Global_Stabilization_of_a_Class_of_Feedforward_Systems_with_Lower-Order_Nonlinearities-0i3

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Technical Notes and CorrespondencesGlobal Stabilization of a Class of Feedforward Systems with Lower-Order NonlinearitiesShihong Ding,Chunjiang Qian,and Shihua LiAbstract—This note considers the problem of global stabilization for aclass of feedforward systems whose nonlinearities are allowed to be lower-order growing,instead of higher-order or linear growing as required by many existing results.To solve the problem,a domination approach is de-veloped to design the stabilizer with a new structure,which is also inte-grated with the adding a power integrator and nested saturation methods.The use of a locally homogeneous stabilizer enables us to relax the growth condition imposed on the nonlinearities and hence enlarges the class of feed-forward systems which are globally stabilizable.Index Terms—Feedforward systems,global stabilization,lower-order nonlinearities.I.I NTRODUCTIONTHIS note considers the global stabilization problem of feedforwardsystems described by_x i =d i x i +1+f i (t;x i +1;...;x n );i =1;...;n 01;_x n =d n u(1)where x =(x 1;...;x n )T2is controlinput,d i ,i =1;...;n are uncertain (possibly time-varying)real pa-rameters satisfying d d i dwith positive constants d and d ,and f i (t;x i +1;...;x n );i =1;...;n 01are continuous unknown func-tions with f i (t;0;...;0)=0.The global stabilization of feedforward systems is a problem of theoretical and practical importance and has at-tracted a great deal of attention from the nonlinear control community.A number of interesting results have been achieved to solve the stabi-lization problem based on two major approaches:the nested saturation design method and the Lyapunov’s direct design method.The nested saturation method was introduced by Teel [1],[2]to globally stabilize a class of nonlinear feedforward systems,and was expanded by many papers,such as [3]–[7].Another effective method for stabilizing feed-forward systems is the forwarding technique [8],[9],[11],[12],which is based on the Lyapunov’s direct design.Both the nested saturation and forwarding methods have been widely used for the stabilization of feedforward systems and many interesting results have been achieved (see surveys in books [13],[14]).Manuscript received March 06,2009;revised August 24,2009.First published January 22,2010;current version published March 10,2010.This work was supported in part by the U.S.National Science Foundation under Grant ECCS-0239105,National Natural Science Foundation of China (Project 60504007),Graduate Innovation Program of Jiangsu Province (Project CX08B_087Z),and the Scientific Research Foundation of Graduate School of Southeast University.Recommended by Associate Editor S.Celikovsky.S.Ding and S.Li are with the School of Automation,Southeast University,Nanjing,Jiangsu 210096,China (e-mail:shihong.ding@;lsh@).C.Qian is with the Department of Electrical and Computer Engineering,Uni-versity of Texas at San Antonio,San Antonio,TX 78249,USA (e-mail:chun-jiang.qian@).Digital Object Identifier 10.1109/TAC.2009.2037455Almost all the aforementioned results on global stabilization of feed-forward systems are based on certain restrictions imposed on the non-linear terms.A common assumption is that the nonlinear functions should satisfy a higher-order growth condition as required in [2].An-other common assumption for the global stabilization of feedforward systems is that the nonlinear terms need some smoothness [3],[6],[15].Nevertheless,the ubiquitous property of all aforementioned conditions is that the nonlinear term f i (t;x i +1;...;x n )should be at