Transactions of Nanjing University of Aeronautics and Astronautics Dynamic Surface Control with Nonlinear Disturbance Observer for Uncertain Flight Dynamic System Li Fei(李飞)1* Hu Jianbo (胡剑波)1 Wu Jun(吴俊)2 Wang Jian-hao(王坚浩)3 1. Equipment Management and Safety Engineering College, Air Force Engineering University, Xi’an 710051, P. R. China; 2. Institute of Cyber-Systems and Control, Zhejiang University, Hangzhou 310027 , P. R. China; 3. Unit93286, People’s Liberation Army, Shenyang 110141, P. R. China. Abstract: A new robust control method of a nonlinear flight dynamic system with aerodynamic coefficients and external disturbance has been proposed. The proposed control system is a combination of the dynamic surface control (DSC) and the nonlinear disturbance observer (NDO). DSC technique provides the ability to overcome the “explosion of complexity” problem in backstepping control. NDO is adopted to observe the uncertainties in nonlinear flight dynamic system. It has been proved that the proposed design method can guarantee uniformly ultimately boundedness of all the signals in the closed-loop system by Lyapunov stability theorem. Finally, simulation results show that the proposed controller provides better performance than the traditional nonlinear controller. Keywords: nonlinear disturbance observer(NDO), dynamic surface control(DSC), uncertainties, flight control. CLC Number: TP273 Document Code: A crucial for a nonlinear control method to have the Nomenclature =aerodynamic forces about the body-fixed frame F Article ID: L, M , N =aerodynamic rolling, pitching, yawing moment ability to handle unknown disturbances. Another important property required for a good control method in aerospace engineering is the robustness against I =moment of inertia unmodeled dynamics and variation of various p, q, r =roll, pitch, yaw rate about the body-fixed frame aerodynamic derivatives[1][2]. q =dynamic pressure T =thrust common method employed in nonlinear flight V =velocity control[3][4], which allows to be performed without , =angle of attack, sideslip angle state transformation. However it requires complete Feedback linearization is probably the most and accurate knowledge of the plant dynamics. To e , a , r =elevator, aileron, rudder angle , solve this issue, adaptive backstepping design methods =roll, pith angle is proposed[5][6]. However, the parameter estimation 1 Introduction error is only guaranteed to be bounded and converging aerospace to an unknown constant value. DSC technique could engineering technology over the past few decades, conquer the problem of complexity in backstepping requirements on the performance of modern flight design resulting from the repeated differentiations by vehicles have become more and more challenging. introducing a first-order filtering of the synthetic input And there have been great interests on flight dynamic at each step[7][8]. NDO could effectively reject model systems due to the facts that unknown disturbances mismatches and external disturbances to compensate widely exist in aerospace engineering, such as wind, timely for the controller by constructing a new friction, systems, dynamic system[9][10][11]. So it can improve the environmental and electromagnetic noise etc. So it is performance of the controller by eliminating the With the fast coupling development effects from of other effects of uncertainties of controlled device. NDO has Foundation items: This work was supported by the open research project of the State Key Laboratory of Industrial Control Technology Zhejiang University China(No. ICT1401) and Shanghai Leading