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The fast mould integrates the systematic precision and guarantees that the system is studied
Summary : Recommend with RP fast of technology and cast maturity manufacture functional CAE system of products of craft, have analysed the factor influencing precision of the products, has proposed analysing the precision at stage of three main shaping with the non-linear finite element, adopt pattern-recognition theory, error theory, neural network method to deal with the error and feedback the problem, carry on error compensate revise and prediction of machining accuracy , improve final method of product quality.
?? Keyword: CAE; RT; Feedback
1 preface
Fast prototype technology (& of Rapid Prototyping Manufacturing, abbreviate RP or RPM as, urged directly by the part CAD model, adopt the material accurate prototype or part manufacturing technology which piles up the complicated three-dimensional entity, it is each on the basis of being dispersed /pile up by shaping manufacturing approach not new-type of the principles.
Rapid shaping technique already can make and include resin, plastics, paper, paraffin wax, pottery prototype of material successfully very much, but can only do the function experiment in the limited metal used for substituting really of occasion and other type function parts as the functional part. With increase and constant development of technology of demand, fast prototype technology to fast prototype / direction in which the part is being made develop.
Utilize RP technological shaping function part, especially metal a kind of main method of part a switch technology, called the fast technology of mould(RT, Rapid tooling). Because traditional mould complicated consuming time and long costly to make course, become design and bottleneck that make often, use RP technology make fast economic mould become RP main motive force of technical development one of. Paul think from PR to RT shaping the second leap of technical development fast.
The final purpose of making and shaping is products and service offered and meet the demands. RP technology become front technology of manufacturing industry with a great deal of superiority their, but because limitation restriction extensive application their of material; Traditional technology such as cast, forge and press and develop for a long time, relatively ripe already, but can't meet the needs of fast flexibility of information age , within future one period, must conbine rapid shaping technique and traditional shaping technology together , realize melting quickly and make.
2 RP and RT system integration
New product development most high cost have, man-hour longest stage to make the physical model involved, namely prototype manufacture process. RP technology use fast inspection and fast manufactures of different model that part design for mainly. Its basic principle and forming process are: Designed the computer three-dimensional curved surface entity's model of the necessary part by CAD software first , namely electronic model; According to technological requirement, go on the strata it according to certain thickness , turn the original three-dimension electronic model into two-dimentional level information (sectional information); And then carry on certain treatment of the data after the strata, put into and process parameters, produce the numerical control code; Until computer control, numerical control system process by level way process each thin layer and make they automatic sticking and shaping in succession in an orderly manner.
RT changes RP prototype into the technology of the tool and mould with various kinds of methods . A method among them is to change the prototype into a pottery type, utilize the method to cast to change into a metal type . And RT manufacture of technology intergration accurate to cast mould method meet modern industry towards many variety, turn batch request of development into with RP prototype, it known as" it is flexible tool " method 5 ,its craft of routes.
3 integrated system make functional precision of product analyse
Can find out from CAD final precision of product decide by manufacture error at each stage from model to fast prototype course in products.
(1)CAD modeling among the course, because of modeling limitation of software, can describe accurate often as to complicated curved surface;
(2)STL division of file among the course, because STL file layout mistake that modelling appear in the course sensitive to geometry, mistake these bring through STL file into and get RP modelling among the craft, some influence RP modelling course 3 these of craft seriously, 4 these . And STL file one approach the wanton surface of the room the triangle with level, therefore can only it is at CAD geometrical property among system therefore 3 on part since express approximate therefore.
(3)RP craft through to pile up and succeed the object prototype next life material, a lot of craft course is also following the changing of the material , such as FDM and SLA craft. So, the forming process of RP is not only that the machinery of a material piles up course, a height coupling, non-linear thermodynamics course. There are thes influence factor of the precision of this course: Thickness of material parameter, power of laser, strata, scanning route,etc..
