Modular decentralized control and design of a reconfigurable redundant manipulator

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Copyright: Vittor, Timothy R.
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Abstract
Soft automation employs multi-functional robotic manipulators where dexterity, versatility and reconfigurability are now becoming key issues. For manipulators used in these circumstances, hyper-redundancy in configuration is essential. A typical application envisaged, is manipulator movement in unpredictable surroundings such as navigation amongst branches and foliage in automated fruit picking. This requires motion of a manipulator in an unmapped dynamic environment to find its way to the goal i.e. the fruit. To date, centralized control architectures have proven impractical for the control of redundant systems because the full environment needs to be mapped, there are time constraints on the computation required and, simultaneously, algorithms for obstacle avoidance must be enacted. Decentralized control of manipulators complements modular construction because the need for a full environment reconstruction in one master controller is bypassed by having localized sub-goals for each module. Time constraints are removed because the control algorithms are much simpler. Obstacle avoidance is localized. Manipulators constructed modularly are effective because they allow for reconfiguration and ease of fault diagnosis. For modular manipulators to be a more effective option as a subclass of robots, the conditions under which the interactive movements of modules are stable become a major issue. When a general review of hyper-redundant manipulators was undertaken, no published implementation of Modular Decentralized Control (MDC) was discovered. This thesis explored the use of a modular decentralized technique to create stable control of a redundant manipulator system. The computational burden was minimized by restricting inverse kinematics to within each module. Advantages of the approach taken were the ease of implementation of obstacle avoidance, reconfigurability and fault tolerance. Having firstly simulated a MATLAB version of stable motion using MDC on a modular manipulator with up to six identical modules, the technique was extended with state space analysis to redefine the limits of stable control of a hyper-redundant manipulator. The MDC study mapped motion profile types that were dependant on the initial manipulator configuration and goal position and, thereby, investigated possible instabilities in the system. A two-link, single degree of freedom system was initially explicated followed by an extension of the stability analysis to an n-module two degree of freedom system. A stability theory utilizing decentralized control was formed. Simulation results showed dynamic motion, path generation and obstacle avoidance capabilities in unmapped environments to be stable. The modeling redefined the bounds of stable control, showing that classical stability via Root Locus, now required only two roots from the characteristic equation to be stable for a selection of path trajectory to the goal to be found. The remaining roots could be unstable in traversing to the goal and settling at a marginal stability point when the goal was reached. The marginal stability was a reflection of the pseudo-independence given to each module in seeking the goal and differed radically from a standard Root Locus analysis and interpretation of stability. A hyper-redundant Reconfigurable Modular Manipulator System (RMMS) was designed and built to implement the MDC technique in a real world environment. From an initial design, five modules were constructed and control algorithms embedded appropriate to their position in a five-segment robotic manipulator. A stereoscopic vision system was attached to the end of the manipulator which supplied real time data on a goal in 3D Cartesian space. The data was supplied to the first module of the arm and subsequently to all others by localized homogenous transformation. The manipulator was tested for goal seeking, path following, obstacle avoidance, fault tolerance and reconfigurability. The arm produced stable motion and satisfied the criteria as hypothesized in the theory.
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Vittor, Timothy R.
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Publication Year
2007
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Thesis
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PhD Doctorate
UNSW Faculty
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