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工程機(jī)械攪拌設(shè)備用稱重傳感器的選型
本文闡述了工程機(jī)械攪拌設(shè)備稱重系統(tǒng)的基本要求和特殊要求，提出了工程機(jī)械攪拌設(shè)備稱重系統(tǒng)選用傳感器時(shí)需要考慮的幾個(gè)問(wèn)題，重點(diǎn)分析了傳感器防護(hù)結(jié)構(gòu)對(duì)工程機(jī)械攪拌設(shè)備運(yùn)行可靠性的影響，指出IP代碼所代表的防護(hù)等級(jí)不能涵蓋工程機(jī)械攪拌設(shè)備對(duì)傳感器的全部防護(hù)要求。建議不同類(lèi)型的攪拌設(shè)備選用不同防護(hù)能力的傳感器。
稱重系統(tǒng)中選用傳感器通常要考慮稱重系統(tǒng)的量限、準(zhǔn)確度、傳感器的安裝空間、周?chē)h(huán)境對(duì)傳感器的可能影響、加載的類(lèi)型以及傳感器的壽命等諸方面因素。工程機(jī)械攪拌設(shè)備的稱重系統(tǒng)也不例外。只是不同的稱重系統(tǒng)由于要求不同，工作條件不同，所要考慮的問(wèn)題側(cè)重點(diǎn)有所不同罷了。
工程機(jī)械攪拌設(shè)備通常包括混凝土攪拌樓（站）、瀝青攪拌站、穩(wěn)定土拌和廠以及海上混凝土攪拌船等，其中以混凝土攪拌樓（站）最為典型。下面就以混凝土攪拌樓（站）為例進(jìn)行分析。
1 混凝土攪拌樓（站）對(duì)稱重系統(tǒng)的基本要求
1.1 準(zhǔn)確稱量誤差對(duì)混凝土的強(qiáng)度影響很大，特別是水灰比計(jì)量精度，因?yàn)閺?qiáng)度和水灰比是線性關(guān)系。相關(guān)國(guó)家標(biāo)準(zhǔn)規(guī)定，水泥、水、外加劑、摻合料的動(dòng)態(tài)計(jì)量精度為±1%，砂、石料的動(dòng)態(tài)計(jì)量精度為±2%。
1.2 快速滿足攪拌樓站工作循環(huán)的要求。
1.3 種類(lèi)多稱量值預(yù)選的種類(lèi)要多，變換要方便，以適應(yīng)多種配比和不同容量的要求。
1.4 結(jié)構(gòu)簡(jiǎn)單稱量裝置要結(jié)構(gòu)簡(jiǎn)單，牢固可靠，性能穩(wěn)定，操作容易。
顯然，采用傳感器電子稱重系統(tǒng)較之機(jī)械秤更能滿足要求。因此，稱重傳感器在混凝土攪拌樓站中得到了越來(lái)越廣泛的應(yīng)用。但是，客觀地說(shuō)，與機(jī)械杠桿秤相比，在“牢固可靠，性能穩(wěn)定”方面，傳感器電子稱重系統(tǒng)還有很多工作要做。
2 混凝土攪拌樓站中稱重傳感器的運(yùn)行條件
與一般用于商貿(mào)計(jì)量的電子秤的一個(gè)很大的不同之處在于，混凝土攪拌樓站中稱重傳感器處于相當(dāng)惡劣的運(yùn)行條件中，應(yīng)力環(huán)境十分復(fù)雜，與一般的電子產(chǎn)品的運(yùn)行環(huán)境相比，有更大的隨機(jī)性。
2.1 環(huán)境溫度和濕度
混凝土攪拌樓站通常是露天安裝，傳感器可能遭受日曬雨淋，溫度劇烈變化。而不少工程建設(shè)項(xiàng)目是在自然條件相當(dāng)惡劣的山區(qū)或邊遠(yuǎn)地區(qū)。所以，必須考慮更大的溫度范圍，更高的濕度條件?；炷猎谏a(chǎn)過(guò)程中需要水。在水的輸送和稱量過(guò)程中也會(huì)產(chǎn)生不少水氣，在一定的小范圍內(nèi)形成較為潮濕的環(huán)境。在溫控?cái)嚢铇侵?，則有高溫工況和低溫工況的不同要求。夏天運(yùn)行在低溫工況時(shí)要通入零度以下的冷風(fēng)以及加冰攪拌，這時(shí)樓內(nèi)會(huì)出現(xiàn)冷凝水，足見(jiàn)樓內(nèi)濕度之高。
2.2 粉塵
混凝土在生產(chǎn)過(guò)程中需要大量的水泥、煤粉灰以及適量的外加劑。這些粉狀物在輸送和稱量過(guò)程中會(huì)產(chǎn)生粉塵。即使是骨料，在輸送過(guò)程中也有粉塵產(chǎn)生。這些粉塵有一部分會(huì)附著在傳感器表面。在粉塵和水氣的共同作用下，傳感器將受到較為嚴(yán)重的腐蝕。所以，粉料秤傳感器的損壞通常要比其他秤的傳感器更頻繁一些。
