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A Term Paper Presentation on (Optical) Encoders

A Term Paper Presentation on (Optical) Encoders. P.P.Ouseph Sanjeeb Kr. Ghosh Alok Kr. Pugalia Vikas Agarwal. Outline. Types of Encoders Optical Encoder Types of Optical Encoder Basic Structure Implementation Issues & solution Quadrature Decoding – Techniques & Issues

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A Term Paper Presentation on (Optical) Encoders

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  1. A Term PaperPresentation on(Optical) Encoders P.P.Ouseph Sanjeeb Kr. Ghosh Alok Kr. Pugalia Vikas Agarwal

  2. Outline • Types of Encoders • Optical Encoder • Types of Optical Encoder • Basic Structure • Implementation Issues & solution • Quadrature Decoding – Techniques & Issues • Resolution – Beyond 4X • The Complete Motor System

  3. Types of Angle Encoders • Potentiometers • Magnetic encoders • Resolvers • Inductosyn • Optical encoders

  4. Potentiometer • Electromechanical transducer • Available in single / multi revolution form. • Limitations: Non linearity Wear and tear Sensitive to voltage variations

  5. Toothed wheel Magnetic encoders • Consists of metallic toothed wheel • Magnetic pick up permanent magnet pole piece and coil • On rotation, discontinuity in the wheel changes the magnetic field - induces voltage in the pickup coil. pickup

  6. Vr sine O Resolvers Vr Vr cos O

  7. Inductosyn • Like Resolver • It has a rotor and stator • No Cu wire windings • PCB pattern rotor sine cosine

  8. Inductosyn advantages • High angular accuracy : 2 arc second • Can withstand wide temperature range, hard vacuum and pressure. • No wear and tear, Light weight • Multi speed 128(2.8 deg), 256, 360 ...

  9. Inductosyn Limitations • Needs slip rings to excite the rotor • Mechanical assembly is very critical (50 microns air gap) • Low transformation ratio - 250 :1 • Shielding, Excitation signal coupling in the harness and in PCB level are very critical that increases the position error.

  10. Optical Encoder

  11. Types of Optical Encoder Absolute Encoders Provides distinct digital code for each particular LS increment of resolution Incremental Encoders Provides a pulse for each increment of resolution but doesn’t distinguish which increment

  12. Incremental Encoders – Basic Parts • 4 basic parts a. light source b. a rotating disk (rotor or commutator) c. a stationary mask (stator) d. a sensor • The disk has alternate opaque and transparent sectors. • The mask is used for increasing resolution.

  13. Limitation on Resolution

  14. Increasing the Resolution

  15. Exploded View of the Encoder

  16. Light Source Collimation

  17. Direction of Rotation

  18. Encoder Assembly

  19. Binary Encoder Gray Encoder Absolute Encoder The most used configurations are: • A: Gray-code • B: Binary code

  20. Single Turn encoder. • Specifies the absolute position for one turn of the shaft (360°). • After one turn the measuring range is completed and starts again from the beginning.

  21. Multi-Turn • Linear systems normally need more than one turn of a shaft. • Several single turn encoders are connected using a reduction gear

  22. Light sources • Incandescent lamp • Failure due to shock/vibration/over-voltage • LED • Extremely shock/vibration proof • Operating spectrum has the peak sensitivity of sensors.

  23. Sensors • Photodiode • Operated in reverse-voltage • High speed response • Low signal level • Small sensitive area (~20*20 mils) • Phototransistor • Medium to high speed response • High signal level • Wide variation in output level • Small sensitive area (~20*20 mils) • Photovoltaic cell • Produces voltage proportional to incident light intensity • Wide spectral response & Good sensitivity uniformity • Small sensitive area (~20*20 mils)

  24. Practical design considerations • Resolution • Window size limited to 5 x 10-4 inches, gives nearly 10,000 ppr • Accuracy/Frequency modulation • Angular accuracies of 2.5 minutes of arc are typical • Frequency modulation of less than 1 % p-p • Inertia • Of the order of 10-5 oz-in-sec -2 • Environmental variables • Operating temperature of 0-50 deg C and storage temp of –20-85 deg C

  25. A B A up Decoder B down Quadrature Decoding

  26. A B Count up A B Count down Decoding the phases

  27. Clk A B S0 S2 • Same states for down counting S1 S3 State Machine – 4X decoding

  28. A B X Count up S2 S0 S3 S1 A Count down S3 S2 R S0 S1 B State Diagram

  29. Digital Filter Latch Latch Latch Noise Filtering

  30. Up Counter O/p Buffer A 4X Decoder Digital Filter B Down Counter O/p Buffer Clk The Complete Decoder Interface

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