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The development trend of the numerical control technology
1. NUMERICAL CONTROL
Numerical control(N／C)is a form of programmable automation in which the processing equipment is controlled by means of numbers，letters，and other symbols．The numbers，letters，and symbols are coded in an appropriate format to define a program of instructions for a particular workpart or job．When the job changes，the program of instructions is changed．The capability to change the program is what makes N／C suitable for low-and medium-volume production．It is much easier to write programs than to make major alterations of the processing equipment．
There are two basic types of numerically controlled machine tools：point—to—point and continuous—path(also called contouring)．Point—to—point machines use unsynchronized motors，with the result that the position of the machining head Can be assured only upon completion of a movement，or while only one motor is running．Machines of this type are principally used for straight—line cuts or for drilling or boring．
The N／C system consists of the following components：data input，the tape reader with the control unit，feedback devices，and the metal—cutting machine tool or other type of N／C equipment．
Data input，also called“man—to—control link”，may be provided to the machine tool manually，or entirely by automatic means．Manual methods when used as the sole source of input data are restricted to a relatively small number of inputs．Examples of manually operated devices are keyboard dials，pushbuttons，switches，or thumbwheel selectors．These are located on a console near the machine．Dials ale analog devices usually connected to a syn-chro-type resolver or potentiometer．In most cases，pushbuttons，switches，and other similar types of selectors aye digital input devices．Manual input requires that the operator set the controls for each operation．It is a slow and tedious process and is seldom justified except in elementary machining applications or in special cases．
In practically all cases，information is automatically supplied to the control unit and the machine tool by cards，punched tapes，or by magnetic tape．Eight—channel punched paper tape is the most commonly used form of data input for conventional N／C systems．The coded instructions on the tape consist of sections of punched holes called blocks．Each block represents a machine function，a machining operation，or a combination of the two．The entire N／C program on a tape is made up of an accumulation of these successive data blocks．Programs resulting in long tapes all wound on reels like motion-picture film．Programs on relatively short tapes may be continuously repeated by joining the two ends of the tape to form a loop．Once installed，the tape is used again and again without further handling．In this case，the operator simply loads and unloads the parts．Punched tapes ale prepared on type writers with special tape—punching attachments or in tape punching units connected directly to a
computer system．Tape production is rarely error-free．Errors may be initially caused by the part programmer，in card punching or compilation，or as a result of physical damage to the tape during handling，etc．Several trial runs are often necessary to remove all errors and produce an acceptable working tape．
While the data on the tape is fed automatically，the actual programming steps ale done manually．Before the coded tape may be prepared，the programmer，often working with a planner or a process engineer, must select the appropriate N／C machine tool，determine the kind of material to be machined，calculate the speeds and feeds，and decide upon the type of tooling needed. The dimensions on the part print are closely examined to determine a suitable zero reference point from which to start the program．A program manuscript is then written which gives coded numerical instructions describing the sequence of operations that the machine tool is required to follow to cut the part to the drawing specifications．
The control unit receives and stores all coded data until a complete block of information has been accumulated．It then interprets the coded instruction and directs the machine tool through the required motions．
The function of the control unit may be better understood by comparing it to the action of a dial telephone，where，as each digit is dialed，it is stored．When the entire number has been dialed，the equipment becomes activated and the call is completed．
Silicon photo diodes，located in the tape reader head on the control unit，detect light as it passes through the holes in the moving tape．The light beams are converted to electrical energy，which is amplified to further strengthen the signal．The signals are then sent to registers in the control unit, where actuation signals are relayed to the machine tool drives．
Some photoelectric devices are capable of reading at rates up to 1000 characters per second．High reading rates are necessary to maintain continuous machine—tool motion；otherwise dwell marks may be generated by the cutter on the part during contouring operations．The reading device must be capable of reading data blocks at a rate faster than the control system can process the data．
A feedback device is a safeguard used on some N／C installations to constantly compensate for errors between the commanded position and the actual location of the moving slides of the machine tool．An N／C machine equipped with this kind of a direct feedback checking device has what is known as a closed-loop system．Positioning control is accomplished by a sensor which，during the actual operation，records the position of the slides and relays this information back to the control unit．Signals thus received ale compared to input signals on the tape，and any discrepancy between them is automatically rectified．
In an alternative system，called an open—loop system，the machine is positioned solely by stepping motor dri
ves in response to commands by a controllers．There are three basic types of NC motions, as follows:
Point-to-point or Positional Control In point-to-point control the machine tool elements (tools, table, etc.) are moved to programmed locations and the machining operations performed after the motions are completed. The path or speed of movement between locations is unimportant; only the coordinates of the end points of the motions are accurately controlled. This type of control is suitable for drill presses and some boring machines, where drilling, tapping, or boring operations must be performed at various locations on the work piece. Straight-Line or Linear Control Straight-Line control systems are able to move the cutting tool parallel to one of the major axes of the machine tool at a controlled rate suitable for machining. It is normally only possible to move in one direction at a time, so angular cuts on the work piece are not possible, consequently, for milling machines, only rectangular configurations can be machined or for lathes only surfaces parallel or perpendicular to the spindle axis can be machined. This type of controlled motion is often referred to as linear control or a half-axis of control. Machines with this form of control are also capable of point-to-point control.
