end wall 35 of the shaft 28. A flat reflector,
preferably in the form of a mirror 40, is secured to the rear end wall 35
with a quarter-wave retardation plate 38 secured to and overlying the
surface of the mirror 34. The four-fold mirror 34 and the quarter-wave
retardation plate 38 and its mirror 40 are aligned along the axis A.sub.x
so that light directed along the path 24 through one of the windows 30
will be reflected by the four-fold mirror 34 in the direction of and
through the quarter-wave retardation plate 38. Similarly, the light
reflected by the mirror 40 will be directed through the quarter-wave
retardation plate 38 to the four-fold mirror 34 for reflection through the
window 30 along the light path 50.
Light is introduced into the shaft 28 by an optical circuit which includes
an optical source 12, such as a laser diode, that directs light along a
path 14 through a linear polarizer 16. The light is preferably
monochromatic and in the 800 nm to 1500 nm wavelength range. In an
aircraft engine application, a wavelength of 800 nm is preferred, since
condensate materials, known as `coke`, that accumulate on surfaces within
the engine are acceptably transmissive to 800 nm energy. The linearly
polarized light is directed to and enters a beam splitter 20 with a
portion of the incident light transmitted through the beam splitter 20
along the light path 24 to the windows 30 of the shaft 28, another portion
reflected along a path 22 to a polarizing beam splitter 54, and another
portion reflected to a diffuser 25 and a reflector in the form of a mirror
26.
The linearly polarized light that is transmitted through the beam splitter
20 along the light path 24 passes through the windows 30 of the rotating
shaft 28 at an angle that is perpendicular to the axis A.sub.x of the
shaft 28. The light transmitted along light path 24 passes through one of
the windows 30 and strikes one of the four reflecting surfaces of the
four-fold mirror 34. The light is reflected at a right angle relative to
the light path 24 and is directed toward the surface of the quarter-wave
retardation plate 38 at the other end of the shaft 28. In FIG. 1, the
direction of polarization of the light travelling along the axis A.sub.x
is represented by the vibration direction 46.
The quarter-wave retardation plate 38 is mounted to the rear end wall 35 of
the shaft 28 so that its fast axis, represented at 42, is at some
predetermined angle .theta., when the shaft 28 is under no torsional load
and in its torsionally undeflected state, with respect to the vibration
direction 46 of the light after passing through one of the windows 30 and
reflection by the four-fold mirror 34, that is, any angle except
0.degree., 90.degree., 180.degree. or 270.degree.. The incident light from
the four-fold mirror 34 enters and passes through the quarter-wave
retardation plate 38 and is reflected by the mirror 40 to again pass
through the quarter-wave retardation plate 38 toward the four-fold mirror
34. The orientation of the quarter-wave retardation plate 38 provides
reflected light from the quarter-wave retardation plate 38 that is
retarded relative the incident light from the four-fold mirror 34. For
example, where the incident light provided along light path 24 has a
0.degree. polarization component only, the reflected and retarded light
from the quarter-wave retardation plate 38 will have both a 0.degree.
polarization component and a 90.degree. polarization component because of
the retardation effect of the quarter-wave retardation plate 38.
In the case where the incident light provided along the light path 24 is
linearly polarized and has a vibration direction 46 with only a 0.degree.
polarization component when it first enters the quarter-wave retardation
plate 38, and, depending upon the initial angle .theta. between the
vibration direction 46 and the fast axis 42 when the incident light from
light path 24 strikes the quarter-wave retardation plate 38, the reflected
and retarded light returned along light path 50 will be linearly polarized
and have a 0.degree. polarization component and a 90.degree. polarization
component having magnitudes that are a function of the angle .theta..
