Fault-Tolerant Flight Control Using One Aerodynamic Control Surface

University of Minnesota Ph.D. dissertation. June 2018. Major: Aerospace Engineering and Mechanics. Advisor: Peter Seiler. 1 computer file (PDF); xiii, 291 pages.Small unmanned aircraft systems (UAS) have recently found increasing civilian and commercial applications. On-board fault management is one of several technical challenges facing their widespread use. The aerodynamic control surfaces of a fixed-wing UAS perform the safety-critical functions of stabilizing and controlling the aircraft. Failures in one or more of these surfaces, or the actuators controlling them, may be managed by repurposing the other control surfaces and/or propulsive devices. A natural question arises in this context: What is the minimum number of control surfaces required to adequately control a handicapped aircraft? The answer, in general, depends on the control surface layout of the aircraft under consideration. For some aircraft, however, the answer is one. If the UAS is equipped with only two control surfaces, such as the one considered in this thesis, then this limiting case is reached with a single control surface failure. This thesis demonstrates, via multiple flight tests, the autonomous landing of a UAS using only one aerodynamic control surface and the throttle. In seeking to arrive at these demonstrations, this thesis makes advances in the areas of model-based fault diagnosis and fault-tolerant control. Specifically, a new convex method is developed for synthesizing robust output estimators for continuous-time, uncertain, gridded, linear parameter-varying systems. This method is subsequently used to design the fault diagnosis algorithm. The detection time requirement of this algorithm is established using concepts from loss-of-control. The fault-tolerant controller is designed to operate the single control surface for lateral control and the throttle for total energy control. The fault diagnosis algorithm and the fault-tolerant controller are both designed using a model of the aircraft. This model is first developed using physics-based first-principles and then updated using system identification experiments. Since this aircraft does not have a rudder, the identification of the lateral-directional dynamics requires some novelty

Vireo Flight 34

VIREO / Day16 / Flight34. TEST: Single surface HINF controller tolerates stuck right elevon, and autolands the aircraft. RESULT: Success. EVENTS: 1. First flight using a single surface HINF controller. 2. Trajectory: left circle, 250 ft AGL, 30 kts, 150 m radius. Controller tracking performance was good. 3. Aborted autoland performed in bravo mode. Aircraft was in single surface mode all the way until glideslope tracking. A few seconds after glideslope tracking began, Curt took over manual control and landed the aircraft

Baldr Flight 6

Brian, Chris, Danny, Parul, and Raghu arrived at the UMore Park Airfield around 8:30am for the fourth, fifth, and sixth flights of Baldr. In addition to Baldr flights, there was one Fenrir flight that took place. A separate flight report describes the purpose of the Fenrir flight. Baldr is the UAV LabÕs newest UltraStick 120 airframe that will be used for aircraft reliability research. Baldr is a modified UltraStick 120 airframe that has split elevators and split rudders, each surface driven by a dedicated servo motor. Recently, efforts have been underway at the University of Minnesota to design fault tolerant control laws for UAVs. Specifically, researchers have been focusing on attempting to control Baldr using only the split elevators, with all other control surfaces locked into their respective trim positions. The key idea in this experiment is controlling a conventional aircraft with two coplanar control surfaces. There are two main motivations that drove this experiment: 1. Exploring the controllability of conventional aircraft (with an empennage) that have been severely handicapped with losses in multiple aerodynamic control channels, and 2. Drawing meaningful conclusions about the controllability of two-surface flying wing aircraft which are subject to faults in any one of the two aerodynamic control surfaces. For this experiment, the performance objectives were tracking phi and theta commands. Hence, only phi and theta tracking control loops were implemented. Researchers decided to modify the baseline loop-at-a-time controller for this experiment. The baseline controller has two independently designed tracking loops. The pitch tracking loop generates an elevator deflection command in order to track a theta reference. The roll tracking loop generates an aileron deflection command in order to track a phi reference. It is important to note that each of the split elevators induce both longitudinal and lateral-directional motions in the aircraft. When both elevators deflect symmetrically, they act as elevators. When both elevators deflect anti-symmetrically, they act as ailerons. Hence, the pitch and roll tracking loops can be retained, with the only addition being a control command reallocation block. The reallocation involves mapping the conventional elevator and aileron commands into left and right elevator commands. Finally, updated input trim settings for all the control surfaces (estimated from Baldr flights 1, 2, and 3) were used in this flight. This experiment used both elevators of Baldr to regulate outputs around trim and track phi and theta

Vireo Flight 32

VIREO / Day16 / Flight32. TEST: Gather data for offline fault detection by injecting stuck surface fault with nominal controller. RESULT: Good flight, but detectability will be low unless changes in trajectory are performed. EVENTS: 1. First flight wherein stuck surface faults are injected WITHOUT controller reconfiguration. The goal is to see what the nominal controller does when a fault is injected. The flight data will be useful for offline fault detection. 2. Aircraft switched into alpha mode (both surfaces operational) and it transitions to a left cirle at 250 ft AGL, 30 kts, 150 m radius. 3. The aircraft is then switched to bravo mode (right elevon stuck at -5 deg TE up) with no control reconfiguration. 4. The aircraft does not exhibit any erratic behavior while in the circle. It was postulated that the circle is too mellow a trajectory to cause the nominal controller to behave erratically. 5. The aircraft was then commanded to follow a box route. The sharp turns required to perform the box excited the controller and the stuck surface turned out to be problematic. To help the aircraft regain control, we needed to switch into alpha. This back and forth between alpha and bravo was repeated till the end of mission

