數(shù)控機(jī)床相關(guān)畢業(yè)設(shè)計(jì)翻譯

1、IMPROVING ACCURACY OF CNC MACHINETOOLS THROUGH COMPENSATIONFOR THERMAL ERRORSAbstract： A method for improving accuracy of CNC machine tools through compensation for the thermal errors is studied. The thermal errors are obtained by 1-D ball array and characterized by an auto regressive model based on

2、 spindle rotation speed. By revising the workpiece NC machining program , the thermal errors can be compensated before machining. The experiments on a vertical machining center show that the effectiveness of compensation is good.Key words : CNC machine tool Thermal error Compensation0 INTRODUCTIONIm

3、provement of machine tool accuracy is essential to quality cont rol in manufacturing processes. Thermally induced errors have been recognized as the largest cont ributor to overall machine inaccuracy and are probably the most formidable obstacle to obtaining higher level of machine accuracy. Thermal

4、 errors of machine tools can be reduced by the st ructural improvement of the machine tool it self through design and manufacturing technology. However , there are many physical limitations to accuracy which can not be overcome solely by production and design techniques. So error compensation techno

5、logy is necessary. In the past several years , significant effort s have been devoted to the study. Because thermal errors vary with time during machining ,most previous works have concent rated on real-time compensation. The typical approach is to measure the thermal errors and temperature of sever

6、al representative point s on the machine tools simultaneously in many experiment s , then build an empirical model which correlates thermal errors to the temperature statues by multi-variant regression analysis or artificial neural network.During machining , the errors are predicted on-line accordin

7、g to the pre-established model and corrected by the CNC cont roller in real-time by giving additional signals to the feed-drive servo loop.However , very few practical cases of real-time compensation have been reported to be applied to commercial machine tools today. Some difficulties hinder it s wi

8、despread application. First , it is tedious to measure thermal errors and temperature of many point s on the machine tools. Second ,the wires of temperature sensors influence the operating of the machine more or less. Third , thereal-time error compensation capability is not available on most machin

9、e tools.In order to improve the accuracy of production-class CNC machine tools , a novel method is proposed. Although a number of heat sources cont ribute to the thermal errors , the f riction of spindle bearings is regarded as the main heat source. The thermal errors are measureed by 1-D ball array

10、 and a spindle-mounted probe. An auto regressive model based on spindle rotation speed is then developed to describe the time-variant thermal error. Using this model , thermal errors can be predicted as soon as the workpiece NC machining program is made. By modifying the program , the thermal errors

11、 are compensated before machining. The effort and cost of compensation are greatly reduced. This research is carried on a JCS2018 vertical machining center.1 EXPERIMENTAL WORKFor compensation purpose , the principal interest is not the deformation of each machine component , but the displacement of

12、the tool with respect to the workpiece. In the vertical machining center under investigation , the thermal errors are the combination of the expansion of spindle , the distortion of the spindle housing , the expansion of three axes and the distortion of the column.Due to the dimensional elongation o

13、f leadscrew and bending of the column , the thermal errors are not only time-variant in the time span but also spatial-variant over the entire machine working space.In order to measure the thermal errors quickly , a simple protable gauge , i. e. , 1-D ball array , is utilized. 1-D ball array is a ri

14、gid bar with a series of balls fixed on it with equal space. The balls have the same diameter and small sphericity errors. The ball array is used as a reference for thermal error measurement . A lot of pre-experiment s show that the thermal errors in z-axis are far larger than those in x-axis and y-

15、axis , therefore major attention is drawn on the thermal errors in z-axis. Thermal errors in the other two axes can be obtained in the same way.The measuring process is shown in Fig.1. A probe is mounted on the spindle housing and 1-D ball array is mounted on the working table. Initially , the coord

16、inates of the balls are measured under cold condition. Then the spindle is run at a testing condition over a period of time to change the machine thermal status. The coordinates of the balls are measured periodically. The thermal drift s of the tool are obtained by subt racting the ball coordinates

