Sensors For Measuring Mechanical States

When designing a robot mechanically it's important to plan for the integration of sensors into the design early on. A good sensor placement provides meaningful information about the current state of the robot or the environment around a robot. These sensors can then allow programming to know these states and make decisions based on them. The primary use case is using sensors as feedback for controllers to move a robot to a desired state. (See Control Theory)

Encoder

• Digital sensor placed on the end of a shaft to measure radial position
• Limited resolution based on the construction of the encoder
• Current position is relative to the position when the program started watching the sensor
• Primary use case: On shafts that can spin infinitely (Ex: Robot wheels)
• Good in combination with another sensor to establish a known zero or starting point (See Limit Switches or Hall Effect Sensors)

Potentiometer

• Analog sensor placed on the end of a shaft to measure radial position
• Limited resolution based on the Analog to Digital converter onboard the microprocessor
• Current position is absolute and tracked while system doesn't have power
• Has a fixed rotational range of motion
• Primary use case: On shaft with mechanically limited movement (Ex: Arm joint)

Limit Switch

• Digital sensor to detect when something depresses the sensor
• Must be securely mounted to ensure full press of the switch
• Comes in button and lever variants
• Good for zeroing a system by designing to be pressed when the robot is in a certain mechanical state

Hall Effect Sensor

• Technically an analog sensor, usually used as a digital sensor to detect nearby magnets
• Acts as a limit switch which is activated by a nearby magnet instead of a press
• Mount the magnet on mechanism which brings the magnet nearby the sensor
• Good for mechanisms which move along a set path to detect a known position in the path (Ex: Elevator)
• More reliable than Limit switches