For example, if the angle .theta. between the vibration direction 46 of the
incident light from the four-fold mirror 34 and the fast axis 42 is equal
to 45.degree., the linearly polarized and retarded light reflected along
the light path 50 toward the four-fold mirror 38 will have equal 0.degree.
and 90.degree. polarization components. The 45.degree. angle .theta.
results in polarization components of equal magnitude since the
quarter-wave retardation plate 38 effectively functions as a half-wave
retardation plate. When the incident light provided along the light path
24 passes through quarter-wave retardation plate 38 twice because of the
mirror 40, the extraordinary component of the incident light along light
path 24 goes through a phase change of 180.degree. before it exits the
quarter-wave retardation plate 38 so that the reflected and retarded light
directed to the four-fold mirror 34 will have a 0.degree. polarization
component and a 90.degree. polarization component of equal magnitude. It
should also be noted that when the incident light along the light path 24
passes through a different one of the four windows 30, its vibration
direction 46 strikes the fast axis 42 at a different angle .theta..
However, the relative position of the vibration direction 46 of the
incident light provided along the light path 24 with respect to the fast
axis 42 is always an equivalent because of the equal circumferential
spacing of the windows 30 on the cylindrical surface 29 of the shaft 28.
Accordingly, the 0.degree. polarization component and the 90.degree.
polarization component will have equivalent magnitudes independent of the
window 30 through which the light directed along light path 24 enters.
After the incident has its polarization state changed, it is reflected by
the same reflecting surface of the four-fold mirror 34 that the incident
light along light path 24 passes through the same window 30 along the path
48 to the beam splitter 20. After being reflected by the beam splitter 20,
the retarded light beam 50 strikes the polarizing beam splitter 54 which
splits the retarded light beam 50 into a 90.degree. polarization component
beam along light path 56 and a 0.degree. polarization component beam along
light path 60. The 90.degree. polarization component beam 56 is detected
by a 90.degree. polarization component detector 58, and, in a similar
manner, the 0.degree. polarization component beam 60 is detected by a
0.degree. polarization component detector 62. The resulting signals are
provided to and processed by a signal processor 64, the function of which
is described more fully below.
When the shaft 28 is in its no-load, torsionally undeflected state, the
signal processor 64 senses and stores the initial values of the 90.degree.
polarization component and the 0.degree. polarization component for use in
computing the torsional deflection using the corresponding values when the
shaft 28 is in a loaded, torsionally deflected state. In addition, values
for the 0.degree. and 90.degree. polarization components for all angles
.theta. between the vibration direction 46 and the fast axis 42 are
likewise determined to correlate sensed 0.degree. and 90.degree.
polarization component values with the initial value to provide a
torsional deflection signal. While rotation of the shaft 28 causes the
linearly polarized light along the light path 24 to strike the
quarter-wave retardation plate 38 four times per revolution, the vibration
direction 46 of the incident light from light path 24 will strike the fast
axis 42 at the same relative angle .theta. every time for any given
torsional deflection.
When the shaft 28 is subjected to a torsional load, the forward and
rearward ends will be rotationally displaced relative to one another to
cause a corresponding relative rotation of the quarter-wave retardation
plate 38, a change in the orientation of the fast axis 42, and a change in
the angle .theta. between the fast axis 42 and the vibration direction 46.
Because of these differences, the values of the 0.degree. polarization
component and the 90.degree. polarization component of the retarded light
returned toward the four-fold mirror 34 from the quarter-wave retardation
plate 38 will also vary from the polarization component values under the
no-load, torsionally undeflected conditions.
The retarded light is reflected by the four-fold mirror 34 to the light
path 50 and presented to the beam splitter 20 and the polarization
splitter 54 and split into a 0.degree. polarization component along light
path 56 and a 90.degree. polarization component along light path 60. The
polarization component along light paths 56 and 60 are then detected by
the detectors 58 and 60, respectively, and the detected signals sent to
the signal processor 64.
The signal processor 64 uses the signal values to calculate the result of
the difference of these values divided by their sum, compare this result
with the predetermined values for all angles .theta. to determine the
detected angle .theta., and provide an output representative of the
angular difference .delta..theta. between the detected angle .theta. and
the angle .theta. sensed under the no-load, torsionally undeflected
conditions. This angular difference, .delta..theta., is the torsional
displacement between the front shaft end 32 and its rear shaft end 35. The
rotational shaft speed is determined by counting the frequency of the
pulses detected by either the detector 58 or the detector 60 per unit
time.
