Computer process control
In computer process control, a digital computer is used to direct the operations of a manufacturing process. Although other automated systems are typically controlled by computer, the term computer process control is generally associated with continuous or semicontinuous production operations involving materials such as chemicals, petroleum, foods, and certain basic metals. In these operations the products are typically processed in gas, liquid, or powder form to facilitate flow of the material through the various steps of the production cycle. In addition, these products are usually mass-produced. Because of the ease of handling the product and the large volumes involved, a high level of automation has been accomplished in these industries.
The modern computer process control system generally includes the following: (1) measurement of important process variables such as temperature, flow rate, and pressure, (2) execution of some optimizing strategy, (3) actuation of such devices as valves, switches, and furnaces that enable the process to implement the optimal strategy, and (4) generation of reports to management indicating equipment status, production performance, and product quality. Today computer process control is applied to many industrial operations, two of which are described below.
The typical modern process plant is computer-controlled. In one petrochemical plant that produces more than 20 products, the facility is divided into three areas, each with several chemical-processing units. Each area has its own process-control computer to perform scanning, control, and alarm functions. The computers are connected to a central computer in a hierarchical configuration. The central computer calculates how to obtain maximum yield from each process and generates management reports on process performance.
Each process computer monitors up to 2,000 parameters that are required to control the process, such as temperature, flow rate, pressure, liquid level, and chemical concentration. These measurements are taken on a sampling basis; the time between samples varies between 2 and 120 seconds, depending on the relative need for the data. Each computer controls approximately 400 feedback control loops. Under normal operation, each control computer maintains operation of its process at or near optimum performance levels. If process parameters exceed the specified normal or safe ranges, the control computer actuates a signal light and alarm horn and prints a message indicating the nature of the problem for the technician. The central computer receives data from the process computers and performs calculations to optimize the performance of each chemical-processing unit. The results of these calculations are then passed to the individual process computers in the form of changes in the set points for the various control loops.
Substantial economic advantages are obtained from this type of computer control in the process industries. The computer hierarchy is capable of integrating all the data from the many individual control loops far better than humans are able to do, thus permitting a higher level of performance. Advanced control algorithms can be applied by the computer to optimize the process. In addition, the computer is capable of sensing process conditions that indicate unsafe or abnormal operation much more quickly than humans can. All these improvements increase productivity, efficiency, and safety during process operation.
Like the chemical-processing industries, the basic metals industries (iron and steel, aluminum, etc.) have automated many of their processes by computer control. Like the chemical industries, the metals industries deal in large volumes of products, and so there is a substantial economic incentive to invest in automation. However, metals are typically produced in batches rather than continuously, and it is generally more difficult to handle metals in bulk form than chemicals that flow.
An example of computer process control in the metals industry is the rolling of hot metal ingots into final shapes such as coils and strips. This was first done in the steel industry, but similar processing is also accomplished with aluminum and other metals. In a modern steel plant, hot-rolling is performed under computer control. The rolling process involves the forming of a large, hot metal billet by passing it through a rolling mill consisting of one or more sets of large cylindrical rolls that squeeze the metal and reduce its cross section. Several passes are required to reduce the ingot gradually to the desired thickness. Sensors and automatic instruments measure the dimensions and temperature of the ingot after each pass through the rolls, and the control computer calculates and regulates the roll settings for the next pass.
In a large plant, several orders for rolled products with different specifications may be in the mill at any given time. Control programs have been developed to schedule the sequence and rate at which the hot metal ingots are fed through the rolling mills. The production control task of scheduling and keeping track of the different orders requires rapid, massive data gathering and analysis. In modern plants this task has been effectively integrated with the computer control of the rolling mill operations to achieve a highly automated production system.
Computer-integrated manufacturing
Since about 1970 there has been a growing trend in manufacturing firms toward the use of computers to perform many of the functions related to design and production. The technology associated with this trend is called CAD/CAM, for computer-aided design and computer-aided manufacturing. Today it is widely recognized that the scope of computer applications must extend beyond design and production to include the business functions of the firm. The name given to this more comprehensive use of computers is computer-integrated manufacturing (CIM).