Academic Discipline Project(No.J50103). *Corresponding author: Li Fei, E-mail: [email protected] Li Fei, et al. Dynamic Surface Control with Nonlinear Disturbance Observer… been widely applied in robot control[10][12][13], missile [14] [15][16] cos sin ( FT Fx ) mV cos sin sin g Fy Fz (cos sin sin (2) mV mV V cos cos sin sin sin cos cos ) p sin r cos control , flight control and so on. Accordingly, in this paper, a new robust control design method for nonlinear flight dynamic system with aerodynamic coefficients and external disturbances is proposed. Firstly, we present the nonlinear flight model with aerodynamic coefficients and external disturbances. Secondly, the design principles of nonlinear disturbance observer are introduced. And the structure of the composite controller is investigated. Then the stability of the proposed control system is analyzed. Finally, simulation results demonstrate that the proposed controller could effectively resist the influence of uncertainties and disturbances on the flight control system. The flight dynamic Eqs.(1–7) can be rearranged as a 2 Problem Formulation kind The control law design is based on the nonlinear model of an F-16 fighter. The task of the controller is to track the commands of ,, and when parameter perturbations and external disturbance exist. The relevant equations of six-degree nonlinear model used for the control design can be written as[17] p cos tan q r sin tan sin cos ( FT Fx ) Fz mV cos mV cos g (sin sin cos cos cos ) V cos p tan (q sin r cos ) (3) p I1qr I 2 pq I 3 L I 4 N (4) q I 5 pr I 6 ( p 2 r 2 ) I 7 M (5) r I 2 qr I8 pq I 4 L I9 N (6) q cos r sin (7) of uncertain non-linear MIMO system considering the external disturbance and aerodynamic coefficients perturbations. x1 x2 x3 1 2 f1 ( x1,x3 ) g1 ( x1,x3 ) x2 1 f 2 ( x1,x2 ) g2 ( x1 )u 2 f 3 ( x1,x2 ) f1 ( x1,x3 ) g1 ( x1,x3 ) x2 [h( x1 ) h( x1 )]u f 2 ( x1,x2 ) g2 ( x1 )u d (t ) (8) (1) Where x , u , f , g , h represent the following terms respectively: x1 , , , T x2 p, q, r , T x3 , T , u e , a , r T ( s i n / c To sCx )[ qS ( ) ] (Czc o qS s / c o s ) ( ) 1 ( c o s s T i n C ) [ qS ( C ) ] qS c o s ( C qSs i n s i n x y z ) mV 0 ( 1 / c o s ) ( s i n s i n c o s c o s c o s ) g c os s i n s i n s c sin o s sin c o sin cos cos V 0 f1 ( x , 1 x ) 3 ( , (9) ) Transactions of Nanjing University of Aeronautics and Astronautics cos tan g1 ( x1,x3 ) sin 1 1 0 sin tan sin tan cos cos tan sin cos 0 Cxq ( )c C z ( )c cos cos q S cos C y p ( )b cos sin Cxq ( )c sin sin C zq ( )c 4m 0 0 cos C yr ( )b 0 0 sin cos Cx ( ) Cz (, ) 0 0 cos e cos e VS h1 ( x1 ) cos sin Cxe ( ) sin sin Cze (, ) cos C ya ( ) cos C yr ( ) 2m 0 0 0 (10) (11) I1qr I 2 pq I 3Cl (, )qsb I 4Cn (, )qsb f 2 ( x1,x2 ) I 5 pr I 6 ( p 2 r 2 ) I 7 Cm ( )qsc I 2 qr I 8 pq I 4 Cl (, )qsb I 9Cn (, )qsb I 3Cl ( )b I 4 Cn ( )b 0 p p VS 0 I 7 Cmq ( )c 4 0 I 4 Cl p ( )b I 9Cn p ( )b 0 g2 ( x1 ) qS I 7 Cm ( )c e 0 f3 ( x1,x2 ) 0 cos (12) I 3Clr ( )b I 4 Cnr ( )b p q 0 I 4Clr ( )b I 9 Cnr ( )b r I 3Cl ( , )b I 4Cn ( , )b r r 0 0 I 4 Cl ( , )b I 9Cn ( , )b I 4Cl ( , )b I 9Cn ( , )b a a r r I 3Cl ( , )b I 4Cn ( )b a a (13) p sin q r (14) 1 and 2 represent the compound uncertainties in m, m and m R such that the magnitudes and the model, f ( x ) , g ( x ) and h( x ) represent the is the external derivatives of f1,f 2,g1 and g2 are bounded for all , and R satisfying m and The following assumptions are used in the design m . Furthermore, there exist gi 0 and gi1 such that 0 gi 0 gi gi1 , i 1, 2 . aerodynamic coefficients , d (t ) disturbance. and analysis processes. Assumption 1: yc [c,c,c ] T Remark 