Study Of Slab Thickness Dynamic Surface Control For MDF Continuous Hot Pressing Hydraulic System

As the share of medium density fiberboard(MDF)is gradually increasing in wood based panel market,the performance of continuous hot pressing machine become the focus in the whole wood industry.Aimed to guarantee productivity and quality,it is essential for the MDF to control its slab thickness in the fixed stage.Nevertheless,due to the existence of parameter perturbations,external load force disturbance in the MDF continuous hot pressing hydraulic system,rapid and precise control for the slab thickness are difficult to realize.Besides,more attention should be paid to alleviate complexities of the control system design as well when aforementioned problems are tackled.Therefore,the dissertation applies the dynamic surface control(DSC)as a main control method with observer techniques and prescribed performance control approach to have a research on tracking control issues in the MDF continuous hot pressing hydraulic system.Owing to improving the steady-state and transient performance by the proposed method,the precision of the MDF slab thickness is obtained in final.The specific researching work are given as follows:To eliminate the common problem of differential terms' expansion in the backstepping controller design,first order low-pass filters are introduced to calculate the derivatives of virtual control inputs when a slab thickness DSC controller is obtained.The calculation burden is reduced by the DSC approach.For parameter perturbations and external load force disturbance in the MDF continuous hot pressing hydraulic system,a linear extended state observer(LESO)based slab thickness DSC method is presented.The compound disturbance,which is composed of the parameter perturbations and external load force disturbance,is extended as a new state for the system.The LESO is constructed to observe the new state.It is proved that the observing error of the LESO can converge to zero asymptotically.The observing result is used in the DSC controller design to enhance the robustness.Considering the issue of input saturation in the MDF continuous hot pressing hydraulic system in practice,a nonlinear disturbance observer(NDO)based slab thickness DSC method is presented.Firstly,with the help of hyperbolic tangent function,a smooth function is given to approximate the discontinuous saturated function in the system model.It is conducive to design a DSC tracking controller like above strategies.Subsequently,the excess of saturation limit is viewed as a part of compound disturbance.A NDO is applied to observe the compound disturbance and the observing result is feedback to the controller.Hence,anti-saturation and robustness of control system is improved.Simultaneously,compared with the LESO,NDO have the advantage of simpler structure and lower calculation amount where the complexity of control system design is further alleviated.Achieving the goal of providing a specific method to adjust the transient behavior in the MDF continuous hot pressing hydraulic control system,a slab thickness DSC method with prescribed performance is proposed.A performance function is constructed to ensure that steady-state error,overshoot and convergence rate can reach a predefined region.After that,by virtue of barrier Lyapunov function,an error transformation function is designed where a performance inequality constrained system is transferred into a system with equality constrain.It is convenient for the DSC tracking controller design.In summary,the proposed methods in the dissertation solve the problems of parameter perturbations,external load force disturbance in the MDF continuous hot pressing hydraulic system effectively,and alleviate the complexity of controller design obviously.Finally,the precise control of MDF slab thickness is realized.To some extent,the research achievements own theoretical value and engineering practice.