Model Equivalence-Based Identiﬁcation Algorithm for Equation-Error Systems with Colored Noise

: For equation-error autoregressive (EEAR) systems, this paper proposes an identiﬁcation algorithm by means of the model equivalence transformation. The basic idea is to eliminate the autoregressive term in the model using the model transformation, to estimate the parameters of the converted system and further to compute the parameter estimates of the original system using the comparative coefﬁcient way and the model equivalence principle. For comparison, the recursive generalized least squares algorithm is given simply. The simulation results verify that the proposed algorithm is effective and can produce more accurate parameter estimates.


Introduction
System modeling and system identification are the prerequisite and foundation of all control issues.System identification has a significant effect on the filtering [1][2][3], state estimation [4][5][6], system control [7][8][9] and optimization [10].For example, Scarpiniti et al. proposed a nonlinear filtering approach based on spline nonlinear functions [11]; Zhuang et al. presented an algorithm to estimate the parameters and states for linear systems with canonical state-space descriptions [12]; Khan et al. discussed the theoretical implementation of robust attitude estimation for a rigid spacecraft system under measurement loss [13].As system identification becomes widely available, many identification methods have been raised, e.g., the gradient identification methods [14,15], the hierarchical identification methods [16][17][18], the auxiliary model identification methods [19,20] and the multi-innovation identification methods [21].
In all of these identification methods, the recursive identification [22][23][24] and the iterative identification methods [25][26][27] constitute two categories of important parameter estimation methods [28].The variable of the recursive identification is about time, so it can be used to estimate the system parameters online.Yu et al. derived the recursive identification algorithm to identify the parameters in the parameterized Hammerstein-Wiener system model [29]; Filipovic presented a robust recursive algorithm for identification of a Hammerstein model with a static nonlinear block in polynomial form and a linear block described by the ARMAX model [30]; Cao et al. studied constrained two-dimensional recursive least squares identification problems for batch processes, which can improve the identification performance by incorporating a soft constraint term in the cost function to reduce the variation of the estimated parameters [31].Liu and Lu derived the mathematical models and presented a least squares-based iterative algorithm for multi-input multirate systems with colored noises by replacing the unknown noise terms in the information vector with their estimates [32].
Some estimation methods focus on the estimation problems of equation error type systems [33][34][35], including the equation-error autoregressive (EEAR) systems, the equation-error moving average systems and the equation-error autoregressive moving average systems.For example, Xiao and Yue derived a filtering-based recursive least squares identification algorithm for nonlinear dynamical adjustment models [36]; Li developed a maximum likelihood estimation algorithm to estimate the parameters of Hammerstein nonlinear CARARMA systems by using the Newton iteration [37]; Ding presented a recursive generalized extended least squares algorithm for identifying controlled ARMA systems [28]; the basic idea is to replace the unknown terms in the information vector with their estimates.On the basis of the work in [28,32], the objective of this paper is to develop new identification algorithms using the model equivalent transformation and to provide more accurate parameter estimates.
The rest of this paper is organized as follows.Section 2 gives the identification model for EEAR systems.Section 3 gives a recursive generalized least squares algorithm, and Section 4 gives a model equivalence-based recursive least squares algorithm.Section 5 computes the parameter estimates of the original system.Section 6 provides numerical examples to prove the validity of the proposed algorithms.Finally, some concluding remarks are made in Section 7.

The Identification Model for an EEAR System
Let us define some notation."A =: X" or "X := A" represents "A is defined as X"; the superscript T denotes the matrix/vector transpose; the norm of a matrix X is defined by X 2 := tr[XX T ]; I stands for an identity matrix of appropriate size; X(t) represents the estimate of X at time t.
Consider the following equation-error autoregressive system, i.e., the controlled autoregressive autoregressive (CARAR) system, in Figure 1, where u(t) and y(t) are the measured input and output of the system, respectively, v(t) represents stochastic white noise with zero mean and variance σ 2 and A(z), B(z) and C(z) denote the polynomials in the unit backward shift operator z −1 [i.e., z −1 y(t) = y(t − 1)]: Suppose that u(t) = 0, y(t) = 0, v(t) = 0 for t 0, the orders n a , n b and n c are known, and Inserting Equation (2) into Equation (1) yields Define the parameter vector θ s and the information vector ϕ s (t) of the system model and the parameter vector θ n and the information vector ϕ n (t) of the noise model as where subscripts s and n denote the first letters of the words "system" and "noise", respectively.ϕ s (t) is the known information vector, which consists of measured input-output data u(t − i) and y(t − i); ϕ n (t) is the unknown information vector, which consists of noise terms w(t − i).By means of the above definitions, Equations ( 2) and ( 3) can be expressed as: This is the identification model for the EEAR system in Equation ( 1).The objective of this paper is to propose new identification algorithms for estimating the parameters of EEAR systems.

The Recursive Generalized Least Squares Algorithm
As we all know, the recursive generalized least squares algorithm can identify CARAR systems [36].The core idea is to substitute their estimates for the unmeasurable noise terms in the information vector.
The following is the recursive generalized least squares (RGLS) algorithm for estimating the parameter vector θ of the EEAR systems: The RGLS algorithm can estimate the parameters of EEAR systems on-line.

