Optical Encoders, LVDT
Rushi Vyas
Xiaoyu Ding
Lei Yang
Rushi Vyas
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.
Rushi Vyas
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.
Rushi Vyas
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.
Rushi Vyas
Optical Encoders: Theory
LED
Code
Disk
Photosensor
Rushi Vyas
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
Rushi Vyas
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
7
Rushi Vyas
Problem with Binary Code
• One angle shift
results in multiple bit
changes.
• Example: 1 => 2
– 001
– 000
– 010
(start at 1)
(turn off bit 0)
(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
Rushi Vyas
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
7
Notice only 1 bit has to be
changed for all transitions.
Rushi Vyas
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
Rushi Vyas
Quadrature
An 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.
Rushi Vyas
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
(
).
Rushi Vyas
Examples
Number 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
Rushi Vyas
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
Rushi Vyas
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
Rushi Vyas
Optical Encoders: Reliability
• Encoder errors
1.Quantization Error – Dependent on digital word
size.
2.Assembly Error – Due to instability in rotational
motion of code disk
3.Manufacturing tolerances – Code printing
accuracy, sensor position, and irregularities in
signal generation.
Rushi Vyas
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.
Rushi Vyas
Applications
• Primarily used in motors for
monitoring velocity and position.
– Robotics
– Conveyor belts
– Locomotives: Automobiles,
planes..
– Tachometers
Rushi Vyas
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/
Linear Variable Differential
Transformer（LVDT）
Lei Yang
Lei Yang
LVDT
•
•
•
•
•
•
•
What is LVDT?
Construction of LVDT
How LVDT works
Support electronics of LVDT
Properties of LVDT
Types of LVDT
Applications of LVDT
Lei Yang
What is a LVDT
• Linear variable differential transformer
• Electrical transformer measuring linear
displacement
Lei Yang
Construction of LVDT
• One Primary coil
• Two symmetric secondary coils
• Ferromagnetic core
Primary coil
•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
Ferromagnetic
core
Secondary
coils
Lei Yang
Recall of conventional transformer
• Mutual induction
• the secondary voltage proportional to the primary
voltage
• The transformer core is fixed
• Energy transferred is high
Lei Yang
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.
Lei Yang
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).
Lei Yang
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).
Lei Yang
How LVDT works
Lei 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
Lei Yang
Properties of LVDT
•
•
•
•
•
•
•
•
Friction-Free Operation
Infinite Resolution
Unlimited Mechanical Life
Single Axis Sensitivity
Environmentally Robust
Null Point Repeatability
Fast Dynamic Response
Absolute Output
Lei Yang
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
Lei Yang
• Free core
•
•
Types of LVDT
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)
Lei Yang
Example of commercial LVDT
• SE-750 Series General Purpose Free Core Single-Ended DC-LVDT Position Sensors
Lei Yang
Applications of LVDT
•
For power generation
1.
2.
Conditioning valves for large and medium steam turbines.
Reheat and stop valves for large and medium steam
turbines.
Feed water boiler pump valve positioning.
Natural gas fuel valve position for gas turbines for throttle
control.
Monitoring hydraulic fluid level in reservoir of feed water
pumps in nuclear reactor core.
3.
4.
5.
Lei Yang
Applications of LVDT
•
For manufacturing
1.
Measuring final height placement for automotive wheel
trim
Measuring injector height for diesel engines Feed water
boiler pump valve positioning.
Thickness measuring in multiple locations of fly-wheel to
insure balance.
Controlling depth of hole during machining operations in a
rotary transfer machine.
Providing indication and feedback position of rocket engine
nozzle actuators during testing.
2.
3.
4.
5.
Lei Yang
Other Applications
•
•
•
•
•
•
•
•
Automation Machinery
Civil / Structural Engineering
Metal Stamping / Forming
OEM
Pulp and Paper
Industrial Valves
R&D and Test
Automotive Racing
Lei Yang
References
• http://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/index.h
tml#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_p
osition_sensors/dc_lvdt/free_core_dc/se_750_single_ended.html
• Alexandre Lenoble’s lecture
Lei Yang
Thank you!