Encoders

Inductive rotary encoders operate without a line-of-sight requirement between sensor and scale, making them immune to particulate and liquid contamination that disables optical systems. This article covers the operating physics, ingress protection ratings, and verified industrial applications where inductive encoders are the technically correct choice.

The three principal optical encoder architectures — transmissive, reflective, and interferential — differ in accuracy, physical layout, and suitability for precision motion applications. Selection depends on resolution requirements, installation envelope, and contamination tolerance.

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.

Single-ended encoder signals degrade over cable lengths that are entirely acceptable for differential pairs. When cable length and data rate require transmission line modeling, unterminated lines produce reflections that corrupt position data. This article provides the engineering criteria for determining when termination is required and how to implement it for RS-422 encoder outputs.

Rotary encoders convert angular position into electrical signals through optical scanning, electromagnetic induction, or capacitive field modulation. Understanding the physics of each sensing method — and the signal processing chain that follows — is the foundation for selecting, installing, and troubleshooting rotary position feedback systems.

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.