17、under the new thermal status f rom the reference coordinates under initial condition. Because it takes only about 1 min to finish one measurement , the thermal drifts of the machine under different z coordinates can be evaluated quickly and easily. According to the rate of change , the thermal error

18、s and the rotation speed are sampled by every 10 min. Since only the drift s of coordinates deviated from the cold condition but not the absolute dimensions of the gauge are concerned , accuracy and precise inst rument such as a laser interferometer is not required. There are only four measurement p

19、oint s z 1 ,z 2 , z 3 , z 4 to cover the z-axis working range whose coordinates are - 50 , - 150 , - 250 , - 350 respectively. Thermal errors at other coordinates can be obtained by an interpolating function.Previous experiment s show that the thermally induced displacement between the spindle housi

20、ng and the working table is the same with that between the spindle and table. So the thermal errorsz measured reflect those in real cutting condition with negligible error.In order to obtain a thorough impression of the thermal behavior of the machine tool andidentify the error model accurately , a

21、measurement strategy is developed. Various loads of the spindle speed are applied. They are divided into three categories as the following : (1) The constant speed ; (2) The speed spect rum ; (3) The speed simulating real cutting condition. The effect of the heat generated by the cutting process is

22、not taken into account here. However , the influence of the cutting process on the thermal behaviour of the total machine structure is regarded to be negligible in finishing process.In this machine , the most significant heat sources are located in the z-axis. Thermal errors in z direction on differ

23、ent x and y coordinates are approximately the same. It implies that the positions of x-carriage and y-carriage have no strong influence on the z-axis thermal errors.Fig.1（L） Thermal error measurement 1.Spindle mounted probe 2.1-D ball array Fig.2 （R）Thermal errors at different z coordinates 1. z = -

24、 50 2. z = - 150 3. z = - 250 4. z = - 350Fig.2 plot s the time-history of thermal drift z at different z coordinates under a test . Itshows that the resultant thermal drift s are obvious position-dependent . The thermal drift s at z 1 ,z 2 , z 3 , z 4 are coincident initially but separate gradually

25、 as time passes and temperature increases.The reason is that , initially most of thermal drift s result f rom the position-independent thermal growth of the spindle housing which would rise fast and go to thermal-equilibrium quickly compared to other machine component s with longer thermal-time-cons

26、tant s. However , as time passes , those position-dependent thermal errors such as the lead screw and the column cont ribute to the resultant thermal drift s of the tool more and more. As a result , the thermal drifts at different z coordinates have different magnitude and thermal characteristics. H

27、owever , the thermal errors at different coodinates vary with z coordinate continuously.2 AR MODEL FOR THERMAL ERRORPrecise prediction of thermal errors is an important step for accurate error compensation.Since the knowledge of the machine structure , the heat source and the boundary condition are

28、insufficient , a precise quantitative prediction based on theoretical heat transfer analysis is quite difficult . On the other hand , empirical-based error models using regression analysis and neural networks have been demonst rated to predict thermal errors with satisfactory accuracy in much applic

29、ation.Thermal errors are caused by various heat sources. Only the influence of the heat caused by the fiction of spindle which is the most significant heat source is considered. The influence of external heat source on machining accuracy can be diminished by environment temperature control.From the

30、obtained data , it is found that thermal errors vary continuously with time. Thevalue of error at one moment is influenced by that of the previous moment and the rotation speed of spindle. So a model representing the behavior of the thermal errors as written is the formwhere z ( t) Thermal error at

31、time tk , m Order of the modelai , bi Coefficient of the modeln ( t - i) Spindle rotation speed at time t - iThe order k and m are determined by the final prediction-error criterion. The coefficients aiand bi are estimated by artificial neural network technique. A neural network is a multiple nonlin

32、ear regression equation in which the coefficient s are called weight s and are t rained with an iterative technique called back propagation. It is less sensitive than other modeling technique to individual input failure due to thresholding of the signals by the sigmoid functions at each node. The ne

