There are many, some may say too many, fieldbuses in use in manufacturing plants. The term fieldbus defines a method of connectivity between the essential elements in geographically enclosed areas, such as, a manufacturing plant or factory.
We use Ethernet LAN for all our normal connectivity in our offices, so why do manufacturing plants not follow the same trend? In the era of Industry 4.0, in which connectivity is a core element and IIoT is on every agenda, how relevant are fieldbuses? If there are so many fieldbuses, how do control elements manage to talk to each other? In other words, how is the serious topic of interoperability addressed?
The original objective of fieldbus was to replace any point-to-point links between sensors to PLCs or CNCs or other controllers. The fieldbus defines the electrical characteristics of the connection and also the protocol. With flexible electronic modules, the attention to the electrical characteristics has diminished, and attention to protocols has increased.
Electrical characteristics, protocol & standardisation
Fieldbuses could be classified by several features or characteristics. Mostly, one checks for speed (bit-rate), number of nodes permitted (single-master, multi-master), flexibility in topology (linear, multidrop, ring, tree, star), enhanced availability (redundancy), methods of data capture (polling, cyclical, event initiated), how new nodes can be added (system reset, on-the-fly), how fall-out of one or more nodes is detected and corresponding recovery procedures and so on.
Communication protocol is the set of rules which define the different syntax elements and the ‘grammar’ – the position of the elements, the allowed values and significance of the same. By following a protocol, it is possible to initiate a communication, flag errors in communication, detect a broken communication, and recover from the break. Also, more than anything else, communication protocols should be common across vendors, to ease the job of maintenance and spares. Yet, the world of fieldbuses is a veritable Tower Of Babel, and the species and sub-species of protocols can easily be compared with the zoological tree, mostly not quite compatible to each other.
Why are there so many fieldbuses?
Due to the wide variety of application areas and equally varied demand on the properties, a large number of fieldbuses have sprung up. Manufacturers defined and developed own protocols in order to obtain the best performance from their products.
The various application areas are process plants, discrete manufacturing, building automation, and to some smaller extent, safety automation. Subspecies emerged from specific industries, like power generation, oil & gas, automobiles, breweries, and so on. The area of sensors (this was called the ‘field’ and hence, comes the name, fieldbus), had their own buses.
With the universal spread of Office LAN, ethernet has obtained a position of dominance in networks. It is important to remember that the ethernet standard is not a complete protocol. In its basic form, ethernet is not even a deterministic protocol. However, with increase in speed (bit-rates), these disadvantages have been overcome by different manufacturers. Famous protocols at this level are PROFINET, Modbus/TCP, EthernetPOWERLINK, EtherCAT, DeviceNet and many others. Now we may reach a situation where such bus systems can become a homogeneous solution all across the plant, from sensors to the management terminals.
As we are all along intoning, there are just too many protocols for comfort. But essentially what is desired is interoperability. That means no matter what bus runs as the backbone in the plant, a freedom should exist to buy and connect a device from any vendor, and the system should be able to talk to this new device and vice-versa.
The interesting thing about this is that interoperability can be achieved at several levels. Here we take a reference model from ISO, the famous 7-layer model. The communication stack is abstracted into seven layers – a standard called The Basic Reference Model for Open Systems Interconnection. What’s more, we can show interoperability for two devices at any one or more of the seven layers. The task of protocol conversion gets shifted to the next higher level.
Over the period, many attempts were made to arrive at a standard protocol. This gave rise to many ‘standard’ protocols – vendors’ standard protocols, buyers’ standard protocols, industry body standard protocols. Some organisations, usually consortiums of vendor manufacturers, have evolved standards – meaning an agreement among their members. In the nature of things, these members, after achieving agreement, went ahead and, during implementation, provided additional features of their own to give special benefits. Unfortunately, these add-ons prevent complete interoperability.
Another approach was from large user companies or communities. They defined standards which they tried to compel all their vendors to follow. However, since different groups of manufacturers framed standards to suit their area of application, this did not result in any major reform.
Open protocols & focus of Industry 4.0
A different approach is to provide the entire protocol stack in the public domain. With this, every manufacturer has a possibility to incorporate compatibility into his devices with such a protocol, without having to pay any license or royalty fees. The open source approach has many enthusiastic followers. But if manufacturers would push in add-on features on the top of this definition, once again we have some incompatibility.
Besides, Industry 4.0 is a new era in manufacturing and business. The focus of today is to extract value out of data for various purposes. One important strategy is aggregation of data from various sources and recording it together with the context of creation of data.
Interplay with data systems
In this era of Industry 4.0, we are hungry for data. The requirement of data for these purposes is very different from the real-time deterministic demands of operation and control. However, the sources of data are quite nearly same, and so we should see many changes in the times to come. Since Industry 4.0 lays a strong emphasis on collaboration end-to-end along the value chain, there has to evolve better ways to share data.
The emergence of the Industrial Internet of Things (IIoT) and new possibilities for connectivity with wireless and ethernet are changing the protocol landscape. New value in integrating public and private enterprise clouds, operational systems and business domains is presenting new opportunities for protocol harmonisation in industrial environments. Deployment of wireless in shopfloor is in the beginning stages. Already a number of protocols have sprung up, such as, LoRA, ZigBee, Wireless HART etc.
OPC-UA and beyond
OPC Unified Architecture (OPC UA) bridges the gap between the IP-based world of IT and the production floor. Interfaces, gateways and the associated loss of information are a thing of the past because all production process data is transferred via a single protocol – within a machine, between machines or between a machine and a cloud database. OPC UA is eliminating the need for traditional factory-level fieldbus systems. The soft facts like easy implementation, openness, vendor independent, risk avoidance, conformity, interoperability, long-term availability, and overall distribution that makes OPC UA gain acceptance in today’s industries.
OPC UA has had its limitations when it comes to complex processes with real-time requirements until now. Adoption of publish-subscribe model (pub/sub) and implementation of IEEE 802.1 standard for time-sensitive networking (TSN) aim to give the OPC UA communication standard real-time capability.
Moore’s Law has been in operation since nearly forty years and shows no signs of relaxation. Just on the horizon are 5G communication, satellite based broadband, rapid advances in computation. A combination of all of these, with Artificial Intelligence assisting development, would change the way communications happen – between sensors, machines, machine and humans, and who knows, humans and humans.
Courtesy: B&R Industrial Automation