Analogue control systems

The -transform was originally devised by control engineers of the early 1950 s who needed to extend the Laplace transform methods, which served well in determining the stability of analogue control systems, to the new world of digital control systems in which time delays played a more important role. 

The VPC can be readily incorporated in a conventional analogue control system. It is frequently also incorporated in advanced control systems. 

Fig. 3.7 shows a comparison between a capillary thermostat in the right part of the figure and the ever more frequently used NTC temperature sensor whose analogue electrical signal can easily be processed by an electronic control system. The NTC sensor type is increasingly used, particularly in modern European machines that always have their own heater element and sometimes also an additional hot water connection. 

Feedback control systems can be either analogue, where the controller is a mechanical device, or digital, where the controller is a computer or... 

As with any body system, there is the possibility of disease and malfunction glands can secrete too much (hypersecretion) or too little (hyposecretion) of their hormones cells in tissues can have altered sensitivity to hormones feedback control systems can break down. In these cases, hormones or their synthetic analogues are used as drugs. In other cases, hormones are used as drugs in ways that differ from their physiological role. 

Typically, the temperature drop between the evaporator and condenser of a heat pipe is of particular interest to the designer of heat pipe thermal control systems and is most readily found by utilizing an analogous electrothermal network. Figure 12.4 illustrates the electrothermal analogue for the heat pipe illustrated in Fig. 12.1. As shown, the overall thermal resistance is composed of nine different resistances arranged in a series-parallel combination. These nine resistances can be summarized as follows ... 

The process becomes even more complicated when interfacing new technology to older sub-systems. For example replacing an aging analogue hoist control system with a new PLC... 

The basic aspects of a control system are the same, independent of the size of the automated system or of the type of the controlled process. The fundamental automation tasks, which are independent of the control system used, have to be described and implemented. An example of such a control task is closed-loop control. The system tries to move a measured process variable (e.g. temperature) towards a desired, typically analogue, value (set point) and keep it there by manipulating an output variable by controlling the current flowing through a heating jacket in a stirred vessel. 

The computer system of the station control and data acquisition is a distributed micro processer based systems. A digital multiplexed control system takes the place of hard wired analogue control. This accounts for a significant reduction in cable usage. Built-in diagnostics and board level maintenance makes restoration of operability of any fault in the system a matter of replacement of printed circuit cards. Automatic control systems and procedures are deployed to simplify these procedures and power level manoeuvers. In case of unsafe conditions the reactor protection system (PMS) takes over and automatically scrams the reactor and actuates the relevant safety systems. Diverse methods are used to assure the shutdown of the reactor in hypothetical situations. The systems also provide for post-accident monitoring. 

The computer system is intensively used in plant control and data acquisition and takes the place of hardware analogue control. This results in a significant reduction in the scope of cabling. 

Another essential component in the functioning of automated processes and servomechanisms is the feedback control systems that provide self-regulation and auto-adjustment of the overall system. Feedback control systems may be pneumatic, hydraulic, mechanical, or electrical in nature. Electrical feedback may be analogue in form, although digital electronic feedback methods provide the most versatile method of output sensing for input feedback to digital electronic control systems. 

An extremely valuable contribution to control system design has been provided by the digital and analogue system simulations developed over the past years. It is, therefore, worthwhile to outline the basic content of the current wide range simulator which is programmed on a PACE analogue computer. 

Naturally-occurring and selection-induced resistance to analogues are two aspects of the same problem which concerns the protection mechanisms developed by cells against otherwise toxic agents. Metabolic adjustments may lead to an overproduction of the natural competitor. Analogues disrupt the control systems which operate by directing active metabolic assimilation (transport mechanisms) or by avoiding a wasteful metabolite oversynthesis, as well as excessive enzyme production. 

Whilst some older plants do not have computer control systems, most new plants and retrofits do. The emphasis in this chapter, therefore, is on the commissioning of control systems that aie computer based. Most of the principles and procedures outlined, however, are just as applicable to analogue systems as to digital ones. 

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