Zhang　Ying-zi,　Chen　Hai-tao,　Hou　Shou-yin,　Ji　Wen-yi,　Ouyang　Bin-lin,　Dun　Guo-qiang,　and　Zhang　Ji-cheng

College　of　Engineering,　Northeast　Agricultural　University,　Harbin　150030,　China

Design and Experiment of Slave Computer Control System for Applying Variable-rate Liquid Fertilizer

ZhangYing-zi,ChenHai-tao*,HouShou-yin,JiWen-yi,OuyangBin-lin,DunGuo-qiang,andZhangJi-cheng

CollegeofEngineering,NortheastAgriculturalUniversity,Harbin150030,China

Inordertoincreasetheapplyingrateofliquidfertilizerandreduceenvironmentalpollution,aslavecomputercontrol systemforapplyingvariable-rateliquidfertilizerwasdesigned.ThesystemusedSMCascoreprocessorandelectricallycontrolled pressureregulatorasexecutioncomponent.Thecharacteristicequationofthesystemwasobtainedbyusingclassicalcontroltheory. Resultsindicatedthatthecharacteristicequationmettherequirementsofrouth-criterion,whichindicatedtheworkingprocessofthe systemwasstable.Performanceoftheslavecomputerwasverifiedviabenchtests.Resultsdemonstratedthattherewasnosignificant influenceontheresponsefrominterclasserror.Thefertilizationerrorwaslessthan0.9,andthefertilizationaccuracywaslargerthan 97%.Theliquidfertilizeremittedbythefertilizingdeviceshadnosignificantdifferenceinuniformity,whichmetthedemandsofthe slavecomputercontrolsystemforapplyingvariable-rateliquidfertilizer.

variable-ratefertilization,controlsystem,mathematicalmodel

Introduction

Inordertohavehigherrateoffoodproduction,many peasanthouseholdsusefertilizersandpesticides blindly,whichcanresultinaseriesofproblems,such asawasteofresourcesandagriculturalecological environmentpollution(Shiet al.,2014;Weiet al., 2013;Fanet al.,2014).ItmaymakesensetoagriculturaldevelopmentinChina,ifstudiesarecarried outonhowfertilizerscanbeusedinareasonable way.Variable-ratefertilizationisdesignedtodivide thewholeplotintosmalloperatingunitsandfertilize withdifferentproportionsofthefertilizers,basedon diversityofthesoilfertilityandcropgrowth(Liu et al.,2012;Liet al.,2010).

Atpresent,variable-ratefertilizationtechnology isstillinlabdevelopingstageinChina.Basedon practicalneedsandcurrenttechnicallevels,anindepthresearchonvariable-ratefertilizationcontrol systemisconducted.Theslavecomputerisaction executioncomponentofthecontrolsystemthat performsoperationsaccordingtotheoperation informationfrommastercomputerviacommunication module.Atheoreticalandexperimentalresearchwas conductedonthefertilizationcontrollerintheslave computercontrolsystem.

Materials and Methods

Equipment and instrument

FertilizationcontrollermanufacturedbyNortheastAgriculturalUniversitywasusedascontroldevice; electricallycontrolledpressureregulatorwithstyle BC44BRL-24-03Dwasusedasexecutiondevice; three-phaseasynchronousmotorwithstyleY100L2-4 wasusedasoutputdeviceofpower;highpressurepowerfertilizationpumpwithstyle3WZ-70 producedbyShanghaiSuNongPump,Inc.was usedasinputdeviceforliquidfertilizer;thepressure transmitterwithstyleTS-800-FproducedbyShanghai MeterFactoryandflowmeterwithstyle801produced byPuriSpraySystem,Inc.wereusedasfeedback device.

Test for uniformity of fertilization

Theimportantperformanceindexoftheslavecomputercontrolsystemwastheuniformityofthefertilization.Theuniformityoffertilizationwasverified bytest.Workingstatusoftheenginewassettoafixed condition.Internalsystempressurewasadjustedto 0.2MPa.Whenthecontrolsystemwasstable,four exitsofliquidfertilizerwereselectedastestingpoints fortheuniformitytestofthefertilization.Thefluxof eachexitwasrecordedforthesameperiodoftimeby usingmeasuringglass.Theexperimentwasrepeated forthreetimes.

