#### Abstract

In this article, we propose a fault tolerant control for multiple-input multiple-output (MIMO) nonlinear systems via model predictive control. The MIMO nonlinear systems are approximated by MIMO ARX-Laguerre multiple models. The latter is obtained by expanding a discrete-time MIMO ARX multiple model parameters on Laguerre orthonormal bases. The resulting model ensures an efficient complexity reduction with respect to the classical MIMO ARX multiple models. This parametric complexity reduction still subjects to an optimal choice of the Laguerre poles defining Laguerre bases. The parameter and structure identifications of the MIMO ARX-Laguerre multiple models are achieved by the recursive method and a metaheuristic algorithm, respectively. The proposed model is built from the system input/output observations and is used to synthesize a MIMO nonlinear fault tolerant control algorithm via MPC. So, we develop a fault detection and isolation (FDI) scheme based on the proposed model. The scheme of the fault detection is applied at every step of MPC control calculation, where we determine the actuator faults and we use it in the MPC optimization problem to determine the new control with respect to the actuator faults. The proposed strategy is tested on numerical simulation and validated on the real system.

#### 1. Introduction

The security of technological systems is a major concern in the last decade [1–4]. Ensuring the security and the environment of a process require the knowledge of its operating status as finely as possible at every moment. In particular, we have to be able to decide if the working system is normal or if a malfunction has occurred. In this case, it is interesting to know the nature of this dysfunction, which is the main objective of the diagnosis. So, in the context of increasing autonomy of the systems, once the dysfunction is detected and identified, this knowledge must be taken into account when calculating a closed-loop control law to counter the influence of defects. This strategy is entitled fault tolerant control (FTC) [5].

The goal of this work is to synthesize fault tolerant control for nonlinear multiple-input multiple-output (MIMO) systems via model predictive control (MPC) based on reduced complexity multiple models. Model-based predictive control is a well-established online control strategy which iteratively computes control signals by solving an optimization problem over a future time horizon under certain process constraints [6–9]. This optimization uses a prediction model of the future plant behavior. The closed-loop performance depends on the choice of an appropriate model for prediction and several tuning parameters. To model MIMO nonlinear systems, several models are used like neural network [10], MIMO nonlinear autoregressive with exogenous input model (NARX) [11], fuzzy logic model [12], and MIMO autoregressive with exogenous input (ARX) multiple models known as MIMO Takagi–Sugeno model [13]. However, these models are constrained by a high parameter number. Furthermore, the complexity of MIMO nonlinear models handicaps the synthesis of a control law by increasing the computation time of the control. To overcome this problem, several works are developed in the literature in the case of single-input single-output (SISO) linear and nonlinear models and MIMO linear model by expansion on Laguerre orthonormal bases [14–18].

In this context, we propose in this paper to reduce the parametric complexity of ARX MIMO multiple models by decomposing its parameter associated with the inputs and the outputs on independent Laguerre orthonormal bases. This decomposition can be realized since the coefficients of the ARX MIMO multiple models are absolutely summable on in the sense of the bounded-input bounded-output (BIBO) stability criterion of the system. The new model, entitled MIMO ARX-Laguerre multiple models, ensures the parameter number reduction with a recursive and easy representation. The proposed model is characterized by a set of poles where an optimal choice of these poles is compulsory to lessen considerably the number of MIMO ARX-Laguerre multiple model parameters. In this paper, we propose to use a recursive method to identify the Fourier coefficients and a metaheuristic algorithm to optimize the MIMO ARX-Laguerre multiple model poles.

The synthesis of a MIMO nonlinear fault tolerant control via MPC (MIMO NFTC-MPC) requires fault detection and estimation procedures. About that, we propose in this paper to use the moving horizon fault estimation (MHE) [19] based on the proposed MIMO ARX-Laguerre multiple models. The MHE is used to estimate the actuator faults from the error between the estimated outputs and the system outputs. The main contributions of this paper is basically threefold. (1) We present a new reduced complexity model for nonlinear MIMO systems by expanding the MIMO ARX multimodel on independent Laguerre bases. The resulting MIMO ARX-Laguerre multiple models ensures the parameter number reduction with a recursive and easy representation. (2) We develop a fault actuator detection and estimation based on the identified MIMO ARX-Laguerre multimodel and using the moving horizon fault estimation. (3) By combining the fault estimation procedure and the model predictive control for MIMO nonlinear system based on the MIMO ARX-Laguerre multiple models, we develop a MIMO nonlinear fault tolerant control via model predictive control.