(4)Change precision loss of craft depend on and change craft material, conversion method used,etc..
(5)It is the stage that the precision loses the most in the whole craft course that the metal pours the course. Usually, the first several in 10*10 error of stage - 2 mm order of magnitude, and metal solidify size change of course in several even a dozen mm.s of order of magnitude, so, the improvement of the precision of the final products depends on this stage to a great extent. There are thes factor influencing its precision: Material nature, for instance material density, elastic mould amount, heat conduction rate, specific heat, line coefficient of expansion,etc., especially the high temperature hot thing parameter of the metal, the heat transfer characteristic between the metal and type, position of watering, rising head ,etc..
4 integrate the precision of the systematic products and guarantee the system
In the actual production, adopt and try on law come and guarantee final precision of product by mistake often. With the development of technology of the computer and to lowering costs, realizing the requests of digitization, course controllability, necessary to adopt computer integrated manufacturing approach, go on computer simulation emulation study to whole course. CAE (Computer Aided Engineering, project computer auxiliary ) including fairy victory make, flexible manufacturing, at the same time project, make etc. fictitiously. Introduce CAE technology, can draw the craft route again. If regard the middle course as " the black box ", from the craft route to the products of CAD model; This fast, many return circuit error with corresponding course controls and the reponse system.
5 errors feedback system research
In this CAE system, the error exists at every stages.
In CAD modelling , divide in the course of STL file , it is because of the limitation of the modelling software that precision is lost, improve the quality of the software, can reduce the error .
At RP prototype fabrication stage , change craft stage and metal to pour stage, involve the hot, strength coupling question, quote the non-linear finite element method , under the three-dimensional flute Karl's coordinate, according to the conservation principle of energy, can draw :
??????????????? (1)
Exert hot stress and external force(such as border terms) that temperature cause as the strength load the object and then, try to get total deformation amount. After drawing the errors of every stages, can set up error reponse system . The error reponse system belongs to and closes the control system of the ring . A main content of it is the research that transmits function out of shape. Under it is at flute not three-dimensional Karl coordinate, whether calculate and error after being the out of shape can express for deformation that get for:
e(x,y,z) =P ideal (x, y, z)- p reality (x, y,z) (2)
If use Δ d (x, y, z) to show the change matrix of the real whole form , g (x, y, z) shows the matrix of controlling, the feedback control of the whole course can be described as:
Δ d (x, y, z) =g(x,y,z) e (x, y,z) (3)
E (x, y, z) is the algebraical sum of each course, namely, if does not consider the error of CAD modelling course , and e 1 (x, y,z),e 2 (x, y,z),e 3 (x, y,z) show respectively there aren't RP the error, the conversion craft error and the metal of course where the prototypes makes,
e(x,y,z) =(x, y,z)+e 3 (x,y, z) (4)
The present question is, set up proper g (x, y, z),make Δ d (x, y, z) can disappear to and smaller than range that a certain error allows δ quickly , namely satisfied:
<δ of Δ d (x, y, z) (5)
G (x, y, z) embodies us to the whole understanding of deforming the course, it must real-time reflection products out of shape with CAD model out of shape coupling relation. And will change with the changes of the border condition out of shape and material parameter .
Because the whole course is that more than one variable, complicated three-dimension input more close the control system of the ring non-linearly, perhaps there is a coupling relation between each variable, so, influence g (x, y,z) factor the a lot of,very it it is difficult to be describe with unified mathematics formula, so, the author puts forward the method to use neural network, train g (x, y,z),make Δ d (x, y,z) reach require.
Utilize the neural network to have advantage taught oneself to practise, can reduce the error and feedback the demand for the craft data of the question greatly , and benefit systematic expansion. And because close correcting function to the error of system of ring , may cause systematic unstability . Adopt neural self-adaptation of network, learning method, can raise system stupid and getting wonderful.