least locally linearly growing in variables x i +1;...;x n .An interesting question is to what extent those restrictions can be relaxed to achieve global stabi-lization for the feedforward system (1).For example,is it possible to achieve global stabilization of the following system_x 1=x 2+x 22+x 2=33;_x 2=x 3;_x 3=u (2)using a state-feedback stabilizer?Apparently,the functionf 1(x 2;x 3)=x 22+x 2=33has a nonlinear term x 2=33which is neither higher-order nor continuously differentiable and hence system (2)cannot be handled by existing design methods.In this note,we aim to tackle this question and shall provide a solution to the problem of global stabilization for (1)under a lower-order growth condition.This problem is of theoretical interest since we can enlarge the class of feedforward systems which are globally stabilizable by state feedback.In this note,we introduce a domination approach based on [16],[17],[19]to solve the global stabilization problem of system (1)with lower-order nonlinear terms,which is not solvable by the existing re-sults including [16],[19].Specifically,we first use the technique of adding a power integrator [17]to design a higher-order controller for the linear nominal system.Then using homogeneous properties,we show that the nonlinear system (1)is locally stabilized by the proposed controller even under a lower-order growth stly,we add a series of nested saturations to the higher-order controller obtained for the nominal system.Under the saturated control law,by tuning the sat-uration level,we show that the states of the closed-loop system will enter and stay in a small region where the controller is no longer satu-rated and will finally stabilize system (1).The novelty of the proposed controller lies in its new structure in which most of the controller co-efficients are preset without knowing the nonlinearities and only the saturation level is tunable.Hence,for a family of feedforward systems whose nonlinearities have different growth rates,we can just use the same stabilizer but with different saturation levels.II.P ROBLEM S TATEMENT AND A P RELIMINARY R ESULTThe objective of this note is to design a state feedback controller u (x )which globally stabilizes system (1)in the absence of the commonly required higher-order or linear growth conditions.Specifically,we will show that global stabilization of (1)is achievable under the following assumption.Assumption 2.1:In a neighborhood of the origin,the following in-equalities hold for i =1;...;n 01:j f i (t;x i +1;...;x n )j (j x i +1jpFor simplicity,we assume =p=q with an even number p and an odd number q.Consequentlyr i:=(i01) +1;i=1;...;n+1(4) are ratios of two odd numbers,which will simplify the notation of the controller since(0s)r=0s r.A similar result can be obtained for any real number 0by defining[s]r=sign(s)j s j r[17].Remark2.1:Note that the higher-order growth condition for global stabilization of(1),such as for i=1;...;n01j f i(t;x i+1;...;x n)j (j x i+1j p+111+j x n j p);p>1(5)used in[16]is a special case of Assumption2.1with =0.A distinc-tive feature of Assumption2.1is theflexibility in choosing which enables us to handle a more general class of systems such as(2).How-ever,it should be pointed out that thefirst term j x i+1j px n+ n01=r=rr=r0 )=r=r=r0 )=r0 )=r0 )=r=r0r1 2+ (2r)=r=r0r2x3+ d j 1=r0r2j r2j2r j 1=r2j r1j r1,y= 1=r=r=r0 )=r3 d(2r n0r2)=(2r n)0r+ 1r2(2r n0 )=(2dr1r n)(3 1r2 =(2dr1r n)) =(2r3 21r2r3=(2dr1r n)=(2r).It isobvious that for any constant 2 32( 1):= 1( 1)+n01,thevirtual controller x33=0 2 r2yields_V2 0(n01)( 2r1+ 2r2)+d2 (2r012(x30x33):Following the same line shown in thefirst two steps,we canfindfunctions 33( 1; 2);...; 3n( 1;...; n01)such