基于CAV424的电容式液位检测系统(1)

:2010-02:(20070212):(1976),,,,,CAV424侯海岭1,解衍江2(1.天津科技大学电子信息与自动化学院,天津300222;2.河北省泊头市第二中学,河北泊头062150):介绍一种基于CAV424的电容式液位检测系统根据电容式液位传感器的工作原理,传感器两个极板间的电容值会随液位的改变而改变,系统利用电容/电压转换芯片CAV424将电容值转化为电压值输出,然后使用16位高精度-模数转换器AD7705再将电压值转换为数字信号,交由单片机对数据做进一步的处理,如数码管显示及电流输出电流输出部分由12位串行数模转换器AD7943和4~20mA的电压/电流转换器组成系统具有RS-485远传接口:电容式液位传感器;CAV424;MCU:TP274:B:1006-2394(2010)04-0001-03CapacitanceLevelTransducerMeasurementSystemBasedonCAV424HOUHa-iling1,XIEYan-jiang2(1.CollegeofElectronicInformationandAutomation,TianjinUniversityofScienceandTechonology,Tianjin300222,China;2.TheSecondMiddleSchoolofBotou,Botou062150,China)Abstract:AkindofcapacitanceleveltransducermeasurementsystembasedonCAV424isintroducedinthispaper.Accordingtotheprincipleofthecapacitanceleveltransducer,thecapacitancevaluebetweenthetwoplatesofthetransducerwillchangealongwithliquidleve.lThesystemtakesadvantageoftheintegratedcircuitCAV424toconvertthecapacitancetovoltagesignals.Aftertha,tthevoltagesignalsareconvertedtodigitalsignalsbythe16-bi,thighprecision-A/DconverterAD7705.DigitalsignalsareprocessedbyMCU,suchasdigitaldisplayandcur-rentoutpu.tThecurrentoutputcircuitiscomposedof12-bi,tserialD/AconverterAD7943anda4~20mAV/Icon-verter.ThesystemhasaRS485longdistancetransmissioninterface.Keywords:capacitanceleveltransducer;CAV424;MCU11,,dD;,图1电容式液位传感器工作示意图0;H,L,:C1=2Hln(D/d)(1):C2=20(L-H)ln(D/d)(2):C=C1+C2=2(-0)Hln(D/d)+20Lln(D/d)(3):C0=20Lln(D/d)(4)H:CX=C-C0=2(-0)Hln(D/d)(5)120104(5),,,,,,22PIC16F876,8KFlash368256EEPROM,,图2系统的硬件框图2.1/AMGCAV424//,,CAV424,A/D,4~20mA3CAV424CAV424:CoscCx1Cx2,Cx1,Cx2Cx1Cx2,5%~100%,Cx110pF~1nF,Cx210pF~2nF3,3dB-CL1CL2(20k),CL1CL2200Cx1RL1RL2:GLP=1+RL1RL2图3CAV424电路原理框图RL1+RL290~200k5:VLPOUT=VDIFF+VM:VMCAV424,2.5V;VDIFF,:VDIFF=GLP38(VCX1-VCX2)CAV424,7,2.32V,0.8mVCosc1.55Cx1~1.65Cx1;Rcosc250k,Rcx1Rcx2500kCAV42456VDIFFA/DADIAD7705A/D,AD7705,CAV4242.24~20mA4~20mAD/AAD7943V/IAD694,4AD794312DAC,,+3.3V/+5V,,,AD754xDACRA2AD794359,RA1AD79437,RA0AD794310STB312STB3AD7943,LD1LD2,DAC,PIC16F876RA,RAADC,PA220104,ADCON1图44~20mA电流传输电路AD7943:voidAD7943Write(unsignedintvalue){unsignedchar;iAD7943_LD=1;value=value4;//4,for(i=0;i12;i++){if(value&0x8000)AD7943_SRI=1;elseAD7943_SRI=0;value=value1;AD7943_STB=1;asm(nop);//AD7943_STB=0;}AD7943_LD=0;//12,asm(nop);AD7943_STB=1;AD7943_LD=1;}4AD694ADI4~20mA,0~20mA:(1):4~20mA0~20mA;(2):0~2V0~10V;(3):4.5~36V;(4)D/A(AD7541AD7943),;(5)AD694:RL=(VS-2)/20mA,12V,5002.3,,MAX7219,/,87LEDMAX7219,,LED35,PIC16F876I/OEEPROM,-,,,AD7705,,EEPROM;,,AD7705A/D,-MAX7219AD7943D/A,AD694V/I4~20mA图5系统软件流程图4CAV424/,,,,CAV424,:[1].[J].,2002(6):15-16.(6)320104,:,,,5图5传感器的安装,6,1,2,2,,2,图6补偿电路调试前后4.21mm,,0.05mm,VrVx,Vz(Vz=V2x+V2r),(7)(8)图7阻抗图图8阻抗幅值与距离之间关系图78,,,,,,0.3mm,0.6mm,,,,,5(1),,(2),,:[1],.[J].,2001,15(1):60-64.[2],,.[J].,2003,25(1):44-46.[3],.[J].,2007,28(9):1718-1720.[4],.MAX038[J].,1999(3):30-32.()(3)[2]TorgStecker,HelmutKremer.CAV424[J].,2003(1):40-42.[3],,,.MCS-51[M].:,1993.()620104