MIMO Precision Motion Control Of Redundantly Driven Gantry Systems With Direct Actuators

With the increasing requirement of high-speed,high-acceleration and high-precision move-ment,the redundantly driven gantry system(RDGS)with direct actuators has become a key com-ponent of the high-end numerical control equipments,in which the use of redundant drives and highly rigid physical connections has the potential to offer better control performance.Howev-er,under high-speed/acceleration movement,the excessive internal forces may be easily generated due to the strong mechanical coupling and the flexible deformation of certain components in linear guides,which has become the main factor affecting the smooth operation(i.e.,the basic require-ment for system reliability)and the lifespan of the system;it also further limits the achievable control performance.Subjected to this synchronization problem between redundant actuators,the traditional contouring control methods only considering the motion coordination between axes obviously cannot meet the final demand of high-performance contouring tracking.With this in mind,a new concept of contouring control not only coordinating the motions between axes but also synchronizing the redundant actuators has been proposed.Along this line,detailed researches have been carried out.At present,the main factor limiting the development of control methods for the RDGS with direct actuators is due to the absence of effective model,which leads to the uncomplete knowl-edge of system characteristics.Therefore,this dissertation firstly explores the systematic method of rigid-flexible coupling modeling and verification through frequency-domain identification,for such kind of multi-axis mechanical system with highly rigid physical connections and relatively flexible components in linear guides.The proposed modeling method gives an accurate descrip-tion of the complete motions of system and the complex coupling between multiple degrees of freedom.It also gives a detailed analysis of the cause and the mechanism of coupling effect.The proposed frequency-domain identification method essentially solves the common distortion prob-lem of frequency response caused by the nonlinear friction in mechanical system.Through this method,the correctness of the proposed modeling is verified effectively,along with the acquire-ment of accurate model information,which can be used to guide the controller design.Then,to meet the prior requirement of smooth operation,a multi-variable synchronization control method is proposed,which focuses on both internal forces regulation and motion tracking.For different load conditions,with the guidance of valid physical model,corresponding mechanisms of accu-rately online adaptation and model compensation are designed properly in the proposed method to deal with parametric uncertainty and coupling effects.In addition,the internal forces are sup-pressed through the active control of rotational dynamics of the crossbeam.Thus,the synchro-nization behaviour is effectively improved to simultaneously achieve high performance of internal forces regulation and motion tracking during both transient and steady-state periods.Finally,to meet the requirement of excellent contouring tracking for such kind of system,an innovative con-touring control method is proposed,which synthesizes the synchronization of redundant actuators and the motion coordination between axes.Aware of the existence of rotation dynamics,the tra-ditional estimation model of contouring error is modified to improve the calculation accuracy in practice.Based on this improved index,the proposed contouring control method then combines the previously designed synchronization method subjected to dynamic load and the efficient mo-tion coordination mechanism based on the global task coordinate frame(GTCF).The proposed method overcomes the weakness of traditional approach based on rigid dynamics assumption,i.e.,the uncertainty of contouring tracking performance in practice.It further enhances and can indeed guarantee the contouring tracking performance of such kind of system subjected to different load conditions and desired contours.The proposed method is verified to be the essential solution to contouring tracking problem of the RDGS.This dissertation consists of the following six chapters:In Chapter 1,the background and development of the RDGS with direct actuators are intro-duced first,after which the mainly associated issues for such kind of system in meeting the prac-tically applied requirements are summarized.Then,a corresponding literature survey is given,including the situation and limitation of present research.Subsequently,it is concluded that high-level synchronization of redundant actuators is the precondition to further pursue high-performance motion control.Therefore,a new concept of contouring control focusing on both synchronization of redundant actuators and motion coordination between axes is proposed.Finally,a brief sum-mary of the research significance and the main issues to be solved is given.In Chapter 2,by analyzing the relative rigidness of the components and connections in RDGS,a complete kinematic description is given first,which contains both the conventional linear motion-s of two axes,and the usually ignored rotational motion of the crossbeam.Then,a three-degree-of-freedom(3DOF)dynamic model is established,which can accurately reflect the rigid-flexible coupling characteristics of the system.Further,the transfer function model of the underlying lin-ear dynamics is derived for verification through frequency-domain identification.For this purpose,the mechanism of nonlinear friction effect on frequency response is detailedly analyzed,to for-m a new nonlinear friction compensation based frequency-domain identification method,which can effectively improve the obtained identification results.Finally,by combining the proposed identification method with the linear transfer functions,the correctness of the proposed modeling is fully verified through experiments,along with the acquirement of accurate model information,which can further motivate the new solutions to synchronization control and contouring control of the studied system.In Chapter 3,the synchronization problem subjected to static load effect,i.e.,only consid-ering the mass variation and static position change of the load locating on the crossbeam,is s-tudied.Firstly,the existing synchronization control method considering the essential problem of internal forces regulation is adopted for case study,which is based on the thrust allocation tech-nique.The accurate frequency-domain identification result obtained previously is used to optimize the parameters design of this controller,to effectively improve and guarantee the synchronization performance.Simultaneously,the weakness of this method is analyzed through comparison ex-periments,which also show the direct relationship between the rotational angle and the constraint internal forces.Keeping this in mind,a two-input two-output synchronization control method with accurate estimation of model parameters(e.g.,the thrust allocation coefficient)and active control of rotational dynamics is proposed.Compared with the optimized thrust allocation based synchro-nization controller,the experimental results completely show the the performance improvement and the practicability of the proposed method,which can also inspire the further study of syn-chronization problem subjected to dynamic load.In Chapter 4,considering the more complex coupling property under dynamic load condition(i.e.,the simultaneous work of two axes),a proper mechanism of online parameter adaptation and model compensation is designed to deal with parametric uncertainty and coupling effects.Based on this,a three-input three-output synchronization control method is proposed,which focuses on both the motion tracking of two axes and the active control of rotational dynamics(i.e.,regulation of internal forces).The comparison experimental results fully validate that the proposed method can simultaneously achieve high tracking accuracy of linear motions and strong suppression ability of rotational mode.In addition,considering the degradation of model compensation and synchro-nization performance due to the commonly incremental measurement in practice,an extended method is then proposed to solve this issue by online adaptation of the mismatch value between the initial position of the load and the absolute zero position.Therefore,the proposed method can avoid the possible offline position calibration in practice,and achieve high-level synchronization performance subjected to arbitrary initial position.The effectiveness and better practicability of the proposed method are verified through experiments.In Chapter 5,the excellent contouring tracking performance of the RDGS is further pursued.Firstly,the problem existing in traditional contouring error estimation is discussed,which is based on the rigid dynamics only and thus cannot accurately reflect the actual contouring tracking perfor-mance.Accordingly,a modified calculation model is proposed,in which the rotational information is employed to measure the actual position of the working point and then derive the exact value of contouring error.With the improved index,the efficient motion coordination mechanism based on GTCF is integrated with the proposed multi-variable synchronization strategy,which forms a novel contouring control method by considering both the synchronization of redundant actuators and the motion coordination between axes.Finally,the proposed method is compared with the traditional contouring controller based on rigid dynamics assumption.The experimental results clearly indicate the performance limitation caused by the flexible rotational dynamics.Additional-ly,it also verifies the superiority of the proposed contouring controller,i.e.,to indeed guarantee the smooth operation of system in practice and further improve the contouring tracking performance.In Chapter 6,the main work and achievements in this dissertation are summarized.The conclusions and innovations are extracted and emphasized.The possible research directions in the future are also briefly discussed.