that for any con-stants 1 31; 2 32( 1);...; n 3n( 1;...; n01)the fol-lowing holds:_Vn(X n)j(6)0(7) 0( 2r1+111+ 2rn)where 1=x1, i=x i0x3i,x3i=0 i01 r i01;i=2;...;n;andV n(X n)=0 )=ri=12ri+n01=r=r0r+111+j x n j p0r).It can be verified thatboth=r=r=rni=12ri+ =rn01i=1(j x i+1jp=r=rn01i=1(j x i+1jp=r=ris homogeneous of degree withrespect to dilation(r)if,for all">0;V("x)="III.G LOBAL S TABILIZATION OF F EEDFORWARD S YSTEMSIn this section,we show that under Assumption 2.1,the problem of global stabilization for system (1)is solvable.Specifically,based on (7)in Section II,we first construct the following bounded control law:u =u n (X n )=0 nr(10)with u 0=0,u i (X i )=0 i [ r )]r,i =1;...;n 01,and (s )=>max ( d +1)=d (r[ d (1+ >max ;...;()+ d+1;2 i n (12);...;r ( d(1+ ());2 j n 01:(13)To show how controller (10)can globally stabilize system (1),we firstintroduce a lemma which will be constantly used in the proof of the main result.Lemma 3.1:Consider the closed-loop system (1)–(10).For i =1;...;n 01,under the condition j x j jr(14)(b )j u i (X i ( t))0u i (X i (t ))j r (1+ j 01);j =i +1;...;n;it canbe seen from (3)thatj f i (1)j rj 1+ i jpr+111+j 1+ n 01j p prpr0rat time instantst k ,i.e.,j x 1(t k )j =rfor all t in a time interval [t k ;t k +1],by continuity ofthe solution,x 1(t k )and x 1(t k +1)have the same sign,i.e.,x 1(t k ) r or x 1(t k ) 0 r ,which in turn implies j u 1(x 1(t k ))0u 1(x 1(t k +1))j =0by the property of saturation function.Hence,we only need to consider the time interval [t k ;t k +1]where j x 1(t )j r.First,by the mean value theorem and the monotone property of s p 01for odd fraction p 1,we have the following inequality:j a p 0b p j p j a 0b j max fj a j p 01;j b j p 01g ;a;b 2I R :(18)Using (18)and the fact that j x r 011j yieldsj u 1(x 1( t 3))0u 1(x 1(t 3))j = 1j x r 1( t 3)0x r 1(t 3)j 1r 2r 1j x 1( t3)0x 1(t 3)j :(19)For j x j (t )j (1+ j 01) rtj d 1x 2(s )+f 1(s;x 2(s );...;x n (s ))j ds[ d(1+ 1)+1] r +( t30t 3)with 1( 1):=( 1r 2=r 1)[ d(1+ 1)+1].This together with (17)and;j =2;...;nj u 1(x 1( t))0u 1(x 1(t ))j 1( 1) r ;j =i;...;nj u i 01(X i 01( t))0u i 01(X i 01(t ))j i 01(1) r ;j =i +1;...;n:(22)Similar to what we did in the initial step,for any final point u i (X i ( t))and starting point u i (X i (t )),we list time series t k ;k =1;2;...;N ,such that j x i (t k )0u i 01(X i 01(t k ))j = rk =0j u i (X i (t k ))0u i (X i (t k +1))j :(23)If j x i (t )0u i 01(X i 01(t ))jr;8t 2[t k ;t k +1].In one of such intervals [t 3; t 3]where j x i (t )0u i 01(X i 01(t ))jr[x i ( t 3)0u i 01(X i 01( t 3))]r0[(x i (t 3)0u i 01(X i 01(t 3)))]ri r i +1ri=r=r:(24)For j x j (t )j (1+ j 01) rtj d i x i +1(s )+f i (s;x i +1(s );...;x n (s ))j ds[ d(1+ i )+1]( t 30t 3) r ;8t 2[t 3; t3],we have j x i (t )j (1+ i 01)r+ ( t30t 3)with i ( 1;...; i )=( i r i +1=r i )[ d(1+ i )+1+ i 01(1)].This together with (23),and+( t 0t )for j x j (t )j (1+ j 01) rg .To this end,we first claim that there exists a time instant t 1such that j x n (t 1)0u n 01(X n 01(t 1))jr=2,for all t 0.We firstconsider the case whenx n (t )0u n 01(X n 01(t ))>r(x n (t )0u n 01(X n 01(t ))=r=rwith which implies that x n (t ) x n (0)0 n r,yieldsrt + n 01r=2;8t 0is also impossible.In conclusion,there exists a time instant t 1such that j x n (t 1)0u n 01(X n 01(t 1))jr;8t t 1:(28)Again,a contradiction argument will be used to prove (28).If (28)is nottrue,there exist two time instants t 012[t 1;+1)and t 0012(t 01;+1)such that x n (t )0u n 01(X n 01(t ))will leave the boundariesr=2at t 01andr at t 001sequentially,i.e.,(a)j x n (t 01)0u n 01(X n 01(t 01))j =r,(c)r;8t 2[t 01;t 001].We first consider the casex n (t 01)u n 01(X n 01(t 01))=r(30)r;t 2[t 01;t 001]:(31)By (31),it can