Slung Load Research Articles

Cooperative Transport of a Slung Load Using Load-Leading Control

A system for cooperative transport of a slung load by a team of autonomous rotorcraft is described. The method described here is hierarchical and centralized, with the payload defined as the leader (here denoted as load-leading control): the payload uses knowledge of its current state and its desired trajectory to determine the net force and moment acting upon its center of gravity required to follow the desired trajectory. Using knowledge of the cable attachment geometry, the required cable forces are computed and transmitted to the rotorcraft. The cable force optimization problem is defined and shown to be nonconvex; constraints that make the problem convex are introduced. Cable forces computed using both the convex and nonconvex formulations are compared. Controllers for both payload and rotorcraft trajectory tracking are designed. Hardware implementation that uses small single-board computers carried onboard the payload and rotorcraft is described. Flight demonstrations conducted in an indoor motion capture studio using preplanned trajectories as well as human-in-the-loop control of the payload are described to show trajectory following and rejection of impulse disturbances. An outdoor flight test showing coordinated transport in the presence of light winds is described. All calculations are hosted on the computers carried on the payload and rotorcraft, and the system is shown to be capable of real-time control and path following both for fixed and in-flight changing formations.

Nonlinear cascade control for a quadrotor transporting a slung load

This paper focuses on the motion control problem for a quadrotor with a slung load (QSL). A dynamic model of a QSL is proposed by Lagrangian approach. We considered the air resistance of the load in the model building. Based on such a dynamical model, we propose a novel nonlinear three-loop cascade controller to realize velocity control for the load of a QSL, and the exponential stability of the whole system is proved. Numerical simulations implemented in a Matlab/SimMechanics environment demonstrate the effectiveness of the designed controller and the proposed model.

Rapid Automatic Slung Load Operations with Unmanned Helicopters

Slung load operations often involve following conservative flight paths to prevent unwanted swing of the load. However, doing so often limits the maneuvering capability of the system and increases the time to complete trajectories. This work investigates methods to rapidly move a slung load with a helicopter with the intent to deliver it quickly and precisely to a fixed point. Inspired by prior work utilizing differential flatness, which is a property of certain systems that allow for advantageous controllers to be developed for nonlinear systems, the controller developed does not require instrumentation of the load or the ability to estimate the state of the load but does not preclude it. A feedforward and feedback controller was developed that modifies the helicopter commands based on the desired load path. The controller is simplified by neglecting selected higher order terms and modeling the load as a point-mass. An adaptive dynamic inversion controller is used to control the helicopter. A fixed downward-facing camera is used to provide estimates of load position for the feedback controller. Simulation and flight-test results are shown using a Yamaha R-MAX helicopter to validate the proposed method.

Pose and position trajectory tracking for aerial transportation of a rod-like object

This paper focuses on a pose tracking problem for a system composed of two connected rigid bodies, namely an aerial vehicle – with hovering capabilities – and a rod-like rigid body. The rod-like rigid body has an axis of axial symmetry, and the joint connecting the aerial vehicle and the rod lies along that axis. The aerial vehicle is meant to transport that object, which can swing/slung with respect to the vehicle: we refer to this as generalized slung load transportation because the object being transported is a rod-like object, as opposed to standard slung load transportation where the object is a point mass with no moment of inertia. Given this system, we consider two tracking problems. Firstly, we assume that a torque input on the joint is available, and, for this scenario, we formulate a semi-pose tracking problem, requiring the rod object to track a desired pose trajectory, apart from rotations around its axis of axial symmetry (and thus the semi qualifier). Secondly, we assume that no such torque input is available, and, for this scenario, we formulate a position tracking problem, requiring a specific point along the axis of axial symmetry of the rod object to track a desired position trajectory. Our approach for solving both these problems lies in finding a state and an input transformations, such that the vector field in the new coordinates is of a known form for which controllers are found in the literature, and which we leverage in this paper. Simulations are presented that validate the proposed algorithms.

Flight Validation of a System for Autonomous Rotorcraft Multilift

A system for rotorcraft dual lift was developed and flight-tested using two Yamaha RMAX helicopters equipped with customized avionics and sling–load hardware. Important features of the system were tested, including precision dual-lift maneuvering, load tension equalization, and load motion damping. Flight-test results are presented that include autonomous takeoff and landing and an oval racetrack pattern flying at 5 m/s carrying 171% of the rated maximum load for one aircraft. Results of formation changes while flying oval patterns are also presented as well as results on active control to equalize the sling tensions between the two aircraft. In addition, sling–load feedback results to increase the sling damping in hover are presented. Finally, the feasibility of migrating the concept to full-scale is shown in simulation using two UH-60 models in a dual-lift configuration carrying 6000 lb at 80 kt.

