The filtering based auxiliary model generalized extended stochastic gradient identification for a multivariate output-error system with autoregressive moving average noise using the multi-innovation theory

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Abstract

This paper studies the parameter estimation algorithms of multivariate output-error autoregressive moving average (M-OEARMA) systems. By means of the filtering technique and the auxiliary model identification idea, this paper gives an auxiliary model generalized extended stochastic gradient (AM-GESG) algorithm for identifying the M-OEARMA system as a comparison. In order to enhance the performance of the AM-GESG algorithm, a modified filtering based AM-GESG algorithm and a filtering based auxiliary model multi-innovation generalized extended stochastic gradient algorithm are proposed. Compared with the AM-GESG algorithm, the proposed two algorithms can generate highly accurate parameter estimates. The simulation examples demonstrate that the proposed algorithms are effective for identifying the M-OEARMA systems.

Introduction

System identification is the modeling theory and methods for researching linear systems [1], [2], [3] and nonlinear systems [4], [5], [6]. For system analysis and design with unknown parameters, the first step is to obtain the parameters of the system models through some identification methods [7]. The recursive identification and the iterative identification are two important types of parameter estimation methods [8]. The iterative algorithms use the batch data to update the parameter estimates and have many applications in off-line identification [9], [10]. In contrast to the iterative algorithms, the recursive algorithms can be used for on-line identification because they can update the parameters with new data as the time moves forward [11], [12]. The stochastic gradient algorithm belongs to the recursive algorithms which updates the estimates along the negative gradient and has been employed in many occasions [13], [14].

The modeling and identification of multivariable systems have received much attention in the field of industrial control [15], [16], [17]. It is worth pointing out that many multivariable systems possess various disturbances which contaminate the measurement outputs of the systems and therefore result in low identification accuracy [18]. To weaken the interference of the noises, one effective way is to apply the filtering technique to change the structure of the disturbance noise model and to improve the identification accuracy. Although the stochastic gradient algorithm can be used for system identification, it cannot achieve high estimation accuracy. In order to solve this issue, we use the multi-innovation identification method to enhance the performance of the stochastic gradient algorithm. The multi-innovation is a powerful approach to enhance the estimation accuracy of the gradient methods. The key is to expand the innovation and to make full use of input–output data. This paper investigates the parameter estimation problem of a multivariate output-error system with autoregressive moving average noise through using the multi-innovation theory and the filtering technique. For one thing, we use the filtering technique to process the noise model of the system; For another, we employ the multi-innovation theory to enhance the performance of the stochastic gradient algorithm. Besides, in order to make the stochastic gradient algorithm realizable, an auxiliary model is established to replace the unknown variables in the identification algorithm with the outputs of the auxiliary model. The main contributions of this paper are as follows.

  • A filtering based auxiliary model generalized extended stochastic gradient algorithm is proposed for the M-OEARMA systems by using the filtering technique and the auxiliary model. Compared with the auxiliary model based stochastic gradient algorithm, the proposed algorithm can generate more accurate estimates.

  • By expanding the identification innovation and introducing the convergence index, a modified filtering based auxiliary model generalized extended stochastic gradient algorithm and a filtering based auxiliary model multi-innovation generalized extended stochastic gradient algorithm are derived in order to improve the performance of the filtering based stochastic gradient algorithm.

The proposed identification algorithms are suitable for multivariate stochastic systems and can be extended to identify bilinear stochastic systems [19], [20], [21] and nonlinear stochastic systems [22] with colored noises, and can be applied to other fields [23], [24], [25], [26] such as engineering application systems [27], [28], [29], [30].

The rest of this paper is organized as follows. Section 2 describes the identification problems for multivariate output-error autoregressive moving average systems. Section 3 introduces an auxiliary model based stochastic gradient algorithm as a comparison. Section 4 presents a filtering based stochastic gradient algorithm and Section 5 proposes a filtering based multi-innovation stochastic gradient algorithm. Section 6 provides an illustrative example to indicate the effectiveness of the proposed algorithms. Finally, some concluding remarks are offered in Section 7.

