外文翻譯---三坐標(biāo)測(cè)量機(jī)測(cè)頭的誤差校準(zhǔn)

1、<p><b>　　附錄2</b></p><p>　　Study and Design no Automation Control System of the Thermal Power Unit from the System Point of view.</p><p>　　Tian Jianyan liu sifeng Pan lizh

2、en</p><p>　?。?. Nanjing University of Aeronautics and Astronautics ,Nanjing 210016 China)</p><p>　?。?. Taiyuan University of Technology ,Taiyuan 030024 China)</p><p>　　Abstract: Fr

3、om the system point of view, the feature of the thermal power unit and the demand on its control system are analyzed and research in details in this paper. The character that the boils and the steam turbine are a whole o

4、f relative independence is fully considered. By coordinate control, the purpose of the optimal whole operating effect of the system is realize adopting the key techniques of communication, computer and intelligent inform

5、ation process etc.... The adaptation and whole p</p><p>　　1Introduction</p><p>　　The system is an organic whole certain structure and function ,which consists of several element with interrelati

6、on , interdependence and interplay .The thermal power unit is a typical system ,composed of generator ,steam turbine ,boiler and many anxiliary equipment .Its main function is to produce the power load according to powe

7、r network requirement .System engineering is the science used to study and design the complex system .In the meantime, It is also the theory and method used to deal wi</p><p>　　2 Study and Design on Automati

8、on Control System of the Thermal Power unit from the System Point of View </p><p>　　2.1 Function of Automation Control System of the Power unit </p><p>　　At first the thermal power unit as the c

9、ontrolled object is deeply studied before its automation control system is studied and designed .The thermal power unit is a huge group of equipments ,which is composed of generator, ,steam turbine ,boiler and many auxil

10、iary equipments. Its process flow is complex and pipelines intervenient .Especially the large-scale power unit is a typical coupling ,nonlinear ,varying complex controlled object with multi-input and multi-output .Thous

11、ands of parameters a</p><p>　?。?.）Load control :Along with the increase of the unit capacity ,early load control mode of “boiler follows steam turbine ”or “steam turbine follows boiler ” has not already sati

12、sfied the requirements so we must adopt more suitable load control scheme ,which is the unit coordinate control .</p><p>　?。?.）Reliability: The design of system must be reliable, ensuring safe and economic

13、operation of the unit .In addition, The tolerance fault, redundant technology ,self-diagnosis of software and hardware must be adopted to improve the reliability of system .</p><p>　　（3.）Communication and m

14、an-machine interface: Because there are many parameters need to be measured and controlled and its varying speed is quick when the large scale unit is running , communication system must have quick real-time response an

15、d high reliability ;man-machine interface must be good at the operator station including operating terminal and large-area display that provides synthetic menus and information about the whole production process for the

16、operators. </p><p>　　2.2 Structure of Automation Control system of the power unit </p><p>　　The design of automation control system of thermal power unit mainly includes control, alarm monito

17、r, protect etc . In accordance with the above analysis .So we divide the complex system into several subsystems in order to make the whole system optimal .</p><p>　?。?.）Data acquisition system(DAS):It is in

18、formation center of whole system and provides credible rapid objective operating records ,which is the base of safe and economical operating of equipments .Function of ,print of accident list and incident review ,prope

19、rty calculate ,operate guidance etc…</p><p>　　（2）Boiler control system :Boiler control system such as boiler combustion ,steam temperature ,feedwater ,auxiliary control system and the boiler safe supervisor

20、y system and boiler combustion management system. There subsystem coordinate each other and make boiler operate in safe ,efficient and steady state to satisfy the load instruction of power network in various operating mo

21、des .</p><p>　?。?.）Steam turbine control system :Steam turbine control system realizes load control and rotative velocity control of steam turbine by DEH (Digital Electro-Hydraulic ).In addition ,the auxilia

22、ry system of steam turbine include the deoxygenation water lever control ,deoxygenation pressure control ,the condenser hot well water level control and the heater water lever control etc </p><p>　?。?.）Aux

23、iliary system :Besides the main systems mentioned above ,the auxiliary system also has by-pass control system etc…</p><p>　　The main system and auxiliary system are a organic whole through close connection o

