Abstract:

Multirotor unmanned aerial vehicles are typically used in military, commercial, or public safety-related applications. In contrast to these applications, this work aims to use multirotor unmanned aerial vehicles as low-cost alternatives to the traditional research platforms that enable microgravity for an onboard payload. Towards this, control and estimation strategies that turn a hexrotor unmanned aerial vehicle into a microgravity enabling platform are designed and verified through simulations.

A 1-D vertical maneuver is proposed which, when executed by a hexrotor, will result in an onboard payload experiencing microgravity for a specified time. The proposed maneuver involves high axial accelerations and velocities, and therefore, the standard steady-state models that predict the thrust generated by a propeller in hover cannot be used to simulate and analyze such a maneuver accurately. Computational fluid dynamic analysis, wind tunnel tests, and flight tests are used to study the thrust variation of propellers with axial speed, and a novel thrust model is developed from the results of these studies.

The proposed maneuver that enables a payload onboard a hexrotor to experience microgravity involves the hexrotor maintaining a certain acceleration level for a specified duration. The variation of the propeller thrust and the drag force with speed in an axial flight makes it challenging to maintain the desired acceleration. To enable the hexrotor to faithfully track the commanded acceleration, a feedback linearization-based acceleration control law is designed and used.

The proposed control law requires the estimates of the system parameters. Therefore, ancillary to this control law, a parameter estimation scheme is proposed. Theoretically, it is shown that, under certain conditions, the proposed control law, along with the parameter estimation scheme, ensures the convergence of the hexrotor acceleration to the desired value. The hexrotor attitude and vertical speed estimates are also required to implement the developed acceleration control law, and therefore, algorithms to estimate these values too are developed.

Finally, simulations are conducted to demonstrate that the developed control and estimation strategies enable a hexrotor to maintain constant microgravity levels despite the drag force and thrust variations due to high-velocity axial airflows.

The simulation results show that the developed control law is capable of maintaining microgravity as well as non-microgravity accelerations. Thus, hypogravity and hypergravity experiments can also be performed using multirotors with the developed control law. Moreover, since the cost involved in developing a hexrotor like the one used in this study is considerably less than that of the traditional microgravity platforms, any research/educational institution can afford to perform microgravity experiments using platforms as such.