Choosing the wrong encoder costs more than the encoder. It costs redesigns, requalification, and downtime on your production line. This guide walks through the seven decisions that matter most — in the order that matters.
Step 1 — Rotary or linear motion?
Rotary encoders measure angular position or speed of a shaft; linear encoders measure straight-line displacement. The decision is usually dictated by your mechanism: servomotors, robot joints, spindles and rotary tables need rotary feedback; linear motors, maglev tracks and precision stages need linear feedback. Some machines need both.
At Ranmin, the RM10 rotary encoder and RM20 linear encoder share the same inductive core technology, so mixed-axis machines keep one supplier and one engineering interface.
Step 2 — Absolute or incremental?
Incremental encoders report relative motion and need a homing routine after power-up. Absolute encoders know their position immediately. For servomotors, robotics and any safety-critical axis, absolute feedback is strongly preferred — the machine powers up ready to work, with no lost-position risk.
The RM series is absolute by design, with 17-bit single- and multi-turn options.
Step 3 — Resolution and accuracy
| Specification | What it means | Typical need |
|---|---|---|
| Resolution | Number of distinct position steps | 17-bit = 131,072 steps/turn is a strong baseline for robotics |
| Accuracy | How close measured position is to true position | 0.001° rotary / 0.03 µm linear for precision motion |
| Repeatability | Ability to return to the same position | Usually 2–5× better than accuracy |
Do not confuse resolution with accuracy. A high-resolution encoder with poor accuracy gives you precise-looking but wrong numbers.
Step 4 — Communication protocol
Your controller determines the protocol family, or the protocol determines your controller. The three most common for servo and industrial motion:
- RS485 (Tamagawa protocol): the standard in many servo ecosystems — a direct drop-in for replacement projects.
- BISS-C: fast, bidirectional, with CRC protection — ideal for high-dynamic robotics.
- SSI: simple and universal for PLC and general industrial systems.
Read our deeper comparison: BISS-C vs SSI vs RS485.
Step 5 — Environment
Machines live in oil, coolant, dust, humidity and vibration. Check:
- Ingress protection (IP rating): IP67 means dust-tight and immersion-safe.
- Temperature range: inductive encoders operate from −40 to +85 °C.
- Contamination: optical encoders fail when dirt blocks the light path; inductive sensing has no optics to block.
- EMI: inductive encoders are immune to magnetic interference, so they sit happily next to motor windings.
Step 6 — Form factor and mounting
Shaft size, hollow shaft, flange pattern, height, connector orientation — match the encoder to your mechanical design. If a standard unit doesn't fit, a supplier with in-house design (like us) can adapt the form factor around your BOM.
Step 7 — Supply security and support
A great encoder from a supplier that can't deliver is a bad encoder. Evaluate lead times, local technical support, customization ability and second-source risk. A self-developed core IC and domestic manufacturing remove export-control and supply-chain surprises.
Quick selection matrix
| Application | Recommended | Key specs |
|---|---|---|
| Servo motor / drive | RM10 · RS485 or BISS-C | 17-bit, 0.001°, 10,000 RPM |
| Robot joint | RM10 · BISS-C | EMI-immune, lightweight, absolute |
| Linear motor / stage | RM20 · BISS-C or SSI | 0.03 µm, infinite cascade |
| Machine tool spindle | RM10 · SSI | IP67, high speed |
| Battery-powered meter | RM10 · low-power | < 10 µA system average |