24、f information communication network .They are inter-permeable and .Inter-dependent so as to form the main part of the automation control system of the large-scale unit.</p><p>　　2.3 Design of Automation Cont

25、rol System of the Thermal Power Unit </p><p>　　From the system point of view, we analyze and study the principle of two basic feedback control method early that “boiler follows steam turbine” and “steam turb

26、ine follows boiler” in order to find out the existent problem ad be able to design more reasonable control method.</p><p>　　2.3.1 Boiler follows steam turbine mode </p><p>　　This kind of control

27、 mode is first to let steam turbine trail the needs of power network load ,then let boiler trail steam turbine .Its advantage is to be able to fully use the heat accumulation of boiler ,make the unit meet the change of p

28、ower network load comparatively quickly.. But because the inertia and delay of boiler are greater ,the regulation of former turbine pressure pt cannot follow the speed of load regulation, thus pt has greater fluctuation,

29、 which is disadvantageous to the steady </p><p>　　2.3.2. Steam turbine follows boiler mode </p><p>　　This kind of control mode is first to let boiler train the needs of power network load, then

30、let steam turbine train boiler .It can be better to maintain steam pressure stabilize and beneficial to boiler steady operation .But it do not fully utilize the heat accumulation f boiler to regulate the unit’s output ,i

31、t is more slowly that the unit meets the load’s demand ,so this control mode is only to be applied to the unit undertaking basic load .</p><p>　　2.3.3 Coordinate control mod</p><p>　　According t

32、o the analysis above, it must use the heat accumulation of boiler fully and reasonably when the unit has better adaptability to power network load in safe operation. That is to let former turbine pressure changes, then t

33、o coordinate the interaction between two loops of power regulation and pressure regulation reasonably, so that unit's output meets the needs of the power network load promptly, and guarantee the control demand of uni

34、t steady operation. The practical method is to lead i</p><p>　　2.3.3.1. Basic function of coordinate control system. </p><p>　　Coordinate control system of the thermal power unit should usually

35、have following function:(l) select different load instruction according to the unit operating condition and the requirement of power network for the unit.(2) restrict the change rate of load instruction.(3). Calculate th

36、e biggest possible output of the unit and restrict the biggest and minimum amplitude of load.(4). Have the function of RUNBACK.(5). Select different running mode according to the unit operating condition.(6). Calc</p&

37、gt;<p>　　2.3.3.2 Basic composition of coordinate control system of the unit.</p><p>　　Now the coordinate control system of the unit is composed of two parts, which are load instruction process and coo

38、rdinate control of boiler and steam turbine, shown as fig.1. </p><p>　　Fig.1 Unit Coordinate Control System</p><p>　　In which, load instruction process is composed of unit calculation loop, unit

39、 Allowed load calculation loop and load restriction loop. Coordinate and steam turbine controller.</p><p>　　2.3.3.3. Implementation of coordinate control system of the thermal power unit Modern large-scale t

40、hermal power unit adopts distributed control system to realize the unit automation control that guarantees safe economic operation of the unit. TEKEPERM-ME of SIEMENS is chosen to realize coordinate control system of the

41、 thermal power unit including several parts shown as Fig.2.</p><p>　　Fig 2 Implementation diagram of unit coordinate control system</p><p>　　When each subsystem of the unit normally run, start t

42、he mode of coordinate control mode. In this mode , steam turbine and boiler accept load instruction in parallel. Boiler maintains main steam pressure through changing combustion rate, and steam turbine pressure and set v

43、alue. When the deviation of former turbine pressure and set value exceeds certain limit value, open degree instruction of steam value given by steam turbine main control loop is restricted, up to this deviation within th

44、e press</p><p>　　3. Conclusion. </p><p>　　In one word, from the system point of view, the automation control system of the thermal power unit is divided into several subsystems as well as the de

45、sign and implementation of unit coordinate control system is demonstrated in this paper. The large scale unit coordinate control has faster trailing performance and better stability and robustness. The purpose of the opt

46、imal whole operation effect of the unit is realized by coordinate the subsystems.</p><p><b>　　Reference</b></p><p>　　[1]Xiong Shuyan , Wangxingye ,Tianyan .Distributed Control System

