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A Decentralized Stable Fuzzy Adaptive Controller for Large Scale Nonlinear Systems



R. Ghasemi, M.B. Menhaj and A. Afshar
 
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ABSTRACT

A new method to design a decentralized Fuzzy Adaptive Controller (FAC) for a class of large scale nonaffine nonlinear systems is proposed in this study. It is assumed that functions of the subsystems and their interactions are unknown. To design controller, the lyapunov function is proposed for the system and then unknown parameters of controller and system are derived based on the stability theory. The robustness against uncertainty and external disturbance, the boundedness of the estimation errors, the convergence of the output error to zero and the lyapunov stability of the closed loop system are guaranteed. To use the knowledge of the experts in FAC is another advantage of controller. Robust adaptive control has been used to avoid chattering in adaptation laws. An illustrative example is given to show the promising performance of the proposed method.

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  How to cite this article:

R. Ghasemi, M.B. Menhaj and A. Afshar, 2009. A Decentralized Stable Fuzzy Adaptive Controller for Large Scale Nonlinear Systems. Journal of Applied Sciences, 9: 892-900.

DOI: 10.3923/jas.2009.892.900

URL: https://scialert.net/abstract/?doi=jas.2009.892.900
 

INTRODUCTION

In the recent years, controller design for Large Scale Systems (LSS) and effort to extend it has attracted much attention. Research in controllers of LSS is motivated by many emerging applications that employ novel actuation devices for active control of industrial automation, cooperating robotic systems, power systems and aerospace processes. Centralized controller for the LSS is usually impractical due to the requirement of a large amount of information exchanges between subsystems and the lack of computing capacity (Karimi et al., 2007).

The tunable structure of the FAC and using the knowledge of experts in the FAC are reasons to attract many researchers to developed appropriate controllers for nonlinear systems especially for LSS (Ioannou and Sun, 1996).

In the recent year, FAC has been fully studied as follow:

In the first case, the TS fuzzy systems have been used to model nonlinear systems and then TS based controllers have been designed with guaranteed stability (Feng, 2002; Fenga et al., 2002). To model affine nonlinear system and to design stable TS based controllers have been employed by Hsu et al. (2003). Designing of the sliding mode fuzzy adaptive controller for a class of multivariable TS fuzzy systems are presented by Cheng and Chien (2006). The nonaffine nonlinear function are first approximated by the TS fuzzy systems and then stable TS fuzzy controller and observer are designed for the obtained model (Goleaa et al., 2003; Park and Park, 2004). In these studies, modeling and controller has been designed simply, but the systems must be linearizable around some operating points.

In the second case, the linguistic fuzzy systems have been used to design controllers for nonlinear systems.

Ying-Guo and Hua-Guang (1998), Jagannathan (1998), Tong et al. (1999), Tong et al. (2000) and Zhang and Bien (2000) have considered linguistic fuzzy systems to design stable adaptive controller for affine systems based on feedback linearization and furthermore, Tong et al. (2000) and Zhang and Bien (2000) has considered that the zero dynamic is stable. Stable FAC based on sliding mode is designed for affine systems by Labiod et al. (2005). Designing of the FAC for affine chaotic systems are presented by Tang et al. (1999) and Chen et al. (1999). To design stable FAC and linear observer for class of affine nonlinear systems are presented by Ho et al. (2005), Zhang (2006), Tong et al. (2004) and Shaocheng et al. (2005). Fuzzy adaptive sliding mode controller is presented for class of affine nonlinear time delay systems by Yu (2004), Chiang (2005) and Jianga et al. (2005). The output feedback FAC for class of affine nonlinear MIMO systems is suggested by Yiqian et al. (2004). The main incompetency of these studies is those restricted conditions on their nonlinear functions.

