陸澤琦 陳立群 ,?,2)
?(上海大學上海市應用數(shù)學和力學研究所,上海200072)?(上海大學力學系,上海200444)
非線性被動隔振的若干進展1)
陸澤琦?陳立群?,?,2)
?(上海大學上海市應用數(shù)學和力學研究所,上海200072)?(上海大學力學系,上海200444)
工程中航空航天、船舶與海洋結構物及其上裝備和精密儀器易受極端環(huán)境干擾和破壞,使得非線性隔振理論在近十年來迅猛發(fā)展;針對日益嚴峻的隔振和抗沖擊等要求,工程師和科學家們已發(fā)展出各種不同的非線性隔振系統(tǒng),包括主動、半主動、被動和復合隔振.利用非線性改善的被動隔振兼具傳統(tǒng)被動隔振的魯棒性和主動隔振的高效性成為振動控制領域的先進技術.本文主要綜述了非線性隔振理論和應用的近十年進展,包括非線性隔振設計、建模、分析、仿真和實驗.在隔振系統(tǒng)的構建中,既考慮了剛度非線性又考慮了阻尼非線性;動力學響應的研究中,既有確定性分析又有隨機分析.首先提出了適用于非線性隔振系統(tǒng)改進的評價方式;其次綜述了高靜態(tài)低動態(tài)剛度隔振及其加強形式非線性阻尼加強和雙層非線性隔振,混沌反控制技術、內共振影響、非線性能量阱應用等振動機制利用型隔振和非線性隔振功能材料.最后,對非線性隔振研究發(fā)展的熱點和關鍵性問題進行了分析和展望.
隔振,非線性振動,高靜態(tài)低動態(tài)剛度,非線性阻尼,隔振材料
在絕大多數(shù)工程振動系統(tǒng)中,振動都被認為是有害的,機械振動會導致軸承磨損過大、緊固件松動、結構性或機構性失效和降低效率.很多方法可以用來減小傳遞到被激振結構的振動.理想的方法是設計低噪聲機械設備,或者通過加入外部設備,比如動力吸振器來減少振動響應.其中在振源和接受結構之間引入隔振元件的方法是目前最為關注的振動控制方法,即通過改變傳遞途徑的方式達到隔離振動的目的,如圖1所示.
圖1 隔振問題Fig.1 Schematic diagram of vibration isolation
被動隔振器是一個承載并且消耗能量的元件,它結構簡單,不需要外部能量,并具一定經(jīng)濟性優(yōu)勢,因此在很多情況下成為解決工程振動傳遞問題的首選方案[1].Piersol[2]和Rivin[3]對被動隔振系統(tǒng)作了全面闡述和分析.但目前廣泛使用的被動隔振存在兩個自身的缺點:一方面,對于低頻外擾的隔振,在實現(xiàn)時會遇到靜變形過大和失穩(wěn)的問題;另一方面,阻尼可以減少共振頻率下的響應,但會降低隔振區(qū)的隔振效率.被動隔振對低頻振動控制不足,可以通過主動隔振來克服,但主動控制技術結構復雜、能耗大、成本較高,并且存在作動器輸出力與響應時間的限制、傳感器探測的信號在測量和傳輸過程中的遲滯等問題[46].除了主動隔振以外,被動隔振的第一個缺點可以通過引入具有高靜態(tài)、低動態(tài)剛度的非線性隔振元件來克服,這類系統(tǒng)是通過彈簧的特定幾何構型來獲得較低的動剛度和固有頻率,同時具有較小的靜態(tài)變形[7].第二個缺點可以通過非線性阻尼隔振克服,例如庫倫阻尼、冪函數(shù)阻尼、平方/立方阻尼、遲滯阻尼[8],從而實現(xiàn)在振動位移較大時(比如共振頻率處)阻尼較大、在振動位移較小時(比如在高頻處)阻尼也較小的目的.
環(huán)境激勵導致結構振動隔離的復雜性.被保護結構服役期間通常會受到幾類環(huán)境激勵.這些激勵通常是自然和動態(tài)的,會導致結構部件的振動和破壞[9-10]:
(1)引起船舶與海洋結構振動,如動態(tài)波浪、風、洋流、冰或地震等載荷[10];
(2)引起航天器微振動,如火箭發(fā)動機振動、太陽能帆板展開和收縮等沖擊載荷、晝夜溫差引起載荷、空間的不穩(wěn)定輻射干擾等[9].
利用非線性以提高被動隔振系統(tǒng)的性能,是隔振領域一個重要的研究方向.特別是在過去十年間非線性隔振在工程各個領域得到廣泛應用.但是除了2008年Ibriham發(fā)表相關綜述[7]外,再沒有新的文章總結在非線性隔振方面取得的最新成績和進展.本文總結了非線性隔振近十年的研究成果,尤其是高靜態(tài)低動態(tài)系統(tǒng)方面的成果,也包括作者的工作.
1.1 航天器隔振
航天器結構在對接過程中會產(chǎn)生振動和沖擊環(huán)境,另外在發(fā)射過程中火箭發(fā)動機推進不可能一直是穩(wěn)定狀態(tài),發(fā)射過程中的振動也會對航天器結構造成影響,甚至其振動會對航天器結構的運行及穩(wěn)定性產(chǎn)生致命影響[9,11].因此航天器結構振動控制對其操作性和穩(wěn)定性有重要意義,減振設計已成為航天器平臺設計的重要內容之一.事實上,由于航天器結構應用環(huán)境的特殊性及其振動精度要求的極端重要性,在應用振動控制技術解決航天器振動控制和結構探測問題時,相當一部分工程問題受到實際條件的制約,由此引出諸多技術難題,形成了當前航天器結構振動控制技術前沿的重要組成部分[12].Zheng等[13]針對整星結構提出Octo-strut主被動一體化非線性隔振平臺,其原理是應用氣動主、被動一體化技術實現(xiàn)結構的振動抑制.Zhang等[1419]研究了星上擾振部件與隔振系統(tǒng)的耦合問題,提出被動隔振裝置參數(shù)設計方法,以及振動抑制和姿態(tài)控制聯(lián)合方法,提高了高精度、高穩(wěn)定度航天器的姿態(tài)快速機動與快速穩(wěn)定的能力.