47、 of thermal power plant ,Scientific Press .</p><p>　　Beijing P.R China ,20003 </p><p>　　[2] Zhong xinyuan .Application of DEB Coordinated Control System in Thermal power plants ,electric power S

48、cience And Engineering ,2003(3):50-53.</p><p>　　Author Biographies </p><p>　　Jianyan Tian , associate professor ,is a Doctoral student in Nanjing University of Aeronautics and Astronautics .Her

49、research is aimed at intelligent control ,System Engineering and Grey System theory .</p><p>　　Lizhen pan is a student for Master Degree in 2002 .Her research is aimed at intelligent control theory and its

50、application.</p><p><b>　　附錄3</b></p><p>　　三坐標(biāo)測(cè)量機(jī)測(cè)頭的誤差校準(zhǔn)</p><p>　　內(nèi)容摘要:經(jīng)過(guò)幾十年的快速發(fā)展，坐標(biāo)測(cè)量技術(shù)已臻成熟，測(cè)量精度得到極大提高，測(cè)量軟件功能更加強(qiáng)大，操作界面也日益完善，生產(chǎn)廠家遍布全球，開(kāi)發(fā)出了適于不同用途的三坐標(biāo)測(cè)量機(jī)型。幾十年的發(fā)展充分證明，現(xiàn)代三坐標(biāo)測(cè)

51、量系統(tǒng)打破了傳統(tǒng)的測(cè)量模式，具有通用、靈活、高效等特點(diǎn)，可以通過(guò)計(jì)算機(jī)控制完成各種復(fù)雜零件的測(cè)量，符合機(jī)械制造業(yè)中柔性自動(dòng)化發(fā)展的需要，能夠滿足現(xiàn)代生產(chǎn)對(duì)測(cè)量技術(shù)提出的高精度、高效率要求。 除用于空間尺寸及形位誤差的測(cè)量外，應(yīng)用坐標(biāo)測(cè)量機(jī)對(duì)未知數(shù)學(xué)模型的復(fù)雜曲面進(jìn)行測(cè)量，提取復(fù)雜曲面的原始形狀信息，重構(gòu)被測(cè)曲面，實(shí)現(xiàn)被測(cè)曲面的數(shù)字化，不僅是坐標(biāo)測(cè)量機(jī)應(yīng)用的一個(gè)重要領(lǐng)域，也是反求工程中的關(guān)鍵技術(shù)之一，近年來(lái)也得到快速發(fā)展。<

52、;/p><p>　　關(guān)鍵詞：補(bǔ)償誤差 探針 找正 校準(zhǔn) 三維特性 觸發(fā)</p><p><b>　　坐標(biāo)測(cè)量機(jī)及其特點(diǎn)</b></p><p>　　坐標(biāo)測(cè)量機(jī)是一種具有很強(qiáng)柔性的尺寸測(cè)量設(shè)備，CMM在工業(yè)界的應(yīng)用開(kāi)始于對(duì)棱柱零件的快速、精確測(cè)量。但隨著CMM各方面技術(shù)的發(fā)展（如回轉(zhuǎn)工作觸發(fā)式測(cè)頭的產(chǎn)生），特別是計(jì)算機(jī)的CMM的出現(xiàn)，目前

53、，CMM已廣泛應(yīng)用于對(duì)各類零件的自動(dòng)檢測(cè)。與投影儀、輪廓測(cè)量?jī)x、圓度測(cè)量?jī)x、激光測(cè)量?jī)x等比較，CMM具有適應(yīng)性強(qiáng)，功能完善等特點(diǎn)。坐標(biāo)測(cè)量機(jī)的出現(xiàn)，不僅提高了檢測(cè)設(shè)備的水平，而且在自動(dòng)化檢測(cè)中也是一個(gè)生重要的突破。</p><p>　　CMM在自動(dòng)化程度方面有很大的差別。計(jì)算機(jī)控制的CMM具有自動(dòng)執(zhí)行檢測(cè)、分析檢測(cè)數(shù)據(jù)和輸出檢測(cè)結(jié)果的功能，而一般的CMM僅有手動(dòng)控制功能或手動(dòng)控制加示教功能。目前，隨著計(jì)算機(jī)硬件性

