Encoders and feedback

An encoder tells the drive or controller where the axis is. In a servo system it closes the position and velocity loops; in a closed-loop stepper it catches lost steps; on a measuring axis it is the reference everything else is judged against. Getting the encoder wrong rarely shows up on the test bench. It shows up as a machine that has to home after every power cut, an axis that drifts when the enclosure warms up, or noise faults once the cable runs alongside the motor power.

This hub covers the decisions in the order we would take them: absolute or incremental, sensing technology, rotary or linear, interface, then accuracy and mounting. For a deeper comparison of the first decision, see absolute vs incremental encoders.

Absolute vs incremental encoders

An incremental encoder outputs a pulse train as it moves. The controller counts pulses from a known reference (an index pulse or home switch), so after power-up the axis must home before it knows where it is. Quadrature incremental encoders use two channels, A and B, 90 electrical degrees apart; which channel leads tells you direction. Counting every edge of both channels (“x4 decoding”) turns a 2,500 PPR encoder into 10,000 counts per revolution, according to Dynapar. A Z (index) channel gives one reference pulse per revolution.

An absolute encoder reports a unique position word at every point, so the axis knows where it is the moment power returns. Single-turn absolute encoders cover one revolution. Multi-turn versions also count revolutions through power loss using a gear train, a backup battery or self-powered counting such as Wiegand energy harvesting, as POSITAL explains. Batteries need replacing; gear trains add size and dislike shock; energy-harvesting designs avoid both but are not available in every format.

When to use which: choose absolute where homing is unsafe, slow or impossible (robot joints, vertical axes, machines that must restart mid-cycle, multi-axis gantries). Choose incremental where cost matters, a homing move is acceptable, or the drive only accepts A/B/Z.

Encoder technologies compared

Technology	Strengths	Watch-outs	Typical use
Optical (disc or scale)	Highest resolution and accuracy; mature, wide choice	Sensitive to contamination, condensation and misalignment; tight mounting tolerances	Precision stages, semiconductor, metrology, high-end servo motors
Magnetic	Tolerant of dirt, oil and shock; compact; low cost	Lower accuracy than optical; can be disturbed by strong external magnetic fields	Motor feedback, AGVs, actuators, linear tape scales
Capacitive	Very thin and light; no magnets in the sensing path; large hollow bores	Check the maker’s environmental ratings; accuracy depends on mounting	Robot joints, gimbals, defence and aerospace pointing systems
Inductive	Very tolerant of misalignment, contamination and temperature; resolver-like robustness	Usually larger than optical for the same accuracy	Harsh-environment rotary axes, replacement for resolvers

Control Engineering makes the point that no single technology wins everywhere: optical leads on performance but suffers in temperature extremes, contamination and shock, while inductive designs use the whole face of the rotor and stator rather than a single read point, so they tolerate misalignment far better.

Example series in each technology: Renishaw RESOLUTE and TONIC (optical), RLS AksIM-4 and the Bogen AKS16 heads (magnetic), Netzer VLX and multi-turn VLM (capacitive), and the Zettlex Mini Ultra IncOder (inductive).

Rotary vs linear, shafted vs bearingless

Rotary encoders come shafted (own bearings, coupled to your shaft), hollow-shaft, or bearingless ring/kit encoders where the rotor fits your shaft and the readhead fits your housing. Shafted units are simple to fit, but a flexible coupling adds lost motion, so they are not the first choice for high-accuracy angle measurement. Bearingless designs suit low axial height and large bores for cables or slip rings.

Linear encoders read a scale along the axis: open optical scales for clean, high-precision stages; enclosed scales such as Renishaw FORTiS-S for machine tools; magnetic tape such as Bogen LMSA for long, dirty axes; and capacitive options such as KappaSense FleXe where mounting tolerance is loose. Measuring at the load with a linear encoder removes screw lead error, backlash and thermal growth from the loop, which a motor-mounted rotary encoder cannot do.

Encoder interfaces: BiSS-C, SSI, EnDat, HIPERFACE DSL and quadrature

The interface must match what the drive reads. This is the most common reason a good encoder ends up on the wrong machine.

Interface	Type	Key characteristics
Quadrature A/B/Z	Incremental, usually RS-422 differential	Universal; no position at power-up; count rate limits top speed
SSI	Absolute, one-way serial	Typically clocked 100 kHz to 2 MHz; optional parity only
BiSS-C	Absolute, bidirectional, open protocol	CRC checking, clock to about 10 MHz, parameter read/write
EnDat 2.2	Absolute, bidirectional (HEIDENHAIN)	Up to 16 MHz clock over cables up to 100 m with propagation-delay compensation; diagnostics and safety-rated versions
HIPERFACE DSL	Absolute, fully digital (SICK)	Two wires inside the motor power cable; up to 100 m; 9.375 Mbaud

Figures from Motion and Mechatronics, HEIDENHAIN and Motion Control Tips. Drive makers also use proprietary serial protocols (Siemens DRIVE-CLiQ, FANUC, Mitsubishi, Panasonic, Yaskawa and others); some encoder ranges, RESOLUTE among them, are offered in these variants so they can plug straight into that drive.

