title: Adaptive control solutions for advanced unmanned underwater vehicle applications
Creators: name: Makavita, CD
Publisher: name: University of Tasmania
Year: 2018

description: Unmanned  Underwater  Vehicles  (UUVs)  have  evolved  from  rudimentary Remotely Operated Vehicles (ROVs) with operator control of the actuator outputs directly to sophisticated ROVs and Autonomous Underwater Vehicles (AUVs) with semi and fully autonomous control systems that require minimal to zero operator input. In  addition,  the rapid progress in technology has made UUVs to evolve from the mammoth work- class ROVs to mini and micro ROVs and AUVs. With these changes in UUVs there has been a parallel evolution of UUV applications culminating in advanced applications  such as operating in cluttered environments in tandem with human divers and being launched and recovered by underwater docking platforms or naval submarines. Thus future UUV  autonomous control systems must precisely manoeuvre UUVs  that  are more susceptible to parameter changes and disturbances while operating under conditions which require large parameter variations.
Adaptive control has been identified as a key enabling technology for all of the above applications. Although proven to be superior to fixed-gain controllers, adoption of adaptive control for UUV applications has been lacklustre in the past due to the lack of demanding applications that justify the added complexity and some inherent limitations. However, it has come to a point that it is no longer feasible to ignore the benefits of adaptive control for future high performance, safety critical UUV applications.
Therefore, this thesis is an effort to design and evaluate adaptive control systems for such future applications. It was identified that to ensure precise manoeuvres throughout the entire operation, the main focus should be on improving transient tracking without control signal oscillation or instability. Also, the controllers are required to show sufficient robustness against measurement noise and time-delay. To this end, three modifications to the standard Model Reference Adaptive Control (MRAC) architecture that improves transient performance without using high learning rates were developed for an existing small ROV/AUV. Composite MRAC (CMRAC) and Predictor MRAC (PMRAC) both use a prediction error in addition to the tracking error to improve transient performance. Command Governor Adaptive Control (CGAC) uses command signal modification to achieve the same end.  The performance improvements of   these architectures  were  all  initially  verified using  simulations  and  then  validated using experiments.  Simulations  and  experiments  were  carried  out  to  investigate transient operations, actuator failures and external disturbances. The acquired data were subjected to a comprehensive analysis in both time domain and frequency domain to provide a compelling quantitative evaluation of the different methods.
The results indicated that significant improvements in transient tracking, fault tolerance and disturbance rejection can be obtained with the proposed solutions, compared to standard MRAC with minimal additional complexity. The transient tracking performance improvement was achieved while reducing the high frequencies in the control signal and with less control effort or less energy usage. It has also been shown that, under partial actuator failures, regulation and tracking task can still be carried out with negligible variations. In addition several forms of disturbances such as large impacts, wave disturbances and tether snags are simulated and tested and significant improvements were observed in reducing maximum deviation, settling time and oscillations at the output. Furthermore, it is shown that some proposed solutions are able to overcome the actuator dead-zone without using an additional dead-zone inverse. Also, introduced in this thesis is a novel adaptive control methodology named Extended CGAC (ECGAC) which increases the robustness to noise and time-delay while  retaining the enhanced performance. In summary, the feasibility of designing adaptive controllers with transient performances equivalent to steady state performances while ensuring much better control signal is verified in this thesis.