Modeling and control of backdrivable flexible systems

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Adams, Christopher James
Singhose, William E.
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Flexible systems deflect, vibrate, or oscillate when moved. This behavior results in decreased performance in the form of inaccurate positioning, stress loading, transient deflection, and residual vibration. In a backdrivable flexible system, coupling between flexible and rigid-body modes also leads to degraded performance of the rigid-body motion. For example, sway of a massive payload can backdrive the position of a crane trolley, forcing it to move in unintended ways. Other examples of backdrivable flexible systems include helicopters carrying suspended loads and spacecraft with large flexible appendages. This thesis investigates dynamic models that capture the fundamental behavior of a variety of backdrivable flexible systems. These models are used to understand and illustrate the conditions under which a system can be classified as backdrivable. Then, the models are studied to identify the range of system parameters that can lead to significant backdrivability and degraded performance. Performance metrics are defined based on analysis of mode shapes and system poles and zeros to quantify the level of backdrivability resulting from a given set of system parameters. The fundamental models are then used to develop and analyze control methods that can mitigate or suppress the performance degradation seen in both the flexible mode(s) and the backdriven rigid-body mode(s). The proposed control methods are illustrated through two demonstrative applications: experiments and simulations of helicopters carrying suspended loads, and as part of an attitude control system for a spacecraft with flexible appendages driven by stepper motors.
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