Continuous Path or Contouring Control In continuous path control the motions of two or more of the machine axes are controlled simultaneously, so that the position and velocity of the can be tool are changed continuously. In this way curves and surfaces can be machined at a controlled feed rate. It is the function of the interpolator in the controller to determine the increments of the individual controlled axes of the machines necessary to produce the desired motion. This type of control is referred to as continuous control or a full axis of control.
Some terminology concerning controlled motions for NC machines has been introduced. For example, some machines are referred to as four-or five-or even six-axis machines. For a vertical milling machine three axes of control are fairly obvious, these being the usual X, Y, Z coordinate directions. A fourth or fifth axis of control would imply some form of rotary table to index the work piece or possibly to provide angular motion of the work head. Thus, in NC terminology an axis of control is any controlled motion of the machine elements (spindles, tables, etc). A further complication is use of the term half-axis of control; for example, many milling machines are referred to as 2.5-axis machine. This means that continuous control is possible for two motions (axes) and only linear control is possible for the third axis. Applied to vertical milling machines, 2.5axis control means contouring in the X, Y plane and linear motion only in the Z direction. With these machines three-dimensional objects have to be machined with water lines around the surface at different heights. With an alternative terminology the same mac
hine could be called a 2CL machine (C for continuous, L for linear control). Thus, a milling machine with continuous control in the X, Y, Z directions could be termed be a three-axis machine or a 3c machine, Similarly, lathes are usually two axis or 2C machines. The degree of work precision depends almost entirely upon the accuracy of the lead screw and the rigidity of the machine structure．With this system．there is no self-correcting action or feedback of information to the control unit．In the event of an unexpected malfunction，the control unit continues to put out pulses of electrical current．If，for example，the table on a N／C milling machine were suddenly to become overloaded，no response would be sent back to the controller．Because stepping motors are not sensitive to load variations，many N／C systems are designed to permit the motors to stall when the resisting torque exceeds the motor torque．Other systems are in use，however，which in spite of the possibility of damage to the machine structure or to the mechanical system，ale designed with special high—torque stepping motors．In this case，the motors have sufficient capacity to“overpower’’the system in the event of almost any contingency．
The original N／C used the closed—loop system．Of the two systems，closed and open loop，closed loop is more accurate and，as a consequence，is generally more expensive．Initially，open—loop systems were used almost entirely for light-duty applications because of inherent power limitations previously associated with conventional electric stepping motors．Recent advances in the development of electro hydraulic stepping motors have led to increasingly heavier machine load applications．
2.NC technology development trends
2.1 NC system developments at home and abroad
With the rapid development of computer technology, the traditional beginning of a fundamental change manufacturing, the industrial developed countries spent huge sums of money on the modern manufacturing technology research and development, to create a new model. In modern manufacturing systems, CNC technology is the key to technology, which combines microelectronics, computers, information processing, automatic detection, automatic control, such as the integration of advanced, a high-precision, high-efficiency, flexible automation, and other characteristics, the manufacturing industry Flexible automation, integrated, intelligent play the pivotal role. At present, NC technology is undergoing a fundamental change, from a special closed-loop control mode to general-purpose real-time dynamic open all closed-loop control mode. In the integrated on the basis of the CNC systems ultra-thin, ultra-light; on the basis of the intelligent, integrated computers, multimedia, fuzzy control, neural network and other technical disciplines, NC system to achieve high-speed, high-precision, Efficient control, automatic processing can be amended to regulate compensation and the parameters for an online intelligent fault diagnosis and treatment of the network based on the CAD / CAM and CNC systems integration as one machine network, makes the central government centralized control of the group control processing.
The application of the technology of numerical control has not only brought the revolutionary change to traditional manufacturing industry, make the manufacturing industry become the industrialized symbol , and with the constant development of the technology of numerical control and enlargement of application, the development of he some important trades (IT , car , light industry , medical treatment ,etc. ) to the national economy and the people''s livelihood plays a more and more important role, because these trade necessary digitization that equipped has already been the main trend of modern development.
