  This paper presents a novel approach for optical flow control of Micro Air
Vehicles (MAVs). The task is challenging due to the nonlinearity of optical
flow observables. Our proposed Incremental Nonlinear Dynamic Inversion (INDI)
control scheme incorporates an efficient data-driven method to address the
nonlinearity. It directly estimates the inverse of the time-varying control
effectiveness in real-time, eliminating the need for the constant assumption
and avoiding high computation in traditional INDI. This approach effectively
handles fast-changing system dynamics commonly encountered in optical flow
control, particularly height-dependent changes. We demonstrate the robustness
and efficiency of the proposed control scheme in numerical simulations and also
real-world flight tests: multiple landings of an MAV on a static and flat
surface with various tracking setpoints, hovering and landings on moving and
undulating surfaces. Despite being challenged with the presence of noisy
optical flow estimates and the lateral and vertical movement of the landing
surfaces, the MAV is able to successfully track or land on the surface with an
exponential decay of both height and vertical velocity at almost the same time,
as desired.
