Электропривод и автоматизация промышленных установок и технологических комплексов

(The Electric Drive and the Automatization of the Commercial Plants and Production Process Complexes)

 

 

AUTOMATION IN INDUSTRY.

FIXED AND PROGRAMMABLE AUTOMATION (7823 characters)

 

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 a specific processing op­eration, so that the part or product is constructed stepwise as it progresses along the line. A raw work part enters at one end of the line, proceeds through each workstation and appears at the other end as a completed product. In the normal operation of the line, there is a work part being processed at each station, so that many parts are being processed simultaneously and a finished part is produced with each cycle of the line. The various opera­tions, part transfers, and other activities taking place on an automated transfer line must all 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, where they are used 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 usually made by this process. In many cases, multiple operations are required to completely shape the part. If the part is mass-produced, an automated transfer line is often the most economical method of pro­duction. Many separate operations are divided among the workstations.

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 More than one processing step is often required to complete a compli­cated part. Several presses are connected together in se­quence by handling mechanisms that transfer the par­tially completed parts from one press to the next, thus creating an automated pressworking line.

 

Numerical Control

 

Numerical control 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 or an alternative storage medium. The initial appli­cation of numerical control was in 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 an NC machine tool. Since this form of numerical control is implemented by computer, it is called computer numerical control, or 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, or DNC.

Many applications of numerical control have been de­veloped since its initial use to control machine tools. Other machines using numerical control include compo­nent-insertion machines used in electronics assembly, drafting machines that prepare engineering drawings, coordinate measuring machines that perform accurate inspections of parts. In these applications coded numeri­cal data are employed to control the position of a tool or workhead relative to some object. Such machines are used to position electronic components (e.g., semiconductor chip modules) onto a printed circuit board (PCB). It is basically an x-y positioning table that moves the printed circuit board relative to the part-insertion head, which then places the individual component into position on the board. A typical printed circuit board has dozens of in­dividual components that must be placed on its surface; in many cases, the lead wires of the components must be inserted into small holes in the board, requiring great precision by the insertion machine. The program that controls the machine indicates which components are to be placed on the board and their locations. This informa­tion is contained in the product-design database and is typically communicated directly from the computer to the insertion machine.

 

Automated Assembly

 

Assembly operations have traditionally been per­formed manually, either at single assembly workstations or on assembly lines with multiple stations. Owing to the high labour content and high cost of manual labour, greater attention has been given in recent years to the use of automation for assembly work. Assembly opera­tions can be automated using production line principles if the quantities are large, the product is small, and the design is simple (e.g., mechanical pencils, pens, and ciga­rette lighters). For products that do not satisfy these conditions, manual assembly is generally required.

Automated assembly machines have been developed that operate in a manner similar to machining transfer lines, with the difference being that assembly operations, instead of machining, are performed at the workstations. A typical assembly machine consists of several stations, each equipped with a supply of components and a mecha­nism for delivering the components into position for as­sembly. A workhead at each station performs the actual attachment of the component. Typical workheads include automatic screwdrivers, welding heads and other join­ing devices. A new component is added to the partially completed product at each workstation, thus building up the product gradually as it proceeds through the line. Assembly machines of this type are considered to be ex­amples of fixed automation, because they are generally configured for a particular product made in high volume. Programmable assembly machines are represented by the component-insertion machines employed in the electron­ics industry.

 

INDUCTION MOTORS FOR SPEED AND POSITION CONTROL

 

From Electric Motor

 

On a constant-frequency supply, an induction motor is essentially a near-constant speed drive. Induction motors, however, can be used to provide accurate speed and position control in either direction of rotation by furnishing a controllable-voltage, controllable-frequency three-phase supply. This is done by means of an electronic inverter. Using semiconductor switches (e.g., transistors or thyristors), the utility supply is converted into a set of three near-sinusoidal inputs of controlled voltage and frequency to the stator winding. The speed of the motor will then approach the synchronous value of 120 f/p revolutions per minute for a controlled frequency of f cycles per second.

Reversal of the phase sequence from abc to acb reverses the direction of the torque. For accurate control of speed or of position, the speed of the shaft can be monitored by a tachometer or position sensor and compared with a signal representing the desired value. The difference is then used to control the inverter frequency. Generally, the voltage varies directly with the frequency to keep the magnitude of the magnetic field constant.

 

 

(Copyright © 1994-2000 Encyclopædia Britannica, Inc.)