Abstract: Atultra-lowspeed，theprecisionofgimbalspeedmeasurementisakeyfactorwhichinfluencesthecontrolaccuracyofacontrolmomentgyro（CMG）gimbalsystem.Focusingontheproblemofnoiseamplificationbroughtbyconventionalfirst-orderbackwarddifferentiatorwhencalculatingultra-lowspeed，areal-timespeedestimationmethodbasedonanonlineartrackingdifferentiatoranditsdiscretetimeexpressionusingthefourthorderRunge-Kuttamethodwasproposed.Acompositespeedmeasuringmethodwaspresentedtosolvethetrackingdelayproblemwhenusingtrackingdifferentiatorsdirectly.Then，thenumericalsimulationandexperimentwereconducted.Thesimulationresultsdemonstratethatthespeedestimationmethodproposedcouldreducethequantizationnoisesbroughtbytachometersandmeasurementnoises.Theexperimentalresultsshowthatbyusingtracking-differentiatorsthefluctuationofgimbalspeedisreducedby67.1%.

Key words: controlmomentgyro（CMG）, gimbal, PMSM, ultra-lowspeed, trackingdifferentiators