Dexterous manipulation with a highly redundant movement system is one of the hallmarks of human motor skills.

High-precision maneuvers at high angles-of-attack are not properly addressed by even the most advanced aircraft control systems. Here we present our control design procedure and indoor experimental results with a small fixed-wing autonomous glider which is capable of executing an aggressive high angle-of-attack maneuver in order to land on a perch. We first acquire a surprisingly accurate aircraft model through unsteady flight regimes from real kinematic flight data taken in a motion-capture arena. This model is then used in a numerical nonlinear (approximate) optimal control procedure which designs a feedback control policy for the elevator deflection. Finally, we report our experimental data demonstrating that this simple glider can exploit pressure drag to achieve a high-speed perching maneuver.

Abstract. Birds routinely execute post-stall maneuvers with a speed and precision far beyond the capabilities of our best aircraft control systems. One remarkable example is a bird exploiting post-stall pressure drag in order to rapidly decelerate to land on a perch. Stall is typically associated with a loss of control authority, and it is tempting to attribute this agility of birds to the intricate morphology of the wings and tail, to their precision sensing apparatus, or their ability to perform thrust vectoring. Here we ask whether an extremely simple fixed-wing glider (no propeller) with only a single actuator in the tail is capable of landing precisely on a perch from a large range of initial conditions. To answer this question, we focus on the design of the flight control system; building upon previous work which used linear feedback control design based on quadratic regulators (LQR), we develop nonlinear feedback control based on nonlinear model-predictive control (NMPC) and "LQR-Trees". Through simulation using a flat-plate model of the glider, we find that both nonlinear methods are capable of achieving an accurate bird-like perching maneuver from a large range of initial conditions; the "LQR-Trees" algorithm is particularly useful due to its low computational burden at runtime and its inherent performance guarantees. With this in mind, we then implement the "LQR-Trees" algorithm on real hardware and demonstrate a 95 percent perching success rate over 147 flights for a wide range of initial speeds. These results suggest that, at least in the absence of significant disturbances like wind gusts, complex wing morphology and sensing are not strictly required to achieve accurate and robust perching even in the post-stall flow regime.

Abstract-Understanding the principles of motor coordination with redundant degrees of freedom still remains a challenging problem, particularly for new research in highly redundant robots like humanoids. Even after more than a decade of research, task space control with redundacy resolution still remains an incompletely understood theoretical topic, and also lacks a larger body of thorough experimental investigation on complex robotic systems. This paper presents our first steps towards the development of a working redundancy resolution algorithm which is robust against modeling errors and unforeseen disturbances arising from contact forces.To gain a better understanding of the pros and cons of different approaches to redundancy resolution, we focus on a comparative empirical evaluation. First, we review several redundancy resolution schemes at the velocity, acceleration and torque levels presented in the literature in a common notational framework and also introduce some new variants of these previous approaches. Second, we present experimental comparisons of these approaches on a seven-degree-of-freedom anthropomorphic robot arm. Surprisingly, one of our simplest algorithms empirically demonstrates the best performance, despite, from a theoretical point, the algorithm does not share the same beauty as some of the other methods. Finally, we discuss practical properties of these control algorithms, particularly in light of inevitable modeling errors of the robot dynamics.

Abstract-The ability of birds to perch robustly and effectively is a powerful demonstration of the capabilities of nature's control systems. Their apparent robustness to gust disturbances is particularly remarkable because when the airspeed approaches zero just before acquiring a perch, the influence of aerodynamic forces, and therefore potentially the control authority, is severely compromised. In this paper we present a simplified closed-form model for a fixed-wing aircraft which closely agrees with experimental indoor perching data. We then carefully examine the LTV controllability along an optimized perching trajectory for three different actuation scenarios -a glider (no powerplant), a fixed propeller, and a propeller with thrust vectoring. The results reveal that while all three vehicles are LTV controllable along the trajectory, the additional actuators allow the perch to be more easily acquired with less control surface deflections. However, in all three cases, disturbances experienced just before reaching the perch cannot be effectively rejected.