Control has traditionally been the domain of engineers and electricians. But as each improvement breeds the next challenge, that picture is changing. Increasingly, the goal of many manufacturers is to foster tighter collaboration between control
engineering and other parts of the manufacturing enterprise—ranging from the IT department to quality, production, finance and even upper management. The goal: to integrate control into the total business process as a way to drive even further improvements to the bottom line.
This means that more non-engineering personnel need to gain a better understanding of control technology basics. Here, we’ll take a look at the most important of these fundamentals.
Control basics
The control part of the automation system is composed of devices and software that read incoming information, analyze data and make decisions. The system then either tells another device to perform an action, or it sends information to a computer database or another application, such as an enterprise resources planning (ERP) system.
Control hardware is available in a variety of sizes, in terms of physical footprint, memory and numbers of inputs and outputs (I/O) handled. Today, almost all control platforms have embedded networking capability. The networks may be specific industrial networks, such as Profibus, DeviceNet or Foundation Fieldbus, or standard networks, such as Ethernet TCP/IP.
Like any computing device, a control system must be programmed to accomplish its tasks. Although some programmers use the C programming language, or even BASIC, most programming is done in controller-specific languages. These languages are evolving toward a standard known as IEC 61131. This European standard is gaining acceptance by most controller manufacturers. Adoption of IEC 61131 has made it much easier to train technicians because of the consistent behavior of programming components.
Pioneers of the IEC 61131 standard envisioned a paradise of total interchangeability of programming software and control hardware. Customers would be able to change platforms or mix platforms within a plant without worrying about incompatible formats. However, this interchangeability would drive all products toward commodity status and thus limit technology providers’ ability to develop products with a competitive advantage. Adoption of the standard has reduced the cost of training, but vendor innovation creating enhanced communication and control function blocks is even more important.
Multilingual
IEC 61131 defines four languages: two are text and two graphical. Structured Text is a textual language similar to Pascal. C programmers should have little trouble learning this standard language, enabling them to broaden their choices of control platforms. Instruction List is a textual language similar to assembly language. Ladder Diagram is a graphic representation of the relay logic wiring diagrams used historically for PLCs. Suppliers have enhanced this language over time with sophisticated function blocks enabling it to handle data, control process loops, perform various math functions and handle networking. Function Block Diagram graphically “wires” defined functions to perform various control algorithms.
In addition to these four, Sequential Function Charts are defined by IEC 61131 as an “organizing principle,” with a function similar to flow-charting. That is, a programmer can organize the design of a control system with a sequence of blocks in which control algorithms written in one of the defined languages are embedded.