6 conclusions
RP technology is a fast-developing new developing manufacturing technology, RP, and RT integrated system that technology combine together to make functional main method of part with RP technology nowadays, the precision of raising this fast flexible system is nowadays urgent problem that is solved . This text has analysed that influences the factor of the precision of the products in this flexible manufacturing system, have proposed analysing the precision at stage of three main shaping with the non-linear finite element, adopt pattern-recognition theory, error theory, neural network method to deal with the error and feedback the problem, carry on error compensate revise and prediction of machining accuracy , improve final method of product quality.
Selected from " the electromechanical project "
快速模具集成系統(tǒng)精度保證體系研究
摘要: 介紹了利用RP技術(shù)的快速性和鑄造工藝的成熟性制造功能性產(chǎn)品的CAE系統(tǒng)，分析了影響產(chǎn)品精度的因素，提出了用非線性有限元分析三個主要成型階段的精度，采用模式識別理論、誤差理論、神經(jīng)網(wǎng)絡(luò)方法處理誤差反饋問題，進(jìn)行誤差補(bǔ)償修正和加工精度的預(yù)報，提高最終產(chǎn)品質(zhì)量的方法。
關(guān)鍵詞：CAE；RT;反饋
1 前 言
快速原型技術(shù)（Rapid Prototyping & Manufacturing,簡稱RP或RPM）是指在計(jì)算機(jī)控制與管理下，由零件CAD模型直接驅(qū)動，采用材料精確堆積復(fù)雜三維實(shí)體的原型或零件制造技術(shù)，是一種基于離散/堆積成型原理的新型制造方法。
快速成型技術(shù)已經(jīng)能非常成功地制作包括樹脂、塑料、紙類、石蠟、陶瓷等材料的原型，但往往不能作為功能性零件，只能在有限的場合用來替代真正的金屬和其它類型功能零件做功能實(shí)驗(yàn)。隨著需求的增加和技術(shù)的不斷發(fā)展，快速原型技術(shù)正向快速原型/零件制造的方向發(fā)展。
利用RP技術(shù)成型功能零件尤其是金屬零件的一種主要方法是轉(zhuǎn)換技術(shù)，稱為快速模具（RT，Rapid tooling）技術(shù)。由于傳統(tǒng)模具制作過程復(fù)雜、耗時長、費(fèi)用高，往往成為設(shè)計(jì)和制造的瓶頸，因此應(yīng)用RP技術(shù)制造快速經(jīng)濟(jì)模具成為RP技術(shù)發(fā)展的主要推動力之一。Paul[1]認(rèn)為從RP到RT是快速成型技術(shù)發(fā)展的第二次飛躍。
制造和成型的最終目的是要提供滿足要求的產(chǎn)品和服務(wù)。RP技術(shù)以其諸多優(yōu)越性而成為制造業(yè)的前沿技術(shù)，但因?yàn)椴牧系木窒扌灾萍s了其更廣泛的應(yīng)用；傳統(tǒng)技術(shù)如鑄造、鍛壓等經(jīng)過長期發(fā)展，已相對成熟，但不能適應(yīng)信息時代的快速柔性要求，在未來一段時期內(nèi)，必須將快速成型技術(shù)與傳統(tǒng)成型技術(shù)結(jié)合起來，實(shí)現(xiàn)敏捷化制造。
2 RP與RT系統(tǒng)集成
新產(chǎn)品開發(fā)中成本最高、工時最長的階段就是制造所涉及的物理模型，即原型制造過程。RP技術(shù)主要用于零件設(shè)計(jì)的快速檢驗(yàn)以及各種模型的快速制造。其基本原理和成型過程是：先由CAD軟件設(shè)計(jì)出所需零件的計(jì)算機(jī)三維曲面實(shí)體模型，即電子模型；然后根據(jù)工藝要求，將其按一定厚度進(jìn)行分層，把原來的三維電子模型變成二維平面信息（截面信息）；再將分層后的數(shù)據(jù)進(jìn)行一定的處理，加入加工參數(shù)，生成數(shù)控代碼；在計(jì)算機(jī)控制下，數(shù)控系統(tǒng)以平面加工方式有序地連續(xù)加工出每個薄層并使它們自動粘接而成型。
RT就是用各種方法把RP原型轉(zhuǎn)換成工模具的技術(shù)。其中一個方法是將原型轉(zhuǎn)換成陶瓷型，再利用鑄造的方法轉(zhuǎn)換成金屬型。利用RP原型與RT技術(shù)集成的制造精密鑄造模具方法適應(yīng)了現(xiàn)代工業(yè)向著多品種、變批量發(fā)展的要求，被稱為“柔性工具”方法[5],其工藝路線.
