Imagine a robotic arm that bends and rotates, with each axis equipped with very precise motor drives, sensors or machine vision, as if playing a symphony of motion. But without a "conductor" to tell each component of the system when and how to perform its respective actions, the arm might make harsh collisions and metallic scrapes.
In previous articles in the Real-Time Control series, we looked at the real-time control (RTC) instruments used for sensing, driving, and processing. To bring them all together requires "command": real-time communications. In this article, we will use Industry 4.0 based on real-time communication and control as a starting point for our discussion.
Factors driving the development of Big Data in automation
Factory operations without human intervention have become popular due to the epidemic. The collection and proper distribution of big data (defined by the Oxford Dictionary as very large data sets that can be analyzed computationally to reveal patterns, trends, and correlations, especially in relation to human behavior and interactions) can support digital twins, metering, service charging, and predictive maintenance. For example, having available big data enables monitoring the performance of robotic arms and system operating conditions, as well as data rates, temperature, humidity, vibration, etc., leading to the development of models that can predict future performance and operating conditions based on AI that learns using big data (digital twins). To take full advantage of these benefits, it is necessary to combine information technology (IT) and operations technology (OT) to be able to support Internet Protocol (IP) as well as the RTC system edge. Logically, this is called IT and OT convergence.
In Ethernet, the network and transport layers of the Open Systems Interconnection (OSI) model support Transmission Control Protocol/Internet Protocol (TCP/IP), so Ethernet is inherently capable of supporting IPv4 (and IPv6). In addition to this, the ability to transfer the required amount of information deterministically is why Industrial Ethernet is becoming a substantial communication standard in the converging fields of industrial automation. Traditional fieldbuses are still used to communicate with edge devices because existing infrastructures typically use two-wire protocols and do not support native TCP/IP. Figure 1 illustrates the current communication methods in industrial automation.
Current communication methods in industrial automation
The way industrial communications are implemented has begun to change. Single-pair Ethernet (SPE) maintains existing two-wire system architectures while supporting the faster speeds and many benefits of Industrial Ethernet. Advanced field diagnostics support both distributed and centralized monitoring and operation. And, of course, SPE can reuse existing two-wire infrastructures built from multiple existing fieldbuses, simplifying convergence-driven upgrades and minimizing costs.
A Deeper Understanding of Ethernet
While Ethernet is open and ubiquitous in enterprise applications, it is not currently available for real-time applications because the transmission of IT Ethernet frames is "best-effort" and uncontrolled; in any case, errors are annoying. For real-time OT, errors can have serious consequences and even be dangerous, and RTC systems need reliable communications as the "conductor" of the system to ensure that the system operates as intended, thus avoiding product failure or damage to the system or injury to personnel. Because IT Ethernet is typically used in enterprise or consumer environments, there are few environmental challenges. In contrast, RTC systems are often in harsh environments.
The need for robustness, deterministic behavior (e.g., reliability over wide temperature ranges, in noisy and dirty environments), and higher data rates has driven the emergence of Industrial Ethernet. Industrial Ethernet is deterministic and robust, providing additional bandwidth and inherent IP connectivity to fully utilize RTC systems.
Here's a look at timing characteristics and how they apply to the Ethernet physical layer (PHY).
Importance of Timing Characteristics
There are three important timing characteristics in an RTC system:
Latency. In this context, it is important to consider delays such as propagation delay: the length of time from when data enters the system, subsystem, or subsystem component until it leaves. For example, TI's DP83826E 10Mbps/100Mbps Ethernet PHY has a round-trip delay of 208ns. Lower latency can reduce cycle time or increase the number of nodes on the bus.
Determinism. It doesn't matter how low the latency is if the arrival time varies greatly each time data passes through the system. This variation in arrival time is known as determinism. Low jitter means good determinism. Low determinism means you need to build less margin into the system to accommodate changing latency. Figure 2 illustrates the latency (208ns) and determinism (±2ns) of the DP83826E. Real-time Ethernet protocols such as EtherCAT can take advantage of the lower, deterministic latency characteristics of Ethernet PHYs.
Delay and its certainty
Synchronization. There are also advantages to tying the timing of an entire system or several complete systems together. In order to maximize efficiency and throughput while ensuring safe operation, different subsystems may need to "know" exactly when another subsystem will perform an operation. Industrial Ethernet protocols all support some kind of synchronization. Time Sensitive Networking (TSN) is an example of time synchronization for RTC systems. The Institute of Electrical and Electronics Engineers (IEEE) 1588v2, Precision Time Protocol (PTP), helps keep multiple devices synchronized with each other, and IEEE 802.1as, also known as generalized PTP (gPTP), further facilitates synchronization for time-sensitive applications such as RTC.
Conclusion
Successful RTC and communications deployments are the cornerstone of Industry 4.0. But more than just enabling Industry 4.0, with deterministic, synchronized, and low-latency communications PHYs and Industrial Ethernet protocols, all instruments can come together to make beautiful music.