数控技术专业英语 教学课件 作者 黄义俊 电子教案统一模板(数控)Unit3.ppt

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1、Unit3TheControlSystemofCNCMachinesLesson5Open-loopControlSystem3.1Lesson6Closed-loopControlSystem3.2ReadingMaterials5CoordinateSystemofCNCMachines3.3ReadingMaterials6TypeofControlSystemforToolMovements3.43.1Lesson5Open-loopControlSystemTherearetwotypesofservo-controlsystemsonNC/CNC

2、machines:open-loopandclosed-loop.Thetypeofservo-controlloopuseddeterminestheoverallaccuracyofthemachine.Aopen-loopsystemutilizessteppingmotorstocreatemachinemovements.Thesemotorsrotateafixedamountforeachpulsereceived.SteppingmotorsaredrivenbyelectricalsignalscomingfromtheMCU.Themot

3、orsareconnectedtothemachinetableball-nutleadscrewandspindle.Theopen-loopcontrolsystemsdoesnotprovidepositioningfeedbacktothecontrolunit．Themovementpulsesaresentoutbythecontrolandtheyarereceivedbyaspecialtypeofservomotorcalledasteppermotor.Thenumberofpulsesthatthecontrolsendstothest

4、eppermotorcontrolstheamountoftherotationofthemotor.Thesteppermotorthenproceedswiththenextmovementcommand.Sincethiscontrolsystemonlycountspulsesandcannotidentifydiscrepanciesinpositioning，themachinewillcontinuethisinaccuracyuntilsomebodyfindstheerror．Theopen-loopcontrolcanbeusedinap

5、plicationsinwhichthereisnochangeinloadconditions，suchastheNCdrillingmachine．Theadvantageoftheopen-loopcontrolsystemisthatitislessexpensive，sinceitdoesnotrequiretheadditionalhardwareandelectronicsneededforpositioningfeedback．Thedisadvantageisthedifficultyofdetectingapositioningerror

6、(Figure3-1).3.2Lesson6Closed-loopControlSystemAclosed-loopsystemutilizestheservomotorwhosetypesincludeACservomotors,DCservomotors,andhydraulicservomotors.Aservomotordoesnotoperatelikeapulsecountingsteppingmotor.ThespeedofanACorDCservomotorisvariableanddependsupontheamountofcurrentp

7、assingthroughit.Thespeedofahydraulicservomotordependsupontheamountoffluidpassingthroughit.ThestrengthofcurrentcomingfromtheMCUdeterminesthespeedatwhichaservomotorrotates.[1]Servomotorsareconnectedtothespindle.Theyarealsoconnectedtothemachinetablethroughtheball-nutleadscrew.Adevicec

8、alledaresolvercontinuously