Modern servo systems do not connect each axis to the controller by a dedicated analog or quadrature cable. Fieldbus networks (EtherCAT, CANopen, PROFINET, and others) connect all drives and encoders to a shared communication medium.
The encoder’s position information is packetized, transmitted over the network, and received by the controller. For high-bandwidth control loops, the communication latency, jitter, and synchronization of encoder data across multiple axes determine control performance.
The encoder’s physical sensing technology is only one element; the network communication protocol is equally important.
CANopen for Servo Drives: Architecture and Timing
Network Architecture
CANopen is a master-slave bus system operating at data rates from 10 kbps to 1 Mbps. In a servo drive network:
- A single CANopen master (motion controller or PLC) manages all nodes.
- Each servo drive is a node; the drive reads the encoder and transmits position data as a Process Data Object (PDO).
- PDO configuration defines which encoder data is transmitted, at what cycle rate, and with what priority.
Process Data Objects (PDOs)
PDOs carry cyclic real-time data (position, velocity, current, status) between the master and each drive. Two PDO types:
- TPDO (Transmit PDO): Drive transmits position/status to master.
- RPDO (Receive PDO): Master sends target position/velocity to drive.
In a position feedback loop, the TPDO carries the actual encoder position. The master receives all axis positions, computes the next trajectory point, and sends new position targets via RPDOs.
Timing: CANopen supports synchronous PDO transmission, where all nodes transmit TPDOs simultaneously in response to a SYNC message from the master. This ensures that all axis positions are captured at the same moment, critical for coordinated multi-axis motion.
Data Rate Constraints
At 1 Mbps (maximum CANopen rate), each PDO carries 8 bytes. With 10 axes, each TPDO at 8 bytes = 80 bytes, plus protocol overhead ≈ 200 bytes per cycle → approximately 1,600 µs per cycle at 1 Mbps. Maximum synchronous cycle rate: approximately 600 Hz.
This limits CANopen to applications with control loop rates ≤ 500 Hz. For high-bandwidth servo control requiring 1–8 kHz position loop rates, EtherCAT is required.
EtherCAT for Servo Drives: Architecture and Performance
Network Architecture
EtherCAT (Ethernet for Control Automation Technology) uses standard Ethernet hardware but a radically different communication model. Instead of point-to-point addressed packets, EtherCAT uses a distributed telegram approach:
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- The master sends a single Ethernet frame that traverses all slave nodes in sequence.
- Each slave node reads its own input data and inserts its output data (encoder position, status) while the frame passes through, on the fly, without buffering.
- The processed frame returns to the master with all slave data filled in.
- Total cycle time for 100 axes: < 100 µs at 100 Mbit/s Ethernet.
This architecture provides substantially higher bandwidth and lower latency than CANopen:
| Parameter | CANopen | EtherCAT |
|---|---|---|
| Network speed | 1 Mbps (max) | 100 Mbps |
| Cycle time (10 axes) | ~2 ms | < 100 µs |
| Control loop rate | ≤ 500 Hz | Up to 8 kHz+ |
| Jitter | ~1 ms | < 1 µs |
| Topology | Bus | Bus, ring, star, tree |
Distributed Clocks
EtherCAT’s distributed clock mechanism synchronizes all nodes to a common time base with < 1 µs jitter across all nodes. This means all axes capture their encoder positions at the same hardware timestamp, not approximately the same time.
For coordinated multi-axis motion (gantry, scara, delta robot kinematics), this synchronization eliminates the position inconsistency that would arise from asynchronous sampling.
EtherCAT Drive Profile (CoE, CANopen over EtherCAT)
Most servo drives implement the CANopen device profile for drives and motion control (CiA 402) over EtherCAT transport, called CoE.
This provides:
- Standardized object dictionary entries for encoder position (position actual value), velocity feedback, torque feedback.
- Cyclic Synchronous Position (CSP), Cyclic Synchronous Velocity (CSV), and Cyclic Synchronous Torque (CST) operation modes.
- Homing mode, Profile Position mode for trajectory-following applications.
Encoder Interface to the Drive
Digital Incremental (ABZ Quadrature)
The drive counts quadrature edges in hardware. This is the simplest and fastest encoder interface, the drive reads position at any time by checking its hardware counter. No communication latency beyond the drive’s position register read cycle.
Limitation: only incremental position data. Absolute position is established at power-up by homing (Z pulse or limit switch) or by a parallel absolute encoder interface.
Serial Absolute (BiSS-C, SSI)
The drive initiates a read of the encoder’s absolute position by clocking a serial communication cycle. BiSS-C at 10 MHz clock requires approximately 3 µs for a 26-bit position word.
The drive must complete this read and have the position value ready for the next network cycle.
For a 1 ms EtherCAT cycle time: the 3 µs BiSS-C read is negligible. For a 100 µs EtherCAT cycle, the 3 µs read consumes 3% of the cycle, still acceptable.
Functional Safety Encoder Interfaces
For SIL2/PLd or SIL3/PLe functional safety applications:
- BiSS Safety: Adds cyclic redundancy check and sequence counter to BiSS-C frame, allowing the drive to detect communication errors
- HIPERFACE DSL: Single-cable interface (motor power + encoder data on the same cable) with safety-rated communication; requires specific cable design
- FailSafe over EtherCAT (FSoE): Safety data tunneled through the EtherCAT network; drive and encoder communicate via standard EtherCAT, and a separate FSoE protocol layer carries safety-critical data
Recommended articles to continue reading:
- RF Rotary Joints: Frequency Range, Loss Specifications, and Integration with Slip Ring Assemblies,
- in addition to our analysis of Incremental vs. Absolute Rotary Encoders: Technical Differences and Selection Criteria
- and our read on Rotary Encoders for SATCOM Antenna Pointing Systems: Requirements and Technology Selection.
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