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　　電阻應(yīng)變式稱重傳感器是基于這樣一個原理：彈性體(彈性元件，敏感梁)在外力作用下產(chǎn)生彈性變形，使粘貼在他表面的電阻應(yīng)變片(轉(zhuǎn)換元件)也隨同產(chǎn)生變形，電阻應(yīng)變片變形后，它的阻值將發(fā)生變化(增大或減小)，再經(jīng)相應(yīng)的測量電路把這一電阻變化轉(zhuǎn)換為電信號(電壓或電流)，從而完成了將外力變換為電信號的過程。

　　由此可見，電阻應(yīng)變片、彈性體和檢測電路是電阻應(yīng)變式稱重傳感器中不可缺少的幾個主要部分。下面就這三方面簡要論述稱重傳感器工作原理。

　　稱重傳感器原理圖

　　一、傳感器電阻應(yīng)變片

　　電阻應(yīng)變片是把一根電阻絲機械的分布在一塊有機材料制成的基底上，即成為一片應(yīng)變片。他的一個重要參數(shù)是靈敏系數(shù)K。我們來介紹一下它的意義。

　　設(shè)有一個金屬電阻絲，其長度為L，橫截面是半徑為r的圓形，其面積記作S，其電阻率記作ρ，這種材料的泊松系數(shù)是μ。當(dāng)這根電阻絲未受外力作用時，它的電阻值為R：

　　R = ρL/S(Ω) (2—1)

　　當(dāng)他的兩端受F力作用時，將會伸長，也就是說產(chǎn)生變形。設(shè)其伸長ΔL，其橫截面積則縮小，即它的截面圓半徑減少Δr。此外，還可用實驗證明，此金屬電阻絲在變形后，電阻率也會有所改變，記作Δρ。

　　對式(2--1)求全微分，即求出電阻絲伸長后，他的電阻值改變了多少。我們有：
　　ΔR = ΔρL/S + ΔLρ/S –ΔSρL/S2 (2—2)
　　用式(2--1)去除式(2--2)得到
　　ΔR/R = Δρ/ρ + ΔL/L – ΔS/S (2—3)
　　另外，我們知道導(dǎo)線的橫截面積S = πr2，則 Δs = 2πr*Δr，所以
　　ΔS/S = 2Δr/r (2—4)
　　從材料力學(xué)我們知道
　　Δr/r = -μΔL/L (2—5)
　　其中，負(fù)號表示伸長時，半徑方向是縮小的。μ是表示材料橫向效應(yīng)泊松系數(shù)。把式(2—4)(2—5)代入(2--3)，有 　　ΔR/R = Δρ/ρ + ΔL/L + 2μΔL/L =(1 + 2μ(Δρ/ρ)/(ΔL/L))*ΔL/L = K *ΔL/L (2--6)
　　其中
　　K = 1 + 2μ +(Δρ/ρ)/(ΔL/L) (2--7)
　　式(2--6))說明了電阻應(yīng)變片的電阻變化率(電阻相對變化)和電阻絲伸長率(長度相對變化)之間的關(guān)系。

　　需要說明的是：靈敏度系數(shù)K值的大小是由制作金屬電阻絲材料的性質(zhì)決定的一個常數(shù)，它和應(yīng)變片的形狀、尺寸大小無關(guān)，不同的材料的K值一般在1.7—3.6之間;其次K值是一個無因次量，即它沒有量綱。

　　在材料力學(xué)中ΔL/L稱作為應(yīng)變，記作ε，用它來表示彈性往往顯得太大，很不方便

　　常常把它的百萬分之一作為單位，記作με。這樣，式(2--6)常寫作： ΔR/R = Kε (2—8)

　　二、稱重傳感器彈性體的原理

　　彈性體是一個有特殊形狀的結(jié)構(gòu)件。它的功能有兩個，首先是它承受稱重傳感器所受的外力，對外力產(chǎn)生反作用力，達(dá)到相對靜平衡;其次，它要產(chǎn)生一個高品質(zhì)的應(yīng)變場(區(qū))，使粘貼在此區(qū)的電阻應(yīng)變片比較理想的完成應(yīng)變棗電信號的轉(zhuǎn)換任務(wù)。

　　以托利多公司的SB系列稱重傳感器的彈性體為例，來介紹一下其中的應(yīng)力分布。

　　設(shè)有一帶有肓孔的長方體懸臂梁。

　　肓孔底部中心是承受純剪應(yīng)力，但其上、下部分將會出現(xiàn)拉伸和壓縮應(yīng)力。主應(yīng)力方向一為拉神，一為壓縮，若把應(yīng)變片貼在這里，則應(yīng)變片上半部將受拉伸而阻值增加，而應(yīng)變片的下半部將受壓縮，阻值減少。下面列出肓孔底部中心點的應(yīng)變表達(dá)式，而不再推導(dǎo)。

　　ε = (3Q(1+μ)/2Eb)*(B(H2-h2)+bh2)/ (B(H3-h3)+bh3) (2--9)
　　其中：Q--截面上的剪力;E--揚氏模量：μ—泊松系數(shù);B、b、H、h—為梁的幾何尺寸。

