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Motors and Numerical Control
Motors change electrical energy into mechanical energy .
This is simply the mechanical forces between magnets .when two magnets (or electromagnets)approach each other ,one will be either pulled toward or pushed away from the other .
1.CONSTRUCTION
Each type of motor is built to do its own special job .All of them have two main parts .One part is called the stator .The stator is stationary ;it does not move .The other part ,the rotor ,is mounted on bearings so it can rotate .The rotor shaft sticks out beyond the housing .For a motor ,the rotor shaft is coupled to a mechanical load .
2.MOTORS
The power input to a motor is electrical .Voltage is applied to a motor’s terminals resulting in a current .The power output of a motor is mechanical .This power is transmitted by the rotor shaft as a torque .This torque tends to rotate a load , such as a fan or pump .
As a motor drives a load , mechanical power is drawn from the motor .The motor ,in turn ,draws electrical power from the source at the same rate
3.NUMERICAL CONTROL
The major disadvantage of machine tool automation thus far described lies in the economics of the process .It is expensive to tool a installations for automatic production .Therefore ,unless the part is to be made in very large numbers ,the cost becomes prohibitive .Great need exists for a method that permits rapid automatic production ,economical in job-lot amounts .The answer has been found in the numerical control of machine tools .
Computer Numerical Control .Numerical control (NC) 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 workpiece or job .When the job changes ,the program of the particular workpiece changes .The capability to change the program is what makes NC suitable for low-volume and medium-volume production ,and it is much easier to write new programs than to make major alterations to the processing equipment .
There is a wide area of performance duplication between numerical control and automatics .Numerical control , however ,offers more flexibility ,lower tooling cost ,quicker changes ,and less machine down-time .
In machining contours ,numerical control can mathematically translate the defined curve into a finished product ,saving time and eliminating templates .This can in turn improve accuracy .Another advantage appears to be great saving of machine time ,the equivalent og increasing productive capacity with no increase in facilities .
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 qppropriate 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/Csuitable 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 ,whit 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 .
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 toll 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 relatibely small number of inputs .Examples of manually operated devices are kryboard dials ,pushbuttons ,switches ,or thumbwheel selectors .These are located on a console near the machine .Dials are analog devices usually connected to a synchro-type resolver or potentiometer .In most cases ,pushbuttons ,switches ,and other similar types of selectors are 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 too 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 are 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 are prepared on typewriters 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 are 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 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 persecond. High reading rates are necessary to maintain continuous installations motion; other-wise,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 compen-sate 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 close-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 are 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 is installations positioned solely by stepping motor drives in response to commands by a controller. The degree of work precision depends almost entirely upon the accuracy of the lead screw and the rigidity of the installations 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 system are in use, however, which, in spite of possibility of damage to the installations structure or to the mechanical system , are 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 system, closed and openloop, 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 electrohydraulic stepping motors have led to increasingly heavier machine load applications.
