Using tfest to find a system model
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Revision as of 14:52, 9 April 2019 by Steven Wasserman (Talk | contribs)
Here is some example code that demonstrates how to use tfest to fit a linear model to frequency response data:
%Generate some synthetic frequency response measurement data s = tf( [ 1 0 ], 1 ); highPass = s / ( s + 1 ) [ magnitude, phase, frequency ] = bode( highPass ); % add some random noise noiseStandardDeviation = 0.05; magnitude = magnitude + noiseStandardDeviation * randn( size( magnitude ) ); phase = phase + noiseStandardDeviation * randn( size( phase ) ); numeratorForm = [ 1 0 ]; % decreasing powers of s -- numerator for HPF has no constant term, so set it to zero (otherwise tfest will fit a constant value) denominatorForm = [ 1 0 ]; % decreasing powers of s -- numerator for HPF has no constant term, so set it to zero (otherwise tfest will fit a constant value) estimatedTransferFunction = FitTransferFunction( magnitude, phase, frequency, numeratorForm, denominatorForm ) function EstimatedTransferFunction = FitTransferFunction( MagnitudeRatio, PhaseDifferenceDegrees, FrequencyHertz, NumeratorForm, DenominatorForm ) % convert magnitude and phase to single complex vector complexResponseData = MagnitudeRatio .* exp( 1i .* PhaseDifferenceDegrees .* pi ./ 180 ); % create optional initial model for tfest so that known zero % coefficients can be constrained initalModel = idtf( NumeratorForm, DenominatorForm ); zeroCoefficients = find( NumeratorForm == 0 ); for ii = 1:numel( zeroCoefficients ) initalModel.Structure.Numerator.Free(ii) = false; end zeroCoefficients = find( DenominatorForm == 0 ); for ii = 1:numel( zeroCoefficients ) initalModel.Structure.Denominator.Free(ii) = false; end frequencyResponseData = frd( complexResponseData, FrequencyHertz, 'FrequencyUnit', 'Hz'); EstimatedTransferFunction = tfest( frequencyResponseData, initalModel ); end