砂輪回轉(zhuǎn)試驗(yàn)機(jī)的設(shè)計(jì)（含CAD圖紙?jiān)次募?/h1>
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砂輪回轉(zhuǎn)試驗(yàn)機(jī)設(shè)計(jì)文獻(xiàn)綜述
摘 要 本文介紹了大型自動(dòng)控制砂輪回轉(zhuǎn)試驗(yàn)機(jī)的設(shè)計(jì)研制,通過(guò)設(shè)計(jì)一體式砂輪卡盤(pán)及新結(jié)構(gòu)防護(hù)罩,采用交流變
頻、計(jì)算機(jī)控制技術(shù),研制出了新一代性能優(yōu)良的大型回轉(zhuǎn)試驗(yàn)機(jī),具有自動(dòng)化程度高,操作維護(hù)簡(jiǎn)便,數(shù)字化顯示主軸
轉(zhuǎn)速,節(jié)能等優(yōu)點(diǎn)。
關(guān)鍵詞 自動(dòng)控制;砂輪;回轉(zhuǎn)試驗(yàn)機(jī)
1 概述
砂輪回轉(zhuǎn)強(qiáng)度是衡量砂輪質(zhì)量的最重要指標(biāo)之一,直接影響到人身及設(shè)備的安全,回轉(zhuǎn)機(jī)是檢測(cè)砂輪回轉(zhuǎn)強(qiáng)度的專(zhuān)用設(shè)備。隨著我國(guó)加入WTO 和機(jī)械行業(yè)的振興,特別是汽車(chē)行業(yè)的發(fā)展,對(duì)曲軸磨等大直徑砂輪的需求迅速增長(zhǎng),因而生產(chǎn)大直徑砂輪廠(chǎng)家的數(shù)量增多,砂輪生產(chǎn)廠(chǎng)家和使用廠(chǎng)家對(duì)大型回轉(zhuǎn)機(jī)的市場(chǎng)需求變大。
我國(guó)目前生產(chǎn)和使用的大型回轉(zhuǎn)機(jī)主要有以下幾種: (1)簡(jiǎn)易回轉(zhuǎn)機(jī):采用人工開(kāi)關(guān)直接控制電機(jī)的啟動(dòng)停止,電機(jī)和回轉(zhuǎn)機(jī)主體分別固定在基礎(chǔ)上,無(wú)測(cè)速及速度控制裝置,速度變化通過(guò)更換皮帶輪來(lái)實(shí)現(xiàn),速度誤差大,安全防護(hù)措施簡(jiǎn)單,屬于五六十年代的水平。(2)使用電磁調(diào)速電機(jī)拖動(dòng)的回轉(zhuǎn)機(jī):通過(guò)控制器控制調(diào)速電機(jī)的運(yùn)行及速度,控制簡(jiǎn)單,便于維修;利用轉(zhuǎn)速表可實(shí)現(xiàn)速度的測(cè)量及顯示。缺點(diǎn)是噪音大,不能實(shí)現(xiàn)速度的閉環(huán)控制,控制精度低;由于電磁調(diào)速電機(jī)要通過(guò)電磁離合器傳遞力矩,故同樣載荷下所需電機(jī)功率較大。( 3)采用直流電機(jī)拖動(dòng)回轉(zhuǎn)機(jī):直流電機(jī)拖動(dòng)的回轉(zhuǎn)機(jī),這種設(shè)備噪音低,升降速平穩(wěn),控制精度高,但線(xiàn)路復(fù)雜,造價(jià)也偏高,維修水平要求也高。
綜上所述,從目前已生產(chǎn)出回轉(zhuǎn)機(jī)的情況來(lái)看還有許多需加以改進(jìn)的地方:(1)主軸系統(tǒng)需重新設(shè)計(jì)。主軸系統(tǒng)是回轉(zhuǎn)機(jī)的關(guān)鍵,現(xiàn)在的主軸系統(tǒng)采用分體結(jié)構(gòu),油浴潤(rùn)滑,結(jié)構(gòu)復(fù)雜,精度不易保證,裝配維修復(fù)雜。(2)防護(hù)罩存在著安全隱患?，F(xiàn)有的回轉(zhuǎn)機(jī)均采用活動(dòng)式防護(hù)罩,沉重的防護(hù)罩僅靠立柱通過(guò)螺栓固定,存在著安全隱患。(3)裝卸砂輪麻煩。由于砂輪沉重,常采用數(shù)人人工裝卸,勞動(dòng)強(qiáng)度大且不安全;條件較好的廠(chǎng)家采用軟帶兜著砂輪而后行車(chē)裝卸,工人勞動(dòng)強(qiáng)度降低了,但行車(chē)裝卸找正不易,最少仍需兩人且配合要特別好。(4)電氣控制部分有的極其簡(jiǎn)單, 有的線(xiàn)路復(fù)雜,造價(jià)也偏高,維修水平要求也高,已不能適應(yīng)信息社會(huì)及行業(yè)發(fā)展的要求。本設(shè)備的推出,對(duì)提高砂輪檢測(cè)設(shè)備的水平,促進(jìn)技術(shù)進(jìn)步,具有較大的積極作用。