1.2 船舶與海洋結構物隔振
各種環(huán)境激勵引起海洋結構振動,降低了海洋平臺的生產(chǎn)能力和安全性,影響了海洋結構物的操縱性;在過去幾十年里已成為船舶與海洋工程結構的事故和破壞的主要因數(shù)[2022].由于自激的非線性水動力、大變形和強非線性響應,船舶與海洋工程振動控制已成為新的挑戰(zhàn).一般振動控制策略已得到廣泛研究并認為能有效緩解結構振動,但強調特定方法的船舶與海洋工程結構振動設計存在局限性.Kandasamy等[10]綜述了振動控制方法以及在船舶與海洋工程中的應用;被動、主動、半主動和復合控制的各種傳統(tǒng)策略和方法以及針對船舶與海洋工程結構振動控制方法.船舶與海洋工程振動控制結構包括夾套結構[23]、張力腿平臺[24]、柱狀支撐結構、浮式生產(chǎn)儲油輪、卸油輪和立管結構[25].
2.1 非線性振動傳遞率
振動傳遞率是評價隔振系統(tǒng)效能的關鍵參數(shù).線性系統(tǒng)力傳遞率定義為傳遞力(ft)與激勵力(fe)的幅值比;位移傳遞率定義為被保護結構位移(xt)與基礎位移(xe)的幅值比.線性系統(tǒng)力和位移傳遞率的數(shù)學表達式是相同的,表示為
其中,ζ為阻尼比,Ω為無量綱激勵頻率.而非線性系統(tǒng)在諧波激勵下,力和位移響應存在除基礎頻率以外的頻率成分.它可能是周期、準周期或混沌響應.因此幅值比用于定義傳遞率存在局限性.Lu等[26]提出了響應與激勵的均方根值之比作為傳遞率的定義
改進的定義有利于保能,因為通過均方根值可以計及響應的其他頻率成分,這將減少能量在定義時的損失.Lu等[26]對雙層非線性隔振系統(tǒng)展開傳遞率定義的適用性研究,用近似方法和數(shù)值方法定量研究高階諧波對傳遞率的影響.最后改進的傳遞率推廣到準周期、混沌響應.
2.2 功率流指標
在很多情況下隔振基礎具有柔性特征,隔振器和基礎之間存在強烈的耦合效應.剛性基礎假設的系統(tǒng)建模忽略了振源和基礎間耦合,導致以往研究不夠精準[27-28].
功率流分析考慮了力、速度和它們之間的相位關系;作為一個獨立描述系統(tǒng)響應的參數(shù),用來評價隔振,可以得到更豐富的動力學特性[29].因此,研究非線性隔振系統(tǒng)功率流很必要.馬業(yè)忠等[30]將柔性基礎簡化為矩形薄板,運用諧波平衡和Newton迭代法研究非線性剛度和阻尼對隔振系統(tǒng)功率流的影響.高書磊等[31]將柔性基礎看作柔性梁,用功率流評價隔振效果,運用諧波平衡和Newton迭代法研究了系統(tǒng)參數(shù)對功率流的影響.Royston等[32]考察了通過非線性隔振器的功率流;建立了振源--非線性路徑--接收結構的運動方程,運用諧波平衡結合弧長延伸算法研究了穩(wěn)態(tài)響應,結構噪聲產(chǎn)生于次噪聲頻率激勵;基礎模型為多自由度,振動諧次與接收結構頻率相符,較高諧次會傳遞較高幅值振動.Xiong等[33]研究了非線性耦合系統(tǒng)的功率流特征,這類系統(tǒng)包括機器、非線性隔振器和受海浪激勵的柔性船;剛度和阻尼非線性度為p和q階,運用諧波平衡法分析穩(wěn)態(tài)響應,研究了由于海浪引起的振源對輸入到整個系統(tǒng)功率流的影響.Kerschen等[34]運用功率流方法研究了非線性保守系統(tǒng)的非線性模態(tài)和頻率能量依賴特點.Yang等[35]運用平均法研究了具有負剛度特征的隔振系統(tǒng)功率流特性.提出了非線性隔振系統(tǒng)優(yōu)化設計以達到最小亞諧波共振.Yang等[36]研究了達芬振子功率流動力學行為,揭示了由于非線性引起的不同功率輸入和耗散特性,運用諧波平衡和龍格庫塔數(shù)值方法研究了平均輸入功率;超/亞諧波周期響應能增加平均輸入功率,輸入功率幅值依賴于初始條件.Yang等[37]基于平均法比較了考慮非線性基礎的非線性隔振器功率流和傳遞率,并得到數(shù)值驗證.無論是漸軟還是漸硬基礎,漸軟非線性隔振器優(yōu)于漸硬隔振器.
