Sensor Selection

This is a practical guide to selecting sensors for your project. For each sensor there is a breakdown of what physical parameters the sensor is measuring, the conditions the sensor works best in, and some things the sensor does well and poorly.

Positioning System Sensors

These sensors are used to generate estimates of the robot’s position state (position, orientation/heading, speed, and/or acceleration).

Wheel 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 commonly attached to the robot's drive motors and used to measure the robot's linear speed, angular speed, and distance traveled.

• Measurement: When attached to the drive motors, measures wheel speed. This information can then be used to track the robot’s linear and angular/rotational speed and subsequently the robot’s position and heading by integrating the data over time
• 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 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 electronics are often needed to keep track of the encoder counts. However, some motor controllers (such as Roboteq models) have this functionality built-in.