Interview
A reliable approach to precise, highly dynamic movements with EtherCAT
EtherCAT is by and large the fastest Industrial Ethernet technology, but it also synchronizes with nanosecond accuracy. The following interview with Dr. Guido Beckmann, Senior Management Control System Architecture at Beckhoff Automation and Head of the Technical Committee of the EtherCAT Technology Group, explains why these properties, among others, are so highly valued, especially in the motion control environment.
EtherCAT has now asserted itself as the most important motion fieldbus. Which technological features were instrumental in making this possible?
Dr. Guido Beckmann: That’s right – it has. We are now aware of over 200 drive manufacturers that support an EtherCAT interface, offering users well over 1,000 different EtherCAT drive devices for their own applications. Fieldbus systems face stringent demands in terms of drive technology, most notably including cycle time, synchronicity, and simultaneity. Typical values for the necessary cycle times lie between 1 ms and 500 µs, with a moderate 2 ms in some applications (in reference to the cyclic position specification with position control in the drive). In extreme cases, however, cycle times of 62.5 μs are also required (for a current control loop closed via bus). While synchronicity describes the temporal jitter during executing the functions in the devices involved (drives and controllers), simultaneity defines the measure of temporal offset of these functions. EtherCAT uses an approach based on what we call "distributed clocks" for synchronization control: All devices have an independent clock as a basis for running local cycles and events. The crucial factor is that all clocks run at the same speed and have the same base time. A special control integrated in the EtherCAT Slave Controller (ESC) ensures that all clocks are guided by a reference clock and are synchronized irrespective of temperature and production tolerances. The necessary simultaneity of all clocks is achieved by means of a run-time measurement of the signal from the reference clock to each synchronized device, which is also supported by the ESC in terms of hardware. Comprehensive measurements have shown that the deviations are well below 100 ns for both synchronization and simultaneity – even for large networks.
High-performance technology should also be as easy to use as possible, which has been achieved exceptionally well with EtherCAT. In relation to motion applications, which attributes are particularly advantageous in terms of aspects such as engineering and diagnostics?
Dr. Guido Beckmann: When it comes to supporting drive technology, key factors of a fieldbus system include the communication protocol and profile used, which are responsible for compatibility and efficient data exchange between controller and drive. The drive profile most commonly used in drive technology is the CiA402 profile developed by CAN in Automation. It was mapped to EtherCAT at an early stage (IEC 61800-7-3) and is supported by virtually all EtherCAT servo drives, which allows them to be automatically detected and integrated into the motion control application. It also meant that the complete tool chain and existing experience with the parameterization of associated drives could be maintained. Unlike traditional fieldbuses, EtherCAT makes it possible to achieve very short cycle times. In response to this capability, ETG introduced new operating modes in the specification to support cycle-synchronous transmission of position, speed, and torque process data. This made it possible to relocate the functionality of the setpoint generator, which was previously integrated into more complex drive controllers, to the central motion controller. It also simplifies the functionality of the drive amplifiers and enables coordinated motion control of multiple (coupled) drives in a machine on the central motion controller.
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