3.1 確定性分析
Ibrahim[7]對高靜態(tài)低動態(tài)非線性剛度隔振做了綜述,比較了許多不同非線性隔振器的優(yōu)劣和發(fā)展,尋找高效緊湊的負剛度結構成為實現(xiàn)高靜態(tài)低動態(tài)非線性剛度隔振的關鍵[3839].Robertson[40]等提出了磁懸浮隔振系統(tǒng),通過磁鐵布置產(chǎn)生負剛度來降低懸浮系統(tǒng)的固有頻率,并討論了磁鐵作用面積和間隙對剛度曲線的影響,為研究磁鐵型高靜態(tài)低動態(tài)隔振系統(tǒng)提供了設計基礎[41].彭超等[42]研制了一種新型高靜態(tài)低動態(tài)剛度隔振系統(tǒng),由3個特定形狀的片彈簧對稱分布構成,可顯著降低系統(tǒng)起始隔振頻率;建立了隔振實驗平臺,驗證了理論結果.路純紅等[43]通過分析正負剛度并聯(lián)機理,也設計了一種新型超低頻被動隔振系統(tǒng).Carrella等[44]研究了準零剛度(qausi zero sti ff ness,QZS)隔振系統(tǒng),它是由一個豎直彈簧和兩個水平彈簧(與傳遞振動的方向垂直的普通線性彈簧)組成,豎直彈簧作為靜變形的支撐剛度,兩個水平彈簧在平衡位置處預壓產(chǎn)生負剛度,由三彈簧組成的隔振系統(tǒng)性能優(yōu)于線性隔振系統(tǒng),而且具有較低的靜態(tài)變形,如圖2所示.在豎直方向上作用力與產(chǎn)生位移間關系可寫為
圖2 準零剛度模型[44]Fig.2 QZS model[44]
Kovacic等[45]和Carrella等[4647]利用三彈簧結構建立了高靜態(tài)低動態(tài)剛度隔振系統(tǒng),如圖3所示.水平放置的彈簧引入了幾何非線性剛度,調節(jié)水平彈簧剛度以減少系統(tǒng)固有頻率,隔振的頻率范圍往低頻擴展.隔振器的受力--位移關系可以寫成多項式形式
其中,k1=kv1-2(lo/l-1)kh1,k3=lo/l3kh1,lo是水平彈簧的初始長度,l是當它處于水平位置時的長度.當lo>l時,水平彈簧kh1可以減小隔振系統(tǒng)線性剛度,讓其小于線性剛度kv1,減小系統(tǒng)的固有頻率,但同時引入立方剛度項.水平彈簧作用包括擴大隔振頻率范圍以及產(chǎn)生漸硬非線性,會導致潛在跳躍現(xiàn)象.
圖3 高靜態(tài)低動態(tài)隔振系統(tǒng)示意圖[46]Fig.3 Schematic of nonlinear vibration isolation system with high-static-low-dynamic sti ff ness[46]
微振動時(x≤0.2l),圖3所示的非線性隔振系統(tǒng)運動方程可以近似并無量綱化為
徐道臨等[48]在磁力式QZS隔振系統(tǒng)中加入調節(jié)技術可以適應于不同加載質量.Sun等[49]在準零剛度隔振系統(tǒng)中加入時滯反饋的主動控制策略,時滯反饋控制不但提高了系統(tǒng)的魯棒性,同時提高了隔振性能.Li等[50]提出了構建負剛度的有效方法,通過設計實驗臺實驗研究了磁力式負剛度隔振器,磁彈簧和橡膠相耦合時產(chǎn)生較低固有頻率和較大靜載能力,固有頻率可達到1.5Hz,在0~100Hz傳遞率最大下降40dB.Huang等[51]使用光滑梁作為負剛度設計了極低固有頻率的隔振系統(tǒng),隔振器剛度和外載缺陷會導致動力學響應呈現(xiàn)出漸軟、漸硬和軟硬共存特性,外載缺陷會使系統(tǒng)呈現(xiàn)漸軟特性,增加剛度缺陷會弱化外載缺陷所引起的漸軟特性,增加激勵幅值會使隔振在不同的剛度特征間復雜轉遷[52].引入剪刀形元件(scissor-like structure,SLS)作為產(chǎn)生幾何非線性特性的構件開始受到關注[53];Zhang等[54]分析和研究了SLS的非線性剛度特性和隔振性能,SLS在機械工程、結構工程和宇航工程中廣泛存在,具有較高剛度和阻尼非線性,以及很高的承載能力和較優(yōu)的平衡穩(wěn)定性;采用攝動法和平均法研究了SLS的不同構型特點,在沒有改變整體結構和外部尺寸下,SLS能顯著地調節(jié)非線性剛度等非線性特征,可以滿足宇航工程對隔振結構的嚴格要求.Friswell等[55]通過調節(jié)軸向靜載以幾何非線性形式改變梁的力位移特征曲線,實現(xiàn)高靜態(tài)低動態(tài)剛度隔振,提出了兩種調節(jié)方式:一是通過端部作動器改變軸向位移和轉角,二是改變復合梁的初始熱應力.Trung等[56]研究了QZS隔振器的Lyapunov函數(shù)主動控制方法,控制力加載在水平方向以抵消豎直方向的振動,基于Lyapunov函數(shù)發(fā)展了QZS隔振主動控制的非線性算法;實驗結果表明,非直接激勵的水平作動器可以通過非線性作用有效抑制豎直方向振動.Le和Ahn[57]使用負剛度主動控制系統(tǒng)成功實現(xiàn)低于5Hz的主動控制,設計模糊滑??刂?FSMC)提高了負剛度主動系統(tǒng)的隔振性能;基于Lyapunov穩(wěn)定性理論建立模糊控制準則,通過設計實驗研究了FSMC控制在不同激勵環(huán)境下對系統(tǒng)的有效性.Danh和Ahn[58]針對車輛座椅的低頻振動問題,設計了基于準零剛度的主動控制系統(tǒng),利用自適應控制算法解決了該系統(tǒng)的時變和非線性問題,實現(xiàn)了高效隔振.多向準零剛度隔振器拓展了準零剛度隔振器的研究[59].徐鑒等[60]研究了多方向準零剛度隔振系統(tǒng),并加入時滯主動控制,有效地抑制了多方向振動[6162].Zhou等[63]設計了6自由度剛體QZS隔振平臺,運用諧波平衡法得到幅頻關系;與線性6自由度隔振平臺相比,能有更高效的低頻隔振特性.雙穩(wěn)板材質輕,可操作性強,近些年倍受到隔振實驗研究者們的青睞和關注.Shaw等[6465]用雙穩(wěn)復合板制作了質輕而有效的高靜態(tài)低動態(tài)剛度的隔振系統(tǒng),用實驗的方法證明此系統(tǒng)比等價的線性系統(tǒng)具有更大的隔振區(qū)域以及更低的固有頻率.
