【導讀】Hereandthere:theyhadjustdiscoveredthataprisonerhadescaped.Atthatmoment,alargeblackcarwithfourofficersinsideit,stoppedatthecampgates.Theofficersgotoutandtheprisonerstoodtoattentionandsalutedastheypassed.Whentheyhadgone,thedriverofthecarcametowardshim.Themanobviouslywantedtotalk.Hewasratherelderlywithgreyhairandclearblueeyes.Theprisonerfeltsorryforhim,buttherewasnothingelsehecoulddo.Asthemancamenear

　　

【正文】 r chitect Cost a, t he new cit y, Br asi li a, r eplaced Ri o de Janei ro as t he capi tal of i n 1960. Br asi li a has been careful ly pl anned f or moder n li ving. It s wide r oads, which can t ake f ourt en l anes of tr af ic, have been kept away f rom li ving areas. Chi ldr en do not have to cr oss busy st r eet s to go t o school . Housewives can vi sit shoppi ng on foot, f or in t hese speci al ly designed li ving areas, cars ar e unnecesary. At f ir st , t he gover nment had gr eat dif fi cul ty i n pers uadi ng peopl e to l eave Ri o and t o set tl e in Br asi li a. Si nce 1960, however, t he popul at ion has been gr owing al l the ti me. Bras il ia has quickly establi shed i tself as the capital of the countr y. The i dea t o have the capital moved so far i nl and wil l have a gr e at ef fect on the f ut ure of Br azi l. Leson91 Three men i n a basket 三人同籃 A pil ot not iced a bal loon whi ch seemed t o be making f or a Royal Ai r Force Stat ion nearby. He inf ormed t he stati on at once, but no one there was abl e to explai n the m yst er y. The of fi cer i n t he contr ol tower was ver y angry when he heard the news, because baloons can be a great danger to ai rcraf t. He said t hat someone might by spyi ng on the st at ion and t he pi lot was order ed to keep tr ack of t he st range object. The pil ot m anaged to ci rcl e the bal oon for som e ti me. He coul d make out thr e men in a basket under i t and one of t hem was holding fi el dglasses. When t he bal loon was over t he st ati on, t he pi lot saw one of the m en t aki ng phot ogr aphs. Soon aft er wards, the bal oon began to descend and i t landed near an ai rf iel d. The poli ce were cal ed in, but they coul d not ar rest anyone, f or the basket cont ai ned two Members of Parl iament and the Comm anding Of f icer of t he stati on! As the Comm anding Of icer expl ai ned later , one half of t he st at ion di d not know what t he other hal f was! Leson92 Aski ng for t rouble 自找麻 煩 It m ust have been about t wo i n the m or ning when I r eturned home. I tr ied t o wake up my wif e by r inging t he door bel , but she was fast asleep, so I got a l adder fr om t he shed i n the gar den, put i t agai nst the wal l, and began cli mbi ng towards t he bedr oom wi ndow. I was alm ost t her e when a sar cast ic voice below sai d, 39。 I don39。t t hi nk the wi ndows need cl eaning at t hi s ti me of t he ni ght.39。 I l ooked down and near ly fel of f the ladder when I saw a pol iceman. I imm edi at ely r egr et ed answeri ng in the way I did, but I sai d, 39。I enjoy cl eani ng windows at night. 39。 So do I, 39。 answered t he pol iceman in the same tone. 39。Excuse m e inter rupti ng you. I hate t o i nt er upt a m an when he39。s busy wor king, but woul d you mind i ng wi th me to t he Wel l , I 39。d pr ef er t o st ay her e, 39。 