【正文】 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 bl 。目前，直線型倒立擺作為一種實(shí)驗(yàn)儀器以其結(jié)構(gòu)相對(duì)簡(jiǎn)單、形象直觀、構(gòu)件參數(shù)易于改變和價(jià)格低廉等優(yōu)點(diǎn)，已經(jīng)廣泛運(yùn)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。擺桿的級(jí)數(shù)越多，控制難度越大，而擺桿的長(zhǎng)度也可能是變化的。在此基礎(chǔ)上，人們又進(jìn)行拓展，產(chǎn)生了多種形式的倒立擺。 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?jī)r(jià)值和意義 [3]。 (6)約束限制由于實(shí)際機(jī)構(gòu)的限制，如運(yùn)動(dòng)模塊行程限制，電機(jī)力矩限制等。 (5)開環(huán)不穩(wěn)定性倒立擺系統(tǒng)有兩個(gè)平衡狀態(tài)：垂直向下和垂直向上。 (4)耦合特性倒立擺擺桿和小車之間，以及多級(jí)倒立擺系統(tǒng)的上下擺桿之間都是強(qiáng)耦合的。之所以采用欠冗余的設(shè)計(jì)是要在不失系統(tǒng)可靠性的前提下節(jié)約經(jīng)濟(jì)成本或者節(jié)約有效的空間。 (2)不確定性主要是指建立系統(tǒng)數(shù)學(xué)模型時(shí)的參數(shù)誤差、量測(cè)噪聲以及機(jī)械傳動(dòng)過程中的減速齒輪間隙等非線性因素所導(dǎo)致的難以量化的部分。 I shout ed. 39。ve f or got t en m y key. He cal l ed. 39。 I sai d. 39。s busy wor king, but woul d you mind i ng wi th me to t he Wel l , I 39。 answered t he pol iceman in the same tone. 39。I enjoy cl eani ng windows at night. 39。t t hi nk the wi ndows need cl eaning at t hi s ti me of t he ni ght.39。 lesson90 f or supper ？ 晚餐吃什么？ Though Br azi l is one of the r ichest count ri es in t he worl d, much of i t has not yet been devel oped. It was mai nl y f or t his r eason that the Br azil an gover nment decided to have a new ci ty bui lt 600 mi les nort h west of Ri o de Janeir o. Desi gned by t he great ar 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。clock t rain di d not run at al . I t broke down at stati on and was taken of f the l ine.39。t hat you are not t el li ng the t rut h. I suggest that you di d not cat ch the o39。t not ice anything unusual . I suggest, 39。 39。 39。cl ock. My employer wil l conf ir m t hat I was t her e on ti me. Would a lat er t rain get you t o work on ti me? asked the i nspector . 39。 answer ed t he m an. 39。 asked the i nspector. 39。 said the man. 39。At the t im e the m ur der was m i t ed, I was tr avel li ng on the 8. 0 o39。 tim e. Even so, peopl e are goi ng to f ind i t dif fi cul t to get to wor k. But so f ar, t he publ ic has expr esed it s grati tude t o the students i n let ters t o t he Pr es. Only one or t wo peopl e have obj ect ed that the student s wi l dri ve too f ast ! Lesson85 Never too ol d to learn 活到老學(xué)到老 I have just recei ved a let er fr om m y old school i nf orm ing me that my f or mer head master , Page, wi l l be r eti ri ng next week. Pupil s of the school , ol d and new, wi l l be sendi ng him a pr esent t o mar k the occasi on. Al l thos