As an alternative to calculating each of the values of the 0.degree.
polarization component and the 90.degree. polarization component and
storing these values in signal processor 64, Mueller calculus can be used
to predict the torsional displacement for detected polarization component
values. The optical power P of the detected signal follows the equation:
P=K[1+cos 4(wt+.delta..theta.)].sup.2 Eq. 1
where
K is a scaling factor,
w is the shaft rotation rate,
.theta. is the torsional displacement, and
.delta..theta. is the change in torsional displacement.
Since the shaft speed w can be determined from the the pulses per unit time
from the detectors 58 and 62 and the scaling factor K and the optical
power P are known, the torsional displacement .theta. can be derived.
The embodiment of FIG. 1 uses a quarter-wave retardation plate 38 with a
mirror 40 because the configuration allows very accurate measurements to
be obtained. The calculation of the difference of the 90.degree.
polarization component and the 0.degree. polarization component divided by
the sum of these components compensates for fluctuations in the intensity
of the light provided by the optical source 12 and provides accurate and
repeatable results. If desired, the quarter-wave retardation plate 38 can
be replaced with a different type of the retardation plate that varies the
phase of the extraordinary ray of the incident light by a different
amount. The magnitudes of the 0.degree. polarization component and the
90.degree. polarization component of the resulting retardation of the
light under no-load conditions can still be predetermined for any angle
.theta. for the retardation plate used. These values can then be compared
to the values actually detected by the 0.degree. polarization component
detector 58 and the 90.degree. polarization component detector 62 to
determine the torsional angular displacement of the shaft 28, although the
retarded light provided by the retardation plate may be linearly,
elliptically, or circularly polarized. Also, the number of windows 30
provided in the shaft 28 is arbitrary, although a greater number of
windows 30 increases the accuracy of the resulting measurement.
A variation of the system of FIG. 1 is shown in FIG. 2 and designated
therein by reference character 10A with like parts designated by like
reference characters. A half-wave retardation plate 39 at the end wall 35
is utilized in a transmission configuration by which the light reflected
from the four-fold mirror 34 is directed through the half-wave retardation
plate 39 with the retarded light transmitted through the half-wave
retardation plate 39 along a light path 50' to the polarization beam
splitter 54 for analysis as described above. The end wall 35 is apertured
(unnumbered) as appropriate to allow light passing through the half-wave
retardation plate to pass along path 50'. As can be appreciated, the light
transmitted through the half-wave retardation plate 39 includes the
0.degree. polarization component and the 90.degree. polarization
component. After detection, the signal outputs of the detectors 58 and 62
are processed in the same way by the signal processor 64 to determine the
speed of the shaft 28 and its torsional displacement.
Another embodiment of the present invention is illustrated in FIG. 3 and is
designated generally therein by the reference character 10B and in which
like reference characters are used for like parts. In the embodiment of
FIG. 3, the windows 30 and the four-fold mirror 34 are not utilized. As
shown, the forward end wall 32 of the shaft 28 is provided with a slit
system 76, including orthogonally crossed slits 78 (represented in dotted
line). The slits 78 serve the same purpose as the equally spaced windows
30 of the embodiments of FIGS. 2 and 3 in allowing the incident light
along the light path 24 to strike the quarter-wave retardation plate 38 at
the same relative angle .theta.. It is noted that the light path is
parallel to but offset from the axis of rotation A.sub.x of the shaft 28.
A further embodiment of the present invention is illustrated in FIG. 4 and
designated by the reference character 10C. As shown, the shaft 28 includes
a first set of diametrically opposed windows 68 and a second set of
diametrically opposed windows 72 (of which only one is shown). A linear
polarizer 66 and the mirror 40 are attached to the rear end wall 35 of the
shaft 28 rather than the quarter-wave retardation plate 38 and mirror 40
of FIG. 1. The linear polarizer 66 is mounted on the rear shaft end 35 so
that when the incident light along light path 24 travels through either of
the two windows 68, the vibration direction 46 of the incident light is
parallel to the transmission axis 70 of the linear polarizer 66. However,
when the shaft 28 is rotated 90.degree. and the incident light passes
through the windows 72, the vibration direction 46 is perpendicular to the
transmission axis 70. When the shaft 28 is torsionally unloaded, the
resulting light along light path 74 will have its largest magnitude when
the incident light along light path 24 travels through the windows 68 and
will have a minimum magnitude, when the incident light from light path 24
travels through the windows 72. Measuring the magnitude of the resulting
light along path 74 for all angles .theta. between the transmission axis
70 and the vibration direction 46 allows for the determination of the
torque load on the shaft 28 since the torsional displacement causes a
different detected magnitude of the light intensity.