A trajectory tracking control law for a quadrotor with slung load

We present a trajectory tracking controller for the full dynamics of a quadrotor vehicle carrying a slung load attached by a string. The full dynamic system is modeled as two connected subsystems, the string–load subsystem, with dynamics identical to that of a standard quadrotor in free flight, and the quadrotor subsystem with attitude kinematics and dynamics. A trajectory tracking controller for the position of the point-mass load is designed based on existing Lyapunov-based trajectory tracking controllers for free flying quadrotors which are further backstepped through the quadrotor attitude dynamics. A parameterized Lyapunov function is provided for the full system dynamics with a negative semi-definite time derivative. The proposed controller is proven to drive the load position error to zero and the origin of the error system is exponentially stable. Simulation results attest the performance of the proposed controller for aggressive trajectories and its robustness and validity are further highlighted by experimental results with a model-scale vehicle and slung load.

Active-Model-Based Control for the Quadrotor Carrying a Changed Slung Load

In this paper, a simple active-model-based control scheme is developed for the quadrotor slung load (QSL) system. The scheme works to improve the rejection of the influences caused by the abruptly changed load as a complementary enhancement while maintaining the structure and parameters of the original controller. A linearized model is first constructed with respect to the hovering state of a quadrotor. Modeling error is then introduced to describe the uncertainties caused by the load change and the simplified model. The modeling error is actively estimated by a Kalman filter (KF), while the estimation is further integrated into a normal controller, to enhance its performance of disturbance rejection. Experiments are conducted on a quadrotor controlled by the Pixhawk, which is one of the most popular controllers commercially available on the market. The improvements of the proposed scheme are shown by the comparisons between the controls with and without the active-model-based enhancement. The experiments also indicate that, with its simple structure and less computational algorithm, this active-model-based enhancement would be a feasible approach to enhance the commercial UAV controller to handle more uncertainties.

Swing angle estimation for multicopter slung load applications

A filtering technique based on a set of recursive equations is analyzed and applied to swing angle estimation for multicopter slung load applications. Starting from the equations of motion of the coupled slung load system, an observation model is derived to autonomously measure the swing angle by means of the data available from the onboard IMU, without the need to rely on extra sensors. Provided the multicopter is subject to known control inputs, data-fusion is performed through a Fading Gaussian Deterministic approach, whose theoretical background was recently investigated by the author. In particular, the algorithm is based on a two-step set of equations derived from the minimization of a cost function where earlier data are progressively de-weighted by a fading factor, making the estimation less prone to problem unknowns. The validity of the approach is investigated by means of numerical simulations, where a tuning criterion is shown to provide the fading factor that best dampens the modeling errors with respect to the measurement noise. Estimated swing angles are then used in a sample feedback control application with the aim to simultaneously perform trajectory tracking and payload swing damping.

Mathematical Modeling of Helicopter Dynamics with an External Sling Load

The article shows the urgency of the development of theoretical methods for studying the dynamics of the flight of a helicopter with an external sling load aimed at ensuring the flight operation safety when transporting loads. A mathematical model of the helicopter flight dynamics with an external sling load is described. The assumptions made are indicated. The load coordinate systems are introduced and described. The systems of equations of acting forces and moments are presented, which are derived considering the accelerated movement of the load's slinging point on the helicopter with its translational and rotational motion.

Four blades rotor model aerodynamic characterization and experimental investigation of rotor wake and sling load interaction

An experimental study has been carried at the Aerodynamic Measurement Methodology laboratory of CIRA (Centro Italiano Ricerche Aerospaziali). The scope of the investigation was the characterization of the loads and of the wake behaviour of a scaled rotor test rig simulating a drop test. A 3D circular cylinder was selected as representative of a typical sling load, the experimental set up having been carefully designed also with the aid of FEM analyses. The overall performances of the scaled rotor have been retrieved by the measure of its Figure of Merit and several measurements have been performed to characterize the wake and the pressure load upon the cylindrical body. The wake flow has been carefully analysed by PIV measurements and a tracking algorithm has been implemented to identify and characterize the tip vortices. The overall properties of the wake have been retrieved and a qualitative description of the tip vortices evolution and of their properties is proposed.