2 調(diào)速技術(shù)簡(jiǎn)述
回轉(zhuǎn)試驗(yàn)機(jī)由于適用的砂輪規(guī)格較多,相關(guān)標(biāo)準(zhǔn)對(duì)砂輪試驗(yàn)時(shí)的速度精度要求嚴(yán)格,所以要求回轉(zhuǎn)試
驗(yàn)機(jī)有很好的調(diào)速和穩(wěn)速性能,調(diào)速范圍從幾百轉(zhuǎn)/分到數(shù)千轉(zhuǎn)/分。常用的調(diào)速技術(shù)有:機(jī)械調(diào)速、液壓氣動(dòng)調(diào)速、電氣調(diào)速,下面簡(jiǎn)要介紹各種調(diào)速技術(shù)的優(yōu)缺點(diǎn)。
⑴機(jī)械調(diào)速通過(guò)采用各種齒輪、鏈輪、蝸輪蝸桿減速器,僅能得到有限的一擋或幾檔速度, 一般用于減速傳動(dòng); 采用摩擦輪的各種機(jī)械式變速器,一般用于低速傳動(dòng),傳遞功率較小。機(jī)械調(diào)速裝置體積大,噪音大,調(diào)速范圍有限。
⑵液壓、氣動(dòng)調(diào)速通過(guò)采用節(jié)流閥或變量泵來(lái)改變供給液壓馬達(dá)的流量,可實(shí)現(xiàn)液壓馬達(dá)轉(zhuǎn)速的無(wú)級(jí)調(diào)整。液壓調(diào)速在速度變化范圍寬時(shí)低速功率損失較大,調(diào)速精度不易保證,液壓調(diào)速裝置易發(fā)生跑冒滴漏,發(fā)熱量大,液壓泵噪音大。液壓調(diào)速?lài)?guó)外只在小型高速輕負(fù)荷的回轉(zhuǎn)試驗(yàn)機(jī)有所應(yīng)用,國(guó)內(nèi)尚無(wú)實(shí)例。氣動(dòng)調(diào)速機(jī)械特性很軟,不能適用于回轉(zhuǎn)試驗(yàn)機(jī),噪音同樣較大。
⑶電氣調(diào)速分交流調(diào)速和直流調(diào)速兩種方式。直流調(diào)速線(xiàn)路復(fù)雜,造價(jià)也偏高?，F(xiàn)主要介紹交流調(diào)速。
近十年來(lái),隨著電力電子技術(shù)、計(jì)算機(jī)技術(shù)、自動(dòng)控制技術(shù)的迅速發(fā)展,電氣傳動(dòng)技術(shù)面臨著一場(chǎng)歷史革命,即交流調(diào)速取代直流調(diào)速和計(jì)算機(jī)數(shù)字控制技術(shù)取代模擬控制技術(shù)已成為發(fā)展趨勢(shì)。電機(jī)交流變頻調(diào)速技術(shù)是當(dāng)今節(jié)電、改善設(shè)備性能以提高產(chǎn)品質(zhì)量和改善環(huán)境、推動(dòng)技術(shù)進(jìn)步的一種主要手段。變頻調(diào)速是通過(guò)改變電機(jī)電流的頻率進(jìn)而改變電機(jī)的轉(zhuǎn)速,已從簡(jiǎn)單的V / f控制到高性能的矢量控制、磁通控制、轉(zhuǎn)矩控制、模糊控制等新的方式; 16 位、32位高速微處理器以及信號(hào)處理器(DSP)和專(zhuān)用集成電路(ASIC)技術(shù)的快速發(fā)展,為實(shí)現(xiàn)變頻器高精度、多功能提供了硬件手段。變頻調(diào)速以其優(yōu)異的調(diào)速和起、制動(dòng)性能,高效率、高功率因數(shù)和節(jié)電效果,廣泛的適用范圍及其它許多優(yōu)點(diǎn)而被國(guó)內(nèi)外公認(rèn)為最有發(fā)展前途的調(diào)速方式。變頻器從模擬式、數(shù)字式、智能化、多功能向集中型發(fā)展。最近,日本安川公司提出了以變頻器、伺服裝置、控制器及通訊裝置為中心的” D&M&C”概念。
我國(guó)變頻器設(shè)計(jì)制造水平、產(chǎn)品質(zhì)量、可靠性、售后服務(wù)水平都有了顯著的提高,價(jià)格迅速降低,性?xún)r(jià)比明顯。變頻調(diào)速技術(shù)的應(yīng)用范圍已有很大發(fā)展,石油、石化、機(jī)械、冶金、紡織等行業(yè)都經(jīng)過(guò)了單系統(tǒng)試用、大量使用和整套裝置系統(tǒng)使用3個(gè)發(fā)展階段。將變頻技術(shù)用于磨料磨具行業(yè)到了水到渠成的時(shí)候。
3 設(shè)計(jì)與研制
