CONTROL SYSTEMS, ROBOTICS, AND AUTOMATION - Vol. V - Recursive Algorithms - Han-Fu Chen

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1 CONROL SYSEMS, ROBOICS, AND AUOMAION - Vol. V - Recursive Algorithms - Han-Fu Chen RECURSIVE ALGORIHMS Han-Fu Chen Institute of Systems Science, Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Beijing, P. R. China. Keywords: least squares, extended least squares, forgetting factor, instrumental variable, stochastic approximation, stochastic gradient, convergence, consistency, rate of convergence, diminishing excitation, least mean squares. Contents. Introduction 2. Recursive Algorithm for Constant Coefficients 2.. Least Squares (LS) 2.2. Extended Least Squares (ELS) 2.3. RLS with Forgetting Factors 2.4. Instrumental Variables Estimate 2.5. Stochastic Approximation Estimate 2.6. Stochastic Gradient Algorithm 2.7. Recursive Algorithms Derived From Off-Line Identification 3. Convergence of Estimates 4. ime-varying Systems 5. Concluding Remars Glossary Bibliography Biographical Setch Summary Various recursive identification algorithms are presented for the discrete-time stochastic time-invariant systems. he convergence issue is addressed: Sufficient conditions for strong consistency of various recursive estimates are given; Convergence rates are also provided for LS and ELS estimates. he diminishing excitation method is introduced, by which the strong consistency of parameter estimates can be achieved for a feedbac control system. SAMPLE CHAPERS racing a time-varying parameter, i.e., identifying time-varying coefficients in discrete-time, stochastic linear systems is an important problem not only for system and control but also for signal processing. KF, LMS and RLS are the most commonly used recursive algorithms in the area. he basic properties are described for these algorithms. Finally, besides the historical issue and continuous-time systems briefly addressed in Concluding Remars, some open problems are pointed out as well.. Introduction Many real-world phenomena can be approximately described by linear models which linearly relate system outputs with inputs by differential or difference equations. he

2 CONROL SYSEMS, ROBOICS, AND AUOMAION - Vol. V - Recursive Algorithms - Han-Fu Chen approximation errors in some cases may be modeled as random variables. As a result, the real system is modeled by a linear stochastic difference (or differential) equation with unnown parameters to be determined. In this case, the topic of system identification is to estimate or define the unnown parameters contained in the model on the basis of the input-output data derived from the observation on the real system under consideration. If a batch of input-output data is collected from the system and used to estimate the model parameters, then such a procedure is called o -line identification. If the estimate for model parameters is updated by a recursive algorithm at each time when some new data becomes available, then such a procedure is called recursive identification. he unnown parameters in the model may be the model-equation coefficients, orders, and time delay. Estimates for orders and time delay for a linear system are normally derived by minimizing some information criteria such as AIC, BIC, Φ IC and CIC etc, but they are hardly to be on-line and recursive. herefore, in all recursive identification methods the system orders and time delay or their upper bounds are usually assumed to be nown, and the main effort of system identification is devoted to estimate the system coefficients by which the system input and output are related. o be precise, let us consider the discrete-time case. Let u, y be system input and output, respectively. he real system may be modeled by y = Ay + A y + + A y + Bu + Bu + 2 p p B + ε qu q+ + SAMPLE CHAPERS l m, () where A, A 2,..., A p, B, B 2,..., B q are unnown coefficient matrices, ε + characterizes the approximation error, and it may be modeled as an m-dimensional random vector. In this case, (p, q) are the system orders, or upper bounds for the true orders. Since the latest input that effects on y + is u, the time delay for system () is one. Notice that A i and B j, are m m and m l matrices, respectively, i =,..., p, j =,..., q. Let us combine all unnown coefficients to a big matrix θ : θ = [ A, A,..., A, B, B,..., B ] (2) 2 p 2 and denote the input-output data by q φ y y y p+ uu u q+ = [,,...,,..., ], (3) where X denotes the transpose of matrix X (or vector X). By Eqs. (2) and (3), the model () can be rewritten as + + y = θ φ + ε. (4)

3 CONROL SYSEMS, ROBOICS, AND AUOMAION - Vol. V - Recursive Algorithms - Han-Fu Chen Recursive algorithms are used to estimate the unnown coefficient matrix θ by using the input-output data, and the estimate is on-line updated while receiving the new inputoutput observation. When θ is time-independent, the system (4) or () is called time invariant system. But θ may vary with time, i.e., θ = θ, then the system + + y = θ φ + ε (5) is called time-varying system. he noise ε + in Eq. () can also be modeled, for example, + sequence of mutually independent random vectors { difference sequence), i.e., ε + = w + + Cw + + Crw r + ε may be driven by a w } (or, more general, a martingale. (6) his ind of noise { ε } is called MA (moving average) process, and system () with ε + } being defined by Eq. (6) is called ARMAX system, where AR (autoregression) { implicates the part in Eq. () containing { y }, and X the part with input terms. If u and { ε } is given by Eq. (6), then Eq. () is called ARMA process. In what follows, the typical recursive estimation algorithms will be given for estimating the constant matrix θ in Eq. (4), and it will be shown how to mae the estimates converge to the true coefficient matrices. he convergence rates will also be presented. For the time varying system (5), recursive estimation algorithms for tracing the timevarying coefficient θ will be addressed. In conclusion, some open problems will be pointed out. 2. Recursive Algorithm for Constant Coefficients Least Squares (LS) SAMPLE CHAPERS For system (4) the parameter matrix θ is to be estimated based on { y +, φ, =,2,..., n }, n =, 2,... A common and natural way is to estimate θ by minimizing the sum of squared errors, i.e., the estimate for θ is selected by minimizing the criterion n ˆ ˆ 2 n+ ( θ) = + θ φ, =,2,... (7) = V y n with respect to ˆ θ. he minimizer denoted by ˆn θ is called the LS estimate.

4 CONROL SYSEMS, ROBOICS, AND AUOMAION - Vol. V - Recursive Algorithms - Han-Fu Chen At time, the model approximation error ε + is unnown. herefore, θ φ is the bestpredicted value for the output at time +, and hence y+ θ φ can be viewed as a prediction error for the system output. o minimize V ˆ n+ ( θ ) is to minimize the prediction errors. In other words, the estimate for θ is chosen such that the identified model best fits the observed data. he criterion V ˆ n+ ( θ ) is a quadratic form with respect to ˆ θ. Consequently, there exists a unique minimum for V ˆ n+ ( θ ) which is achieved at ˆ n n n+ = i i iy i+ i= i= (8) ˆ θ φθ φ provided the inverse exists. his is the well nown least-squares (LS) estimate. Note that the LS estimate ˆn θ + given by Eq. (8) depends on n +, the data size, and is in an o -line form. For the on-line identification, it is convenient to express { ˆ θ } in the recursive way: ˆ θ = θ + φ ( φ θ ) (9) + ap y+ φφ P + = P a P P, () a = ( + φ Pφ ). () his is the recursive LS algorithm for identifying system () or (4) given initial values ˆ θ and P. he LS estimate may give a good estimate for θ only in the case where { ε } is a SAMPLE CHAPERS sequence of uncorrelated random vectors. If { ε } is a correlated sequence, the LS may not give a satisfactory estimate. his is why one has to apply the extended least squares estimate Extended Least Squares (ELS) Now, assume ε + is given by Eq. (6), i.e., the real system is modeled by an ARMAX process y = A y + + A y + Bu + + p p+ + Bu + w + Cw + + Cw q q+ + r r+ (2) and all matrix coefficients A,..., A p, B,..., B q, C,..., C r have to be identified. In this

5 CONROL SYSEMS, ROBOICS, AND AUOMAION - Vol. V - Recursive Algorithms - Han-Fu Chen case, similar to Eq. (2) the unnown coefficients are still denoted by θ. θ = [ A, A,..., A, B,..., B, C,..., C ]. (3) 2 p q r Since { w } is not observed, θ cannot be estimated by Eqs. (9)-(), if φ is simply r extended by adding [ w,..., w ]. It is a natural way to replace e.g. w by its + estimate wˆ y θ φ and define φ ˆ ˆ y y p+ u u p+ w w r+ = [,...,,,...,,,..., ] (4) hen the recursive algorithm (9)-() with φ defined by Eq. (4) can still be used to estimate θ given by Eq. (3). his is the extended least-squares (ELS) estimate for θ given by Eq. (3) Bibliography O ACCESS ALL HE 7 PAGES OF HIS CHAPER, Clic here Benveniste A., M. Métivier and P. Priouret. (99). Adaptive Algorithms and Stochastic Approximations, 365pp. Berlin:Springer-Verlag. [his boo presents stochastic approximation methods applied to tracing time-varying parameters. he analysis is based on the ODE method.] Box E. P. and G. Jenins.(97). ime Series Analysis; Forecasting and Control, 553pp. San Francisco:Holder-Day.. [his boo provides practical methods for identifying ARMA processes and demonstrates many application examples.] Chen H. F. and Guo L. (99) Identification and Stochastic Adaptive Control, 435pp. Boston:Birhäuser. [his boo presents detailed convergence analysis for LS, ELS and SG algorithms, gives their convergence rates, and shows how to use the diminishing excitation method to derive strongly consistent estimates for feedbac control systems. he boo also provides estimation methods for orders and timedelay with detailed analysis. Continuous-time systems are also addressed.] Chen, H. F. (22) Stochastic Approximation and Its Applications, 357pp. Boston: Kluwer Academic Publishers. [his boo presents the stochastic approximation algorithm with expanding truncations and uses the trajector-subsequence (S) method for analyzing the path-wise convergence of the algorithm. Applications to various problems arising from optimization, signal processing, and systems and control are demonstrated.] Guo L. and Ljung L.(995) Performance analysis of general tracing algorithms. IEEE rans. on Automatic Control, 4, (8), [his paper presents theoretical analysis for tracing timevarying parameters.] SAMPLE CHAPERS Hannan E. J. and Deistler M. (988). he statistical heory of Linear Systems, 38pp. New Yor: Wiley. [his boo concerns estimation of coefficients, orders and time-delay for ARMAX systems with input being independent of system noise, i.e., the feedbac control is excluded from consideration.]

6 CONROL SYSEMS, ROBOICS, AND AUOMAION - Vol. V - Recursive Algorithms - Han-Fu Chen Kushner H. J. and Yin G. (997). Stochastic Approximation Algorithm and Applications, New Yor: Springer-Verlag. [his boo presents stochastic approximation methods and analyses the convergence of algorithms based on the ODE and wea convergence methods.] Ljung L. and Söderström. (983). heory and Practice of Recursive Identification, 529pp. Cambridge, Massachusetts: MI Press. [his boo provides various recursive algorithms for identifying system s parameters and many guides for their applications in practice.] Solbrand G, Ahlen A., and. Ljung, L. Recursive. (985). Methods for On-line Identification. Int. J. of Control, 4 (), [On off-line recursive computation of estimates.] P. C. Young (999). Nonstationary time series analysis and forecasting, Progress in Environmental Science,, [On off-line estimation and recursive fixed interval smoothing for time-varying systems.] Biographical Setch Han-Fu Chen received the Diploma from the Department of Mathematics and Mechanics, Leningrad (St. Petersburg) University, Russia. He joined the Institute of Mathematics, Chinese Academy of Sciences in 96. Since 979 he has been with the Institute of Systems Science, which now belongs to the Academy of Mathematics and Systems Science, Chinese Academy of Sciences. He currently is a Professor of the Laboratory of Systems and Control of the Institute. He was the Director of the Institute of Systems Science for the period of His research interests are mainly in stochastic systems including identification, adaptive control, recursive estimation, stochastic approximation and its applications to system, control, and signal processing. He has authored and coauthored more than 5 journal papers and 7 monographs. Han-Fu Chen is the Editor of the journals Systems Science and Mathematical Sciences, and Control heory and Applications and is involved in the editorial boards of several international and domestic journals. He was elected to a Member of the Chinese Academy of Sciences in 993 and to an IEEE Fellow in 996. He serves as a member of the IFAC Council (22-25), served as the President of the Chinese Association of Automation (993-22) and on the echnical Board of IFAC (996-22). He was the IPC Chair of the 4th IFAC World Congress held in 999. SAMPLE CHAPERS