Cooperative load transportation using multiple UAVs

The aim of this paper is cooperative task assignment to multiple unmanned aerial vehicles (UAV) for load transportation. The main goal is to transport a slung load safely with minimal swing. To this end, for each UAV, which is regarded as an agent, a distributed controller is proposed. The proposed controller guarantees a fixed formation, which in turn achieves the main objective. A model of the system is obtained using the Udwadia–Kalaba method for an arbitrary number of UAVs and one slung load with ropes. This method leads to a novel multi-agent system model with interactions between neighbor and non-neighbor agents. The control law is then proposed based on sub-optimal LQR-PID for the extended system. Simulation results are presented to verify the ability of the proposed method to keep the formation of the agents, and to guide the load in the desired direction.

Sliding mode approach for formation control of multi-agent systems with unknown nonlinear interactions

This paper proposes nonlinear control approaches to solve a leader-follower formation of multi-agent system with unknown nonlinear interactions. Two distributed sliding mode control approaches are suggested here to track a leader in a desired formation with compensating unknown nonlinear terms. The nonlinear interaction terms can appear in multi-agent systems due to physical connections or cooperation between agents. Also the uncertainty in coefficient of control input is considered. Super twist algorithm is suggested for investigating this problem. Some Lyapunov functions are modified and employed to prove maintaining the formation of group, using the proposed sliding mode controllers. A simulation result for slung load transporting with quad-rotors is presented to demonstrate the capability of the proposed approaches.

Harmonic Extended State Observer Based Anti-Swing Attitude Control for Quadrotor with Slung Load

During the flight of the quadrotor, the existence of a slung load will exert a swing effect on the system and the motion of which will significantly change the dynamics of the quadrotor. The external torque caused by the slung load can be considered as a kind of disturbance and it is a threat to the attitude control stability of the system. In order to solve this problem, a high precision disturbance compensation method is presented in this paper, based on the harmonic extended state observer (HESO). Firstly, a generic mathematical model for the quadrotor-slung load system is obtained via the Lagrangian mechanics, and according to the analysis of the slung load motion, we obtain the disturbance as a form of periodic equation. Secondly, based on the dynamic model of the disturbance, we propose a HESO to achieve high precision disturbance estimation and its stability is proved by Lyapunov theory. Thirdly, we designed an attitude tracking controller based on backstepping method, and discussed the stability of the entire system. Finally, numerical simulations and real time experiments are carried out to evaluate the performance of the proposed method. Our results show that the robustness of the quadrotor subject to slung load has been improved.

Testing-Based Approach to Determining the Divergence Speed of Slung Loads

When a rotorcraft carries an external slung load, flight speed is often limited by the fear of divergent oscillations, rather than vehicle performance. Since slung objects can be of any shape, incorporating the aerodynamics with sufficient accuracy to predict safe speed has been a problem. The uncertainty forces certifying authorities to set conservative limits on speed to avoid divergence. Obtaining the aerodynamic coefficients of bluff bodies was excessively time-consuming in experiments, and impractical in computations. This review traces the evolution of progress in the area. Prior thinking was to use computations for prediction, with the computational codes validated using a few samples of experiments. This approach has not led to valid general predictions. Data were sparse and a-priori predictions were rarer. A continuous rotation approach has enabled swift measurements of 6-degrees-of-freedom aerodynamic load maps with high resolution about several axes of rotation. The resulting knowledge base in turn permits a swift determination of dynamics up to divergence, with wind tunnel tests where necessary to fill interpolation gaps in the knowledge base. The essence of efficient and swift dynamics simulation with a few well-tested assumptions is described. Under many relevant conditions, the vehicle flight dynamics can be safely decoupled from those of the slung load. While rotor wake swirl causes the payload to rotate at liftoff and landing, this effect can be incorporated into the simulation. Recent success in explaining two well-documented flight test cases provides strong evidence that predictions can be made for most missions swiftly.

Modeling and control for cooperative transport of a slung fluid container using quadrotors

In this paper, dynamic modeling and control problem for transfer of a sloshing liquid container suspended through rigid massless links from a team of quadrotors are investigated. By the proposed solution, pose of the slung container and fluid sloshing modes are stabilized appropriately. Dynamics of the container-liquid-quadrotors system is modeled by Euler-Lagrange method. Fluid slosh dynamics is included using multi-mass-spring model. According to derived model, a proper control law is designed for a system with three or more quadrotors. Implementing the proposed control law, quadrotors can control pose of the container, directions of the links and liquid sloshing modes simultaneously. Stability of closed loop system of tracking errors and sloshing modes are demonstrated using a theory of singularly perturbed systems and Lyapunov stability theorem. Also, the capability of the proposed feedback control laws in solving a formerly organized transport problem of a liquid filled container has been demonstrated in simulations. Moreover, priority of the proposed control scheme to an existing slung load controller in the literature is demonstrated.