2.3 沖擊與振動(dòng)
在進(jìn)料過(guò)程中，砂石料會(huì)產(chǎn)生沖擊。傳感器應(yīng)能承受5g的加速度。在攪拌過(guò)程中，會(huì)產(chǎn)生持續(xù)的振動(dòng)，而振動(dòng)會(huì)產(chǎn)生疲勞破壞。
2.4 人為環(huán)境
人為環(huán)境是產(chǎn)品可靠性設(shè)計(jì)時(shí)必須考慮的因素之一?；炷翑嚢铇且话惆惭b在施工現(xiàn)場(chǎng)。工地上大量使用臨時(shí)工，其中相當(dāng)多的臨時(shí)工文化水平較低，缺少必要的技能。在設(shè)備的維修和清洗等工作中，很有可能發(fā)生傳感器受到高壓水的濺射，誤操作引起過(guò)載等情況。顯然，傳感器要在這樣的環(huán)境條件下長(zhǎng)期可靠的運(yùn)行，是要進(jìn)行一些特殊設(shè)計(jì)的。
上述基本要求和運(yùn)行條件可以作為混凝土攪拌樓（站）用稱重傳感器的選型的依據(jù)。
3 混凝土攪拌樓（站）用稱重傳感器選型時(shí)需要考慮的幾個(gè)問(wèn)題
3.1稱重傳感器載荷容量的確定
稱重傳感器載荷容量通常按下式計(jì)算
傳感器額定載荷=料斗自重＋額定稱重量
（0.6~0.7）×傳感器只數(shù)
事實(shí)上人們?cè)谶x擇傳感器容量時(shí)往往還要綜合考慮沖擊載荷的大小以及選定安全系數(shù).安全系數(shù)的選擇又與傳感器的靈敏度有密切關(guān)系．國(guó)內(nèi)外常見(jiàn)的應(yīng)變式稱重傳感器靈敏度多數(shù)為2mV/V，但是也有1mV/V的，如柱式傳感器；也有3mV/V的，如部分懸臂梁式傳感器和板環(huán)式傳感器；扭環(huán)式傳感器則通常是2.85mV/V。目前在攪拌樓上使用的傳感器基本上都是2mV/V的。
3.2稱重傳感器準(zhǔn)確度的選擇
傳感器準(zhǔn)確度的選擇以滿足稱量系統(tǒng)的準(zhǔn)確度要求為準(zhǔn)，不必片面追求過(guò)高的傳感器準(zhǔn)確度等級(jí)。在多只傳感器組合使用時(shí)，其綜合誤差按下式計(jì)算
δr=δ／√n
式中，δ為單只傳感器的準(zhǔn)確度，n為組合使用的傳感器只數(shù)。
目前攪拌樓上常用的S型傳感器、懸臂梁式傳感器、板環(huán)式傳感器，其線性、滯后、重復(fù)性、靈敏度溫度影響、蠕變等主要指標(biāo)絕大多數(shù)廠家均優(yōu)于0.05%，大多數(shù)廠家優(yōu)于0.03%，部分廠家優(yōu)于0.02%。其單只傳感器的綜合誤差都接近或優(yōu)于0.1%。多只傳感器組合后其綜合誤差就更小了?？梢哉f(shuō)一般稱重傳感器生產(chǎn)廠家的產(chǎn)品都能滿足要求。
以前，不少攪拌樓生產(chǎn)廠家規(guī)定其稱重系統(tǒng)的靜態(tài)精度分別為0.1%和0.3%。這樣，對(duì)于使用單只傳感器的秤而言，單項(xiàng)指標(biāo)為0.05%的傳感器就可以滿足0.3%精度的秤的要求。對(duì)于使用三只以上傳感器的秤而言，單項(xiàng)指標(biāo)0.05%的傳感器也能滿足0.1%精度的要求。
需要指出的是，上述精度是稱重系統(tǒng)的靜態(tài)精度，而混凝土國(guó)家標(biāo)準(zhǔn)要求的是動(dòng)態(tài)精度，這是由于原材料不斷地向稱量機(jī)構(gòu)供料在重力的沖擊下稱量誤差明顯增加。上述0.1%和0.3%的靜態(tài)精度能否保證分別達(dá)到1%和2%的動(dòng)態(tài)精度還與供料系統(tǒng)的設(shè)計(jì)有關(guān)。
現(xiàn)在，不少?gòu)S家規(guī)定其稱重系統(tǒng)的精度在0~額定稱量值的整個(gè)稱量范圍內(nèi)為水泥、水、外加劑±1%，砂、石料±2%。須知，預(yù)拌混凝土國(guó)家標(biāo)準(zhǔn)要求的是實(shí)物計(jì)量精度，而一般計(jì)量?jī)x器在0~20%范圍相對(duì)誤差較大，因此，還是混凝土攪拌樓（站）的行業(yè)標(biāo)準(zhǔn)中規(guī)定的20~100%的稱量段滿足水泥、水、外加劑±1%，砂、石料±2%的計(jì)量允許偏差更為實(shí)際一些。更為合理的精度標(biāo)注方式是采用衡器行業(yè)的術(shù)語(yǔ)來(lái)表述，即作為累計(jì)料斗秤，應(yīng)采用自動(dòng)稱量準(zhǔn)確度的等級(jí)標(biāo)志1.0和2.0。這類(lèi)秤的檢定項(xiàng)目與用途相適應(yīng)，既包括物料試驗(yàn)（確定累計(jì)誤差）也包括靜態(tài)檢定。
