The shift from analog to digital hardware is providing greater capability for automation 6 The use of more digitally oriented process-control equipment tends to over­come the disadvantages in cost or reliability of an analog predecessor

7- 2.1 Analog Control Systems

Before the advent of the transistor, analog controllers, independently serving individual process loops, became the design standard As shown in Fig 8.1, this system provides process control under one set of conditions Changes in the operating status of the process are accommodated by manually adjusting the set point and the analog controller characteristics These devices incorporate proportional, integral, and derivative control action as appropriate to the process

When the transistor introduced solid-state control elec­tronics more-elaborate analog systems became feasible. Figure 8 2 shows schematically the components of such a system. Automation is achieved through relieving the operators of having to adjust set points for changes m process conditions A distinguishing feature of this system
is that the analog computer has available to it all significant process variable signals This permits feed-forward, cascade, and multivariable control modes

In contrast to the controller bank of Fig 8 1, the number of operators needed does not necessarily grow with increasing size and complexity of the process under control The control system grows, and the cost of installing and maintaining an extensive configuration limits large-scale analog automation even with solid-state hardware

8- 2.2 Hybrid Control Systems

Although the transistor gave needed improvement to analog devices its effect in the digital field was outstanding Vacuum-tube digital computers required a great deal of maintenance and almost continuous adjusting As transis tors improved in time response and reliability, larger and faster machines were built The continued advance into integrated circuits lowered the cost of smaller models into the realm of process control

The utility of the digital computer lies in its inherent time-shared nature and its memory One set of arithmetic elements serves the calculating needs of all control loops one at a time, and the computer remembers the inputs, outputs, limit values, and decisional requirements of each loop between turns These features make the digital computer superior to analog computing elements, the configuration shown in Fig 8 3 was that seen in most of the early digital applications Since the digital computer was added to feedback control systems already in opera­tion, it was visualized as “supervising” the process by monitoring and alarming out-of-limits conditions, adjusting analog controller set points, and generally performing the simpler tasks of a human operator Thus the hybrid arrangement is often called “supervisory control,” although the term will not be used here because it has several other meanings

As seen in Fig 8 3, the hybrid approach adds even more equipment to the system But the multiplexer, which scans the inputs and distributes the set-point signals, is the only major component whose size is proportional to the number of measurements and control loops In a basic setup there is one set of mput/output equipment to change analog inputs to digital, digital outputs to analog, and to perform the required amplification, there is one computer to do the timing, logic, and arithmetic The cost of these two items rises slowly with greater system size Hence, as the controlled process gets larger, there exists a point where the hybnd becomes less expensive than the analog system, this is one reason for using hybrid automatic control

8- 2.3 Digital Control Systems

The analog controllers in a hybrid system are usually there because they were already there, they are familiar to and are trusted by the plant operators, and they do a job that would otherwise require a bigger computer and more programming Their inputs are differences between mea­sured variables and corresponding set points, and their outputs are signals to process actuators, such as valve motors and heater relays The controller amplifies, inte­grates, or differentiates the input and combines the results to produce an output that makes a portion of the process respond to set points and process disturbances in a stable fashion

The function of the controller can be done easily by the digital computer, so the analog feedback-loop hardware need not be present 7 The result is a digital automatic control system as illustrated in Fig 8 4 The defining characteristic of the digital configuration is that all major control loops pass through the computer The system is often called “direct digital control,” which, because it implies the exclusive use of digital control signals, will not be used in this chapter In the practical case digital actuators for some process components have not yet been developed

The schematic diagram of the digital system indicates that less equipment is needed for a large number of control loops than with the analog or hybrid However, the complexities of design are still there As will be shown later, they have been largely transferred from the hardware to the computer programs

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