電梯轎廂內安裝監控如何抗干擾?
在(zai)閉路監控(kong)工程中,電梯監控(kong)視頻干擾(rao)(rao)問題,一直是最(zui)常見、最(zui)難對付、也是最(zui)受(shou)關注的(de)問題之一。本文闡明:只要(yao)掌握了干擾(rao)(rao)產生(sheng)原理,電梯抗(kang)干擾(rao)(rao)工程問題也將迎刃而解。
1.掌握常用同軸電(dian)纜類型(xing)及特點
1)考慮(lv)傳(chuan)輸(shu)衰減:當樓層(ceng)很高(gao),距離監控(kong)中心又較遠的(de)情況下,應(ying)慎重考慮(lv)傳(chuan)輸(shu)衰減問題。選擇電(dian)(dian)纜(lan)(lan)(lan)(lan)時(shi),大家都(dou)知道粗纜(lan)(lan)(lan)(lan)優(you)(you)于(yu)(yu)(yu)細(xi)纜(lan)(lan)(lan)(lan),但(dan)還應(ying)了解SYWV物理(li)發泡電(dian)(dian)纜(lan)(lan)(lan)(lan)優(you)(you)于(yu)(yu)(yu)實心SYV電(dian)(dian)纜(lan)(lan)(lan)(lan),高(gao)編電(dian)(dian)纜(lan)(lan)(lan)(lan)優(you)(you)于(yu)(yu)(yu)低編電(dian)(dian)纜(lan)(lan)(lan)(lan),銅芯纜(lan)(lan)(lan)(lan)優(you)(you)于(yu)(yu)(yu)"銅包鋼"纜(lan)(lan)(lan)(lan),銅編網優(you)(you)于(yu)(yu)(yu)鋁鎂合金編網;
2)關(guan)注高(gao)頻(pin)(pin)衰(shuai)減(jian)(jian):低頻(pin)(pin)成分的亮度(du)/對比度(du)衰(shuai)減(jian)(jian)容易發現和(he)(he)解決,電(dian)(dian)纜(lan)最重(zhong)要的傳輸特(te)性(xing)就是(shi)頻(pin)(pin)率(lv)越(yue)高(gao)衰(shuai)減(jian)(jian)越(yue)大,高(gao)頻(pin)(pin)衰(shuai)減(jian)(jian)主要影響清(qing)晰(xi)度(du)和(he)(he)分辨率(lv),要特(te)別注意總(zong)結圖像質量的觀察方(fang)法。這方(fang)面電(dian)(dian)纜(lan)特(te)點(dian)和(he)(he)規律是(shi):粗(cu)纜(lan)優于(yu)細纜(lan),發泡(pao)優于(yu)實心,但同(tong)型號的"高(gao)編和(he)(he)低編高(gao)頻(pin)(pin)衰(shuai)減(jian)(jian)一樣";
3)考慮(lv)電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)壽命:軟性電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)優于(yu)普通電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan),細(xi)纜(lan)(lan)(lan)優于(yu)粗纜(lan)(lan)(lan);還(huan)有一(yi)(yi)個最易被忽視的(de)問題:電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)各(ge)(ge)層(ceng)(ceng)間(jian)的(de)粘合(he)力,即當電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)各(ge)(ge)層(ceng)(ceng)之間(jian)縱向(xiang)(xiang)相反方向(xiang)(xiang)受力時,是否會發(fa)生相對滑動,高(gao)層(ceng)(ceng)電(dian)(dian)(dian)(dian)梯纜(lan)(lan)(lan)長(chang)可達100米(mi)垂(chui)直(zhi)布(bu)線,電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)外護套(tao)固定(ding)在隨行電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)上,這(zhe)是一(yi)(yi)種(zhong)"軟固定(ding)",固定(ding)時不(bu)允許(xu)電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)變(bian)