AUTOMATED PRODUCTION LINES
THE FEEDBACK PRINCIPLE
Essential to all automatic-control mechanisms is the feedback principle, which enables a designer to endow a machine with the capacity for self-correction. A feedback loop is a mechanical, pneumatic, or electronic device that senses or measures a physical quantity such as position, temperature, size, or speed, compares it with a preestablished standard, and takes whatever preprogrammed action is necessary to maintain the measured quantity within the limits of the acceptable standard.
The feedback principle is used in all automatic-control mechanisms when machines have ability to correct themselves. The feedback principle has been used for centuries. An outstanding early example is the flyball governor, invented in 1788 by James Watt to control the speed of the steam engine. The common household thermostat is another example of a feedback device.
In manufacturing and production, feedback loops require that acceptable limits or tolerances be established for the process to be performed. Besides, these physical characteristics should be measured and compared with the set of limits. Finally, that the feedback system be capable of correcting the process so that the measured items comply with the standard. Through feedback devices, machines can start, stop, speed up, slow down, count, inspect, test, compare, and measure. These operations are commonly applied to a wide variety of production operations that can include milling, boring, bottling, and refining.
Computers have greatly facilitated the use of feedback in manufacturing processes. Computers gave the rise to the development of numerically controlled machines. The motion of these machines is controlled by punched paper or magnetic tapes. In numerically controlled machining centers machine-tools can perform several different machining operations.
More recently the introduction of microprocessors and computers have made possible the development of computer-aided design and computer-aided manufacture (CAD and CAM) technologies. When using these systems a designer draws a part and indicates its dimensions with the help of a mouse, a light pen or other input device. After the drawing has been completed the computer automatically gives the instructions that direct a machining center to machine the part.
AUTOMATION
Automation is the system of manufacture performing certain tasks, previously done by people, by machines only. The sequences of operations are controlled automatically. The most familiar example of a highly automated system is an assembly plant for the automobiles or other complex products.
The term automation is also used to describe non-manufacturing systems in which automatic devices can operate independently of human control. Such devices as automatic pilots, automatic telephone equipment and automated control systems are used to perform various operations much faster and better than could be done by people.
Automated manufacturing had several steps in its development. Mechanization was the first step necessary in the development of automation. The simplification of work made it possible to design and build machines that resembled the motions of the worker. These specialized machines were motorized and they had better production efficiency.
Industrial robots, originally designed only to perform simple tasks in environments dangerous to human workers, are now widely used to transfer, manipulate and position both light and heavy workpieces performing all the functions of a transfer machine.
In the 1920s the automobile industry for the first time used an integrated system of production. This method of production was adopted by the most car manufacturers and became known as Detroit automation.
Computers gave the rise to the development of numerically controlled machines. In numerically controlled machining centers machine-tools can perform several different machining operations.
More recently the introduction of microprocessors and computers have made possible the development of computer-aided design and computer-aided manufacture (CAD and CAM) technologies. When using these systems a designer draws a part and indicates its dimensions with the help of a mouse, a light pen or other input device. After the drawing has been completed the computer automatically gives the instructions that direct a machining center to machine the part.
Another development using automation are the flexible manufacturing systems (FMS). A computer in FMS can be used to monitor and control the operation of the whole factory.
Automation has also had an influence on the areas of the economy other than manufacturing. Small computers are used in systems called word processors, which are rapidly becoming a standard part of the modern office. They are used to edit texts, to tape the letters and so on.
The concept of automation is evolving rapidly, partly because the applications of automation techniques vary both within a plant or industry and also between industries. The oil and chemical industries, for example, have developed the continuous-flow method of production, owing to the nature of the raw materials used. In a refinery, crude oil enters at one point and flows continuously through pipes in cracking, distillation, and reaction devices as it is being processed into such products as gasoline and fuel oil. An array of automatic-control devices governed by microprocessors and coordinated by a central computer is used to control valves, heaters, and other equipment, thereby regulating both the flow and reaction rates.
In the steel, beverage, and canned food industries, on the other hand, some of the products are produced in batches. For example, a steel furnace is charged (loaded with the ingredients), brought up to heat, and a batch of steel ingots produced. In this phase very little automation is evident. These ingots, however, may then be processed automatically into sheet or structural shapes by being squeezed through a series of rollers until the desired shape is achieved.
The automobile and other consumer product industries use the mass production techniques of step-by-step manufacture and assembly. This technique approximates the continuous-flow concept but involves transfer machines. Thus, from the point of view of the auto industry, transfer machines are essential to the definition of automation.
Each of these industries uses automated machines in all or part of its manufacturing processes. As a result, each industry has a concept of automation that fits its particular production needs. More examples can be found in almost every phase of commerce. The widespread use of automation and its influence on daily life provides the basis for the concern expressed by many about the influence of automation on society and the individual.
