An absolute encoder is a sensor that tells a robot the exact angular position of a shaft or joint. Unlike an incremental encoder, it does not need to move to a home switch after power is turned on. This matters in robotics because a joint must know where it is before it can move safely, especially in arms, grippers, wheels, and surgical or industrial machines.
Absolute encoders make robot motion more reliable by giving a unique digital position value for each angle.
Key Facts
- Angular resolution = 360° / 2^n, where n is the number of encoder bits.
- An n-bit absolute encoder can represent 2^n unique positions per revolution.
- For a 12-bit encoder, positions = 2^12 = 4096 and resolution = 360° / 4096 = 0.0879°.
- Gray code changes only one bit between neighboring positions, reducing errors during transitions.
- Single-turn absolute encoders measure position within one 360° revolution.
- Multi-turn absolute encoders measure position within a revolution plus the number of completed turns.
Vocabulary
- Absolute encoder
- A position sensor that outputs a unique digital code for each shaft angle.
- Gray code
- A binary coding system in which adjacent values differ by only one bit.
- Resolution
- The smallest angle change that an encoder can distinguish.
- Single-turn encoder
- An absolute encoder that reports the shaft angle within one complete revolution.
- Multi-turn encoder
- An absolute encoder that reports both the angle within a revolution and the number of full revolutions.
Common Mistakes to Avoid
- Confusing absolute encoders with incremental encoders is wrong because an absolute encoder gives a unique position immediately, while an incremental encoder counts changes from a reference point.
- Using ordinary binary code without considering transition errors is wrong because multiple bits can change at once, causing a temporary false position reading.
- Ignoring resolution is wrong because a low-bit encoder may not measure small joint angle changes accurately enough for the robot task.
- Treating a single-turn encoder as a multi-turn encoder is wrong because it cannot distinguish 10° from 370° without extra turn-tracking information.
Practice Questions
- 1 A robotic joint uses a 10-bit absolute encoder. How many unique positions can it report in one full revolution, and what is its angular resolution in degrees?
- 2 A 14-bit absolute encoder is mounted on a robot elbow joint. Calculate the angle per count in degrees, using resolution = 360° / 2^n.
- 3 Explain why Gray code is useful in an absolute encoder mounted in a moving robotic joint, especially when the disk is between two neighboring positions.