OSU ECE 5463 - Position-Sensing (1) (10 pages)

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Position-Sensing (1)



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Position-Sensing (1)

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Pages:
10
School:
Ohio State University
Course:
Ece 5463 - Introduction to Real Time Robotics Systems
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Position Sensing Take class notes Type of Position Sensing There are five fundamental types of position sensing elements that are used in robots Potentiometers Linear Variable Differential Transformer LVDT Resolvers Absolute Optical Encoder Incremental Optical Encoder Potentiometers There are two types linear and rotary The latter is more often Use mechanically variable resistors Material wire wound thin films of carbon metal or other conductive materials Linear 0 1 accuracy Rotary cannot turn beyond 360 in fact it is often less space occupied by contacts Alternate 5 to 10 turn helix or with gearing which converts linear motion to rotary motion Provide absolute position when power is off Low cost 5 25 Life is less than non contacting sensor LVDT Linear Variable Differential Transformer LVDT Made up of 3 coils AC supplied and DC out DC linear function of displacement of core coupling changes Note no voltage when core is symmetrically placed Good no contacting parts bad more expensive Keep absolute position even with power off Resolver AC input amplitude of sine and cosine outputs vary with the angle Complex electronics including analog and digital resolve digital Special IC circuits are available for the purpose Analog Devices Variable Resolver to Digital converters AD2S81A AD2S82A 12 bit resolution 1 in 4096 No problem with power off 0 1 accuracy Brushless resolvers eliminates arc and wear 2 fixed coils 90o apart stator Rotor coil produces The voltages of the stator coils and Absolute Optical Encoders Multiple concentric rings of binary code are photographed etched on the code wheel radical line angle absolute value of the position No problem with power off very accurate 10 20 rings 210 to 220 resolutions Photolithography expensive Start at 300 Need N light sources and N detectors N is the number of rings Gray code could be used leads to 1 bit change at a time more accurate Consider moving from 0111 1000 Only increased by 1 but all the bits changed A3 Using Gray code we can have 0111 0101 Motion Only one bit change A2 A1 A0 Incremental Shaft Encoders Installed on the motor shaft Simpler and less expensive Two photo cells sense direction individual steps Two track structure with slots Each slot width 2 sensor photo cell width Incremental Shaft Encoders 2 One track structure 2 photo cells sensors slot width displaced usually by 1 slot V A V B In general small gear ratio between joint and encoder high positional accuracy for reasonable encoder size Visual Sensors Visual position sensors have been used in robotics for a long time Most of the visual sensors such as a cameras are used for navigating a mobile robot low precision in a few centimeters Machine vision is a term saved for robots and factory automation high precision up to fractions of a millimeter provide imaging based automatic inspection provide robot guidance in industry vision based feedback control Vision based Control Robotic system Input PID control Camera What about Velocity Sensing Several approaches Tachometer a small DC generator Use time between encoder pulses Back EMF of driving motor Derivative of position signal


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