Industrial Encoder Selection: Incremental vs. Absolute
1. Function and Classification of Encoders
An encoder converts mechanical angular or linear displacement into electrical signals, and is a key feedback element for closed-loop control in servo systems, CNC machine tools, cranes and metallurgical lines. By signal principle encoders divide into incremental and absolute types; by sensing method, optical and magnetic.
2. Incremental Encoders
Incremental encoders output A/B quadrature pulses plus a Z index pulse, and a counter accumulates pulses to derive position. Advantages: simple structure, low price and a wide choice of resolutions. Disadvantage: position is lost at power-off, requiring a homing routine after every power-up. Typical applications: VFD speed feedback, conveyor counting and ordinary positioning. The HEIDENHAIN ERN 430 series (1024 lines, HTL output, hollow shaft) is widely used for speed feedback on cranes and metallurgical equipment.
3. Absolute Encoders
Every angular position of an absolute encoder corresponds to a unique digital code; single-turn types resolve 2ⁿ positions per revolution, while multi-turn types record revolutions via gearing. The biggest advantage is that position is retained after power-off — the current absolute position is available immediately on power-up without homing, greatly improving equipment safety and startup efficiency. Typical applications: CNC feed axes, robot joints, automated warehouses, wind turbine pitch control and elevator leveling.
4. Selection Points
- Resolution: for incremental types look at pulses per revolution (PPR) — 2500 PPR or above for servo feed; for absolute types look at bit width — 17–23 bits single-turn is common
- Output type: HTL (push-pull) offers strong noise immunity and long transmission distance for heavy industry; TTL (differential 5 V) suits CNC systems; bus types (PROFIBUS, PROFINET, CANopen, EtherCAT) suit modern automation networks
- Shaft type: solid shafts need a coupling; hollow/blind shafts mount directly onto the measured shaft — simpler and space-saving
- Protection and environment: choose IP65+ for dusty, wet environments; magnetic types for high temperature (they tolerate heat better than optical)
- Supply voltage: commonly 5 VDC or 10–30 VDC — match the control system when ordering