A Review on Control Methods for Multi-Lift Rotorcraft Systems
---
paper_title: Nonlinear adaptive control of a twin lift helicopter system

paper_content:

The tracking control of a twin-lift helicopter system in the presence of parametric uncertainty is considered. A nonlinear model is used to describe the dynamics of a twin-lift helicopter configuration in the lateral/vertical plane, and a controller is synthesized using an input-output feedback linearization technique in conjunction with an adaptation algorithm. The control scheme does not require any knowledge of the bound of uncertainties present and drives the output tracking error to zero asymptotically. The performance of the controller is illustrated by simulating the nonlinear model of the twin-lift system. >
---
paper_title: Multi-UAV cooperation and control for load transportation and deployment

paper_content:

This paper deals with the cooperation and control of multiple UAVs with sensing and actuation capabilities. An architecture to perform cooperative missions with a multi-UAV platform is presented. The interactions between UAVs are not only information exchanges but also physical couplings required to cooperate in the joint transportation of a single load. Then, the paper also presents the control system for the transportation of a slung load by means of one or several helicopters. Experimental results of the load transportation system with one and three helicopters are shown. On the other hand, the UAVs considered in the platform can also deploy small objects, such as sensor nodes, on different locations if it is required. This feature along with the whole platform architecture are illustrated in the paper with a real multi-UAV mission for the deployment of sensor nodes to repair the connectivity of a wireless sensor network.
---
paper_title: Pilot/vehicle analysis of a twin-lift helicopter configuration in hover

paper_content:

A pilot/vehicle analysis of a twin-lift helicopter configuration carrying a load suspended from a rigid spreader bar is undertaken. The task is one of lateral/vertical control in hover. A two-pilot manual control structure is formulated in which the pilot of the "master" helicopter maneuvers his vehicle in response to internally generated position commands. The pilot of the "slave" helicopter senses master helicopter transnational motion and uses it as a position command to his own vehicle. The required pilot equalization in all feedback loops is established using classical frequency domain pilot/vehicle analysis techniques based upon the crossover model of the human pilot. Closed-loop performance is assessed through computer simulation.
---
paper_title: Multivariable Control of a Twin Lift Helicopter System using the LQG/LTR Design Methodology

paper_content:

This paper presents guidelines for developing a multivariable centralized automatic flight control system (AFCS) for a twin lift helicopter system (TLHS). Singular value ideas are used to formulate performarce and stability robustness specifications. A Linear-Quadratic-Gaussian with Loop Transfer Recovery (LQG/LTR) design is obtained and evaluated.
---
paper_title: Equilibrium Computation of Close-coupling Multiple Helicopters in Hover Condition

paper_content:

Helicopter is a complex system and the harmony operations of close-coupling multiple helicopters have their particularities.Through analyzing the harmony operations of close coupling multiple helicopters in hover condition,the nonlinear force model was established.Using Newton iterative method,the proposed model was computed and the perfect control inputs and poses in equilibrium hovering were attained.The results can be regarded as the reference control inputs of the flight controller system in hover condition and the base of dynamic modeling and stabilization analyzing.
---
paper_title: Autonomously Flying VTOL-Robots: Modeling and Control

paper_content:

In this paper an approach for control of autonomously flying robots with vertical take off and landing capabilities (VTOL) is presented. After reviewing that the motion description for different VTOL-robots is very similar, the general control scheme for VTOL-robots is presented. This scheme is based on linearisation and decoupling using inversion of the system model blocks. To compensate the model uncertainties and disturbances two additional parts are included into the controller: a reduced state observer based on a robot motion model as well as a disturbances observer and compensator for orientation control. The presented approach was applied to two different VTOL-robots: a helicopter and a quad-rotor. In real flight experiments it was verified that the presented general but simple controller provides sufficient performance for a wide range of practical applications.
---
paper_title: Autonomous Transportation and Deployment with Aerial Robots for Search and Rescue Missions

paper_content:

It is generally accepted that systems composed of multiple aerial robots with autonomous cooperation capabilities can assist responders in many search and rescue (SAR) scenarios. In most of the previous research work, the aerial robots are mainly considered as platforms for environmental sensing and have not been used to assist victims. In this paper, outdoor field experiments of transportation and accurate deployment of loads with single/multiple autonomous aerial vehicles are presented. This is a novel feature that opens the possibility to use aerial robots to assist victims during rescue phase operations. Accuracy in the deployment location is a critical issue in SAR scenarios in which injured people may have very limited mobility. The presented system is composed of up to three small-size helicopters and features cooperative sensing, using several different sensor types. The system supports several forms of cooperative actuation as well, ranging from the cooperative deployment of small sensors/objects to the coupled transportation of slung loads. The complete system is described, outlining the hardware and software framework used, as well as the approaches for modeling and control used. Additionally, the results of several flight field experiments are presented, including a description of the worldwide first successful autonomous load transportation experiment, using three coupled small-size helicopters (conducted in December 2007). During these experiments strong, steady winds and wind gusts were present. Various solutions and lessons learned from the design and operation of the system are also provided. © 2011 Wiley Periodicals, Inc.
---
paper_title: Coordinated Transport of a Slung Load by a Team of Autonomous Rotorcraft

paper_content:

This paper describes a hierarchical approach to coordinated transportation of a slung load by a team of autonomous helicopters. A closed-loop controller computes net force and moments on the load’s center of gravity so a desired trajectory is followed; cable forces at each attachment point are computed so that the net force and moment on the center of gravity equal the desired values from the controller; nally each helicopter’s position, velocity, and thrust are computed to obtain the desired attachment force, assuming a compliant cable connects the helicopter to the load. Results of simulations showing four degree of freedom transport of a load (North, East, Down position and yaw angle) show the utility of the proposed approach.
---
paper_title: Cooperative Manipulation and Transportation with Aerial Robots

paper_content:

In this paper we consider the problem of controlling multiple robots manipulating and transporting a payload in three dimensions via cables. We develop robot configurations that ensure static equilibrium of the payload at a desired pose while respecting constraints on the tension and provide analysis of payload stability for these configurations. We demonstrate our methods on a team of aerial robots via simulation and experimentation.
---
paper_title: Load transportation system based on autonomous small size helicopters

paper_content:

This paper is devoted to modelling and control algorithms for a slung load transportation system composed of one or multiple helicopters, where the load is coupled by the means of flexible ropes (see Fig. 1). The coupled helicopter system overcomes the payload limitation of a single small size helicopter, while keeping most of its advantages: small costs of operation, low maintenance costs and increased safeness. Therefore, the system can be utilised whenever the use of full size helicopters is impossible, too expensive or prohibited by law. We focus on the deployment and repairing of distributed sensor networks, using a transportation system based on multiple small size helicopters. A possible real world application is the deployment of fire fighting equipment, where space limitation of the fire trucks prohibits the application of bigger UAVs and using full size helicopters is too dangerous. The problem of load transportation using one or two full size helicopters (twin lift helicopter system), connected to the load by means of flexible ropes, has been discussed in the aerospace research community at least since 1960. We have shown in our previous work that there is a fundamental difference in the mathematical description between small and full size helicopters. Therefore, also the control design for the case of small size helicopters needs to be different. To our knowledge, the control of a slung load transportation system composed of multiple small size helicopters has not been studied until now. In this paper, the complete mechanical setup of the slung load transportation system based on one or more small size helicopters is presented. This includes a short description of the used UAVs, the additionally required sensors, and how the load is mounted. A model of one/multiple helicopters transporting a load is introduced. This model is used in a simplified form for the controller design and in full form for simulation. The controller for one and two helicopters, which is based on a state feedback controller, as well as the controller for three and more helicopters, which is based on a non linear controller, are explained in detail. Both controllers utilise an underlying non-linear orientation controller. We propose a feedback loop, based on forces measured in the ropes, to compensate for the influence of the rope. The controllers were tested in simulation and in real flight experiments. The world wide first flight experiment with three coupled helicopters was successfully conducted at the end of 2007.
---
paper_title: Models and Analysis for Twin-Lift Helicopter Systems

paper_content:

Multiple-lift schemes using two or more helicopters to lift payloads too large for a single helicopter have been tested on several occasions. However, the manual control tasks involved demanded a considerable amount of flying skill and cooperative abilities from the pilots. Initial to the design of automatic control is the development of reasonable models of the twin-lift system, which is discussed. An appropriate analytical model is described.
---
paper_title: Nonlinear adaptive control of a twin lift helicopter system

paper_content:

The tracking control of a twin-lift helicopter system in the presence of parametric uncertainty is considered. A nonlinear model is used to describe the dynamics of a twin-lift helicopter configuration in the lateral/vertical plane, and a controller is synthesized using an input-output feedback linearization technique in conjunction with an adaptation algorithm. The control scheme does not require any knowledge of the bound of uncertainties present and drives the output tracking error to zero asymptotically. The performance of the controller is illustrated by simulating the nonlinear model of the twin-lift system. >
---
paper_title: The Inverse Kinematics of 3-D Towing

paper_content:

This work addresses the kinematics of 3-D towing where multiple aerial robots are used to cooperatively transport a payload using cables. In 3-D towing, it is important to determine the relative positions of the robots for the desired position and orientation of the payload. This is actually the inverse kinematics problem of the 3-D towing. In the case with three robots, when the tensions are given, the inverse kinematics problem has finite number of solutions. In order to obtain all possible solutions, an efficient analytic algorithm based on dialytic elimination is presented. An example is used for illustration.
---
paper_title: Pilot/vehicle analysis of a twin-lift helicopter configuration in hover

paper_content:

A pilot/vehicle analysis of a twin-lift helicopter configuration carrying a load suspended from a rigid spreader bar is undertaken. The task is one of lateral/vertical control in hover. A two-pilot manual control structure is formulated in which the pilot of the "master" helicopter maneuvers his vehicle in response to internally generated position commands. The pilot of the "slave" helicopter senses master helicopter transnational motion and uses it as a position command to his own vehicle. The required pilot equalization in all feedback loops is established using classical frequency domain pilot/vehicle analysis techniques based upon the crossover model of the human pilot. Closed-loop performance is assessed through computer simulation.
---
paper_title: Equilibrium Computation of Close-coupling Multiple Helicopters in Hover Condition

paper_content:

Helicopter is a complex system and the harmony operations of close-coupling multiple helicopters have their particularities.Through analyzing the harmony operations of close coupling multiple helicopters in hover condition,the nonlinear force model was established.Using Newton iterative method,the proposed model was computed and the perfect control inputs and poses in equilibrium hovering were attained.The results can be regarded as the reference control inputs of the flight controller system in hover condition and the base of dynamic modeling and stabilization analyzing.
---
paper_title: Stability, Control, and Simulation of a Dual Lift System Using Autonomous R-MAX Helicopters

paper_content:

Cargo missions of the near future will require transportation of increasingly heavy loads to remote locations that are at distances farther, and at altitudes higher, than what can be accomplished by current helicopters. Using two helicopters to carry a single load, termed dual lift, provides heavy lift on demand and avoids the very costly design and development of a special purpose heavy lift helicopter. This paper discusses the stability and control aspects of a dual lift system using autonomous R-MAX helicopters. Unique dual lift modes lie in a frequency range that affect helicopter attitude responses and should be considered while designing an inner-loop (attitude) dual lift controller. One such mode is unstable and is characterized by the helicopters drifting away from each. In order to stabilize this mode, helicopter separation feedback is used. Unlike in a single lift system, there exists only one pendulum mode in a dual lift configuration and a cable angle that is relative to a no swing plane is needed to isolate the swinging motion and improve damping of the payload. With the identified control challenges as a foundation, a multi-objective parametric optimization approach is used to design a controller for a dual lift system that meets a comprehensive set of ADS-33E stability and performance specifications. NOTATION N~a B1cm Acceleration of point B1cm in reference frame N, f t sec2 I B1 B1cm Inertia dyadic of B1 about B1cm, slugf t2 Ib1x b1x Roll moment of inertia of B1 about B1cm, slugf t2 Ib1y b1y Pitch moment of inertia of B1 about B1cm, slugf t2 Presented at the AHS 70th Annual Forum, Montreal, Quebec, Canada, May 20–22, 2014. This is a work of the U.S. Government and is not subject to copyright protection in the U.S. DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited. Ib1z b1z Yaw moment of inertia of B1 about B1cm, slugf t2 ~r B1cm No Position vector from point No to B1cm, f t N~v B1cm Velocity of point B1cm in reference frame N, f t sec N~α B1 The angular acceleration of B1 in N, rad sec2 N~ ω B1 The angular velocity of B1 in N, rad sec ADS Aeronautical Design Standard AFCS Automatic Flight Control System AFDD Aeroflightdynamics Directorate ARP Autonomous Rotorcraft Project DOF Degree of Freedom DRB Disturbance Rejection Bandwidth, rad sec DRP Disturbance Rejection Peak, dB FBD Free Body Diagram
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paper_title: Equations of motion of slung-load systems, including multilift systems

paper_content:

General simulation equations are derived for the rigid body motion of slung-load systems. This work is motivated by an interest in trajectory control for slung loads carried by two or more helicopters. An approximation of these systems consists of several rigid bodies connected by straight-line cables or links. The suspension can be assumed elastic or inelastic. Equations for the general system are obtained from the Newton-Euler rigid-body equations with the introduction of generalized velocity coordinates. Three forms are obtained: two generalize previous case-specific results for single-helicopter systems with elastic and inelastic suspensions, respectively; and the third is a new formulation for inelastic suspensions. The latter is derived from the elastic suspension equations by choosing the generalized coordinates so that motion induced by cable stretching is separated from motion with invariant cable lengths, and by then nulling the stretching coordinates to get a relation for the suspension forces. The result is computationally more efficient than the conventional formulation, is readily integrated with the elastic suspension formulation, and is easily applied to the complex dual-lift and multilift systems. Results are given for two-helicopter systems; three configurations are included and these can be integrated in a single simulation. Equations are also given for some single-helicopter systems, for comparison with the previous literature, and for a multilift system. Equations for degenerate-body approximations (point masses, rigid rods) are also formulated and results are given for dual-lift and multilift systems. Finally, linearlized equations of motion are given for general slung-load systems are presented along with results for the two-helicopter system with a spreader bar.
---
paper_title: Cooperative quadrotors carrying a suspended load

paper_content:

This work proposes the representation of the high level dynamic modelling of a formation of two quadrotor UAVs cooperatively carrying a suspended load. The two quadrotors are attached to the load by massless and stretchable cables, which can slack and taut during the task accomplishment. The cable is modeled as a high stiffness spring with a damping coefficient, according to the Hooke's law, and its behavior is taken into account in the disturbance estimation. A fictitious repulsion force is also introduced to minimize the risk of a collision between the UAVs during lateral displacements. Finally, to dealt with disturbance rejection caused by the load compensation and the cable tension variations, a nonlinear controller is proposed to guide the UAVs cooperatively during positioning and trajectory-tracking tasks, considering that the movement of the UAVs is restricted to the XZ-plane. Finally, simulated results are presented, in order to validate the proposal.
---
paper_title: Cooperative Manipulation and Transportation with Aerial Robots

paper_content:

In this paper we consider the problem of controlling multiple robots manipulating and transporting a payload in three dimensions via cables. We develop robot configurations that ensure static equilibrium of the payload at a desired pose while respecting constraints on the tension and provide analysis of payload stability for these configurations. We demonstrate our methods on a team of aerial robots via simulation and experimentation.
---
paper_title: Models and Analysis for Twin-Lift Helicopter Systems

paper_content:

Multiple-lift schemes using two or more helicopters to lift payloads too large for a single helicopter have been tested on several occasions. However, the manual control tasks involved demanded a considerable amount of flying skill and cooperative abilities from the pilots. Initial to the design of automatic control is the development of reasonable models of the twin-lift system, which is discussed. An appropriate analytical model is described.
---
paper_title: COMPARISON OF NONLINEAR CONTROLLERS FOR TWIN-LIFT CONFIGURATIONS

paper_content:

The stability and control problems associated with twinlift helicopter configurations are analyzed. For the two twin-lift configurations viz. twin-lift with spreader bar and twin-lift without spreader bar, a nonlinear control scheme based on feedback linearization is presented. The performance of the resulting closed-loop systems in terms of trim attitudes and required control travel for carrying out a typical twin-lift mission is evaluated through nonlinear simulation. Also, the effect of controller and sensor degradations on the overall system performance is discussed.
---
paper_title: Nonlinear adaptive control of a twin lift helicopter system

paper_content:

The tracking control of a twin-lift helicopter system in the presence of parametric uncertainty is considered. A nonlinear model is used to describe the dynamics of a twin-lift helicopter configuration in the lateral/vertical plane, and a controller is synthesized using an input-output feedback linearization technique in conjunction with an adaptation algorithm. The control scheme does not require any knowledge of the bound of uncertainties present and drives the output tracking error to zero asymptotically. The performance of the controller is illustrated by simulating the nonlinear model of the twin-lift system. >
---
paper_title: Multivariable Control of a Twin Lift Helicopter System using the LQG/LTR Design Methodology

paper_content:

This paper presents guidelines for developing a multivariable centralized automatic flight control system (AFCS) for a twin lift helicopter system (TLHS). Singular value ideas are used to formulate performarce and stability robustness specifications. A Linear-Quadratic-Gaussian with Loop Transfer Recovery (LQG/LTR) design is obtained and evaluated.
---
paper_title: Dynamic collaboration without communication: Vision-based cable-suspended load transport with two quadrotors

paper_content:

Transport of objects is a major application in robotics nowadays. While ground robots can carry heavy payloads for long distances, they are limited in rugged terrains. Aerial robots can deliver objects in arbitrary terrains; however they tend to be limited in payload. It has been previously shown that, for heavy payloads, it can be beneficial to carry them using multiple flying robots. In this paper, we propose a novel collaborative transport scheme, in which two quadrotors transport a cable-suspended payload at accelerations that exceed the capabilities of previous collaborative approaches, which make quasi-static assumptions. Furthermore, this is achieved completely without explicit communication between the collaborating robots, making our system robust to communication failures and making consensus on a common reference frame unnecessary. Instead, they only rely on visual and inertial cues obtained from on-board sensors. We implement and validate the proposed method on a real system.
---
paper_title: Collision avoidance for quadrotor UAVs transporting a payload via Voronoi tessellation

paper_content:

This paper is focused on the formation planning and control of an arbitrary number of quadrotors transporting a cable-suspended payload. A payload that is modeled as a point mass is connected to quadrotors via rigid links, and a desired trajectory of the payload is prescribed. Assuming that the location and shape of obstacles are available, the desired formation of quadrotors relative to the payload is constructed to avoid collision with the obstacles, via the centroidal Voronoi tessellation. Then, these are integrated with a geometric tracking control system. A numerical example illustrates that quadrotors transports a payload along a desired trajectory, while flying through a narrow gap between obstacles to avoid a collision.
---
paper_title: A survey of consensus problems in multi-agent coordination

paper_content:

As a distributed solution to multi-agent coordination, consensus or agreement problems have been studied extensively in the literature. This paper provides a survey of consensus problems in multi-agent cooperative control with the goal of promoting research in this area. Theoretical results regarding consensus seeking under both time-invariant and dynamically changing information exchange topologies are summarized. Applications of consensus protocols to multiagent coordination are investigated. Future research directions and open problems are also proposed.
---
paper_title: Collaborative transportation using MAVs via passive force control

paper_content:

This paper shows a strategy based on passive force control for collaborative object transportation using Micro Aerial Vehicles (MAVs), focusing on the transportation of a bulky object by two hexacopters. The goal is to develop a robust approach which does not rely on: (a) communication links between the MAVs, (b) the knowledge of the payload shape and (c) the position of grasping point. The proposed approach is based on the master-slave paradigm, in which the slave agent guarantees compliance to the external force applied by the master to the payload via an admittance controller. The external force acting on the slave is estimated using a non-linear estimator based on the Unscented Kalman Filter (UKF) from the information provided by a Visual-Inertial (VI) navigation system. Experimental results (online video [1]) demonstrate the performance of the force estimator and show the collaborative transportation of a 1.2 m long object.
---
paper_title: Autonomous Transportation and Deployment with Aerial Robots for Search and Rescue Missions

paper_content:

It is generally accepted that systems composed of multiple aerial robots with autonomous cooperation capabilities can assist responders in many search and rescue (SAR) scenarios. In most of the previous research work, the aerial robots are mainly considered as platforms for environmental sensing and have not been used to assist victims. In this paper, outdoor field experiments of transportation and accurate deployment of loads with single/multiple autonomous aerial vehicles are presented. This is a novel feature that opens the possibility to use aerial robots to assist victims during rescue phase operations. Accuracy in the deployment location is a critical issue in SAR scenarios in which injured people may have very limited mobility. The presented system is composed of up to three small-size helicopters and features cooperative sensing, using several different sensor types. The system supports several forms of cooperative actuation as well, ranging from the cooperative deployment of small sensors/objects to the coupled transportation of slung loads. The complete system is described, outlining the hardware and software framework used, as well as the approaches for modeling and control used. Additionally, the results of several flight field experiments are presented, including a description of the worldwide first successful autonomous load transportation experiment, using three coupled small-size helicopters (conducted in December 2007). During these experiments strong, steady winds and wind gusts were present. Various solutions and lessons learned from the design and operation of the system are also provided. © 2011 Wiley Periodicals, Inc.
---
paper_title: Coordinated Transport of a Slung Load by a Team of Autonomous Rotorcraft

paper_content:

This paper describes a hierarchical approach to coordinated transportation of a slung load by a team of autonomous helicopters. A closed-loop controller computes net force and moments on the load’s center of gravity so a desired trajectory is followed; cable forces at each attachment point are computed so that the net force and moment on the center of gravity equal the desired values from the controller; nally each helicopter’s position, velocity, and thrust are computed to obtain the desired attachment force, assuming a compliant cable connects the helicopter to the load. Results of simulations showing four degree of freedom transport of a load (North, East, Down position and yaw angle) show the utility of the proposed approach.
---
paper_title: Geometric control of multiple quadrotor UAVs transporting a cable-suspended rigid body

paper_content:

This paper investigates tracking controls for an arbitrary number of cooperating quadrotor unmanned aerial vehicles with a suspended rigid body payload. Assuming that a rigid body is connected to multiple quadrotors by rigid massless links, control systems for quadrotors are constructed such that the rigid body asymptotically follows a given desired position and attitude trajectory. These are developed in a coordinate-free fashion to avoid singularities and complexities associated with local parameterizations.
---
paper_title: Load transportation system based on autonomous small size helicopters

paper_content:

This paper is devoted to modelling and control algorithms for a slung load transportation system composed of one or multiple helicopters, where the load is coupled by the means of flexible ropes (see Fig. 1). The coupled helicopter system overcomes the payload limitation of a single small size helicopter, while keeping most of its advantages: small costs of operation, low maintenance costs and increased safeness. Therefore, the system can be utilised whenever the use of full size helicopters is impossible, too expensive or prohibited by law. We focus on the deployment and repairing of distributed sensor networks, using a transportation system based on multiple small size helicopters. A possible real world application is the deployment of fire fighting equipment, where space limitation of the fire trucks prohibits the application of bigger UAVs and using full size helicopters is too dangerous. The problem of load transportation using one or two full size helicopters (twin lift helicopter system), connected to the load by means of flexible ropes, has been discussed in the aerospace research community at least since 1960. We have shown in our previous work that there is a fundamental difference in the mathematical description between small and full size helicopters. Therefore, also the control design for the case of small size helicopters needs to be different. To our knowledge, the control of a slung load transportation system composed of multiple small size helicopters has not been studied until now. In this paper, the complete mechanical setup of the slung load transportation system based on one or more small size helicopters is presented. This includes a short description of the used UAVs, the additionally required sensors, and how the load is mounted. A model of one/multiple helicopters transporting a load is introduced. This model is used in a simplified form for the controller design and in full form for simulation. The controller for one and two helicopters, which is based on a state feedback controller, as well as the controller for three and more helicopters, which is based on a non linear controller, are explained in detail. Both controllers utilise an underlying non-linear orientation controller. We propose a feedback loop, based on forces measured in the ropes, to compensate for the influence of the rope. The controllers were tested in simulation and in real flight experiments. The world wide first flight experiment with three coupled helicopters was successfully conducted at the end of 2007.
---
paper_title: COMPARISON OF NONLINEAR CONTROLLERS FOR TWIN-LIFT CONFIGURATIONS

paper_content:

The stability and control problems associated with twinlift helicopter configurations are analyzed. For the two twin-lift configurations viz. twin-lift with spreader bar and twin-lift without spreader bar, a nonlinear control scheme based on feedback linearization is presented. The performance of the resulting closed-loop systems in terms of trim attitudes and required control travel for carrying out a typical twin-lift mission is evaluated through nonlinear simulation. Also, the effect of controller and sensor degradations on the overall system performance is discussed.
---
paper_title: Dynamic collaboration without communication: Vision-based cable-suspended load transport with two quadrotors

paper_content:

Transport of objects is a major application in robotics nowadays. While ground robots can carry heavy payloads for long distances, they are limited in rugged terrains. Aerial robots can deliver objects in arbitrary terrains; however they tend to be limited in payload. It has been previously shown that, for heavy payloads, it can be beneficial to carry them using multiple flying robots. In this paper, we propose a novel collaborative transport scheme, in which two quadrotors transport a cable-suspended payload at accelerations that exceed the capabilities of previous collaborative approaches, which make quasi-static assumptions. Furthermore, this is achieved completely without explicit communication between the collaborating robots, making our system robust to communication failures and making consensus on a common reference frame unnecessary. Instead, they only rely on visual and inertial cues obtained from on-board sensors. We implement and validate the proposed method on a real system.
---
paper_title: Collaborative transportation using MAVs via passive force control

paper_content:

This paper shows a strategy based on passive force control for collaborative object transportation using Micro Aerial Vehicles (MAVs), focusing on the transportation of a bulky object by two hexacopters. The goal is to develop a robust approach which does not rely on: (a) communication links between the MAVs, (b) the knowledge of the payload shape and (c) the position of grasping point. The proposed approach is based on the master-slave paradigm, in which the slave agent guarantees compliance to the external force applied by the master to the payload via an admittance controller. The external force acting on the slave is estimated using a non-linear estimator based on the Unscented Kalman Filter (UKF) from the information provided by a Visual-Inertial (VI) navigation system. Experimental results (online video [1]) demonstrate the performance of the force estimator and show the collaborative transportation of a 1.2 m long object.
---
paper_title: Autonomous Transportation and Deployment with Aerial Robots for Search and Rescue Missions

paper_content:

It is generally accepted that systems composed of multiple aerial robots with autonomous cooperation capabilities can assist responders in many search and rescue (SAR) scenarios. In most of the previous research work, the aerial robots are mainly considered as platforms for environmental sensing and have not been used to assist victims. In this paper, outdoor field experiments of transportation and accurate deployment of loads with single/multiple autonomous aerial vehicles are presented. This is a novel feature that opens the possibility to use aerial robots to assist victims during rescue phase operations. Accuracy in the deployment location is a critical issue in SAR scenarios in which injured people may have very limited mobility. The presented system is composed of up to three small-size helicopters and features cooperative sensing, using several different sensor types. The system supports several forms of cooperative actuation as well, ranging from the cooperative deployment of small sensors/objects to the coupled transportation of slung loads. The complete system is described, outlining the hardware and software framework used, as well as the approaches for modeling and control used. Additionally, the results of several flight field experiments are presented, including a description of the worldwide first successful autonomous load transportation experiment, using three coupled small-size helicopters (conducted in December 2007). During these experiments strong, steady winds and wind gusts were present. Various solutions and lessons learned from the design and operation of the system are also provided. © 2011 Wiley Periodicals, Inc.
---
paper_title: The Inverse Kinematics of 3-D Towing

paper_content:

This work addresses the kinematics of 3-D towing where multiple aerial robots are used to cooperatively transport a payload using cables. In 3-D towing, it is important to determine the relative positions of the robots for the desired position and orientation of the payload. This is actually the inverse kinematics problem of the 3-D towing. In the case with three robots, when the tensions are given, the inverse kinematics problem has finite number of solutions. In order to obtain all possible solutions, an efficient analytic algorithm based on dialytic elimination is presented. An example is used for illustration.
---
paper_title: Dynamic collaboration without communication: Vision-based cable-suspended load transport with two quadrotors

paper_content:

Transport of objects is a major application in robotics nowadays. While ground robots can carry heavy payloads for long distances, they are limited in rugged terrains. Aerial robots can deliver objects in arbitrary terrains; however they tend to be limited in payload. It has been previously shown that, for heavy payloads, it can be beneficial to carry them using multiple flying robots. In this paper, we propose a novel collaborative transport scheme, in which two quadrotors transport a cable-suspended payload at accelerations that exceed the capabilities of previous collaborative approaches, which make quasi-static assumptions. Furthermore, this is achieved completely without explicit communication between the collaborating robots, making our system robust to communication failures and making consensus on a common reference frame unnecessary. Instead, they only rely on visual and inertial cues obtained from on-board sensors. We implement and validate the proposed method on a real system.
---
paper_title: Stability, Control, and Simulation of a Dual Lift System Using Autonomous R-MAX Helicopters

paper_content:

Cargo missions of the near future will require transportation of increasingly heavy loads to remote locations that are at distances farther, and at altitudes higher, than what can be accomplished by current helicopters. Using two helicopters to carry a single load, termed dual lift, provides heavy lift on demand and avoids the very costly design and development of a special purpose heavy lift helicopter. This paper discusses the stability and control aspects of a dual lift system using autonomous R-MAX helicopters. Unique dual lift modes lie in a frequency range that affect helicopter attitude responses and should be considered while designing an inner-loop (attitude) dual lift controller. One such mode is unstable and is characterized by the helicopters drifting away from each. In order to stabilize this mode, helicopter separation feedback is used. Unlike in a single lift system, there exists only one pendulum mode in a dual lift configuration and a cable angle that is relative to a no swing plane is needed to isolate the swinging motion and improve damping of the payload. With the identified control challenges as a foundation, a multi-objective parametric optimization approach is used to design a controller for a dual lift system that meets a comprehensive set of ADS-33E stability and performance specifications. NOTATION N~a B1cm Acceleration of point B1cm in reference frame N, f t sec2 I B1 B1cm Inertia dyadic of B1 about B1cm, slugf t2 Ib1x b1x Roll moment of inertia of B1 about B1cm, slugf t2 Ib1y b1y Pitch moment of inertia of B1 about B1cm, slugf t2 Presented at the AHS 70th Annual Forum, Montreal, Quebec, Canada, May 20–22, 2014. This is a work of the U.S. Government and is not subject to copyright protection in the U.S. DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited. Ib1z b1z Yaw moment of inertia of B1 about B1cm, slugf t2 ~r B1cm No Position vector from point No to B1cm, f t N~v B1cm Velocity of point B1cm in reference frame N, f t sec N~α B1 The angular acceleration of B1 in N, rad sec2 N~ ω B1 The angular velocity of B1 in N, rad sec ADS Aeronautical Design Standard AFCS Automatic Flight Control System AFDD Aeroflightdynamics Directorate ARP Autonomous Rotorcraft Project DOF Degree of Freedom DRB Disturbance Rejection Bandwidth, rad sec DRP Disturbance Rejection Peak, dB FBD Free Body Diagram
---
paper_title: Collaborative transportation using MAVs via passive force control

paper_content:

This paper shows a strategy based on passive force control for collaborative object transportation using Micro Aerial Vehicles (MAVs), focusing on the transportation of a bulky object by two hexacopters. The goal is to develop a robust approach which does not rely on: (a) communication links between the MAVs, (b) the knowledge of the payload shape and (c) the position of grasping point. The proposed approach is based on the master-slave paradigm, in which the slave agent guarantees compliance to the external force applied by the master to the payload via an admittance controller. The external force acting on the slave is estimated using a non-linear estimator based on the Unscented Kalman Filter (UKF) from the information provided by a Visual-Inertial (VI) navigation system. Experimental results (online video [1]) demonstrate the performance of the force estimator and show the collaborative transportation of a 1.2 m long object.
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paper_title: Equations of motion of slung-load systems, including multilift systems

paper_content:

General simulation equations are derived for the rigid body motion of slung-load systems. This work is motivated by an interest in trajectory control for slung loads carried by two or more helicopters. An approximation of these systems consists of several rigid bodies connected by straight-line cables or links. The suspension can be assumed elastic or inelastic. Equations for the general system are obtained from the Newton-Euler rigid-body equations with the introduction of generalized velocity coordinates. Three forms are obtained: two generalize previous case-specific results for single-helicopter systems with elastic and inelastic suspensions, respectively; and the third is a new formulation for inelastic suspensions. The latter is derived from the elastic suspension equations by choosing the generalized coordinates so that motion induced by cable stretching is separated from motion with invariant cable lengths, and by then nulling the stretching coordinates to get a relation for the suspension forces. The result is computationally more efficient than the conventional formulation, is readily integrated with the elastic suspension formulation, and is easily applied to the complex dual-lift and multilift systems. Results are given for two-helicopter systems; three configurations are included and these can be integrated in a single simulation. Equations are also given for some single-helicopter systems, for comparison with the previous literature, and for a multilift system. Equations for degenerate-body approximations (point masses, rigid rods) are also formulated and results are given for dual-lift and multilift systems. Finally, linearlized equations of motion are given for general slung-load systems are presented along with results for the two-helicopter system with a spreader bar.
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paper_title: Autonomous Transportation and Deployment with Aerial Robots for Search and Rescue Missions

paper_content:

It is generally accepted that systems composed of multiple aerial robots with autonomous cooperation capabilities can assist responders in many search and rescue (SAR) scenarios. In most of the previous research work, the aerial robots are mainly considered as platforms for environmental sensing and have not been used to assist victims. In this paper, outdoor field experiments of transportation and accurate deployment of loads with single/multiple autonomous aerial vehicles are presented. This is a novel feature that opens the possibility to use aerial robots to assist victims during rescue phase operations. Accuracy in the deployment location is a critical issue in SAR scenarios in which injured people may have very limited mobility. The presented system is composed of up to three small-size helicopters and features cooperative sensing, using several different sensor types. The system supports several forms of cooperative actuation as well, ranging from the cooperative deployment of small sensors/objects to the coupled transportation of slung loads. The complete system is described, outlining the hardware and software framework used, as well as the approaches for modeling and control used. Additionally, the results of several flight field experiments are presented, including a description of the worldwide first successful autonomous load transportation experiment, using three coupled small-size helicopters (conducted in December 2007). During these experiments strong, steady winds and wind gusts were present. Various solutions and lessons learned from the design and operation of the system are also provided. © 2011 Wiley Periodicals, Inc.
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paper_title: COMPARISON OF NONLINEAR CONTROLLERS FOR TWIN-LIFT CONFIGURATIONS

paper_content:

The stability and control problems associated with twinlift helicopter configurations are analyzed. For the two twin-lift configurations viz. twin-lift with spreader bar and twin-lift without spreader bar, a nonlinear control scheme based on feedback linearization is presented. The performance of the resulting closed-loop systems in terms of trim attitudes and required control travel for carrying out a typical twin-lift mission is evaluated through nonlinear simulation. Also, the effect of controller and sensor degradations on the overall system performance is discussed.
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Title: A Review on Control Methods for Multi-Lift Rotorcraft Systems

Section 1: Introduction 
Description 1: Provide an overview of the research topic, including its significance and the historical context of multi-lift rotorcraft systems.

Section 2: Plant Modeling
Description 2: Discuss the dynamics modeling of Multi-Lift Rotorcraft (MLR) systems, including common assumptions and methodologies used in this research field.

Section 3: Coordinated Control
Description 3: Describe various coordinated control strategies utilized in MLR systems, both linear and nonlinear, and highlight their effectiveness and challenges.

Section 4: Experimental Studies
Description 4: Summarize the experimental studies conducted on MLR systems, highlighting key findings and methodologies used in these experiments.

Section 5: Major Problems and Prospect
Description 5: Outline the main challenges and potential future directions in research for MLR systems, focusing on dynamic modeling, system configuration, and control strategies.

Section 6: Conclusions
Description 6: Conclude the paper by summarizing the main findings and emphasizing the remaining challenges and future opportunities in the field of MLR systems.