3 集成系統(tǒng)制造功能性產(chǎn)品的精度分析
從CAD模型到快速原型到產(chǎn)品的過程可以看出，最終產(chǎn)品的精度是由每個階段的制造誤差決定的。
(1)CAD建模過程中，由于建模軟件的局限性，對于復(fù)雜的曲面常常不能精確地加以描述；
(2)STL文件的劃分過程中，由于STL文件格式對幾何造型過程中出現(xiàn)的錯誤不敏感，這些錯誤通過STL文件帶入到RP造型工藝中，有的將嚴(yán)重影響RP工藝的造型過程[3，4]。而且，STL文件用平面三角形面片來逼近空間的任意表面，因而只能近似地表示零件在CAD系統(tǒng)中的幾何特征[3]。
(3)RP工藝是通過材料的堆積來生成物體原型，許多工藝過程還伴隨著材料的相變，如FDM和SLA工藝。所以，RP成型過程不只是一個材料的機(jī)械堆積過程，還是一個高度耦合、非線性的熱力學(xué)過程。這一過程的精度影響因素有：材料參數(shù)、激光功率、分層厚度、掃描路徑等。
(4)轉(zhuǎn)換工藝中的精度損失取決于轉(zhuǎn)換工藝所使用的材料、轉(zhuǎn)換方法等。
(5)金屬澆注過程是整個工藝過程中精度損失最大的階段。通常，前幾個階段的誤差在10×10-2mm數(shù)量級，而金屬凝固過程的尺寸變化在幾個甚至十幾個毫米數(shù)量級，因此，最終產(chǎn)品精度的提高很大程度上取決于這個階段。影響其精度的因素有：材料性質(zhì)，如材料密度、彈性模量、導(dǎo)熱率、比熱、線膨脹系數(shù)等，尤其是金屬的高溫?zé)嵛镄詤?shù)，金屬與型腔之間的傳熱特征，澆、冒口的位置等。
4 集成系統(tǒng)產(chǎn)品精度保證體系
實(shí)際生產(chǎn)中，常常采用試錯法來保證最終產(chǎn)品的精度。隨著計(jì)算機(jī)技術(shù)的發(fā)展以及對降低成本、實(shí)現(xiàn)數(shù)字化、過程可控性的要求，有必要采用計(jì)算機(jī)集成制造方法，對整個過程進(jìn)行計(jì)算機(jī)模擬仿真研究。CAE（Computer Aided Engineering,計(jì)算機(jī)輔助工程）包括靈捷制造、柔性制造、同時工程，虛擬制造等。引進(jìn)CAE技術(shù)，可以把工藝路線重新繪制。如果把中間過程看作“黑匣子”，則由CAD模型到產(chǎn)品的工藝路線；此過程相應(yīng)的快速、多回路誤差控制與反饋系統(tǒng)。
5 誤差反饋系統(tǒng)研究
在此CAE系統(tǒng)中，誤差存在于每一階段。
在CAD造型、劃分STL文件的過程中，精度的丟失是由于造型軟件的局限性，提高軟件的質(zhì)量，可以降低誤差。
在RP原型制造階段、轉(zhuǎn)換工藝階段和金屬澆注階段，涉及熱、力耦合問題，引用非線性有限元方法，在三維笛卡爾坐標(biāo)下，根據(jù)能量守恒原理，可以得出：
???????????????? (1)
再將溫度引起的熱應(yīng)力和外力（如邊界條件）之和作為力載荷施加到物體，求得總變形量。得出每一階段的誤差之后，可以建立誤差反饋系統(tǒng)。誤差反饋系統(tǒng)屬閉環(huán)控制系統(tǒng)。它的一個主要內(nèi)容是變形傳遞函數(shù)的研究。在三維笛卡爾坐標(biāo)下，由計(jì)算得到的變形前和變形后的誤差可以表示為：