Lightweight design of airlift provision for Korean light tactical vehicle using approximate optimization

A tactical vehicle should have air transportability for rapid attack and should satisfy its basic role for ground transportation. Airlift provision must be developed for aerial transportation of tactical vehicles by helicopter. The design points of front and rear lifting devices should be constructed, and the sling loads should be calculated according to various flight conditions to maintain a stable position in the air transportation of a military vehicle. The sling loads that act on the design points of airlift provision can be calculated by the analytic method defined in MIL-STD-209K. However, the actual sling loads could be different from theoretical sling loads due to vehicle specification, airlifting mechanism, and flight environment. Therefore, a virtual analysis environment for the development of airlift devices should be constructed. To establish an optimal design of airlift provision, accurate sling loads should be calculated through flight simulation, and the base model of airlift provision should be constructed based on calculated loads. In this study, we proposed an integrated process for the selection of design parameters, design of experiment, and approximate optimal design of a Korean light tactical vehicle with lightweight airlift provision.

Nonlinear forward path tracking controller for helicopter with slung load

In this paper, a tracking controller is designed for a helicopter with a slung load. The designed controller is nonlinear and is based on the backstopping method. First, we developed a comprehensive nonlinear mathematical model of the helicopter and designed a nonlinear backstepping controller to track the trajectory of the helicopter alone. Then, we developed a mathematical model of the slung load and integrated it with the helicopter system. In the next step, we designed an anti-swing delayed feedback controller to damp the load oscillation. This controller works independently of the nonlinear tracking controller. Moreover, its performance is optimized using the differential evolution (DE) technique. The simulation results show the effectiveness of the designed control system.

Using iterative LQR to control two quadrotors transporting a cable-suspended load

This paper is focused on the tracking control problem of two quadrotors carrying one cable-suspended payload. The paper main contributions include a dynamic model constructed by Euler-Lagrange equations for this system, an iterative linear quadratic regulator (iLQR) optimal controller, which is able to track the desired trajectory for the payload position and attitude and a simulation for tracking a spiral trajectory. The nonlinear dynamic model is linearised by considering the angles of connected links from the payload to each quadrotor as the equilibrium point. The iLQR control algorithm is developed for the full system consisting of two quadrotors connected with a point mass load in order to improve the tracking performance and avoid the slung load swing. The simulation results demonstrate that the iLQR approach is able to improve the tracking performance compared with an LQR controller.

Flight Evaluation of a Flexible Fabric Stabilizer for Sling Loads

Helicopter sling load is the most accurate form of aerial delivery in the military due to the ability to air-land materiel in an exact location; however, some missions have a tendency for the payloads to become unstable due to both pilot-in-the-loop and aerodynamic effects. Past research demonstrated that allowing the container to rotate freely in yaw stabilizes pendulum motions. Other research utilized rigid fins affixed to the rear of the container. These methods work during tests; however, they become difficult to use in an operational environment. This paper discusses tests using a flexible fabric stabilizer that can be temporarily added to any payload. The flight tests were conducted at Moffett Field, CA, using the same payload as the previously mentioned research. Tests showed that the flexible stabilizer provided an intermediate level of performance eliminating sling wind up and stabilizing pendulum motions out to the aircrafts' power limit in exchange for very little operational overhead. The material in this paper concerns loads carried in a single point suspension.

Design of an automatic load positioning system for hoist operations

The operation of a helicopter with an externally slung load is highly demanding for the flight crew. Without having a direct view on the load, the pilot needs assistance by a crew member for load handling, e.g., load positioning. During helicopter hoist operations (HHO), the hoist operator gives instructions to the pilot and stabilizes the load motion. An automatic load positioning system for HHO is designed with the aim to reduce the workload of the flight crew and to improve the load handling performance. The control system consists of a multiloop architecture that is designed using multi-objective optimization. The system was designed to achieve best performance in load positioning for the entire cable length range of the rescue hoist. Load positioning performance is a function of the cable length; faster automatic load positioning can be achieved with the shorter cable lengths. This paper describes the overall design process covering the tools, the control law architecture and optimization, and the validation using DLR’s simulator.