其自動(dòng)稱量時(shí)的最大允許誤差見(jiàn)表一。
在用標(biāo)準(zhǔn)載荷進(jìn)行靜態(tài)檢定和進(jìn)行非自動(dòng)稱量時(shí)，應(yīng)滿足表二的規(guī)定。
通常將1.0級(jí)秤設(shè)計(jì)為1000分度，2.0級(jí)秤設(shè)計(jì)為500分度。可以看出，在高量程端1.0級(jí)秤的誤差為0.15%，2.0級(jí)秤的誤差為0.3%，并不比以前廠家規(guī)定的0.1%和0.3%靜態(tài)精度高。但是在低量程段精度要求確實(shí)是提高了。在這種情況下需要確認(rèn)所選用的傳感器在低量程段能否也滿足要求。
按秤的分度數(shù)選擇傳感器最簡(jiǎn)單的方法就是1000分度的秤選用1000分度的傳感器，500分度的秤選用500分度的傳感器。稱重傳感器國(guó)家標(biāo)準(zhǔn)中準(zhǔn)確度就是用分度數(shù)來(lái)表示的，但是由于多方面的原因，目前大多數(shù)生產(chǎn)廠仍用單項(xiàng)指標(biāo)表示傳感器的精度。用戶再根據(jù)單項(xiàng)精度指標(biāo)算出綜合誤差。選用起來(lái)麻煩一點(diǎn)。
國(guó)外發(fā)達(dá)國(guó)家的混凝土攪拌樓稱重系統(tǒng)精度一般用分度數(shù)來(lái)表示，規(guī)定為１000分度?？赡芘c他們使用高性能混凝土所占比例高有關(guān)。
隨著經(jīng)濟(jì)和技術(shù)的不斷發(fā)展，超高層建筑、超長(zhǎng)橋梁、大型水利工程以及其他暴露在嚴(yán)酷環(huán)境中的建筑對(duì)混凝土的性能提出了越來(lái)越高的要求，混凝土技術(shù)也進(jìn)入了高科技時(shí)代，高性能混凝土的應(yīng)用比例不斷提升。生產(chǎn)高性能混凝土除了要正確選用原材料、確定合理的工藝參數(shù)外，施工工藝的控制也是十分重要的。混凝土攪拌樓中配料系統(tǒng)的準(zhǔn)確度是其中重要的一環(huán)。
3.3傳感器結(jié)構(gòu)形式的選擇
常用的拉式傳感器有Ｓ形，板環(huán)式以及中心十字筋板環(huán)式等。中心十字筋板環(huán)式傳感器精度高、抗偏載性能優(yōu)異，但是價(jià)格較高，通常只在高精度測(cè)量場(chǎng)合使用。攪拌樓上常用的是Ｓ型傳感器和板環(huán)式傳感器。其中Ｓ型傳感器因其精度高、抗偏載能力強(qiáng)、同時(shí)可以帶過(guò)載保護(hù)、量程范圍寬等優(yōu)點(diǎn)用得最多。
常用的壓式傳感器有懸臂梁式、輪輻式、柱式、橋式、扭環(huán)式等。綜合考慮精度、量程范圍、安裝方式、價(jià)格等因素，絕大多數(shù)攪拌樓生產(chǎn)廠選擇了懸臂梁式。
3.4載荷類(lèi)型的考慮
混凝土生產(chǎn)主要使用砂、石子、水泥以及水、外加劑、摻合料。幾種材料中數(shù)石子秤的沖擊為最大。在電子秤大家族中，這類(lèi)沖擊不算最大，一般傳感器均能承受過(guò)載情況。
從筆者調(diào)查的情況來(lái)看，傳感器因過(guò)載而損壞的情況時(shí)有發(fā)生。比如控制系統(tǒng)出故障，造成大量物料傾瀉而下，造成過(guò)載。也有使用過(guò)程中人為因素造成的過(guò)載，特別是小量程傳感器因操作者踩踏秤架而過(guò)載損壞的事時(shí)有發(fā)生。因此傳感器的過(guò)載能力、傳感器有無(wú)過(guò)載保護(hù)對(duì)稱重系統(tǒng)的可靠運(yùn)行還是有一定影響的。
傳感器的性能指標(biāo)中有兩項(xiàng)與此有關(guān)，一是允許過(guò)負(fù)荷，一是極限過(guò)負(fù)荷。