　　需要說明的是，上面分析的應(yīng)力狀態(tài)均是“局部”情況，而應(yīng)變片實際感受的是“平均”狀態(tài)。

　　三、檢測電路原理

　　檢測電路的功能是把電阻應(yīng)變片的電阻變化轉(zhuǎn)變?yōu)殡妷狠敵?。因為惠斯登電橋具有很多?yōu)點，如可以抑制溫度變化的影響，可以抑制側(cè)向力干擾，可以比較方便的解決稱重傳感器的補償問題等，所以惠斯登電橋在稱重傳感器中得到了廣泛的應(yīng)用。

　　因為全橋式等臂電橋的靈敏度最高，各臂參數(shù)一致，各種干擾的影響容易相互抵銷，所以稱重傳感器均采用全橋式等臂電橋。

　　其實稱重傳感器原理上就是壓力傳感器，形狀不一樣而已，通常有很多種方法傳感的，但我見到，用得比較多，比如地磅用的那些，一般為電渦流式。

　　也就是說，他有一個電渦流觸發(fā)繞組，然后還有一個傳感器感應(yīng)電渦流強度。

　　由于這個傳感器整體是金屬封裝，電渦流在其內(nèi)部，受到壓后形變，渦流就發(fā)生變化，放大后就可以讀到數(shù)據(jù)了。

　　然后，封裝這個東西的材料，通常選用剛性材料， 總之，就是一般的金屬，比如鋼，但肯定不會用很軟的東西的。至少電渦流方式傳感的壓力傳感器，是不會用軟金屬制造的。

　　因為即使是鋼，就算受到壓力形變那么幾微米，那么電渦流的變化也足夠感應(yīng)出到底變化了多少而且如果是軟金屬，稱很重東西的時候，可能很容易出問題。

　　稱重傳感器的使用知識

　　隨著技術(shù)的進(jìn)步,由稱重傳感器制作的電子衡器已廣泛地應(yīng)用到各行各業(yè),實現(xiàn)了對物料的快速、準(zhǔn)確的稱量，特別是隨著微處理機的出現(xiàn)，工業(yè)生產(chǎn)過程自動化程度化的不斷提高，稱重傳感器已成為過程控制中的一種必需的裝置，從以前不能稱重的大型罐、料斗等重量計測以及吊車秤、汽車秤等計測控制，到混合分配多種原料的配料系統(tǒng)、生產(chǎn)工藝中的自動檢測和粉粒體進(jìn)料量控制等，都應(yīng)用了稱重傳感器，目前，稱重傳感器幾乎運用到了所有的稱重領(lǐng)域。

　　本系統(tǒng)由微機控制稱重傳感器的稱重和比較，并輸出控制信號，執(zhí)行定值稱量，控制外部給料系統(tǒng)的運轉(zhuǎn)，實行自動稱量和快速分裝的任務(wù)。

　　系統(tǒng)采用MCS-51單片機和V/F電壓頻率變換器等電子器件，其硬件電路框圖如圖1所示，用8031作為中央處理器，BCD拔碼盤作為定值設(shè)定輸入器，物料裝在料斗里，其重量使傳感器彈性體發(fā)生變形，輸出與重量成正比的電信號，傳感器輸出信號經(jīng)稱重變送器放大后，輸入V/F轉(zhuǎn)換器進(jìn)行A/D轉(zhuǎn)換，變送成的頻率信號直接送入8031微處理器中，其數(shù)字量由微機進(jìn)行處理。微機一方面把物重的瞬時數(shù)字量送入顯示電路，顯示出瞬時物重，另一方面則進(jìn)行稱重比較，開啟和關(guān)閉加料口和變送器，放料于箱中等一系列的稱重定值控制。

　　傳統(tǒng)概念上，負(fù)荷傳感器是稱重傳感器、測力傳感器的統(tǒng)稱，用單項參數(shù)評價它的計量特性。舊國標(biāo)將應(yīng)用對象和使用環(huán)境條件完全不同的“稱重”和“測力”兩種傳感器合二為一來考慮，對試驗和評價方法未給予區(qū)分。舊國標(biāo)共有21項指標(biāo)，均在常溫下進(jìn)行試驗;并用非線性、滯后誤差、重復(fù)性誤差、蠕變、零點溫度附加誤差以及額定輸出溫度附加誤差6項指標(biāo)中的最大誤差，來確定稱重傳感器準(zhǔn)確度等級，分別用0.02、0.03、0.05......1.0表示。

　　稱重傳感器的原理是在被測物體上的重力按一定比例轉(zhuǎn)換成可計量的輸出信號?？紤]到不同使用地點的重力加速度和空氣浮力對轉(zhuǎn)換的影響，稱重傳感器的性能指標(biāo)主要有線性誤差、滯后誤差、重復(fù)性誤差、蠕變、零點溫度特性和靈敏度溫度特性等。在各種衡器和質(zhì)量計量系統(tǒng)中，通常用綜合誤差帶來綜合控制傳感器準(zhǔn)確度，并將綜合誤差帶與衡器誤差帶(圖1)聯(lián)系起來，以便選用對應(yīng)于某一準(zhǔn)確度衡器的稱重傳感器。國際法制計量組織(OIML)規(guī)定，傳感器的誤差帶δ占衡器誤差帶Δ的70%，稱重傳感器使用知識的線性誤差、滯后誤差以及在規(guī)定溫度范圍內(nèi)由于溫度對靈敏度的影響所引起的誤差等的總和不能超過誤差帶δ。這就允許傳感器制造廠對構(gòu)成計量總誤差的各個分量進(jìn)行調(diào)整，從而獲得期望的準(zhǔn)確度。