be concluded that for t 2[t 01;t 001],_x n (t )=0d n n [r)]r 0 nr=r (t 0010t 01).This,together with (29),leads tor(t 0010t 01)r(t 0010t 01):(32)Noting the fact that j u n 01(X n 01(t ))j n 01r;8t 2[t 01;t 001].Under thiscondition,it follows from Lemma 3.1thatu n 01(X n 01(t 001))0u n 01(X n 01(t 01))rr +( n 01( 1;...; n 01)0 n )(t 0010t 01)<r.Hence,for 8t t 1X n (t )2Q n =f X n (t ):j x n (t )0u n 01(X n 01(t ))j <rg :(34)Next we will prove that the above relation will also hold at step i .Similar to the first step,we first show that there exists a time instant t i t i 01such that j x n 0i +1(t i )0u n 0i (X n 0i (t i ))jr=2.To give a com-plete picture of the negative case which was not elaborated in the initial step,here we consider in detail the following case x n 0i +1(t )0u n 0i (X n 0i (t ))<0 r12=r0 drUsing (34),for t t i 01,we have j x j (t )j r;tt i 01.By (35),for 8t t i 01,the following inequality holds:_x n 0i +1(t )12=rr=r>0.Itfollows that x n 0i +1(t ) x n 0i +1(t i 01)+ n 0i +1r2>x n 0i +1(t )0u n 0i (X n 0i (t )) x n 0i +1(t i 01)+ n 0i +1rwhich leads to a contradiction as the time goes to infinity.A proof sim-ilar to the initial step can be used to show that the case x n 0i +1(t )0u n 0i (X n 0i (t ))>r=2.Following the line of the proof of the initial step,next we use a con-tradiction argument to prove that j x n 0i +1(t )0u n 0i (X n 0i (t ))j <r=2,(b)j x n 0i +1(t 00i )0u n 0i (X n 0i (t 00i ))j =r=2j x n 0i +1(t )0u n 0i (X n 0i (t ))jr2(37)x n 0i +1(t 00i )0u n 0i (X n 0i (t 00i ))=0rx n 0i +1(t )0u n 0i (X n 0i (t ))0 r ;t 2[t 0i ,t 00i ],which leadsto x n 0i +1(t 00i ) x n 0i +1(t 0i )+ n 0i +1r=x n 0i +1(t 00i )0u n 0i (X n 0i (t 00i ))x n 0i +1(t 0i)0u n 0i (X n 0i (t 0i ))+u n 0i (X n 0i (t 0i ))0u n 0i (X n 0i (t 00i ))+ n 0i +1r20j u n 0i (X n 0i (t 00i))0u n 0i (X n 0i (t 0i ))j + n 0i +1r,it can beconcluded thatj x n 0i +1(t )j (1+ n 0i ) r(1+ j 01),j =n 0i +1;...;n .Hence,by Lemma 3.1,the following holds:u n 0i (X n 0i (t 00i ))0u n 0i (X n 0i (t 0i ))rr( n 0i +10 n 0i (1))(t 00i 0t 0i )>0r=2x n 0i +1(t )0u n 0i (X n 0i (t ))r,or X n 0i +1(t )2Q n 0i +1=f X n 0i +1(t ):j x n 0i +1(t )0u n 0i (X n 0i (t ))j < rgfor n 0i +1 j n ,which completes the inductive proof.Final Step.Proceeding by this way,we can obtain there exists t n 01>0such that for t t n 01;j =2;...;nX j (t )2Q j =f X j (t ):j x j (t )0u j 01(X j 01(t ))j < r(1+ j 01),j =2;...;n .Therefore,by Lemma 3.1,j f 1(t;x 2;...;x n )j rfort t n 01.With the help of these relations,it can be concluded from _x 1(t )=d 1u 1(x 1(t ))+f 1(t;x 2;...;x n )+d 1[x 2(t )0u 1(x 1(t ))]that (a)_x 1(t ) 0 1 r =2,or (b)_x 1(t ) 1 r=2,where 1=d 1(1=2)r 0 d01>0guaranteed by (11).Hence,in either case,there exists a time instant t n such thatfor t t n ,j x 1(t )j r,which implies x 1(t )2Q 1=f x 1:j x 1(t )j <r,j x 2(t )0u 1(x 1(t ))j <rg in which by the construction defined as (10)it is clearthat u i (X i )=0 i [ r)]r =0 i (x i 0x 3i 01)r;i =1;...;n .It implies the saturated control law (10)reduces to the homogeneous controller (7)for X n (t )2Q;8t t n .In addition,by tuning the parameter we can assure that Q where is defined in Lemma ing Lemma 2.1,we obtain that the closed-loop system (1)–(7)is locally strongly stable.Thus it can be concluded that system (1)is globally (strongly)stabilized by controller (10).Remark 3.1:The idea of our proof of the main result is originally inspired by [16]and [17].In this note,we first design a controller for system (6)using the adding a power integrator technique proposed in [17].Note that the adding a power integrator technique in [17]is a top-to-bottom method which is mainly used to design stabilizers for lower-triangular systems.In order to