Multi-Axis Synchronization: How Many Axes Can One Fieldbus Carry Reliably?

There is no fixed answer to “how many axes one fieldbus can carry” in a multi-axis synchronised system; it is set by the relation among three time quantities: sensor update period, bus cycle time and controller position-loop period. The three must satisfy the time budget “sensor update period ≤ bus cycle time ≤ controller position-loop period”; if any link is exceeded, synchronisation accuracy degrades to “they appear to be moving, but each is going its own way”. Synchronisation also depends on the protocol’s own mechanism — EtherCAT distributed clocks (DC) and PROFINET isochronous real-time (IRT) can guarantee that all slaves sample at the same instant, whereas a polling bus with no synchronisation mechanism can only guarantee sequential reading. Fieldbus practice is in How to Connect Magnetostrictive Displacement Sensors to a Fieldbus.

Three sources of synchronisation error

• Inconsistent sampling instants: the axis sensors do not sample at the same instant, so even if every datum is “fresh” it reflects a different moment. The higher the axis speed, the larger this error — position error is approximately axis speed times sampling-instant offset. This is the core problem that DC / IRT exist to solve.
• Stale data: when the bus cycle is longer than the controller cycle, the controller reuses data from the previous cycle, equivalent to inserting pure delay. Closed-loop phase margin falls, and a slightly high gain oscillates.
• Jitter: an unstable period is more troublesome than a long period, because the controller’s derivative term amplifies jitter into noise. Excessive bus load and aperiodic messages (diagnostics, parameter read/write) crowding the bandwidth are common causes of jitter.

The update rate on the magnetostrictive sensor side falls as range increases; long-stroke multi-axis systems especially must check this first — see Matching Response Time and Refresh Rate to the Control Cycle. Power-loss retention and synchronous refresh on fieldbus types are in Power-Loss Retention and Synchronous Refresh on Fieldbus Types.

Synchronisation capability and scale of each fieldbus

The “scale limits” in the table are qualitative. The actual number of axes must be measured on the project: load all nodes and the actual process-data volume at the target cycle time, and observe whether cycle time and jitter remain stable. A cross-protocol comparison is in Comparing the Whole Fieldbus Range: How to Pick CANopen / Profibus / EtherCAT / PROFINET.

Two hard constraints on CANopen multi-axis

CANopen is common on mobile machinery and small-to-medium systems, but two constraints cannot be ignored when adding axes:

First, baud rate is inversely related to bus length. A high baud rate requires a short trunk; long runs can only drop baud rate, which directly compresses available bandwidth and therefore the number of axes that can be refreshed synchronously. This constraint is often underestimated on large machines.

Second, telegram bandwidth is limited. Each axis must send at least one position PDO per cycle, so bus load rises linearly with axis count. In practice keep a margin on the steady-state load ratio and allow for bursts of aperiodic diagnostics and parameter traffic; otherwise the bus is fine in normal running and jitters as soon as diagnostics start. Configuration practice is in CANopen Node Address and Baud Rate Configuration in Practice; terminating resistors and cable length are in Terminating Resistors and Bus Length: Why It Has to Be 120 Ω.

Engineering practice for multi-axis systems

1. Make a time-budget table first: list each axis’s sensor update period, bus cycle time and controller period; confirm that the three close, with margin;
2. Synchronisation before speed: when axis-to-axis synchronisation is critical, rather relax the cycle by one step than omit DC / IRT / SYNC;
3. Control bus load: leave ample margin on steady-state load so that diagnostic telegrams do not cause jitter;
4. Unify sensor type and range: use the same model and range on every axis in a synchronisation group so that update rates match and a fast axis is not waiting for a slow one;
5. Calibrate zeros together: align each axis’s mechanical zero once at installation and record it. Synchronisation control compares relative positions; misaligned zeros will be mistaken for synchronisation error;
6. Topology and earthing: use a specified topology (line/star as the protocol requires); earth the shield at one end or both as specified. Interference troubleshooting is in Troubleshooting Fieldbus Interference: Grounding, Shielding, Topology.

For product choice, high-speed synchronisation prefers Series 197 EtherCAT (DC synchronisation) or the compact, wiring-saving Series 198 single-M12 EtherCAT; Siemens architectures can use Series 199PROFINET (IRT); mobile machinery and small-to-medium systems can use Series 194 CANopen.