This paper is organized as follows: in Section 2, we present the modeling of MIMO nonlinear systems where we recall the principle of MIMO multiple model approach and we give the definition of the MIMO ARX multiple models. In Section 3, we present the MIMO ARX-Laguerre multiple models obtained by the expansion of MIMO ARX multiple models on independent Laguerre bases. In Section 4, we propose the identification procedure of the MIMO ARX-Laguerre multiple models, where we develop a recursive method to identify the Fourier coefficients and we use a metaheuristic algorithm to optimize the poles. Section 5 is devoted to the development of a MHE to detect and estimate the actuator faults of the MIMO nonlinear system using the MIMO ARX-Laguerre multiple models. In Section 6, we synthesize the MIMO nonlinear fault tolerant control via MPC where we develop the *j*-step ahead predictor of the MIMO ARX-Laguerre multiple model outputs by taking into account the actuator faults and we present the control calculation by taking into account the constraint on the inputs and the outputs by resolving an optimization problem. Finally, Section 7 illustrates the proposed MIMO nonlinear fault tolerant control via MPC by a numerical example.

#### 2. Modeling of MIMO Nonlinear Systems

##### 2.1. Principle of Multiple Model Approach

A multiple model is a set of LTI (linear time invariant) and causal submodels aggregated by an interpolation mechanism to characterize the dynamic behavior of the overall nonlinear system. It is characterized by the number of submodels, their structure, and the choice of weighting functions. A multiple model structure is represented bywhere is the multiple model output, *L* is the submodel number, is the weighting function associated to the submodel, is the decision variable in general is selected as the input or the output of the system, and is the output of the submodel. The weighting functions allow to determine the relative contribution of each submodel according to the zone where the system operates, and they respect the convexity properties given as follow:

The weighting functions can be constructed from continuous functions derivatives such as Gaussian functions as follows:where and are, respectively, the dispersion and the center of the indexed variable .

##### 2.2. MIMO ARX Multiple Models

A strictly causal discrete time MIMO nonlinear system with *p* inputs and *m* outputs can be represented by a MIMO ARX multiple models where each nonlinear multiple-input single-output (MISO) system for can be represented by a MISO ARX multiple models written aswhere is the output of the submodel for the MISO model, is the weighting function associated with the *s*^{th} submodel for the MISO model, and is the decision variable selected as the inputs or the outputs of the system:where for , , if the decision variable is selected as the inputs and for , , if the decision is variable selected as the outputs. By defining the following matrices,where and are, respectively, the center, , and the dispersion of the decision variable, , where is defined as

Then, the weighting function is defined as

The MISO linear submodel can be described by its output equation given by a MISO autoregressive with exogenous input (MISO ARX) model as follows:where and are the model parameters of the MISO ARX submodel, and for and are, respectively, the system inputs and the system outputs, and and are the model orders associated, respectively, with the inputs and the outputs of every MISO ARX submodel. The MISO ARX submodel can be written in the matrix form aswiththen, the MISO ARX multiple models given by relation (4) can be rewritten as

The output of the MISO ARX multiple models can be rewritten aswithwhere each MISO ARX multiple models is characterized by a parameters number determined as follows:

Then, the MIMO ARX multiple models are characterized by a parameter number. From relation (15), we can conclude that the complexity of the MIMO ARX multiple models increases according to the submodel orders and . In order to reduce the number of parameters, we will proceed with the decomposition of these coefficients and for of the MIMO ARX multiple models given by (9), for , on Laguerre orthonormal bases.

#### 3. Expansion of MIMO ARX Multiple Models on Laguerre Orthonormal Bases

In this section, we use the Laguerre orthonormal bases to reduce the parametric complexity of the MIMO ARX multiple models defined by (9) [15, 18, 20]. This choice is due to the capability of Laguerre base on parametric reduction and for the classical recurrent representation. According to the stability condition of the system in the sense of bounded-input bounded-output criterion (BIBO), the coefficients and are absolutely summable and they satisfy

Therefore, these coefficients belong to the Lebesgue space . Noting that the orthogonal Laguerre functions form an orthogonal base belong also to the Lebesgue space, the coefficients and can be, respectively, developed on the Laguerre bases and as follows [20, 21]:where for and are the Fourier coefficients, and *i* are the Laguerre functions, and and are the poles defining, respectively, the orthogonal bases and . Taking into account the stability condition (16), the MISO ARX multiple models given by (9) can be written aswhere if and if .

By substituting and given by (17) in the MISO ARX submodel defined by (18), the resulting submodel can be written asfor .