e(x,y,z)=p理想(x,y,z)-p實(shí)際(x,y,z)??????????????? (2)
如果用Δd（x,y,z）表示實(shí)際整體形狀的變化矩陣，g(x,y,z)表示控制矩陣，則整個過程的反饋控制可以描述為：
Δd（x,y,z）=g(x,y,z) e (x,y,z)?????????????????(3)
e(x,y,z)是每個過程的代數(shù)和，即，如果不考慮CAD造型過程的誤差，且e1(x,y,z)，e2(x,y,z)，e3(x,y,z)分別表示RP原型制造的誤差、轉(zhuǎn)換工藝過程的誤差和金屬澆注過程的變形量，則
e(x,y,z)=e1(x,y,z)+e2(x,y,z)+e3(x,y,z)?????????????(4)
現(xiàn)在的問題就是，設(shè)置適當(dāng)?shù)膅(x,y,z)，使Δd（x,y,z）能夠很快地收斂到小于某個誤差允許的范圍δ，即滿足：
Δd（x,y,z）＜δ??????????????????????? (5)
g(x,y,z)體現(xiàn)出我們對整個變形過程的理解，它必須實(shí)時的反映產(chǎn)品變形和CAD模型變形的耦合關(guān)系。而且還要隨變形邊界條件和材料參數(shù)的變化而變化。
由于整個過程是一個多變量、多輸入的復(fù)雜三維非線性閉環(huán)控制系統(tǒng)，各個變量之間可能存在耦合關(guān)系，因此，影響g(x,y,z)的因素很多，很難用統(tǒng)一的數(shù)學(xué)公式描述，因此，作者提出應(yīng)用神經(jīng)網(wǎng)絡(luò)的方法，訓(xùn)練g(x,y,z)，使Δd（x,y,z）達(dá)到要求。
利用神經(jīng)網(wǎng)絡(luò)具有自學(xué)習(xí)的優(yōu)點(diǎn)，可以大大減少誤差反饋問題對工藝數(shù)據(jù)的需求，并且便于系統(tǒng)的擴(kuò)展。而且由于閉環(huán)系統(tǒng)對誤差的校正作用，可能會導(dǎo)致系統(tǒng)的不穩(wěn)定。采用神經(jīng)網(wǎng)絡(luò)的自適應(yīng)、自學(xué)習(xí)方法，可以提高系統(tǒng)的魯棒性。
6 結(jié)論
RP技術(shù)是一個正在快速發(fā)展的新興制造技術(shù)，RP和RT技術(shù)相結(jié)合的集成系統(tǒng)是當(dāng)今利用RP技術(shù)制造功能性零件的主要方法，提高此快速柔性系統(tǒng)的精度是當(dāng)今急迫解決的問題。本文分析了影響此柔性制造系統(tǒng)中產(chǎn)品精度的因素，提出了用非線性有限元分析三個主要成型階段的精度，采用模式識別理論、誤差理論、神經(jīng)網(wǎng)絡(luò)方法處理誤差反饋問題，進(jìn)行誤差補(bǔ)償修正和加工精度的預(yù)報，提高最終產(chǎn)品質(zhì)量的方法。
選自《機(jī)電工程》
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