Computer-controlled Systems

Three types of computer control systems are commonly used for pilot-plant instmmentation. The first is a centralized system, usually based on a minicomputer or occasionally a mainframe. These systems have large storage capacities, substantial memories, and much associated equipment. They typically control all the pilot plants in an area or faciUty. Centralized systems are economical if a large number of units are involved but are becoming less common due to their high installation and maintenance costs as well as the limitation that any failure of the central system shuts down all pilot plants involved. 

Advanced Computer Control Systems and Training Simulators... 

One such approach is called cascade control, which is routinely used in most modern computer control systems. Consider a chemical reactor, where reac tor temperature is to be controlled by coolant flow to the jacket of the reac tor (Fig. 8-34). The reac tor temperature can be influenced by changes in disturbance variables such as feed rate or feed temperature a feedback controller could be employed to compensate for such disturbances by adjusting a valve on me coolant flow to the reac tor jacket. However, suppose an increase occurs in the... 

A real-time optimization (RTO) system determines set point changes and implements them via the computer control system without intervention from unit operators. The RTO system completes all data transfer, optimization c culations, and set point implementation before unit conditions change and invahdate the computed optimum. In addition, the RTO system should perform all tasks without upsetting plant operations. Several steps are necessaiy for implementation of RTO, including determination of the plant steady state, data gathering and vahdation, updating of model parameters (if necessaiy) to match current operations, calculation of the new (optimized) set points, and the implementation of these set points. 

The computer control system shown in Figure 7.26 has a sampling time of 0.5 seconds... 

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

Astrom, K.J. and Wittenmark, B. (1984) Computer Controlled Systems, Prentice-Hall, Inc., Englewood Cliffs, NJ. 

The first set of case studies illustrates errors due to the inadequate design of the human-machine interface (HMI). The HMI is the boundary across which information is transmitted between the process and the plant worker. In the context of process control, the HMI may consist of analog displays such as chart records and dials, or modem video display unit (VDU) based control systems. Besides display elements, the HMI also includes controls such as buttons and switches, or devices such as trackballs in the case of computer controlled systems. The concept of the HMI can also be extended to include all means of conveying information to the worker, including the labeling of control equipment components and chemical containers. Further discussion regarding the HMI is provided in Chapter 2. This section contains examples of deficiencies in the display of process information, in various forms of labeling, and the use of inappropriate instrumentation scales. 

Have proper revisions been made to die process control logic, instrumentation set points and alarm points, especially for computer control systems to properly respond to the modification ... 

Figure 26 is the schematic diagram of the FFM designed by Lu (one of the present authors) et al. [9] in 1995. It consists of a stage system, circuitry controlling system, and computer controlling system. 

Computer/control system experts who know how to implement an actual control scheme after a functional specification has been developed in cooperation with process and production experts. 

Computer-controlled systems [54] are commonly used in clinical trials to control dispensing and manage site inventories of trial supplies. Such systems are implemented with telephone voice-based or Internet web-based systems. 

Software to continuously update and optimise plant performance can be incorporated in the computer control systems. 

Two potential disadvantages of temperature programming are the inevitable delay between consecutive chromatographic runs whilst the oven is cooled down and a stable starting temperature re-established, and the possible decomposition of thermally-labile compounds at the higher temperatures. Computer-controlled systems have improved the reproducibility of temperature programming, and automated forced cooling of the oven... 

The most utilized and reliable process control in the petrofeum and related industries is human observation and surveillance. Local pressure and level gages along with control room instrumentation are provided so that human observation and actions can occur to maintain the proper process conditions. First stage process alarms are provided to alert operators to conditions that they may not have already noticed. Typically when secondary alarm stages are reached, computer control systems employed to automatically implement remedial actions to the process. 

Automation and control of processing equipment by highly sophisticated computer control systems is becoming the standard at most hydrocarbon facilities. Automatic control provides for closer control of the process operating conditions and therefore increased efficiencies. Increased efficiencies allow higher production outputs. Automation is also thought to reduce operator manpower requirements. However other personnel are still needed to inspect and maintain the automatic controlling system. All process control systems should be monitored by operators and have the capability for backup control or override commands by human operators. 

Diagram showing the combination of real-time optimization and model predictive control in a computer control system. 

Computer controls, blast furnace, 14 508 Computer control systems, ethylene plant, 10 622... 

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. 

The equation describing the closedloop digital-computer control system of Sec. 18.7 was... 

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,. 

Lightning strikes have resulted in fires in processing facilities. They can also be the cause of electrical and computer control system malfunctions and result in process upsets. 

Another new challenge for incident investigators is capturing, preserving, and retrieving electronically stored data. As previously mentioned, electronic computer control systems have limited storage capacity for detailed process information. Capturing this detailed information is often an overlooked task. 

The AFM has a number of elements common to STM the piezoelectrc scanner for actuating the raster scan and z positioning, the feedback electronics, vibration isolation system, coarse positioning mechanism, and the computer control system. The major difference is that the tunneling tip is replaced by a mechanical tip, and the detection of the minute tunneling current is replaced by the detection of the minute deflection of the cantilever. 

Full process control computerization for a multipurpose plant is much more complex and therefore will be also be much more expensive than for a dedicated single-product plant. Whenever possible, all efforts have to be made to choose standard process control systems and to apply standard control software this is a proven measure to control the investment costs in this segment and will also minimize the risk of having excessive investment and startup costs due to initiating problems with the computer control system. 

High transmission rates can be achieved, if necessary, over the relatively short distances required in a process plant. The PCM equivalent of the 4-20 mA analog transmission system shown in Fig. 6.1 can operate at up to 9,600 baud for distances up to 3000 m. The standard RS-232C transmission link is limited to about 15 m at rates up to 20,000 baud. Higher speed interfaces (such as versions of the IEEE-488 connection) used for computer control systems can handle up to 20,000 bytes/s (which for an 8-bit system is about 1.6 x 105 baud). However, in this case, the distance between devices is limited to about 2 m(4). The more recent RS-422A standard allows the transmission of data rates of 107 baud over distances not exceeding 16.4 m and 105 baud over distances not exceeding 1220 m(9S). 

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