Test for precision of fertilization

Theimportantperformanceindexoftheslave computercontrolsystemwastheprecisionofthe fertilization.Theprecisionofthefertilizationwas verifiedbytest.Workingfrequencyoftheconverter wassetto40Hz.Internalsystempressurewas adjustedto0.2MPa.Fertilizationcontrollerreceived 10setsoftargetfertilizingamountdatafrommaster PCandeachsetofrealfertilizingamountdatawas recorded.Contrastedthetargetfertilizingamountdata withrealfertilizingamountdataandthencalculated theprecisionofthefertilization.

Results

ResultsoftheuniformityofthefertilizationexperimentsareshowninTable1andresultsofANOVAfor uniformityofthefertilizationexperimentsareshown inTable2.Forthegivensignificantlevelof0.01,the valueofProb＞Flessthan0.01indicatedthattherewas nosignificantdifferenceintheflowofthefourexitsof theliquidfertilizer.

Table 1　Test factors and data

Table 2　Analyses of variance

Resultsoftheprecisionofthefertilizationexperiments areshowninTable3andresultsofANOVAforprecision ofthefertilizationexperimentsareshowninTable4. AsshowninTable3,fertilizationaccuracycouldreachatleast97.00%.Thevalueoftheminimumerrorwas 0.1,andthevalueofmaximalerrorwas0.9.Asshown inTable4,forthegivensignificantlevelof0.01,the valueofProb＞Flessthan0.0001indicatedthatinterclass errorhadnosignificantinfluenceontheresponseto fertilizationaccuracy.Theresultsindicatedthatthe hardwareandsoftwaredesignsoftheslavecontrol systemwerereasonable.

Table 3　Test factor and data

Table 4　Analysis of variance

Discussion

Operation theory of system

Theslavecomputercontrolsystemwasconsistedoffertilizationpump,fertilizationcontroller,electricregulating valve,pressuretransmitterandflowmeter(Fig.1).

Whileworking,powerproducerdrovethefertilizationpump.Thefertilizationpumptookouttheliquid fertilizerfromfertilizerbins.Signalofthetarget fertilizationvolumefrommastercomputerwassentto thefertilizationcontrollerviaserialcommunication. Thefertilizationcontrolleramplifiedthecontrolsignal, smoothedthefluctuation,calculatedandchangedthe sizeoftheoutputvoltageandchangedtheaperture ofthevalveport.Thefeedbacksignalofthevessel's liquidpressurefrompressuretransmitterwassentto thefertilizationcontrollerintheformoftheelectrical signal.Thefeedbacksignalofthevessel'sliquid flowfromflowmeterwassenttothefertilization controllerintheformoftheelectricalsignal.Itserved asthesystem'sfeedbackcorrectionsegmenttomake thecontrolsystemasaclosedloop.Therefore,the controlledfertilizervolumecouldbechangedaccording tothetargetfertilizervolumeandtheerrorcausedby thedisturbancecouldbecorrected.Thepurposeofthe intelligentautomaticfertilizervolumeadjustmentcould beachieved(Oliveret al.,2010;Wanget al.,2009; Yanget al.,2006).

Fertilizationcontrollerwasthecoreoftheslave computercontrolsystem.Inthisresearch,singlechip micyoco(SCM)withstyleSTC12C5412ADproducedbyAtmelCo.,Ltd.actedasthecoreoffertilization controlleranditsupperfrequencycouldreach35MHz. SCMwasequippedwithserialperipheralinterface(SPI), analogdigital(AD)andmanyotherperipheralmodules, whichgreatlysimplifieddesignofperipherycircuit. SCMcommunicatedwithA/DtransformerandD/A transformerviaSPIbus.Thefertilizationcontrollerwas usedforcomparingfeedbacksignalsofliquidpressure andflowanddrivingelectricregulatingvalve(Camp et al.,2000).Shiftregisterchip74HC595,digital-toanalogueconversionchipDAC0832,operational amplifierμA741wereusedforconstructingdriven circuit.OptoelectronicisolationchipTLP521andSchmitt trigger74HC14wereusedforconstructingdigitalsignal regulatorcircuit(Zhuet al.,2007;Yanget al.,2006). OperationalamplifierOP07andhigh-precisionvoltage chipTL431wereusedforconstructinganalogsignal regulatorcircuit,whichcollectedandprocessedsignal frompressuresensor(Yanget al.,2001;Lin,et al.,2006; Yanget al.,2004).

