飼料稱量包裝的準(zhǔn)確與否將直接影響到企業(yè)的信譽(yù)和經(jīng)濟(jì)效益。過去采用機(jī)械稱量、人工裝袋，勞動(dòng)強(qiáng)度大、速度慢、精度低。KULICKE & SOFFA 05100-4200-000-45近幾年，采用電子稱量裝置雖然可使其靜態(tài)稱量精度大大提高，但在飼料加工連續(xù)生產(chǎn)過程中，其動(dòng)態(tài)精度仍不能保證。因此，在快速自動(dòng)稱量中如何提高動(dòng)態(tài)稱量精度，一直是飼料加工企業(yè)急需解決的難題。
本文是作者在研制飼料自動(dòng)化生產(chǎn)設(shè)備中，為了與配料過程相協(xié)調(diào)，實(shí)現(xiàn)飼料生產(chǎn)的全部自動(dòng)化，應(yīng)用PLC作為動(dòng)態(tài)稱量包裝測(cè)控設(shè)備，在硬件和軟件設(shè)計(jì)中采用了一些措施和動(dòng)態(tài)控制方法，較好地兼顧了稱量速度與精度的矛盾，實(shí)現(xiàn)了飼料連續(xù)生產(chǎn)中動(dòng)態(tài)稱重計(jì)量的精度。
一、KULICKE & SOFFA 05100-4200-000-45系統(tǒng)工作原理
用稱重傳感器、放大器和PLC組成測(cè)控系統(tǒng)來完成飼料的稱重、計(jì)量、包裝的生產(chǎn)工藝過程，該系統(tǒng)以PLC為核心，配以稱重傳感器、放大器、各種電動(dòng)執(zhí)行器和機(jī)械裝置，實(shí)現(xiàn)飼料的動(dòng)態(tài)在線稱重計(jì)量和包裝工作。
稱斗的上方是成品倉，該倉中的原料是來至飼料混合工序生產(chǎn)的飼料散狀成品料。成品倉下是一臺(tái)電動(dòng)機(jī)驅(qū)動(dòng)的螺旋進(jìn)料裝置，啟動(dòng)電動(dòng)機(jī)，則該成品倉中的粉狀飼料就隨著傳輸絞籠的旋轉(zhuǎn)而進(jìn)入稱斗中稱量。稱斗上裝有三個(gè)S梁式應(yīng)變式拉力傳感器，稱斗的重量信號(hào)直接由該傳感器組轉(zhuǎn)換成與之對(duì)應(yīng)的電壓信號(hào)，經(jīng)放大器把該電壓信號(hào)放大后送入PLC中進(jìn)行數(shù)據(jù)處理，當(dāng)達(dá)到預(yù)定值時(shí)，PLC控制停止下料，然后PLC控制開稱斗門，并控制打包機(jī)自動(dòng)裝袋后由傳送帶送出。于是，就完成了飼料稱量打包的自動(dòng)化過程。
二、KULICKE & SOFFA 05100-4200-000-45提高動(dòng)態(tài)稱重精度的硬件措施
該系統(tǒng)用三個(gè)拉力傳感器將飼料重量W變換為成線性關(guān)系的電壓信號(hào)Ux，并通過兩級(jí)放大器進(jìn)行放大。Un表示等效到放大器輸入端的噪聲和干擾電壓，ΔUi表示等效到輸入端的漂移電壓。設(shè)兩級(jí)放大器的放大倍數(shù)為A，則Uo=A(Ux+Un+ΔUi)=AUx+AUn+A·ΔUi式中的第2項(xiàng)主要影響靈敏度，第三項(xiàng)主要影響系統(tǒng)的精度。
1、影響傳感器的因素及解決辦法
傳感器輸出信號(hào)的穩(wěn)定性除決定傳感器本身的性能外，還與供電電源和傳感器的安裝有密切關(guān)系。本系統(tǒng)采用UH61-100u型三只稱重傳感器，每只傳感器單獨(dú)供電，通過調(diào)節(jié)其橋路電壓使三只傳感器的輸出靈敏度K相同，三只傳感器串聯(lián)輸出的電壓為Ux=K(E1+E2+E3)。為了提高每個(gè)傳感器供橋電源的穩(wěn)定性采用二次穩(wěn)壓，并對(duì)元器件進(jìn)行老化、測(cè)試后進(jìn)行選配，特別是對(duì)基準(zhǔn)穩(wěn)壓管2DW233的老化處理和時(shí)漂測(cè)試，選擇時(shí)漂小的通過調(diào)節(jié)其工作電流使其工作在接近零溫度系數(shù)（<2ppm/℃）下，使整個(gè)傳感器電源的溫度穩(wěn)定性優(yōu)于10ppm/℃。三只拉力傳感器安裝在稱斗和稱架之間。如果傳感器承受的重量與傳感器軸線存有ɑ角，則將產(chǎn)生橫向分力而引入誤差ΔW=W-Wcosɑ.這對(duì)于每次稱重25Kg，在稱斗皮重為100Kg的情況下，即三只100Kg的傳感器實(shí)際荷重為125Kg。當(dāng)ɑ=4°時(shí)，稱重誤差就為0.43Kg。因此，安裝傳感器時(shí)應(yīng)設(shè)法確保傳感器都能垂直受力。
2、KULICKE & SOFFA 05100-4200-000-45影響系統(tǒng)靈敏度的因素及解決辦法
