Ethernet is becoming an increasingly important network in industrial applications. In motion control, Ethernet, fieldbus, and other technologies (such as peripheral component interconnects) have historically competed for control of some of the most demanding workloads in industrial automation and control systems. Motion control applications require determinism (ensuring the network can deliver workloads to designated nodes in a timely manner), which is essential for maintaining position, and in turn, for precise stopping of drives, appropriate acceleration/deceleration, and other tasks.
The standard IEEE 802.3 Ethernet has never met these requirements. Even though full-duplex switching and isolated collision domains have rendered the obsolete CSMA/CD data link layer obsolete, it still lacks predictability. Furthermore, the high complexity of TCP/IP in a typical stack is not optimized for reliable delivery of real-time traffic. Therefore, fieldbus and PC control architectures with ASIC-based PCI cards have remained common motion control solutions.
Ethernet solutions, from EtherNet/IP® to EtherCAT®, overcome these shortcomings in unique ways. While Industrial Ethernet offers several other advantages over alternative technologies, it is far from dominant in motion control. Let's look at three reasons why it has been and will become increasingly accepted in the coming years.
1. Integrate rather than add complexity
As interconnectivity between enterprise IT and factories increases over time, systems become more complex, often mixing standard Ethernet and industrial Ethernet with fieldbuses. For example, machines might utilize:
*Applicable to SERCOS1 for communicating with servers
*PROFIBUS® for networked inverter drives
*SafetyBUSp for fail-safe fieldbus communication
*Applicable to DeviceNet connected to sensors
*Suitable for Ethernet networks that send data to end users and are accessed via a gateway.
Such networks are complex, and their setup and maintenance are expensive. Each protocol requires its own implementation procedures, installers, and training. In contrast, Ethernet offers the possibility of consolidating different networks suitable for sports, security, and other applications into a cost-effective infrastructure that is easier to cable, has broad vendor support, and can adapt to future requirements.
Ethernet offers the possibility of convergence of different networks.
The EtherNet/IP protocol demonstrates how convergence can be fully leveraged in practice. By using standard Ethernet technologies such as TCP/IP and UDP/IP, supplemented by features like CIPSync (for implementing distributed clock synchronization using the IEEE 1588 precision time protocol), integrated switching systems can adapt to both commercial and industrial applications.
2. Determinism is applicable to motion control applications.
Motion control relies on precise communication. This precision is supported by time-slot-based scheduling, where each device has a schedule table for communicating with other devices within the scheduling strategy. These servo drives and controllers calculate their respective timings, from which the ∆T value of the control function can be calculated. However, if data transmission becomes unpredictable, results may be lost, thus determinism is needed to ensure loop stability.
Ethernet enables demanding motion control applications in factories. In some cases, implementing IEEE 1588 in EtherNet/IP via accelerator circuitry directly integrated into Intel® chips is just one common mechanism for Ethernet solutions to enforce determinism. EtherCAT's high-speed real-time processing is another example of how consistent predictive performance can be achieved in motion control applications. EtherCAT overcomes the strict physical limitations of centralized PCI-based communication, which requires rapid communication between machine processing units and servo processors over short distances.
In a 2010 MachineDesign article, Jason Goerges explained: "EtherCAT-based distributed processor architectures offer wide bandwidth, synchronization, and physical flexibility, comparable to centralized control while also possessing the advantages of distributed networks."
"In fact, some processors that use this method can control up to 64 highly coordinated axes (including position, speed, and current loops, as well as commutation) at a sampling rate and update rate of 20 kHz."
3. Long-term feasibility for IIoT
Since its inception as a local area network (LAN) technology, Ethernet has undergone a series of developments. Given the current small manufacturing scale of traditional fieldbus components and the risk that PCI is gradually becoming an obsolete industrial standard architecture, Ethernet has been continuously developed and is now fully capable of serving the IP-based Industrial Internet of Things (IIoT).
Upcoming improvements (such as time-sensitive networking complementing IEEE 1588 and supporting network convergence) also make Ethernet an ideal choice for current and future motion control. This doesn't mean fieldbuses and PCI will disappear; it simply means that Ethernet's advantages will continue to grow as the automation industry moves towards the Internet of Things (IIoT).
References
1. Paul Brooks. “EtherNet/IP Applications in Motion Control”, Industrial IP Advantage, October 2015
2 "Successfully Implementing Motion Control via Standard Ethernet" Industrial Ethernet Book, Vol. 48, No. 71
3. Jason Goerges. “EtherCAT Enables High-Performance Motion Control” MachineDesign, November 2010.
