Digital control

All of the control schemes mentioned previously can be accomplished using digital control or the traditional analog control methods. For many processes there is at present no economic justification for a digital control system if only physical disturbances are to be considered. One exception to this is a multiproduct batch processing plant, in which a number of operations are required. 

Digital computers, however, can do some things the analog equipment cannot. They offer the opportunity of using more advanced control concepts. These are not necessary for 90% of the processes, but they can increase production and reduce waste for some hard-to-manage processes. For multiproduct plants, direct digital 

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Tj = integral time constant and = derivative time constant. Upon the advent of digital control devices, this basic control algorithm was implemented as a digital approximation ... 

Coupling digital controls with networking technology permits information to be passed from level-to-level within a corporation at high rates of speed. This technology is capable of presenting the measured variable brom a flow transmitter installed in a plant in a remote location anywhere in the world to the company headquarters in less than a second. 

Although digital control technology was first apphed to process control in 1959, the total dependence of the early centralized architectures on a single computer for all control and operator interface functions resulted in complex systems with dubious rehability. Adding a second processor increased both the complexity and the cost. Consequently, many installations provided analog backup systems to protect against a computer malfunction. 

Remote control units. These units are used to control unit processes. Basic control functions such as the PID algorithm are implemented here. Depending on other hardware components used, data acquisition capability may be required to perform digital control. They may be configured to supply process set points to single-loop controllers. Radio telemetiy may be installed to communicate with MUX units located at great distances. 

Initially, the microprocessor-based single-loop controllers made the power of digital control affordable to those with small processes. To compete with these products in small applications, the DCS suppliers have introduced micro-DCS versions of their products. As a PC-based operator interface is usually a component of the micro-DCS, there is sometimes little distinction between a micro-DCS and a system consisting of single-loop controllers coupled to a PC-based operator interface. 

Distributed Control System (DCS) A system that divides process control functions into specific areas interconnected by communications (normally data highways) to form a single entity. It is characterized by digital controllers, typically administered by central operation interfaces and intermittent scanning of the data highway. 

Digital control methodologies similar to the Elliott Digital System program evolved from successful and proven reliable high-speed air compressor management and control approaches. 

The Turbolog DSP digital control system was installed to meet these requirements. This system is based on off-the-shelf hardware that is adapted to this application, running GHH-Borsig s custom-designed... 

Figure 7.13 shows the general form of a digital control system. The pulse transfer function of the digital controller/compensator is written... 

This tutorial looks at the application of MATLAB to digital control system design, using the problems in Chapter 7 as design examples. 

This is a closed-loop digital control system as shown in Figure 7.14, with a plant transfer function... 

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