Labiod and Guerra (2007) and Tong et al. (2004) are involved stable FAC for class of nonaffine nonlinear systems. The deficiencies of these studies are bad performance of the controller when the controller has not been adjusted. Stable adaptive controller for class of linear LSS is proposed by Pagilla et al. (2007), Ioannou and Ponte (1988), Shi and Singh (1992) and Yousef and Simaan (1991). Chiang and Lu (2007) delt with designing FAC based on sliding mode for class of large scale affine nonlinear systems. (Zhang et al., 2002) presented decentralized sliding mode fuzzy adaptive tracking for a class of affine nonlinear systems in large scale systems. Wu (2002) designed FAC for a class of affine nonlinear time delayed systems. These studies have many restricted conditions on their nonlinear function.

FAC has been never applied to nonaffine nonlinear large scale systems. In this study, the stable decentralized robust adaptive controller has been designed based on fuzzy systems for a class of large scale nonaffine nonlinear systems. The controller is robust against uncertainties, external disturbances and approximation errors.

PROBLEM STATEMENT

Consider the following large scale nonaffine nonlinear system.

(1)

where, is the state vector of the system which is assumed available for measurement, ui∈R is the control input, yi∈R is the system output, fi (xi, ui) is an unknown smooth nonlinear function, mi (x1, x2,…xN) is an unknown interconnection term and di(t) is bounded disturbance.

The control objective is to design an adaptive fuzzy controller for system (1) such that the system output yi(t) follows a desired trajectory yd(t) while all signals in the closed-loop system remain bounded.

In this study, the following assumptions have been considered concerning the system (1) and the desired trajectory yd(t).

Assumption 1: Without loss of generality, it is assumed that the nonzero function satisfies the following condition:

(2)

where, fdm ∈ R is nonzero, known and constant.

Assumption 2: The desired trajectory and its time derivatives are all smooth and bounded.

Assumption 3: the interconnection term satisfies the following:

(3)

where, ξij is an unknown time varying parameters.

Assumption 4: The disturbance in the above equation is bounded by:

(4)

Define the tracking error vector as:

(5)

Where:

(6)

Taking the nith derivative of both sides of the Eq. 6, the following equation can be derived.

(7)

Use Eq. 5 to rewrite the above equation as:

(8)

where, Ai0 and bi are defined below:

(9)

Consider the vector be coefficients of and chosen so that the roots of this polynomial are located in the open left-half plane. This makes the matrix be Hurwitz. Thus, for any given positive definite symmetric matrix Qi, there exists a unique positive definite symmetric solution Pi for the following Lyapunov equation:

(10)

Let vi be defined as:

(11)

By adding and subtracting the term from the right-hand side of Eq. 8, the following equation is obtained.

(12)

Using assumption 1, Eq. 11 and the signal vi which is not explicitly dependent on the control input ui, the following inequality is satisfied:

(13)

Invoking the implicit function theorem, it is obvious that the nonlinear algebraic equation fi (xi, ui)–vi = 0 is locally soluble for the input ui for an arbitrary (xi, vi). Thus, there exists some ideal controller satisfying the following equality for a given :

(14)

As a result of the mean value theorem, there exists a constant λ in the range of f <λ1 , such that the nonlinear function f i (xi, ui) can be expressed around as:

(15)

Where:

Using Eq. 15 and 14 to rewrite Eq. 12 as follow:

(16)

However, the implicit function theory only guarantees the existence of the ideal controller for Eq. 14 and does not recommend a technique for constructing solution even if the dynamics of the system are well known. In the following, a fuzzy system and classic controller will be used to obtain the unknown ideal controller.

FUZZY SYSTEMS

Figure 1 shows the basic configuration of the fuzzy systems considered in this study. Here, fuzzy systems can be considered as a multi-input, single-output: x ∈ U ⊂ Rn→y ∈ V ⊂ R. Consider that a multi-output system can be separated into a group of single-output systems.

Fig. 1: Configuration of fuzzy system

The fuzzifier performs a mapping from a crisp input vector x = [x1 ,x2 ….,xn]T to a fuzzy set, where the label of the fuzzy set are such as small, medium, large, etc.

The fuzzy rule base is consisted of a collection of fuzzy IF-THEN rules. Assume that there are M rules and the lth rule is:

(17)

where, x = [x1 ,x2 ….,xn]T and y are the crisp input and output of the fuzzy system, respectively. Alj and Bl are fuzzy membership function in Uj and V, respectively.