Another embodiment of the present invention is illustrated in FIGS. 5 and 6
and is designated generally by the reference character 10D with like parts
designated by like reference characters. In the embodiment of FIG. 5,
optical sensor heads, described below, direct light into and receive light
from the shaft 28. In addition, the shaft 28 is provided with a linear
polarizer 66 and underlying mirror 40 on the rear end wall 35 with the
forward end wall of the shaft 28 defined, as shown in FIG. 5A, by a light
chopper 110 in the form of an annular rim having diametrically opposed
non-reflective, i.e., light-absorbing, sectors alternated with reflective
sectors 114.
As shown, a light source 80 generates and introduces light into the core of
an optical fiber 84 and, in a similar manner, a light source 82 generates
and introduces light into the core of the optical fiber 86. The optical
sources 80 and 82 can take the form of light emitting or laser diodes. The
light from the optical fibers 84 and 86 is transferred to passive,
bi-directional optical couplers 88 and 90, respectively, which provide a
50/50 split ratio. The couplers 88 and 90 each have three ports arranged
so that 50% of the incoming light introduced through one port is
transmitted to each of the remaining two ports. A portion of the light
generated from the light source 80 passes through the coupler 88 into the
optical fiber 91 and travels to a sensor head 94, and, in a similar
manner, a portion of the light from the light source 82 passes through the
coupler 90 to the optical fiber 92 to the sensor head 96.
Both sensor heads 94 and 96 are similar and include a bulkhead connection
98, an optical fiber stub 100, and a lens 102. Each lens 102 is designed
with a focal length that focuses the return light back into its respective
optical fiber stub 100. The sensor head 94 also contains a linear
polarizer 104 having a center bandwidth that matches the center bandwidth
of the light emitted from the light source 80. In addition, the diameter
of the polarizer 104 matches the diameter of the lens 102 in the sensor
head 94.
The polarized light emitted from the polarizer 104 is transmitted along
light path 106 and passes through the interior of the shaft 28 in a
direction parallel to the longitudinal axis A.sub.x of shaft 28. The
polarized light irradiates and passes through the polarizer 66 and is
returned along light path 107 by the mirror 40 as twice-polarized light
that passes through the polarizer 104, is focused by the lens 102 into the
optical fiber stub 100, and passes through the bulkhead connection 98 and
the optical fiber 91 into the coupler 88. A portion of the twice-polarized
return light then travels through the optical fiber 118 and is detected by
the detector 120 which provides an output along circuit path 122 to the
signal processor 130.
The sensor head 96 directs light along light path 108 toward the forward
end of the shaft 28 (FIG. 5A) and irradiates the two non-reflective areas
112 and the two alternating reflective areas 114, the non-reflective and
reflective sectors subtending the same arc. When the light from the sensor
head 96 along light path 108 irradiates the reflective areas 114, a
portion is reflected back along light path 109 toward the sensor head 96,
is focused by the lens 102 into the optical fiber stub 100, and then
passes through the bulkhead connection 98 and the optical fiber 92 into
the coupler 90. A portion of the reflected light passes through the
optical fiber 124 and is detected by the detector 126 which provides an
output along circuit path 128 to the signal processor 130.
The detectors 120 and 126 convert the detected optical signals from the
twice-polarized return light along light path 107 and the reflected light
along light path 109 into electrical signals along circuit paths 122 and
128, respectively. Both of the electrical signals along circuit paths 122
and 128 are sent to the signal processor 130 so that the shaft speed and
torsional displacement of shaft 28 can be determined, as explained below
in relationship to FIG. 6.
