【正文】 vement, or while only one motor is of this type are principally used for straight—line cuts or for drilling or N/C system consists of the following ponents：data input, the tape reader with the control unit, feedback devices, and the metal—cutting machine tool or other type of N/C input, also called “man—to—control link”, may be provided to the machine tool manually, or entirely by automatic methods when used as the sole source of input data are restricted to a relatively small number of of manually operated devices are keyboard dials, pushbuttons, switches, or thumbwheel are located on a console near the ale analog devices usually connected to a synchrotype resolver or most cases, pushbuttons, switches, and other similar types of selectors are digital input input requires that the operator set the controls for each is a slow and tedious process and is seldom justified except in elementary machining applications or in special practically all cases, information is automatically supplied to the control unit and the machine tool by cards, punched tapes, or by magnetic —channel punched paper tape is the most monly used form of data input for conventional N/C coded instructions on the tape consist of sections of punched holes called block represents a machine function, a machining operation, or a bination of the entire N/C program on a tape is made up of an accumulation of these successive data resulting in long tapes all wound on reels like motionpicture on relatively short tapes may be continuously repeated by joining the two ends of the tape to form a installed, the tape is used again and again without further this case, the operator simply loads and1unloads the tapes ale prepared on type writers with special tape—punching attachments or in tape punching units connected directly to a puter production is rarely may be initially caused by the part programmer, in card punching or pilation, or as a result of physical damage to the tape during handling, trial runs are often necessary to remove all errors and produce an acceptable working the data on the tape is fed automatically, the actual programming steps ale done 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 dimensions on the part print are closely examined to determine a suitable zero reference point from which to start the 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 control unit receives and stores all coded data until a plete block of information has been then interprets the coded instruction and directs the machine tool through the required function of the control unit may be better understood by paring it to the action of a dial telephone, where, as each digit is dialed, it is the entire number has been dialed, the equipment bees activated and the call is photo diodes, located in the tape reader head on the control unit, detect light as it passes through the holes in the moving light beams are converted to electrical energy, which is amplified to further strengthen the signals are then sent to registers in the control unit, where actuation signals are relayed to the machine tool photoelectric devices are capable of reading at rates up to 1000 characters per reading rates are necessary to maintain continuous machine—tool motion；otherwise dwell marks may be generated by the cutter on the part during contouring reading device must be capable of reading data blocks at a rate faster than the control system can process the feedback device is a safeguard used on some N/C installations to constantly pensate for errors between the manded position and the actual location of the moving slides of the machine N/C machine equipped with this kind of a direct feedback checking device has what is known as a closedloop control is acplished by a sensor which, during the actual operation, records the position of the slides and relays this information back to the control thus received ale pared to input signals on the tape, and any discrepancy between them is automatically an alternative system, called an open—loop system, the machine is positioned solely by stepping motor drives in response to mands by a is one basic type of NC or Positional Control In pointtopoint control the machine tool elements(tools, table, etc.)are moved to programmed locations and the machining operations performedafter the motions are path or speed of movement between locations is unimportant。only the coordinates of the end points of the motions are accurately 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 or Linear Control StraightLine 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 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 type of controlled motion is often referred to as linear control or a halfaxis of with this form of control are also capable of pointtopoint original N/C used the closed—loop the two systems, closed and open loop, closed loop is more accurate and, as a consequence, is generally more , open—loop systems were used almost entirely for lightduty applications because of inherent power limitations previously associated with conventional electric stepping advances in the development of electro hydraulic stepping motors