Robust Adaptive Control of a Multirotor with an Unknown Suspended Payload A.P. Erasmus * H.W. Jordaan ** * Electrical and Electronic Engineering Department, Stellenbosch University, South Africa (email: 18177654@sun.ac.za) ** Electrical and Electronic Engineering Department, Stellenbosch University, South Africa (email: wjordaan@sun.ac.za) Abstract: This paper addresses the problem of a multirotor carrying an unknown suspended payload, allowed to swing in one axis. The payload is unknown in the sense that its parameters, such as the payload mass and cable length, are unknown and its state, the swing angles, are not available for measurement. The suspended payload alters the flight dynamics of the vehicle considerably and the flight controllers need to minimize this effect. A robust model reference adaptive control technique is designed and implemented to minimize the effect of the suspended payload on the vehicle while allowing the controller to adapt to account for the unknown payload. The controller is modified to reject external disturbances and to be robust in the presence of sensor noise and drift. In simulation, this technique proves to dampen the oscillations caused by the payload. A quadrotor was built to practically demonstrate the effectiveness of the controller. The PX4 flight control stack is considered for the firmware of the vehicle. The model reference adaptive controller was implemented and succeeded to dampen the oscillations caused by the payload in a practical flight. Keywords: Learning and adaptation in autonomous vehicles, Multirotor, UAV, Suspended payload, Model reference adaptive control, Robust. 1. INTRODUCTION The use of multirotors, known as rotary-wing Unmanned Aerial Vehicles (UAVs), for tasks that involve transport offers several advantages over conventional ground trans- port systems. These advantages include travel to hard-to- reach locations, shorter travel distances and avoiding traf- fic. The applications include consumer deliveries, military transport, medical and emergency needs, and search and rescue. Consumer deliveries with UAVs are becoming quite attrac- tive with companies such as Flirtey [Mack, 2017], Alpha- bet [Hern, 2019] and Amazon [Hern, 2016] investigating the potential of the market. The use of multirotors for transportation regarding medical needs are also gaining attraction [Zraick, 2019]. These projects involve a grasped payload rigidly attached to the body of the vehicle. This approach restricts the shape and size of the payload. Suspended payloads can be used to overcome this limitation. Suspended payloads are attached to the multirotor through a rope, cable or rigid rod. These payloads are free to swing underneath the multirotor and significantly alters the flight characteristics of the vehicle. Suspended pay- loads dominate the literature because of the advantages they have over grasped payloads and the difficulty of the problem. Researchers consider both known and unknown suspended payloads, outlined in Fig. 1. Multirotor with Payload Grasped Payload Suspended Payload Known Payload Unknown Payload Known Payload Unknown Payload Unknown Parameters Unknown States Unknown States and Parameters Focus of paper Fig. 1. Summary of literature study on multirotors with payloads. Preprints of the 21st IFAC World Congress (Virtual) Berlin, Germany, July 12-17, 2020 Copyright lies with the authors 9567