Section snippets

The system description

Let us introduce some notations. ``A ≕ X” or ``X ≔ A” stands for ``A is defined as X”; the superscript T represents the vector/matrix transpose; the symbol In denotes an identity matrix of size n × n; ϑ^(t) means the estimate of ϑ at time t; 1n stands for a n-dimensional column vector whose elements are 1; the norm of a matrix (or a column vector) X is defined by ‖X2 ≔ tr[XXT].

Consider the following multivariate output-error autoregressive moving average (M-OEARMA) system:y(t)=x(t)+w(t)=Φ1(t)θA

The auxiliary model based generalized extended stochastic gradient algorithm

This section extends the AM-GSG algorithm for M-OEAR systems in [31] to present an auxiliary model based generalized extended stochastic gradient algorithm for M-OEARMA systems in Eq. (1) as a comparison. According to the identification model in Eq. (8), we can derive the auxiliary model based generalized extended stochastic gradient (AM-GESG) algorithm:ϑ^(t)=ϑ^(t1)+Φ^T(t)r(t)[y(t)Φ^(t)ϑ^(t1)],r(t)=r(t1)+Φ^(t)2,Φ^(t)=[Φ1(t),Φ^x(t),Φ^n(t)],Φ^x(t)=[x^(t1),x^(t2),,x^(tna)],Φ^n(t)=[w^(

The filtering based auxiliary model generalized extended stochastic gradient algorithm

The stochastic gradient algorithm has a slow convergence rate and cannot reach a satisfactory estimation accuracy. In this section, a linear filter L(z) ≔ C(z) is introduced to deal with the colored noises and enhance the estimation accuracy of the AM-GESG algorithm.

Refer to the method in [31], and define the filtered output vector yf(t) and the filtered information matrix Φf(t) asyf(t):=C(z)y(t)=y(t)+c1y(t1)+c2y(t2)++cncy(tnc)Rm,Φf(t):=C(z)Φ1(t)=Φ1(t)+c1Φ1(t1)+c2Φ1(t2)++cncΦ1(tnc)Rm×n

The filtering based multi-innovation generalized extended stochastic gradient algorithm

Based on the F-SG algorithm in Eqs. (35)–(47), we employ the multi-innovation theory to derive a filtering based multi-innovation generalized extended stochastic gradient algorithm to improve the parameter estimation accuracy in this section.

Define p as the innovation length. Based on the F-SG algorithm in Eqs. (35)–(47), expand the innovation vector e(t):=y(t)Ψ^(t)ϑ^(t1) in Eq. (35) into a large innovation vector:E(p,t):=[y(t)Ψ^(t)ϑ^(t1)y(t1)Ψ^(t1)ϑ^(t2)y(tp+1)Ψ^(tp+1)ϑ^(tp)]Rmp.

Example

Consider the following M-OEARMA system:y(t)=x(t)+w(t),x(t)=Φ1(t)θA(z),w(t)=D(z)C(z)v(t),Φ1(t)=[y2(t1)u2(t1)cos(t/π)u1(t2)u2(t2)sin(t/π)u1(t1)y1(t2)],A(z)=1+0.12z1+0.39z2,C(z)=10.58z1+0.35z2,D(z)=1+0.09z1+0.23z2,θ=[θ1,θ2,θ3]T=[0.42,0.93,0.09]T,ϑ=[0.42,0.93,0.09,0.12,0.39,0.58,0.35,0.09,0.23]T.

In simulation, the inputs u1(t) and u2(t) are taken as two uncorrelated stochastic signal sequences with zero mean and unit variances; v1(t) and v2(t) are taken as two white noise sequences

Conclusions

This paper uses the filtering technique and the multi-innovation theory to investigate the identification problems of M-OEARMA systems and presents a filtering based auxiliary model generalized extended stochastic gradient algorithm and a filtering based auxiliary model multi-innovation generalized extended stochastic gradient algorithm. The simulation results indicate that the proposed algorithms have good performances after introducing the innovation length and the convergence index and the

Declaration of Competing Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

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    This work was supported by the National Natural Science Foundation of China (No. 61472195) and by Key Program Special Fund in XJTLU (No. KSF-E-12).

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