The polarizer 66 is mounted on the rear end wall 35 of the shaft 28 so that
its transmission axis 70 is aligned with the transition between the
reflective areas 114 and the non-reflective areas 112. This orientation
allows for the detection of the twice-polarized light returned along light
path 107 during those times that light along light path 108 is being
absorbed by the non-reflective area 112 of the chopper 110. When light is
reflected along light path 109 by the reflective area 114 and is detected,
the transmission axis 103 of the polarizer 104 is perpendicular to the
transmission axis 70 of the polarizer 66. Accordingly, the twice-polarized
light returned along light path 107, having a magnitude of zero, is
extinguished to result in two pulses per revolution per channel occurring
at equally spaced intervals when the shaft 28 is rotating at a constant
speed.
The speed of rotation of the forward portion of the shaft 28 can be
determined by measuring the time between the pulses sensed by the detector
126. Similarly, the speed of rotation of the rearward portion of the shaft
28 can be determined using the time between pulses detected by the
detector 120. The torsional displacement of the forward portion of the
shaft 28 with respect to the rearward portion can be determined by
detecting the time difference between the pulses detected by the detector
120 and the detector 126. Dividing the time between these pulses by the
period of rotation and then multiplying by a scaling factor yields the
torsional displacement of the front and rear portions of the shaft 28.
The signal processor 130 is shown in functional block diagram form in FIG.
6, and, as shown, the electrical signals provided on the circuit paths 122
and 128, which correspond to the signals detected by the detectors 120 and
126, respectively, are each sent through respective amplifiers 132 and
132' into respective zero-crossing detectors 138 and 138'. The
zero-crossing detectors 138 and 138' transform the analog signals along
circuit paths 134 and 136 into digital signals along circuit paths 140 and
142 that are sent to respective pulse shapers 144 and 144'. Each pulse
shaper 144 and 144' produces a short pulse on the rising or leading edge
of the respective digital signal from the zero-crossing detectors 138 and
138'. Each pulse shaper 144 and 144' outputs a pulse with a fixed duration
sufficiently short so that a pulse on circuit path 146, obtained from the
digital signal on circuit path 140, does not overlap a pulse on circuit
path 148, obtained from the digital signal on the circuit path 142. The
pulse shapers 144 and 144' can be designed, if desired, to provide a pulse
on both the rising and falling edges of the digital signals on the circuit
paths 140 and 142 to double the number of measurements that can be
obtained in a single revolution of the shaft 28. The pulse output on the
circuit path 146 is sent to one input of an OR gate 150 and the set input
of a RS flip-flop 152. The pulse on circuit 148 path is sent to another
input of the OR gate 150 and the reset input of the RS flip-flop 152.
Whenever the OR gate 150 detects an input pulse, an output pulse is
provided along circuit path 151 to a counter 154, which receives its
timing signals from an oscillator 156, and determines the time that the
pulse was sent by the OR gate 150. A computer 158 reads and stores this
time, determines the time duration between successive pulses, and uses
these times to compute the shaft speed with the torsional displacement of
the shaft 28 presented on a display 160.
If a distinction between the speed of the forward end of the shaft 28 and
rearward end is desired, the output waveform along circuit path 153 from
the RS flip-flop 152 can be sent to the computer 158 to indicate whether
the computer 158 is reading a pulse originating from the detector 120 or
from the detector 126. Furthermore, overspeed and underspeed conditions of
the shaft 28 can be determined by comparing the detected shaft speed with
predefined upper and lower shaft speed limits.
FIG. 6 also includes the analog equivalent of the calculation circuitry
which normally is not included with the digital equivalent described
above. The analog circuitry requires the use of the R-S flip-flop 152 and
low-pass filters 162 and 166. Pulses along circuit paths 146 and 148,
which are provided to the set and reset inputs of the R-S flip-flop 152,
generate an output waveform along circuit path 153 that is proportional to
any phase changes between the pulses along circuit paths 146 and 148. The
output waveform along circuit path 153 is then sent to the low-pass filter
162 to convert the phase information of the output waveform along circuit
path 153 into an appropriate voltag | | |
