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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 drives 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 machine 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.
數(shù)控技術(shù)
數(shù)控是可編程自動(dòng)化技術(shù)的一種形式，通過(guò)數(shù)字、字母和其他符號(hào)來(lái)控制加工設(shè)備。數(shù)字、字母和符號(hào)用適當(dāng)?shù)母袷骄幋a為一個(gè)特定工件定義指令程序。當(dāng)工件改變時(shí)，指令程序就改變。這種改變程序的能力使數(shù)控適合于中、小批量生產(chǎn)，寫(xiě)一段新程序遠(yuǎn)比對(duì)加工設(shè)備做大的改動(dòng)容易得多。
數(shù)控機(jī)床有兩種基本形式：點(diǎn)位控制和連續(xù)控制(也稱(chēng)為輪廓控制)。點(diǎn)位控制機(jī)床采用異步電動(dòng)機(jī)，因此，主軸的定位只能通過(guò)完成一個(gè)運(yùn)動(dòng)或一個(gè)電動(dòng)機(jī)的轉(zhuǎn)動(dòng)來(lái)實(shí)現(xiàn)。這種機(jī)床主要用于直線切削或鉆孔、鏜孔等場(chǎng)合。
數(shù)控系統(tǒng)由下列組件組成：數(shù)據(jù)輸入裝置，帶控制單元的磁帶閱讀機(jī)，反饋裝置和切削機(jī)床或其他形式的數(shù)控設(shè)備。
數(shù)據(jù)輸人裝置，也稱(chēng)“人機(jī)聯(lián)系裝置”，可用人工或全自動(dòng)方法向機(jī)床提供數(shù)據(jù)。人工方法作為輸人數(shù)據(jù)唯一方法時(shí)，只限于少量輸入。人工輸入裝置有鍵盤(pán)，撥號(hào)盤(pán)，按鈕，開(kāi)關(guān)或撥輪選擇開(kāi)關(guān)，這些都位于機(jī)床附近的一個(gè)控制臺(tái)上。撥號(hào)盤(pán)通常連到一個(gè)同步解析器或電位計(jì)的模擬裝置上。在大多數(shù)情況下，按鈕、開(kāi)關(guān)和其他類(lèi)似的旋鈕是數(shù)據(jù)輸入元件。人工輸入需要操作者控制每個(gè)操作，這是一個(gè)既慢又單調(diào)的過(guò)程，除了簡(jiǎn)單加工場(chǎng)合或特殊情況，已很少使用。幾乎所有情況下，信息都是通過(guò)卡片、穿孔紙帶或磁帶自動(dòng)提供給控制單元。在傳統(tǒng)的數(shù)控系統(tǒng)中，八信道穿孔紙帶是最常用的數(shù)據(jù)輸入形式，紙帶上的編碼指令由一系列稱(chēng)為程序塊的穿孔組成。每一個(gè)程序塊代表一種加工功能、一種操作或兩者的組合。紙帶上的整個(gè)數(shù)控程序由這些連續(xù)數(shù)據(jù)單元連接而成。帶有程序的長(zhǎng)帶子像電影膠片一樣繞在盤(pán)子上，相對(duì)較短的帶子上的程序可通過(guò)將紙帶兩端連接形成一個(gè)循環(huán)而連續(xù)不斷地重復(fù)使用。帶子一旦安裝好，就可反復(fù)使用而無(wú)需進(jìn)一步處理。此時(shí)，操作者只是簡(jiǎn)單地上、下工件。穿孔紙帶是在帶有特制穿孔附件的打字機(jī)或直接連到計(jì)算機(jī)上的紙帶穿孔裝置上做成的。紙帶制造很少不出錯(cuò)，錯(cuò)誤可能由編程、卡片穿孔或編碼、紙帶穿孔時(shí)的物理?yè)p害等形成。通常，必須要試走幾次來(lái)排除錯(cuò)誤，才能得到一個(gè)可用的工作紙帶。
雖然紙帶上的數(shù)據(jù)是自動(dòng)進(jìn)給的，但實(shí)際編程卻是手工完成的，在編碼紙帶做好前，編程者經(jīng)常要和一個(gè)計(jì)劃人員或工藝工程師一起工作，選擇合適的數(shù)控機(jī)床，決定加工材料，計(jì)算切削速度和進(jìn)給速度，決定所需刀具類(lèi)型，仔細(xì)閱讀零件圖上尺寸，定下合適的程序開(kāi)始的零參考點(diǎn)，然后寫(xiě)出程序清單，其上記載有描述加工順序的編碼數(shù)控指令，機(jī)床按順序加工工件到圖樣要求。
控制單元接受和儲(chǔ)存編碼數(shù)據(jù)，直至形成一個(gè)完整的信息程序塊，然后解釋數(shù)控指令，并引導(dǎo)機(jī)床得到所需運(yùn)動(dòng)。
為更好理解控制單元的作用，可將它與撥號(hào)電話進(jìn)行比較，即每撥一個(gè)數(shù)字，就儲(chǔ)存一個(gè)，當(dāng)整個(gè)數(shù)字撥好后，電話就被激活，也就完成了呼叫。