I sai d. 39。You see, I 39。ve f or got t en m y key. He cal l ed. 39。My key, 39。 I shout ed. 39。 For t unat el y, t he shout i ng woke up m y wi f e who opened t he wi ndow j ust as t he pol i cem an had st ar t ed t o cl i m b t owar ds m e. Lesson93 A nobl e gi f t 崇高的禮物 One of t he m ost f am ous m onum ent s i n t he wor l d, t he St at ue of Li ber t y, was pr esent ed t o t he Uni t ed St at es of Am er i ca i n t he ni eent h cent ur y by t he peopl e of Fr ance . The gr eat st at ue, whi ch was desi gned by t he scul pt or August Bar t hol di , t ook t en year s t o pl et e. The 第 2 頁 共 44 頁 的近似模型，線性化處理后再進行控制，也可以利用非線性控制理論對其進行控制，倒立擺的非線性控制正成為一個研究的熱點。 (2)不確定性主要是指建立系統(tǒng)數(shù)學模型時的參數(shù)誤差、量測噪聲以及機械傳動過程中的減速齒輪間隙等非線性因素所導致的難以量化的部分。 (3)欠冗余性一般的，倒立擺控制系統(tǒng)采用單電機驅(qū)動，因而它與冗 余機構(gòu)，比如說冗余機器人有較大的不同。之所以采用欠冗余的設計是要在不失系統(tǒng)可靠性的前提下節(jié)約經(jīng)濟成本或者節(jié)約有效的空間。研究者常常是希望通過對倒立擺控制系統(tǒng)的研究獲得性能較為突出的新型控制器設計方法，并驗證其有效性及控制性能。 (4)耦合特性倒立擺擺桿和小車之間，以及多級倒立擺系統(tǒng)的上下擺桿之間都是強耦合的。這既是可以采用單電機驅(qū)動倒立擺控制系統(tǒng)的原因，也是使得控制系統(tǒng)的設計、控制器參數(shù)調(diào)節(jié)變得復雜的原因。 (5)開環(huán)不穩(wěn)定性倒立擺系統(tǒng)有兩個平衡狀態(tài)：垂直向下和垂直向上。垂直向下的狀態(tài)是系統(tǒng)穩(wěn)定的平衡點 (考 慮摩擦力的影響 )，而垂直向上的狀態(tài)是系統(tǒng)不穩(wěn)定的平衡點，開環(huán)時微小的擾動都會使系統(tǒng)離開垂直向上的狀態(tài)而進入到垂直向下的狀態(tài)中。 (6)約束限制由于實際機構(gòu)的限制，如運動模塊行程限制，電機力矩限制等。為制造方便和降低成本，倒立擺的結(jié)構(gòu)尺寸和電機功率都盡量要求最小，行程限制對于倒立擺的擺起尤為突出，很容易出現(xiàn)小車的撞邊現(xiàn)象。 倒立擺的以上特性增加了倒立擺的控制難度，也正是由于倒立擺的這些特性，使其更具有研究價值和意義 [3]。 倒立擺系統(tǒng)的分類 倒立擺系統(tǒng)誕生之初為單級直線形式，即僅有的一級擺桿一端自 由，另一端鉸接于可以在直線導軌上自由滑動的小車上。在此基礎(chǔ)上，人們又進行拓展，產(chǎn)生了多種形式的倒立擺。 按照基座的運動形式，主要分為三大類：直線倒立擺、環(huán)形倒立擺和平面倒立擺，每 種形式的倒立擺再按照擺桿數(shù)量的不同可進一步分為一級、二級、三級及多級倒立擺等 [4]。擺桿的級數(shù)越多，控制難度越大，而擺桿的長度也可能是變化的。多級擺的擺桿之間屬于自有連接 (即無電動機或其他驅(qū)動設備 )。目前，直線型倒立擺作為一種實驗儀器以其結(jié)構(gòu)相對簡單、形象直觀、構(gòu)件參數(shù)易于改變和價格低廉等優(yōu)點，已經(jīng)廣泛運用Her e and t here: they had just di scovered t hat a pr isoner had escaped. At t hat mom ent , a large bl ack car wit h f our of fi cer s inside i t, stopped at t he camp gat es. The off icers got out and t he pri soner st ood to at tenti on and salut ed as t hey passed. When t hey had gone, the dri ver of t he car came t owar ds him. The man obviously want ed to t alk. He was r at her el der ly wi t h grey hair and clear blue eyes. The pri soner felt sorr y f or hi m, but t her e was nothi ng el se he coul d do. As the man came near, the pr isoner knocked hi m t o t he gr ound wit h a shar p bl ow. Then, jumpi ng into the car, he drove of f as quickly as he could. ？ Fishermen and sail or s som etim es cl ai m to have sen m onster s in t he sea. Though people have oft en laughed at stor ies tol d by s eamen, it i s now known that many of these 39。monst ers39。 which have at t imes been si ghted are si mply st range fi sh. Occasi onal y, unusual creatur es ar e washed t o t he shore, but they are r arely caught out at sea. Some ti me ago, however, a pecul iar fi sh was caught near Madagascar . A smal l f i shing boat was car r ied mi les out to sea by the power ful fi sh as it pulled on the li ne. Reali zing t hat thi s was no or di nar y f ish, t he fi sher man made ever y ef ort not to damage i t i n any way. When it was event ualy brought t o shor e, i t was found t o be over t hi r teen feet long. I t had a head l ike a hor se, big