參照國(guó)內(nèi)外現(xiàn)有相近的回轉(zhuǎn)試驗(yàn)機(jī),結(jié)合我們多年的實(shí)際經(jīng)驗(yàn),經(jīng)過(guò)分析和論證,我們確定了下述技術(shù)方案:
⑴重新設(shè)計(jì)主軸系統(tǒng),提高其裝配精度;重新設(shè)計(jì)新結(jié)構(gòu)的防護(hù)罩,消除安全隱患;設(shè)計(jì)新結(jié)構(gòu)的砂輪卡盤(pán),便于砂輪裝卸。
⑵采用先進(jìn)的變頻調(diào)速技術(shù),實(shí)現(xiàn)電機(jī)轉(zhuǎn)速的無(wú)級(jí)調(diào)整。
⑶采用微機(jī)解決相應(yīng)的軟硬件問(wèn)題,通過(guò)人機(jī)對(duì)話(huà)界面輸入砂輪有關(guān)的實(shí)驗(yàn)參數(shù),能自動(dòng)檢查、提示;設(shè)備運(yùn)行狀態(tài)有指示燈顯示;實(shí)時(shí)數(shù)字顯示主軸的瞬間轉(zhuǎn)速;試驗(yàn)數(shù)據(jù)可按用戶(hù)的要求進(jìn)行查詢(xún)、打印。
⑷采用單片機(jī)為前置機(jī),接受微機(jī)Rs232口發(fā)出的控制指令,控制變頻器的運(yùn)行;單片機(jī)把主軸運(yùn)行狀態(tài)通過(guò)微機(jī)Rs232口在主控界面上顯示出來(lái),從而由微機(jī)實(shí)現(xiàn)檢驗(yàn)過(guò)程的全自動(dòng)控制。根據(jù)技術(shù)方案,進(jìn)行了設(shè)計(jì)與研制:
3. 1 機(jī)械設(shè)計(jì)
3. 1. 1 主電機(jī)功率確定
本設(shè)備適用砂輪的直徑范圍: 300～1100mm;最高轉(zhuǎn)速6000r /min;最大負(fù)荷150kg;砂輪最大使用速度100m / s。采用變頻調(diào)速技術(shù)后根據(jù)計(jì)算和經(jīng)驗(yàn),確定主電機(jī)功率2.2kW,轉(zhuǎn)速2840 r /min。
3. 1. 2 軸承的選擇
軸承是主軸系統(tǒng)的關(guān)鍵件,由于軸承要承受砂輪高速旋轉(zhuǎn)狀態(tài)下不平衡所產(chǎn)生的巨大離心力,又要高速旋轉(zhuǎn),為避免因配合精度導(dǎo)致主軸旋轉(zhuǎn)過(guò)程中溫度過(guò)高及平穩(wěn)性受影響,以往采用高精度進(jìn)口軸承,價(jià)格昂貴。為了在滿(mǎn)足設(shè)備性能需要的同時(shí)又降低成本和便于采購(gòu),我們考察了國(guó)內(nèi)主要的軸承生產(chǎn)廠(chǎng)家,我們選用了角接觸軸承7022AC /P5。為了滿(mǎn)足軸承的潤(rùn)滑要求,降低噪聲,我們優(yōu)選了高性能的軸承潤(rùn)滑脂,該潤(rùn)滑脂耐160℃的溫度,可滿(mǎn)足高速大負(fù)荷軸承的潤(rùn)滑要求,可長(zhǎng)期使用,避免了采用油浴潤(rùn)滑易于泄露污染的缺點(diǎn)。
3. 1. 3 主軸的結(jié)構(gòu)
鑒于試驗(yàn)的砂輪孔徑不同, 其孔徑從D127 ～D305mm,主軸的結(jié)構(gòu)有兩種方案可供選擇。一:主軸采用通軸結(jié)構(gòu),不同孔徑通過(guò)更換卡盤(pán)的方式來(lái)解決,主軸精度高,容易加工。二:主軸采用錐軸結(jié)構(gòu),不同孔徑的砂輪可用強(qiáng)度相匹配的芯軸,但是加工難度大,精度不易保證。經(jīng)過(guò)分析論證,我們采用了方案一進(jìn)行主軸的設(shè)計(jì)。為減小高速回轉(zhuǎn)砂輪產(chǎn)生的振動(dòng),應(yīng)保證砂輪裝卡過(guò)程中其對(duì)軸心的同軸度,我們提高了砂輪卡盤(pán)內(nèi)孔和主軸配合精度。主軸材料為40Cr,主軸加工采取通軸精磨。為避免裝卸砂輪對(duì)主軸表面的磨損及磕碰,影響其安裝精度,造成偏心力增大而引起劇烈振動(dòng),主軸表面高頻淬火提高其表面硬度。
3. 1. 4 設(shè)計(jì)新結(jié)構(gòu)的砂輪卡盤(pán),便于砂輪裝卸。
我們?cè)O(shè)計(jì)新結(jié)構(gòu)的一體式砂輪卡盤(pán)。安裝砂輪時(shí)首先將砂輪卡盤(pán)和砂輪固定在一起,然后用吊鉤鉤著卡盤(pán),平衡吊車(chē)輕松找正,一人即可操作。既減輕了工人勞動(dòng)強(qiáng)度且安全性提高,又保證了砂輪內(nèi)孔在裝卸中不受破壞。