33、ural network for this problem is shown in Fig.3. ( k = 1 , m = 0) . The number of hidded nodes is determined by a trial-and error procedure.Using the data obtained (thermal errors and correspondence speed) , four models for the errors at z 1 , z 2 , z 3 and z 4 are established. Thermal errors at pos

34、itions other than z 1 , z 2 , z 3 , z 4 are calculated by an interpolating function. So the errors at any z coordinates can be obtained.In order to verify the prediction accuracy of the model , a number of new operation conditions are used. Fig14 shows an example of predicted result on a new conditi

35、on. It shows that the auto regressive model based on speed can descibe thermal errors well in a relative stable environment .Fig.3 A neural network for thermal errorsFig.4 Thermal error predicting 1.Measuring results 2Predicting results3 PRE-COMPENSATION FOR THERMAL ERRORSThe principle of pre-compen

36、sation for thermal errors is shown in Fig.5. The spindle rotation speed and the z coordinates are known as soon as the workpiece NC machining program is made.By , for example , every 10 min , the thermal errors z are calculated by the model. Then the program is corrected by adding the calculated z t

37、o the original z . So the thermal errors are compensated before machining.The effectiveness of the error compensation is verified by many cutting test s. Several surfaces are milled under cold start and after 1 h run with varying speeds. As shown in Fig.6 , the depth difference of the milled surface

38、 is used to evaluate the compensation result of the thermal errors in z direction. It shows that the difference is reduced from 7m to 2m.Fig.5 Compensation for thermal errors by revising machining programFig.6 The effectiveness of compensation4 CONCLUSIONSA novel method for improving the accuracy of

39、 CNC machine tools is discussed. The core of the study is an error model based on spindle rotation speed but not on temperature like conventional approach. By revising the NC workpiece machining program , the thermal errors can be compensated before machining but not in real-time. By using the metho

40、d , the accuracy of machine tools can be increased economically.References1 Chen J S , Chiou G. Quick testing and modeling of thermally-induced errors of CNC machine tools. InternationalJournal of Machine Tools and Manufacture , 1995 , 35(7) 1 0631 0742 Chen J S. Computer-aided accuracy enhancement

41、for multi-axis CNC machine tool. International Journal of Machine Tools and Manufacture , 1995 , 35(4) 5936053 Donmez M A. A general methodology for machine tool accuracy enhancement by error compensation. Precision Engineering , 1986 , 8 (4) 1871964 Lo C H. An application of real-time error compens

42、ation on a turning center. International Journal of Machine Tools and Manufacture , 1995 , 35(12) 1 6691 682.5 Yang S. The Improvement of thermal error modeling and compensation on machine tools by CMAC neural network. International Journal of Machine Tools and Manufacture , 1995 , 36(4) 5275376 李書和

43、1 數(shù)控機(jī)床誤差補(bǔ)償?shù)难芯坎┦繉W(xué)位論文1 天津天津大學(xué),19961通過熱量誤差補(bǔ)償來改善數(shù)控機(jī)床的精確度摘要：通過熱量誤差補(bǔ)償來改變數(shù)控機(jī)床的精度是一種可行的方法。熱量誤差的獲得是通過1-D滾珠排列和建立在錠子轉(zhuǎn)速基礎(chǔ)上的自動(dòng)退刀的表征。通過改變工件的數(shù)控程序，熱量誤差在機(jī)加工以前可以被補(bǔ)償。試驗(yàn)表明直立的加工中心的實(shí)際補(bǔ)償是可行的。關(guān)鍵詞：數(shù)控加工中心, 熱量誤差,補(bǔ)償0.引言：數(shù)控機(jī)床精確度的改善是生產(chǎn)過程中質(zhì)量控制的根本。熱量誤差已經(jīng)被作為機(jī)器精確度失衡的最大誘因，而且可能也是機(jī)器獲取更高精確度的最大障礙。數(shù)控機(jī)床的熱量誤差可通過機(jī)床本身的結(jié)構(gòu)設(shè)計(jì)和生產(chǎn)技術(shù)的改善而降低。盡管如此，還是有