允許過(guò)負(fù)荷是指卸去這一負(fù)荷后，傳感器的性能指標(biāo)不變。極限過(guò)負(fù)荷是指在這一負(fù)荷下傳感器不會(huì)產(chǎn)生有害的永久性機(jī)械變形。
一般傳感器的允許過(guò)負(fù)荷為150%，極限過(guò)負(fù)荷在200%~300%之間。有些帶過(guò)載保護(hù)的傳感器則可能超過(guò)著一范圍。如莆田傳感器廠的CFCKN-1型傳感器因其特殊設(shè)計(jì)，允許過(guò)負(fù)荷高達(dá)500%。此類(lèi)傳感器在頻繁過(guò)載的情況下也能可靠地工作。
3.5傳感器的防護(hù)等級(jí)
傳感器的防護(hù)等級(jí)通常用IP表示。一般傳感器廠家均稱自己的產(chǎn)品達(dá)到IP67水平，少數(shù)廠家的部分產(chǎn)品達(dá)到IP68水平。
我們知道，IP代碼所表示的是電壓不超過(guò)72.5kV電氣產(chǎn)品的外殼防護(hù)等級(jí)。國(guó)家標(biāo)準(zhǔn)《GB4208-93外殼防護(hù)等級(jí)》中規(guī)定，IP67是指產(chǎn)品能防塵、防短時(shí)間浸水影響；IP68是指產(chǎn)品能防塵、防持續(xù)潛水影響。需要指出的是，這樣的防護(hù)中未包括機(jī)械損壞、銹蝕、潮濕等外部影響或環(huán)境條件。涉及這類(lèi)保護(hù)通常由有關(guān)產(chǎn)品標(biāo)準(zhǔn)規(guī)定。稱重傳感器與一般的電器產(chǎn)品和二次儀表產(chǎn)品不同的是，它同時(shí)還是一個(gè)受力構(gòu)件，在運(yùn)行中不斷地受到力的作用并產(chǎn)生變形。此外還有可能受到震動(dòng)、沖擊或撞擊這樣的機(jī)械損傷，粉塵和水氣共同作用下的較為嚴(yán)重的腐蝕以及在非常潮濕的環(huán)境條件下運(yùn)行。這與GB4208-93規(guī)定的試驗(yàn)條件有很大的不同。
曾碰到這樣的情況：某礦山的礦石秤上選用了進(jìn)口傳感器，某大型鋼廠的鋼包秤上選用了一種國(guó)際知名品牌的傳感器，均是焊接密封，非常漂亮，達(dá)到了IP68級(jí)。但是在現(xiàn)場(chǎng)實(shí)際使用壽命都很短。在不得已的情況下，試用了莆田傳感器廠的產(chǎn)品。結(jié)果出乎他們的意外，使用壽命都超過(guò)了進(jìn)口傳感器。其中在某大型鋼廠鋼包秤上的傳感器已運(yùn)行了7年，至今還在運(yùn)行中。
在這兩個(gè)例子中，說(shuō)穿了主要是防護(hù)上的區(qū)別。一是有外殼還是沒(méi)有外殼；二是焊縫設(shè)計(jì)。三是密封材料。
一般說(shuō)來(lái)，有外殼的設(shè)計(jì)優(yōu)于無(wú)外殼的設(shè)計(jì)；在焊縫設(shè)計(jì)方面則要盡可能避免焊縫受力。在無(wú)法避免的情況下，則要校核焊縫強(qiáng)度，特別是疲勞強(qiáng)度。疲勞引起的細(xì)微裂紋是導(dǎo)致潮氣進(jìn)入，傳感器失效的重要原因。前面提到的某國(guó)際知名品牌焊接密封傳感器在現(xiàn)場(chǎng)使用壽命短的原因就是過(guò)分依賴焊接密封，內(nèi)部應(yīng)變片僅薄薄一層面膠密封，一旦焊縫裂紋，傳感器就在潮氣的作用下迅速失效。在密封材料方面需要提醒的是，能通過(guò)IP67半小時(shí)浸水試驗(yàn)的傳感器未必能通得過(guò)傳感器標(biāo)準(zhǔn)中的12周期濕熱試驗(yàn)。應(yīng)該說(shuō)傳感器標(biāo)準(zhǔn)中規(guī)定的濕熱試驗(yàn)更符合傳感器的使用環(huán)境要求，其嚴(yán)酷程度并不低于IP67。
受價(jià)格競(jìng)爭(zhēng)的影響，不少傳感器廠取消了保護(hù)外殼，拉式傳感器尤甚。目前國(guó)內(nèi)生產(chǎn)的S型傳感器、板環(huán)式傳感器幾乎都取消了外殼。
確實(shí)，在很多場(chǎng)合這樣做沒(méi)有什么問(wèn)題。但是在混凝土攪拌樓這樣的環(huán)境條件下，未必是最好的選擇。我廠為三峽工程攪拌樓設(shè)計(jì)生產(chǎn)的CFCKN-1型外殼焊封傳感器使用壽命長(zhǎng)達(dá)十年。我們所看到的美日歐混凝土攪拌樓上用的拉式傳感器多數(shù)也是帶保護(hù)外殼的。他們認(rèn)為傳感器應(yīng)是半永久性器件，使用壽命應(yīng)不低于10６次。
