Encoder Selection
An encoder is the position and speed feedback device that closes the control loop on a servo axis or monitors the position of a conveyor, rotary table, or linear actuator. Selecting the wrong encoder — wrong resolution, wrong interface protocol, wrong IP rating, or wrong shaft loading — is one of the most common causes of servo system instability, positioning errors, and premature bearing failure in special purpose machines. The encoder selection must be coordinated with the servo drive, PLC motion module, and mechanical design before any motion axis is finalised.
- Servo axes on SPMs: Rotary incremental or absolute encoders mounted directly on servo motor shafts (typically via motor feedback port) — selection is usually determined by the servo system brand (Siemens, Mitsubishi, Yaskawa) and communicated via proprietary protocol (EnDat, HIPERFACE, A-quad-B).
- Conveyor speed and position feedback: Separate shaft encoders (hollow shaft or solid shaft, TTL or HTL output) mounted on conveyor tail shafts or drive rollers — used for synchronisation and web tension control.
- Rotary index tables: High-resolution absolute encoders (SSI or BiSS-C interface) to maintain position reference across power cycles without homing routines.
- Linear position measurement: Linear encoders (optical or magnetic scale) used on press slides, welding heads, and dispensing gantries — resolution typically 1 µm to 10 µm depending on process requirements.
Encoder selection parameters: (1) Type — incremental (outputs A, B, Z pulse trains; loses position on power-down, requires homing) vs. absolute (outputs full position value; retains position across power cycles). (2) Resolution — pulses per revolution (PPR) for incremental, or bits for absolute (12-bit = 4096 positions/rev, 17-bit = 131072 positions/rev). Higher resolution is not always better — the mechanical coupling backlash and system noise floor must be considered. (3) Interface — TTL (5V, up to 100m cable), HTL (10–30V, better noise immunity, used in industrial environments), SSI (synchronous serial, used for absolute encoders), EnDat 2.2 / HIPERFACE DSL (single-cable motor feedback, proprietary to drive brand), BiSS-C (open protocol absolute). (4) Environmental — IP rating (IP67 for washdown, IP65 for general industrial), operating temperature range, and shaft seal type for food/pharma applications.
- Selecting a TTL-output encoder for a long cable run (>10m) in a panel with VFDs — TTL signals on long cables in high-EMI environments produce false counts and positioning errors.
- Specifying an incremental encoder on an axis that must retain position across power cycles (e.g., a vertical axis with a brake) — on power restore, the axis must home before it can move, creating production downtime.
- Mismatching encoder PPR to the servo drive's electronic gear ratio settings — an incorrect ratio causes the drive to report wrong position values, leading to positioning errors that appear intermittent.
- Exceeding the encoder's maximum shaft loading (axial and radial) with a direct-coupled mechanical load — premature encoder bearing failure within 3–6 months of commissioning.
- Not specifying the connector type and cable exit direction during mechanical design — the encoder is selected correctly but the cable exits toward a guard or heat source, requiring an expensive redesign.
ClusterVise cross-references servo drive selection with compatible encoder types and interface protocols — ensuring that the encoder specified in the BOM matches the drive's feedback requirements. For axes where the servo motor includes an integrated encoder (as is standard for major brands), ClusterVise includes the correct motor-encoder combination as a paired line item rather than two separate selections, eliminating the most common encoder specification error in early-stage BOMs.
