Sensors
Bryson Cook
James Wyler
Hao Phan
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Bryson Cook
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
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Bryson Cook
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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Bryson Cook
Optical Encoders
• Use light & photosensors to produce digital code
• Most popular type of encoder.
• Can be linear or rotary.
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Bryson Cook
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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Bryson Cook
Optical Encoders: Theory
LED
Code
Disk
Photosensor
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Bryson Cook
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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Bryson Cook
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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Bryson Cook
Standard Binary Encoding
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
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*Note: Extremely simplified encoder
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Bryson Cook
Gray Encoding
Angle
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
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Notice only 1 bit has to be
changed for all transitions.
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James Wyler
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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James Wyler
Quadrature
• Standard Encoder Track
– Gives velocity and
position but not
direction
• Quadrature Encoder
Track
– Gives velocity, position
AND direction
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James Wyler
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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James Wyler
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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James Wyler
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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James Wyler
LVDT
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•
•
•
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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Hao Phan
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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Hao Phan
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 voltage
indicates 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
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•
•
•
Crankshaft Balancing
Testing Soil Strength
Automated Part Inspection
Automotive Damper Velocity
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References
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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-lvdtsnew-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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