Paul E. I. Pounds

Grasping from the air: Hovering capture and load stability

This paper reports recent research efforts to advance the functionality of Unmanned Aerial Vehicles (UAVs) beyond passive observation to active interaction with and manipulation of objects. The archetypical aerial manipulation task — grasping objects during flight — is difficult due to the unstable dynamics of rotorcraft and coupled object-aircraft motion. In this paper, we analyze key challenges encountered when lifting a grasped object and transitioning into laden free-flight. We demonstrate that dynamic load disturbances introduced by the load mass will be rejected by a helicopter with PID flight control. We determine stability bounds in which the changing mass-inertia parameters of the system due to the grasped object will not destabilize this flight controller. The conditions under which transient partial contact mechanics of objects resting on a surface will not induce instability are identified. We demonstrate grasping and retrieval of a variety of objects while hovering, without touching the ground, using the Yale Aerial Manipulator testbed.

Stability of small-scale UAV helicopters and quadrotors with added payload mass under PID control

The application of rotorcraft to autonomous load carrying and transport is a new frontier for Unmanned Aerial Vehicles (UAVs). This task requires that hovering vehicles remain stable and balanced in flight as payload mass is added to the vehicle. If payload is not loaded centered or the vehicle properly trimmed for offset loads, the robot will experience bias forces that must be rejected. In this paper, we explore the effect of dynamic load disturbances introduced by instantaneously increased payload mass and how those affect helicopters and quadrotors under Proportional-Integral-Derivative flight control. We determine stability bounds within which the changing mass-inertia parameters of the system due to the acquired object will not destabilize these aircraft with this standard flight controller. Additionally, we demonstrate experimentally the stability behavior of a helicopter undergoing a range of instantaneous step payload changes.

Design principles of large quadrotors for practical applications

Virtually all quadrotors used in research weigh less than 2 kg, and carry payload measured in hundreds of grams. To be useful platforms for expanded operations, these vehicles must be capable of carrying greater weight. Several obstacles in aerodynamics, design and control must be overcome to enable the construction of larger craft with payloads in excess of 1 kg. We report the key design considerations essential for the construction of heavy quadrotor MAVs and demonstrate a 4 kg quadrotor with 1 kg payload.

Hovering Stability of Helicopters With Elastic Constraints

Aerial vehicles are difficult to stabilize, especially when acted upon by external forces. A hovering vehicle in contact with objects and surfaces must maintain flight stability while subject to forces imparted to the airframe through the point of contact. These forces couple with the motion of the aircraft to produce distinctly different dynamics from free flight. While external contact is generally avoided, extending aerial robot functionality to include contact with the environment during flight opens up new and useful areas such as perching, object grasping and manipulation. In this paper, we present a general elastic contact constraint model and analyze helicopter stability in the presence of those contacts. As an example, we evaluate the stability of a proof-of-concept helicopter system for manipulating objects using a compliant gripper that can be modeled as an elastic linkage with angular reaction forces. An off-the-shelf PID flight controller is used to stabilize the helicopter in free flight, as well as during the aerial manipulation task. We show that the planar dynamics of the object-helicopter system in vertical, horizontal and pitch motion around equilibrium are shown to remain stable, within a range of contact stiffnesses, under unmodified PID control.

Stability of Helicopters in Compliant Contact Under PD-PID Control

Aerial vehicles are difficult to stabilize, especially when acted upon by external forces. A hovering vehicle interacting with objects and surfaces must be robust to contact forces and torques transmitted to the airframe. These produce coupled dynamics that are distinctly different from those of free flight. While external contact is generally avoided, extending aerial robot functionality to include contact with the environment during flight opens up new and useful areas such as perching, object grasping, and manipulation. These mechanics may be modeled as elastic couplings between the aircraft and the ground, represented by springs in R3×SO(3). We show that proportional derivative and proportional integral derivative (PID) attitude and position controllers that stabilize a rotorcraft in free flight will also stabilize the aircraft during contact for a range of contact displacements and stiffnesses. Simulation of the coupled aircraft dynamics demonstrates stable and unstable modes of the system. We find analytical measures that predict the stability of these systems and consider, in particular, the planar system in which the contact point is directly beneath the rotor. We show through explicit solution of the linearized system that the planar dynamics of the object-helicopter system in vertical, horizontal, and pitch motion around equilibrium remain stable, within a range of contact stiffnesses, under unmodified PID attitude control. Flight experiments with a small-scale PID-stabilized helicopter fitted with a compliant gripper for capturing objects affirm our models stability predictions.

