Modal Space Decoupling Control Strategy For6DOF Motion Simulation Platform

Hydraulically driven motion simulation platform with6DOF have beenwidely used for the simulation of aircraft motion, because of its obviousadvantages. However, the realization of the full potentials of its applicationthrough control strategies is yet to be full achieved. This is because of its highkinematic and dynamic coupling effects. Conventionally, joint space controller isemployed where each joint axis is regarded as a linear single-input/single-outputsystem with the dynamic coupling effects treated as disturbances. Withoutcompensating for these coupling effects the control performance could beconsiderably degraded.Addressing these coupling problems necessitates an effective decouplingcontrol strategy that will increase the system bandwidth, compensate for couplingeffects and ensure that each degree of freedom can be independently tuned inmodal space. In the research work presented in this dissertation, attention isfocused on the study of coupling effects on the control performance ofhydraulically driven6DOF motion simulation platform, as well as thecompensation of these effects through control strategies. The method is based onsingular value decomposition (SVD) to the properties of a joint space inversemass matrix using a transformation matrix. The elements of each column of thematrix formulate the coupling effects in modal space coordinates with respect tothe physical task space frame.A mathematical description of the method is given and supported withsimulation and experimental results. These results show good agreement, thusreflecting the potential of the proposed method. The results show a relationshipbetween the coupling effects and frequency, revealing that the dynamic coupling effects of the6DOF motion simulation platform increase with frequency. In thelow frequency region, the coupling is small, but as the frequency increases, thecoupling effects become greater. And when the frequency is close to the naturalfrequency of the system, the coupling effects reach a maximum. As the frequencyof the movement increase again, the coupling effects decrease. The results alsoreveal that only heave and yaw are independent of the other DOF and can betuned individually in task space.Furthermore, a simulation model to analyze the coupling effects betweenactuators was established. To analyze the results, the actuators were divided intoeven and odd numbered actuators. From the simulation results, the couplingeffects were found to be heavier between even and odd numbered actuators.In view of the limitations of conventional joint space controllers to addressthe coupling effects in the control of the6DOF motion simulation platform, anovel method, modal space decoupling control (MSDC) was developed. Thismethod transforms the highly coupled six-input six-output dynamics to sixindependent single-input single-output (SISO)1DOF hydraulically drivenmechanical systems. This is through the mapping of the control and feedbackvariables from the joint space to the decoupling modal space. The advantages ofthe proposed method are that each degree of freedom can almost be tunedindependently, and their bandwidths can be raised to near the eigenfrequencies.The proposed control strategy allows the control design of a complexstructure to follow the same procedure as employed in the design of single-degree-of-freedom systems. Since the control variables can be tunedindependently for each degree of freedom, degree of freedom motion couplingeffects can be effectively reduced. Comparing a modal space decoupledcontroller with a conventional joint space showed that the modal space controller has a better performance.This study has provided a method for the formulation of an index andevaluation of coupling effects of6DOF motion simulation platform. The resultsare expected to be useful to, and provide guidance to, mechanism and controllerdesigners; and assess the value of the coupling effects at the initial design stageof6DOF mechanisms in which the decoupling of the displacement in regard toDOF motion is desired. The study also demonstrated the applicability of themodal space decoupled controller and its effectiveness over conventional jointspace controllers.