Robotics: Absolute Encoder

Understanding Robotics: Absolute Encoder

Inside an absolute encoder is a patterned disk or magnetic ring attached to the rotating shaft. A sensor reads the pattern and converts it into a set of digital bits. Each shaft angle has its own bit pattern, like a numbered seat in a stadium.

Optical models use light shining through coded tracks on a disk. Magnetic models sense changes in a magnetized ring.

Magnetic encoders are common where dust, vibration, or oil could make optical reading difficult. The controller receives the bit pattern through wires or a communication link, then turns that reading into a joint angle for its motion software.

The number of bits sets how finely the rotation is divided. More bits create smaller angle steps, but resolution is not the same as accuracy. A sensor may report very small changes while still having an offset from its true physical angle.

Shaft wobble, mounting errors, temperature changes, electrical noise, and imperfect magnetic fields can all affect accuracy. Robot builders therefore calibrate the encoder against a known mechanical reference during assembly. They store an offset so that a reported zero matches the real zero position of the joint.

Students should keep resolution, accuracy, and repeatability separate. Repeatability means the encoder gives nearly the same result whenever the shaft returns to one position.

Many absolute encoders use Gray code in their internal pattern. Ordinary binary counting can change several bits at once between adjacent positions. During movement, one sensor channel may switch slightly before another.

For a brief moment, the controller could read a false number far from the real position. Gray code avoids this problem because neighboring positions differ by only one bit. A changing reading can then be interpreted more safely.

This idea shows why digital systems depend on physical timing. Bits appear clean in a diagram, yet real sensors, wires, and moving parts do not change at exactly the same instant.

Single-turn devices are useful when a joint never needs to distinguish between separate full rotations. A steering joint, camera mount, or robot arm wrist may only operate within part of one turn. A multi-turn device keeps track of full revolutions as well as the angle within the current revolution.

This is important for winches, lifting mechanisms, cable spools, and long-travel rotary axes. Some multi-turn encoders use gears and a small counter. Others use battery backup or energy captured from shaft motion to preserve the turn count without external power.

In a real robot, encoder information must still be checked against mechanical limits. A loose coupling, stripped gear, or collision can make the shaft position differ from the position the robot assumes for the larger mechanism.