Modeling and observer-based robust control design for energy-dense monopropellant powered actuators
This dissertation presents the development of a monopropellant-based power supply and actuation system for human scale robots that is energy and power dense with the ability to be controlled accurately at a high bandwidth. This kind of actuation system is known to have an actuation potential an order of magnitude better than conventional battery-DC motor based actuation systems. Though a monopropellant-based actuator has the appeal of being simple in design, it is fairly complex in terms of the physics of its operation. The complex interaction between several energy domains and the nonlinear nature of many of them necessitates a model-based control design to provide adequately accurate, high-bandwidth, efficient, and stable operation as generally required of a mobile robot platform. In order to obtain a model-based controller, a physics-based model of this kind of a system is derived in this work. The control architecture of the centralized configuration is then presented which is shown to provide stable servo tracking of the system. This model-based controller is designed on the basis of Lyapunov stability-based sliding mode control theory to control the inertial mass. A model-based predictive controller is additionally implemented for the control of rate of pressurization and regulation of the supply pressure in the reservoir. Since the model-based control of the actuators necessitates the use of two high-temperature pressure sensors, these sensors add substantial cost to the monopropellant-based servo system. In order to make the chemofluidic system more cost effective and economically viable, a nonlinear pressure observer is developed in this work. This observer utilizes the available knowledge of other measurable states of the system to reconstruct the pressure states. The elimination of pressure sensors reduces the initial cost of the system by more than fifty percent. Additionally, the use of pressure observers along with the design of a robust controller results in lower weight, more compact and lower maintenance system. The development of two Lyapunov-based nonlinear pressure observers for pneumatic systems is also presented in this work. The implementation of pressure observers instead of expensive pressure sensors reduces the cost of the system by nearly thirty percent. These savings are achieved without any compromise on the quality of servo tracking of the system. The results presented demonstrate that the tracking performance using pressure observers versus using pressure sensors is in essence indistinguishable.