3. 1. 5 設(shè)計(jì)新結(jié)構(gòu)的防護(hù)罩。
砂輪檢驗(yàn)時(shí)在回轉(zhuǎn)過(guò)程中經(jīng)常破碎,直接影響到人身及設(shè)備的安全,防護(hù)罩是提供保護(hù)的最重要部件。
現(xiàn)有的回轉(zhuǎn)機(jī)均采用活動(dòng)式防護(hù)罩,沉重的防護(hù)罩僅靠立柱通過(guò)螺栓固定機(jī)座或基礎(chǔ)上,存在著安全隱患,某廠(chǎng)就發(fā)生過(guò)立柱斷裂情況,險(xiǎn)些傷人。防護(hù)罩本身有數(shù)百公斤重,砂輪檢驗(yàn)在回轉(zhuǎn)過(guò)程中破碎,由動(dòng)能定律知,碎片擊打防擴(kuò)罩的能量相當(dāng)高,可達(dá)數(shù)噸/米。固定防護(hù)罩的立柱呈懸臂結(jié)構(gòu),砂輪經(jīng)常破碎所產(chǎn)生的強(qiáng)大沖擊力使立柱發(fā)生疲勞破壞,固定螺栓易于松動(dòng),危及人身及設(shè)備的安全。同時(shí)數(shù)百公斤重的防護(hù)罩開(kāi)關(guān)亦不方便。我們?cè)O(shè)計(jì)的防護(hù)罩牢固地焊接在機(jī)座上,從根本上保證了人身及設(shè)備的安全。防護(hù)罩采用雙層結(jié)構(gòu),內(nèi)外圈之間加減振材料,減少噪聲。防護(hù)罩上裝有輕便的安全門(mén),砂輪裝卸打開(kāi)安全門(mén)即可,操作方便。
3. 2 自動(dòng)控制系統(tǒng)設(shè)計(jì)與研制
經(jīng)過(guò)分析和論證,我們認(rèn)為控制系統(tǒng)應(yīng)實(shí)現(xiàn)以下功能:
( l)實(shí)時(shí)數(shù)字顯示主軸的瞬間轉(zhuǎn)速。
(2)設(shè)備運(yùn)行狀態(tài)有指示燈顯示。
(3)主軸運(yùn)行曲線(xiàn)調(diào)整。
(4)電氣能耗剎車(chē)。
(5)輸入?yún)?shù)自動(dòng)檢查、提示。
(6)試驗(yàn)數(shù)據(jù)可按用戶(hù)的要求進(jìn)行查詢(xún)、打印。
自動(dòng)控制系統(tǒng)由微機(jī)和單片機(jī)聯(lián)合控制實(shí)現(xiàn)設(shè)備的自動(dòng)工作。此系統(tǒng)中,單片機(jī)部分作為前置機(jī),通過(guò)
串口接受微機(jī)發(fā)出的控制指令,微機(jī)通過(guò)Rs232口再把單片機(jī)傳送過(guò)來(lái)的主軸運(yùn)行狀態(tài)顯示出來(lái)在主控界面上,從而由微機(jī)實(shí)現(xiàn)檢驗(yàn)過(guò)程的全自動(dòng)控制。微機(jī)通過(guò)人機(jī)對(duì)話(huà)界面輸入砂輪的外徑、工作線(xiàn)
速度或檢驗(yàn)轉(zhuǎn)速等與砂輪有關(guān)的實(shí)驗(yàn)參數(shù),若待檢砂輪的實(shí)驗(yàn)參數(shù)輸入錯(cuò)誤、不符合標(biāo)準(zhǔn),可自動(dòng)檢查、提示;設(shè)備運(yùn)行狀態(tài)有指示燈顯示;實(shí)時(shí)數(shù)字顯示主軸的瞬間轉(zhuǎn)速;試驗(yàn)數(shù)據(jù)可按用戶(hù)的要求進(jìn)行查詢(xún)、打印。
人機(jī)對(duì)話(huà)界面
單片機(jī)系統(tǒng)采用價(jià)廉、抗干擾性能好的51系列單片機(jī),控制系統(tǒng)根據(jù)設(shè)定轉(zhuǎn)速值和升速時(shí)間,計(jì)算出用
于控制D /A轉(zhuǎn)換的單位時(shí)間的遞增值,D /A輸出的電壓控制變頻器的輸出頻率,從而控制主軸按設(shè)定的升速曲線(xiàn)將主軸的轉(zhuǎn)速?gòu)牧銊蛩偕猎O(shè)定轉(zhuǎn)速。轉(zhuǎn)速升到設(shè)定轉(zhuǎn)速值,啟動(dòng)維持時(shí)間定時(shí)器,開(kāi)始計(jì)時(shí),進(jìn)入維持時(shí)間。在維持轉(zhuǎn)速時(shí)間內(nèi)根據(jù)設(shè)定轉(zhuǎn)速與實(shí)際轉(zhuǎn)速的誤差,應(yīng)用PID方法調(diào)節(jié),使轉(zhuǎn)速維持在1%的誤差范圍內(nèi)。
4 設(shè)備技術(shù)指標(biāo)