For a long time, China''s CNC system for traditional closed architecture, but only as a non-intelligent CNC machine controller. Process variables based on experience in the form of pre-fixed parameters, processing procedures before the actual processing by hand or through CAD / CAM and automatic programming system prepared. CAD / CAM and CNC have no feedback control link, the entire manufacturing process CNC is a closed ring-opening implementing agencies. In a complex and changing environment under the conditions of processing tool in the process of composition, workpiece material, spindle speed, feed rate, tool path, cutting depth, step, allowance and other processing parameters, not at the scene circumstances under external interference and real-time dynamic random factors, not by random amendment feedback control link CAD / CAM settings volume, in turn, affect the work of CNC machining efficiency and product quality. Clearly, the traditional fixed CNC system that controlled mode and closed architecture, limiting the CNC to the development of more intelligent control variables, can no longer meet the increasingly complex manufacturing process, therefore, the CNC technology in the potential for change inevitable.
The technology of numerical control of our country started in 1958, the development course in the past 50 years can roughly be divided into 3 stages: The first stage is from 1958 to 1979, i.e. closed developing stage. In this stages, technology of foreign countries blockade and basic restriction of terms of our country, the development of the technology of numerical control is comparatively slow. During " Sixth Five-Year Plan Period " , " the Seventh Five-Year Plan Period " of the country in second stage and earlier stage in " the Eighth Five-Year Plan Period ", introduce technology , digest and assimilate, the stage of establishing the system of production domesticization arisesing tentatively. At this stage , because of reform and opening-up and national attention , and study the improvement of the development environment and international environment, research , development and all making considerable progress in production domesticization of the products of the technology of numerical control of our country. The third stage is that on the later stage in " the Eighth Five-Year Plan Period " of the country and during the " Ninth Five-Year Plan Period ", implement the research of industrialization , enter market competition stage. At this stage, made substantive progress in industrialization of the domestic numerical control equipment of our country. In " the Ninth Five-Year Plan " latter stage,
domestic numerical control domestic market share of lathe reach 50% , mix domestic numerical control system (popular ) to up to 10%.
Review the development course in the past 50 years of technology of numerical control of our country, especially pass the brainstorm of 4 Five-Year Plans, all in all has made following achievements.
a. Have establish the foundation of the technical development of numerical control, has basically mastered the technology of modern numerical control. Our country know from numerical control system , servo urge , numerical control host computer , special plane and their basic of fittings basically already now, among them most technology have already possessed the foundation that is commercialized and developed , some technology has already, industrialization commercialized.
a.Have formed the industrial base of numerical control tentatively. In tackling key problems the foundation that the achievement and some technology commercialize , set up the systematic factories of numerical control with production capacity of batch such as numerical control of Central China , spaceflight numerical control etc.. Electrical machinery plant of Lanzhou, a batch of servo systems and first machine tool plant , first machine tool plant of Jinan of servo electrical machinery factory and Beijing ,etc. several numerical control host computer factories such as the numerical control in Central China. These factories have basically formed the numerical control industrial base of our country.
b.Have set up research of a numerical control, development , managerial talent''s basic team . Though has made considerable progress in research and development and industrialization of the technology of numerical control, but we will realize soberly, the research and development of the technology of advanced numerical control of our country, there is greater disparity between current situation and current demand of our country of engineering level especially in industrialization. Though very fast from longitudinal development to watch our country, horizontal until (contrast with foreign countries ) , the engineering level has disparity, there is disparity too in development speed in some aspects, i.e. the disparity of engineering level between some high-grade , precision and advanced numerical control equipment has the tendency to expand . Watch from world , estimate roughly as follows about the numerical control engineering level of our country and industrialization level.
2.2 Strategic consideration and Development tactics
Our country of strategic consideration makes the big country , should try hard to accept the front instead of the back transformation in the world industry shifts , should master and make key technology advancedly , otherwise in a new round of international industrial structure adjustment, of our country manufacturing industry step forward " the empty core ". We regard resource , environment , market as cost, possibility got to exchange " assemble the centre ", but not master the status of the manufacturing center of key technology , will so influence the development process of the modern manufacturing industry of our country seriously.
We should stand in the height of national security strategy paying attention to the technology of numerical control and industry''s question , at first seen from social safety, because manufacturing industry whether our country obtain employment most populous trade, the development of manufacturing industry not only can improve the people''s living standard but also can relieve the pressure of employment of
our country , ensure the stability of the society; Secondly seen from security of national defence, the western developed country has classified all the high-grade , precision and advanced numerical control products as the strategic materials of the country, realize embargo and restriction to our country, " Toshiba incident " and " Cox Report " is the best illustration.
Proceed from the angles of the fundamental realities of the country of our country, regard the strategic demand of the country and market demand of the national economy as the direction, regard improving our country and making the comprehensive competitive power of equipping industry and industrialization level as the goal, use the systematic method , be able to choose the support technology that the initial our country makes the key technology upgraded in development of equipping industry and supports the development of industrialization in 21st century of leading factor, the ability to supply the necessary techno