Abstract:
This paper considers the problem of controlling the attitude and altitude of a quadrocopter in the presence of uncertainties in the plant model. When solving this problem, it is especially important to consider the peculiarities of the plant: strong susceptibility to roll, pitch, and altitude oscillations due to the quadrocopter design and motor dynamics (with yaw being the least susceptible to oscillations due to motor dynamics compared with other controllable variables). To achieve high control quality in the presence of uncertainties, combined control is usually applied. It is constructed as a sum of two parts: a basic stabilizing part and a part compensating uncertainties with the help of a disturbance observer. Typically, both parts contain linear feedback. However, when the output variables of the plant track non-smooth reference signals, linear feedback can cause overshooting and increased oscillations. To prevent these problems, we propose a combined control law with smooth and bounded feedback in the form of the hyperbolic tangent. This feedback is used by both the controller and the disturbance observer. In this case, the control synthesis is based on the structural properties of the plant using the block approach. Its application provided invariance of the output variables with respect to not only matched but also unmatched uncertainties, and also allowed to construct a disturbance observer of the minimum possible order. In addition, to reduce the oscillations, a part with plant accelerations was introduced into the control law. To realize the proposed approach, it is sufficient to know the nominal values of some parameters of the plant and the permissible bounds of uncertainty variation. We present the results of experiments on a quadrocopter with an F450 frame and the results of a comparative analysis of the proposed approach with the one using linear control.
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