CAD/CAM is based on the capability of a computer system to process, store, and display large amounts of data representing part and product specifications. For mechanical products, the data represent graphic models of the components; for electrical products, they represent circuit information; and so forth. CAD/CAM technology has been applied in many industries, including machined components, electronics products, and equipment design and fabrication for chemical processing. CAD/CAM involves not only the automation of the manufacturing operations but also the automation of elements in the entire design-and-manufacturing procedure.
Computer-aided design (CAD) makes use of computer systems to assist in the creation, modification, analysis, and optimization of a design. The designer, working with the CAD system rather than the traditional drafting board, creates the lines and surfaces that form the object (product, part, structure, etc.) and stores this model in the computer database. By invoking the appropriate CAD software, the designer can perform various analyses on the object, such as heat transfer calculations. The final object design is developed as adjustments are made on the basis of these analyses. Once the design procedure has been completed, the computer-aided design system can generate the detailed drawings required to make the object.
Computer-aided manufacturing (CAM) involves the use of computer systems to assist in the planning, control, and management of production operations. This is accomplished by either direct or indirect connections between the computer and production operations. In the case of the direct connection, the computer is used to monitor or control the processes in the factory. Computer process monitoring involves the collection of data from the factory, the analysis of the data, and the communication of process-performance results to plant management. These measures increase the efficiency of plant operations. Computer process control entails the use of the computer system to execute control actions to operate the plant automatically, as described above. Indirect connections between the computer system and the process involve applications in which the computer supports the production operations without actually monitoring or controlling them. These applications include planning and management functions that can be performed by the computer (or by humans working with the computer) more efficiently than by humans alone. Examples of these functions are planning the step-by-step processes for the product, part programming in numerical control, and scheduling the production operations in the factory.
Computer-integrated manufacturing includes all the engineering functions of CAD/CAM and the business functions of the firm as well. These business functions include order entry, cost accounting, employee time records and payroll, and customer billing. In an ideal CIM system, computer technology is applied to all the operational and information-processing functions of the company, from customer orders through design and production (CAD/CAM) to product shipment and customer service. The scope of the computer system includes all activities that are concerned with manufacturing. In many ways, CIM represents the highest level of automation in manufacturing.
Automation in daily life
In addition to the manufacturing applications of automation technology, there have been significant achievements in such areas as communications, transportation, service industries, and consumer products. Some of the more significant applications are described in this section.
Communications
One of the earliest practical applications of automation was in telephone switching. The first switching machines, invented near the end of the 19th century, were simple mechanical switches that were remotely controlled by the telephone user pushing buttons or turning a dial on the phone. Modern electronic telephone switching systems are based on highly sophisticated digital computers that perform functions such as monitoring thousands of telephone lines, determining which lines require service, storing the digits of each telephone number as it is being dialed, setting up the required connections, sending electrical signals to ring the receiver’s phone, monitoring the call during its progress, and disconnecting the phone when the call is completed. These systems also are used to time and bill toll calls and to transmit billing information and other data relative to the business operations of the phone company. In addition to the various functions mentioned, the newest electronic systems automatically transfer calls to alternate numbers, call back the user when busy lines become free, and perform other customer services in response to dialed codes. These systems also perform function tests on their own operations, diagnose problems when they arise, and print out detailed instructions for repairs.
Other applications of automation in communications systems include local area networks, communications satellites, and automated mail-sorting machines. A local area network (LAN) operates like an automated telephone company within a single building or group of buildings. Local area networks are generally capable of transmitting not only voice but also digital data between terminals in the system. Communications satellites have become essential for communicating telephone or video signals across great distances. Such communications would not be possible without the automated guidance systems that place and retain the satellites in predetermined orbits. Automatic mail-sorting machines have been developed for use in many post offices throughout the world to read codes on envelopes and sort the envelopes according to destination.
