A linear encoder measures position along a rigid scale with resolutions typically from 0.001 mm to 0.05 mm and stroke lengths capped below ~3 m on exposed glass or magnetic scales, while a draw-wire sensor extends that envelope to 1.25–60 m by translating linear pull into rotary encoder counts on a spring-loaded drum.
The choice is governed by four specs: required stroke, required resolution, mounting envelope, and environmental rating. Where stroke is short and accuracy is non-negotiable, a linear scale wins; where the moving mass travels beyond the practical reach of a rigid scale, the wire-pull architecture is the only cost-effective option [S1][S3].
Operating Principle and Resolution Stack
A linear encoder reads graduations on a fixed scale using optical, magnetic, or inductive pickup; the read head resolves the grating period, typically 4 µm–40 µm on glass scales, then interpolates internally to 0.05 µm–1 µm output steps [S5].
A draw-wire sensor wraps a stainless-steel or polyamide-coated wire on a precision drum coupled to a rotary encoder; one drum revolution is the mean length per revolution (e.g. 125 mm on the SICK EcoLine), and the rotary counts are multiplied by that constant to give linear output [S2]. Resolution is therefore the product of drum circumference and rotary encoder resolution, with the SICK EcoLine listed at 0.02 mm over 1.25 m of measuring range and the LARM LU0 series specified at 0.02 mm resolution with a 0.36 mm polyamide-coated stainless-steel wire [S1][S2].
Stroke Range and Mounting Footprint
Glass-scale linear encoders listed on the open market span sub-metre to ~3 m strokes, with housing lengths fixed to the measurement range; the Lika SME11 LINEPULS magnetic ring encoder is sold as a non-contact read head plus a flexible magnetic ring, allowing the linear motor stroke to dictate the scale length [S4][S5].
Draw-wire sensors trade the rigid scale for a small housing and a long wire: TR-Electronic SL30 series housings of 80–190 mm diameter deliver 2 m to 60 m measuring length, and the Kübler D125 reaches 94 m, but the wire must be routed in a straight, unobstructed path with a defined return spring force. For long-stroke applications such as crane boom position, hydraulic cylinder stroke, or elevator car location, no glass or magnetic scale of equivalent length exists as a stock part [S1].
Accuracy, Linearity, and Repeatability
Linear encoders on glass scales routinely quote linearity of ±2–5 µm/m and repeatability at sub-micron level, because the scale graduation is the only error source and the read head is contactless; the TE Connectivity ED34 magneto-resistive incremental linear encoder uses a 1 mm pole pitch on a magnetised scale with up to 0.4 mm air gap, with an internal sine/cosine interpolation unit driving an A/B quadrature output [S7].
Draw-wire sensors carry extra error sources: wire stretch under load, drum eccentricity, spring hysteresis, and the nonlinearity of the wound layers. TR-Electronic's SX80 series quotes linearity of max. ±0.02 % of full scale over 1000–3000 mm, with housing temperature range of −40 °C to +120 °C and outputs spanning analog 0–10 V, 4–20 mA, potentiometer, TTL, HTL, SSI, Profibus, CANopen, DeviceNet, EtherCAT, and Profinet. POSITAL FRABA's draw-wire line, by contrast, lists 0.02 mm resolution but a wider −20 °C to +80 °C process-temperature window [S1].
Output Protocols and Integration
Both technologies expose the same downstream protocols because both end in a rotary encoder block. Incremental TTL/HTL, SSI absolute, and the major fieldbuses — Profibus, CANopen, DeviceNet, Profinet, EtherCAT — are listed across vendors in the SICK, Kübler, TR-Electronic, and POSITAL draw-wire ranges [S1][S2].
Linear encoders share that interface set, with magneto-resistive heads (TE ED34) emitting A/B quadrature directly, and glass-scale absolute encoders (GIVI MISURE, Magnescale Europe, Precizika Metrology, TR-Electronic) offering both incremental and absolute variants. The Lika SME11 product description notes status-LED clearance error feedback and optional tape-cleaning wipers, a feature specific to exposed scale service in linear-motor applications [S4][S5][S7].
Environmental and Mechanical Limits
Draw-wire sensors depend on a return spring and a tensioned wire; dust, ice, side-load, and shock shorten service life. The SICK EcoLine miniature draw-wire sensor is rated precision, miniature, and shock-proof with 1.25 m–10 m range, while the Kübler C60 carries IP69K protection and a −40 °C to +85 °C window for harsh-environment mounting [S1][S2].
Linear encoders, particularly sealed glass-scale units, tolerate coolants, chips, and high-pressure washdown at IP67 with the right housing; exposed-scale designs like the SME11 add optional wipers to clear swarf from the tape. A summary of how a rotary encoder inside the draw-wire housing sets the absolute position count is detailed in the Lika and TR-Electronic datasheets, where the rotary model dictates maximum speed, output protocol, and shaft load limits [S1][S4].
Selection Matrix: When to Specify Each
Use a linear encoder when the stroke stays under ~3 m, the read head is mounted to a moving slide or linear-motor forcer, and the application requires micron-level repeatability (machine tools, EDM, semiconductor stages, CMMs). Use a draw-wire sensor when the measurement length exceeds ~2 m, the moving point is a cable-pulled end on a cylinder, crane, or actuator, and ±0.02 % full-scale linearity is acceptable [S1][S3].
Hybrid installations are common: a long-stroke draw-wire sensor provides the coarse absolute position of, for example, a linear actuator rod, while a short-stroke linear encoder read head closed-loop on the actuator's output flange corrects the wire-drum hysteresis. Buyers comparing linear guides carriage feedback against rod-end feedback will usually pick linear encoders on the carriage and draw-wire sensors on the rod, with linear bearings sized for the resulting side-load budget. For deeper guidance on the encoder side of the stack, the 2026 buying guide at Linear Encoder Buying Guide 2026 walks the same stroke-versus-resolution trade-off, while the LVDT-versus-displacement comparison at Laser vs LVDT Displacement Sensor is a useful cross-reference when sub-micron non-contact sensing is on the table [S1][S3].