Optical Encoders, LVDT

Rushi VyasXiaoyu Ding

Lei Yang

Outline

• Optical Encoders: Theory and applications– Fundamental Components– Theory– Types of optical encoders– Quadrature– Errors– Applications

Rushi Vyas

What are Encoders

• An accessory to a mechanical device that translates mechanical motion into a measurable electrical signal Digital or Analog (preferably digital).

• Optical Encoders– Use light & photosensors to produce digital code – Most popular type of encoder.

• Can be linear or rotary.

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Optical Encoders: Components

• Code Disk: Used to produce different light patterns on a photo detector assembly from a stationary light source.

• Code Disk: Determines the Optical Encoder type.

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Optical Encoders: Components

• Light source(s)– LEDs or IR LEDs provide light source.– Light is collimated using a lens to make the beams

parallel.• Photodetector(s)

– Either Photodiodes or Phototransistors.• Opaque disk (Code Disk)

– One or more “tracks” with slits to allow light to pass through.

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Optical Encoders: TheoryLED Code

DiskPhoto-sensor

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Optical Encoder Types• Incremental Encoders: Mechanical motion computed by

measuring consecutive “on” states. • Absolute Encoders: Digital data produced by code disk, which

carries position information.

Incremental Encoder code Disk

Absolute Encoder code Disk

Lab 3

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Standard Binary EncodingAngle Binary Decimal

0-45 000 0

45-90 001 1

90-135 010 2

135-180 011 3

180-225 100 4

225-270 101 5

270-315 110 6

315-360 111 7

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Problem with Binary Code

• One angle shift results in multiple bit changes.

• Example: 1 => 2– 001 (start at 1)– 000 (turn off bit 0)– 010 (turn on bit 1)

Angle Binary Decimal

0-45 000 0

45-90 001 1

90-135 010 2

135-180 011 3

180-225 100 4

225-270 101 5

270-315 110 6

315-360 111 7

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Gray EncodingAngle Binary Decimal

0-45 000 0

45-90 001 1

90-135 011 2

135-180 010 3

180-225 110 4

225-270 111 5

270-315 101 6

315-360 100 7

Notice only 1 bit has to be changed for all transitions.

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Quadrature

• ❖ Quadrature describes two signals 90° out of phase• ❖ Used to determine direction of measurement• ❖ Only two directions possible, A leads B or B leads

A

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QuadratureAn incremental rotary encoder, also known as a quadrature encoder or a relative rotary encoder, has two outputs called quadrature outputs that are 90 deg out of phase. Direction of rotation can be determined from output sequence.

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Encoder Resolution:

• Absolute Optical Encoder– Resolution = 360º/(2n)– n = number of encoder bits– Measures the rotational displacement that can be

measured per bit change.• Incremental Optical Encoder

– Resolution = 360/n– N = number of windows on code disk– Resolution can be increased by reading both rising and

falling edges ( ) and by using quadrature ( ).

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ExamplesNumber of bits on encoder code disk n = 3

Resolution = 360º/23 = 45º

Number of bits on encoder code disk n = 4

Resolution = 360º/24 = 22.5º

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Example:• What resolution absolute optical encoder is

needed to be able to measure rotational displacements of 1.5 degrees? – N = ?– Resolution = 1.5 degrees

For absolute optical encoder:

Resolution=360/2N =1.5 → N = 7.91 ≈ 8 bits

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Example:• What number of slits (windows) are needed

on the code disk of an incremental optical encoder to be able to measure rotational displacements of 1.5 degrees? – N = ?– Resolution = 1.5 degrees

For incremental optical encoder

Resolution=360/N =1.5 → N = 240 windows

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Optical Encoders: Reliability

• Encoder errors1.Quantization Error – Dependent on digital word

size.2.Assembly Error – Due to instability in rotational

motion of code disk3.Manufacturing tolerances – Code printing

accuracy, sensor position, and irregularities in signal generation.

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Optical Encoders: Reliability

4. Structural Limitations – Disk Deformation, physical loads on shaft.

5. Coupling Error – Gear backlash, belt slippage, etc…

6. Ambient Effects – Vibration, temperature, light noise, humidity, etc…

7. Diffraction of light: occurs due to edge of codes disk windows. Fixed in newer encoders by using mask and minimizing distance to photodetector.