圖4 不同非線性剛度下力傳遞率幅值|TF|[47]Fig.4 E ff ect on the force transmissibility when the nonlinear sti ff ness is changed[47]
3.2 隨機分析
針對實際環(huán)境中隨機非平穩(wěn)的激勵源,有學者提出了在隨機白噪聲激勵下的非線性隔振系統(tǒng)模型[7].彈簧位移的概率密度方程可以通過???-普朗克--柯爾莫哥洛夫(Fokker-Planck-Kolmogorov,F(xiàn)PK)方程的解析解獲得,位移或加速度響應的均方根值被用來評價3類不同的非線性剛度:(a)立方漸硬彈簧;(b)立方漸軟彈簧;(c)正切彈簧.立方漸軟彈簧能有效減少振動傳遞率.Le和Ahn[66]用數(shù)值方法研究了車輛座椅在隨機激勵下時域響應,額外加入與正剛度彈簧平行的負剛度結構,建立了在低頻激勵下非線性隔振模型,并比較了單頻、多頻以及隨機3種情況下隔振性能.結果表明,隔振頻帶均比未加入負剛度時寬,并用實驗的方法驗證了這一結果.橡膠隔振在工程結構中受到廣泛使用,針對不同激勵環(huán)境呈現(xiàn)出非線性和不確定因素.采用單自由度質量--彈簧--阻尼模型來簡化橡膠隔振系統(tǒng),其中剛度和阻尼是關于相對位移的多項式,多項式系數(shù)通過實驗數(shù)據(jù)識別;主分量分析和蒙特卡洛仿真研究了系數(shù)不確定度,仿真和理論結果都表明,多項式非線性剛度非線性阻尼模型和不確定評價可以有效地預報振動特性和隔振系統(tǒng)不確定度[67].然而,很難獲得非線性系統(tǒng)在隨機激勵下動力學分析的頻域解析解,尤其是阻尼、剛度同為非線性的情況.因此,關于高靜態(tài)低動態(tài)隔振系統(tǒng)的隨機動力學行為的研究較少.
3.3 抗沖擊性能
在水下潛器和航天器的振動抑制中,隔振器的抗沖擊性能和隔振作用同等重要[68].沖擊具有激勵幅值高和瞬時特點,若不隔離會導致較大傳遞力和位移.傳統(tǒng)沖擊隔離是通過剛度元件變形吸收能量和阻尼耗散殘余振動實現(xiàn).Ledezma-Ramirez等[69]利用非線性剛度特別是低動態(tài)剛度實現(xiàn)沖擊隔離,非線性剛度能減小絕對位移和加速度響應.非線性抗沖也可通過Euler屈曲梁實現(xiàn),調節(jié)屈曲梁的曲率可以使隔振器處于QZS,Duffing型和Helmholtz-Duffing型3種工作模式,在較小激勵下,與其他兩類隔振模式相比,QZS并不是隔離沖擊的最優(yōu)形式,Duffing型含有微小正剛度能提高沖擊隔離能力,Helmholtz-Duffing型隔離沖擊會隨著外載增加而惡化[70].
4.1 剛度線性
Peng等[7172]和Guo等[73]使用輸出頻響函數(shù)較為系統(tǒng)地研究了立方非線性阻尼隔振器的力傳遞率,增加立方非線性阻尼能夠減小共振頻率處的力傳遞率并且在其他頻區(qū)不受影響.Laalej等[74]實驗研究了隔振系統(tǒng)中非線性阻尼的作用.Peng,Guo和Laalej的研究結果與Lang等[75]在早些年的研究結論相一致.Tang和Brennan[76]比較了水平阻尼(與傳遞振動方向垂直放置的線性阻尼[77])和立方非線性阻尼兩類不同非線性阻尼的單層隔振系統(tǒng)的傳遞率特征,對于力傳遞率,兩類阻尼都比線性阻尼具有更好的隔振特性;但對于位移傳遞率,水平阻尼相比立方非線性阻尼具有更多優(yōu)良的特性.Xiao等[78]對比了有與沒有線性項的立方非線性阻尼對單層隔振的影響,沒有阻尼線性項的單純立方非線性阻尼有更好的隔振特性,它能讓力和位移傳遞率的共振峰壓低成類似過阻尼的線性隔振系統(tǒng).Lang等[79]將非線性黏性阻尼加入多自由度隔振系統(tǒng),它可以描述為樓層型剪切模型,基于輸出頻響函數(shù)法,首次將線性黏性阻尼多自由度隔振推廣到非線性情況,得到了與先前研究單自由度一致的結論.Peng等[80-81]運用輸出頻響函數(shù)的概念研究了多自由度隔振系統(tǒng)中不對稱非線性阻尼作用,也得出了類似的結論.L¨u等[82]在非線性阻尼隔振的基礎上,研究了周期解的穩(wěn)定性.Huang等[83]研究了同時依賴速度和位移的非線性阻尼(VDD),分別用位移和力傳遞率評價了VDD隔振系統(tǒng)性能,單自由度基礎激勵實驗驗證了理論結果.Lu等[84]實驗研究了非線性阻尼的推廣的麥克斯韋模型(generalized maxwell model,GMM),阻尼非線性是通過流體阻尼的長沖程實現(xiàn),GMM模型是彈簧和阻尼串聯(lián)的模型.實驗結果表明,GMM模型在仿真流體阻尼時在很寬的頻帶上都非常精確,然而麥克斯韋模型和黏滯阻尼模型只在特定頻率下比較精確;進行了振動臺測試實驗,流體阻尼作為隔振系統(tǒng)的補充阻尼,GMM能夠更準確地預測阻尼的能量耗散和峰值偏移.Mokni等[85]引入時滯非線性阻尼以加強隔振性能,時滯非線性阻尼通過快速參變阻尼以減小傳遞率,非線性阻尼能提高隔振性能.為了對非線性隔振狀態(tài)監(jiān)測,需要實驗識別.Mu等[86]綜述和比較了幾種非線性阻尼隔振系統(tǒng)的結構故障診斷.