44、許多物理性限制因素使得精確度不能通過生產(chǎn)和設(shè)計(jì)技術(shù)而單獨(dú)克服。因此，誤差補(bǔ)償技術(shù)是很必要的。在過去的幾年里，對(duì)此技術(shù)的研究已經(jīng)獲得重大成果。由于熱量誤差在加工時(shí)隨時(shí)間而變化，許多前人的工作都集中在實(shí)際時(shí)間的的補(bǔ)償比率上。典型的方法是對(duì)機(jī)床幾個(gè)有代表性的點(diǎn)進(jìn)行熱量誤差和溫度的同步試驗(yàn)，然后建立一個(gè)與熱量誤差和溫度的試驗(yàn)?zāi)Ｐ蛯?duì)多種變化進(jìn)行回歸分析或是人工網(wǎng)絡(luò)分析。在加工期間，誤差是根據(jù)之前建立的模型進(jìn)行預(yù)測(cè)并通過在實(shí)際過程中用額外的信號(hào)和自由回路進(jìn)行改正的。但是，目前只有很少被報(bào)道的實(shí)際過程補(bǔ)償案例適用于商業(yè)機(jī)床。首先，對(duì)機(jī)床的多個(gè)點(diǎn)進(jìn)行熱量誤差和溫度的測(cè)量是不可取的。其次，溫度傳感器的線會(huì)或多或

45、少影響機(jī)器的運(yùn)轉(zhuǎn)。第三，實(shí)際操作中的誤差補(bǔ)償功能在許多的機(jī)器上是不可用的。為了改善數(shù)控機(jī)床生產(chǎn)的精確度，有個(gè)方法是值得嘗試的。盡管許多的熱源都能引起熱量誤差，但是環(huán)形軸承的摩擦被認(rèn)為是最主要的熱源。熱量誤差是由1-D滾珠排列來衡量的。一個(gè)自動(dòng)回歸模型是以錠子轉(zhuǎn)速然后被發(fā)展到描述那時(shí)的熱量錯(cuò)誤為基礎(chǔ)的。利用這個(gè)模型，熱量誤差能夠在機(jī)械加工程序制造的時(shí)候被預(yù)測(cè)出來。通過對(duì)程序的修訂，熱量誤差能夠在加工之前得到補(bǔ)償。那么補(bǔ)償?shù)拇鷥r(jià)就大大的減輕了。1.試驗(yàn)工作為了達(dá)到補(bǔ)償目的，重要的部分不是每個(gè)機(jī)器的零部件，而是工件的位移。在調(diào)查的線性機(jī)械加工中心中，熱量誤差是由錠子膨脹、錠子固件變形和三個(gè)軸空間的變

46、形一起引起的。由于導(dǎo)桿的伸長和欄的彎曲，熱量誤差并不只是在時(shí)間上的改變，而且還是機(jī)械加工在空間上的變化。為了能夠快速的測(cè)量熱量誤差，一些簡單的量規(guī)是可以使用的，例如：滾珠排列。滾珠排列是把一系列的滾珠按相等的間隔固定在頂梁上。由于滾珠的直徑相等，球狀的誤差比較小，因此，滾珠排列被用于熱量誤差測(cè)量的一個(gè)參考。大量的之前試驗(yàn)數(shù)據(jù)表明在光軸上的熱量誤差遠(yuǎn)遠(yuǎn)高于在橫軸和縱軸。所以，熱量誤差主要關(guān)注在光軸上。同理，也可以用相同的辦法得到其他兩個(gè)軸上的熱量誤差數(shù)據(jù)。測(cè)量的過程如圖1所示：剛開始，滾珠的坐標(biāo)是處在低溫狀態(tài)的，然后錠子在試驗(yàn)狀態(tài)下改變機(jī)器的熱量。滾珠溫度的測(cè)量是周期性的。熱量的轉(zhuǎn)移是通過用最初

