Sensors

Bryson CookJames Wyler

Hao Phan

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Outline• Optical Encoders: Theory and applications

–Types of encoders –Fundamental Components –Quadrature –Errors –Applications

• LVDT (Linear Variable Differential Transformer)–What is a LVDT –Types of LVDT –How do they work? –Applications

Bryson Cook

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What are Encoders• For our class, an encoder is a device that senses position or

orientation for use as a reference or active feedback to control position.– Most are either:

• Rotary: converts rotary position to an analog or electronic signal.

• Linear: converts linear position to an electronic signal.– Are also either absolute or incremental.

• Absolute gives the absolute position and knowledge of the previous position is not needed.

• Incremental encoders is more ambiguous and requires counting of cycles to determine absolute position.

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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

• 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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Rotary 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

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Binary and Gray Encoding

• In some devices, Binary Encoding is used to keep track of the various positions. The areas of the disk are named counting in binary.– This can cause problems since multiple bits can

change from one successive area to the next, such as in 011 to 100 all three bits change.

• Gray Encoding is a binary system where the adjacent areas only differ in one bit.

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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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*Note: Extremely simplified encoder 9

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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Quadrature

• Standard Encoder Track– Gives velocity and

position but not direction

• Quadrature Encoder Track– Gives velocity, position

AND direction

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Quadrature – How It Works

• Grey Encoding• Identical tracks

– Phase offset of 90º• Two sensors• Current state vs. next

state

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James Wyler

Quadrature – Rotary Encoders

• Examples of Quadrature Rotary Encoders

2 Bit Wheel 64 Bit Wheel

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Optical Encoder Errors

1. Quantization Error – Dependent on resolution of sensor

2. Assembly Error – Disk not positioned correctly with respect to sensor

3. Manufacturing Error – Tolerances of sensor positioning and code printing lead to inaccurate signals

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Optical Encoder Errors – Cont.

4. Structural Limitations – Loading on shaft or disk deformation

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

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

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James Wyler

Optical Encoder Applications

• Coordinate Measuring Machine (CMM)

• Digital Calipers• CNC Machining• Electric Motors• Robotics

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LVDT

• What is a LVDT • Types of LVDT • How do they work? • Applications

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James Wyler

What is a LVDT

• Linear Variable Differential Transformer• Electrical transformer used to measure linear

displacement

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James Wyler

Construction of LVDT

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

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James Wyler

Types of LVDT

• Power supply :– DC– AC

• Type of armature :– Free (Unguided)– Captive (Guided)– Spring-extended

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Hao Phan

Power supply : DC LVDT

• Easy to install• Signal conditioning easier• Can operate from dry cell batteries• High unit cost

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Hao Phan

Power supply : 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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Hao Phan

Armature : Free Core (Unguided)• Core is completely separable from the transducer

body• Well-suited for short-range applications • high speed applications (high-frequency vibration)

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Hao Phan

Captive Core (Guided)• Core is restrained and guided by a low-friction

assembly• Both static and dynamic applications• Long range applications• Preferred when misalignment may occur

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Hao Phan

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• Medium range applications

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Hao Phan

How do they work?

• An alternating current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary.

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Hao Phan

How do they work?

• The coils are connected in reverse series• The output voltage is the difference (differential)

between the two secondary voltages

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Hao Phan

Null Position

• When the core is in its central position, it is placed equal distance between the two secondary coils.

• Equal but opposite voltages are induced in these two coils, so the differential voltage output is zero.

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In Phase Voltage• Displacing the core to the left causes the first

secondary to be more strongly coupled to the primary than the second secondary.

• The higher voltage of the first secondary in relation to the second secondary causes an output voltage that is in phase with the primary voltage.

• The phase of the voltageindicates the direction of the displacement.

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Hao Phan

Out of Phase Voltage

• Displacing the core to the right causes the second secondary to be more strongly coupled to the primary than the first secondary.

• The greater voltage of the second secondary causes an output voltage to be out of phase with the primary voltage.

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Hao Phan

How do they work?

• The magnitude of the output voltage is proportional to the distance moved by the core, which is why the device is described as "linear".

• Note that the output is not linear as the core travels near the boundaries of its range.

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Hao Phan

LVDT Applications

• Crankshaft Balancing• Testing Soil Strength• Automated Part Inspection• Automotive Damper Velocity

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Hao Phan

References• http://www.macrosensors.com/lvdt_tutorial.html• http://zone.ni.com/devzone/cda/tut/p/id/3638#toc3• http://en.wikipedia.org/wiki/Linear_variable_differential_transformer• http://prototalk.net/forums/showthread.php?t=78\• http://www.transtekinc.com/support/applications/LVDT-applications.html• http://www.sensorsmag.com/sensors/position-presence-proximity/modern-lvdts-

new-applications-air-ground-and-sea-7508• http://www.macrosensors.com/lvdt_tutorial.html• http://zone.ni.com/devzone/cda/tut/p/id/3638#toc3• http://en.wikipedia.org/wiki/Linear_variable_differential_transformer• Sensors Lecture: Fall ME6405 2009• http://electricly.com/absolute-optical-encoders-rotary-encoders

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