4.2 剛度非線性
Kovacic和Brennan[8]綜述了具有非線性阻尼的3類達芬系統(tǒng)(漸硬、漸軟和雙穩(wěn))動力學特性;指出了冪函數(shù)非線性阻尼對分岔結構和通向混沌途徑的影響依賴于非線性剛度特征;非線性阻尼對漸軟Duffing系統(tǒng)的諧波響應和分岔結構,隨著激勵幅值的增加,兩條分離的幅頻曲線聚合并只存在上跳頻率;上跳頻率對阻尼的非線性階次不敏感,而在較高階次下,下跳頻率隨阻尼系數(shù)發(fā)生劇烈地改變,因此可以通過增加阻尼階次來消除小阻尼帶來的不穩(wěn)定諧波響應;對稱與不對稱系統(tǒng)在非線性阻尼情況下,系統(tǒng)共振所屬的分岔結構不受阻尼階次影響,但是發(fā)生不同類型分岔的臨界參數(shù)依賴于阻尼系數(shù)和階次;漸硬Duffing振子在庫倫阻尼作用下存在一些有趣的響應特征.Awrejcewicz等[87]運用諧波平衡法研究了逃脫頻率(break-loose frequency),發(fā)現(xiàn)激勵頻率大于逃脫頻率時會持續(xù)較長一段時間的黏滑運動(stick-slip motion)[88].Cveticanin[89]用諧波平衡法和數(shù)值方法分別研究了正負阻尼系數(shù)與非線性階次對振動頻率的相互影響,以及極限環(huán)運動的參數(shù)邊界對非線性階次的依賴關系,數(shù)值與理論相吻合.Sharma等[90]研究了非線性阻尼對強迫達芬系統(tǒng)分岔與混沌特性的影響,研究結果表明,非線性阻尼能使系統(tǒng)首次進入混沌狀態(tài)的閾值減小,并且能夠增加發(fā)生混沌的參數(shù)空間,影響系統(tǒng)進入混沌的途徑.Ho等[91]用輸出頻響函數(shù)法研究了非線性剛度和非線性阻尼對Duffing系統(tǒng)在不同頻率范圍內的影響.非線性黏性阻尼能夠抑制共振頻率處的響應,同時對高頻處的響應影響較小.盡管非線性阻尼、非線性剛度系統(tǒng)在理論和數(shù)值上都得到了大量的研究,并且發(fā)現(xiàn)了一些特殊的動力學現(xiàn)象,但不涉及到隔振應用.
針對同時應用非線性阻尼和非線性剛度在諧波激勵下的隔振問題,Ho等[92]研制了單自由度非線性阻尼、非線性剛度隔振系統(tǒng),比較了線性和非線性阻尼對傳遞力曲線的不同影響,如圖5所示.隨著線性阻尼的增加,傳遞率峰值減小,但是高頻傳遞率增加;隨著非線性阻尼的增加,傳遞率峰值和跳躍頻率減小,同時保持較優(yōu)的高頻特性[93].
圖5 比較線性和非線性阻尼對傳遞率的影響Fig.5 Comparison of e ff ect on transmissibility between linear and nonlinear damping
雙層隔振系統(tǒng)由于具有一些較突出的特性得到了廣泛的應用[1-3,94].與單層隔振系統(tǒng)相比,雙層隔振系統(tǒng)有額外的質量與剛度,因此也就有了額外的共振頻率(圖6).對于單層隔振系統(tǒng),傳遞的振動在大于隔振開始頻率時以40dB/oct下降,而對于雙層隔振系統(tǒng),它的下降率卻達到80dB/oct.非線性剛度隔振系統(tǒng)有相對較好的低頻隔振效率、較小的靜態(tài)變形、不失穩(wěn)性的特點,非線性阻尼能有效減小共振頻率附近響應,同時對隔振區(qū)的傳遞率沒有影響的優(yōu)點.在雙層線性隔振系統(tǒng)的上、下兩層引入非線性剛度和非線性阻尼對隔振效率的影響的研究具有重要的工程與學術價值.