設(shè)備達(dá)到的技術(shù)指標(biāo)如下:
⑴回轉(zhuǎn)砂輪最大直徑: 1100mm
⑵最高轉(zhuǎn)速4000 r /min
⑶最大負(fù)荷150kg
⑷主軸徑向跳動(dòng)0. 02mm
⑸轉(zhuǎn)速調(diào)整精度≤0. 5%
⑹轉(zhuǎn)速靜態(tài)穩(wěn)定度≤0. 5%
⑺空載時(shí)最大噪音80分貝
⑻轉(zhuǎn)速變化率≤1%
⑼延時(shí)精度≤1%
⑽電機(jī)功率11kW
5 使用結(jié)果
設(shè)備經(jīng)一汽集團(tuán)大連柴油機(jī)廠(chǎng)、二汽發(fā)動(dòng)機(jī)廠(chǎng)等用戶(hù)使用,用于曲軸磨等大直徑砂輪的試驗(yàn),普遍反映該設(shè)備具有自動(dòng)化程度高、操作簡(jiǎn)便、數(shù)值化顯示主軸轉(zhuǎn)速準(zhǔn)確直觀(guān)、維護(hù)簡(jiǎn)單方便、節(jié)能、便于管理,其性能優(yōu)于國(guó)內(nèi)其他同類(lèi)型砂輪回轉(zhuǎn)機(jī)。
6 結(jié)論
通過(guò)重新設(shè)計(jì)主軸系統(tǒng),采用交流變頻、微機(jī)控制技術(shù),研制出了新一代性能優(yōu)良的大型回轉(zhuǎn)試驗(yàn)機(jī),具有操作簡(jiǎn)單、維護(hù)簡(jiǎn)單方便、節(jié)能降耗、試驗(yàn)數(shù)據(jù)可按用戶(hù)的要求進(jìn)行查詢(xún)打印等特點(diǎn),所研制的自動(dòng)控制大型砂輪回轉(zhuǎn)強(qiáng)度試驗(yàn)機(jī)先進(jìn)適用,該設(shè)備的研制成功提高了我國(guó)砂輪檢測(cè)設(shè)備的技術(shù)水平。
參考文獻(xiàn)
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畢業(yè)設(shè)計(jì)
文獻(xiàn)綜述
院（系）名稱(chēng)
工學(xué)院機(jī)械系
專(zhuān)業(yè)名稱(chēng)
機(jī)械設(shè)計(jì)制造及其自動(dòng)化
學(xué)生姓名
指導(dǎo)教師
年 03 月 10 日
Application and development
Of case based reasoning in fixture design
Abstract: Based on the case based designing (CBD) methodology, the fixture similarity is in two respects: the function and the structure information. Then, the computer aided fixture design system is created on case based reasoning (CBR),in which the attributes of the main features of workpiece and structure of fixture as case index code are designed for the retrieve of the similar cases, and the structure and hierarchical relation of case library are set up for store. Meanwhile, the algorithm based on the knowledge guided in the retrieve of the similar cases, the strategy of case adapt at ion and case storage in which the case ident if cat ion number is used to distinguish from similar cases are presented. The application of the system in some projects improves the design efficiency and gets a good result .
Keywords: case based reasoning ;fixture design; computer aided design(CAD)
