How to Choose a Rotary Encoder: Incremental vs Absolute, Resolution and Interfaces
A rotary encoder turns shaft rotation into an electrical signal your PLC, drive or counter can read. Choosing the wrong one usually shows up later as lost position after a power cut, noisy signals on long cable runs, or a bearing that fails early because the shaft load was never considered. This guide walks through the decisions in the order an application engineer makes them.
1. Incremental or absolute?
Incremental encoders output a stream of pulses as the shaft turns. Resolution is given in pulses per revolution (ppr). The controller counts pulses to work out position and speed, so after a power loss the machine must run a homing routine to find its reference point again. They are simple, economical and ideal for speed feedback, conveyor tracking, cut-to-length lines and any axis that homes at start-up.
Absolute encoders give a unique position value for every shaft angle, so the position is known the moment power returns, with no homing. Resolution is given in bits: a 14-bit singleturn encoder resolves 16,384 positions per revolution.
- Singleturn absolute encoders know the angle within one revolution. Use them when the axis never turns more than once, such as a swivel, a valve or a rotary table index.
- Multiturn absolute encoders also count complete revolutions. Use them on lead screws, cranes, hoists, storage and retrieval systems and long-travel axes where position must survive a power cut.
Rule of thumb: if re-homing after a stoppage is unsafe, slow or impossible (a loaded crane, a half-filled mould, a robot track), choose absolute. Otherwise incremental is usually the economical choice.
2. Resolution: how much do you really need?
Work backwards from the accuracy you need at the load. If a 5 mm-pitch lead screw must position to 0.01 mm, you need at least 500 distinguishable steps per revolution, and quadrature evaluation in the controller multiplies incremental ppr by four. Buying far more resolution than needed raises the signal frequency, which can exceed the input frequency of your counter or PLC card at high speed. Check maximum speed × ppr ÷ 60 against the input limit of the receiving electronics.
3. Interface and output
| Interface | Typical use |
|---|---|
| HTL push-pull (10–30 V) | Incremental, robust against noise, common on PLC high-speed counter inputs |
| TTL / RS422 (5 V differential) | Incremental, high frequencies and longer cables, motor drives |
| SSI | Absolute, simple point-to-point link to PLCs and motion controllers |
| CANopen, PROFINET, EtherCAT and other fieldbuses | Absolute, networked machines, diagnostics, fewer cables |
| IO-Link | Parameterisation and diagnostics over a standard 3-wire sensor cable |
| Analog (4–20 mA, 0–10 V) | Direct connection to analog inputs where digital interfaces are not available |
Match the interface to what the controller already supports. A fieldbus encoder such as the Kübler Sendix F58 PROFINET encoder removes the need for a separate counter card on PROFINET machines.
4. Shaft and mounting
- Solid shaft encoders connect through a flexible coupling. They isolate the encoder from motor shaft run-out and are the safest choice where alignment is uncertain.
- Hollow shaft / blind hollow shaft encoders slide straight onto the motor or machine shaft and are held by a stator coupling. Mounting is faster and the assembly is shorter, which suits motors and compact machines.
- Large hollow shaft designs fit directly on big motor and generator shafts. The Kübler A020 takes up to a 42 mm hollow shaft, and the H100 and H120 are built for large motors and generators.
Always check the permitted radial and axial shaft load. Belt-driven pulleys mounted directly on an encoder shaft are a common cause of early bearing failure.
5. Environment
Steel plants, paper machines, wind and mobile hydraulics are hard on encoders. Look at the IP rating, operating temperature range, shock and vibration ratings and the bearing design. Encoders described as particularly resistant, with sealed housings and robust bearing structures, such as the Kübler 5020 and 5026, are designed for these conditions. Magnetic scanning, as on the M5868 and M3678, tolerates dirt, condensation and shock better than optical scanning, while optical scanning generally offers the highest precision.
6. Quick selection checklist
- Must position survive a power cut? Absolute (multiturn if the axis turns more than once), otherwise incremental.
- Required accuracy at the load, converted to steps per revolution.
- Maximum speed, checked against the controller input frequency.
- Controller interface: HTL, TTL/RS422, SSI, fieldbus, IO-Link or analog.
- Shaft type and size, flange, and available mounting space.
- Supply voltage, cable length and connector or cable exit.
- IP rating, temperature, shock and vibration.
Encoders available from Rajdeep
Rajdeep Automation supplies Kübler and SIKO encoders across India, from economical incremental models like the KIS40 and KIS50 to high-resolution absolute encoders such as the 5852, 5873 and F5868. Browse the full range under positioning solutions.
Not sure which model fits? Send us your shaft size, required resolution, interface and operating conditions through the enquiry form and an application engineer will recommend a model and share the datasheet and price.