Sensors
Optical Encoders and Linear Variable Differential Transformers
Chris Davidson
Ari Kapusta
Overview
• Optical Encoders
– What is an optical encoder
– Types of optical encoders
– Components
– How do they work?
• LVDT (Linear Variable Differential Transformer)
– What is a LVDT
– Types of LVDT
– How do they work?
– Applications
Optical Encoders
• An electro-mechanical device that senses
angular (or linear) position or motion
Types of Encoders
• Rotary: Converts rotational position/velocity to
an analog or digital signal
• Linear: 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 point
Fundamental Components
• Light Source(s): Light provided by LED and
focused through a lens
• Photosensor(s): Photodiode or
Phototransistor used to detached light
• Opaque 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.
Fundamental Components
Absolute Encoders
Advantages
Disadvantages
• A missed reading does
• More expensive/complex
not affect the next
reading
• Only needs power when
on when taking a
reading
Binary Encoding
Note: Simplified Encoder (3 Bit)
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
Gray Encoding
Notice only 1 bit has to be changed for all
transitions.
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
Gray Code to Binary Code
1.) Copy MSB
2.) Perform XOR (Exclusive OR) between bi and gi+1
3.) Repeat
Example: Gray Code to Binary Code
Convert the gray code value of 0101 to binary
code.
Solution: 0110
Incremental Encoders
Advantages
• Cost
Disadvantages
• Must be “zeroed” at a
reference location for
each startup
Incremental Disk
Incremental Disk
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 resolution
Encoder Resolution
• Absolute Optical Encoder
360°
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = 𝑛
2
𝑛 = Number of Encoder Bits
Encoder Resolution
• Incremental Optical Encoder
360°
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =
𝑛
𝑛 = Number of Windows on Code Disk per Channel
If we read the rising edge of Channel A and Channel B,
360°
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =
2𝑛
If we read the falling edge of Channel A and Channel B,
360°
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =
2𝑛
If we read the rising edge and the falling edge of Channel A and Channel B,
360°
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =
4𝑛
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,
360°
360°
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 =
=
= .18°
4𝑛
4 ∗ 500
Error and Reliability
• Quantization Error – Dependent on sensor resolution
• Assembly 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…
Applications
• Old Computer Mice
• Speed Feedback from Motor/Gearbox
• Angular Position of Robotic Arm
LVDT (Linear Variable Differential
Transformer)
Presenter: Ari Kapusta
•
•
•
•
What is a LVDT
How do they work?
Types of LVDT
Applications
What is a LVDT
• Linear Variable Differential Transformer
• Electrical transformer used to measure linear
displacement
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.
Ferromagnetic
core
Secondary
coils
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.
How LVDT works
• The coils are connected in reverse series
• The output voltage is the difference (differential)
between the two secondary voltages
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.
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).
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).
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.
LVDT Types
- Distinction by :
- Power supply :
- DC
- AC
- Type of armature :
- Unguided
- Captive (guided)
- Spring-extended
Power supply : DC LVDT
•
•
•
•
Easy to install
Signal conditioning easier
Can operate from dry cell batteries
High unit cost
Power supply : AC LVDT
•
•
•
•
Small size
Very accurate –Excellent resolution (0.1 μm)
Can operate with a wide temperature range
Lower unit cost
Armature : Free Core (Unguided)
• Core is completely separable from the transducer
body
• Well-suited for short-range applications
• high speed applications (high-frequency vibration)
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
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
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 feedback
LVDT Applications
•
•
•
•
Crankshaft Balancing
Testing Soil Strength
Automated Part Inspection
Automotive Damper Velocity
Questions?
Presenter of Optical Encoder: Chris Davidson
Presenter of LVDT: Ari Kapusta
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-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