The fuzzy inference performs a mapping from fuzzy sets in U to fuzzy sets in V, based on the fuzzy IF-THEN rules in the fuzzy rule base.

The defuzzifier maps fuzzy sets in V to a crisp value in V. The configuration of Fig. 1 shows a general framework of fuzzy systems, because many different choices are allowed for each block in Fig. 1 and various combinations of these choices will construct different fuzzy systems (Wang, 1997). Here, the sum-product inference and the center-average defuzzifier are used for fuzzy system. Therefore, the fuzzy system output can be expressed as:

(18)

where, is the membership degree of the input xi to fuzzy set Alj and yl is the point at which the membership function of fuzzy set Bl achieves its maximum value.

The fuzzy systems in the form of Eq. 18 are proven by Wang and Mendel (1993) to be a universal approximator if their parameters are properly chosen.

Theorem 1: Suppose f(x) is a continuous function on a compact set U (Wang, 1997). Then, for any ∈>0, there exists a fuzzy system like Eq. 18 satisfying:

(19)

The output given by Eq. 18 can be rewritten in the following compact form:

(20)

where, θ = [y1 y2…yM] is a vector grouping all consequent parameters and is w(x) = [w1(x) w2(x)…wM(x)]T a set of fuzzy basis functions defined as:

(21)

The fuzzy system (Eq. 18) is assumed to be well defined so that for all x ∈ U.

FUZZY ADAPTIVE CONTROLLER DESIGN

Here, it has been shown how to develop a fuzzy system to adaptively approximate the unknown ideal controller.

The ideal controller can be represented as:

(22)

where, , and are consequent parameters and a set of fuzzy basis functions, respectively. ∈iu is an approximation error that satisfies |∈iu|≤∈max and ∈max>0. The upid is the primary controller that developed properly to initially control the underlying system and parameters are determined through the following optimization.

(23)

Denote the estimate of as θi1 and uirob as a robust controller to compensate approximation error, uncertainties, disturbance and interconnection term to rewrite the controller given in Eq. 22 as:

(24)

In which uirob is defined below.

(25)

In Eq. 23, approximates the ideal controller, tries to estimate the interconnection term, uicom compensates for approximation errors and uncertainties, uir is designed to compensate for bounded external disturbances and is estimation of . Define error vector and use Eq. 24 and 25 to rewrite the error Eq. 16 as:

(26)

Consider the following update laws.

(27)

where, are constant parameters.

In following equation, λmin, λmax and svdmax(.) are the minimum, maximum eigenvalue and maximum singular value decomposition, respectively.

Lemma 1: The following inequality holds if .

(28)

Proof: From assumption 1 and the lemma 1, it is obvious that:

(29)

This in turn leads to the following inequality.

(30)

After some algebraic manipulations, the following inequality is obtained.

(31)

Use Eq. 31 to have the following which completes the proof.

(32)

Lemma 2: based on lemma 1 and Eq. 10, the following inequality holds.

(33)

Proof: Using Eq. 10 and after some algebraic manipulations, the following inequality is obtained.

(34)

Using the Eq. 34, the following equation can be derived.

(35)

Use Eq. 29 and 35 to have the following which completes the proof.

(36)

Theorem 2: Consider the error dynamical system given in Eq. 26 for the large scale system (1) satisfying assumption 1, interconnection term satisfying assumption 3, the external disturbances satisfying assumption 4 and a desired trajectory satisfying assumption 2, then the controller structure given in Eq. 24, 25 with adaptation laws Eq. 27 makes the tracking error converge asymptotically to a neighborhood of origin and all signals in the closed loop system be bounded.

Proof: Consider the following lyapunov function.

(37)

Where:

The time derivative of the lyapunov function becomes.