影響系統(tǒng)靈敏度的主要因素是檢測(cè)電路的內(nèi)部噪聲和外部干擾電壓Un，它與放大器所工作的頻帶相關(guān)。在研制中，通過選擇低噪聲器件，在滿足采集速度所需足夠?qū)挼念l帶的前提下，通過選配電阻來提高放大電路本身的共模抑制能力，整個(gè)檢測(cè)系統(tǒng)采用雙層屏蔽，采樣時(shí)間選為工頻周期整數(shù)倍等項(xiàng)措施，使整個(gè)系統(tǒng)獲得了能分辨5g重量的靈敏度。
3、影響準(zhǔn)確度的主要因素及解決辦法
影響準(zhǔn)確度的主要因素是整個(gè)檢測(cè)系統(tǒng)的非線性和漂移ΔUi。其中系統(tǒng)的非線性，在選配元器件校正的基礎(chǔ)上，采用了軟件修正；而對(duì)于隨溫度和時(shí)間產(chǎn)生的漂移電壓ΔUi，主要采用元器件的老化、測(cè)試與分選工藝，篩選掉時(shí)漂大的，然后選配溫度系數(shù)進(jìn)行補(bǔ)償，使整個(gè)系統(tǒng)的靜態(tài)精度達(dá)到了0.07%，為實(shí)現(xiàn)動(dòng)態(tài)稱量精度奠定了基礎(chǔ)。
三、KULICKE & SOFFA 05100-4200-000-45提高動(dòng)態(tài)稱重精度的軟件措施
影響動(dòng)態(tài)稱重精度的主要因素是被稱物料的比重、流量和落差的大小，它是由成品料倉的壓力和PLC所控制的進(jìn)料驅(qū)動(dòng)裝置產(chǎn)生的。因此，改進(jìn)控制思路，借鑒靜態(tài)稱量精度高的特點(diǎn)是提高動(dòng)態(tài)計(jì)量精度的關(guān)鍵。為此，我們選用雙速變徑變距螺旋加料機(jī)，采用"先快后慢，最后點(diǎn)動(dòng)"的控制下料方式，如圖2所示。圖中：Wx0為稱量前PLC所采集的稱斗皮重；由此PLC按照每包計(jì)量凈重量的90%、95%和100%算出快速下料的終了值Wx1、慢速下料終了值Wx2和稱量終了值Wx。其控制過程可以這樣簡單說明：在下料開始一段時(shí)間，PLC控制絞籠電動(dòng)機(jī)快速下料，當(dāng)檢測(cè)達(dá)到快速下料的終了值Wx1時(shí)，PLC控制絞籠電動(dòng)機(jī)開始慢速下料；當(dāng)檢測(cè)達(dá)到慢速下料的終了值Wx2時(shí)，PLC關(guān)閉絞籠電動(dòng)機(jī)后采用"點(diǎn)動(dòng)"下料，當(dāng)達(dá)到或接近期望值Wx時(shí)為止。
按照上述思路，通過編寫全動(dòng)態(tài)控制加料的快慢、速加料和點(diǎn)動(dòng)下料的軟件模塊；以及為消除空中落料對(duì)稱量精度的影響，所編寫的自動(dòng)尋找提前停機(jī)量的軟件模塊等軟件措施。該系統(tǒng)使用一年來，經(jīng)計(jì)量部門兩次測(cè)試，整個(gè)系統(tǒng)動(dòng)態(tài)計(jì)量準(zhǔn)確度優(yōu)于0.2%。
四、KULICKE & SOFFA 05100-4200-000-45結(jié)論
采用PLC控制進(jìn)行飼料稱重包裝，具有結(jié)構(gòu)簡單、計(jì)量準(zhǔn)確、工作可靠，較好的兼顧了動(dòng)態(tài)稱重計(jì)量的精度和速度，滿足了在線快速動(dòng)態(tài)重量計(jì)量的要求，對(duì)水泥、食鹽、面粉等的稱重包裝有借鑒作用和推廣價(jià)值。
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Details about BENTLY NEVADA 3300 XL 8MM/330130-080-00-00 NEW
RELIANCE ELECTRIC 22492-29T USPP 2249229T
Details about FURNAS ELECTRIC CO 3RT1033-1AK60 FNFP 3RT10331AK60
Details about GENERAL ELECTRIC CR106D000BWA USPP CR106D000BWA
Details about CATERPILLAR 119-4740 NSFP 1194740
Details about ASCO USF8257A001 NSPP USF8257A001
Details about VERIS H8041-0800-3 USPP H804108003
Details about WIX FILTERS 33358 NSFP 33358
Details about FURNAS ELECTRIC CO 50D26699007 NSPP 50D26699007
