Geraint Thomas from Control Techniques offers some insight into fieldbus systems, and explains how and why they benefit machine builders.
Fieldbus systems have grown into an essential part of any device used today in homes and workplaces; from smart energy meters showing where we are using our electricity, to a printing machine that prints the daily newspapers. This is also a hot topic at the moment with the Internet of Things (IoT), where a system of computing devices, mechanical and digital machines, objects, people that can provided the ability to transfer data over a network without requiring any interaction from humans. Now this is achieved by the use of modern fieldbus networks.
Fieldbus is the name given to a digital communication network that is used in automation and control. Where fieldbus networks are best used is within industrial environments, such as factories and manufacturing plants.
In the past, control systems were implemented using point to point wiring. This meant every part of the machine that was to be monitored would have to be individually connected to a central controller, creating miles of cabling. Using a fieldbus system the complexity of point to point wiring can be drastically reduced, using a single communication cable and the use of remote I/O hubs to relay signals back to the controller.
There are two types of fieldbus control, centralized and decentralized. Traditionally most complex machine designs relied on centralised control with one programmable logic controller (PLC) coordinating the function of the entire machine. With the increasing complexity of certain applications, designers are now implementing distributed control schemes allowing complex functions in the machine to be managed at a local level.
Using the example of a concert; the centralised control strategy can be considered a ‘one man band’, where all instruments are played by one person. This can put considerable demands on the musician, trying to play all instruments in perfect tune and in time. With a distributed control approach each instrument is played by an individual musician with only a single task to perform in a similar way to an orchestra. The system can then be coordinated using the fieldbus.
Let’s look at these two types of control using industrial equipment. Centralised control systems usually require a powerful controller to coordinate the machine and perform all the tasks of the machine concurrently. The control is orchestrated from one central controller. All the programming will come from the controller allowing complex programs to be controlled in one programme. When doing this the controller can become bottle-necked when commissioning the machine, as all testing must be performed centrally. The network can have processing delays as all the functions come from the controller.
Decentralised networks require local intelligence in each device, or ‘automation intelligence’. Some co-ordination is required within the machine, however no specific node (a node being a connection or redistribution point) has overall control. Allowing peer-to-peer communications (where devices exchange information) creates the opportunity for faster response times with reduced network traffic. A decentralised system typically has a lower cost implication compared to a dedicated PLC as the PLC’s full functionality is not always required.
Since the early 1970’s various networking topologies have evolved. Some of these systems have survived on their technical merits alone while others have persisted with effective marketing. Industry acceptance and geographical preferences have been a key driving force behind various fieldbus systems in industry.
Early fieldbus systems were predominantly used to replace large cable looms for input, output and control. Initially used to reduce production and maintenance costs, the technology evolved to encompass control loop synchronisation and higher level programming of devices using a single network.
The next generation of fieldbus utilises Ethernet to expand on the previous features adding a more coherent communication flow from the shop floor to board room. Modern Ethernet-based systems allow high speed determinism, clock sharing and control loop synchronisation while simultaneously allowing information flow from the cell level to manufacturing/order processing systems and beyond.
This leaves the route open for effective process and production control, allowing management to effectively control production, optimise processes and reduce down-time through preventative maintenance. The future will see truly efficient and proactive manufacturing processes, not only within single companies but also extending to group companies and effective supply chain integration.