The Yale Aerial Manipulator: Grasping in flight

This video demonstrates a helicopter Unmanned Aerial Vehicle (UAV) research platform for grasping objects while in flight. Typically, helicopters avoid interacting with objects in their surroundings due to the unstable flight dynamics of rotorcraft and coupled mechanics encountered during contact. We introduce the Yale Aerial Manipulator and demonstrate stable grasping of a range of objects both when landed and while hovering. We discuss the platforms underactuated gripper and its contribution to aircraft stability while grasping.

UAV rotorcraft in compliant contact: Stability analysis and simulation

A hovering vehicle interacting with objects and surfaces must be robust to contact forces and torques transmitted to the airframe, which produce coupled dynamics distinctly different from those of free flight. These mechanics may be modeled as elastic couplings between the aircraft and the ground, represented by a 6-DOF spring in ℝ3×SO(3).We show that Proportional Derivative attitude and position controllers that stabilize a rotorcraft in free flight will also stabilize the aircraft during contact for a range of contact displacements and stiffnesses. Simulation of the coupled aircraft dynamics demonstrates stable and unstable modes of the system.

System Identification and Control of an Aerobot Drive System

Fast thrust changes are important for authoritive control of VTOL micro air vehicles. Fixed-pitch rotors that alter thrust by varying rotor speed require high-bandwidth control systems to provide adequate performace. We develop a feedback compensator for a brushless hobby motor driving a custom rotor suitable for UAVs. The system plant is identified using step excitation experiments. The aerodynamic operating conditions of these rotors are unusual and so experiments are performed to characterise expected load disturbances. The plant and load models lead to a proportional controller design capable of significantly decreasing rise-time and propagation of disturbances, subject to bus voltage constraints.

Attitude control of rigid body dynamics from biased IMU measurements

Commercially viable aerial robotic vehicles require low-cost attitude stabilisation systems that are robust to noise and sensor bias. A typical attitude stabilisation system consists of MEMs accelerometers, gyroscopes linked to separate attitude estimator and attitude controller algorithms. This paper proposes a non-linear attitude stabiliser for low-cost aerial robotic vehicles that combines attitude and bias estimation with control. The attitude control algorithm is based on a non-linear control Lyapunov function analysis derived directly in terms of the rigid-body attitude dynamics and measurement signals.

The Triangular Quadrotor: A More Efficient Quadrotor Configuration

We describe a new configuration of fixed-pitch miniature robot rotorcraft that combines the energetic efficiency of a helicopter and the mechanical simplicity of a quadrotor. The large power required to hover is proportional to the inverse of the rotor radius; thus, for a given diameter footprint, a single large rotor will energetically outperform several smaller rotors within the same boundary. However, smaller rotors are able to respond more quickly than large rotors, which require complex actuation to provide control. Our “triangular quadrotor” configuration uses a single large rotor for lift and three small rotors for control, gaining the benefits of both. The small rotors are canted slightly to also provide the same service as a conventional helicopters tail rotor. Momentum theory analysis shows that a triangular quadrotor may provide a 20% reduction in required hover power, compared with a quadrotor of the same mass and footprint. This is particularly valuable for flying robots working indoors where maximum rotor size is constrained. Using conventional quadrotor and a triangular quadrotors constructed to be a similar as possible, we demonstrate that the triangular quadrotor uses 15% less power, without optimization. A power efficiency budget is provided, and the influence of drive system efficiency is explored. We present a dynamic model and demonstrate experimentally that the aircraft can be stabilized in flight with simple PID control.