形(xing)(破壞(huai)同軸性),這(zhe)樣一(yi)(yi)來(lai),在電(dian)(dian)(dian)(dian)梯反復(fu)運動中電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)內(nei)部層(ceng)(ceng),在重(zhong)力作用下(xia),會逐漸(jian)"下(xia)滑",慢慢拉(la)斷編織(zhi)網或(huo)芯線,表現為信號逐步減弱,干擾越來(lai)越大;目前還(huan)沒有這(zhe)項電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)技(ji)術(shu)標準,簡單(dan)檢查(cha)方法是取一(yi)(yi)米(mi)電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan),在一(yi)(yi)頭剝開各(ge)(ge)層(ceng)(ceng),一(yi)(yi)人用手握住電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)兩端,另一(yi)(yi)人用鉗子拉(la)電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)的(de)內(nei)層(ceng)(ceng):依次(ci)拉(la)芯線,絕緣層(ceng)(ceng),編織(zhi)網,體驗粘合(he)力的(de)大小,做出合(he)理估計(ji),粘合(he)力差、易滑動的(de)盡量不(bu)選(xuan)用。這(zhe)項性能很多電(dian)(dian)(dian)(dian)纜(lan)(lan)(lan)并不(bu)好,應慎重(zhong)選(xuan)擇。
2.干擾產生原(yuan)理簡介
1) 電(dian)(dian)梯井內(nei)通常布置了動(dong)(dong)力、照明、風扇、控(kong)制、通信等(deng)線(xian)纜,各種電(dian)(dian)纜都會(hui)(hui)產生(sheng)電(dian)(dian)磁輻(fu)射。與天線(xian)接收(shou)原(yuan)理(li)相同(tong)(tong),同(tong)(tong)軸電(dian)(dian)纜也會(hui)(hui)"接收(shou)"這些干(gan)(gan)擾(rao),即干(gan)(gan)擾(rao)電(dian)(dian)磁場在電(dian)(dian)纜上產生(sheng)干(gan)(gan)擾(rao)感應電(dian)(dian)流,這個干(gan)(gan)擾(rao)感應電(dian)(dian)流也就會(hui)(hui)在電(dian)(dian)纜外導體(ti)(編織網(wang))縱向電(dian)(dian)阻上產生(sheng)干(gan)(gan)擾(rao)感應電(dian)(dian)壓(ya)(電(dian)(dian)動(dong)(dong)勢),這個干(gan)(gan)擾(rao)感應電(dian)(dian)壓(ya)剛好串(chuan)聯在視頻信號傳輸(shu)回路"長長的地線(xian)"中,形成干(gan)(gan)擾(rao);
2)更(geng)重(zhong)要的是這(zhe)些(xie)隨(sui)行(xing)電纜(lan)都是與視頻(pin)電纜(lan)并(bing)行(xing),且近(jin)距離捆扎在(zai)一起。這(zhe)就形成(cheng)了接(jie)近(jin)"最佳(jia)最有效的"干(gan)擾耦(ou)合關系。在(zai)一般工程(cheng)中可以(yi)采用穿金屬管或走金屬槽的屏蔽干(gan)擾辦法,但(dan)在(zai)電梯隨(sui)動(dong)的環境中,這(zhe)種(zhong)方(fang)法無(wu)能為(wei)力。所(suo)以(yi)電梯環境下的抗干(gan)擾難(nan)度很大(da),只(zhi)能選擇較好的設計(ji)和施(shi)工方(fang)法;
3.常(chang)用銅(tong)軸電纜(lan)傳輸方案的抗干擾(rao)措施
