Why the Machine Lost Its Position After a Power Cycle
A packaging line I worked on used an incremental encoder on the feed axis. Every morning, the operator turned on the machine, and the axis started wandering — it moved to the wrong position, the bags were mis-sealed. The root cause wasn’t mechanical. It was the encoder type. An incremental encoder counts pulses, but it has no memory of where it is when the power drops. After every power cycle, the controller had to re-home the axis — and nobody had programmed that step. We ended up replacing the encoder with an absolute one, and the morning chaos stopped overnight.
This article walks through the real engineering difference between incremental and absolute encoders, when to use which, and the hidden pitfalls that bite you on the shop floor.
Incremental Encoder: Pulses, No Memory
An incremental encoder outputs two square-wave channels (A and B) as the shaft rotates. By counting pulses and reading the A/B phase relationship, the controller knows direction and relative position. But it has no clue where “zero” is after a power cycle. The count starts at whatever the register happens to hold. To find a reference point, you need a home sensor and a homing routine.
Strengths: cheap, simple, high resolution (up to millions of counts per revolution), rugged.
Weaknesses: requires homing after every power-up; lost counts on electrical noise; multi-turn position is gone when power drops.
Absolute Encoder: Position is Encoded on the Disk
An absolute encoder has a unique digital code for every angular position on its disk. When you power it up, it immediately reports where it is — no homing. A single-turn absolute encoder covers one revolution (0–360°). A multi-turn absolute encoder tracks how many revolutions have happened, often backed by a battery in the feedback cable or a self-contained energy-harvesting mechanism.
Strengths: power-up position is known instantly; no homing routine; safe for machines that must not move when energized.
Weaknesses: more expensive; slower data rates (because the word is wider); battery replacement adds maintenance.
How to Choose: Three Practical Questions
1. Does the machine have to be safe on power-up?
If the axis must not move on its own after a power cycle (a vertical axis holding a load, a robot arm over a conveyor), you want an absolute encoder. An incremental encoder forces a homing move — and that move can be dangerous if the axis is suspended.
2. Is downtime acceptable?
An incremental system requires a homing sequence every morning. If that adds 30 seconds per shift and risks mis-homing (a dirty home sensor, a mis-triggered limit), an absolute encoder pays for itself. On high-utilization lines, the avoided downtime often justifies the cost.
3. What is the noise environment?
Incremental signals (5 V TTL) are vulnerable to noise — a stray pulse is a lost count, and position drifts. If the encoder cable runs near a servo motor cable, use differential line drivers (RS-422) and shielded cable. An absolute encoder over a serial bus (BiSS, SSI, EtherCAT) is inherently more noise-immune because the position is a complete word, not a pulse train.
The rule of thumb: If the axis is horizontal, short-stroke, and you can tolerate a 10-second homing routine, incremental is fine. If the axis is vertical, multi-turn, or must be safe on power-up, spend the money on absolute. The morning-after problem almost always traces back to choosing incremental where absolute was required.
Single-Turn vs Multi-Turn: Don’t Mix Them Up
This is the most common spec mistake. A single-turn absolute encoder knows the angle within one revolution, but if the shaft turns 5 revolutions while the machine is off, it has no idea how many times. For a linear axis driven by a ball screw (where 10 revolutions = 100 mm of travel), a single-turn encoder is useless — you need multi-turn. Always calculate the maximum travel in revolutions and choose accordingly.
A Decision Checklist
- Does the axis move vertically or hold a load? If yes → absolute.
- Does power-up need to be instant (no homing move)? If yes → absolute.
- Is the travel more than one revolution? If yes → multi-turn absolute.
- Is the cable routed near servo power? If yes → differential (incremental) or serial (absolute).
- Can you accept a daily homing routine? If yes → incremental is fine.
- Is the budget tight? Incremental saves 30–50%.
- Is the application safety-relevant? Absolute is nearly always required.
The Bottom Line
The encoder you pick shapes the machine’s behavior. Incremental is a cheap, workable choice when you can home every cycle and noise is controlled. Absolute is the right call when power-up must be safe, multi-turn position matters, or downtime is expensive. The line that wandered every morning didn’t need better software — it needed an encoder that remembered where it was.
