Linear Enc
Reads incremental or absolute position from a fixed precision scale (glass-chrome stripe pattern or magnetised steel tape) and outputs digital quadrature pulses or a serial absolute position word.
An LED illuminates a Diadur (chromium-on-glass) scale through a fine reticle on the read-head; phase-shifted photodiode pairs detect the moiré pattern and produce A/B quadrature signals. Interferential interpolation electronics divide the basic 20 µm grating period into 10,000 counts, yielding 2 nm resolution. Absolute encoders read a coded track (e.g., pseudo-random) through additional photodetectors so the chip knows position the moment power comes up — no homing pass needed.
In plain terms
Like a runner's lap-counter at a track — every centimetre marker on the rail clicks once as it passes a fixed sensor, and the counter accumulates total distance at any speed.
Why designers use it
- Sub-µm precision unobtainable with motor-side resolvers — semiconductor lithography stages need 1–2 nm position knowledge.
- Direct measurement on the moving carriage — eliminates ball-screw backlash and thermal expansion error.
- Fast — analog interpolation supports velocities of metres/second without losing counts.
- Robust to vibration and power glitches — magnetic absolute encoders read position immediately after power-up.
Best for
- CMM stages
- Semiconductor
- CNC mills
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- In contaminated environments where a glass scale would foul — pull a magnetic encoder or sealed enclosed scale.
- On long (>30 m) travels where a single piece of scale becomes impractical — use rotary encoder + ball screw or laser interferometer.
Common mistakes
- Mounting the read-head off-axis to the scale — Abbe error multiplies misalignment by lever arm and the position reads wrong by tens of µm.
- Routing encoder cables through the bend radius of a high-flex cable carrier — copper fatigue cuts traces in months.
Where you will find it
- An ASML NXT:1980Di lithography scanner uses a Heidenhain LIP 481R interferential linear encoder on its wafer stage: the encoder's 2 nm resolution per axis is what holds overlay accuracy below 5 nm across a 300 mm wafer at 200 wph throughput.
- A Hardinge Bridgeport 3-axis VMC mill uses Heidenhain LS 187 sealed glass linear encoders for X/Y/Z position: the encoder's 0.1 µm resolution closes the position loop directly on the table, so backlash in the ball screw never reaches the workpiece.
- A Renishaw RTS rotary tool-setter integrated with a Mazak Integrex i-630V VTL uses a Renishaw RGH22 magnetic linear encoder strip running along the tool magazine: the magnetic readhead reads through coolant, swarf, and oil with 5 µm absolute repeatability.
A short history
Linear position encoders convert linear displacement directly into digital or analogue position information by reading a precision scale. Heidenhain (Germany) introduced the first optical linear encoder for machine tools in 1952 using their Diadur chromium-on-glass scale process. Photoelectrically scanned incremental linear encoders followed in 1961. Linear encoders achieve resolutions below 1 nm using interferometric grating interpolation (Heidenhain's 1987 LIP series) and are indispensable in semiconductor lithography stages, CMM machines, and high-precision laser cutters. Magnetic linear encoders (using magnetoresistive reading heads against a steel scale) provide a robust alternative for harsh-environment applications.