VTOL Control & Airspeed Fault Detection

Roman Bapst - 21.06.2019

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Content

● Introduction● Current VTOL Control Architecture and next Steps● Airspeed Fault Detection

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VTOL Types

Tailsitter Tiltrotor QuadPlane

● Difficult to control (wind)● Less actuators● Simplicity

● Easier to control● Mechanically more

complex (tilt)

● Easy to control● Many actuators● Fuel engine

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VTOL Introduction

● Ability to hover

● Land almost everywhere

● Fly efficient● Fly fast● Payload

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Control Architecture

Advantages:● Rate controllers always control

same set of actuators → no re-tuning required

● Enables advanced safety features: stall prevention in fixed wing mode

● Control surfaces aid during hover in strong wind

Challenges:● We create a MISO system

which is more difficult to tune

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Future Work: Towards Control Allocation

Mixer system offers great abstraction but could be even more flexible.

Example: Tiltrotor transition● As motors tilt, mixer geometry

changes● Roll / yaw coupling ● Reduced control authority during

transitions

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Future Work: Towards Control Allocation

● Enhance mixer to do control allocation● Advantage: Controllers remain independent from

specific vehicle configurations● More precise control during transitions (e.g.

eliminate roll / yaw coupling)● Fault tolerant control

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VTOL features and documentation

Tuning information

We need more docs!Search on VTOL in user guide for more info.

Airframe references

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Airspeed fault detection and handling

Why we use airspeed sensor?● Efficiency: Optimize flight efficiency● Safety: Prevent stalls and structural damage due to exceeding load factor● Flight Performance: Gain scheduling (scaling of signals to control surfaces)● Robustness: Limit attitude drift in case of GPS loss (EKF)

What are the problems:● Sensor is exposed to direct airflow and is thus vulnerable to blockage● False airspeed data can upset control system and in the worst case lead to flight into

terrain, stall or overspeeding

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Current airspeed fault handling

● First stab at providing airspeed fault detection made by Paul Riseborough

● Check data timeout● Check EKF airspeed innovation● Check load factor consistency

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Wind estimator

Wind north

Wind east

Airspeed scale

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Airspeed fault detection and handling

Pitot tube 1

Pitot tube 2

Zero sideslip

Airspeed innovation

Airspeed innovation

Sideslip innovation

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Excessive reading on one sensor

Airspeed 1 Airspeed 2

Wind estimator 1 innovations

Wind estimator 2 innovations

Selected Index Airspeed Validated

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Partial blockage of both sensors

Airspeed 1 Airspeed 2

Wind estimator 1 innovations

Wind estimator 2 innovations

Selected Index

Airspeed validated

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Gradual blockage of both sensors

Airspeed 1 Airspeed 2

Wind estimator 1 innovations

Wind estimator 2 innovations

Selected IndexAirspeed validated

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Conclusion

● Success in detecting multiple fault conditions● Can be extended to any number of airspeed sensors● Lightweight wind-estimator● Requires careful handling of wind estimator resets● Pull request to PX4 in the coming days● Real world flight testing with dual airspeed

Thank you!