Issues need to be 2 * 2 matrix Nyquist stability analysis in matlab, the article has mentioned the need to use the generalized Nyquist stability criterion, but when calculating matrix feature worth to transfer function to convert the data types, namely tf sym, this part has been resolved, but in the use of Nyquist statements remind sym type data cannot be used, need to convert them into tf type, question here, edge BBS wu, urgent answer, transfer function of tf to root operation could not be converted to sy type, but how to transfer from sys to tf type and draw the Nyquist curve