1)常(chang)用銅軸(zhou)電(dian)纜:不管是(shi)多(duo)層(ceng)(ceng)高(gao)(gao)編(bian)銅編(bian)網(wang)電(dian)纜、"鋁箔-編(bian)網(wang)"的雙屏蔽(bi)電(dian)纜、還(huan)是(shi)"鋁箔-編(bian)網(wang)--鋁箔-編(bian)網(wang)"的四(si)屏蔽(bi)電(dian)纜,電(dian)氣上都屬于一個(ge)屏蔽(bi)層(ceng)(ceng)。干(gan)(gan)擾(rao)(rao)感應電(dian)壓(ya),都是(shi)直接串(chuan)聯在視(shi)頻(pin)信號傳輸回路(lu)中。只是(shi)多(duo)層(ceng)(ceng)高(gao)(gao)編(bian)電(dian)纜的外導體(ti)電(dian)阻(zu)小(xiao),形(xing)成的干(gan)(gan)擾(rao)(rao)感應電(dian)壓(ya)也相對較低(di)一些。這對抗低(di)頻(pin)電(dian)源干(gan)(gan)擾(rao)(rao)、電(dian)機(ji)電(dian)火花干(gan)(gan)擾(rao)(rao)等有一定效果(幾十(shi)kHz以下的干(gan)(gan)擾(rao)(rao))。但對高(gao)(gao)頻(pin)干(gan)(gan)擾(rao)(rao),由于"趨膚效應",高(gao)(gao)頻(pin)阻(zu)抗與低(di)編(bian)電(dian)纜相同,抗干(gan)(gan)擾(rao)(rao)效果也基(ji)本一樣;所以應該清醒看到(dao):高(gao)(gao)編(bian)電(dian)纜只有適當降低(di)低(di)頻(pin)干(gan)(gan)擾(rao)(rao)的作用,防強干(gan)(gan)擾(rao)(rao)和高(gao)(gao)頻(pin)干(gan)(gan)擾(rao)(rao)還(huan)是(shi)無能為力;
2) 電梯布線方式的抗干擾措施(shi):
視頻(pin)電(dian)(dian)纜(lan)走出電(dian)(dian)梯(ti)井的位置選擇:理想的選擇應在井的中部,因為這時(shi)井內隨行視頻(pin)電(dian)(dian)纜(lan)長度,大約只有井深的一半多(duo)一點(dian),最短,自然引入的干擾(rao)也最小;但工程上這種(zhong)出線要(yao)求(qiu),只能看(kan)情況爭取,實際工程不一定(ding)允許(xu)。
過去,在不明白原理(li)的(de)情(qing)(qing)況(kuang)下(xia),多(duo)數(shu)出線(xian)位置都是(shi)和其他隨(sui)(sui)行(xing)電(dian)纜一(yi)起走(zou)(zou),從(cong)電(dian)纜井(jing)的(de)頂(ding)部(bu)或(huo)底(di)部(bu)走(zou)(zou)出。這種情(qing)(qing)況(kuang)下(xia),考慮到只(zhi)有(you)一(yi)半(ban)(ban)電(dian)纜是(shi)隨(sui)(sui)行(xing)運動(dong)的(de),另一(yi)半(ban)(ban)只(zhi)是(shi)固定延伸(shen)連接,不運動(dong),我們把(ba)這部(bu)分叫著"不動(dong)電(dian)纜";這就提供了(le)一(yi)種可(ke)能(neng)(neng):那(nei)一(yi)半(ban)(ban)隨(sui)(sui)行(xing)運動(dong)電(dian)纜只(zhi)能(neng)(neng)與其他隨(sui)(sui)行(xing)電(dian)纜一(yi)起捆綁走(zou)(zou)線(xian);而另一(yi)半(ban)(ban)不動(dong)電(dian)纜可(ke)以選(xuan)擇遠(yuan)離隨(sui)(sui)行(xing)電(dian)纜單獨走(zou)(zou)線(xian)的(de)方法(fa),在電(dian)梯井(jing)內把(ba)視頻線(xian)緊貼井(jing)璧垂直走(zou)(zou)線(xian),并(bing)把(ba)這部(bu)分電(dian)纜穿金屬管或(huo)走(zou)(zou)金屬槽,以屏(ping)蔽(bi)干擾對這部(bu)分電(dian)纜的(de)影響(xiang),比較有(you)效。
隨(sui)(sui)行運(yun)動部分的(de)(de)視頻電(dian)(dian)(dian)纜(lan)與其他(ta)隨(sui)(sui)行電(dian)(dian)(dian)纜(lan)捆(kun)扎時(shi),設計者應充分了解其他(ta)隨(sui)(sui)行電(dian)(dian)(dian)纜(lan)的(de)(de)結構和分布情(qing)況,捆(kun)扎時(shi)視頻電(dian)(dian)(dian)纜(lan)應盡量遠離電(dian)(dian)(dian)流大(da)、頻率高(gao)的(de)(de)電(dian)(dian)(dian)纜(lan),靠近電(dian)(dian)(dian)流小頻率低的(de)(de)電(dian)(dian)(dian)纜(lan)捆(kun)扎;這里,哪怕有1厘米(mi)的(de)(de)選擇可能(neng)也要爭(zheng)取,因為干擾影響大(da)小至少與距離平方成反比;
