The focus of this study is the dynamic behaviour of a novel aerial positioning system, which consists of a lighter-than-air aerostat attached to a series of actuated tethers in the form of a tripod. The objective of the positioning system is to achieve accurate station-keeping of a payload at heights up to 500 m. This work encompasses comprehensive dynamics modeling, experimental investigation, and advanced controller development and analysis of the tri-tethered aerostat system. The system's characteristics and positioning objectives are targeted to receiver placement during the operation of a large-scale radio telescope. / The experimental test facility, built at one-third scale of the proposed radio telescope, was constructed for the dual purpose of measuring the precision of the positioning system and providing a basis for validating our computational model. The main components of the experimental apparatus are a 18-m helium aerostat, three synthetic braided tethers---hundreds of meters in length, an instrumented payload, and three computer-controlled winches. The dynamics model of the tri-tethered aerostat system is achieved by discretizing it into a series of lumped-masses, and estimating the aerodynamic properties of each of its physical elements. The disturbance input to the model is provided by a wind model that includes stochastic turbulent gusts. / A series of experimental flight tests were conducted over a two-year period to study both the uncontrolled and controlled response of the system. This leads to an incremental model validation process where the passive elements of the model, such as the tethers and the aerostat are validated prior to the verification of the entire closed-loop system. The tether model shows excellent agreement with the measured response at a temporal level, while the validation of the aerostat model is conducted at a statistical level due to the uncertainty associated with recreating the actual test wind conditions. / The controlled experiments were performed using basic proportional, integral, and derivative (PID) feedback gains applied to the position error of the payload. The closedloop system effectively reduces the standard deviation of the payload deflections to less than 10 cm over the range of operating conditions tested. A comparison of the predicted closed-loop behaviour of the dynamics shows favourable agreement to the experimental results, indicating that the tether actuation system is modeled appropriately. / To improve on the PID controller tested in the field, various optimal control strategies are investigated that exploit the availability of our validated dynamics model. The optimal controllers, which are designed and simulated using linear time invariant versions of the dynamics model, result in approximately double the precision of the PID controller. The control system is improved further by incorporating a feedforward control input based on measurements of the primary disturbance force acting on the system. In general the advanced control techniques, tested in simulation, offer encouraging results that suggest the positioning system should exceed the requirements of the radio telescope application.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.102994 |
| Date | January 2006 |
| Creators | Lambert, Casey. |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Mechanical Engineering.) |
| Rights | © Casey Lambert, 2006 |
| Relation | alephsysno: 002590019, proquestno: AAINR32203, Theses scanned by UMI/ProQuest. |
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