Fig. 1　Control system structure diagram

Fertilizationdatawastransferredfromparallelportof materPCtoslavePCSTC12C5412AD(SCM).Fertilizationcontrollertransformedfertilizationamount fromdigitaldatatoanalogvalueviausingD/Aconverter.Byamplifyingthedrivingfertilizationsignal, fertilizationcontrollercontrolledtheopeningdegree oftheelectro-hydraulicproportionalvalve.Theslave computercontrolsystemcontrolledtheflowofthe liquidfertilizerandachievedthegoalofthevariable controlofthefertilization.Feedbacksignalacquisition moduleincludedflowmeterandpressuretransmitter. Thismoduletransformedtheflowandpressurechange intovoltagechange.Throughtheprocessofthesignal amplificationandnoisereduction,A/Dconverter convertedtheanalogsignalintodigitalsignalandsent thesignalbacktoSCM.SCMadjustedtheopening degreeoftheelectricallycontrolledpressureregulating valveaccordingtothedeviationbetweenthecalculated targetfertilizationamountandtheactualamount(Li, 2006).

System stability analysis

Duringtheworkingprocess,flowmeterwasusedfor detectingtheflowoffertilizerpipeanddetermining whetherthefluxreachesthevaluesetbyPC.Ifnot, thisinformationwassentbacktoSCM.Incremental proportion-integration-differentiation(PID)algorithm wasusedforadjustingthecontrolsystemtoachievethe goalofprecisecontroleffectofthefertilization(Guo, 2006).Electricallycontrolledpressureregulatorwasthe executionunit.Fromprincipleoftheautomaticcontrol system,thedeviationequationwasshownasthe following:

Intheequationabove,tindicatedworkingtimeof slavecomputercontrolsystem;e(t)indicateddeviation;uo(t)indicatedthereferenceortargetvalue,whichwasthe presetfertilizationamount;u(t)indicatedsample,which wastheactualfertilizationamountinthiscase(Chen et al.,2011).PIDcontrollerdeviationequationwas shownasthefollowing:

Intheequationabove,ea(t)wasthedeviationafter correction;Kpwasaproportionalityfactor;Kiwas theintegrationtimeconstant;Kdwasthedifferential timeconstant.Controlvalueea(t)wasactuallylinear combinationofproportion,integrationanddifferentiation. Theslavecomputercontrolsystemwascarriedoutby electricallycontrolledpressureregulator.

Theslavecomputercontrolsystemmainlyconsisted offertilizationcontroller,electricallycontrolledpressure regulatoranddetectionfeedbackcomponents.Asthe componentofexecutionunit,electricallycontrolled pressureregulatorhaduniquepropertyandstructureand playedanimportantrole(Liet al.,2009;Xu,2009;Fan et al.,2014).Electricallycontrolledpressureregulator dynamicequationwasshownasthefollowing:

Intheequationabove,Va(t)wastheinputvoltageof theamplifiercircuit;Kawastheamplifierfactorofthe amplifiercircuit.TheLaplacetransformofequation (2)wascombinedwithequation(3).Wecouldgetthe transferfunctionoftheelectricallycontrolledpressure regulatorasthefollowing:

Intheequationabove,Va(s)wastheformofLaplace transformaftertransformedbyVa(t).Thissystemused theincrementalPIDalgorithm(Wanget al.,2008). Electricallycontrolledpressureregulatorwascontrolled by Δu(n).nwasthesamplenumber.n=0,1,2,…(Zhang et al.,2003).Wegotdifferenceequationshownasthe following:

