  In this paper, we present a real-time system identification method based on
relay feedback testing with applications to multirotor unmanned aerial
vehicles. The proposed identification method provides an alternative to the
expensive lab testing of certain UAV dynamic parameters. Moreover, it has the
advantage of identifying the parameters that get changed throughout the
operation of the UAV, which requires onboard identification methods. The
modified relay feedback test (MRFT) is used to generate stable limit cycles at
frequency points that reveal the underlying UAV dynamics. The locus of the
perturbed relay system (LPRS) is used to predict the exact amplitude and
frequency of these limit cycles. Real-time identification is achieved by using
the homogeneity properties of the MRFT and the LPRS which are proven in this
paper. The proposed identification method was tested experimentally to estimate
the aerodynamic parameters as well as the onboard sensor's time delay
parameters. The MRFT testing takes a few seconds to perform, and the
identification computations take an average of 0.2 seconds to complete in
modern embedded computers. The proposed identification method is compared
against state-of-the-art alternatives. Advantages in identification accuracy
and quantification of uncertainty in estimated parameters are shown.