Automation has made a major contribution toward increases in both free time and real wages enjoyed by most workers in industrialized nations. Automation has greatly increased production and lowered costs, thereby making automobiles, refrigerators, televisions, telephones, and other goods available to more people. It has allowed production and wages to increase, and at the same time the work week has decreased in most Western countries from 60 to 40 hours.
ROBOTS
Today most robots are used in manufacturing processes. The robot applications can be divided into three categories: material handling, processing operations, assembly and inspection.
Material handling is the transfer of the material and loading and unloading of machines. Material-transfer applications require the robot to move materials or work parts from one to another, many of these tasks are relatively simple. But there are some complex, such as placing parts in an arrangement that can be calculated by the robots. Machine loading and unloading operations utilize a robot to load and unload parts. This requires the robot to be equipped with a gripper that can grasp parts.
In processing operations the robot manipulates a tool to perform a process on the work part. Examples of such applications include spot and arc welding, spray painting and polishing. All these operations are very complex; that's why different robots are used for particular cases.
The third application area of industrial robots is assembly and inspection. More and more often robots will be used in assembly because of the high cost of manual labour. Assembly methods that are satisfactory for humans are not suitable for robots. Inspection is another area of factory operations which involves the use of robots. Each production process step is supervised by a robot and it determines whether the part answers the quality specifications.
TYPES OF AUTOMATION
Manufacturing is one of the most important application areas for automation technology. There are several types of automation in manufacturing. Here are the examples of automation systems that we use today in production.
Fixed or hard automation is referred to as a process performed by machines in which the equipment configuration allows particular sequence of processing operations. They are programmed by engineers to make only certain processing operation. Its sequence can not be easily changed over from one product style to another.
Programmable automation is a form of automation for producing products in large quantities, ranging from several dozen to several thousand units a time. For each new product the production equipment should be particularly reprogrammed and adjusted. This reprogramming and changeover take a period of non-productive time. Production rates are lower in this case than in hard automation because the equipment is designed to facilitate product changeover rather than for product specialization.
Flexible automation is a kind of programmable automation. Flexible automation requires time to reprogram and change over the production equipment for each series of new product. It is expensive. Flexible automation allows a mixture of different products to be produced one right after another.
AUTOMATED PRODUCTION LINES
An automated production line consists of a series of workstations connected by a transfer system to move parts between the stations. This is an example of fixed automation, since these lines are set up for long production runs, making large number of product units and running for several years between changeovers. Each station is designed to perform to perform a specific processing operation. The various operations, part transfers and other activities taking place on an automated transfer line should be sequenced and coordinated properly for the line to operate efficiently.
Modern automated lines are controlled by programmable logic controllers which are special computers that can perform timing and sequencing functions required to operate such equipment. Automated production lines are utilized in many industries, mostly automobile, for processes such as machining and pressworking.
Machining is a manufacturing process in which metal is removed by a cutting or shaping tool, so that the remaining work part is the desired shape. Machinery and motor components are made by this process. In many cases multiple operations are required to completely shape the part. Many separate operations are divided among the workstation. An automated transfer line is the most economical production method for the mass-produced parts.
Pressworking operations involve the cutting and forming of parts from sheet metal. Examples of such parts include automobile body panels, outer shells of laundry machines and metal furniture.
NUMERICAL CONTROL
Numerical control (NC) is a form of programmable automation in which a machine is controlled by numbers (and other symbols) that have been coded on punched paper tape of an alternative storage medium. The initial application of numerical control was the machine tool industry to control the position of a cutting tool relative to the work part being machined. The NC part program represents the set of machining instructions for the particular part. The coded numbers in the program specify x-y-z coordinates in a Cartesian axis system defining the various positions of the cutting tool in relation to the work part. By sequencing these positions in the program the machine-tool is directed to accomplish the machining of the part.
A position feedback control system is used in most NC machines to verify that the coded instructions have been correctly performed. Today a small computer is used as the controller in a NC machine-tool. Since this form of numerical control is implemented by computer it is called computer numerical control (CNC).
Another variation in the implementation of numerical control involves sending part programs over telecommunications lines from a central computer to individual machine tools in the factory. This form of numerical control is called direct numerical control (DNC).
Many applications of numerical control have been developed since its initial use to control machine tools.
sequence - последовательность
assembly plant - сборочный завод
flyball governor - центробежный регулятор
CAD - автоматизированное проектирование computer-aided design and computer-aided manufacture
CAM - автоматизированное производство
FMS - гибкая производственная система flexible manufacturing system
handling - манипулирование
processing - обработка
assembly - сборка
welding - сварка
painting - окраска
polishing - полирование
changeover - переход, переналадка
storage medium - носитель (информации)
Cartesian axis system — прямоугольная система координат