Fixtures are devices that serve as the purpose of holding the workpiece securely and accurately, and maintaining a consistent relationship with respect to the tools while machining. Because the fixture structure depends on the feature of the product and the status of the process planning in the enterprise, its design is the bottleneck during manufacturing, which restrains to improve the efficiency and leadtime. And fixture design is a complicated process, based on experience that needs comprehensive qualitative knowledge about a number of design issues including workpiece configuration, manufacturing processes involved, and machining environment. This is also a very time consuming work when using traditional CAD tools (such as Unigraphics, CATIA or Pro/E), which are good at performing detailed design tasks, but provide few benefits for taking advantage of the previous design experience and resources, which are precisely the key factors in improving the efficiency. The methodology of case based reasoning (CBR) adapts the solution of a previously solved case to build a solution for a new problem with the following four steps: retrieve, reuse, revise, and retain [1]. This is a more useful method than the use of an expert system to simulate human thought because proposing a similar case and applying a few modifications seems to be self explanatory and more intuitive to humans .So various case based design support tools have been developed for numerous areas[2-4], such as in injection molding and design, architectural design, die casting die design, process planning, and also in fixture design. Sun used six digitals to compose the index code that included workpiece shape, machine portion, bushing, the 1st locating device, the 2nd locating device and clamping device[5]. But the system cannot be used for other fixture types except for drill fixtures, and cannot solve the problem of storage of the same index code that needs to be retained, which is very important in CBR[6].
