Coordinating optimization-based sliding mode variable structure control for electro-hydraulic se

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 ̄umal ofControl Theory andApplications 2(2006)168-174 Coordinating optimization—based sliding mode variable structure control for electro—hydraulic servo system 

YongYANG1,2 AnLUO 3,HuaHAN1 

(1.College of Information Science and Engineering,Central South University,Changsha Hunan 410083,China; 2.Changsha University of Science and Technology,Changsha Hunan 410076。China; 3.College of Electric and Information Engineering,Hunan University,Changsha Hunan 410082,China) 

Abstraet:A sliding mode variable structure control(SMVSC)based on a coordinatin2 optimization algorithm has been developed.Steady state error and control switching frequency are used to constitute the system performance indexes in the coordinating optimization,while the tuning rate of boundary layer width(BLW)is employed as the optimization parameter.Based on the mathematical relationship between the BU and steady—state error,an optimized BU tuning rate is added to the nonlinear control term of SMVSC.Simulation experiment results applied to the positioning control of an electro.hydraulic servo system show the comprehensive superiority in dynamical and static state performance by using the proposed controller is better than that by using SMVSC without optimized BU tullin2 rate.This Succeeds in coordinately considering both chattering reduction and high—precision control realization in SMVSC. Keywords:Sliding mode variable structure control(SMVSC);Varying boundary layer;Chattering reduction;Steady— state performance;Coordinating optimization(CO):Electro—hydraulic servo system(EHSS) 

1 Introduction Electro—hydraulic servo system(EHSS)[1】is widely used in mobile vehicles,mining engineering,and aerospace ex— plorations.These servo systems are typically nonlinear and imperatively need control techniques with stronger robust— ness and higher precision. Based on a high—frequency switching control,sliding mode variable structure control(SMVSC)【2】can turn a nonlinear system into a linear system when the motion state arrives at the sliding mode surface.Theoretically,SMVSC can strengthen the robustness against external disturbances and system parameter uncertainties+However,no ideal slid— ing modes but quasi—sliding modes occur in real systems, and high-・frequency chattering happens in the SMVSC sys-- tem.This consequently limits the application of SMVSC to EHSS,and it is necessary to take measures to reduce chattering.At present,using saturation function to replace the switching function and introducing boundary layer(BL) are two main methods of smoothing chattering in SMVSC 【2 7】+However,there exists one design conflict between 

requirement on the smoothness of control signals and that on the steady-state performance[6】.Some methods have been used to solve such paradoxical problem.Pushkin,et al+ [3】employed a BL with a fixed width instead of with satu- ration characteristics.Xu,et a1.【4】designed a BL with see- tor characteristies instead of saturation characteristics+Bon. tolini【5】presented a measure to make chattering happen on the visual derivation of control value to smooth chattering. Chen,et a1.【6】proposed an on-line method for adjusting the BLWbasedonthe statenOHTIforanuncertainlinear system. Feng,et a1.【7】set up a mathematical relationship between the steady-state error and the BLW,which not only reduces chattering but also satisfies the requirements on specified steady-state error, Many optimization tools have been applied to practical engineering designs[8--d 0]+Because of the ability to lo- cate both global and local Pareto frontiers in a single opti- mization and the merit of less specific tuning of parameters, such an optimization tool as multi-objective struggle genetic algorithm[8,1 0】can combine the Pareto-based ranking op- timization with the struggle crowding genetic algorithm.In 

Received 28 June 2oo5:revised 18 March 20O6. This work was supported by the Provincial Natural Science Foundation of Hunan(No.04JJ6033)and the Research Foundation of Hunan Education Bureau(No.03C066). 

维普资讯 http://www.cqvip.com Y YANG et a1./Journal ofControl Theory and Applications 2(2006)168—174 this paper,an improved SMVSC based on coordinating opti- mization is proposed.The BLW tuning rate of sliding mode phase plane is optimized through a GA-based coordinating optimization algorithms.Simulation results verify the effec。 tiveness of the proposed SMVSC. 2 Design of coordinating optimization・based SMVSC 2.1 c!onvenfional SMVSC Consider the following single-input nonlinear system r圣1(£): 2(t), {圣2(t): 3(t), (1) L 23(t):f(xl,x2,x3,t)+“(t)+d(t), where x(t)=[ 1(t)x2(t) 3(t)]T is the state vector which is constituted of error function 1(t)and its derivatives,and Xl(t)=e(£)=Xpr(t)一Xp(t), Xpr(t)and Xp(t)are the desired output and the practical output signals of the system at the time t,respectively. “(t)∈ isthecontrolvariable,f(xl,x2,x3,t)anditsesti— mate f(xl,x2,x3,t)are the nonlinear function matrices of the system,respectively,and 1f(xl,x2,x3,t)一f(xl,x2,x3,t)l≤F(xl,x2,x3,t). d(t)is unknown bounded disturbance and ld(t)l≤D(t). A switching surface based on system error is defined as 8(t)=Cx(t)=ClXl(t)+c2x2(t)+ 3(£), (2) where C=[c1,c2,1】∈ 。is a sliding surface matrix determined by means of arbitrary pole assignment. In order to guarantee that the trajectory ofthe error vec- tor (t)converges to zero asymptotically,the control action must satisfy the following sliding condition. 8(t)- (t)≤0,for l8(t)l≠0. (3) Ideally,the equation for the switching surface of sliding mode canbeexpressed as 8(t):Cx(t)=0 However,only quasi.sliding modes occur in real SMVSC systems because of switching delays and negligible time constant.This SMVSC is in essence a discontinuous con— tro1.High—frequency chattering around the desired equilib‘ rium point and excitation of the unmodeled high—frequency dynamics of the system may happen. The strategies of introducing a BL around the sliding sur- face and exerting continuous control in the BL have been proposed to reduce chatmring of SMVSC.It can be guaran‘ teed that all the trajectories outside the BL are attracted to the BL if the following condition is satisfied. Is㈤I≥ )==}去面d s㈤ ≤( ㈤一町)ls㈤I, (5)