Baldr Flight 11

Apurva, Brian, Chris, Danny, Julian, Laura, and Raghu arrived at the UMore Park Airfield around 8:30am for the eleventh flight of Baldr. In addition to Baldr flights, there were several Fenrir flights that are summarized in separate flight reports. Baldr is the UAV LabÕs newest UltraStick 120 airframe that will be used for aircraft reliability research. Baldr is a modified UltraStick 120 airframe that has split elevators and split rudders, each surface driven by a dedicated servo motor. Recently, efforts have been underway at the University of Minnesota to design fault tolerant control laws for UAVs. Specifically, researchers have been focusing on attempting to control Baldr using only the split elevators, with all other control surfaces locked into their respective trim positions. The key idea in this experiment is controlling a conventional aircraft with two coplanar control surfaces. There are two main motivations that drove this experiment: 1. Exploring the controllability of conventional aircraft (with an empennage) that have been severely handicapped with losses in multiple aerodynamic control channels, and 2. Drawing meaningful conclusions about the controllability of two-surface flying wing aircraft which are subject to faults in any one of the two aerodynamic control surfaces. For this experiment, the performance objectives were tracking phi and theta commands. Hence, only phi and theta tracking control loops were synthesized and implemented. It is important to note that each of the split elevators induce both longitudinal and lateral-directional motion in the aircraft. As a consequence, researchers were specifically interested in synthesizing multi-input, multi-output control laws (as opposed to the conventional loop-at-a-time designs). For this experiment, researchers synthesized a linear quadratic Gaussian (LQG) controller, with the primary performance objective being output regulation. A secondary performance objective was tracking phi and theta commands. In order to track commands, two integrators were added to the synthesized LQG controller on the roll and pitch channels. The integrators effectively ensure that the steady-state tracking error is as close to zero as possible. In addition, the baseline controller runs for the first 2 seconds before the LQG controller is engaged. This simulates a realistic scenario wherein the flight control law has to switch from the baseline to the backup after a fault has been detected. The LQG controller was designed in Simulink and subsequently autocoded using Simulink coder. In addition, updated input trim settings for all the control surfaces (estimated from Baldr flights 1 through 6) were used in this flight. This experiment used ONLY the left elevator of Baldr to regulate outputs around trim and track phi and theta

Vireo Flight 1

VIREO / Day1 / Flight1. TEST: This was the first flight of the Vireo. This flight was a total system checkout. RESULT: Success

Baldr Flight 7

Chris, Danny, Parul, and Raghu arrived at the UMore Park Airfield around 9am for the seventh, eighth, and ninth flights of Baldr. Baldr is the UAV LabÕs newest UltraStick 120 airframe that will be used for aircraft reliability research. Baldr is a modified UltraStick 120 airframe that has split elevators and split rudders, each surface driven by a dedicated servo motor. Recently, efforts have been underway at the University of Minnesota to design fault tolerant control laws for UAVs. Specifically, researchers have been focusing on attempting to control Baldr using only the split elevators, with all other control surfaces locked into their respective trim positions. The key idea in this experiment is controlling a conventional aircraft with two coplanar control surfaces. There are two main motivations that drove this experiment: 1. Exploring the controllability of conventional aircraft (with an empennage) that have been severely handicapped with losses in multiple aerodynamic control channels, and 2. Drawing meaningful conclusions about the controllability of two-surface flying wing aircraft which are subject to faults in any one of the two aerodynamic control surfaces. For this experiment, the performance objectives were tracking phi and theta commands. Hence, only phi and theta tracking control loops were synthesized and implemented. It is important to note that each of the split elevators induce both longitudinal and lateral-directional motion in the aircraft. As a consequence, researchers were specifically interested in synthesizing multi-input, multi-output control laws (as opposed to the conventional loop-at-a-time designs). For this experiment, researchers synthesized an H-infinity controller, with the primary performance objective being output regulation. A secondary performance objective was tracking phi and theta commands. The H-infinity controller was designed in Simulink and subsequently autocoded using Simulink coder. In addition, updated input trim settings for all the control surfaces (estimated from Baldr flights 1, 2, and 3) were used in this flight. This experiment used ONLY the left elevator of Baldr to regulate outputs around trim and track phi and theta

Vireo Flight 9

VIREO / Day7 / Flight9. TEST: Closed-loop control law validation. RESULT: Good flight for control law validation. EVENTS: 1. Tested control law with new PID values, found in "vireo_v3_RV.JSON". 2. Performed an autonomous left-circle hold at 300 ft AGL and 30 kts. 3. Performed step changes in altitude (300->350->250->300) ft AGL. 4. Performed step changes in airspeed (30->33->27->30) kts. 5. Changed direction to right-circle hold

Vireo Flight 19

VIREO / Day11 / Flight19. TEST: Closed-loop control law validation. RESULT: Good flight for control law validation. EVENTS: 1. Turned off all longitudinal control loops except total energy to throttle. Elevator was held fixed at -5 deg TE up. 2. Under the degraded conditions, the altitude tracker was assisted by an airspeed damper. 3. The addition of airspeed damper to the altitude tracker was beneficial in keeping the airspeed from diverging. However, the performance was still not as good as TECS. 4. We completed the autoland