Chris Davidson Ari Kapusta Optical Encoders and Linear Variable Differential Transformers Overview Optical Encoders What is an optical e ncoder Types of o ptical encoders Components ID: 274430
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Slide1
Sensors
Chris DavidsonAri Kapusta
Optical Encoders and
Linear Variable Differential
TransformersSlide2
Overview
Optical EncodersWhat is an optical encoder
Types of
o
ptical encoders
Components
How do they work?
LVDT (Linear Variable Differential Transformer)
– What
is a LVDT
– Types
of LVDT
– How
do they work?
– ApplicationsSlide3
Optical Encoders
An electro-mechanical device that senses angular (or linear) position or motionSlide4
Types of Encoders
Rotary: Converts rotational position/velocity to an analog or digital signalLinear: Converts linear position/velocity to an analog or digital signal
Absolute: Gives absolute position and knowledge of the previous position is not needed
Incremental: Measures displacement relative to a reference pointSlide5
Fundamental Components
Light Source(s): Light provided by LED and focused through a lensPhotosensor(s): Photodiode or Phototransistor used to detached lightOpaque Disk (Code Disk): One or more tracks with slits to allow light to pass through them
Masking Disk: Stationary track(s) that are identical to the Code Disk. Slide6
Fundamental ComponentsSlide7
Absolute Encoders
AdvantagesA missed reading does not affect the next readingOnly needs power when on when taking a reading
Disadvantages
More expensive/complexSlide8
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
1015270-3151106315-3601117
Note: Simplified Encoder (3 Bit)Slide9
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
1115270-3151016315-3601007
Notice only 1 bit has to be changed for all transitions. Slide10
Gray Code to Binary Code
1.) Copy MSB2.) Perform XOR (Exclusive OR) between bi and g
i+1
3.) RepeatSlide11
Example: Gray Code to Binary Code
Convert the gray code value of 0101 to binary code.
Solution: 0110Slide12
Incremental Encoders
AdvantagesCost
Disadvantages
Must be “zeroed” at a reference location for each startupSlide13
Incremental DiskSlide14
Incremental DiskSlide15
Quadrature
Quadrature describes two signals 90° out of phase
Used to determine direction of measurement
Only two possible directions: A leads B or B leads A
Provides up to 4 times the resolutionSlide16
Encoder Resolution
Absolute Optical Encoder
Slide17
Encoder Resolution
Incremental Optical Encoder
If we read the rising edge of Channel A
and
Channel B,
If we read the
falling
edge
of Channel
A
and
Channel B
,
If we
read the
rising
edge
and
the falling edge of
Channel A
and Channel B, Slide18
Example: Resolution
What is the best resolution you can get on an incremental encoder with 500 windows per channel?
By counting both
rising and falling edges
of both channels,
Slide19
Error and Reliability
Quantization Error – Dependent on sensor resolutionAssembly Error – Dependent on eccentricity of rotation
(Is
track center of
rotation the same as the center
of rotation of
disk)
Manufacturing
Tolerances
– Code printing accuracy, sensor position, and irregularities in signal
generation
Mechanical
Limitations – Disk
deformation
, physical loads on
shaft, rotation
speed (bearings
)
Coupling Error – Gear backlash, belt slippage, etc…
Ambient Effects – Vibration, temperature, light noise, humidity,
dust, etc…Slide20
Applications
Old Computer MiceSpeed Feedback from Motor/GearboxAngular Position of Robotic ArmSlide21
LVDT (Linear Variable Differential Transformer
)
Presenter: Ari
Kapusta
What is a LVDT
How do they work?
Types of LVDT
ApplicationsSlide22
What is a LVDT
Linear Variable Differential TransformerElectrical transformer used to measure linear displacementSlide23
Construction of LVDT
One Primary coilTwo symmetric secondary coils
Ferromagnetic core
The primary coil is energized with a A.C.
The two secondary coils are identical, symmetrically distributed
.
Primary coil
Secondary coils
Ferromagnetic core Slide24
How LVDT works
A current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary.Slide25
How LVDT works
The coils are connected in reverse seriesThe output voltage is the difference (differential) between the two secondary voltagesSlide26
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 t
he differential voltage output is zero.Slide27
Moving Core Left
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). Slide28
Moving Core Right
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).Slide29
Output
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.Slide30
LVDT Types
- Distinction by :
- Power supply :
- DC
- AC
Type of armature :
- Unguided
- Captive (guided)
- Spring-extendedSlide31
Power supply : DC LVDT
Easy to installSignal conditioning easierCan operate from dry cell batteriesHigh unit costSlide32
Power supply : AC LVDT
Small sizeVery accurate –Excellent resolution (0.1 μm)Can operate with a wide temperature rangeLower unit costSlide33
Armature : Free Core (Unguided)
Core is completely separable from the transducer body
Well-suited for short-range applications
high speed applications (high-frequency vibration)Slide34
Captive Core (Guided)
Core is restrained and guided by a low-friction assemblyBoth static and dynamic applications
Long range applications
Preferred when misalignment may occurSlide35
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
applicationsSlide36
LVDT Applications
LVDT measures absolute position. Can be sealed against environment.LVDT is useful for any application where you want linear position.
Machine tool position
Robot arm position
Forklift/hydraulic actuator position
Often used for position feedbackSlide37
LVDT Applications
Crankshaft BalancingTesting Soil StrengthAutomated Part InspectionAutomotive Damper VelocitySlide38
Questions?
Presenter of Optical Encoder: Chris DavidsonPresenter of LVDT: Ari KapustaSlide39
References
http://www.macrosensors.com/lvdt_tutorial.htmlhttp://zone.ni.com/devzone/cda/tut/p/id/3638#toc3http://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