The Encoder That Lost Position Every Power Cycle
A servo axis used an incremental encoder. Every time the machine was powered down and back up, the axis had to home — the drive didn’t know where it was. The homing routine took 10 seconds per axis. With four axes, every startup was 40 seconds of homing before production. The customer asked why we didn’t just use absolute encoders. We had specified incremental because they were cheaper. The cost savings was about $80 per axis. The daily cost of the 40-second homing delay was more than that in a month. We switched to absolute encoders. Startup was instant. The drive remembered the position through power cycles.
Encoder selection for motion control isn’t just picking a resolution. The encoder type (incremental vs. absolute), the communication protocol, the resolution, and the mounting all determine whether the system positions correctly and recovers from power cycles. This article is how I pick encoders that match the application.
Incremental vs. Absolute: The Fundamental Choice
Incremental Encoder
An incremental encoder outputs pulses as the shaft turns. Channel A and B pulses (90° out of phase) indicate direction. A zero pulse (Z) gives a reference once per revolution.
The drive counts pulses relative to a home position. When power is lost, the count is lost. On restart, the drive must find the home reference (homing routine) to know where it is.
Best for: Axes that home once on startup and then don’t need to remember position. Low cost. Simple. But requires a homing routine.
Absolute Encoder
An absolute encoder outputs the actual position value (as a number) with every read. Even when power is off, the encoder retains its position (via a battery or by the absolute position being inherent in the code disk). On power-up, the drive reads the position directly — no homing needed.
Best for: Axes that need to remember position across power cycles, machines where startup time matters, and axes that can’t home safely (vertical axes with gravity load).
| Feature | Incremental | Absolute (battery or multiturn) |
|---|---|---|
| Position on startup | Unknown (must home) | Known immediately |
| Homing routine | Required every power-up | Not needed |
| Cost | Low (half to 1/3 of absolute) | Higher |
| Battery required | No | Single-turn no; multiturn yes (or battery-free absolute) |
| Noise immunity | Susceptible (pulse counting errors) | Digital protocol (more robust) |
| Best for | Cost-sensitive axes, single-axis homing | Multi-axis machines, safe startup, vertical axes |
Resolution: More Isn’t Always Better
The encoder resolution (counts per revolution, or CPR) determines the position feedback granularity. But more resolution isn’t always better.
How Much Resolution Do You Need?
The resolution must be finer than the required positioning accuracy. If the axis needs ±0.01 mm, the encoder resolution must be at least 0.001 mm (10× finer for control loop resolution). A 2,500 CPR encoder on a 10 mm/rev screw gives 0.004 mm per count. That’s enough for ±0.01 mm positioning. A 10,000 CPR encoder gives 0.001 mm per count — finer than needed, more expensive, and more susceptible to noise.
Calculate the required resolution from the accuracy requirement and the mechanical ratio. Don’t just buy the highest CPR you can find.
Interpolation (Multiplication)
Modern drives interpolate the encoder signal electronically. A 2,500 CPR encoder with 4× interpolation (quadrature) gives 10,000 counts. Some drives interpolate further (×4096 or more). This gives high resolution without a high-CPR encoder. But interpolation amplifies noise — a dirty or damaged encoder track gives interpolated noise.
For most applications, 2,500–5,000 CPR with drive interpolation is sufficient. Higher resolution encoders (20,000+ CPR) are for very high-precision applications (micrometer positioning, optics).
Communication Protocol: How the Encoder Talks to the Drive
Modern encoders use serial protocols (not just A/B/Z pulses). The protocol sends position data, status, and diagnostics digitally.
Common Protocols
- BISS-C: Open-source, high-speed, digital. Used by many servo drives. Single cable (power + data).
- EnDat: Heidenhain’s protocol, high precision, used on high-end encoders. Includes diagnostics and temperature.
- Hiperface: SICK / Stegmann’s protocol, digital absolute.
- Proprietary (Maker-specific): Mitsubishi, Yaskawa, Panasonic each have their own encoder protocol. The encoder must match the drive brand.
For a custom machine, match the encoder to the servo drive you’re using. If you use a Yaskawa servo, use a Yaskawa-compatible encoder. Mixing brands (third-party encoder on a proprietary drive) usually doesn’t work.
