1、NumericalControl机械英语文章翻译Numerical ControlOne of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control. Controlling a machine tool using a punched tape or stored program is known as numerical control (NC). NC has been defined by the Electronic Industrie
2、s Association (EIA) as a system in which actions are controlled by the direct insertion of numerical dada at some point .the system must automatically interpret at least some portion of this data. the numerical data required to produce a part is known as a part program. A numerical control machine t
3、ool system contains a machine control unit (MCU) and the machine tool itself. The MCU is further divided into two elements: the data processing unit (DPU) and the control loops unit (CLU). The DPU processes the coded data from the tape or other media and passes information on the potions of each axi
4、s, required direction of motion, feed rate, and auxiliary function control signals to the CLU. The CLU operates the drive mechanisms of the machine, receives feed back signals concerning the actual position and velocity of each of the axes, and signals the completion of operation. The DPU sequential
5、ly reads the data. When each line has completed execution as noted by the CLU, anther line of data is read. A data processing units consists of some or all of the following parts: 1) Data input device such as a paper tape reader, magnetic tape reader, RS232-C port, etc 2) Data-reading circuits and p
6、arity-checking logic 3) Decoding circuits for distributing data among the controlled axes 、4) An interpolator, which supplies machine-motion commands between data points for tool motion A control loops unit, on the other hand consists of the following: 1) Position control loops for all the axes of m
7、otion, where each axis has a separate control loop 2) Velocity control loops, where feed control is required 3) Deceleration and backlash take up circuits 4) Auxiliary functions control, such as coolant on/off, gear change, spindle on/off control Geometric and kinematic data are typically fed from t
8、he DPU to the CLU. The CLU then governs the physical system based on the data from the DPU. Numerical control was developed to overcome the limitation of human operators, and it has done so. Numerical control machines are more accurate than manually operated machines, they can produce parts more uni
9、formly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology: l Electric discharge machining 2 Laser-cutting 3 Electron beam welding Numerical control has also made machine tools more versatil
10、e than their manually operated predecessors. An NC machine tool can automatically produce a wide variety of parts, each involving an assortment of widely varied and complex machining processes. Numerical control has allowed manufacturers to undertake the production of products that would not have be
11、en feasible from an economic perspective using manually controlled machine tools and processes. Historical Development of NC Like so many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of Technology. The concept of NC was developed in the early 1950s with fundi
12、ng provided by the U.S. Air Force. In its earliest stages, NC machines were able to make straight cuts efficiently and effectively. However, curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve. The shorte
13、r the straight lines making up the steps, the smoother is the curve. Each line segment in the steps shown in the close up in Fig.2.17 had to be calculated. This was a cumbersome approach that had to be overcome if NC was to develop further. This problem led to the development in 1959 of the Automati
14、cally Programmed Tools (APT) language. This is a special programming language for NC that uses statements similar to English language to define the part geometry, describe the cutting tool configuration, and specify the necessary motions. The development of the APT language was a major step forward
15、in the development of NC technology. The original NC systems were vastly different from those used today. The machines had hardwired logic circuits. The instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret
16、the instructions written on the tape for the machine. Together, all of this represented a giant step forward in the control of machine tools. However, there were a number of problems with NC at this point in its development. A major problem was the fragility of the punched paper tape medium. It was
17、common for the paper containing the programmed instructions to break or tear during a machining process. This problem was exacerbated by the fact that each successive time a part was produced on a machine tool, the paper tape carrying the programmed instructions had to be rerun through the reader. I
18、f it was necessary to produce 100 copies of a given part, it was also necessary to run the paper tape through the reader 100 separate times. Fragile paper tapes simply could not withstand the rigors of a shot floor environment and this kind of repeated use. This led to the development of a special m
19、agnetic plastic tape. Whereas the paper tape carried the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of magnetic dots. The plastic tape was much stronger than the paper tape, which solved the problem of frequent tearing and
20、breakage. However, it still left two other problems. The most important of those was that it was difficult or impossible to change the instructions entered on the tape. To make even the most minor adjustments in a program of instructions, it was necessary to interrupt machining operations and make a
21、 new tape. It was also still necessary to run the tape through the reader as many times as there were parts to be produced. Fortunately, computer technology became a reality and soon solved the problem of NC associated with punched paper and plastic tape. 1) Advent of Direct Numerical Control The de
22、velopment of a concept known as direct numerical control (DNC) solved the paper and plastic tape problems associated with numerical control by simply eliminating tape as the medium for carrying the programmed instructions. In direct numerical control, machine tools are tied, via a data transmission
23、link, to a host computer (Fig 2.18). Programs for operating the machine tools are stored in the host computer and fed to the machine tool as needed via the data transmission linkage. Direct numerical control represented a major step forward over punched tape and plastic tape. However, it is subject
24、to the same limitations as all technologies that depend on a host computer, the machine tools also experience downtime. This problem led to the development of computer numerical control. 2) Advent of Computer Numerical Control The development of the microprocessor allowed for the development of prog
25、rammable logic controllers (PLCs) and microcomputer. These two technologies allowed for the development of computer numerical control (CNC). With CNC, each machine tool has a PLC or a microcomputer that serves the purpose. This allows programs to be input and stored at each individual machine tool.
26、It also allows programs to be developed off-line and downloaded at the individual machine tool. CNC solved the problems associated with downtime of the host computer, but it introduced another problem known as data management. This is a problem all work settings dependent on microcomputers have. The
27、 same program might be loaded on ten different microcomputers with no communication among them. This problem is the process of being solved by local area networks that connect microcomputers for better data management. The problem of data management led to the development of distributed numerical co
28、ntrol. 3) Advent of Distributed Numerical Control Distributed numerical control (also called DNC) takes advantage of the best aspects of direct numerical control and computer numerical control. With distributed numerical control there are both host computers and local computers at the individual mac
29、hine tools (Fig 2.19). This allows the programs to be stored in the host computers and, thereby, better managed. However, it also allows them to be downloaded to local microcomputers or PLCs. It also allows for local input and interaction through microcomputers or PLCs at the machine levels. NC Mach
30、ine Components There are four components in any NC machine: l The actual NC tool 2 The machine control unit (MCU) 3 The communication interface between the NC machine and the MCU 4 A variety of accessories for performing specific jobs on the NC machine The actual NC machine may be a milling machine,
31、 lathe, drill, or any other type of machine tool. The MCU is the control unit that holds the programs that instruct the NC machine. The MCU also has various devices available for operator input. Information contained in the MCU is carried to the activators on the NC machine through the communication
32、 interface. These activators receive the electronic signals from the MCU and cause the mechanical apparatus of the NC machine to operate. Less sophisticated NC machines have open-loop activators. An open-loop activator can receive a signal and carry out the instructions contained in that signal, but cannot feed back to the MCU to show that instructions carried in the signal have been properly completed. More sophisticated NC machine use closed-loop activators. A closed-loop activator can receive and carry out a signal and f