攝像(xiang)(xiang)機金(jin)屬外殼、NC頭(tou)的(de)外殼、同軸電纜的(de)外導體等(deng)視頻信號的(de)"地",和電梯(ti)轎(jiao)廂、導軌等(deng)要絕緣,這在(zai)安裝(zhuang)攝像(xiang)(xiang)機時要特別(bie)注意。
攝像機供電(dian)應(ying)優選集中直流供電(dian)方式,其(qi)次是選擇轎廂照明電(dian),不能用動(dong)力電(dian)。
供電、控制(zhi)等監控用電纜,盡量選用帶屏蔽(bi)的(de)電纜,防止干擾信號(hao)向外泄露。
從電(dian)梯(ti)井出口(kou)到控制中(zhong)心(xin)的視頻電(dian)纜,應走金屬管或走金屬槽,以(yi)屏(ping)蔽(bi)沿途環境干擾對這部(bu)(bu)分(fen)電(dian)纜的影響(xiang),并注意這部(bu)(bu)分(fen)屏(ping)蔽(bi)與電(dian)梯(ti)井內的屏(ping)蔽(bi),應做好電(dian)氣連接。
4.應用(yong)抗干擾同(tong)軸(zhou)電纜
抗(kang)干(gan)(gan)擾(rao)(rao)同(tong)軸(zhou)電(dian)(dian)纜(lan)(lan)是(shi)一(yi)種"雙絕緣雙屏(ping)蔽的(de)(de)(de)(de)同(tong)軸(zhou)電(dian)(dian)纜(lan)(lan)",其里面的(de)(de)(de)(de)芯線、絕緣層、屏(ping)蔽層仍(reng)然(ran)是(shi)標準的(de)(de)(de)(de)75歐(ou)姆電(dian)(dian)纜(lan)(lan),沒有區別。不同(tong)的(de)(de)(de)(de)是(shi),在(zai)原來屏(ping)蔽層外(wai),又增加(jia)了第二絕緣層和(he)第二屏(ping)蔽層,外(wai)面再加(jia)上(shang)護套。從上(shang)面干(gan)(gan)擾(rao)(rao)產(chan)生(sheng)原理分析(xi)已經(jing)知道,干(gan)(gan)擾(rao)(rao)在(zai)傳統同(tong)軸(zhou)電(dian)(dian)纜(lan)(lan)外(wai)層上(shang)產(chan)生(sheng)的(de)(de)(de)(de)感(gan)應(ying)(ying)電(dian)(dian)壓,串聯在(zai)視頻信(xin)號傳輸回路" 長(chang)長(chang)的(de)(de)(de)(de)地(di)線"中,從而(er)形成(cheng)干(gan)(gan)擾(rao)(rao)的(de)(de)(de)(de)。但采(cai)用抗(kang)干(gan)(gan)擾(rao)(rao)同(tong)軸(zhou)電(dian)(dian)纜(lan)(lan)后,情(qing)況有了質的(de)(de)(de)(de)變(bian)化:干(gan)(gan)擾(rao)(rao)感(gan)應(ying)(ying)電(dian)(dian)壓只(zhi)能形成(cheng)在(zai)"第二屏(ping)蔽層"上(shang),并(bing)由里面的(de)(de)(de)(de)"第二絕緣層"把它與視頻信(xin)號傳輸回路"長(chang)長(chang)的(de)(de)(de)(de)地(di)線"絕緣隔離開,把干(gan)(gan)擾(rao)(rao)排除(chu)在(zai)視頻信(xin)號傳輸回路之外(wai),達到抗(kang)干(gan)(gan)擾(rao)(rao)的(de)(de)(de)(de)目(mu)的(de)(de)(de)(de)。
這種(zhong)抗干(gan)擾(rao)電(dian)(dian)(dian)纜的特性(xing),對于電(dian)(dian)(dian)梯(ti)環境下(xia)的超強低頻動力電(dian)(dian)(dian)源干(gan)擾(rao),電(dian)(dian)(dian)機(ji)電(dian)(dian)(dian)火(huo)花干(gan)擾(rao),變頻電(dian)(dian)(dian)機(ji)干(gan)擾(rao),控制信號干(gan)擾(rao)等幾十千赫(he)以下(xia)的干(gan)擾(rao),抗干(gan)擾(rao)性(xing)能十分突出。
在傳輸線路(lu)較長的工(gong)程設計(ji)中,采用"雙絕緣雙屏(ping)蔽的同(tong)軸電纜(lan)"后,傳統工(gong)程上的一些(xie)抗干(gan)擾措施也(ye)可以大大化簡(jian),并能有(you)效降(jiang)低工(gong)程總造價。
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