The relation (19) can be written, for , as

By analogy to the development given by Mbarek et al. [22] for the MIMO case and from relation (A.3) and (A.4) given in Appendix A, the MISO ARX-Laguerre multiple model can be described by the following recursive representation:where , for and , are the parameter vectors regrouping the Fourier coefficients , for and , and , for and .and the matrices are defined bywhere the matrices , and , are defined according to the poles and as given, respectively, by relations (A.13) and (A.14).

The matrices and are defined bywhere and are defined as follows:where the vectors , and , are defined according to the poles and as given, respectively, by relations (A.15) and (A.16).

The output of the MISO ARX-Laguerre multiple models given by (21) can be written aswhere and are defined as follows:

The MISO ARX-Laguerre multiple models given by relation (21) is characterized by parameters as given by relation (27) and poles defined as follows:

The proposed MIMO ARX-Laguerre multiple models given, for , by the MISO ARX-Laguerre multiple modes (21) can be represented, as in Figure 1, by a parallel structure diagram in the case of 2 inputs/2 outputs where every MISO model is decomposed into *L* submodels.

By defining the output vectors and asthe MIMO ARX-Laguerre multiple models given by relation (21) can be written aswith

Then, the MIMO ARX-Laguerre multiple models can be written in the following recursive representation:where the state vector and the matrices , , , and are defined, respectively, as

#### 4. Identification of the MIMO ARX-Laguerre Multiple Models

A strictly causal discrete time nonlinear MIMO system with *p* inputs and *m* outputs can be described by a MIMO ARX-Laguerre multiple models where each MISO system, for , is represented by the recursive representation given by relation (21) and the output is rewritten by the vectorial representation as given in relation (26). Each MISO ARX-Laguerre multiple model is characterized by the parameter vector defined by relation (27) and the poles vector given by relation (29). To ensure a significant parametric reduction, it is necessary to optimize the MIMO ARX-Laguerre multiple model poles. The identification of the parameter vectors and the optimization of the poles are presented in the next sections.

##### 4.1. Recursive Identification of the Fourier Coefficients

This method proposed by Abdelwahed et al. [21] is based on the minimization of a regularized square error given aswhere(i) and are, respectively, the output system and the output of the MIMO ARX-Laguerre multiple models(ii), , and , are, respectively, the Fourier coefficients at time instants and *h*(iii), , is a regularization constant which regulates the importance that we give either to the quadratic error between and or to that between the Fourier coefficients.

From relation (26) and at time instant *h*, the square error can be written in matrix form aswhere and are the parameter vectors regrouping the Fourier coefficients associated with the output at time instant and *h*, respectively, and the vector and the matrix are defined as

The optimal parameter vector of each MISO system is obtained by minimizing the quadratic regularised square error given by relation (38). The gradient of the criterion with respect to is written as

From relation (40), we can calculate the estimated parameter vector of each MISO system as

The recursive parametric identification of the MIMO ARX-Laguerre multiple models representing a nonlinear MIMO system of *p* inputs and *m* outputs where each MISO system is decomposed into *L* submodel is described by the following algorithm.

##### 4.2. Pole Optimization of the MIMO ARX-Laguerre Multiple Models Using Metaheuristic Algorithms

To guarantee a significant parametric reduction of the proposed MIMO ARX-Laguerre multiple models, it is necessary to optimize the Laguerre bases poles. Several optimization methods are proposed in the literature in order to put the Laguerre poles into their optimal values, such as the gradient algorithm, the Newton–Raphson algorithm, and the Bouzrara el al.’s method. In this paper, since we have many poles to be optimized, poles, we propose the use of genetic algorithms (GAs) as a metaheuristic optimization method. The values of the MIMO ARX-Laguerre multiple models poles are optimized by minimizing the normalized mean squared error of each MISO ARX-Laguerre multiple models given bywhere and are the output system and the output of the MIMO ARX-Laguerre multiple models, respectively.

The pole optimization algorithm of the proposed MIMO ARX-Laguerre multiple models using the genetic algorithm is summarized in the following algorithm.

