Real-Time Control: A Beginner's Overview

Introduction to 'Real-Time' Concepts in Controlling Environments

Real-Time Ethernet is an advanced communication system designed to handle critical tasks that require not just speed, but also precision and reliability. At its core, real-time means determinism, low latency, and low jitter, which are essential for the precise control of devices like servo motors.

Real-Time Control Systems

Pros:

• Determinism: These systems ensure predictable execution times for tasks, providing reliability in time-sensitive operations.
• Low Jitter: They maintain minimal variance in task execution timing, crucial for applications requiring consistent timing.
• Low Latency: Designed for rapid response to inputs or events, they are ideal for applications where immediate reaction is critical.

Cons:

• Lower Throughput: Focused on timely execution rather than volume, they may process fewer tasks over time compared to desktop systems.
• Lower Energy Efficiency: Operating at constant speeds to maintain accuracy, they lack energy-saving features such as sleep states or dynamic frequency scaling, leading to higher energy use.

General Purpose / Desktop Systems

Pros:

• Higher Throughput: Capable of handling a wide variety of tasks efficiently, they prioritize completing a large volume of tasks.
• Energy Efficiency: With the ability to adjust operating power based on workload, including the use of sleep states and frequency scaling, they can significantly reduce energy consumption.

Cons:

• Lower Determinism: These systems cannot guarantee task completion within a tight timeframe, affecting predictability.
• High Jitter: The variability in task execution timing is higher, which, while typically unnoticed by users, can be detrimental for precision-controlled applications.

In conclusion, the choice between a real-time control system and a desktop system depends on the specific requirements of the application in question. Real-time control systems excel in environments where the accuracy of timing is paramount, sacrificing throughput and energy efficiency for precision and predictability. Desktop systems, conversely, offer versatility and energy efficiency, making them well-suited for a broad range of general-purpose applications where high throughput is valued over precise timing control.

Differences Between Real-Time Ethernet and Standard Ethernet

While standard Ethernet is sufficient for everyday internet and network tasks, it lacks the precision and reliability needed for controlling servo motors and other real-time applications. Here are the key differences:

• Predictability: Real-Time Ethernet provides a predictable communication channel, where messages are guaranteed to be delivered within a tightly controlled timeframe. Standard Ethernet does not guarantee when a message will be delivered, which can vary due to network traffic.
• Synchronization: Real-Time Ethernet allows for the synchronization of devices across a network, ensuring they operate in unison. This is essential for applications requiring coordinated actions, such as robotic arms or automated machinery. Standard Ethernet lacks this level of synchronization.
• Priority and Traffic Control: Real-Time Ethernet networks can prioritize critical data packets, ensuring they are not delayed by less important data. This is not typically available in standard Ethernet networks, where all data packets compete equally for bandwidth.

In essence, Real-Time Ethernet is the backbone of modern industrial and robotics applications, providing the deterministic, low-latency, and low-jitter communication necessary for the precise control of servo motors. It ensures that complex, synchronized movements are executed flawlessly, which is not achievable with standard Ethernet.

What is a Fieldbus?

Fieldbus technology represents a network system for real-time, distributed control among various industrial devices such as sensors, actuators, and controllers. Unlike traditional point-to-point communication, where each device requires a separate connection, fieldbus allows multiple devices to communicate over a single network, enhancing efficiency, reducing wiring complexity, and providing more precise control.

A Brief History of Fieldbus Development

The concept of fieldbus emerged in the 1980s as industries sought more efficient ways to connect and control the growing number of electronic devices in automation systems. Early fieldbus systems aimed to replace the cumbersome and expensive point-to-point wiring with a more streamlined and cost-effective communication system.

In the 1990s, the field saw significant development with the establishment of several standards, including Profibus and Foundation Fieldbus. These developments were driven by the need for interoperability among devices from different manufacturers and the desire for more advanced diagnostics and control capabilities.

The introduction of industrial Ethernet protocols like EtherCAT in the early 21st century marked a significant evolution, combining the universal applicability of Ethernet with the real-time control requirements of fieldbus systems. This allowed for greater data speeds, more devices on the network, and improved diagnostics and control.

Throughout its history, the evolution of fieldbus technology has been characterized by a continuous drive towards greater efficiency, interoperability, and real-time control capabilities, reflecting the growing complexity and demands of industrial automation systems.

Some Examples of Real-Time Networks:

Several real-time fieldbus protocols have been developed to meet the demands of industrial automation:

• Profibus (Process Field Bus): One of the earliest and most widely adopted fieldbus standards, designed for a wide range of applications in manufacturing and process automation.
  
  
• Modbus: Originally developed in the late 1970s, Modbus is a simple, robust protocol used in a wide variety of industrial electronic devices.
  
  
• CANbus (Controller Area Network): Developed in the mid-1980s, CANbus is widely used in automotive applications for connecting electronic control units (ECUs) and in industrial automation.
  
  
• PROFINET is an industrial Ethernet standard offering real-time data exchange for a wide range of automation tasks. It supports scalable real-time communication classes and integrates seamlessly with PROFIBUS devices, making it ideal for flexible and scalable network architectures in industrial environments.
  
  
• EtherCAT (Ethernet for Control Automation Technology): Introduced in the early 2000s, EtherCAT is a high-speed, Ethernet-based fieldbus system that offers low latency and high efficiency, particularly suited for real-time applications.
  
  
• Ethernet/IP uses the Common Industrial Protocol (CIP) over Ethernet to enable communication among industrial devices. It supports explicit and implicit messaging, accommodating both non-time-critical and time-sensitive applications, leveraging standard Ethernet infrastructure for automation.
  
  
• Powerlink: Provides deterministic communication over Ethernet, optimizing real-time performance and scheduling.
  
  
• SERCOS III: Combines the real-time capabilities of the SERCOS interface with Ethernet, designed for synchronized communication in automation.
  

Why IO Builder uses EtherCAT?

IO Builder uses EtherCAT because it offers exceptional speed and efficiency for real-time control and communication among various devices in an industrial setting.

Unlike traditional Ethernet, where each device requires a home run cable to the switch, EtherCAT devices can be connected in a series (daisy chain) from one device to the next. This significantly reduces wiring complexity and cost, making network expansion easier and more flexible.

EtherCAT has a unique "processing on the fly" principle: as an EtherCAT frame passes through each device, the device reads data addressed to it and immediately appends its response to the frame. This approach minimizes latencies because the frame does not need to stop at each node; data exchange occurs in a single pass, enhancing real-time performance. This makes it highly suitable for applications requiring fast, deterministic responses, such as those found in automation, robotics, and motion control systems.

If you want to learn more about EtherCAT watch this video