Industrial control systems - SCADA , PLC and Embedded control

Industrial control systems 

In the nineteenth century, during the Industrial Revolution in Great Britain, machines were invented to mechanize the production of textiles and tools, and other such items. Industry emerged world as an important sector of human society. Production and transport processes gradually began to be carried out by machines rather than by human hands. In order to free human eyes and brains as much as possible from operating and monitoring machine processes, Industrial Control and Automation developed as industrial technology.

In the 1980s, industrial control systems only encompassed supervisory control and data acquisition (SCADA) systems, and programmable logic controllers (PLC). As microprocessors and program mable integrated circuits developed in the 1990s, industrial control systems began to incorporate computers. Computerized control systems are powerful and efficient, and thus have found more and more applications across many industries, such as electricity, water, oil and gas, chemical, trans portation, and manufacturing. Computerized control systems are different from computer control systems. In a computer control system, the computer takes the role of a supervisor separate from the controlled objects. In contrast, a computerized control system incorporates hardware and software into the system to be controlled, thus creating a single unified system. In order to differentiate, the term the two “embedded control” is used for computerized control systems.

There are two important types of embedded controls; real-time control and distributed control. Both of these have extended the scope of industrial control applications significantly. Nowadays, real time and distributed controls have become the governing control concepts and rationales of a vast range of systems, from medical instruments in hospitals to satellites in the sky. This chapter discusses three important industrial control systems; embedded control systems, real time control systems, and distributed control systems. This chapter goes through the principles and functions, the architectures and components, and the implementation techniques for each of the three control systems.

Definition and functions

All of us are familiar with personal computers (PCs), laptop computers, industrial computer workstation, supercomputers and so on. All computers have a common feature; they contain both hardware and software, integrated and packaged into one device. In hardware, modern computers have micropro cessors and programmable integrated circuits to perform computing functions. In software, computers have both machine-level instructions and advanced programming languages to produce layers of program structure composed of firmware, the operating system and application programs. In industry control systems are used in a large range of applications. For example, in factories, chemical reactors can be connected to a computer to monitor the volumes of solutions; in petrol stations, fuel pumps are connected to computers to display the volumes and prices of fuel taken; in supermarkets, cameras are connected to computers to monitor customers’ activities. All of these examples are defined as computer control.

In computer control, the computers playing the role of controller are neither inside the controlled devices nor incorporated into the controlled systems. In this case, both the input and output ports are connected to the devices or systems to be controlled. Using these interfaces, computers can read and send digital data to and from the controlled device/system. With such communications, computers fulfil their control functions. In the 1970s, modern computers with microprocessors and programmable integrated-circuits first became available, and industries started using them in some production processes. However, there were very few applications where the computers were not incorporated into the controlled system. In most cases, both hardware and software were built into the systems, becoming system components; in other words, they were embedded control systems.

The first recognizable embedded control system was the Autonetics D-17 guidance computer for the Minuteman II missile, delivered in 1966. This was the first built with integrated circuits for processor and memory. In 1978, the National Engineering Manufacturers Association of America released a standard for a programmable microcontrollers including single-board computers, numerical controllers and logic controllers able to perform control instructions. About 10 years later, in the late 1980s, microprocessor chipsets were produced, and embedded controls became widely applicable. As modern computer and electronic techniques developed in the following years, embedded control has become the most important industrial control technology available.

An embedded control system is a specially organized computer system designed for some dedi cated control function or task. Its distinct feature is that its input/output system is not connected to an external computer; the microprocessor that controls the system is actually embedded in the input/output interface itself. Though the hardware differences between an embedded controller and a standard computer are obvious, the differences in software are also substantial. While most computers are based on operating systems requiring large memory size, such as the Windows or Linux operating systems, the typical embedded control system uses a smaller operating system, which has been developed to provide a simple and powerful graphical user interface (GUI).

Although in embedded control, the central processing unit (CPU) will mainly run independently of any supervisory controller, it is always linked to other parts of the controlled system to obtain digital status information. In this sense, embedded control can be consider as a subset of the overall data acquisition process. In summary, an embedded control system refers to the computer hardware and software which are physically embedded within a large industrial process or production system. Its function is to maintain some property or relationship to other components of the system, in order to achieve the overall system objective. Embedded control systems are designed to perform a specific control task, rather than multiple tasks as a general-purpose computer does. Embedded control systems are not always separate devices or components, but are often integral to the controlled devices. The software designed for embedded control systems comprises embedded firmware, embedded operating systems, and special application programs. Compared with general computer software packages, embedded system software is rather smaller in size, and thus able to run with limited hardware resources, and without the keyboard and screen needed for most applications. Usually, the embedded control software is stored in memory chips rather than on a hard disk. Due to these differences, embedded control systems have achieved wide applicability in industry world-wide.