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Applications• Primarily used in motors for

monitoring velocity and position.– Robotics– Conveyor belts– Locomotives: Automobiles,

planes..– Tachometers

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References• Kawasaki Industries Optical Encoders: www.khi.co.jp• Compumotors: www.compumotor.com• ME class notes: Dr. Kurfess, Georgia Tech• www.motioncontrol-info.com• Sensors: Fall 08. ME6405• Wikipedia• Computer Optical Products: http://www.opticalencoder.com/

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Linear Variable Differential Transformer （ LVDT）

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LVDT

• What is LVDT?• Construction of LVDT• How LVDT works• Support electronics of LVDT• Properties of LVDT• Types of LVDT• Applications of LVDT

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What is a LVDT

• Linear variable differential transformer• Electrical transformer measuring linear

displacement

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Construction of LVDT

• One Primary coil• Two symmetric secondary coils• Ferromagnetic core

•The primary coil is energized with a A.C.

•The two secondary coils are identical, symmetrically distributed.

•The two secondary coils are connected in opposition

Primary coil

Secondary coils

Ferromagnetic core

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Recall of conventional transformer

• Mutual induction• the secondary voltage proportional to the primary

voltage• The transformer core is fixed• Energy transferred is high

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How LVDT works• If the core is located midway

between S1 and S2

• Equal flux is coupled to each secondary.

• Voltage E1 and E2 are equal.• The differential voltage output, (E1 -

E2 ), is zero.• This core position is called null

point.

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How LVDT works• If the core is moved closer to

S1 than to S2

• More flux is coupled to S1 than S2 .

• The induced voltage E1 is increased while E2 is decreased.

• The differential voltage is (E1 - E2).

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How LVDT works• If the core is moved closer to

S2 than to S1

• More flux is coupled to S2 than to S1 .

• The induced E2 is increased as E1 is decreased.

• The differential voltage is (E2 - E1).

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How LVDT worksLei Yang

Support electronics of LVDT• LVDT signal conditioning equipment• Supplying excitation power for an LVDT • typically 3 V rms at 3 kHz • Converting AC output into DC signals with

directional information from the 180 degree output phase shift

External electronics

Self-contained electronics e.g. DC-LVDT

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Properties of LVDT• Friction-Free Operation• Infinite Resolution • Unlimited Mechanical Life• Single Axis Sensitivity • Environmentally Robust • Null Point Repeatability • Fast Dynamic Response • Absolute Output

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Types of LVDT

• DC LVDT• Signal conditioning easier• Can operate from dry cell batteries• High unit cost

• AC LVDT• Small size• Very accurate – Excellent resolution (0.1 µm)• Can operate with a wide temperature range• Lower unit cost

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Types of LVDT• Free core• Core is completely separable from the transducer body• Well-suited for short-range (1 to 50mm), high speed

applications (high-frequency vibration)

• Guided core• Core is restrained and guided by a low-friction assembly• Both static and dynamic applications• working range (up to 500mm)

• Spring-extended core• Core is restrained and guided by a low-friction assembly• Internal spring to continuously push the core to its fullest

possible extension• Best suited for static or slow-moving applications• Lower range than guided core(10 to 70mm)

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Example of commercial LVDT• SE-750 Series General Purpose Free Core Single-Ended DC-LVDT Position Sensors

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Applications of LVDT

• For power generation1. Conditioning valves for large and medium steam turbines. 2. Reheat and stop valves for large and medium steam

turbines. 3. Feed water boiler pump valve positioning. 4. Natural gas fuel valve position for gas turbines for throttle

control. 5. Monitoring hydraulic fluid level in reservoir of feed water

pumps in nuclear reactor core.

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Applications of LVDT

• For manufacturing1. Measuring final height placement for automotive wheel

trim 2. Measuring injector height for diesel engines Feed water

boiler pump valve positioning. 3. Thickness measuring in multiple locations of fly-wheel to

insure balance. 4. Controlling depth of hole during machining operations in a

rotary transfer machine. 5. Providing indication and feedback position of rocket engine

nozzle actuators during testing.

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Other Applications

• Automation Machinery• Civil / Structural Engineering• Metal Stamping / Forming • OEM• Pulp and Paper• Industrial Valves• R&D and Test• Automotive Racing

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References• http://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/

index.html#automation• http://en.wikipedia.org/wiki/Linear_variable_differential_transformer• http://www.rdpe.com/displacement/lvdt/lvdt-principles.htm• http://www.directindustry.com/industrial-manufacturer/lvdt-73930.html• http://www.macrosensors.com/lvdt_macro_sensors/lvdt_products/

lvdt_position_sensors/dc_lvdt/free_core_dc/se_750_single_ended.html• Alexandre Lenoble’s lecture

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Thank you!

Lei Yang