Modal Space Composite Control For Hydraulic Driven 6-DOF Parallel Mechanisms

Hydraulic driven 6-DOF parallel mechanisms are widely used in fields of flight simulator and driving simulator due to their unique multi DOF motion ability,strong carrying capacity,high stiffness and high precision.However,the closed kinematic chain structure makes their dynamic equations complex with strong coordinate coupling.Therefore,it is difficult to obtain ideal control quality with conventional control strategies.Aiming at this situation,scholars introduced modal space coordinate transformation in vibration mechanics to the research on control strategies of parallel mechanisms,and some good results were achieved.However,following key problems still exist,the identification of inertia parameters and viscous friction parameters for modal space controllers’ design are easily affected by joint Coulomb frictions,which lead to inaccurate identification results,the existing control methods based on modal space have insufficient adaptability to the complex trajectories of parallel mechanisms and the control effect needs to be improved,the calculated modal frequencies are often inconsistent with the actual modal frequencies in parallel mechanisms’ engineerings and flexible modes often appear,which reduces the dynamic characteristics of lateral motions.These problems restrict further improvements of their performances.Although the control strategy based on modal space improves the dynamic characteristics of parallel mechanisms,the disadvantages of centralized servo valve controlled hydraulic systems which are often adopted in parallel mechanism engineering projects such as complex structure,poor maintainability and low reliability also limit their further developments and applications.Aiming at the problems above,this paper researches in four aspects,including design of a new parameter identification method,design of hybrid control methods with decoupling function,rigid-flexible coupling modal analysis of parallel mechanisms,design of high-performance direct drive parallel mechanisms.Accurate identification of inertia parameters and viscous friction parameters is a prerequisite for high-performance modal space controllers’ design and rapid system debugging.At present,there are some defects in the existing identification method that the Coulomb frictions which widely exist in joints of parallel mechanisms are not fully considered.For this reason,a new inertia parameters and viscous friction parameters identification method which can eliminate effects of Coulomb frictions in joints is proposed in this paper.The complete dynamic model of the 6-DOF parallel mechanism including joint Coulomb frictions is established.The manifestation of each component of the load force under small range sinusoidal excitation is analyzed,and the interference mechanism of joint Coulomb frictions on inertia parameters identification and viscous friction parameters identification is obtained.On this account,a wavelet function is selected as the signal processing function to construct the identification equations which can eliminate the Coulomb friction term,and the parameter identification error caused by joint Coulomb frictions is eliminated from mechanism.The effectiveness of the identification method is verified by simulation results.Due to the complexity and strong coordinate coupling of dynamic equations,scholars introduced modal space coordinate transformation of vibration mechanics to the research on control strategies of parallel mechanisms,and control strategies based on modal space are established.However,the existing control methods based on modal space mainly focus on dynamic decoupling and regulating the damping ratios of systems,whose control effects are often unsatisfactory with common input signals such as step signals and low and medium frequency periodic signals.In view of the above situation,a modal space predictive functional control method is proposed for the step signals,and a modal space repetitive control method is also proposed for the medium and low frequency periodic signals based on the input signals and the complete dynamic equations of parallel mechanisms.Experimental verifications are carried out.In view of the fact that the calculated modal frequencies of parallel mechanisms are not equal with the actual modal frequencies and flexible modes appear,a research method in which the flexibility of the parallel mechanism is simplified and software aided modeling analysis is adopted is proposed in this paper.The flexibilities of parallel mechanisms are classified to the flexibility of the connecting sleeves with the joints and the flexibility of the hydraulic cylinder,an equivalent physical model of the hydraulic system with structural flexibility is built and the structural flexibility of the hydraulic cylinder is taken as the main research object.The structural flexibility of the hydraulic cylinder is further classified and simplified,and simulation models are built.By comparison between simulation models and the actual model,it is found that the rubber flexibility of O-rings in glyd rings of the guide sleeve is the main reason for the above phenomenon.A rigid flexible coupling dynamic model considering the rubber flexibility of O-rings is established,and the influences of rubber hardness of O-rings and structural parameters of parallel mechanisms on modal frequencies are analyzed.Improvement measures are put forward afterwards.Compared with centralized servo valve controlled parallel mechanisms,direct drive parallel mechanisms own advantages of simple structure,strong maintainability and high reliability,but their dynamic characteristics are poor,and their applications are limited.In view of the above situation,the hydraulic system,control scheme and control strategy of high-performance direct drive 6-DOF parallel mechanisms are studied in this paper.A high-performance direct drive leg hydraulic system is proposed.Based on this,a control scheme of direct drive parallel mechanisms is proposed combined with modal space coordinate transformation,which takes currents of servo motors as the control input and can eliminate both dynamic coupling and current coupling of servo motors.Finally,a hybrid control method of PD control and dynamic pressure feedback control is proposed to improve the dynamic characteristics of direct drive parallel mechanisms.Simulation and experimental verification are carried out in the end.