【正文】 o the action of a dial telephone， where， as each digit is dialed， it is stored． When the entire number has been dialed， the equipment bees activated and the call is pleted． 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 pensate for errors between the manded 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 closedloop system． Positioning 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 unit． Signals thus received ale pared 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 mands by a controllers． There are three basic types of NC motions, as follows: Pointtopoint or Positional Control In pointtopoint control the machine tool elements (tools, table, etc.) are moved to programmed locations and the machining operations performed after the motions are pleted. The path or speed of movement between locations is unimportant。這種機(jī)床主要用于直線 切削或鉆孔、鏜孔等場(chǎng)合。 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。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。 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。因?yàn)榭刂葡到y(tǒng)不需要監(jiān)聽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 環(huán)操作造成一個(gè)回應(yīng)環(huán)，步進(jìn)電機(jī)是理想的。因?yàn)?，步進(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)。 最初的數(shù)控系統(tǒng)采用開環(huán)系統(tǒng)。 這些申請(qǐng)的大部分是公開 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。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。 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，決定所需刀具類型，仔細(xì)閱讀零件圖上尺寸，定下合適的程序開始的零參考點(diǎn)，然后寫出程序清單，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。每一個(gè)程序塊代表一種加工功能、一種操作或