Acceleration compensation for gravity sense using an accelerometer in an aerodynamically stable UAV

This paper considers the problem of compensating for vehicular accelerations in an inertial sensor, in order to obtain a sense of the gravitational field. The gravity sense can then be used to estimate the relative attitude of the sensor with respect to the field. However, the accelerometer in inertial measurement units (IMUs) measures the sum of the inertial acceleration and the gravitational field, and the measurement cannot be directly decomposed into the two components. Separating the gravitational component out is therefore crucial to the use of IMUs for attitude estimation or determination. The separation has typically been accomplished by making a steady turn assumption in unmanned aerial vehicles (UAVs). This paper introduces a new assumption that the aerodynamic forces are nearly constant in the body frame. This assumption leads to significant improvement in estimator performance during perturbations from a nominal motion, when restricted to UAVs that possess inherently stable aerodynamics. The stability properties of the compensator are analyzed to prove that the compensator retains the property of asymptotic stability under a steady turn assumption, and that it performs better despite not being asymptotically stable when the assumption is withdrawn. The resulting improvement in performance is demonstrated both in simulations as well as experiments.

I. Introduction

It may be shown by a simple dimensional analysis [1] that the time scale of vehicle dynamics varies as the inverse of the square root of the length scale for fixed-wing airplanes in incompressible flow under a constant external gravitational field. Control of small unmanned aerial vehicles (UAVs) therefore demands faster response times from the state estimator. This provides the motivation for the design of a low-cost, low-weight, low-latency attitude estimator.