If your controller needs safety functions based on position (safe speed, safe limited position), you need a safety-rated encoder and interface, for example EnDat with its functional safety option, BiSS Safety or a safety-rated HIPERFACE DSL device.

How to select an encoder: resolution vs accuracy

Resolution is the smallest step reported; accuracy is the worst-case error against the true position. The RLS encoder handbook notes that higher resolution does not automatically mean higher accuracy, and gives a rule of thumb that repeatability is typically 5 to 10 times better than accuracy. That matters: many machines need repeatability more than absolute accuracy, and can be calibrated.

Useful conversions for rotary axes (there are 1,296,000 arc-seconds in a revolution):

• 17-bit (131,072 counts) is about 9.9 arc-sec per count
• 20-bit (1,048,576 counts) is about 1.24 arc-sec per count
• 23-bit (8,388,608 counts) is about 0.15 arc-sec per count

Worked example. A rotary table must place a part within ±5 µm at a 300 mm radius. The allowable angular error is 5 µm ÷ 300 mm = 16.7 µrad, about 3.4 arc-sec. The installed encoder accuracy therefore needs to be comfortably better than ±3.4 arc-sec, leaving budget for bearing runout and thermal effects. Resolution should be several times finer than that (a rule of thumb, so the servo loop can hold position without hunting), which points to 21 bits or more. A large-diameter ring encoder will usually be needed, because larger rings achieve better angular accuracy: Renishaw quotes RESOLUTE RESA30 ring system accuracy from about ±4.3 arc-sec on the smallest rings to about ±0.4 arc-sec on the largest.

Mounting tolerance and installation

Datasheet accuracy assumes the encoder is installed to its tolerances. Control Engineering notes that eccentricities below 0.025 mm are common requirements for optical ring encoders. By contrast, the Zettlex IncOder MINI guide allows 0.20 mm rotor-to-stator non-concentricity and a 0.30 to 0.70 mm axial gap on the 37 mm size. Design the housing, bearing and shaft so the encoder sees its required gap and concentricity across the full temperature range and under load, not just when the machine is cold and stationary.

Common encoder specification mistakes

• Choosing on resolution alone. A high bit count says nothing about installed accuracy.
• Interface mismatch. Check the drive’s exact supported protocol, clock rate and supply voltage before ordering.
• Single-turn on a multi-turn axis. If travel exceeds one motor revolution, plan for multi-turn or homing.
• Ignoring the environment. Coolant mist, condensation and swarf defeat open optical encoders; strong motor or brake fields can upset magnetic ones.
• Cable routing. Run encoder cables screened and away from motor power, or choose a one-cable system designed for it.
• Measuring at the wrong place. If accuracy matters at the tool, put a linear or load-side encoder there.

What to send us for an encoder quote

• Rotary or linear; shaft, bore or scale length; space envelope
• Accuracy and repeatability required at the load, and the radius or stroke
• Maximum speed (rpm or m/s)
• Absolute or incremental; single- or multi-turn; number of turns
• Drive or controller make and model, and required interface
• Environment: temperature range, IP rating, contamination, shock and vibration, magnetic fields
• Any functional safety requirement (SIL or PL)
• Annual volumes and target cost

You can capture these in the spec builder and our engineers will propose suitable options from our supply network. Related families: servo drives, servo and stepper motors and frameless torque motors.

Guides

Where encoders and feedback are used

What we need to know to specify encoders and feedback

These are the questions an applications engineer works through first. Approximate answers are fine: ranges narrow the options quickly, and we confirm the details with you.

• Motion: Rotary, Linear
• Output: Absolute, Incremental
• Resolution: Standard, High, Ultra-high
• Interface: BiSS-C, SSI, EnDat, A/B/Z, Not sure
• Mounting: Hollow shaft / ring, Shaft, Open scale and readhead

We also ask about the environment (washdown, outdoor, subsea, ATEX, vacuum, shock and EMC), annual quantities and timescale, because these decide whether a standard, modified or custom design is the best fit.

106 series compared

Series from 17 manufacturers, grouped by type. Figures are the highest published for each series. Open a series for the full specification and, for brands outside our supply network, the alternatives that meet or exceed it.

Magnetic rotary encoder (32)

Optical absolute encoder (24)

Optical incremental encoder (18)

Magnetic linear encoder (12)

Capacitive rotary encoder (9)

Enclosed linear encoder (5)

Inductive encoder (4)

Capacitive linear encoder (2)

Summarised from manufacturers' published data. Always confirm against the current datasheet; an engineer will check the fit before recommending anything.

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