(38)

Use Eq. 26, to rewrite above equation as:

(39)

Using assumption 1 yields and by assumptions 3 and 4, to rewrite Eq. 39 as follow:

(40)

Equation 40 can be rewritten as below:

(41)

Using Eq. 27, the above inequality rewrites as:

(42)

Use the lemma 1, are satisfied. Using Barbalet’s lemma, it is guaranteed the tracking error asymptotically to the neighborhood of the origin. Furthermore, the boundedness of the coefficient parameters is guaranteed. It completes the proof.

Remark 1: The term tanh(.) is a smooth approximation of the discontinuous term sign(.). The sign(.) function is not used in the study due to avoiding chattering in the response.

Remark 2: It is very important to select properly the controller parameters to gain a satisfactory performance. Here, at this stage, number of the rules and the input membership functions are obtained by trial and error.

Remark 3: To guarantee the boundedness of the parameters in the presence of the unavoidable approximation error, the proposed adaptive laws Eq. 27 is modified it by introducing a σ-modification term as follows:

(43)

SIMULATION RESULTS

Here, the proposed decentralized fuzzy model reference adaptive controller is applied to a two-inverted pendulum problem (Karimi et al., 2007) in which the pendulums are connected by a spring as shown in Fig. 2. Each pendulum may be positioned by a torque input ui applied by a servomotor and its base. It is assumed that the angular position of pendulum and its angular rate are available and can be used as the controller inputs. The pendulums dynamics are described by the following nonlinear equations.

(44)

where, y1, y2 are the angular displacements of the pendulums from vertical position. m1 = 2 kg, m2 = 2.5 kg are the pendulum end masses j1 = 0.5 kg, j2 = 0.62 kg are the moment of inertia, k = 100 N m-1 is spring constant, r = 0.5 m is the height of the pendulum, g = 9.81 m sec-2 shows the gravitational acceleration, l = 00.5 m is the natural length of spring, α1, α2 = 25 are the control input gains and b = 0.4 m presents distance between the pendulum hinges.

Fig. 2: Two inverted pendulum connected by a spring

Fig. 3: Performance of the PID controller in first subsystem

The desired value of the outputs are zero (yid = 0 for i = 1, 2). As discussed earlier, the following primary PI controller are obtained after some trials and errors.

(45)

Figure 3 and 4 show the outputs of the system where only the controller defined in Eq. 45 is applied to the system.

Obviously the primary controller by itself is not admissible. Now the proposed controller defined in Eq. 24, 25 has been applied to mentioned system. Initially the PID controller keeps the states of system xi1, xi2 in the range of [–1, 1], [–5, 5]. Let xi = [xi1, xi2]T, z =[xi1, xi2, vi]T and vi are defined over [–45, 45]. For each fuzzy system input, it is defined 6 membership functions over the defined sets. Consider that all of the membership functions are defined by the Gaussian function:

where, c is center of the membership function and δ is its variance.

Fig. 4: Performance of the PID controller in second subsystem

Fig. 5: Performance of the proposed controller in first subsystem

Fig. 6: Performance of the proposed controller in second subsystem

It has been assumed that the initial value of θi1(0), θi2(0), uir(0), uicom(0) and be zero. Furthermore, it has been assumed that fmin = 1, Γ1 = 10, λξi0. = 10, , , . In Eq. 43 and remark 1, σ = 0.1, ∈ = 0.01 has been considered.

Fig. 7: Control input u1

Fig. 8: Control input u2

The parameters fdm, fmin and the vector ki = [ki1, ki2,…,ki,ni]T have been chosen so that the lemma 2 holds.

As shown in Fig. 3-6, it is obvious that the performance of the proposed controller is promising. Based on these simulation results, the controller can stabilized the closed loop system. It can decrease error estimation and disturbances effect in the output of the subsystems. Figure 7 and 8 show the total input of each subsystem. It is shown that ripple in the input controller can decrease error estimation and disturbances attenuation.

CONCLUSION

A decentralized fuzzy model reference adaptive output tracking controller is proposed for a class of large scale nonaffine nonlinear systems in this study. Fuzzy systems used to approximate the knowledge of the experts in the controller design procedure. It has been shown that the derived adaptation laws guaranty the Lyapunov’s stability of closed-loop system. Asymptotic convergence of the tracking error to zero is guarantied. Robustness against external disturbances and approximation errors, relaxing the conditions and using knowledge of experts are the merits of the proposed controller.

