Encoders

Accuracy, resolution, and repeatability are three independent performance parameters of a rotary encoder. All three appear in datasheets, but they measure different things. Confusing them leads to incorrect encoder selection and missed specifications. This article defines each parameter with measurement context and explains how they interact in a complete position feedback system.

Electric encoder technology uses capacitive field modulation — rather than optical or magnetic principles — to measure absolute angular position. The holistic sensing approach measures the entire encoder area simultaneously, providing inherent immunity to localized contamination and relaxed installation tolerances without sacrificing accuracy.

Optical encoder alignment tolerances for small-diameter rotary scales (≤ 20 mm) are significantly tighter than for linear scales or large-diameter rotary discs. This article defines the radial (Y-axis) tolerance reduction for small scales, explains the optical geometry responsible for the tighter requirement, and provides mounting design recommendations.

Encoder interface cables carry differential position signals that are highly susceptible to noise, crosstalk, and impedance discontinuities. Correct cable selection — including characteristic impedance, conductor gauge, shielding type, and jacket material — determines whether the encoder achieves its specified noise immunity and signal quality at the required cable length.

Cable shield termination and ground loop prevention are the most common sources of encoder signal noise in servo motor installations. This article defines the correct shielding method for motor and encoder cables, identifies the conditions that create ground loops, and describes practical mitigation techniques.

Glass scales outperform metal tape scales in optical encoders across three critical parameters: coefficient of thermal expansion, grating uniformity, and sustainable accuracy at elevated temperatures. This article establishes when glass is technically required and when tape scales are an acceptable alternative.

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.

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.