54、能的提高和價(jià)格降低，絕大部分CMM均配有計(jì)算機(jī)，利用計(jì)算機(jī)可對(duì)測(cè)量所得的數(shù)據(jù)進(jìn)行在線分析，以判別被控件是否合格。同時(shí)也可以使用統(tǒng)計(jì)技術(shù)來(lái)確定工藝能力是否滿足，分析誤差等來(lái)源。</p><p>　　除了再質(zhì)量檢測(cè)方面使用CMM外，CMM還可以用于對(duì)實(shí)物的仿物的信制加工中，即所謂逆向工程。在這種情況下，由CMM測(cè)量實(shí)際工件，并將測(cè)量所得的數(shù)據(jù)傳到系統(tǒng)中，由CAD/CAM系統(tǒng)對(duì)這些數(shù)據(jù)進(jìn)行加工處理，建立CAD模型，并進(jìn)

55、一步生成加工指令來(lái)指導(dǎo)加工。 CMM測(cè)頭</p><p>　　測(cè)頭是CMM非常重要的部件，可以這樣說(shuō)，測(cè)頭的發(fā)展先進(jìn)程度就標(biāo)志著CMM的發(fā)展先進(jìn)程度。CMM可以配置不同類型的測(cè)頭傳感器。接觸類的測(cè)頭主要包括觸發(fā)式、模擬式兩種。非接觸式包括激光三角測(cè)量、激光成像、機(jī)器視覺(jué)等。最初人們使用CMM時(shí)，由操作人員移動(dòng)坐標(biāo)軸，所用的測(cè)頭是剛性的，當(dāng)剛性測(cè)頭以一定的接觸力接觸到被測(cè)表面時(shí)，人為記錄下各坐標(biāo)軸的坐標(biāo)值

56、。這種初期的 CMM不可能具有自動(dòng)檢測(cè)的能力，使用范圍受到了極大的限制。但是由于它具有了三坐標(biāo)的雛形，在使用測(cè)頭鉆孔的位置時(shí)也相當(dāng)有效。</p><p>　　CMM能被廣泛地應(yīng)用，其主要的一個(gè)原因是發(fā)明了觸發(fā)式測(cè)頭，觸發(fā)式測(cè)頭的最大功能是它的觸發(fā)功能，即當(dāng)探針接觸被測(cè)表面并有一定的微笑位移時(shí)，測(cè)頭就發(fā)出一電信號(hào)，利用此信號(hào)可以立即鎖定當(dāng)前坐標(biāo)軸的位置，從而自動(dòng)記錄坐標(biāo)值。觸發(fā)式測(cè)頭是由雷尼紹公司發(fā)明的，現(xiàn)在該公司

57、生產(chǎn)一系列的觸發(fā)式測(cè)頭，可用于CNC加工中心，雷尼紹公司的生產(chǎn)的CMM測(cè)頭現(xiàn)已成為行業(yè)標(biāo)準(zhǔn)配置，廣泛地用于各大生產(chǎn)廠家的CMM上。</p><p>　　常見(jiàn)的測(cè)頭再運(yùn)動(dòng)學(xué)上，探針處于由三個(gè)圓柱棒6個(gè)球組成的6個(gè)觸點(diǎn)唯一確定的位置上，用一個(gè)輕型的有參緊力的彈簧維持這一位置。6個(gè) 觸點(diǎn)圖中的方式依次連接，并接一個(gè)恒定電流電源。當(dāng)探針接觸被測(cè)表面，并產(chǎn)生微小位移時(shí)，6個(gè)觸點(diǎn)中將有一個(gè)或一個(gè)以上的觸點(diǎn)斷開(kāi)，從而使回路中的

58、電阻迅速增大。當(dāng)回路中的電阻增大到一定數(shù)值時(shí)，兩端超過(guò)一定數(shù)值的電壓將將起到開(kāi)關(guān)電路發(fā)出信號(hào)。利用此信號(hào)就可以讀取當(dāng)前的測(cè)量位置數(shù)據(jù)。這種測(cè)頭的特點(diǎn)是具有三維特性，即X,Y,Z三個(gè)方向的移動(dòng)接觸均能引起觸發(fā)。因此，可以從不同的方向接觸被測(cè)表面，而不會(huì)影響測(cè)量結(jié)果。</p><p>　　在觸發(fā)式測(cè)頭進(jìn)行測(cè)量的過(guò)程中，探針必須偏移一個(gè)固定的數(shù)量才會(huì)觸發(fā)開(kāi)關(guān)，因此，測(cè)量結(jié)果中要對(duì)這段偏移進(jìn)行補(bǔ)償。探針接觸被測(cè)表面后，為

