Encoder types and configuration for servo drives
What are encoders?
Encoders are position sensors that communicate the current angular position and speed of the rotor to the servo drive. They are indispensable for field-oriented control (FOC), position and speed control, and functional safety. Different encoder types and communication protocols are used depending on the application.
Why is the choice of encoder important?
The choice of encoder directly influences the performance of the entire drive system:
- Resolution — higher resolution enables smoother speed control (less quantization noise), higher controller gains, and better torque quality at low speeds
- Commutation — FOC requires the electrical angle. Absolute encoders provide this immediately upon switch-on, while incremental encoders require offset detection.
- Accuracy — Resolution and accuracy are not the same thing. A 20-bit encoder has 1,048,576 positions per revolution, but actual accuracy depends on manufacturing.
- Safety — SIL 2/3 and PLd/e require redundant dual encoder configurations with cross-validation.
- Costs — from simple Hall sensors to high-resolution absolute encoders with safety protocol
How does it work?
Incremental encoders (ABZ):
Two rectangular signals (A and B) offset by 90° generate quadrature pulses. The phase relationship is used to detect the direction of rotation. One index pulse (Z) per revolution serves as a reference point. Typical resolution: 1024–8192 PPR (×4 through quadrature decoding). Position is lost when power is turned off.
Absolute encoder (single-turn/multi-turn):
Provide a unique digital position for each angle — the position is retained even without power. Multi-turn encoders also count complete revolutions (via gear, Wiegand effect, or battery).
BiSS-C (Bidirectional Synchronous Serial):
Open, bidirectional serial protocol with clock rates up to 10 MHz, CRC error checking, and register access. Hardware compatible with SSI. Preferred for high-resolution absolute encoders.
SSI (Synchronous Serial Interface):
Unidirectional serial protocol (read only, no writing), up to 1.5 MHz. Simple and robust, but without diagnostic or configuration options.
Hall sensors:
Three sensors offset by 120° provide rough position information (60° electrical resolution, 6 states per electrical cycle). Cost-effective and sufficient for block commutation, but too inaccurate for high-precision FOC alone.
Hall + ABI combination:
SOMANET enables the combination of Hall sensors (for immediate commutation when switched on) with an incremental encoder (for high resolution during operation). As soon as the index pulse is detected, the electrical angle is tracked to the calibrated offset value.
How does SOMANET implement this?
SOMANET drives support two independent encoder ports (0x2110 and 0x2112) with the following interfaces: BiSS-C, SSI, incremental ABZ, Hall sensors, SinCos, Nikon, and REM 16MT (integrated magnetic sensor). Each encoder is assigned one of three functions: commutation, motion control feedback, or monitoring.
The SOMANET Circulo integrates up to two magnetic absolute encoders (up to 20-bit resolution) directly in the drive housing—one on the motor side and one on the output side. This enables true dual-loop control without external encoder cabling.
Commutation offset detection (0x2009) offers three methods: current injection sweep, PID-controlled alignment, and static injection with the brake applied.
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