In many process plants, the PLC simply plays the role of an ‘interlock’ or PID code repository, with most of the system intelligence based in the external scada, MES or analysis software application zones.
Volumes of I/O and tag variables are scaled and mapped to higher-level systems, essentially just providing the process image to the intelligence layers. In this context, the computational power of the PLC is less important than its memory capacity and its communication throughput to the control platforms. It makes complete sense to use a PLC more as an I/O interface than as a process controller here, since the intricacies of large scale process plants often fall outside the scope of the traditional PLC programmer, or PLC for that matter. The ability to adapt to complex control cycles easily through parameter exchange rather than PLC program development is final justification for these types of control systems.
Parameter exchange vs machine control
The automotive industry is an example of more intense PLC utilisation. In this and similar industries, the PLC often acts as a ‘hub’ for connecting separate intelligent systems. This integration between systems and machines involves a higher degree of communication flexibility, as well as rigorous integration with safety logic. In production, it is common to see PLC connections to vision or inspection systems, conveyors, barcode and RFID readers, bolting and press fit machines as well as marking and printing facilities. Always, there are numerous systems with their own intelligence, while the exchange of typical Start/Stop/Done parameters still happens through hard-wired I/O or fieldbusses. When compared to typical process plant PLC implementations, the variety of distributed control and intelligence is higher. It is also more common to see production variations implemented at PLC level. Producing several different components on one machine or production line is common, but involves many mechanical and electrical adaptations more easily accomplished through PLC code integration rather than the parameter adjustments common in process plants.
OEMs demanding more
A unique level of PLC utilisation can be found in OEM machine building. Whether you are manufacturing specialised welding, cutting, injection moulding or test and verification beds – your PLC requirements are even more demanding than those just described. The combination of fast motion control and high accuracy of movement requires multi-faceted controllers. Even though many servo drives are capable of standalone operation, full scale integration and rapid response make the use of combined PLC/NC controllers a viable option. OEMs need to keep costs and component count down, leaving little room for large scale use of ‘black-box’ solutions. All sensors and actuators need direct termination to PLC I/O as efficiently as possible – the use of standard controllers across different machines is required. This simplifies inventory to the choice of a memory card to determine the destined machine function of a PLC. Visualisation is also required to be more local and less complex. Combined with data storage and remote software serviceability, these all form excellent arguments for PC based control.
New PLC capabilities mean new markets
Technologies such as combined PLC and HMI elements, TCP/IP and wireless communication have lead to new opportunities for PLCs. Industries where test and measurement tasks often relied on dedicated equipment with high integration costs are now easily within reach of many modern medium level devices. The technology drive of the IT industry is constantly pushing the price of semiconductors down and performance levels up to the point where full-scale adoption by the PLC market must become inevitable. Chipsets with built in serial transmission and Ethernet connectivity mean reduced size and implementation costs in PLC design. Modern IT chipsets like that of the ARM and Intel Atom configurations allow for high performance control, while still maintaining temperature and environmental tolerances well within the levels required of a PLC. For many manufacturers, I believe the intellectual property held in their communication and multitasking functionality will soon exceed the importance of pure memory and number crunching abilities. Older PLC CPUs with limited amounts of timer/counter/memory blocks cannot compete with modern integrated solutions for high-end applications. The flexibility of modern controllers has opened up applications like building automation, wind energy generation, robotics, medical engineering and many others.
Actual machine benefits
Let us consider the functions a PLC must fulfil in what seems like a simple application. A bagging machine needs to weigh product as it travels, close a flap at the correct weight, and then seal the top of the bag. But, the machine builder wants to offer more to his customers without having to add much complexity to the machine. Among his wish list are things like: more accurate filling; historic logging; recipes for different sized bags; simple visualisation; remote diagnostics; easy parameter adjustments; notifications and plant integration functionality. The goals are clear, make a machine that is easy to use and understand, will push information to the relevant personnel and has the ability to adapt to the needs of different customers with minimum re-engineering. Modern PLCs easily cater for all these needs.
Data-logging
Over history, machines are now capable of recording predicted versus achieved fill weights. Simple changes to the flap closure rate then allow the machine to self-tune for different bag variations. Not only is the historic data valuable for self-tuning, but it also allows accurate data storage to satisfy FDA and similar regulations. The addition of an RFID or barcode scanner to the system allows direct variant identification and exact product traceability.
Recipe control
This could easily be handled by the PLC through XML data files. A normal database could handle this as well of course, but many manufacturers prefer independent XML files due to the ease of distributing complex recipes to different machines. PLCs now have the ability to read (and write) XML files as well as CSV, XLS and any other text-formatted files. This allows modification from user inputs on the HMI or across the network from a PC. New recipe files can even be stored on an FTP site, allowing the PLC to retrieve updated recipe files at set intervals.
Visualisation
Machine control solutions seldom require 3D graphics or vector scaled images. A simple diagram outlining the machines components, operating state, alarms, I/O checklist and recipe configuration is mostly all that is needed. Modern PLCs allow for onboard visualisations, either through DVI/USB connections to touch panels or through built-in displays. Another convenience to the machine builder is the benefit of being able to stream HMI across the network/Internet for authorised use by personnel in remote locations. The bag machines live view, its I/O list, its alarm table, its function view, its historical data, its auto-tuned closing performance as well as its recipe management; can all easily be implemented in the same software environment that the IEC-61131-3 programming takes place.
Standardised software development makes it all possible
To implement all these added features and benefits in the modern PLC, we need an open, easy to use IEC 61131-3 PLC programming environment. The fact that approved Function Blocks already exist for most of these advanced functions then makes implementation quick and easy. Open integration into the .NET framework for expansion and external code integration future proofs your software and keeps one integrated platform for different modules. Module-based automation in software means: real-time performance; motion control; safety; hardware management; data-logging; remote connections and all forms of extensions realised in one easy to manage open PLC software development tool.
Due to the modular structure of the interfaces already predefined for many applications, there is now the possibility to instance the different controllers on the machine’s central control hardware. These can also now be created independently of one another, and in different programming languages allowing the large selection of existing (or self-developed) basic modules to form an automation kit from which new applications can be created easily.
This is the key to extended PC based control functionality.
Instrumentation and Control Systems 