Study of Conditional ML Estimators in Time and Frequency domain System Identification.

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Keywords: linear identification, stochastics, estimation
Abstract
This paper studies the impact of replacing the full covariance matrix by its main diagonal in maximum likelihood estimation (MLE) of linear dynamic systems in the time or frequency domain. First the source of the nondiagonal entries is studied, next the impact of neglecting these terms on the efficiency of the estimators is analyzed. Finally the equivalence between the time and frequency domain formulation is shown.
–1 –1
e t(t) = 0
t≥0
(2)
Exact calculation of expressions (3) requires the past values of the noise e(t) for t < 0 . In practice only information for t ≥ 0 is
Study of Conditional ML Estimators in Time and Frequency domain System Identification.
J. Schoukens, R. Pintelon and Y. Rolain Vrije Universiteit Brussel, Department ELEC, Pleinlaan 2, 1050 Brussels, Belgium email: jschouk@vnet3.vub.ac.be; tel: (02) 629 29 44 fax: (02) 629 28 50
tages [7]. The covariance matrix after a DFT is dominated by its diagonal so that the full MLE can be approximated by a weighted least squares estimator using the diagonal terms as a nonparametric weighting. These weighting coefficients (being the variances of the measured spectra) can be easily obtained from a small number of repeated measurements if the excitation is periodic [8]. This need for periodic excitations is the major disadvantage of this approach to frequency domain identification. A detailed analysis shows that the neglected nondiagonal entries in the covariance matrix can be explained as transient phenomena at the beginning and the end of the experiment, completely similar to the time domain situation so that also in the frequency domain the correct name for the method would be CMLE. Also in this case it is shown that the CMLE converges to the MLE under the same conditions.
Define . t<0 ˆ (t t – 1) of y(t) is [4] The one-step-ahead prediction y = e(t) ˆ (t t – 1) = H (q) G(q) u(t) + [ 1 – H (q) ] y(t) with y ˆ (t t – 1) = e(t) . y(t) – y (3)
1
Introduction
The mainstream in time domain system identification is given by the prediction error methods, minimizing the prediction errors using a least squares cost function. These methods can identify simultaneously the system model and a noise model. When both are correctly chosen it is shown that the prediction errors are white so that the least squares method seems to be the MLE if the noise is normally distributed. A careful analysis shows that this is not completely true due to the presence of the initial conditions of the noise filter. The transient response of the noise filter correlates the prediction errors in the beginning of the experiment so that nondiagonal entries appear in the covariance matrix. In practice it is difficult to deal with the full problem [3], [11] and for that reason the nondiagonal terms are almost never considered. This results in a conditional maximum likelihood estimator (CMLE). Many times the CMLE is called the MLE without mentioning the additional constraints. It is shown in this paper that under the standard conditions (persistent and quasi stationary excitation) the CMLE converges to the MLE so that no price is paid (asymptotically) for the simplification. System identification can also be done in the frequency domain. Very often the time-frequency transform is made using the discrete Fourier transform (DFT) which is an orthogonal transformation. This suggests a complete equivalence between time and frequency domain techniques. Although this is formally true it turns out that there can exists significant differences in practice, offering important advanThis work is supported by the Belgian National Fund for Scientific Research, the Flemish government (GOA-IMMI), and the Belgian government as a part of the Belgian programme on Interuniversity Poles of attraction (IUAP50) initiated by the Belgian State, Prime Minister’s Office, Science Policy Programming.
y(t) = G(q) u(t) + H(q) e(t) (1)
with u(t) the input, G(q) the system, H(q) the noise filter (with all poles and zeros inside the unit circle), and e(t) an independent noise source with variance λ . This expression not only depends on the known input ( u(t) , t ≥ 0 ) but also on the initial conditions or the excitation in the past ( u(t) , t ≤ 0 ). For stable plants the transient response due to the initial conditions is exponentially damped and hence it follows immediately that it does not change the asymptotic behavior of the estimation process if the system is persistently and quasi stationary excited. For simplicity reasons it is assumed (without any loss of generality) that the initial conditions of the system G(q) are zero ( u(t) = 0 for t < 0 ). Also the response of the noise filter H depends on the past values of the driving noise source which cannot be put equal to zero. Their impact is explicitly studied here.