59、了避免移動(dòng)過(guò)量而折段，探針需要反方向退出一定距離。因此測(cè)量速度比硬探頭掃描測(cè)量速度低。</p><p>　　CMM是用探針端部球的中心坐標(biāo)值作為點(diǎn)的輸入數(shù)據(jù)。因此，在測(cè)量時(shí)必須用恰當(dāng)?shù)姆椒ㄍ茢鄿y(cè)頭端部球與被測(cè)零件的觸點(diǎn)位置。在非CAD指導(dǎo)的檢測(cè)系統(tǒng)中，通常在觸點(diǎn)附近作三點(diǎn)測(cè)量，從而近似地找出通過(guò)該三點(diǎn)的平面法線，這不僅要耗費(fèi)很多時(shí)間，從而測(cè)量精度也比較低。在CAD指導(dǎo)的檢測(cè)系統(tǒng)中可以根據(jù)被測(cè)工件的CAD模型直接計(jì)

60、算出被測(cè)點(diǎn)法向，讓測(cè)頭從法向接觸被測(cè)點(diǎn)，這樣就比較容易判斷觸點(diǎn)的位置。如圖所為觸點(diǎn)坐標(biāo)，為測(cè)頭端部球心坐標(biāo)，8，a分別為測(cè)點(diǎn)法向與z軸夾角及法向在xy平面內(nèi)投影與y軸夾角，則接觸點(diǎn)的坐標(biāo)可表示為：</p><p>　　x=x’-R sinB.sina</p><p>　　y=x’-R sinb.sina</p><p>　　z=X’-R sinb.sina</

61、p><p>　　圖所示為探針的形狀。它的作用是為紅寶石球提供一個(gè)固定的支撐，當(dāng)探針接觸被測(cè)表面時(shí)，探針的微笑移動(dòng)可觸發(fā)開(kāi)關(guān)，從而發(fā)出信號(hào)。探針有不同的類型。如圖所示，根據(jù)不同的需要可以選擇不同的類型的探針。為了獲得較高的測(cè)量精度，建議在實(shí)際測(cè)量時(shí)遵循以下兩條原則：1、盡量使用長(zhǎng)度短的和剛性好的探針。測(cè)量時(shí)探針的彎曲越大，偏移越大，測(cè)量的重復(fù)精度就越低。2、盡量選用直徑大的紅寶石探針。選用直徑大的紅寶石探針，一方面可以

62、減小加工表面缺陷時(shí)測(cè)量精度的影響，另一方面可以增大探針的有效工作長(zhǎng)度。如圖7-8所示。</p><p>　　1 測(cè)頭的分類 測(cè)量頭作為測(cè)量傳感器，是坐標(biāo)測(cè)量系統(tǒng)中非常重要的部件。三坐標(biāo)測(cè)量機(jī)的工作效率、精度與測(cè)量頭密切相關(guān)，沒(méi)有先進(jìn)的測(cè)量頭，就無(wú)法發(fā)揮測(cè)量機(jī)的卓越功能。坐標(biāo)測(cè)量機(jī)的發(fā)展促進(jìn)了新型測(cè)頭的研制，新型測(cè)頭的開(kāi)發(fā)又進(jìn)一步擴(kuò)大了測(cè)量機(jī)的應(yīng)用范圍。按測(cè)量方法，可將測(cè)頭分為接觸式（觸發(fā)式）和非接觸式兩