#### 5. Moving Horizon Fault Estimation Based on MIMO ARX-Laguerre Multiple Models

In the following, we propose to use the moving horizon fault estimation (MHE) based on the MIMO ARX-Laguerre multiple models to solve the fault estimation of the MIMO nonlinear system. The MHE is used to estimate the actuator faults from the error between the estimated output and the system output . The MIMO ARX-Laguerre multiple models taking account of actuator faults can be written from (33) aswhere and are the estimated state vector and the estimation outputs model, respectively, when the actuator faults are defined as

The estimate actuators faults are obtained by an online minimization at every sample time of the quadratic cost function defined as follows:where is the estimation fault horizon, is the estimation outputs model when the actuator faults are considered as given by (43), and is the outputs system defined as

The quadratic criterion given by (45) can be rewritten aswhere is the fault actuators to be optimized using the MHE method:

The MHE method is formulated by minimizing the criterion given by (47) at every sample time:where is defined as follows:and is the admissible set of fault actuators constraints defined aswhere and are -dimensional vectors defined as follows:where and are the bounds of actuator faults that can be chosen from the physical constraints defined as

#### 6. MIMO Nonlinear Fault Tolerant Control via Model Predictive Control

In this paper, we propose a MIMO nonlinear fault tolerant control via model predictive control using MIMO ARX-Laguerre multiple models (MIMO NFTC-MPC). The proposed MIMO NFTC-MPC is applied to a nonlinear system and used to compensate online the actuator faults by including the effects of the faults in the model predictive control optimization problem. Then, the MIMO NFTC-MPC strategy requires a fault estimation unit to estimate the actuator faults and a MIMO ARX-Laguerre multiple models observer to estimate the system state. The MIMO NFTC-MPC strategy is described by a block diagram as represented in Figure 2.

The MIMO NFTC-MPC is characterized by three essential steps. The first step is devoted to the calculation of the system output prediction to determine the process behavior on the prediction horizon depending on the inputs applied to the process until the instant . The second step is dedicated to the actuator fault optimization until the instant , the outputs measured until the instant , and the equations describing the model, *T*, is the sample time. The third step is dedicated to the optimization of a criterion to determine a future control sequence over the control horizon .

##### 6.1. The *j*-Step Ahead Predictor including the Actuator Faults

The MIMO ARX-Laguerre multiple models taking into account actuator faults can be written from relation (31) aswhere is the vector of actuator faults estimated using the MHE method. To calculate the *j*-step ahead predictor on the MIMO ARX-Laguerre multiple models output taking into account actuator faults, , we use the incremental form given from (54) aswhere , , , , and are the control increments, the output increments, the actuator faults increments, the state vector increments, and the weighting function increments, respectively, defined as

By successive substitutions and after some reorganization, we give the following proposition.

Proposition 1. *The j-step ahead predictor including the actuator faults using the MIMO ARX-Laguerre multiple models, , is given aswhere is the -dimensional identity matrix, is the truncation order of the L submodel for each MISO submodel, and is defined as*

*See proof in Appendix B.*

The *j*-step ahead predictor given by (57) is split into two components, the free and the forced components:where is the free components determined using the measured outputs until and the controls up to :

The free component given by relation (60) can be rewritten aswhere is defined asand from relation (A.25), the forced component resulting from the action of future control can be written as

For , relation (59) becomeswhere

From relations (71) and (73) and for , the vectors and can be written aswhere(i) is a -dimensional vector computed from relation (72) and defined as(ii) is a -dimensional vector defined as(iii) is a -dimensional future control increments vector where is the control horizon, defined as(iv) is a -dimensional actuator faults increments vector given by(v) and are, respectively, -dimensional matrix and -dimensional matrix given as follows:where is a -dimensional vector of zeros.where is a -dimensional vector of zeros.

According to relations (71) and (73), the components and and can be written as

Then, from relation (66), the prediction vector given by (54) can be written in matrix form as

##### 6.2. Control Calculation

The control calculation of a MIMO system with *p* inputs and *m* outputs is based on the minimization of the following performance quadratic criterion:where , are the reference trajectories at time instant , are the weights on the tracking error considered constant and equal to *ρ*, and are positive weights generally considered constant and equal to .

The quadratic criterion given by (75) can be rewritten on the following compact matrix:where is the -dimensional vector of predicted outputs defined in relation (65) and the vector is defined from the reference trajectories as follows:

is the vector of future control increments given by (69), and are positive definite matrices of dimension, respectively, and , defined as follows:where

By replacing by its expression given by (74), the matrix form of criterion given by (76) becomes

The model predictive control methods allow to take into account some constraints optimization problem subject to physical constraints on the inputs and the outputs due to the actuator technology, the control system security, or the quality desired for the output of the controlled process. In the case of MIMO system of *m* outputs and *p* inputs, the constraints are given asby analogy to the matrix form of constraints given by Mbarek et al. [22], these constraints can be expressed in terms of future control increments and written in matrix form as