Mounting: How the Encoder Attaches to the Motor
The encoder mounting determines its reliability. A poorly mounted encoder introduces errors.
Motor-Mounted (Integral)
The encoder is built into the servo motor (on the rear shaft). It’s factory-aligned and sealed. Standard for most servo motors. The encoder comes with the motor — you don’t select it separately.
External Encoder (Linear or Rotary)
For axes where the motor isn’t on the load (e.g., a ball screw driven by a belt, or a linear motor), an external encoder measures the actual load position, not just the motor rotation. This is “direct feedback” — the control loop closes on the actual position, not the motor’s estimate.
External encoders are mounted to the machine frame. A coupling connects the encoder shaft to the load. The coupling must be bellows-type (zero backlash) and aligned. A sloppy coupling introduces error between the encoder reading and the actual position.
Direct vs. indirect feedback: If the encoder is on the motor, the drive knows the motor position but not the load position (belt stretch, screw backlash, coupling windup). For high-precision axes, mount an external encoder directly on the load. The drive closes the loop on the load. This eliminates transmission error.
Environment: IP Rating and Temperature
The encoder is an electronic device. The environment determines which type works.
- Temperature: Most encoders work 0–60°C. For cold rooms or ovens, specify an extended-temperature encoder.
- IP rating: Standard encoders are IP40 (not sealed). For washdown or dusty environments, use IP65 encoders (sealed housing). Integral motor encoders inherit the motor’s IP rating.
- Vibration: Encoders are sensitive to vibration. A machine with high vibration (punch presses, forming equipment) can damage an encoder. Use a heavy-duty encoder with shock and vibration ratings.
Battery-Backed Absolute: Know the Trade-off
Multiturn absolute encoders need to remember position over multiple revolutions. Two approaches:
- Battery-backed multiturn: A small battery powers the multiturn counter when power is off. The battery lasts 5–10 years. But if the battery dies, the encoder forgets position — you have to home. Keep spare batteries and replace them on schedule.
- Battery-free multiturn: The encoder uses a mechanical gear or energy-harvesting (Wiegand effect) to track turns without a battery. More reliable, no battery maintenance. Slightly more expensive.
For machines where unexpected homing is a problem (vertical axes, multi-axis cells), use battery-free encoders. The battery-backed type is fine for low-risk axes, but set a battery replacement schedule.
Signal Integrity: Cables and Shielding
Encoder signals are low-level (millivolts for analog, or digital for serial). The cable is as important as the encoder.
- Shielded cable: The encoder cable must be shielded. Unshielded cables pick up noise from servo power cables, causing position errors.
- Separate routing: Route encoder cables away from motor power cables (at least 100 mm separation). Cross at 90° if they must cross.
- Connector quality: Use industrial connectors (M12, D-sub) with locking screws. A loose connector causes intermittent signal loss.
An Encoder Selection Checklist
- Does the axis need to remember position across power cycles? (Absolute vs. incremental.)
- What is the required positioning accuracy? (Calculate needed resolution.)
- What is the mechanical ratio? (Screw lead, belt reduction.)
- Does the encoder protocol match the servo drive brand?
- Is the feedback direct (on the load) or indirect (on the motor)?
- For absolute: battery or battery-free multiturn?
- What is the environment? (Temperature, IP rating, vibration.)
- Is the encoder cable shielded and routed away from power cables?
- For external encoders: is the coupling bellows-type and aligned?
- What is the resolution vs. the control loop requirement? (Don’t over-buy.)
The Bottom Line
Encoder selection for motion control isn’t buying the highest resolution available. For multi-axis machines or axes that must start without homing, use absolute encoders. For cost-sensitive single axes, incremental works if you accept the homing routine. Match the protocol to the drive, use direct feedback on high-precision axes, and route the encoder cable away from power. The 40-second startup delay wasn’t a software problem — it was an incremental encoder on a machine that needed absolute position. The encoder that remembers where it is through a power cycle isn’t a luxury; it’s the difference between a machine that starts in 2 seconds and one that sits there homing for a minute every morning.