圖6 單層與雙層隔振系統(tǒng)力傳遞率比較(單層:紅色實線,雙層:藍色虛線)[95]Fig.6 Comparison of the transmissibility of a single-stage and a two-stage linear isolator(single-stage:red solid line,two-stage:blue dashed line)[95]
Lu等[95]將高靜態(tài)剛度引入到雙層隔振系統(tǒng)中,以克服大的靜態(tài)變形以及在高頻處傳遞率曲線下降速率較緩的問題.這些幾何非線性剛度是通過加入與承載剛度垂直的附加彈簧獲得的,這些彈簧起負剛度的作用,因此對隔振的有效頻帶產(chǎn)生影響.這里只考慮水平彈簧能使隔振系統(tǒng)表現(xiàn)為漸硬特征的情況[95].用諧波平衡法得到單純高靜態(tài)低動態(tài)剛度雙層隔振系統(tǒng)的力與位移傳遞率解析表達式,該系統(tǒng)的隔振效率優(yōu)于雙層線性隔振系統(tǒng).隔振系統(tǒng)運動方程的矩陣形式為
微振動情況下可以近似為兩個耦合的達芬(Duffing)方程,無量綱矩陣形式如下
運用諧波平衡法,假設無量綱位移的向量具有如下形式
幅頻矩陣方程可以表示為
其中
Yang等[96]研究了雙層隔振系統(tǒng)的穩(wěn)態(tài)性能,它由雙穩(wěn)層和線性層組成,以隔振為出發(fā)點,增加雙穩(wěn)層的阻尼可以改善單周期和阱內行為,但是未得到有關隔振加強的非線性正效應.Lu等[9798]在將上層非線性剛度連接到中間質量而非基礎上,建立了改進的雙層非線性隔振系統(tǒng)模型,如圖7所示.與原雙層非線性隔振系統(tǒng)相比,原系統(tǒng)只有改變下層的非線性剛度對改善隔振效率有效,而變化改進系統(tǒng)上、下層的非線性剛度對改善隔振效率均有效.雙層非線性隔振兼具較低的隔振開始頻率和較高隔振區(qū)傳遞率下降速率,受到了許多隔振研究者的關注[99].Lu等[100]還構建了緊湊的雙穩(wěn)復合板型負剛度單元,并將它引入到雙層隔振系統(tǒng),通過實驗方法研究了雙層高靜態(tài)低動態(tài)隔振系統(tǒng)的隔振性能.每一層含有高靜態(tài)低動態(tài)剛度;每一層中正剛度是由金屬板提供,相應負剛度是由C-F雙穩(wěn)復合板提供.除去隔振系統(tǒng)中雙穩(wěn)復合板退化成線性雙層線性隔振系統(tǒng),與線性系統(tǒng)相比,雙層非線性隔振系統(tǒng)在100Hz處下降13dB.
圖7 改進的雙層非線性隔振系統(tǒng)模型Fig.7 Improved two-stage nonlinear isolation system
圖7 改進的雙層非線性隔振系統(tǒng)模型(續(xù))Fig.7 Improved two-stage nonlinear isolation system(continued)
6.1 混沌隔振
混沌是非線性系統(tǒng)特有的動力學行為[101].當隔振系統(tǒng)的參數(shù)處于混沌范圍時,會處于混沌狀態(tài)下工作.在單頻激勵下,非線性系統(tǒng)在較寬頻帶內呈現(xiàn)混沌動力學行為.Lou和Zhu[102]利用混沌進行隔振器設計,討論了如何減小水下潛器的輻射噪聲線譜;為確保系統(tǒng)處于混沌態(tài)工作,確定了設計參數(shù)可取范圍;傳統(tǒng)方法是運用非線性理論分析系統(tǒng)和預測混沌參數(shù)的范圍;但由于非線性系統(tǒng)的復雜性,預測參數(shù)和實際參數(shù)存在顯著差別;因此有必要設置隔振系統(tǒng)參數(shù)可調.Liu等[103]為了確保系統(tǒng)在混沌狀態(tài)下有效工作,提出混沌反控制方法;提出非線性隔振系統(tǒng)反饋模型,分析了倍周期分叉特性,通過仿真得出不同參數(shù)下多種動力學行為,最后結合混沌識別技術實驗研究了自制臺架(圖8)在混沌狀態(tài)下的隔振性能[103].
Lou和Zhu[104]還評估了非線性隔振器在混沌狀態(tài)下的隔振性能,發(fā)現(xiàn)線譜幅值明顯減少.非線性阻尼可以作為被動裝置來抑制振動,Harvey等[105]提出了滾動擺隔振系統(tǒng)的非線性模型,研究了特定參數(shù)的滾動擺隔振系統(tǒng)在諧波激勵下的混沌行為,并對激勵幅值與激勵頻率的影響展開了研究.他們發(fā)現(xiàn),在大幅值低頻率激勵下會出現(xiàn)混沌行為,沖擊會導致周期放大和反對稱分叉.最后從工程的角度提出避免隔振系統(tǒng)出現(xiàn)混沌的方法,一方面增加隔振系統(tǒng)可發(fā)生的位移容量,另一方面通過增加阻尼來減小實際振動位移.Farshi等[106]發(fā)展了混沌隔振系統(tǒng),使用一些線性彈簧逐次進入作用的方法提供系統(tǒng)分段非線性,通過判斷Lyapunov指數(shù)以描述系統(tǒng)混沌行為;通過算法比較,連續(xù)Simplex算法為結構修改的最優(yōu)算法.
