Quadrotors are a popular type of unmanned aerial vehicles (UAVs) that have been widely used and extensively researched in recent years. Quadrotors, while exhibiting high agility in maneuvering, generally lack stability. There exist several types of instability in modeling and control of quadrotors. During an aggressive maneuver process, a traditional quadrotor model developed based on Euler angles may encounter singularities due to the Euler sequence. This type of instability can be avoided via quaternion-based modeling.

Although quaternion-based models can eliminate the singularities due to the Euler sequence, there still exist other types of singularities for maneuvers in head-direction and thrust-direction. A UAV usually equips with a camera or other type of sensing device, which is mounted on the front or top of the U AV and which points towards the forward direction. Keeping those sensing devices pointing at the desired orientation can be a difficult task because the definition of reference heading direction vector introduces singularity and renders the mixer of the quadrotor saturated. As a result, the UAV becomes unstable.

In this article, a special strategy in dealing with the head-direction-induced singularity and saturation is proposed. In this strategy, pre-yawing is adopted when a quadrotor’s current heading direction is approaching singularity. As shall be shown, by pre-yawing before the quadrotor passes through the singularity, the quadrotor can converge toward the heading direction faster after exiting the singularity. Also, the pre-yawing strategy can reduce or avoid mixture saturation by reducing the feedback error. Consequently, the proposed strategy can also reduce the deviation from the desired trajectory. The benefits of this new control strategy in tracking are demonstrated through comparison with existing quadrotor models in the literature.

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