　　剪切梁式稱重傳感器原理是彈性體受力的作用后，需要測量的不是其正應(yīng)力，而是由剪切力引起的切應(yīng)力。但切應(yīng)力本身是測量不出來的，它只能產(chǎn)生于與工字梁中心軸 線成45“的互相垂直的主應(yīng)力，也就是產(chǎn)生于由切應(yīng)力而引起的拉伸應(yīng)力及壓縮應(yīng)力。因此，將4片應(yīng)變計分別貼在工字梁腹板的兩面，并與中心軸線成45。的 相互垂直的位置上，如圖1所示。這4個應(yīng)變計組成全橋，當(dāng)傳感器承受載荷時，應(yīng)變計R,,R3的電阻值增大，R2,R;的電阻值減小，其結(jié)果在電橋的對角 線上產(chǎn)生與載荷成正比的不平衡輸出。利用這一原理，便可測出力或載荷的大小。

　　2005年期間，某電廠#4爐給煤機稱重傳感器故障率較高，有一個傳感器輸出信號超出使用范圍0-30mV，有一個輸出信號時而超出使用范圍，時而在正常范圍內(nèi)，最近，又有兩個稱重傳感器輸出信號不隨稱重變化發(fā)生明顯變化的現(xiàn)象，工作人員認(rèn)為問題比較嚴(yán)重，做初步分析，可能原因如下：

　　該時期機組負(fù)荷高，煤質(zhì)差，煤量大，給煤機稱重傳感器長時間處在高負(fù)載的運行工況下，同時給煤機內(nèi)部濕度較大，有時溫度稍高，可能超過60℃(稱重傳感器工作溫度不超過60℃)，不同程度影響使用壽命。

　　從#4爐密封風(fēng)機改造后，故障率比改造前明顯增加，懷疑改造后給煤機內(nèi)部工作環(huán)境發(fā)生變化，影響稱重傳感器的使用壽命。

　　經(jīng)咨詢廠家，建議將給煤機內(nèi)部的出煤擋板降低，減少出煤量，稱重傳感器負(fù)載相對降低，為保證目標(biāo)給煤量，給煤機的轉(zhuǎn)速自然會提高，將目前的高負(fù)載低轉(zhuǎn)速改為低負(fù)載高轉(zhuǎn)速運行方式，會延長重量傳感器的使用壽命。

　　該電廠采取的措施是將壞稱重傳感器切除，維持單只給煤機傳感器運行。

　　#4機組小修進(jìn)入第2天，認(rèn)真進(jìn)行鍋爐熱風(fēng)門執(zhí)行機構(gòu)檢修和給煤機稱重傳感器更換工作，截止目前，三臺磨煤機熱風(fēng)門執(zhí)行機構(gòu)全部拆除完畢，A1、A2給煤機4臺稱重傳感器更換接線完畢，B1給煤機2臺稱重傳感器已經(jīng)更換，B2給煤機拆除2臺原稱重傳感器。

　　三臺熱風(fēng)門執(zhí)行機構(gòu)位于12米平臺下方，位置狹小，灰塵多，光線暗，檢修小組克服困難，首先在集控室將熱風(fēng)門全部關(guān)閉，然后通過爬梯到達(dá)檢修場地，清掃積聚的灰塵，打開定位器端蓋，記錄接線端子編號與對應(yīng)的端子，解除接線用絕緣膠布包好，拆除氣源管路，用白膠布包好敞開的管口，松開固定底座上的四個螺絲并保管好，抬好執(zhí)行機構(gòu)并使之脫離底座，用準(zhǔn)備好的繩索系牢靠后，慢慢下放到地面，最后用三輪車?yán)睫k公樓，克服執(zhí)行機構(gòu)笨重等困難，一鼓作氣抬到二樓檢修間進(jìn)行檢修。

　　給煤機稱重傳感器更換工作進(jìn)展順利，首先清除煤灰，拆開保護(hù)罩，松開銷子和壓緊螺絲帽，去除稱重托輥，接著松開固定傳感器的四個螺絲，特別是靠近皮帶的里側(cè)2個螺絲，由于生銹特別難拆，尤其是接近下班時間，看不到螺絲，手電因為已經(jīng)使用一天不能發(fā)亮，只能憑手感覺，本著高度的責(zé)任心和高超的技能，盡管煤粉弄黑了衣袖和雙手，松動劑帶著煤粉刺了一身，大家沒有一句怨言，沒有一個退卻，心中只有一個信念：更換給煤機傳感器，讓入爐煤計量準(zhǔn)確，以便為供電煤耗等指標(biāo)的計算提供可靠依據(jù)。