REFERENCES
1:  Chen, L., G. Chen and Y.W. Lee, 1999. Fuzzy modeling and adaptive control of uncertain chaotic systems. Inform. Sci., 121: 27-37.
CrossRef  |  

2:  Cheng, C.C. and S.H. Chien, 2006. Adaptive sliding mode controller design based on T–S fuzzy system models. Automatica, 42: 1005-1010.
CrossRef  |  

3:  Chiang, C.C., 2005. Adaptive fuzzy sliding mode control for time-delay uncertain large-scale systems. Proceeding of the 44th Conference on Decision and Control and the Eur. Control Conference, December 12-15, 2005, CDC-ECC, Seville, Spain, pp: 4077-4082.

4:  Chiang, C.C. and W.H. Lu, 2007. Decentralized adaptive fuzzy controller design for uncertain large-scale systems with unknown dead-zone. Proceedings of the International Conference on Fuzzy Systems, July 23-26, 2007, London, pp: 1-6.

5:  Feng, G., 2002. An approach to adaptive control of fuzzy dynamic systems. IEEE Trans. Fuzzy Syst., 10: 268-275.
CrossRef  |  Direct Link  |  

6:  Fenga, G., S.G. Caoa and N.W. Reesb, 2002. Stable adaptive control of fuzzy dynamic systems. Fuzzy Sets Syst., 131: 217-224.
Direct Link  |  

7:  Goleaa, N., A. Goleaa and K. Benmahammed, 2003. Stable indirect fuzzy adaptive control. Fuzzy Sets Syst., 137: 353-366.
Direct Link  |  

8:  Ho, H.F., Y.K. Wong, A.B. Rad and W.L. Lo, 2005. State observer based indirect adaptive fuzzy tracking control. Simulation Modeling Practice Theory, 13: 646-663.
Direct Link  |  

9:  Hsu, Y.C., G. Chen, S. Tong and H.X. Li, 2003. Integrated fuzzy modeling and adaptive control for nonlinear systems. Inform. Sci., 153: 217-236.
Direct Link  |  

10:  Ioannou, P. and M.D.J. Ponte, 1988. Adaptive control techniques for a class of large-scale systems. IEEE Conference, America.

11:  Ioannou, P. and J. Sun, 1996. Robust Adaptive Control. 1st Edn., Prentice-Hall, Inc., Englewood Cliffs, NJ..

12:  Jagannathan, S., 1998. Adaptive fuzzy logic control of feedback linearization discrete time dynamical systems under persistence of excitation. Elsevier Science. Automatica, 34: 1295-1310.
CrossRef  |  

13:  Jianga, X., W. Xua and Q.L. Hanb, 2005. Observer-based fuzzy control design with adaptation to delay parameter for time-delay systems. Fuzzy Sets Syst., 152: 637-649.
Direct Link  |  

14:  Karimi, B., M.B. Menhaj and I. Saboori, 2007. Decentralized adaptive control of large-scale nonaffine nonlinear systems using radial basis function neural network. IEICE Trans. Fundementals, 10: 2239-2247.

15:  Labiod, S. and T.M. Guerra, 2007. Adaptive fuzzy control of a class of SISO nonaffine nonlinear systems. Fuzzy Sets Syst., 158: 1126-1137.
CrossRef  |  

16:  Labiod, S., M.S. Boucherit and T.M. Guerra, 2005. Adaptive fuzzy control of a class of MIMO nonlinear systems. Fuzzy Sets Syst., 151: 59-77.
CrossRef  |  

17:  Pagilla, P.R., R.V. Dwivedula and N.B. Siraskar, 2007. A decentralized model reference adaptive controller for large-scale systems. IEEE/ASME Trans. Mechatronics, 12: 154-163.
CrossRef  |  Direct Link  |  