63、大類。觸發(fā)式測(cè)量頭又分為機(jī)械接觸式測(cè)頭和電氣接觸式測(cè)頭；非接觸式測(cè)頭則包括光學(xué)顯微鏡、電視掃描頭及激光掃描頭等。本文討論的重點(diǎn)為觸發(fā)式測(cè)頭。 (1)機(jī)械接觸式測(cè)頭 接觸式測(cè)頭又稱為“剛性測(cè)頭”、“硬測(cè)頭”，一般用于“靜態(tài)”測(cè)量，大多作為接觸元件使用。這種測(cè)頭沒(méi)有傳感系統(tǒng)，無(wú)量程、不發(fā)訊，只是一個(gè)純機(jī)械式接觸頭。機(jī)械接觸式測(cè)頭主要用于手動(dòng)測(cè)量。由于人工直接操作，故測(cè)頭的測(cè)量力不易控制，只適于作一般精度的測(cè)量。由于其明顯

64、的缺點(diǎn)，目前這種測(cè)頭已很少使用。 (2)電氣接觸式測(cè)頭 電氣接觸式測(cè)頭又稱為“軟測(cè)頭”，適于動(dòng)態(tài)測(cè)量。這種測(cè)頭作為測(cè)量傳感器，是唯一與工件接觸的部件，每測(cè)量一個(gè)點(diǎn)時(shí)，測(cè)頭傳感部分總有一個(gè)“接觸—偏轉(zhuǎn)—發(fā)訊—回復(fù)”的過(guò)程，測(cè)頭的測(cè)端與</p><p><b>　?。?）探針的校準(zhǔn)</b></p><p>　　在對(duì)工件進(jìn)行實(shí)際檢測(cè)之前，首先要對(duì)測(cè)量過(guò)程

65、中用到的探針進(jìn)行校準(zhǔn)。因?yàn)樵谠S多尺寸的測(cè)量當(dāng)中需要沿不同的方向進(jìn)行探測(cè)，系統(tǒng)記錄的是探針中心的坐標(biāo)而不是接觸點(diǎn)的坐標(biāo)。為了獲得接觸點(diǎn)的坐標(biāo)，必須對(duì)探針半徑進(jìn)行補(bǔ)償。因此首先必須對(duì)探針進(jìn)行校準(zhǔn)。一般使用校準(zhǔn)球來(lái)校準(zhǔn)探針。校準(zhǔn)球是一個(gè)已知直徑的校準(zhǔn)球，校準(zhǔn)探針的過(guò)程實(shí)際上就是對(duì)這個(gè)已知直徑的標(biāo)準(zhǔn)進(jìn)行測(cè)量直徑的過(guò)程，如圖7-1所示。該球的測(cè)量值等于校準(zhǔn)球的直徑加探針直徑，這樣也就可以確定探針直徑，將探針直徑除以2，得出探針半徑，系統(tǒng)用這個(gè)值就

66、可以對(duì)測(cè)量結(jié)果進(jìn)行補(bǔ)償。校準(zhǔn)的具體操作步驟一般如下：將探頭正確的安裝在CMM的主軸上；將探針在工件表面移動(dòng)，看是否均能測(cè)得到，檢查探針是否清潔。記住，一旦探針的位置發(fā)生改變，就必須重新校準(zhǔn)；將校準(zhǔn)球裝在工作臺(tái)上，要確保不用移動(dòng)校準(zhǔn)球上打點(diǎn)，測(cè)點(diǎn)當(dāng)選最少為五個(gè)；給定的點(diǎn)當(dāng)數(shù)測(cè)完后，就可以得到測(cè)量所得的校準(zhǔn)球的位置、直徑、形狀偏差，由此可以得到探針的半徑值。</p><p>　　測(cè)量過(guò)程中所有要用到的探針都要進(jìn)行校準(zhǔn)

67、，而且一旦探針改變位置，或者取下后下次再用時(shí)要重新進(jìn)行校準(zhǔn)，這樣一來(lái)在探針的校準(zhǔn)方面要用去大量的時(shí)間。為解決這一問(wèn)題，有的CMM上配有測(cè)頭庫(kù)和測(cè)頭自動(dòng)交換裝置。測(cè)頭庫(kù)中的測(cè)頭經(jīng)過(guò)一次校準(zhǔn)后可重復(fù)交換使用而無(wú)需重新校準(zhǔn)。</p><p><b>　　工件的找正</b></p><p>　　我們知道CMM有其本身的機(jī)器坐標(biāo)系，而在進(jìn)行檢測(cè)規(guī)劃時(shí)，檢測(cè)點(diǎn)數(shù)量及其分布的確定以

