Stabilization of the FO-BLDCM Chaotic System in the Sense of Lyapunov

Based on an integer-order Brushless DC motors (IO-BLDCM) system, we give a fractional-order Brushless DC motors (FOBLDCM) system in this paper. There exists a chaotic attractor for fractional-order 0.95 < q ≤ 1 in the FO-BLDCM system. Furthermore, using the Lyapunov direct method for fractional-order system, a control scheme is proposed to stabilize the FOBLDCM chaotic system in the sense of Lyapunov. Numerical simulation shows that the control scheme in this paper is valid for the FO-BLDCM chaotic system.


Introduction
Brushless DC motors (BLDCM) system [1][2][3][4] has several advantages over brushed DC motors, like elimination of ionizing sparks, overall reduction of electromagnetic interference, reduced noise, longer lifetime, increased efficiency and reliability, and so forth; BLDCM has been widely applied in positioning and actuation systems, motion control systems, radio controlled cars, and industrial automation design.Recently, some results have shown that the chaotic motion can be presented in BLDCM system.However, the chaotic motion in BLDCM system is not acceptable in practical situations, because it can destroy the stable operation of the BLDCM system and can lead to system malfunction in practical applications.So, BLDCM system stability is usually a prerequisite of practical application.Up to now, in order to control the chaotic motion in BLDCM system, some schemes have been presented [2][3][4].
On the other hand, it has been recognized that many real-world physical systems can be described by fractionalorder differential equations, such as the fractional-order telegraph system [5], the fractional-order heat conduction system [6], the fractional-order diffusion and superdiffusion system [7,8], the fractional-order Chua system [9], the fractional-order Duffing system [10], the fractional-order cellular neural network [11], the fractional-order gyroscopes system [12], and the fractional-order microelectromechanical system [13].Meanwhile, it is well known that many fractional-order systems exhibit chaotic behavior, such as the fractional Lorenz chaotic system [14], the fractionalorder Chua chaotic system [9], the fractional-order Duffing chaotic system [10], the fractional-order Volta chaotic system [15], the fractional-order gyroscopes chaotic system [12], the fractional-order microelectromechanical chaotic system [13], and the fractional-order chaotic electronic circuit [16].Furthermore, control of the fractional-order chaotic systems has been attracting more attention in recent years [17][18][19][20].
Motivated by the above considerations, a FO-BLDCM system under loading conditions is presented.By numerical calculation, we find that the FO-BLDCM system exhibits a chaotic attractor, and we obtain the largest Lyapunov exponent of the FO-BLDCM system.Furthermore, based on the Lyapunov direct method for fractional-order system [21][22][23], we propose a control scheme to stabilize the FO-BLDCM chaotic system.The result in this paper shows that the FO-BLDCM chaotic system can be stable in the sense of Lyapunov.

A FO-BLDCM Chaotic System
Ge and Chang [2] proposed a mathematical model for a BLDCM system under loading conditions and found that the BLDCM system creates chaotic behavior.Based on the mathematical model in [2], we present a FO-BLDCM system model which is described as follows: where 0 <  ≤ 1 is the fractional order and ). Constants   ,   are related to the fictitious inductance on the quadrature-axis and direct-axis, respectively; constant   is related to the external load, Coulomb friction, cogging effect, and so forth; constants ,  are related to the fictitious inductance on the quadratureaxis and direct-axis; constant  is related to the inertia of rotator and the viscous damping coefficient; constant  is a free parameter.In this paper, we choose system parameters in FO-BLDCM system (1) as follows:   = 0.168,   = 20.66,  = 0.53,  = 0.875,  = 0.26,  = 4.56, and  = 60.
By the improved version of the Adams-Bashforth-Moulton numerical algorithm [24] for fractional-order nonlinear system, the fractional-order system (1) can be discretized as follows: The largest Lyapunov exponent of FO-BLDCM system (1) with respect to the fractional-order . ( The error is where ℎ is step size.Recently, Zhou and Huang [25] introduced one numerical method to calculate the largest Lyapunov exponent (LLE) for the fractional-order nonlinear system.Now, we can calculate the largest Lyapunov exponent (LLE) of FO-BLDCM system (1) for difference fractional-order . Figure 1 displays the LLE of FO-BLDCM system (1) with respect to the fractional-order .
According to Figure 1, the LLE of FO-BLDCM system (1) is larger than zero for 0.95 <  ≤ 1, which implies that chaotic behavior will emerge for 0.95 <  ≤ 1. Figure 2 shows a chaotic attractor of the FO-BLDCM system (1) for  = 0.96, in which the LLE is 1.4972.Conversely, the LLE of FO-BLDCM system (1) is less than zero for  ≤ 0.95, which implies that FO-BLDCM system (1) is stable.For example, the LLE is −0.4246 for  = 0.94, and the evolution of state of  1 ,  2 , and  3 is shown in Figure 3.

Control of the FO-BLDCM Chaotic System
In this section, control of the FO-BLDCM chaotic system will be discussed.

Theorem 1. Give the controlled FO-BLDCM system as
and ) is a scalar controller,   ( = 1, 2, 3) is the equilibrium point of FO-BLDCM system (1), and 4) is stable in the sense of Lyapunov.
According to Figures 4-6, we can obtain that the proposed theorem is valid for FO-BLDCM chaotic system.

Conclusions
A FO-BLDCM system is proposed in this paper.The chaotic motion can be presented in the FO-BLDCM system for 0.95 <  ≤ 1.The chaotic phase portraits for  = 0.96 and the largest Lyapunov exponent with varying the fractional order are obtained by numerical calculation.Based on the Lyapunov direct method for fractional-order system, we proposed a control scheme to stabilize the FO-BLDCM chaotic system in the sense of Lyapunov.The simulation results show that the proposed scheme is effective.