One critical decision machine builders must make is what industrial protocol to use. The 2 most common industrial protocols that I personally come across are EtherNet/IP™ and EtherCAT. EtherNet/IP because of its prevalence in North America and EtherCAT because that is the protocol that KEB has standardized on and many machine builders in our served industries are adopting it in some capacity.
This post compares these two industrial protocols from the lens of a machine builder performing synchronized motion control. I find that it is usually helpful to look at extremes and synchronized multi-axis motion control is an extreme in automation. It is very challenging for both the hardware and the industry protocols they run on.
For sure, the analysis below might be a bit different for SCADA and process automation where cycle times are typically longer and real-time performance is not required.
EtherNet/IP – The basics
EtherNet/IP is ODVA’s implementation of Common Industrial Protocol (CIP™) over Ethernet. It is mainly pushed and associated with Rockwell Automation. You can find plenty of detailed information on ODVA’s website.
EtherNet/IP is an application layer implemented over standard TCP/IP protocol and is transmitted via the Ethernet physical layer. This means that EtherNet/IP utilizes the same physical hardware as office networks. Each EtherNet/IP device can send and receive messages from any other EtherNet/IP device.
On the positive side, this means inexpensive CAT5 network cables can be used for connecting devices, instead of legacy serial based protocols; IT departments and engineers will be familiar with its wiring and general operation; and control and standard network traffic can coexist on the same cable.
On the other hand, this also means EtherNet/IP inherently cannot provide real-time performance – or guaranteed execution within a certain time frame. TCP/IP packets arrive at any time, in any order, from any device. Furthermore, jitter (repeatability between cycles) can be quite large, varies heavily, and depends on available bandwidth, which means that other network traffic can affect the machine control. This has negative implications for machines requiring repeatable performance over time.
EtherCAT – The basics
EtherCAT was started by Beckhoff and is now managed by the EtherCAT Technology Group (ETG). Like EtherNet/IP, EtherCAT is transmitted over the Ethernet physical layer which means inexpensive CAT5 cabling can be used.
EtherCAT does not use TCP/IP, but uses a “processing-on-the-fly” approach where an EtherCAT master sends out a main telegram. As the telegram passes through each node, that node reads its own input data and can add output data to the telegram before it goes to the next node. Ultimately, the telegram makes a cycle through all the connected nodes and returns to the EtherCAT master.
This processing is facilitated by special ASIC chips that are present on each EtherCAT slave device. One EtherCAT telegram contains data for and from each device on the EtherCAT network, without any crisscross messaging between devices. The result is a very fast and efficient means to transmit data. Additionally, EtherCAT devices can operate in synchronous mode, bringing jitter down to a few microseconds (millionth of a second).
EtherNet/IP can support many different network topologies like star, tree and ring. Each device must have an IP address and must be on the same subnet as the other devices (must be programmed by the machine builder).
Implementing a star topology with Ethernet/IP requires the use of an additional managed switch. In general, EtherNet/IP with managed switches provides a lot of flexibility and certainly has the advantage if you were networking a complete plant floor or connecting multiple large machines together.
EtherCAT, by design, does not use IP addresses. Each device is defined by the order of network wiring. Each slave device has an In and Out port – no managed switches are required. Single panel machines can simply daisy-chain from device to device. Star and tree topologies are supported with EtherCAT Extenders. Ring topologies are supported by EtherCAT masters with dual EtherCAT ports when redundant systems are required.
Additionally, the slave devices have an embedded termination resistor so no additional network termination is required. This simplicity gives EtherCAT the network advantage for basic machines.
Performance: Synchronized Motion Control
Vanilla EtherNet/IP is not a deterministic protocol and does not handle synchronized motion control tasks well. Because of this a special profile called CIP Motion™ was developed with multi-axis motion control applications in mind.
CIP Motion uses a time-based model to execute motion control tasks. Therefore it is often coupled with CIP Sync™ which achieves IEEE 1588 time synchronization between axes by adding special hardware at each node which processes a common clock signal to ensure real-time performance. The improved performance comes with a significant price tag due to the added dedicated clock hardware.
Alternatively, EtherCAT is very well suited for synchronized multi-axis motion control right out of the box. Because of the efficient handling of telegram information and the dedicated ASIC on each EtherCAT slave, sub millisecond update rates are achievable. IEEE 1588 synchronization is achieved without any additional hardware or clock synchronization.
EtherCAT is also known for its low jitter (time difference between cyclic telegrams) which is measured in the μseconds. Low jitter will allow for a higher level of synchronization between multiple axes. The tested jitter information is often made available by EtherCAT component vendors.
If your machine needs functional safety, both EtherCAT (FSoE) and EtherNet/IP (CIP Safety) offer networked functional safety options which allow safety I/O, safety switches, and safety drives to communicate safety parameters over the bus.
Both safety protocols are independently certified to a SIL3 level.
The advantage for both of these platforms is the possibility to reduce discrete safety wiring and transfer that information over the network.
Ubiquity of Suppliers
The ETG has done an incredible job of attracting active vendor members. It claims to the have the largest number of members of any fieldbus organization in the world. A quick scan of members on the ETG site shows there are 500+ vendor member companies offering EtherCAT products.
Part of this success is due to the high performance of EtherCAT itself. Another part is due to the openness of ETG, free ETG membership, and royalty-free implementation of EtherCAT.
The result from having so many vendor members is that machine builders have many, many different product options to choose from. They are not tied into one large vendor which carries cost and obsolescence risks. EtherCAT customers have many options regarding controls, drives, I/O, encoders, safety, etc. Performance aside, the rapidly increasing number of EtherCAT products becomes an important advantage to machine builders.
Value – Price to Performance ratio
A simple value equation has performance on top and price on the bottom. By increasing performance, a product manager will increase value.
Similarly, decreasing cost increases value. Value dramatically goes up when both performance increases and price goes down.
For motion control applications, EtherCAT is clearly the winner in performance. The protocol is designed for low jitter and real-time performance without any special hardware or clock synchronization. On the device level, there is some added cost needed to integrate the ASIC with each EtherCAT slave. However, the ASICs are commercially available from multiple vendors and the prices have gone down as the number of available EtherCAT devices continues to go up.
Looking holistically, machine builders save money by not having to implement expensive managed switches. Furthermore, the wide availability of EtherCAT products drives down price which reduces costs across the board.
Judging value to machine builders, based on performance and price, EtherCAT offers a clear advantage.
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