裝在控制單元里的紙帶閱讀機(jī)，通過(guò)其內(nèi)的硅光二極管，檢測(cè)到穿過(guò)移動(dòng)紙帶上的孔漏過(guò)的光線，將光束轉(zhuǎn)變成電能，并通過(guò)放大來(lái)進(jìn)一步加強(qiáng)信號(hào)，然后將信號(hào)送到控制單元里的寄存器，由它將動(dòng)作信號(hào)傳到機(jī)床驅(qū)動(dòng)裝置。
有些光電裝置能以高達(dá)每秒1000個(gè)字節(jié)的速度閱讀，這對(duì)保持機(jī)床連續(xù)動(dòng)作是必須的，否則，在輪廓加工時(shí)，刀具可能在工件上產(chǎn)生劃痕。閱讀裝置必須要能以比控制系統(tǒng)處理數(shù)據(jù)更快的速度來(lái)閱讀數(shù)據(jù)程序塊。
反饋裝置是用在一些數(shù)控設(shè)備上的安全裝置，它可連續(xù)補(bǔ)償控制位置與機(jī)床運(yùn)動(dòng)滑臺(tái)的實(shí)際位置之間的誤差。裝有這種直接反饋檢查裝置的數(shù)控機(jī)床有一個(gè)閉環(huán)系統(tǒng)裝置。位置控制通過(guò)傳感器實(shí)現(xiàn)，在實(shí)際工作時(shí)，記錄下滑臺(tái)的位置，并將這些信息送回控制單元。接受到的信號(hào)與紙帶輸入的信號(hào)相比較，它們之間的任何偏差都可得到糾正。
在另一個(gè)稱(chēng)為開(kāi)環(huán)的系統(tǒng)中，機(jī)床僅由響應(yīng)控制器命令的步進(jìn)電動(dòng)機(jī)驅(qū)動(dòng)定位，工件的精度幾乎完全取決于絲杠的精度和機(jī)床結(jié)構(gòu)的剛度。有幾個(gè)理由可以說(shuō)明步進(jìn)電機(jī)是一個(gè)自動(dòng)化申請(qǐng)的非常有用的驅(qū)動(dòng)裝置。對(duì)于一件事物，它被不連續(xù)直流電壓脈沖驅(qū)使,是來(lái)自數(shù)傳計(jì)算機(jī)和其他的自動(dòng)化的非常方便的輸出控制系統(tǒng)。當(dāng)多數(shù)是索引或其他的自動(dòng)化申請(qǐng)所必備者的時(shí)候，步進(jìn)電機(jī)對(duì)運(yùn)行一個(gè)精確的有角進(jìn)步也是理想的。因?yàn)榭刂葡到y(tǒng)不需要監(jiān)聽(tīng)就提供特定的輸出指令而且期待系統(tǒng)適當(dāng)?shù)胤磻?yīng)的公開(kāi)- 環(huán)操作造成一個(gè)回應(yīng)環(huán)，步進(jìn)電機(jī)是理想的。 一些工業(yè)的機(jī)械手使用高抬腿運(yùn)步的馬乘汽車(chē)駕駛員，而且步進(jìn)電機(jī)是有用的在數(shù)字受約束的工作母機(jī)中。 這些申請(qǐng)的大部分是公開(kāi)- 環(huán) ,但是雇用回應(yīng)環(huán)檢測(cè)受到驅(qū)策的成份位置是可能的。 環(huán)的一個(gè)分析者把真實(shí)的位置與需要的位置作比較，而且不同是考慮過(guò)的錯(cuò)誤。 那然后駕駛員能發(fā)行對(duì)步進(jìn)電機(jī)的電脈沖，直到錯(cuò)誤被減少對(duì)準(zhǔn)零位。在這個(gè)系統(tǒng)中，沒(méi)有信息反饋到控制單元的自矯正過(guò)程。出現(xiàn)誤動(dòng)作時(shí)，控制單元繼續(xù)發(fā)出電脈沖。比如，一臺(tái)數(shù)控銑床的工作臺(tái)突然過(guò)載，阻力矩超過(guò)電機(jī)轉(zhuǎn)矩時(shí)，將沒(méi)有響應(yīng)信號(hào)送回到控制器。因?yàn)椋竭M(jìn)電機(jī)對(duì)載荷變化不敏感，所以許多數(shù)控系統(tǒng)設(shè)計(jì)允許電機(jī)停轉(zhuǎn)。然而，盡管有可能損壞機(jī)床結(jié)構(gòu)或機(jī)械傳動(dòng)系統(tǒng)，也有使用帶有特高轉(zhuǎn)矩步進(jìn)電機(jī)的其他系統(tǒng)，此時(shí)，電動(dòng)機(jī)有足夠能力來(lái)應(yīng)付系統(tǒng)中任何偶然事故。
最初的數(shù)控系統(tǒng)采用開(kāi)環(huán)系統(tǒng)。在開(kāi)、閉環(huán)兩種系統(tǒng)中，閉環(huán)更精確，一般說(shuō)來(lái)更昂貴。起初，因?yàn)樵葌鹘y(tǒng)的步進(jìn)電動(dòng)機(jī)的功率限制，開(kāi)環(huán)系統(tǒng)幾乎全部用于輕加工場(chǎng)合，最近出現(xiàn)的電液步進(jìn)電動(dòng)機(jī)已越來(lái)越多地用于較重的加工領(lǐng)域。