Encoders

Robotic joint encoders operate in constrained envelopes with simultaneous demands for absolute position, high resolution, low profile, contamination resistance, and no homing requirement. The selection between optical, inductive, and capacitive technologies for robot joints is driven by how each handles these competing constraints.

EtherCAT and CANopen fieldbus networks define how servo drives and encoders communicate with motion controllers. The encoder’s position data must be synchronized with the network’s communication cycle to ensure that the control loop operates on consistent, time-stamped position values. This article covers the communication architecture, timing requirements, and encoder interface options for EtherCAT and CANopen servo networks.

Interferential optical encoders achieve nanometer-level position resolution using diffraction gratings and coherent laser sources. The operating principle — VCSEL illumination of a grating, Talbot plane selection, and differential photodetection — enables interpolation factors that conventional LED-based encoders cannot match.

Eccentricity between the encoder disc center and the axis of rotation produces a once-per-revolution sinusoidal position error. The magnitude is directly calculable from eccentricity and scale radius. Two-readhead averaging and mechanical hub design are the primary mitigation strategies.

Magnetic rotary encoders use Hall-effect sensors and permanent magnets to detect angular position. Their low cost and compactness make them the dominant technology in cost-sensitive applications. However, susceptibility to external magnetic fields, reduced accuracy-to-size ratio, and temperature sensitivity limit their use in high-precision or EMI-heavy environments.