Figure 1.

Hierarchical flight speed control scheme. An outer visual loop sets the desired pitch angle (θ_ref) according to the error between the preferred speed v_p and the measured ground speed v_g. In the inner pitch control loop (inset), the pitch controller C sets the pitch angle θ to the desired value θ_ref by producing a torque M around the pitch axis. P represents the plant for pitch rotation. The sensor S, representing the halteres and the visual system, measures the rotational speed which is then integrated to get the measured pitch angle θ_meas. The pitch angle θ is converted to v_g via the pitch-to-speed plant.