電動機和數(shù)控技術(shù)
電動機將電能轉(zhuǎn)換為機械能。
簡單來說電動機就是兩塊磁鐵間的機械力作用。當兩塊磁鐵（或電磁鐵）相互靠近時，一塊將吸引或排斥另一塊。
1.結(jié)構(gòu)
人們制造的各種類型的電動機都有其特定的功能。它們都由兩大部分構(gòu)成：一部分稱為定子。定子是靜止的，它不產(chǎn)生運動。另一部分稱為轉(zhuǎn)子，它安裝在軸承上，可以轉(zhuǎn)動。轉(zhuǎn)軸伸出機殼外。對于電動機而言，轉(zhuǎn)軸聯(lián)接機械負荷。
2.電動機
輸入電動機的是電能。施加到電動機端子上的電壓產(chǎn)生電流。電動機輸出機械功率。機械功率通過轉(zhuǎn)軸以轉(zhuǎn)矩形式輸出。轉(zhuǎn)矩可以驅(qū)動負荷轉(zhuǎn)動，諸如一臺泵。
需要一定的轉(zhuǎn)矩才能驅(qū)動負荷以特定的速度轉(zhuǎn)動。若電動機輸出的轉(zhuǎn)矩足夠大，負載將轉(zhuǎn)動。若電動機輸出的轉(zhuǎn)矩太小，則不能驅(qū)動負載。在選擇電動機時所考慮的最重要的一個指標就是轉(zhuǎn)矩要求。
電動機驅(qū)動負載時，電動機輸出機械功率。而電動機又從電源得到等量的電功率。
3.數(shù)控技術(shù)
迄今所敘述的設備的主要不利條件是這項技術(shù)的經(jīng)濟性。裝配一臺進行自動化生產(chǎn)的設備是很費錢的。因此，除非設備就會因成本過高而邊的不可行。非常需要一種既快速，在單批量生產(chǎn)中又經(jīng)濟的自動化生產(chǎn)方法。這種方法已經(jīng)有了，這就是數(shù)控技術(shù)。
數(shù)控（NC）是可編程自動化的一種表現(xiàn)形式，它以數(shù)字，字母和其他符號來控制加工設備。這些數(shù)字、字母和符號按適當?shù)母袷骄幋a，形成用于某一工件或任務的加工程序。當任務改變了，某一工件的加工程序也隨之改變。這種改變程序的能力使得NC適用于中小批量生產(chǎn)，更新程序要比大量改變加工設備要容易的多。
數(shù)控原理首先用銑削加工中，然后用在車削加工、火花切削、鉆削和磨削之中，數(shù)控基數(shù)越來越多地應用在其他的加工過程中，如成形加工（精密制造，滾壓等加工）、雕刻或激光切割。
目前數(shù)控基數(shù)相對成熟，許多數(shù)控設備具有多種加工功能，如銑削中心可以進行垂直和水平銑削、鉆削、鉸孔、插削、成形和車削等加工過程，當然，在配有大容量的自動化刀具庫后，CNC機床的功能將更加豐富
數(shù)控和自動化之間很多性能是一樣的。然而，數(shù)控技術(shù)更靈活，裝配設備的花費更少，更換更快，而且停機維修時間更短。在加工外形輪廓時，數(shù)控能夠用數(shù)學方法將確定的曲線轉(zhuǎn)換成成品，即節(jié)省時間，又無需樣板，還可提高加工精度。另一個優(yōu)點是能大大節(jié)省機時，也就是說不增添設備就能提高生產(chǎn)率。
數(shù)控是可編程自動化技術(shù)的一種形式，通過數(shù)字、字母和其他符號來控制加工設備。數(shù)字、字母和符號用適當?shù)母袷骄幋a為一個特定工件定義指令程序。當工件改變時，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)，寫一段新程序遠比對加工設備做大的改動容易得多。
數(shù)控設備有兩種基本形式：點位控制和連續(xù)控制（也稱為輪廓控制）。點位控制設備采用異步電動機，因此，主軸的定位只能通過完成一個運動或一個電動機的轉(zhuǎn)動來實現(xiàn)。
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，反饋裝置和數(shù)控設備。
數(shù)據(jù)輸入裝置，也稱“人機聯(lián)系裝置”，可用人工或全自動方法向機床提供數(shù)據(jù)。人工方法作為輸入數(shù)據(jù)唯一方法時，只限于少量輸入。人工輸入裝置有鍵盤，撥號盤，按鈕，開關(guān)或撥輪選擇開關(guān)，這些都位于機床附近的一個控制臺上。撥號盤通常連到一個同步解析器或電位計的模擬裝置上。在大多數(shù)情況下，按鈕、開關(guān)和其他類似的旋鈕是數(shù)據(jù)輸入元件。人工輸入需要操作者控制每個操作，這是一個即慢又單調(diào)的過程，除了簡單加工場合或特殊情況，已很少使用。
