Multi-axis motion control typically uses event-based synchronization. This level of synchronization is defined as scheduled, absolute hard delivery of time-critical cyclic data across the network. Delivery variability must be less than 1 µs, which is also known as jitter.
While a number of networks exist for “motion control,” it is important to define the time delivery needs for the given application. Synchronized multi-axis motion will have different timing needs than a divert actuator on a conveyor system.
A requirement for jitter-free motion can narrow network choices. Here is a brief synopsis of a few common networks used in motion control applications.
Ethernet for Control Automation Technology (EtherCAT) was developed by the Beckhoff company. It is fast and deterministic, and processes data using dedicated hardware and software
It uses a full duplex, master-slave configuration, and accommodates any topology. It can process 1,000 I/O points in 30 µs and communicate with 100 servo axes in 100 µs. Axes receive set values and control data and report actual position and status. A distributed clock technique that is a simple version of IEEE 1588 synchronizes the axes with less than 1 µs of jitter.
This protocol can deliver fast throughput because messages are processed in hardware before they are forwarded to the next slave. Slaves read data relevant to them as the data frame passes and they insert new data into that same data stream on the fly. This procedure does not depend on the run-time of the protocol stack, so processing delays are typically just a few nanoseconds.
Standard Ethernet cannot guarantee jitter of less than 1 µs because of the data layer’s use of Carrier Sense Multiple Access/Collision Detection (CSMA/CD) techniques to control packet transmission. To overcome this issue, ODVA developed EtherNet/IP, and it did so without changing any of the four lower layers of Ethernet.
EtherNet/IP is an industrial application layer protocol operating over the Ethernet medium and used for communication between industrial control systems and their components, such as programmable automation controllers, programmable logic controllers or I/O systems. The “IP” stands for “Industrial Protocol,” referring to Rockwell Automation’s adoption of Common Industrial Protocol (CIP™) standards as EtherNet/IP was developed.
EtherNet/IP with CIP Motion as the application layer removes the requirement for strict determinism from the network infrastructure and entrusts the end devices with the timing information necessary to handle the real-time control needs of the application. Thus, this network can deliver the high performance, deterministic control required for closed-loop drive operation using standard, unmodified Ethernet (complying with Ethernet standards, including IEEE 802.3 and TCP/IP). CIP is also used by DeviceNet and ControlNet, so these networks are interoperable.
CIP Motion accomplishes real time data transmission through the application profiles that define the technology. Those profiles allow position, speed and torque loops to be set within a drive. This protocol makes use of CIP Sync technology, the IEEE-1588 compliant Precision Clock Synchronization, which is also mapped into the CIP object model, to coordinate precise, synchronized motion control.
CIP Sync consists of a Time Sync object and associated services for synchronizing nodes to within ± 100 ns of one another. EtherNet/IP with CIP Motion allows 100 axes to be coordinated with a 1 ms network update to all axes.
Sercos is a digital bus that interconnects motion controllers, drives, I/O, sensors and actuators for numerically controlled machines and systems. It is designed for high-speed serial communication of standard closed-loop real-time data over a noise-immune, fiber optic ring (Sercos I & II) or Industrial Ethernet cable (Sercos III).
In a Sercos interface system, all servo loops are normally closed in the drive to reduce the computational load on the motion controller and synchronize more motion axes than it otherwise could. In addition, closing all the servo loops in the drive reduces the effect of the transport delay between the motion control and drive.
Sercos III is the open, IEC-standard third-generation version that transmits data over Industrial Ethernet cabling and protocol for real-time control. It combines the best of both Ethernet and previous Sercos designs for deterministic bi-directional real time motion and I/O control. It delivers rich I/O communication capabilities while enabling all conventional protocols to be transmitted over the same Ethernet network efficiently in parallel with Sercos real-time communication.
Sercos III achieves cycle times as low as 31.25 µsec. It supports up to 511 slave devices in one network, with multiple networks possible in a system.