18:  Park, C.W. and M. Park, 2004. Adaptive parameter estimator based on T–S fuzzy models and its applications to indirect adaptive fuzzy control design. Inform. Sci., 159: 125-139.
CrossRef  |  Direct Link  |  

19:  Shaocheng, T., C. Bin and W. Yongfu, 2005. Fuzzy adaptive output feedback control for MIMO nonlinear systems. Fuzzy Sets Syst., 156: 285-299.
CrossRef  |  Direct Link  |  

20:  Shi, L. and S.K. Singh, 1992. Decentralized adaptive controller design for large-scale systems with higher order interconnections. IEEE Trans. Automatic Control, 37: 1106-1113.
CrossRef  |  Direct Link  |  

21:  Tang, Y., N. Zhang and Y. Li, 1999. Stable fuzzy adaptive control for a class of nonlinear systems. Elsevier Science. Fuzzy Sets Syst., 104: 279-288.
CrossRef  |  

22:  Tong, S., H.X. Li and W. Wang, 2004. Observer-based adaptive fuzzy control for SISO nonlinear systems. Fuzzy Sets Syst., 148: 355-376.
CrossRef  |  Direct Link  |  

23:  Tong, S., J. Tang and T. Wang, 2000. Fuzzy adaptive control of multivariable nonlinear systems. Elsevier Science. Fuzzy Sets Syst., 111: 153-167.
Direct Link  |  

24:  Tong, S.C., Q. Li and T. Chai, 1999. Fuzzy adaptive control for a class of nonlinear systems. Elsevier Science. Fuzzy Sets Syst., 101: 31-39.
CrossRef  |  

25:  Wang, L.X. and J.M. Mendel, 1993. Fuzzy basis function, universal approximation, and orthogonal least square learning. IEEE Trans. Neural Network, 3-5: 807-814.
CrossRef  |  

26:  Wang, L.X., 1997. A Course in Fuzzy Systems and Control. 1st Edn., Prentice-Hall Inc., Upper Saddle River, NJ., USA., ISBN: 0135408822, Pages: 424.

27:  Wu, H., 2002. Decentralized adaptive robust control for a class of large-scale systems including delayed state perturbations in the interconnections. IEEE Trans. Automatic Control, 47: 1745-1751.
CrossRef  |  Direct Link  |  

28:  Ying-Guo, P. and Z. Hua-Guang, 1998. Design of fuzzy direct adaptive controller and stability analysis for a class of nonlinear system. Proceeding of the American Control Conference, Volume 4, June 24-26, 1998, Philadelphia, Pennsylvania, pp: 2274-2275.

29:  Yiqian, T., W. Jianhui, G. Shusheng and Q. Fengying, 2004. Fuzzy adaptive output feedback control for nonlinear MIMO systems based on observer. Proceedings of the 5th World Congress on Intelligent Control and Automation Hangzhou, June 15-19, 2004, WCICA, Peoples Republics of China, pp: 506-510.

30:  Yousef, H. and M.A. Simaan, 1991. Model reference adaptive control for large-scale systems with application to power systems. Control Theory Applied IEE Proc., 138: 321-326.
Direct Link  |  

31:  Yu, W.S., 2004. Model reference fuzzy adaptive control for uncertain dynamical systems with time delays. IEEE International Conference on Systems, Man and Cybernetics, Taiwan, pp: 5246-5251.

32:  Zhang, H. and Z. Bien, 2000. Adaptive fuzzy control of MIMO nonlinear systems. Fuzzy Sets Syst., 115: 191-204.
Direct Link  |  

33:  Zhang, L., 2006. Stable fuzzy adaptive control based on optimal fuzzy reasoning. Proceeding of the 6th International Conference on Intelligent Systems Design and Applications (ISDA), October 1, 2006, IEEE Computer Society Washington, DC, USA., pp: 299-304.

34:  Zhang, Y., B. Chen, S. Liu and S. Zhang, 2002. Fuzzy indirect adaptive sliding mode tracking costrol for a class of nonlinear similar composite large-scale systems. Proceeding of the American Control Conference Anchorage, May 8-10, 2002, AK, America, pp: 2961-2967.

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