幾乎所有情況下，信息都是通過卡片、穿孔紙帶或磁帶自動提供給控制單元。在傳統(tǒng)的數(shù)控系統(tǒng)中，八信道穿孔紙帶或磁帶自動提供給控制單元。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為程序塊的穿孔組成。每一個程序塊代表一種加工功能、一種操作或兩者的組合。紙帶上的整個數(shù)控程序由這些連續(xù)數(shù)據(jù)單元連續(xù)而成。帶有程序的長帶子象電影膠片一樣繞在盤子上，相對較短的帶子上的程序可通過將紙帶兩端連續(xù)形成一個循環(huán)而連續(xù)不斷地重復使用。帶子一旦安裝好，就可反復使用而無需進一步處理。此時，操作者只是簡單的上、下工件。穿孔紙帶是在帶有特制穿孔附件的打字機或直接連到計算機的紙帶穿孔裝置上作成的，紙帶制造很少不出錯，錯誤可能由編程、卡片穿孔或編碼、紙帶穿孔時的物理損害等形成。通常，必須要試走幾次來排除錯誤，才能得到一個可用的工作紙帶。
雖然紙帶上的數(shù)據(jù)是自動進給的，但實際編程卻是手工完成的，在編碼紙帶做好前，編程者經(jīng)常要和一個計劃人員或工藝工程師一起工作，選擇合適的數(shù)控設備，決定加工材料，計算升降速度，決定所需電動機。
控制單元接受和存儲編碼數(shù)據(jù)，直至形成一個完整的信息程序塊，然后解釋數(shù)控指令，并引導設備得到所需運動。
為了更好理解控制單元的作用，可將它與撥號電話進行比較，即每撥一個數(shù)字，就存儲一個，當整個數(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，通過其內(nèi)的硅光二極管，檢測到穿過移動紙帶上的孔漏過的光線，將光束轉(zhuǎn)變成電能，并通過放大來進一步加強信號，然后將信號送到控制單元里的寄存器，由它將動作信號傳到設備驅(qū)動裝置。
有些光電裝置能以高達每秒1000個字節(jié)的速度閱讀，這對保持設備連續(xù)動作是必須的，否則，在輪廓加工時，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)更快的速度來閱讀數(shù)據(jù)程序塊。
反饋裝置是用在一些數(shù)控設備上的安全裝置，它可連續(xù)補償控制位置與設備運動滑臺的實際位置之間的誤差。裝有這種直接反饋檢查裝置的數(shù)控設備有一個閉環(huán)系統(tǒng)裝置。位置控制通過傳感器實現(xiàn)，在實際工作時，記錄下滑臺的位置，并將這些信息送回控制單元。接受到的信號與紙帶輸入的信號相比較，它們之間的任何偏差都可得到糾正。
在另一個稱為開環(huán)的系統(tǒng)中，設備僅由響應控制器命令的步進電動機驅(qū)動定位，繞線部分的升降幾乎完全取決于絲杠的精度和設備結(jié)構(gòu)的剛度。在這個系統(tǒng)中，沒有信息反饋到控制單元的自矯正過程。出現(xiàn)誤動作時，控制單元繼續(xù)發(fā)出電脈沖。比如，一臺數(shù)控銑床的工作臺突然過載，阻力矩超過電機轉(zhuǎn)矩時，將沒有響應信號送回到控制器。因為，步進電動機對載荷變化不敏感，所以許多數(shù)控系統(tǒng)設計允許電動機停轉(zhuǎn)。然而，盡管有可能損壞設備機構(gòu)或機械傳動系統(tǒng)，也有使用帶有特高轉(zhuǎn)矩步進電機的其他系統(tǒng)，此時，電動機有足夠能力來應付系統(tǒng)中任何偶然事故。
最初的數(shù)控系統(tǒng)采用開環(huán)系統(tǒng)。在開、閉環(huán)兩種系統(tǒng)中，閉環(huán)更準確，一般說來更昂貴。起初，因為原先傳統(tǒng)的步進電動機的功率限制，開環(huán)系統(tǒng)幾乎全部用于輕加工場合，最近出現(xiàn)的電液步進電動機已越來越多地用于較重的加工領(lǐng)域。