Encoder Explained: Absolute vs Incremental, Multi-Turn & More
An encoder is a feedback device that measures shaft rotation — converting mechanical motion into an electrical signal a PLC, motion controller, or servo drive can read. Encoders come in three crucial dimensions: absolute vs incremental, single-turn vs multi-turn, and (for multi-turn absolutes) mechanical vs battery-backed. Picking the wrong combination leads to expensive surprises that often only surface at commissioning or after a power cycle.
Absolute vs incremental — the fundamental choice
| Aspect | Incremental | Absolute |
|---|---|---|
| Output | Two square waves (A & B), 90° out of phase, plus optional Z (index) pulse | Position word (parallel, SSI, BiSS, EnDat, PROFIBUS, EtherCAT) |
| Position after power-up | Unknown — must home/reference | Known immediately |
| Resolution | Pulses per revolution (PPR), typically 100-10,000 | Bits per revolution, typically 12-25 bits (4,096 to 33M counts) |
| Counting | PLC must count pulses (HSC card) | PLC reads position directly |
| Cost | Lower | 2-5x higher |
| Best for | Speed feedback, simple positioning with homing routine | Anything that must know position after power loss without homing |
Single-turn vs multi-turn (for absolute encoders)
- Single-turn absolute — reports position within one revolution (0-359° to 25-bit precision). After power loss, the encoder still knows its position within that one revolution.
- Multi-turn absolute — additionally counts revolutions (typically 12-bit = 4,096 turns or 16-bit = 65,536 turns). After power loss, the encoder still knows both position-within-revolution AND total revolutions.
If your axis travels more than one revolution (most do — leadscrews, rotary tables, conveyor drives), you need multi-turn. If it's a rotary axis returning to the same physical position each cycle (a single rotary index station), single-turn is enough.
Mechanical multi-turn vs battery-backed multi-turn (the trap)
For multi-turn absolutes, there are two technologies, and the difference is huge:
- Mechanical multi-turn — uses a gear train internally to count revolutions. The position is preserved mechanically regardless of power state. No battery. No backup needed. Survives indefinitely.
- Battery-backed multi-turn — counts revolutions electronically using a small lithium battery to keep the count alive when power is off. Lower cost than mechanical, but: when the battery dies, the multi-turn count is lost. The encoder still knows position within one revolution, but the count of how many revolutions is forgotten. The system has to be re-homed.
The "Absolute Encoder Lie": a battery-backed multi-turn encoder is sold as "absolute" but isn't truly absolute — it depends on a battery. Battery life is typically 5-10 years, but battery replacement requires the encoder to be powered up before swapping (or the count is lost). Mechanical multi-turn is genuinely absolute. For long-lifecycle assets (20+ years), mechanical wins despite the higher cost.
Optical vs magnetic vs capacitive
- Optical — light source through a slotted disk to a photodetector. Highest resolution (up to 25-bit). Most common in servo systems and precision positioning. Sensitive to dust, vibration, oil contamination.
- Magnetic — Hall-effect or magnetoresistive sensors reading a magnetised disk. Robust against dust, oil, water, vibration. Lower resolution (typically 12-17 bit). Common in heavy industry, agricultural equipment, mobile machinery.
- Capacitive — capacitive coupling between rotor and stator. Newer technology. Combines optical-class resolution with magnetic-class robustness. Premium pricing.
Selection decision framework
- Does the system need to know position after power loss without homing? Yes → absolute. No → incremental is cheaper.
- Does the axis travel more than one revolution? Yes → multi-turn. No → single-turn.
- Is the asset expected to run for 10+ years without battery service? Yes → mechanical multi-turn. No (or 5-year service interval acceptable) → battery-backed multi-turn is fine.
- Is the environment harsh (dust, oil, vibration, washdown)? Yes → magnetic. No → optical for highest resolution.
- What protocol does the PLC/drive support? Match to SSI, BiSS, EnDat, PROFINET, EtherCAT, or whatever the controller speaks.
Frequently asked questions
What is an encoder?
An encoder is a feedback device that measures shaft rotation, converting mechanical motion into an electrical signal that a PLC, motion controller, or servo drive can read. Encoders come in two main types: incremental (outputs pulses you must count) and absolute (outputs the current position directly). Used for speed feedback, position control, and motion synchronisation.
What is the difference between absolute and incremental encoders?
An incremental encoder outputs pulses (A and B square waves 90° out of phase) that you count to track position. After power loss, position is unknown until you home. An absolute encoder outputs the current position directly as a digital word, so position is known immediately on power-up without homing. Absolute encoders cost 2-5x more but eliminate the homing step, which is critical for safety-related axes and unattended assets.
What is the difference between single-turn and multi-turn absolute encoders?
A single-turn absolute encoder reports position within one revolution (0-359°). A multi-turn absolute encoder additionally counts revolutions (typically 4,096 to 65,536 turns). If your axis travels more than one revolution (leadscrews, rotary tables, conveyor drives), you need multi-turn. If it returns to the same physical position each cycle (a single rotary index), single-turn is enough.
What is a battery-backed multi-turn encoder?
A multi-turn absolute encoder that counts revolutions electronically using a small lithium battery to maintain the count when power is off. Cheaper than mechanical multi-turn but the count is lost when the battery dies. Typical battery life is 5-10 years. Battery replacement usually requires the encoder to be powered up to avoid losing the count.
What is mechanical multi-turn?
A multi-turn absolute encoder that uses an internal gear train to count revolutions mechanically. The count is preserved regardless of power state — no battery, no electronic memory. Survives indefinitely. Costs 30-50% more than battery-backed multi-turn but is genuinely absolute. Recommended for long-lifecycle assets (20+ years) and mission-critical positioning.