Intheequationabove,nwasthemaximumnumberof samples; Δu(n)wastheincrement;e(n),e(n–1)ande(n–2)werethepreviouserror,thefirsttwoerrorsandthefirst threeerrors,respectively.Thiscontrolsystemhadtwo kindsofworkingmode.Closed-loopwasobtainedintest workingmode(Wang et al.,2004),soastodetermine thecriticalstabilityoscillationcycleTc=100andcritical oscillationfactorKc=20.Evaluatingexpressionswere shownasthefollowings:

Intheequationsabove,Tdwasthesamplingperiod;Tnwastheintegrationtime;andTvwasthederivativetime. WegotthatKpequaledto12,Kiequaledto240,andKdequaledto0.144.Thus,thetransferfunctionwasshown asthefollowing:

Theoutputofthefertilizationpumpwasfertilization amount.Theinputofthefertilizationpumpwasthe speedofmotor.Themodelcouldberegardedasafirstordersystemanditstransferfunctionwasshownasthe following:

Intheequationabove,K1wasthegainoffertilizer;and T wasthetimeconstant.Theinputformoffertilization pumpwasstepfunction.Theequationwasshownasthe following:

Intheequationabove,N(s)wastheLaplacecomplex variableformcorrespondington(t).Thefertilization amountequationwasshownasthefollowing:

Intheequationabove,Q(s)wasLaplacecomplex variableformcorrespondingtoq(t).Equation(13)was theinverseLaplacetransformcorrespondingtoequation (12).Weobtainedtherelationshipbetweenfertilization amountandtime.

Intheequationabove,q(t)wasthetimedomainform offertilizationamount.Whenthefertilizationpump workedstably,Kindicatedelectricrotatingspeedand equaledto250.InordertoobtainthetimeconstantT andfertilizergainK1,anexperimenthadbeendone accordingtoequation(13).ThoughMATLABcurve fittingfunctionscombiningwiththetestdata,wegot thatK1equaledto0.0029andT equaledto0.0001. Electricallycontrolledpressureregulatorandfertilizationpumpcharacteristicequationswereshownasthe followings:

Intheequationsabove,D1(s)andD2(s)represented electricallycontrolledpressureregulatorandfertilization pumpcharacteristicequations,respectively.

Eachscalefactorinengineeringapplicationswas positive.Itwasfoundthateachscalefactorinthefirst columnofrouth-listwaspositive,meetingrouth-criterion.Electricallycontrolledpressureregulatorand fertilizationpumpmathematicalmodelswerestable(Lang et al.,2013).

Conclusions

TheslavecomputercontrolsystemforapplyingvariablerateliquidfertilizerwithSMCasthecoredevicewas designedbyusingelectricallycontrolledpressure regulatorasexecutioncomponent.Thesystemachieved thepurposeofadjustingthefertilizingamountaccording tothegivenfertilizationsignalsautomatically.The performanceoftheslavecomputercontrolsystemwas verifiedviabenchtests.Theresultsshowedthatthe fertilizationerrorwaslessthan0.9,andthefertilization accuracywaslargerthan97%.Itwasdemonstrated thatinterclasserrorhadnosignificantinfluenceonthe response.Theliquidfertilizeremittedbythefertilizing deviceshadnosignificantdifferenceinuniformity,which metthedemandsoftheslavecomputercontrolsystem forapplyingvariable-rateliquidfertilizer.

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TP29 Document code: A Article ID: 1006-8104(2015)-02-0073-07

16January2015

SupportedbytheScienceandTechnologyResearchProjectofthe12thFive-yearPlan(2011BAD20B03-01)

ZhangYing-zi(1989-),female,Master,engagedintheresearchofagriculturalelectrificationandautomation.E-mail:18746048532@163.com

*.ChenHai-tao,professor,supervisorofPh.Dstudent,engagedintheresearchofagriculturalmechanizationengineering.E-mail: htchen@neau.edu.cn