多年來(lái)的實(shí)踐證明，CFCKN-1型傳感器與普通S型傳感器相比，至少有以下幾個(gè)優(yōu)點(diǎn)：
①外殼厚，防機(jī)械損傷，特別是防以外撞擊能力強(qiáng)；
②焊縫深，耐蝕穿時(shí)間長(zhǎng)；
③過(guò)載保護(hù)間隙也在外殼內(nèi)，不會(huì)因粉塵積聚或雜物受堵；
稱重傳感器合理的外殼設(shè)計(jì)是稱重裝置“牢固可靠、性能穩(wěn)定”的重要保證，選型時(shí)不可不認(rèn)真考慮。
3.6不同類(lèi)型攪拌樓對(duì)稱重傳感器選型的細(xì)分
大中型水利工程的混凝土攪拌樓、城市商品混凝土攪拌樓、小型水利工程和縣級(jí)公路建設(shè)中使用的混凝土攪拌樓在規(guī)格大小、使用連續(xù)時(shí)間、安裝地點(diǎn)的環(huán)境條件都有很大的不同，所生產(chǎn)的混凝土性能要求也有很大差別。因此，對(duì)傳感器的可靠性、防護(hù)等級(jí)等性能事實(shí)上也有不同的要求，應(yīng)區(qū)別對(duì)待。對(duì)連續(xù)運(yùn)行時(shí)間長(zhǎng)、環(huán)境條件惡劣以及生產(chǎn)高性能混凝土的攪拌樓應(yīng)選擇可靠性、防護(hù)等級(jí)高的傳感器。其余的則可適當(dāng)降低要求。對(duì)于運(yùn)行在海上的混凝土攪拌船，則還需要考慮防鹽霧的要求，這里就不再一一細(xì)說(shuō)了。
Mechanical mixing equipment works with the selection of load cell
This paper engineering machinery mixing equipment weighing system and the basic requirements for special requirements, proposed construction machinery mixing equipment weighing system used to be considered when the sensor several issues, the focus of sensor protective structure on the construction machinery mixing equipment reliability of operation, pointing out that the code generation IP Table protective levels can not cover mechanical mixing equipment works full protection of the sensor requirements. Recommendations different types of mixing equipment selection of different defensive capabilities of the sensor.
???? Weighing System sensors normally used in the weighing system to consider the amount of limited accuracy, sensor installation space, the environment may affect the sensor, load and the type of sensor, such as the life span of various factors. Engineering machinery mixing equipment weighing system is no exception. The weighing system is different because of different demands, different working conditions, to be considered by focusing on different issues only.
