A study of a Gough-Stewart platform-based manipulator for applications in biomechanical testing.
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Date
2014
Authors
Ding, Boyin
Editors
Advisors
Cazzolato, Benjamin Seth
Costi, John Jack
Grainger, Steven Drummond
Costi, John Jack
Grainger, Steven Drummond
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Thesis
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Abstract
This thesis investigates the development and application of a robotic system for in-vitro biomechanical testing to study the mechanisms leading to human joint injury and degeneration in an ethical and safe manner. Six degree of freedom (6-DOF) robotic-based systems, in particular Gough-Stewart platform-based systems, have been increasingly used in applications of biomechanical testing where 6-DOF mobility, large load capacity, and high stiffness and positioning accuracy are required from the testing machine. This study proposes a novel Gough-Stewart platform-based manipulator with ultra-high stiffness and accuracy for use in biomechanical testing and investigates its mechanism and control. Not only restricted to biomechanical testing, the proposed manipulator concept can also be applied to other robotic-based applications, particularly those requiring ultra-high accuracy positioning under large external loads (e.g machining). Four main features of the proposed manipulator are individually studied in this thesis: namely, stiffness and control of a non-collocated actuator-sensor mechanism, active preload control using actuation redundancy for backlash elimination, adaptive velocity-based load control of human joints for unconstrained testing, and reproducing the in-vivo measured kinematics on human cadaveric joints. Stiffness and Control of the Non-collocated Actuator-Sensor Mechanism A novel Gough-Stewart platform-based mechanism is proposed with a fully decoupled actuator-sensor arrangement for passively compensating the structural compliance of the manipulator. The stiffness of the robot load frame and the sensing frame are respectively quantified using the robot kinematics error model combined with finite element analysis (FEA) on the top and bottom assemblies. Numerical results demonstrate that the proposed mechanism improves the stiffness of the robotic testing system in excess of an order of magnitude on the translational axes and two orders of magnitude for rotational axes compared to a traditional actuator-sensor collocated design. Control disturbances arising from actuator-sensor non-collocation is addressed using decoupled control. Experimental results show that the proposed decoupled control algorithm improves the dynamic accuracy of the manipulator by approximately 25% on average. Active Preload Control Using Actuation Redundancy for Backlash Elimination This thesis investigates combining the benefits of both active and passive preload control methods, using actuation redundancy to prevent backlash on a general Gough- Stewart platform. Both the mechanical configuration and the dynamics model of the redundant manipulator are investigated for the ease of control. A novel online optimization algorithm combined with a feedback force control scheme is formulated to achieve a real-time method which is robust to both model inaccuracy and load disturbance. Simulation results demonstrate an effective preload efficacy by the redundant arrangement within the workspace of the robot. Simulation results also show that the proposed method can effectively achieve backlash-free positioning of the manipulator under large 6-DOF external loads. Experimental results further prove that the proposed method can eliminate backlash instabilities from control and consequently higher bandwidth control can be achieved by the robot with improved accuracy. Adaptive Velocity-based Load Control of Human Joint for Unconstrained Testing A novel adaptive velocity-based load control method is proposed in this thesis to more effectively achieve pure force or moments on human joints under unconstrained testing compared to existing methods. The force/moment control gains are designed to vary adaptively based on the tracking performance of the force/moment to make a compromise between load following and control stability, which makes the proposed method self-adaptive to unknown joint dynamics. Sheep functional spinal units are used to experimentally validate the method on the custom-built Gough-Stewart platform-based manipulator. Experimental results illustrate the efficiency of the proposed method, which can be further improved when overcoming certain limitations of the system (e.g. load sensor noise, position inaccuracy arising from backlash, etc.) Reproducing the In-vivo Measured Kinematics on Human Cadaveric Joints This thesis develops a method to scientifically reproduce the general in-vivo kinematics measured from a living human on human cadaver joints using the custom-built Gough-Stewart platform-based manipulator. A human wrist is used as a typical example to elaborate the theory of the method and to assess the fidelity of the method. The proposed method uses a 3-D motion capture system to collect the in-vivo wrist kinematics from 12 patients undertaking hammering motion. In parallel, CT scans and static motion capture are undertaken on 8 cadaveric human wrist specimens in an effort to define the locations of the coordinate systems. Consequently the in-vivo measured wrist kinematics is transformed to the kinematics of the robotic testing system, which is used to reproduce the hammering motion. Experimental results show that the accuracy of the reproduced motion on the cadaveric samples is of similar magnitude to the measurement error of the motion capture system. Experimental results also show that the assumption of fixed wrist joint centre of rotation is valid for motion reproduction.
School/Discipline
School of Mechanical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2014
Provenance
This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals