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 • Resolution – Beyond 4X • The Complete Motor System

Types of Angle Encoders • • Potentiometers • Magnetic encoders • Resolvers • Inductosyn

Optical encoders

Potentiometer • Electromechanical transducer • Available in single / multi revolution form.

• Limitations: Non linearity Wear and tear Sensitive to voltage variations

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

Vr Resolvers Vr sine O Vr cos O

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

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 ...

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.

Optical Encoder

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

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.

Limitation on Resolution

Increasing the Resolution

Exploded View of the Encoder

Light Source Collimation

Direction of Rotation

Encoder Assembly

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

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.

Multi-Turn • Linear systems normally need more than one turn of a shaft.

• Several single turn encoders are connected using a reduction gear

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

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)

Practical design considerations • Resolution – Window size limited to 5 x 10 -4 nearly 10,000 ppr inches, gives • 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

A B Quadrature Decoding A B Decoder up down

A B A B Decoding the phases Count up Count down

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

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

X

S2 S1

Noise Filtering Digital Filter Latch Latch Latch

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