handle the upper-triangular system (1),we first generalize the adding a power integrator technique [17]with the added flexibility in tuning the controller coefficients and then integrate it with the nested saturation scheme [16].The key toolRenabling us to handle the lower-order nonlinearities in this technical note is the use of the higher-order homogeneous controller,while the controller in[16]will still reduce to a linear one.Moreover,a new controller structure is developed by presetting most of the controller coefficients,i.e., i’s,and leaving only one gain,i.e., ,adjustable to obtain the global stabilization results.Specifically,the parameters i’s arefirst determined without knowing the detailed nonlinearities. Then we tune the single gain in the controller to globally stabilize the system.Hence,for a family of feedforward systems whose non-linearities have the same but different growth rates ’s in(3),our domination approach enables us to use the same parameters i’s in the controllers and only tune different ’s to dominate the nonlinearities. With the help of Theorem3.1,now it is possible to design a global stabilizer for system(2).Example3.1:Due to the fact that the nonlinear term f1(x2;x3)= x2=33+x22has a lower-order growing term,the global stabilization problem of system(2)remained unsolved prior to this work.By se-lecting =2,one can verify that p13=2=3>r2=r3=3=5with (r1;r2;r3;r4)=(1;3;5;7),which in turn implies that f1(1)satis-fies Assumption2.1.Therefore,by Theorem3.1,with appropriate con-stants 1; 2; 3and ,system(2)can now be globally stabilized by the following controller:u=0 3 05x3+2 03x2+ 1 01x135=37=5:Next we show that Theorem3.1leads to an interesting result for the strictly feedforward system_x i=d i x i+1+f i(t;x i+2;...;x n);i=1;...;n02;_x n01=x n;_x n=u(44) where d i are uncertain parameters satisfying d d i d with con-stants d>0; d>0,and f i(1);i=1;...;n02are continuous func-tions with f i(t;0;...;0)=0.Unlike most of the existing results that required Lipschitz continuity or higher-order growth condition,now we show that global stabilization of(44)can be achieved under a linear growth condition.Corollary3.1:The strict feedforward system(44)can be globally strongly stabilized if the nonlinear function satisfies a linear growth condition around the origin.Proof:By the linear growth condition,around the origin j f i(t;x i+2;...;x n)j (j x i+2j+111+j x n j); >0where p ij= 1>(i +1)=((j01) +1),i=1;...;n02;j=i+2;...;n for any positive .Hence Assumption2.1holds for(44).It implies Corollary3.1follows immediately by Theorem3.1.Corollary3.1allows us to solve the global robust stabilization problem for the following system.Example3.2:Consider the following system with uncertainties1 d1 2and j d(t)j 1_x1=d1x2+d(t)x3+x23;_x2=x3;_x3=u:(45)It can be verified that around the origin the nonlinear functions satisfy the linear growth condition.Hence,we can designu=0 3 011=7x3+2 09=7x2+ 1 01x19=711=913=11with appropriate coefficients i’s and saturation constant ,which globally stabilizes system(45).Due to the nature of the adding a power integrator technique,the parameters i’s increase significantly along with the system dimension.However,it should be pointed out that our design method in choosing parameters is very conservative for the neatness of the proof.In practice,following the general rules suggested by the proof,we can actually try and test some smaller parameters.This is quite practicable since in our controller the number of the design parameters has been reduced(i.e.,n coefficients i’s and only one ).For system(45),we only need to choose four parameters and one such choice is 1=5; 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