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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 motors 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 passesthrough 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. 电动机和数控技术 电动机将电能转换为机械能。 简单来说电动机就是两块磁铁间的机械力作用。当两块磁铁（或电磁铁）相互靠近时，一块将吸引或排斥另一块。1.结构 人们制造的各种类型的电动机都有其特定的功能。它们都由两大部分构成：一部分称为定子。定子是静止的，它不产生运动。另一部分称为转子，它安装在轴承上，可以转动。转轴伸出机壳外。对于电动机而言，转轴联接机械负荷。2.电动机 输入电动机的是电能。施加到电动机端子上的电压产生电流。电动机输出机械功率。机械功率通过转轴以转矩形式输出。转矩可以驱动负荷转动，诸如一台泵。 需要一定的转矩才能驱动负荷以特定的速度转动。若电动机输出的转矩足够大，负载将转动。若电动机输出的转矩太小，则不能驱动负载。在选择电动机时所考虑的最重要的一个指标就是转矩要求。 电动机驱动负载时，电动机输出机械功率。而电动机又从电源得到等量的电功率。3.数控技术 迄今所叙述的设备的主要不利条件是这项技术的经济性。装配一台进行自动化生产的设备是很费钱的。因此，除非设备就会因成本过高而边的不可行。非常需要一种既快速，在单批量生产中又经济的自动化生产方法。这种方法已经有了，这就是数控技术。 数控（NC）是可编程自动化的一种表现形式，它以数字，字母和其他符号来控制加工设备。这些数字、字母和符号按适当的格式编码，形成用于某一工件或任务的加工程序。当任务改变了，某一工件的加工程序也随之改变。这种改变程序的能力使得NC适用于中小批量生产，更新程序要比大量改变加工设备要容易的多。数控原理首先用铣削加工中，然后用在车削加工、火花切削、钻削和磨削之中，数控基数越来越多地应用在其他的加工过程中，如成形加工（精密制造，滚压等加工）、雕刻或激光切割。目前数控基数相对成熟，许多数控设备具有多种加工功能，如铣削中心可以进行垂直和水平铣削、钻削、铰孔、插削、成形和车削等加工过程，当然，在配有大容量的自动化刀具库后，CNC机床的功能将更加丰富 数控和自动化之间很多性能是一样的。然而，数控技术更灵活，装配设备的花费更少，更换更快，而且停机维修时间更短。在加工外形轮廓时，数控能够用数学方法将确定的曲线转换成成品，即节省时间，又无需样板，还可提高加工精度。另一个优点是能大大节省机时，也就是说不增添设备就能提高生产率。 数控是可编程自动化技术的一种形式，通过数字、字母和其他符号来控制加工设备。数字、字母和符号用适当的格式编码为一个特定工件定义指令程序。当工件改变时，指令程序就改变。这种改变程序的能力使数控适合于中、小批量生产，写一段新程序远比对加工设备做大的改动容易得多。 数控设备有两种基本形式：点位控制和连续控制（也称为轮廓控制）。点位控制设备采用异步电动机，因此，主轴的定位只能通过完成一个运动或一个电动机的转动来实现。 数控系统由下列组件组成：数据输入装置，带控制单元的磁带阅读机，反馈装置和数控设备。 数据输入装置，也称“人机联系装置”，可用人工或全自动方法向机床提供数据。人工方法作为输入数据唯一方法时，只限于少量输入。人工输入装置有键盘，拨号盘，按钮，开关或拨轮选择开关，这些都位于机床附近的一个控制台上。拨号盘通常连到一个同步解析器或电位计的模拟装置上。在大多数情况下，按钮、开关和其他类似的旋钮是数据输入元件。人工输入需要操作者控制每个操作，这是一个即慢又单调的过程，除了简单加工场合或特殊情况，已很少使用。 几乎所有情况下，信息都是通过卡片、穿孔纸带或磁带自动提供给控制单元。在传统的数控系
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