EtherCAT (Ethernet Control Automation Technology) is a high-performance, industrial communication protocol for deterministic Ethernet. It extends the IEEE 802.3 Ethernet standard to transfer data with predictable timing and precise synchronization. Managed by the EtherCAT Technology Group, this open standard has been published as part of the IEC 61158 and is commonly used in applications such as machine design and motion control.
EtherCAT implements a master/slave architecture over standard Ethernet cabling. EtherCAT masters from National Instruments consist of real-time controllers with dual Ethernet ports such as NI CompactRIO, PXI, and industrial controllers. Each NI slave also contains two ports that permit daisy chaining from the master controller.
The EtherCAT protocol transports data directly within a standard Ethernet frame without changing its basic structure. When the master controller and slave devices are on the same subnet, the EtherCAT protocol merely replaces the Internet Protocol (IP) in the Ethernet frame.
Figure 3. Ethernet Frame Structure with EtherCAT
Data is communicated between master and slaves in the form of process data objects (PDOs). Each PDO has an address to one particular slave or multiple slaves, and this “data and address” combination (plus the working counter for validation) makes up an EtherCAT telegram. One Ethernet frame can contain multiple telegrams, and multiple frames may be necessary to hold all the telegrams needed for one control cycle.
EtherCAT is designed for reaching high performance and high-channel counts for single-point applications such as control. Because slave reads and writes can occur in the same frame, the EtherCAT telegram structure is optimized for decentralized I/O. Plus, the complete protocol processing takes place within hardware and is thus independent of the run time of protocol stacks, CPU performance, or software implementation.
Timing and Synchronization
Another factor in achieving deterministic networks is the master controller’s responsibility to synchronize all slave devices with the same time using distributed clocks. Out of all the slave devices, one of them must contain the master clock that synchronizes the other slave devices’ clocks. For NI implementation, the first slave device is designated with the master clock, and the master controller sends a special synchronization telegram to read the master clock in every scan cycle. This telegram then updates and realigns the clocks on all other slave devices to prevent drifting.
Accurate synchronization is particularly important in cases where widely distributed processes require simultaneous actions, such as coordinated motion between motion axes. NI uses timestamps to measure the time difference between the leaving and returning frame. In this way, propagation delay is calculated between nodes, and precise synchronization (less than 1 µs) can be achieved by the exact adjustment of distributed clocks.
Due to high-speed performance and its tight timing and synchronization features, EtherCAT is an ideal solution for networked or distributed motion control applications where a powerful real-time controller acts as the EtherCAT master and runs the motion control application communicating to external, distributed EtherCAT slave drives.
National Instruments EtherCAT Servo Motor Drives
NI EtherCAT servo drives combine performance with flexibility, scalability, and power range to meet the unique performance requirements of nearly any application – from basic torque and velocity applications to multiaxis motion control using graphical programming with NI LabVIEW and the LabVIEW NI SoftMotion Module.
Standard Ethernet cables significantly simplify the cabling and the capability to daisy chain up to 128 axes. A high-performance real-time controller allows the setup of a distributed motion control system within minutes. Using the LabVIEW project to configure and validate the motion control system simplifies the setup and graphical programming. With the high-level motion functions or the property invoke node API, customers can implement custom motion applications through the ease of use of graphical programming.
Figure 4. Graphical Axis Configuration through the LabVIEW Project
NI real-time controllers, AKD EtherCAT servo drives, NI LabVIEW, and the LabVIEW NI SoftMotion Module make EtherCAT technology easily accessible so customers can implement networked or distributed motion control applications.