???? Engineering machinery mixing concrete mixing equipment usually include floor (stations), the asphalt mixing plant, stabilized soil mixing concrete mixing plants and ships at sea, mixing them with concrete floor (stations) the most typical. Below the concrete mixing plant (stations) as an example for analysis.
1 concrete mixing plant (station) on the basic requirements Weighing System
1.1 accurate weighing error on the strength of concrete is strongly influenced, in particular water-cement ratio measurement precision, because strength and water-cement ratio is linear. The relevant national standards, cement, water, additives, the admixture of dynamic measurement accuracy of ± 1%, sand, stone dynamic measurement accuracy of ± 2%.
1.2 Quick meet mixing station duty cycle requirements.
1.3 variety that money to the type of pre-selection, to facilitate the transformation to adapt to a variety of different ratio and capacity requirements.
1.4 simple structure weighing devices to simple structure, solid and reliable, stable performance, easy operation.
???? Clearly, the use of electronic weighing system sensor machinery weighing more than meet the requirements. Therefore, the load cell floor in the concrete mixing stations to be more widely used. However, objectively speaking, compared with the mechanical lever scales in the "solid and reliable, stable performance," as a sensor electronic weighing system has much work to do.
2 concrete mixing stations floor in the load cell operating conditions
???? For commerce and the general measurement of electronic scales a great difference is, in concrete mixing stations floor load cell in a very adverse operating conditions, stress environment is very complex, and the general electronic products compared to the operating environment, a greater randomness.
2.1 ambient temperature and humidity
???? Concrete mixing station is normally open floor installation, sensor may be subject to the sun and rain, temperature dramatic changes. And many construction projects are in very bad natural conditions in the mountains or remote areas. Therefore, we must consider greater temperature range and higher humidity conditions. Concrete in the production process needs water. In the delivery of water and weighing process and also have a lot of water in the gas, in some form within relatively small humid environment. Stir in the temperature control in the House, there are high-temperature and low-temperature conditions of the different requirements of condition. Running in the summer when low-temperature condition to the zero-below cold wind and加冰stirring, then building a condensed water will be well demonstrated high humidity building.
2.2 dust
???? Concrete in the production process requires a large amount of cement, coal ash and the appropriate amount of admixture. These were in the transportation and powder weighing process will produce dust. Even the aggregate, in the course of transmission have also dust. Some of these dust sensor will be attached to the surface. The dust and water vapor in the common role, the sensor will be more serious corrosion. Therefore, the damage powder weighing sensors usually weigh than the other sensors more frequently.
2.3 Shock and Vibration
???? In the feed process, the impact of aggregate will have. Sensors should be able to bear 5 g acceleration. Stir in the process, will have sustained vibration, and vibration would have fatigue damage.
2.4 man-made environment
???? Man-made environment is the product reliability design one of the factors that must be considered. Concrete Mixing floor installed at the construction site in general. Large-scale use of temporary workers on site, of which a considerable number of temporary low level of education, lack of the necessary skills. In terms of facilities, such as the maintenance and cleaning work, the sensor may be in the high-pressure water sputtering, misoperation caused overload, and so on. Clearly, the sensor should be in such an environment and a long-term reliable under the conditions of the operation is to carry out some special design.
????? These basic requirements and operating conditions can be used as concrete mixing plant (stations) by the load cell selection basis.
Three concrete mixing plant (stations) to use when weighing sensor to be considered by the Selection Problems
3.1 weighing sensor payload capacity determination
???? Weighing sensor payload capacity usually
???? Hopper sensor rated load = + rated themselves that weight
???? (0.6 ~ 0.7) × only a few sensors
????? In fact people in the choice of sensor integrated capacity even when they consider the size and impact load selected safety factor. Safety factor with the choice of sensor sensitivity closely related. Domestic and foreign common strain load sensor sensitivity majority of 2 mV / V, but there are 1 mV / V, such as column sensor; Also 3 mV / V, as some cantilever sensor and sensor plate Central; Twisted ring sensor usually 2.85 mV / V. Stir upstairs in the use of sensors are basically 2 mV / V.
3.2 weighing the choice of sensor accuracy
???? The choice of sensor accuracy of the weighing system to meet the accuracy requirements, is not one-sided pursuit of the high level sensor accuracy. In multi-sensor combination used only when its integrated error
Δ r = δ / √ n
Where, δ only a single sensor accuracy, the use of combinations of n is only a few sensors.