68、及檢測(cè)路徑的生成及信真等都是在CAD中工件坐標(biāo)系下進(jìn)行的。因此在進(jìn)行實(shí)際檢測(cè)之前首先要確定工件坐標(biāo)系在CMM機(jī)器坐標(biāo)系中的位置關(guān)系。即首先要在CMM機(jī)器坐標(biāo)系中對(duì)工件進(jìn)行找正。通常采用6點(diǎn)找正法，即“3-2-1”方法對(duì)工件進(jìn)行找正。如圖7-12所示，首先通過(guò)在指定平面上測(cè)量三點(diǎn)或三點(diǎn)以上的點(diǎn)校準(zhǔn)基準(zhǔn)面；其次通過(guò)測(cè)量?jī)牲c(diǎn)或兩點(diǎn)以上的點(diǎn)來(lái)校準(zhǔn)基準(zhǔn)軸；最后再測(cè)一點(diǎn)來(lái)計(jì)算原點(diǎn)。在以上三步操作中檢測(cè)點(diǎn)位置的確定都是依據(jù)工件坐標(biāo)系來(lái)選擇的?！?lt

69、;/p><p>　　工件在工作臺(tái)上的擱置方式一般有兩種。一種是通過(guò)專用夾具或自動(dòng)裝卸裝置，將工件放在工作臺(tái)沙鍋內(nèi)的某儀固定位置。這樣，通過(guò)一次工件找正，再以后測(cè)量同批工件時(shí)，由于工件的位置基本上是確定的，故無(wú)須再對(duì)工件進(jìn)行找正，直接就可進(jìn)行測(cè)量；另一種是通過(guò)肉眼的觀察直接將工件放在工作臺(tái)的某一合適位置，在這種情況下每測(cè)一工件都必須首先對(duì)其在工作臺(tái)上進(jìn)行找正。</p><p>　　(2)測(cè)球半徑

70、補(bǔ)償誤差 當(dāng)測(cè)針接觸到工件時(shí)，三坐標(biāo)測(cè)量機(jī)接收的的坐標(biāo)值應(yīng)是紅寶石球頭中心點(diǎn)坐標(biāo)，顯然，測(cè)量軟件將自動(dòng)沿著測(cè)針從接觸點(diǎn)回退的方向加上一個(gè)測(cè)球半徑值作為測(cè)量值。但該測(cè)量值是一個(gè)與測(cè)頭的機(jī)械慣性有關(guān)的動(dòng)態(tài)值。實(shí)際上，測(cè)量作為一個(gè)動(dòng)態(tài)過(guò)程，其測(cè)量值應(yīng)該考慮到從測(cè)頭采點(diǎn)到實(shí)際向系統(tǒng)傳送該點(diǎn)坐標(biāo)值時(shí)發(fā)生的機(jī)器空間移動(dòng)距離。盡管這個(gè)距離極小，但對(duì)系統(tǒng)計(jì)算動(dòng)態(tài)尺寸有一定影響。 在實(shí)際測(cè)量時(shí)，每測(cè)量一個(gè)元素，系統(tǒng)都可以自動(dòng)區(qū)分測(cè)球半

71、徑的補(bǔ)償方向，計(jì)算正確的補(bǔ)償半徑。在采點(diǎn)開(kāi)始后，測(cè)量軟件將在沿著測(cè)針接觸工件的方向上對(duì)測(cè)球進(jìn)行半徑補(bǔ)償。但被補(bǔ)償點(diǎn)并非真正的接觸點(diǎn)，而是測(cè)頭沿著測(cè)針接觸工件方向的延長(zhǎng)線上的一個(gè)點(diǎn)。這樣就造成了補(bǔ)償誤差，產(chǎn)生誤差的大小與測(cè)球的半徑及該工件被測(cè)面與笛卡爾坐標(biāo)軸的夾角有關(guān)，夾角越大，誤差越大。 ①測(cè)球半徑r對(duì)補(bǔ)償誤差的影響 補(bǔ)償誤差δ與測(cè)球半徑r成正比關(guān)系，即測(cè)球半徑r越小，補(bǔ)償誤差δ也越小。因此當(dāng)用三坐標(biāo)測(cè)量機(jī)進(jìn)行點(diǎn)位測(cè)