The Model Equivalence-Based Recursive Least Squares Algorithm
For the EEAR system in Equation (1), the information vector ϕ(t) of the recursive generalized least squares algorithm contains the unknown noise terms w(t − i).The solution is replacing the unknown noise terms w(t − i) with their estimates.However, the existence of the unknown noise terms in the information vector ϕ(t) affects the accuracy of the parameter estimates to some extent.
The method proposed in this paper is transforming the original system with colored noise into an equation-error system using the model equivalent transformation, so that the information vector in the identification model is composed of the available input u(t − i) and output y(t − i).Since there are no noise terms to be estimated in the information vector, the identification accuracy can be improved.Consider the CARAR system in Figure 1, which is rewritten as follows, Multiplying both sides of it by C(z) makes For simplicity, let n p := n a + n c and n q := n b + n c ; define the polynomials: Inserting Equations ( 16) and ( 17) into Equation ( 15) yields It is clear that Equation ( 14) reduces to an equation-error model, whose parameters can be estimated by the recursive least squares algorithm.Define the parameter vector ϑ and the information vector φ(t) as In this case, Equation ( 18) can be equivalently written as That is the identification model of Equation (18).Let θ(t) be the estimate of ϑ at time t.We obtain the recursive least squares algorithm for identifying ϑ in Equation (19): From Equations ( 20)-( 25), we can compute the parameter estimate θ(t), i.e., the estimates of the parameters p i and q i .The following derives the model equivalence-based recursive least squares algorithm.

The Parameter Estimation of the Original System
According to the acquired estimates pi (t) and qi (t) of the parameters p i and q i , we can compute the parameter estimates âi (t), bi (t) and ĉi (t) of the original system.The key idea is using the coefficient equivalent principle, and the details are as follows.
Assume that the estimates of A(z), B(z) and C(z) are According to Equations ( 16) and ( 17), we can approximately suppose Based on the above assumptions, we let Using B(t, z), P (t, z), Â(t, z) and Q(t, z) gives Expanding the above equation and comparing the coefficients of the same power of z −1 on both sides, we can set up (n b + n p ) equations: which can be written in a matrix form, where It is easy to know that From Equation ( 29), we can get the estimates âi (t) and bi (t) of parameters a i and b i from θ1 (t).According to the definition of P (t, z) in Equation ( 26), similarly, expanding the equation and comparing the coefficients on both sides of it gives the matrix equation, Then, we obtain Based on Equation ( 30), we can obtain the estimates ĉi (t) of c i from θ2 (t).Hence, we obtain all of the parameter estimates âi (t), bi (t) and ĉi (t).
According to the above derivation, it is clear that the model equivalence-based recursive least squares (ME-RLS) algorithm in Equations ( 20)-( 25) and ( 29)-( 30) increases the complexity of computation compared with the RGLS algorithm.However, as the information vector of the ME-RLS algorithm does not contain noise vectors to be estimated, the estimation errors become smaller.

Numerical Example
Consider the following CARAR system, Here, the input {u(t)} is taken as an uncorrelated persistent excitation signal sequence with zero mean and unit variance, {v(t)} is a stochastic white noise sequence with zero mean and variances σ 2 = 0.10 2 and is independent of the input {u(t)}.
Using the model equivalence-based recursive least squares (ME-RLS) algorithm and the recursive generalized least squares (RGLS) algorithm to estimate the parameters of this system, the parameter estimates and their errors are shown in Tables 1-3, and the estimation errors δ versus the data length t are shown in Figure 2, where δ 1 := θ(t) − ϑ / ϑ and δ 2 := θ(t) − θ / θ are the estimation errors of the ME-RLS algorithm and the RGLS algorithm, when σ 2 = 0.10 2 ; the system noise-to-signal ratio is δ ns = 35.01%.
From Tables 1-3 and Figure 2, we can obtain the following conclusions.
• The estimation errors of the ME-RLS algorithm become smaller, and the estimates converge to their true values with the data length increasing (i.e., the proposed algorithm works well).• The estimation errors of the ME-RLS algorithm are smaller than those of the RGLS algorithm, which means that the parameter estimates given by the ME-RLS algorithm have higher accuracy than the RGLS algorithm for CARAR systems.
Table 1.The estimates and errors of the parameters p i and q i .

Conclusions
This paper derives the recursive least squares algorithm based on the model transformation principle for CARAR.Compared with the LS identification algorithms, the algorithms presented in this paper reduce the number of the noise items to be estimated, and so, can generate more accurate parameter estimates.The proposed algorithm can be used to study the identification problems for other systems with autoregressive items.

Figure 1 .
Figure 1.A system described by the equation-error autoregressive (EEAR) model.

Table 2 .
The model equivalence-based recursive least squares (ME-RLS) estimates and errors.

Table 3 .
The recursive generalized least squares (RGLS) estimates and errors.
δ Figure 2. The parameter estimation errors δ versus t.