1: Note that g2 represents the input The desired trajectory angular rates p,q and r . Because the control are bounded as [ yc,yc,yc ] 0 (15) where 0 R is a known positive constant and denotes the 2-norm of a vector or a matrix. surfaces of the aircraft are designed to control each axes’ angular rate of aircraft independently, the input matrix g2 is invertible for all cases. Assumption 2: The total velocity and dynamic pressure are constant. V 0,q 0 matrix of the control u to the dynamics of the Remark 2: Note that h1 represents the aerodynamic force component caused by the control (16) surface deflection. The magnitude of h1 is small Assumption 3: There exist positive constants compared to other aerodynamic terms in the dynamic Li Fei, et al. Dynamic Surface Control with Nonlinear Disturbance Observer… equation[17]. Therefore, it can be assumed that h1 is one part of model uncertainties. 3 Nonlinear Disturbance Observer This paper refers to the idea of [17]: the dynamic So NDO in the inner loop of the flight dynamic system can be designed as: ψˆ 2 2 P2 ( x2 ) 2 L2 ( x2 )[ 2 P2 ( x2 ) f 2 ( x1 , x2 ) g2 ( x1 )u] surface control law is designed by separating the flight dynamics into fast and slow dynamics. So we design NDOs in the inner and the outer loop respectively [13][15] : ψˆ1 L1 ( x1 )ψˆ1 L1 ( x1 )[ x1 f1 ( x1 , x3 ) g1 ( x1 , x3 ) x2 ] (17) ψˆ 2 L2 ( x2 )ψˆ 2 L2 ( x2 )[ x2 f2 ( x1 , x2 ) g2 ( x1 )u] (18) where, ψ̂1 , ψ̂2 represent the estimates of ψ1 , ψ 2 respectively. Li ( x ), i 1, 2 is the gain matrix. From (8), the dynamics of the above NDOs can be rewritten as: ψˆ1 L1 ( x1 )ψˆ1 L1 ( x1 )ψ1 L1 ( x1 )endo1 ψˆ 2 L2 ( x2 )ψˆ 2 L2 ( x2 )ψ2 L2 ( x2 )endo2 where, endoi ψi ψˆ i , i 1, 2 . (28) Remark 3: From the dynamics of NDO (19) and (20), we can know that the output of NDOs ψ̂1 and ψ̂2 will converge to ψ1 and ψ2 via choosing appropriate values of L1 ( x1 ) and L2 ( x2 ) . 4 Controller Design The structure of the two-timescale controller for flight control system is shown in Fig.1. In each feedback loop, control laws x2c and u are designed separately. (19) (20) It is assumed that the uncertainties are slowly time varying compared with the dynamics of NDO. So Fig.1 ψ1 0, ψ 2 0 . Structure of the two-timescale controller In this paper, the dynamic surface control method Then (19) and (20) can be rewritten as: endo1 L1 ( x1 )endo1 0 (21) is used to design a controller. The dynamic surface endo 2 L2 ( x2 )endo 2 0 (22) control method can be viewed as the two-timescale In order to avoid the angular acceleration approach because the fast states x2 are used as measurement, an auxiliary nonlinear function vector is control input for the slow states x1 intermediately. However, this methodology considers the transient defined to improve the NDO in the inner loop. 2 ψˆ 2 P2 ( x2 ) (23) responses of the fast states and, therefore, does not require the timescale separation assumption. where dP2 ( x2 ) L2 ( x2 ) x2 dt From (23), we have: 2 ψˆ 2 P2 ( x2 ) The controller design procedure can be given as (24) follows. Step 1: define error surface S1 x1 x1c , where (25) x1c yc . Its derivative is From (20) and (24), the derivative of 2 can be expressed as: S1 x1 x1c 2 L2 ( x2 )endo 2 L2 ( x2 ) x2 L2 ( x2 )(ψ2 ψˆ 2 ) L2 ( x2 ) x2 (26) =f1 g1 x2 ψ1 yc (29) f1 g1 ( x2 c e2 ) ψ1 yc where, x 2c is the expected virtual control of the inner From (8), (26) can be rewritten as: 2 L2 ( x2 )( f 2 ( x1 , x2 ) g2 ( x1 )u) L2 ( x2 )ψˆ 2 L2 ( x2 )( f 2 ( x1 , x2 ) g2 ( x1 )u) L2 ( x2 )( 2 P2 ( x2 )) (27) loop, e2 x2 x2c . From (17), we have S1 f1 g1 ( x2c e2 ) ψˆ1 endo1 yc We choose