????? Currently stirring upstairs popular S-type sensor, the cantilever sensor, the sensor plate ring, linear, hysteresis, repeatability, and temperature sensitivity effects of creep, and other key indicators vast majority of manufacturers are is better than 0.05%, the majority of manufacturers better than 0.03%, some manufacturers better than 0.02%. Its integrated single sensor error of close to or better than 0.1%. Multi-sensor combination only after its integrated error even smaller. It can be said that the general load cell manufacturers of products to meet the requirements.
????? Previously, many manufacturers stirring floor, its provisions static accuracy of the weighing system, respectively 0.1% and 0.3% respectively. Thus, the use of single-sensor weighing only, individual indicators of 0.05% of the sensor will satisfy 0.3% precision weighing requirements. The use of three over the weigh-sensor, single indicators 0.05% sensors can also meet 0.1% accuracy requirements.
???? It must be pointed out is that the accuracy of the weighing system is static precision, while the concrete is the national standards require dynamic accuracy, which is due to the constant weighing of raw materials for the material in the body of gravity weighing the impact of errors increased significantly. Above 0.1% and 0.3% respectively, the ability of static accuracy reached 1% and 2% of the dynamic accuracy for feeding system with the design.
????? Now, many manufacturers provisions of the accuracy of its weighing system in the 0-rated weighing that money within the cement, water, additives ± 1%, sand, stone ± 2%. Notes, ready-mixed concrete national standards require that physical measurement accuracy, and general measurement instruments 0 ~ 20% of the relative error greater, or concrete mixing plant (station) provides the industry standard in the 20 ~ 100% of the weighting of the meet cement, water, additives ± 1%, sand, stone ± 2% of the more practical measures to allow some deviation. A more reasonable approach is the use of precision weighing instrument tagging industry terminology to express that as a cumulative hopper scales should be used in automatic weighing accuracy of the grading marks 1.0 and 2.0. Such scale test project in line with the purposes, including both materials testing (determining the accumulated error) also includes static test.
?? Automatic Weighing at its maximum allowable error in Table 1.
Criteria used in a static load test and non-automatic weighing, should meet the requirements in Table 2.
????? Usually 1.0 weigh design for the 1000 points, 2.0 Balance of design for 500 points. We can see that the range in the high-1.0 scale error is 0.15%, 2.0 Balance of error is 0.3%, not manufacturers than before provisions of 0.1% and 0.3% static high accuracy. But in the low range of accuracy is indeed increased. Under such circumstances the need to confirm the selection in the low range of the sensor can also meet the requirements.
???? According to weigh the degree of choice of the simplest sensor is 1,000 points method of selection of 1000 at the weigh-in sensors, 500 hours of the weigh-selection 500 points of the sensors. National standards in weighing sensor accuracy is to use the degree to express, but because of various reasons, most production plants that still use single indicator sensor accuracy. According to individual users to calculate indicators integrated precision error. Choose up trouble point.
????? The concrete mixing of foreign developed countries floor general with precision weighing system to the degree that, in the 1000 Indexing. They may be related to the use of high-performance concrete high proportion of the.
????? As the economy and the continuous development of technology, ultra-high-rise building, long bridges, large-scale water conservancy projects, as well as other exposed to the harsh environment in the construction of concrete performance raised higher and higher demands, concrete technology has also entered the high-tech age, the proportion of high-performance concrete continuous improvement. In addition to producing high-performance concrete correct selection of raw materials, determine a reasonable process parameters, the construction technology of control is also very important. Stir ingredients in the concrete floor, the accuracy of the system is important.
3.3 sensor structure forms of choice
????? Commonly used a pull-type S-shaped sensors, the plate center ring and ring cruciate tendons, such as plate. Cross center ring reinforcement plate high accuracy sensors, anti - partial contains excellent performance, but higher prices are normally only used in high-precision measurement occasions. Mixing upstairs is commonly used S-type sensors and sensor plate Central. S-type sensor which its high accuracy, anti - partial-carrying capacity, at the same time can bring overload protection, the wide range of use of the advantages of the largest.
???? Popular pressure sensor of a cantilever beam, Spoke, column, bridge, such as ring-twisting. Considered accuracy, range of installation, price and other factors, the vast majority of production plants to choose stirring floor of the cantilever.
3.4 load type of consideration
???? Concrete production mainly uses sand, gravel, cement and water, additives, admixture. Several materials weigh the impact of a few stone as the greatest. In the large family of electronic scales, such is not the greatest impact, the sensor can generally bear overload situation.
