Introduction

Before anyone debates resolution, protocol, or price, every encoder specification starts at a more basic fork: what is the machine actually measuring — rotation or translation? Linear vs rotary encoder is not a quality tier; it is a geometry decision. Pick wrong and no amount of accuracy will help, because the sensor is answering a question the machine never asked. For machine builders and system integrators, getting this fork right is the first step in any motion-design bill of materials.

This article separates the two geometries in operational terms, adds the often-overlooked third category — the point/displacement gauge — and anchors each to real TIMSENS time-grating product families so the choice lands on concrete numbers rather than category labels.

What a Rotary Encoder Measures: Angle

A rotary encoder reports angular position of a rotating shaft — the angle θ from a defined zero. It is the natural sensor wherever motion is fundamentally rotational: a motor shaft, a ballscrew driven through a coupling, a rotary table, a robot joint, a spindle. The output is an angle (typically 0–360°, absolute by nature on TIMSENS units), optionally plus a multiturn count of how many full revolutions the shaft has made.

TIMSENS time-grating rotary families cover the full mechanical envelope:

• SEF (high-precision angle) — outer diameters 90–375 mm, hollow bores 35–285 mm, IP64; the metrology-grade choice for high-end CNC rotary axes and calibration tables.
• AEF (integrated) — a compact integrated angle module for robots and rotary tables.
• UEF (UEF072) — ultra-thin at ≤81 mm outer diameter, built for space-limited smart-manufacturing axes where a conventional encoder will not fit.
• TEF (economy) — outer diameters 106–170 mm, the general-industrial middle ground.

All are absolute 0–360° by nature, multiturn up to 65,535 turns, and retain position through power-down with no battery — so the angle is always known on power-up without homing.

What a Linear Encoder Measures: Length

A linear encoder reports position along a straight axis — a length, not an angle. It is the right sensor wherever the thing you care about moves in a line: a machine-tool table, a gantry bridge, a measuring column, a stage. Instead of reading a rotating scale, it reads a scale fixed to the reference member while a readhead travels with the moving member.

The TIMSENS LSA linear scale is the linear workhorse: delivered in a closed metal housing with precision bearings, strokes from 140 mm to 1840 mm, scanning speed up to 180 m/min, accuracy ±3 μm/m at 10 nm resolution. The closed housing is the point — it keeps the scale and bearings clean on a working machine where coolant and chips are in the air.

The Third Category: Point / Displacement Gauge

Not every measurement is a full axis. Sometimes you need to know the gap, thickness, or deflection at a single point. The TIMSENS GED is a point-measurement displacement gauge with ranges of 12 / 32 / 50 mm, accuracy 1 μm (P-P) at 0.1 μm resolution, IP67 sealed, with a direct PLC link and no subdivision box — fewer inline electronics to mount. It is neither rotary nor linear-axial; it is a single-point probe, and treating it as a "mini linear encoder" under-specifies what it does.

The Decision Framework: What Is the Axis Doing?

Use this three-question filter before opening a catalog:

1. Does the axis rotate? → rotary encoder (SEF / AEF / UEF / TEF). Size to outer diameter and bore: large metrology tables need SEF's 90–375 mm envelope; cramped servo axes need UEF's ≤81 mm.
2. Does the axis translate in a straight line over a span? → linear scale (LSA), sized to stroke (140–1840 mm) and required accuracy (±3 μm/m).
3. Do you need a single-point gap/thickness/deflection reading? → point gauge (GED), sized to range (12/32/50 mm) and IP67 if the environment is wet or dirty.

The geometry is set by the machine, not the sensor. A ballscrew-driven table, for example, could be measured two ways — a rotary encoder on the screw (cheap, indirect, depends on screw lead accuracy) or a linear scale on the table (direct, immune to screw error). That is a second decision layered on top of the geometry one, and it is where accuracy requirements — not geometry — take over.

How the Reading Principle Differs

The math is the cleanest way to see the split. A rotary encoder's reported position is effectively R × θ (radius times angle) when you care about the linear result at a given radius; a linear encoder reports length directly. Time grating's coupled traveling-field principle applies to both: in the rotary SEF the field travels around the ring, in the linear LSA it travels along the scale. Because the measurement comes from coupled electromagnetic fields rather than a physically printed grating read by sealed optics, both geometries share the same structural benefits — no fragile optics to contaminate, no battery-backed memory, absolute by nature — which simplifies the BOM regardless of which geometry you pick.

When to Combine Both

Real machines rarely pick one. A typical high-precision machining center pairs a rotary encoder on the ballscrew or servo motor (SEF/AEF, absolute, multiturn) with a linear scale on the table (LSA) for direct position closure — the rotary drives, the linear verifies. Knowing the two geometries and their product families lets you spec the pair deliberately instead of defaulting to "one encoder per axis" and hoping the screw lead is good enough.

Real Specifications: TIMSENS Time-Grating Families by Geometry