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Difference between revisions of "Encoders"

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#REDIRECT [[Encoder Support]]
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[[File:TD-044-078 ig42 motor encoders.jpg|thumb|191x191px|The encoder is inside the black end cap. Extra wires are present that connect with the encoder.]]
 
[[File:TD-044-078 ig42 motor encoders.jpg|thumb|191x191px|The encoder is inside the black end cap. Extra wires are present that connect with the encoder.]]
 
Rotary encoders are devices that generate electrical pulses as they rotate. The angle or rate of rotation that the encoder is experiencing can be measured by monitoring the number or frequency of the pulses. In robotics, encoders are most commonly attached to the robot's drive motors and used to measure the robot's linear speed, angular speed, and distance traveled. Drive motor encoders can also be used to perform [[Speed Control#Closed Loop|closed loop speed control]] on the wheels. More generally, encoders can be attached to any of the robot's joints to track its speed and/or angle, such as a rotating joint in a robotic arm.
 
Rotary encoders are devices that generate electrical pulses as they rotate. The angle or rate of rotation that the encoder is experiencing can be measured by monitoring the number or frequency of the pulses. In robotics, encoders are most commonly attached to the robot's drive motors and used to measure the robot's linear speed, angular speed, and distance traveled. Drive motor encoders can also be used to perform [[Speed Control#Closed Loop|closed loop speed control]] on the wheels. More generally, encoders can be attached to any of the robot's joints to track its speed and/or angle, such as a rotating joint in a robotic arm.
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*Quite accurate over short time periods/distances
 
*Quite accurate over short time periods/distances
 
*Works indoors and outdoors, day or night
 
*Works indoors and outdoors, day or night
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*Good for mitigating discrete jumps in position and orientation estimates when fused with other sensors
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*Good for mitigating discrete jumps in [[Positioning System|position]] and orientation estimates when [[Data Filtering|fused]] with other sensors
 
*Great fallback sensor when things go wrong
 
*Great fallback sensor when things go wrong
 
*Can be used for [[Speed Control#Closed Loop|closed loop speed control]] of the wheels
 
*Can be used for [[Speed Control#Closed Loop|closed loop speed control]] of the wheels
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*Requires initial calibration between encoder counts and amount of robot movement
 
*Requires initial calibration between encoder counts and amount of robot movement
 
*Assumes no slip between robot wheel and ground. An unstable or inconsistent surface beneath the robot can lead to wheel slippage. Skid-steer robots also experience wheel slippage while turning. This causes error in the estimated robot speed and position because the wheel moves but the robot doesn’t.
 
*Assumes no slip between robot wheel and ground. An unstable or inconsistent surface beneath the robot can lead to wheel slippage. Skid-steer robots also experience wheel slippage while turning. This causes error in the estimated robot speed and position because the wheel moves but the robot doesn’t.
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*Position errors from wheel slippage and imperfect calibration accumulate over time/distance to give a progressively worse position estimate. *When used to measure speed instead of position this is less of an issue.
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*[[Positioning System|Position]] errors from wheel slippage and imperfect calibration accumulate over time/distance to give a progressively worse position estimate. When used to measure speed instead of position this is less of an issue.
 
*Additional hardware is often needed to keep track of the encoder counts. However, some motor controllers (such as Roboteq models) have this functionality built-in.
 
*Additional hardware is often needed to keep track of the encoder counts. However, some motor controllers (such as Roboteq models) have this functionality built-in.
  
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[[Category:Sensors]]
 
[[Category:Sensors]]
 
[[Category:Encoders]]
 
[[Category:Encoders]]
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[[Category:Electrical Components]]

Latest revision as of 15:25, 16 April 2021

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The encoder is inside the black end cap. Extra wires are present that connect with the encoder.

Rotary encoders are devices that generate electrical pulses as they rotate. The angle or rate of rotation that the encoder is experiencing can be measured by monitoring the number or frequency of the pulses. In robotics, encoders are most commonly attached to the robot's drive motors and used to measure the robot's linear speed, angular speed, and distance traveled. Drive motor encoders can also be used to perform closed loop speed control on the wheels. More generally, encoders can be attached to any of the robot's joints to track its speed and/or angle, such as a rotating joint in a robotic arm.







Sensor Overview

Measurement: Motor rotation distance. When attached to the drive motors, used to measure wheel speed and distance.

Ideal operating conditions: Robot operating on smooth/even ground where wheels maintain constant rolling contact with no slip.

Sensor Pros:

  • Quite accurate over short time periods/distances
  • Works indoors and outdoors, day or night
  • Good for mitigating discrete jumps in position and orientation estimates when fused with other sensors
  • Great fallback sensor when things go wrong
  • Can be used for closed loop speed control of the wheels

Sensor Cons:

  • Requires initial calibration between encoder counts and amount of robot movement
  • Assumes no slip between robot wheel and ground. An unstable or inconsistent surface beneath the robot can lead to wheel slippage. Skid-steer robots also experience wheel slippage while turning. This causes error in the estimated robot speed and position because the wheel moves but the robot doesn’t.
  • Position errors from wheel slippage and imperfect calibration accumulate over time/distance to give a progressively worse position estimate. When used to measure speed instead of position this is less of an issue.
  • Additional hardware is often needed to keep track of the encoder counts. However, some motor controllers (such as Roboteq models) have this functionality built-in.

Encoder Output

Quadrature Encoder Pulses

Encoders typically output what is known as a quadrature signal. A quadrature signal is comprised of two channels (Channel A and Channel B). Channel B is 90 degrees out of phase from channel A. This allows the circuitry watching the output signal to know what direction you are traveling. If B trails A then your motor is moving clockwise, if A trails B then your motor is moving counter clockwise.


Adding Encoders to a Robot

We have written a practical guide on this topic here.

Quick Links to our Encoders and Accessories:

Encoder Buffer and Pull-up Boards:

Motors with Encoders:

Motor Controllers with direct encoder feedback:

Encoder support:

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