????? From the author of the survey, the sensor overload and damage due to the situation from time to time. For example, a control system fault, causing heavy material disposal and the next, causing overload. Also the use of human factors during the overload, especially the small range sensor for the operator stampede Scale Frame and overload damage things happen from time to time. Therefore sensor, sensors whether Weighing System Overload Protection for the reliable operation of a certain influence.
Sensor performance indicators have two with this, first allowing overload, first limit overload.
Allow overload refers to shed the load, sensor performance indicators unchanged. Limit overload refers to the load sensor does not produce harmful permanent mechanical deformation.
???? General overload sensor allows for 150%, and overload limit in 200% ~ 300%. Some of the sensor with overload protection are likely to exceed the scope of one. Sensors such as Putian factory CFCKN-1 - because of their special sensor design, allowing the load as high as 500%. Such frequent overload sensor in the case can reliably work.
3.5 sensor protection rating
???? Sensor IP protection levels that are usually used. General sensor manufacturers said their products have reached IP67 level, a small number of manufacturers of some products to IP68 level.
????? We know that the IP code is expressed by not more than 72.5 kV voltage electrical products protective shell of grading. National standards "GB4208-93 shell protection levels", IP67 means the product can dust, and against short-term immersion; IP68 means the product can dust, and against continued diving impact. It must be pointed out is that such protection does not include mechanical damage, corrosion, or external influences such as humid environmental conditions. Usually involving such protection provided by the relevant product standards. Weighing sensors and general electrical products and secondary products of different instruments, it is also a force component in the operation of constantly being produced and the role of deformation. There is also likely to be shock, impact or impact such a mechanical damage, dust and water vapor to the more common under severe corrosion and in extremely humid conditions running. This is GB4208-93 under the test conditions are quite different.
???? Have encountered such a situation: a weigh-ore mines on selected imports sensor, a large steel ladle weigh on the selection of an internationally renowned brands sensors, are welded, sealed and very beautiful, to IP68 level. But in actual life very short scene. In the last resort circumstances, the trial of Putian sensor plant products. The accident came as the result of their service life in excess of imports sensor. One in a large steel ladle weigh on the sensor has been in operation for seven years, are still running.
In these two examples, the fact is the protection of distinction. First, there were still no shell casing; Second, weld design. Third, sealing materials.
???? Generally speaking, the design is better than no shell casing design; The design of the weld seam should be avoided as far as possible by force. In the unavoidable circumstances, will have to check weld strength, in particular the fatigue strength. Fatigue cracks are caused minor tidal lead entry, the important reasons for sensor failure. The aforementioned welding of a world-renowned brand sensor at the scene sealed the reasons for the short life of excessive reliance on the welding is sealed, the internal strain tablets only a thin plastic seal level, once weld cracks, moisture sensor on the role in the rapid failure. In sealing materials need to be reminded that, through IP67 half-hour soaking test sensors may not be able to know, sensor-standard damp heat in the 12-cycle test.
???? Subject to the impact of price competition, many canceled sensor plant protection frames, pull-type sensor is even worse. The current domestic production of S-type sensor, the sensor plate Central almost canceled shell.
???? Indeed, on many occasions there is nothing wrong with doing so. However, in such a concrete mixing plant environmental conditions, may not be the best choice. I plant for the design of the Three Gorges Project mixing plant production CFCKN-1 Case closure welding sensor service life up to 10 years. We have seen the United States and Japan upstairs Europe concrete mixing with the pull-type sensor is with the protection of the majority of the shell. They think that should be a semi-permanent sensor devices, life should not be less than 106 times.
??? Years of practice has proved that, CFCKN-1-S-type sensor and sensor ordinary compared to a minimum the following advantages:
① shell thickness, and against mechanical damage, in particular the strong anti-collision outside;
② weld depth, corrosion resistance to wear a long time;
③ overload protection gap is within the shell, do not dust or debris accumulated by blocking;
Weighing sensor design is a reasonable shell weighing device "solid and reliable, stable performance," an important guarantee for selection, must be seriously considered.
360 different types of mixing the selection of load cell breakdown
???? Large and medium-sized water conservancy projects of concrete mixing plant, urban commercial concrete mixing plant, small irrigation works and highway construction in the county used in the concrete mixing plant size specifications, the use of continuous-time, the installation locations environmental conditions are very different from the production of concrete performance requirements are very different . Therefore, the reliability of the sensor, the protective properties in grades also have different requirements, should be treated differently. Long hours of continuous operation, harsh environmental conditions and the production of high-performance concrete mixing plant should choose reliability, a high level of protection sensors. The remaining requirements can be appropriately reduced. For running on the concrete mixing vessel at sea, it is also necessary to consider the request of anti-salt spray, not individually elaborate here on the.