1 Construction of a Case Index and Case Library
1.1 Case index
The case index should be composed of all features of the workpiece, which are distinguished from different fixtures. Using all of them would make the operation in convenient. Because the forms of the parts are diverse, and the technology requirements of manufacture in the enterprise also develop continuously, lots of features used as the case index will make the search rate slow, and the main feature unimportant, for the reason that the relative weight which is allotted to every feature must diminish. And on the other hand, it is hard to include all the features in the case index.
Therefore, considering the practicality and the demand of rapid design, the case index includes both the major feature of the workpiece and the structure of fixture. The case index code is made up of 16 digits: 13 digits for case features and 3 digits for case identification number.
The first 13 digits represent 13 features. Each digit is corresponding to an attribute of the feature, which may be one of“*”, “？”, “1”, “2”,…,“A”,“B”,…, “Z”,…, etc. In which, “*” means anyone, “?” uncertain, “0” nothing.
The system rules: fixture type, workpiece shape, locating model cannot be “*”or“?”. When the system is designed, the attribute information of the three items does not have these options, which means the certain attribute must be selected.
The last three digits are the case identification number, which means the 13 digits of the case feature are the same, and the number of these three digits is used for distinguishing them.
The system also rules: “000” is a prototype case, which is used for retrieval, and other cases are “001”,“002”,…, which are used for reference cases to be searched by designers. If occasionally one of them needs to be changed as the prototype case, first it must be required to apply to change the one to “000”, and the former is changed to referential case automatically.
The construction of the case index code is shown in Fig.1.
1.2 Case library
The case library consists of lots of predefined cases. Case representation is one of the most important issues in case based reasoning. So compounding with the index code,.
1.3 Hierarchical form of Case
The structure similarity of the fixture is represented as the whole fixture similarity, components similarity and component similarity. So the whole fixture case library, components case library, component case library of fixture are formed correspondingly. Usually design information of the whole fixture is composed of workpiece information and workpiece procedure information, which represent the fixture satisfying the specifically designing function demand. The whole fixture case is made up of function components, which are described by the function components’ names and numbers. The components case represents the members. (function component and other structure components， main driven parameter, the number, and their constrain relations.) The component case (the lowest layer of the fixture) is the structure of function component and other components. In the modern fixture design there are lots of parametric standard parts and common non standard parts. So the component case library should record the specification parameter and the way in which it keeps them.
2 Strategy of Case Retrieval
In the case based design of fixtures ,the most important thing is the retrieval of the similarity, which can help to obtain the most similar case, and to cut down the time of adaptation. According to the requirement of fixture design, the strategy of case retrieval combines the way of the nearest neighbor and knowledge guided. That is, first search on depth, then on breadth; the knowledge guided strategy means to search on the knowledge rule from root to the object, which is firstly searched by the fixture type, then by the shape of the workpiece, thirdly by the locating method. For example, if the case index code includes the milling fixture of fixture type, the search is just for all milling fixtures, then for box of workpiece shape, the third for 1plane+ 2pine of locating method. If there is no match of it, then the search stops on depth, and returns to the upper layer, and retrieves all the relative cases on breadth.
Retrieval algorithms:
1)According to the case index information of fixture case library, search the relevant case library;
2)Match the case index code with the code of each case of the case library, and calculate the value of the similarity measure;
3)Sort the order of similarity measure, the biggest value, which is the most analogical case.
Similarity between two cases is based on the similarity between the two cases. features. The calculation of similarity measure depends on the type of the feature. The value of similarity can be calculated for numerical values, for example, compareWorkpiece with the weight of 50kg and 20kg. The value can also be calculated between non numerical values, for example, now the first 13 digits index code is all non numerical values. The similarity measure of a fixture is calculated as follows:
where S is the similarity measure of current fixture, n is the number of the index feature, is the weight of each feature, is the similarity measure of the attribute of the i2th feature with the attributeof relative feature of the j-th case in the case library. At the same time, , the value counts as follows:
.