the virtual control as: (30) Transactions of Nanjing University of Aeronautics and Astronautics 1 x2c g1 (k1 S1 f1 ψˆ1 yc ) ψˆ1 L1 ( x1 )ψˆ1 L1 ( x1 )[ x1 f1 ( x1 , x3 ) g1 ( x1 , x3 ) x2 ] F ( constant 2 0 to obtain the filtering virtual control is continuous. Consider Lyapunov x2 c x2c x2 c 2 Step 2: Define the error surface S2 x2 x2c , its derivative is S2 x2 x2c f2 g2 u ψ2 x2c (33) The NDO (28) is used to observe the uncertainty ψ 2 in the inner loop: follows: k23 is a k1 3I L2 1 , k2 ( g112 1) I 4 , L1 1 I 4 , g2 1 1 I , ( 11 1) such that the errors of states 4 4 2 are uniformly ultimately bounded and may be kept arbitrary small. u g21 (k2 S2 f 2 ψˆ 2 x2 c ) x2c x2c x2 c 2 x2c g11 (k1 S1 f1 ψˆ 1 yc ) k2 diag k21 k22 positive number (i.e., V ). Theorem 1: Suppose that nonlinear model of an F-16 aircraft (8) with aerodynamic coefficients and external disturbances is controlled by the proposed controller (35). In addition, if the proposed control system satisfies Assumptions 1~4, there exist ki , Li , (34) Choose an actual control u to drive S2 0 as (35) Differentiating the Lyapunov candidate function (42) and substituting (21), (22), (37), (39), and (40), we can obtain positive V S1T (k1 S1 g1 S 2 g1 y2 endo1 ) 2 T T S 2T (k2 S 2 endo2 ) endo, i Li endo,i y 2 ( constant. i 1 5 Stability Analysis min (k1 ) S1 S1 g1 S 2 S1 g1 y2 S1 endo1 min (k2 ) S 2 errors to represent the closed-loop system as follows: S1 f1 g1 ( S2 x2c ) ψ1 yc (36) S2 k2 S2 endo2 (37) y2 x2 c x2 c =g11 (k1 S1 f1 ψˆ 1 yc ) x2 c (38) Using (17) and (38), (36) can be rewritten as follow: (39) Differentiating (38), we can obtain y2 F ( S1,S2,y2,yc,yc,yc ) τ2 1 y2 τ2 2 2 2 S 2 endo2 min ( Li ) endo,i 2 i 1 y2 F (43) Then, the boundary error is defined as: S1 k1 S1 g1 ( S2 y2 ) endo1 y2 F) τ2 2 We first describe the derivative of the surface where (42) Assumption 4: Assume that the Lyapunov satisfying S2 x2 x2c f2 g2 u ψˆ 2 endo2 x2c y2 function candidate function (42) is bounded by any given Therefore where following 1 2 T T V [ ( SiT Si endo, i endo,i ) y2 y2 ] 2 i 1 in each step. (32) the candidate x2c , which could solve the computational complexity 2 x2c x2c x2c,x2c (0) x2c (0) (41) f f g (k1 S1 1 x1 1 x3 ψˆ 1 yc ) x1 x3 1 1 (31) Where, k1 diag k11 k12 k13 , k1i 0, i 1, 2,3 . We pass x2c through a first-order filter with time g11 g 1 x1 1 x3 )(k1 S1 f1 ψˆ 1 yc ) x1 x3 (40) from Assumption 1 that [ yc,yc,yc ] T is bounded, thus there exists a positive constant 1 such that F1 1 . therefore, from Assumption 3 and applying Young’s inequality, we have Li Fei, et al. Dynamic Surface Control with Nonlinear Disturbance Observer… V min (k1 ) S1 S1 2 2 1 endo1 4 S1 2 1 2 g11 S 2 4 min (k2 ) S 2 2 2 min ( Li ) endo,i 2 i 1 1 y2 τ2 2 2 2 S1 S2 y2 2 2 2 1 2 g11 y2 4 1 endo2 4 2 simulation on the attitude maneuver flight control system of F-16 aircraft. The following command 2 values of d , d , and d are applied to the aircraft in a steady-state level flight of V 200 m s , 1 12 4 h 4000m , FT 60kN : (44) k min (k1 ) 3 , choose 2 1 2 1 k min (k2 ) g112 1 , 4 2 1 g112 1 1 , and Li min ( Li ) , i 1, 2 yields 4 4 2 V ki Si 2 i 1 Li endo,i 2 2 y2 where ki 0 , Li 0 , 2 0 . Then, choose the constant 1 12 4 2 yc can be obtained from yd by the following command filterer: satisfying the 0 2 min[k1,k2, 2,L1,L2 ] (46) Hence, (30) can be rewritten as (45) The integration of (45) is multiplied by e t is: 2 1 (46) V (t ) [V (0) 1 ]e t 12 4 4 Accordingly, the surface errors S1 and S2 , the nonlinear disturbance observer errors endo1 and endo2 , the boundary layer error y2 are uniformly ultimately bounded in the following compact set : {S1 , S2 , endo1,endo2,y2 | ( S Si e i 1 when k1 3I e 2 ) y y2 1 } 2 T 2 1 k2 ( g112 1) I 4 , We assume that the uncertainties are time-varying. , L1 (47) 1 I 4 external disturbance are given as C j [1 0.5sin(0.5 t )]C rj , j x, y, z, l , m, n 2 This relation implies that V 0 when V 1 . 