圖8 混沌反控制隔振實驗臺[103]Fig.8 Nonlinear vibration isolation experimental rig for chaos anti-control technology[103]
6.2 內共振隔振
內共振是典型的非線性振動現(xiàn)象[107]. 涉及內共振的非線性動力系統(tǒng)的隔振問題得到了廣泛研究[7],這些文獻都考察了達芬型剛度基礎和裝置剛性假設的非線性隔振系統(tǒng)動力學響應,剛度非線性能夠在系統(tǒng)模態(tài)間引起耦合.1:1內共振混合模態(tài)的周期性響應會出現(xiàn)與線性和非線性模態(tài)共存的現(xiàn)象.Chen等[108]使用了增量諧波平衡法研究了非線性隔振系統(tǒng)的傳遞率和參數(shù)影響.三自由度隔振系統(tǒng)剛體瞬態(tài)非線性振動存在2:1:1的內共振[109],該內共振是由于剛體的豎直振動、水平振動和繞質心轉動三個自由度耦合產(chǎn)生.當不平衡轉子的轉速達到一定值時,通過共振的剛體振動非常明顯.
6.3 非線性能量阱隔振
非線性能量阱(NES)是種有發(fā)展前景的減振技術[110111].當NES應用于基礎激勵系統(tǒng)的減振時,客觀上起到了隔振的作用.Yang等分別對單自由度[112]和多自由度系統(tǒng)[113]提出了一種基于非線性輸出頻響函數(shù)的振動傳遞率表達式,首次在頻域內使用該傳遞率概念評價NES的減振效果.研究系統(tǒng)為一個安裝有NES的兩自由度系統(tǒng),對該系統(tǒng)進行了數(shù)值仿真分析.另外,還考察了NES的參數(shù)對該非線性系統(tǒng)振動傳遞率的影響.通過增大NES的質量和黏性阻尼,同時減小其非線性剛度,可以在所有的共振頻率處減小該系統(tǒng)的振動傳遞率.因此,給出了一種在頻域內設計NES非線性減振器的隔振思路.Yang等[114]針對帶適配器的整星系統(tǒng),設計了NES減振器;對實驗縮比模型系統(tǒng)進行了有限元建模,并根據(jù)有限元模型得到了等效低維模型.實驗結果、有限元仿真、基于等效模型的能量分析和非線性輸出響應函數(shù)分析均表明,NES減振器能夠有效減少基礎激勵向星體傳遞.
非線性隔振功能材料是非線性隔振的載體[115116],目前在隔振領域較為關注的功能材料有形狀記憶合金[117121]、雙穩(wěn)復合板[122]、電流和磁流變液[123127]和壓電晶體等.形狀記憶合金等智能材料由于它的超彈性和形狀記憶作用特征彌補了傳統(tǒng)材料在隔振中的缺陷,目前使用形狀記憶合金代替橡膠用于高速橋梁隔振已取得顯著效果.通過非線性隨機振動分析,形狀記憶合金隔振能最小化隔振器位移的同時最大化隔振效率.參數(shù)研究表明,這類隔振系統(tǒng)還具有較強的魯棒性[120].Xia等[119]實驗研究了相位可變NiTi形狀記憶合金弦線非線性扭轉振動的動力學響應,通過同步獲取外激勵下扭角和溫度以測量NiTi弦線作為非線性阻尼彈簧的熱機響應,固定激勵幅值測量頻率響應曲線(FRCs),固定激勵頻率測量幅值響應曲線(ARCs),隨著NiTi弦線變形進入漸軟非線性相位轉遷區(qū)域,在FRCs或ARCs分支上的穩(wěn)定響應會逐漸變得不穩(wěn)定并進入到新的其他任意分支;利用實驗識別了FRCs和ARCs任意穩(wěn)定狀態(tài)間的跳躍現(xiàn)象和上下跳躍遲滯.通過實驗識別穩(wěn)定區(qū)域使得避免NiTi弦線作為非線性阻尼彈簧振動系統(tǒng)出現(xiàn)不期望的動力學響應.Ozbulut提出了形狀記憶合金彈簧隔振[128],使用形狀記憶合金彈性元件實現(xiàn)多自由度剪切振動隔振.Choi等[129]使用形狀記憶合金提出了高速橋梁的隔振軸承,通過1:1尺度模型實驗研究了形狀記憶合金隔振的有效性.Ozbulut等[130]綜述了基于形狀記憶合金的隔振系統(tǒng),以及形狀記憶合金在被動隔振中的應用.束立紅等[131]研究了聚氨酯隔振器的彈性、彈塑性和黏彈性特點,分析了激振幅值、激振頻率和預加載情況對隔振器靜動態(tài)特性的影響;通過簡單形狀試驗的靜態(tài)和動態(tài)試驗結果,建立了由彈簧、阻尼器和摩擦元件等多組機械元件所構成的聚氨酯隔振器“非線性廣義模型”.具有高靜態(tài)低動態(tài)剛度(HSLDS)的隔振系統(tǒng)是通過減少固有頻率來提高隔振性能的,并保持一定的承載能力.高靜態(tài)低動態(tài)剛度是通過在水平方向上連接線性彈簧構成突變結構(snap-through)這一常用的方法獲得的,突變結構一方面增加系統(tǒng)的非線性,另一方面可以在一定運動區(qū)域內提供負剛度,同時線性彈簧使負剛度區(qū)域穩(wěn)定化并支撐靜載.由于有兩個穩(wěn)定的平衡形態(tài),并且兩個形態(tài)之間可以來回突變,并在突變附近板中心受力與發(fā)生橫向位移存在反比關系,因此雙穩(wěn)復合板也可以作為負剛度裝置.文獻[132-137]研究了雙穩(wěn)板的突變響應,將這一效應用于驅動結構,研制了相應的驅動裝置,這一裝置能夠實現(xiàn)多個結構狀態(tài)之間轉化,在轉化過程中沒有能量消耗,之后雙穩(wěn)板被廣泛應用到氣動變形裝置上.Pirrera等[138]對雙穩(wěn)復合板的力位移關系曲線進行了研究,得到了負剛度曲線.使用彈性復合板殼來代替復雜的彈簧機構,具有重量輕、可操作性強的優(yōu)點[139141].Shaw等[142]采用雙穩(wěn)板構建了負剛度結構,通過實驗的方法得到了雙穩(wěn)板的準零力位移關系曲線.Lu等[100]構建了緊湊的雙穩(wěn)復合板型負剛度單元,并將它引入到雙層隔振系統(tǒng).