Where is the value of the index attribute of the i-th feature, and is the value of attribute of the relative i-th feature of the j-th case in case library.
So there are two methods to select the analogical fixture. One is to set the value. If the values of similarity measure of current cases were less than a given value, those cases would not be selected as analogical cases. When the case library is initially set up, and there are only a few cases, the value can be set smaller. If there are lots of analogical cases, the value should get larger. The other is just to set the number of the analogical cases (such as10), which is the largest value of similarity measure from the sorted order.
3 Case adaptation and Case Storage
3.1 Case adaptation
The modification of the analogical case in the fixture design includes the following three cases:
1) The substitution of components and the component;
2) Adjusting the dimension of components and the component while the form remains;
3) The redesign of the model.
If the components and component of the fixture are common objects, they can be edited, substituted and deleted with tools, which have been designed.
3.2 Case storage
Before saving a new fixture case in the case library, the designer must consider whether the saving is valuable. If the case does not increase the knowledge of the system, it is not necessary to store it in the case library. If it is valuable, then the designer must analyze it before saving it to see whether the case is stored as a prototype case or as reference case. A prototype case is a representation that can describe the main features of a case family. A case family consists of those cases whose index codes have the same first 13 digits and different last three digits in the case library. The last three digits of a prototype case are always “000”. A reference case belongs to the same family as the prototype case and is distinguished by the different last three digits.
From the concept that has been explained, the following strategies are adopted:
1) If a new case matches any existing case family, it has the same first 13 digits as an existing prototype case, so the case is not saved because it is represented well by the prototype case. Or is just saved as a reference case (the last 3 digits are not “000”, and not the same with others) in the case library.
2) If a new case matches any existing case family and is thought to be better at representing this case family than the previous prototype case, then the prototype case is substituted by this new case, and the previous prototype case is saved as a reference case.
3) If a new case does not match any existing case family, a new case family will be generated automatically and the case is stored as the prototype case in the case library.
4 Process of CBR in Fixture Design
According to the characteristics of fixture design, the basic information of the fixture design such as the name of fixture, part, product and the designer, etc. must be input first. Then the fixture file is set up automatically, in which all components of the fixture are put together. Then the model of the workpiece is input or designed. The detailed information about the workpiece is input, the case index code is set up, and then the CBR begins to search the analogical cases, relying on the similarity measure, and the most analogical case is selected out. If needed, the case is adapted to satisfy the current design, and restored into the case library. The flowchart of the process is shown in Fig.3.
5 Illustrating for Fixture Design by CBR
This is a workpiece (seeFig.4). Its material is 45# steel. Its name is seat. Its shape is block, and the product batch size is middle, etc. A fixture is turning fixture that serves to turn the hole, which needs to be designed.
The value of feature, attribute, case index code and weight of the workpiece is show n in Tab.2.
Through searching, and calculating the similarity, the case index code of the most similar case is 19325513321402000, and the detailed information is show n in Tab. 3.
The similarity is calculated as follows:
So the value of similarity measure of the fixture which needs to be designed with the most analogical case in case library is 0.806, and the structure of the most analogical case is shown in Fig.5.
After having been substituted the component, modified the locating model and clamp model, and adjusted the relative dimension, the new fixture is designed, and the figure is show n in Fig.6.
As there is not the analogical fixture in the case library, the new fixture is restored in to the case library. The case index code is 19325513311402000.
6 Conclusion
CBR, as a problem solving methodology, is a more efficient method than an expert system to simulate human thought, and has been developed in many domains where knowledge is difficult to acquire. The advantages of the CBR are as follows: it resembles human thought more closely; the building of a case library which has self learning ability by saving new cases is easier and faster than the building of a rule library; and it supports a better transfer and explanation of new knowledge that is more different than the rule library. A proposed fixture design framework on the CBR has been implemented by using Visual C ++, UG/Open API in U n graphics with Oracle as database support, which also has been integrated with the 32D parametric common component library, common components library and typical fixture library. The prototype system, developed here, is used for the aviation project, and aids the fixture designers to improve the design efficiency and reuse previous design resources.
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