Encoders

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. 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, can be used to generate position and heading estimates. In most practical applications encoders are best used to track linear and angular velocities instead.

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 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 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.