接受結構與振源之間插入隔振元件以減小振動傳遞是目前最普遍的振動控制方式;高靜態(tài)低動態(tài)剛度隔振系統(tǒng)能夠克服線性隔振低頻缺陷,是振動控制領域研究的主要熱點之一;非線性阻尼能夠抑制線性系統(tǒng)共振頻率處響應,同時保持較優(yōu)高頻隔振性能.本文綜述了非線性隔振的新進展,主要包括非線性隔振評價方式、高靜態(tài)低動態(tài)剛度隔振及其加強形式非線性阻尼加強和雙層非線性隔振、混沌、內共振和非線性能量阱在隔振中的應用和非線性隔振功能材料.
非線性隔振應用背景廣泛如整星隔振、船舶海洋工程結構物隔振等,根據(jù)不同應用背景特點,非線性隔振器大小不同、形狀各異.非線性隔振由于出現(xiàn)倍周期、準周期和混沌等豐富的非線性動力學行為,通常需要響應與激勵的均方根值之比代替幅值比以評價非線性隔振效果,對于柔性基礎,還需要從功率流角度來評估.動力學響應的研究中,既有確定性分析又有隨機分析;在研究方法的應用中,有諧波平衡法等解析方法、有限元法等數(shù)值方法和實驗方法.非線性隔振的實現(xiàn)結構有幾何非線性和材料非線性,幾何非線性結構通常是金屬彈簧、梁、板和殼等通過幾何構造實現(xiàn)非線性隔振功能、材料非線性結構通常是利用雙穩(wěn)復合板、形狀記憶合金、電流/磁流變液等新材料實現(xiàn)特殊非線性剛度和非線性阻尼功能,提高隔振性能.
本文針對非線性隔振理論與應用開展了全面綜述和深入分析,在此基礎上,今后還可以在以下幾方面繼續(xù)開展研究:
(1)非線性隔振系統(tǒng)的主共振、亞諧共振和超諧共振穩(wěn)定性及分岔,從穩(wěn)定性裕度、吸引域角度提高非線性隔振系統(tǒng)的設計水平;
(2)非線性剛度非線性阻尼隔振系統(tǒng)抗沖擊特性,沖擊激勵具有作用時間短、阻尼作用不明顯等特點;
(3)在結構和隔振器發(fā)生內共振情況下隨機振動傳遞特性,非線性模態(tài)間作用對隔振性能的影響,明確在隨機壞境下內共振的動力學機制;
(4)金屬彈簧、梁、板和殼的各種不同屈曲構型特點以實現(xiàn)非線性隔振,碳纖維--壓電晶體復合板材等新材料的特殊非線性剛度、非線性阻尼特性以克服傳統(tǒng)金屬材料隔振缺陷;
(5)非線性隔振與能量采集一體化,適應集成化、輕質小體化發(fā)展,結構兼具隔振和能量采集等多種用途.
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SOME RECENT PROGRESSES IN NONLINEAR PASSIVE ISOLATIONS OF VIBRATIONS1)
Lu Zeqi?Chen Liqun?,?,2)?(Shanghai Institute of Applied Mathematics and Mechanics,Shanghai University,Shanghai 200072,China)?(Department of Mechanics,Shanghai University,Shanghai 200444,China)
Vibrations in aircraft and marine structures,due to various extreme environmental loads,have been attributing factors in accidents and failures.Over the last decade,as the demands for vibration and shock isolation performance increasing,the general approaches following the conventional categorization of passive,active,semi-active and hybrid has been extensive presented.Nonlinear passive vibration isolation is the state of the art of vibration control techniques for combining robustness of the passive device and high performance of the active devices.This paper surveys theoretical and practical advances in nonlinear passive isolation of vibration in recent ten years.Sti ff ness and damping both nonlinearities is considered in modeling of vibration isolation system;Deterministic and stochastic analysis are both conducted on the investigation of the dynamic behavior.Initially,a review of a general approach to quantify the e ff ectiveness of nonlinear vibrations isolation is presented.This is then followed by a review of high-static-low-dynamic sti ff ness vibration isolation,damping nonlinearity vibration isolation,two-stage nonlinear vibration isolation and nonlinear vibration isolation materi-als.The other vibration isolation methods considered in this review include chaotic anti-control technology,influenc of internal resonance and usage of nonlinear energy sink.The article is closed by conclusions,which highlight resolved and unresolved problems and recommendations for future research treads.
vibration isolation,nonlinear vibration,high static low dynamic sti ff ness,nonlinear damping,isolation materials
O328
:A
10.6052/0459-1879-17-064
2017–03–01 收稿,2017–04–20 錄用,2017–04–21 網(wǎng)絡版發(fā)表.
1)國家自然科學基金重點項目(11232009)和國家自然科學基金項目(11502135,11572182)資助.
2)陳立群,教授,主要研究方向:非線性動力學和振動控制.E-mail:lqchen@sta ff.shu.edu.cn
陸澤琦,陳立群.非線性被動隔振的若干進展.力學學報,2017,49(3):550-564
Lu Zeqi,Chen Liqun.Some recent progresses in nonlinear passive isolations of vibrations.Chinese Journal of Theoretical and Applied Mechanics,2017,49(3):550-564