Computer Control Loops

Computer-assisted molecular modeling package, 10 331-333 Computer-assisted tomography (CAT), sodium iodide in, 22 827 Computer-automated image analysis, as a particle counting method, 18 146-149 Computer-controlled viscometers, 21 732 Computer controller, closed loop fuel metering system, 10 57 Computer control loop, components of, 20 677-678... 

Chapter 26 will present the various hardware elements required by a computer control loop and will identify the new control design questions. 

Figure 26.5 summarizes the foregoing changes and indicates all hardware components present in a single computer control loop. We observe that both continuous and discrete-time signals are present in the loop. They are denoted by c and d , respectively. It should also be noted that the set point values, as well as the values of the adjustable control parameters (e.g., gains, reset or rate-time constants, etc.), are now introduced by the operator through a typewriter terminal. 

The introduction of a digital computer in a process control loop changes the picture because a computer can handle information on a discrete-time basis only (i.e., at particular time instants). As we can see from Figure 26.5, in a computer control loop we have both continuous-and discrete-time signals present. The implication of this feature is twofold ... 

Digital-analog converter, 560 Digital computer (see Computer, digital) Digital computer control loops, 561-66 Digital control algorithms, 634-38, 640-47, 650-54... 

Figure 26.5 Hardware components of digital computer control loop.

Computer controls and advanced controls are outside the scope of this text. Although these controls are widespread and can be of primary importance for column optimization, their role in setting stable operation is usually secondary. Advanced controls often enhance column stability, but they seldom assure that the primary stability objectives are met. A troublesome computer control loop can usually be taken off control, and stable (albeit nonoptimum) operation can be restored. On the other hand, an unstable basic control loop usually means an unstable column, even when computer control is used. 

Considerable work has been done on mathematic models of the extmsion process, with particular emphasis on screw design. Good results are claimed for extmsion of styrene-based resins using these mathematical methods (229,232). With the advent of low cost computers, closed-loop control of... 

EGR can seriously degrade engine performance, especially at idle, under load at low speed, and during cold start. Control of the amount of EGR during these phases can be accompHshed by the same electronic computer controller used in the closed loop oxygen sensor TWC system. Thus the desired NO reduction is achieved while at the same time retaining good driveabiUty. 

While the single-loop PID controller is satisfactoiy in many process apphcations, it does not perform well for processes with slow dynamics, time delays, frequent disturbances, or multivariable interactions. We discuss several advanced control methods hereafter that can be implemented via computer control, namely feedforward control, cascade control, time-delay compensation, selective and override control, adaptive control, fuzzy logic control, and statistical process control. 

Some plants have been using computer control for 20 years. Control systems in industrial use typically consist of individual feedback and feedforward loops. Horst and Enochs [Engineering h- Mining]., 181(6), 69-171 (1980)] reported that installation of single-variable automatic controls improved performance of 20 mineral processing plants by 2 to 10 percent. But interactions among the processes make it difficult for independent controllers to control the circuit optimally. 

A unity feedback computer control system, has an open-loop pulse transfer function... 

Chapter 5 comprises the computer simulation examples. The exercises are by no means mandatory or restrictive. Most instructive is to study the influence of important model parameters, using the interactive and graphical features of ISIM. Working through a particular example will often suggest an interesting variation, such as a control loop, which can then be inserted into the model. In... 

Instruments are provided to monitor the key process variables during plant operation. They may be incorporated in automatic control loops, or used for the manual monitoring of the process operation. They may also be part of an automatic computer data logging system. Instruments monitoring critical process variables will be fitted with automatic alarms to alert the operators to critical and hazardous situations. 

Importance. The control room is the major interface with the plant. Automation is increasingly common in all degrees of sophistication, Irom single-loop systems to computer-control systems. 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

In a digital computer-control system, the feedback controller has a pulse transfer function. What we need is an equation or algorithm that can be programmed into the digital computer. At the sampling time for a given loop, the computer looks at the current process output x, compares it to a setpoint, and calculates a current value of the error. This error, plus some old values of the error and old values of the controller output or manipulated variable that have been stored in computer memory, are then used to calculate a new value of the controller output m,. 

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