47、的參考坐標(biāo)減去在新的熱量狀態(tài)下滾珠坐標(biāo)來實(shí)現(xiàn)的。由于這種測(cè)量只需要一分鐘，機(jī)器在不同坐標(biāo)下的熱量轉(zhuǎn)移能夠更快更容易的被顯現(xiàn)出來。根據(jù)轉(zhuǎn)動(dòng)速率的變化，熱量誤差和轉(zhuǎn)速是每十分鐘就是一個(gè)循環(huán)。坐標(biāo)的唯一偏離是在低溫狀態(tài)下完成的，而不是在所關(guān)注的獨(dú)立的量規(guī)尺寸下。象激光干涉儀這樣的精確度和準(zhǔn)確度裝置并不做要求。只有四個(gè)測(cè)量點(diǎn)z1，z2，z3，z4來覆蓋坐標(biāo)為-50，-150，-250，-350的z坐標(biāo)的工作范圍。在其他的坐標(biāo)中熱量誤差可以通過一個(gè)插值函數(shù)來獲得。上述的試驗(yàn)說明了在錠子位置和工作臺(tái)之間的派生位移與錠子和臺(tái)之間是一致的。因此熱量誤差z的測(cè)量反映了在真正的切割條件下誤差是可以忽略的。為了能夠

48、獲得機(jī)床熱量行為的全面理解以及正確的判斷誤差模型，形成了一種測(cè)量方法。錠子轉(zhuǎn)速的多種加載方式是可用的。他們被分為如下三類：1，常規(guī)轉(zhuǎn)速，2，轉(zhuǎn)速范圍，3，真正切割狀態(tài)下的同步轉(zhuǎn)速。此處，由切割過程而引起的熱量作用沒有被考慮進(jìn)來。不過，切割過程對(duì)整個(gè)機(jī)床機(jī)構(gòu)的熱量的影響在最終的過程中是可以忽略的。在這種機(jī)床中，最大的熱源來自于z軸。熱量誤差在z方向和不同的x和y坐標(biāo)方向大約是相同的。也就是說x軸和y軸的位置對(duì)z軸的熱量誤差沒有重大影響。1. 圖1（左）熱量誤差測(cè)量 錠子傳感器2.1-D 滾珠排列圖2（右） 在不同z坐標(biāo)中的熱量誤差1. z = - 50 2. z = - 150 3. z = -

49、 250 4. z = - 350圖2 在測(cè)試中不同z 坐標(biāo)中熱量轉(zhuǎn)移時(shí)間過程圖的繪制上圖表明合成的熱量轉(zhuǎn)移明顯是由所在決定的。在z1，z2，z3，z4點(diǎn)上的熱量轉(zhuǎn)移剛開始是一樣的，然后隨著時(shí)間的流逝和溫度的增加而逐漸分離。原因在于最初大量的熱量轉(zhuǎn)移是由于錠子位置的增長造成的，和其他的耐熱時(shí)間較長的機(jī)床部件相比，這個(gè)位置能更快的達(dá)到熱量平衡。然而，隨著時(shí)間的過去，那些象導(dǎo)螺桿和欄這樣由位置決定熱量誤差的部件越來越多的引起合成熱量的轉(zhuǎn)移。結(jié)果，在不同的z坐標(biāo)中熱量的轉(zhuǎn)移具有不同的大小和熱量特性。但是，不同坐標(biāo)中的熱量轉(zhuǎn)移是隨z坐標(biāo)不斷改變的。2.熱量誤差的回歸模型熱量誤差的準(zhǔn)確預(yù)測(cè)是精確誤差補(bǔ)償?shù)闹匾h(huán)節(jié)。由于對(duì)機(jī)床結(jié)構(gòu)的認(rèn)識(shí)和熱源以及界限條件的不充分，根據(jù)熱量傳遞分析得出精確的數(shù)量測(cè)量是非常困難的。另外，在眾多的實(shí)用中，利用以經(jīng)驗(yàn)為基礎(chǔ)的誤差模型進(jìn)行回歸分析和網(wǎng)絡(luò)分析來準(zhǔn)確預(yù)測(cè)熱量誤