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Birefringent Sensors for Motors
Dukki Chung, Francis L. MeratCase Western Reserve University
Fred M. Discenzo, James H. HarrisRockwell Automation
Presented at SPIE Photonics East �98: Metrology and Inspection � Three-Dimensional Imaging, Optical Metrology, and Inspection, Boston, November 1998.
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Introduction
� Strain Measurement
� strain gage� provides point-by-point data
� photoelasticity� provides point-by-point data or full-field data
� non-destructive measurement
� static and dynamic measurements
� higher bandwidth
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Photoelasticity
� Birefringent materials have the ability to resolve an impinginglight vector into two orthogonal circularly polarized componentswhich propagate with different velocities through the material.
� Transparent photoelastic materials such as some polymericplastics or glasses become birefringent when stress is applied.
� When linearly polarized light is transmitted through abirefringent plastic relative phase retardation will occur.
� The light exiting a birefringent plastic can be passed through alinear polarizer to convert the phase retardation into a two-dimensional intensity patterns.
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Optical Strain Analysis
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Photoelastic Strain Analysis
� primarily used to provide a qualitative analysis ofthe deformation or residual strain in a component
� typically needs human interpretation for properanalysis
� neural network image processing is proposed foranalyzing the fringe patterns of a shaft couplermade from birefringent plastic.
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Sensor Description
� polycarbonate plastic coupling with a high strain-opticalcoefficient
� coating of aluminum filled epoxy on the inner surface ofthe plastic coupling to reflect light back out
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Conceptual Optical Torque Sensor
PhotosensorArray
Neural NetworkTorque Estimator
EstimatedTorque
Polarized Light
PolarizationFilter
PhotoelasticPlastic
Coupling
LoadMotorShaft
DriveMotorShaft
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Neural Network
� nonlinear processing elements operating in parallel andarranged roughly similar to biological neural networks.
� processing elements (nodes) are connected via weights(synapses) that are typically adapted during training phase.
� incoming signals (stimulus) are multiplied by the weights,and summed at the processing elements, or nodes.
� can learn a mapping between the given inputs andcorresponding outputs through training samples.
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Neural Network
...
...
......
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Experimental Setup (Static Test)
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Typical Optical Torque Sensor Images
(a) 20 pound-inches (b) 40 pound-inches (c) 60 pound-inches
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Image Pre-Processing
(254,64)
15 32x32 virtualsensor cells
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Experimental Setup (Dynamic Test)
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Static Test Result
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70
Torque (lb-in)
To
rqu
e /
NN
Ou
tpu
t
0.0%
2.0%
4.0%
6.0%
8.0%
10.0%
12.0%
14.0%
Err
or
(%)
Target Training Output Test 1 Output Test 2 OutputTraining Error (%) Test 1 Error (%) Test 2 Error (%)
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Dynamic Test Result (Low Speed)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70
Torque (lb-inch)
NN
Ou
tpu
t
0%
10%
20%
30%
40%
50%
60%
Err
or
(%)
Target Training Output @ 400 rpm Te st Output @ 400 rpm LB
UB Training Er ror @ 400 rpm Te st Error @ 400 rpm
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Dynamic Test Result @ 900 RPM
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70
Torque (lb-inch)
NN
Ou
tpu
t
0%
5%
10%
15%
20%
25%
30%
35%
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Err
or
(%)
Target Training Output Test Output LB UB Training Error Test Error
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Dynamic Test Result @ 1500 RPM
0.3
0.4
0.5
0.6
0.7
0.8
0.9
30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70
Torque (lb-inch)
NN
Ou
tpu
t
0%
5%
10%
15%
20%
25%
30%
35%
40%
Err
or
(%)
Target Training Output Test Output LB UB Training Error Test Output
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Conclusions
� accurately measure static and dynamic shaft torque values
� static test results showed less than 1% error.
� dynamic test results showed 3.3% error at 900 rpm and3.5% error at 1500 rpm
� for these dynamic tests, the torque fluctuation from thetest apparatus was reflected to the results.
� use non-contacting optical sensing to produce a motortorque sensor
� replace CCD camera by a linear array of photodetectorswith a considerable reduction in system complexity