4 T ndo,i ndo,i [ yc ]i n2 2 ,n 4,n 0.8,i 1, 2,3 [ yd ]i s 2snn n2 The time-varying aerodynamic coefficients and 1 V V 12 4 T i 3 , 0 t 2s 0,0 t 2 s 10 ,2 s t 5s 30 ,2 s t 5s d 0,5s t 10s , d 0 , d 0,5s t 10s s t 15s 10 ,10 30 ,10 s t 15s 0,15s t 20 s 0,15s t 20s (45) following condition: 2 approach. The proposed controller is tested in , g2 1 1 L2 I , ( 11 1) . Besides, The compact set (47) 4 4 2 can be kept arbitrarily small by adjusting k1 , k2 , L1 , L2 , and . That is, the tracking error S1 can be made arbitrarily small. d (t ) [0.5 1 1.5] 104 sin(t ) where Cxr , C yr , Czr , Clr , Cmr , and Cnr are the standard aerodynamic coefficients. The final design parameters are chosen as k1 10 I , k2 5 I , 2 0.05 , where I represents the 3 3 observer gains are chosen as L1 (, , ) diag[2(1 2 ), 2(1 2 ), 2(1 2 )] L2 ( p,q,r ) diag[5(1 p 2 ), 5(1 q 2 ), 5(1 r 2 )] Simulation results are shown in Fig.2–7. Subscript 1 represents the simulation results of dynamic surface controller with NDO, and subscript 2 represents the simulation results of dynamic surface controller without NDO. These figures show that under uncertainties, the proposed NDO-enhanced dynamic surface 6 Simulation Results identity matrix. Nonlinear disturbance control method could signals in the closed-loop system. application of the better performance and guarantee the boundedness of the This section presents the simulation results from the exhibit nonlinear disturbance observer-enhanced dynamic surface flight control Transactions of Nanjing University of Aeronautics and Astronautics 15 30 αd α1 α2 qd q1 q2 20 10 q/(deg/sec) α/deg 10 5 0 -10 0 -20 -5 0 5 10 t/sec 15 -30 0 20 Fig.2 The curves of the attacking angle tracking( ) 5 10 t/sec 15 20 Fig.6 The curves of the pitching rate tracking( q ) 0.5 8 βd 0.4 β1 rd β2 r1 r2 6 0.3 4 r/(deg/sec) 0.2 β/deg 0.1 0 2 0 -0.1 -2 -0.2 -4 -0.3 -0.4 0 5 10 t/sec 15 -6 0 20 Fig.3 The curves of the sideslip angle tracking( ) 5 10 t/sec 15 20 Fig.7 The curves of the yawing rate tracking( r ) 35 Φd Φ1 Φ2 30 7 Conclusions 25 A Φ/deg 20 disturbance observer-enhanced dynamic surface flight control approach for nonlinear 15 flight dynamic system with aerodynamic coefficients 10 5 and external disturbance uncertainties has been 0 -5 0 nonlinear proposed. This approach could solve the problem of 5 10 t/sec 15 20 Fig.4 The curves of the rolling angle tracking( ) “explosion of complexity” caused by the traditional backstepping method and exhibited excellent disturbance attenuation ability and robustness against 80 pd p1 uncertainties. All the signals in the closed-loop system p2 60 are guaranteed to be globally uniformly ultimately p/(deg/sec) 40 20 bounded. And the tracking error of the system is 0 proven to be converged to a small neighborhood of the -20 origin. The dynamic surface control with NDO -40 approach proposed in this paper may also be used in -60 -80 0 5 10 t/sec 15 20 Fig.5 The curves of the roll rate tracking( p ) the design of the controllers for other dynamic systems with model mismatches and external disturbances. References [1] Qiao B, Liu Z Y, Hu B S, et al. Adaptive angular velocity tracking control of spacecraft with dynamic uncertainties[J]. Transactions of Nanjing University of Li Fei, et al. 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