Alejandro Suarez

Lightweight compliant arm for aerial manipulation

This paper presents the design and experimental validation of a low weight compliant arm for aerial manipulation, which uses a linear actuator to move the elbow joint. The proposed arm design allows for estimating payload mass, as well as detecting collisions against obstacles in the forearm. The compliant arm design is intended to be applied in aerial manipulation and transportation, being mounted on the base of an Unmanned Aerial Vehicle (UAV) like a multirotor or a small autonomous helicopter. The payload mass estimation can be useful in the adaptation and stabilization of UAV altitude and attitude controllers when objects of unknown weight are grasped by the arm, while compliance reduces the effect of physical interaction when lifting an object on UAV stability. The paper addresses the mechanical design and construction of a three DoF arm prototype (elbow, wrist roll and pitch), deriving the equations for estimating the payload mass. A set of experiments for payload mass estimation and collision detection and reaction have been performed to test the validity of the approach.

Cooperative sensor fault recovery in multi-UAV systems

This paper presents the design and experimental validation of a Fault Detection, Identification and Recovery (FDIR) system intended for multi-UAV applications. The system exploits the information provided by internal position, attitude and visual sensors onboard the UAVs of the fleet for detecting faults in the measurements of the position and attitude sensors of any of the member vehicles. Considering the observations provided by two or more UAVs in a cooperative way, it is possible to identify the source of the fault, but also implement a Cooperative Virtual Sensor (CVS) which provides a redundant position and velocity estimation of the faulty UAV that can be used for replacing its internal sensor. The vision-based FDIR system has been validated experimentally with quadrotors in an indoor testbed. In particular, fault detection and identification has been evaluated injecting a fault pattern offline on the position measurements, while the CVS has been applied in real time for the recovery phase.

Lightweight and human-size dual arm aerial manipulator

This paper presents the design of a dual-arm aerial manipulator consisting of a multi-rotor platform with an ultra-lightweight (1.8 Kg) human-size dual arm prototype and its control system. Each arm provides three degrees of freedom (DOF) for positioning the end- effector, and two DOF for orientation. As most model-based controllers assume that joint torque feedback is available, a torque estimator for the arms is developed. Note that low cost servos used for building low weight manipulators do not provide any torque feedback or control capability. The redundant DOFs in the dual arm prototype are exploited for generating coordinated motions during contact-less phases in such a way that reaction torques can be partially canceled. Preliminary flight tests have been conducted in outdoors, evaluating the torque compensation capability in test-bench. The influence of the reaction torques exerted by the arms over the UAV controller is also analyzed in simulation.

Lightweight compliant arm with compliant finger for aerial manipulation and inspection

This paper presents the design and experimental validation of a compliant and lightweight 3-DOF robotic arm - shoulder yaw, shoulder pitch and elbow pitch joints - equipped with a compliant finger module intended for aerial inspection and manipulation in contact with the environment. A simple transmission mechanism consisting in a pair of compression springs and a flange bearing is integrated in the shoulder pitch and elbow pitch joints between the servo shaft and the output frame. Joint deflection measurement with potentiometer allows joint torque but also contact force estimation and control. The low stiffness of the compliant finger has been exploited for soft collision detection and obstacle localization, in such a way that the contact forces do not significantly disturb the UAV. Fixed-base experiments have been performed with the arm, including the characterization of the compliant joints and the control of the contact force at wrist point.

Compliant and Lightweight Anthropomorphic Finger Module for Aerial Manipulation and Grasping

In this paper, a single DOF anthropomorphic finger module specifically designed for aerial manipulation and grasping is presented, where low weight and compliance are required features for safer performance of the aerial platform. The under-actuated mechanism consists of a high torque micro motor with a small reel for driving a tendon that moves the three joints of the finger. An elastic element maintains the finger bones tied together and extended by default, providing a compliant response against collisions with walls, the floor or any object in the environment during the grasping operation. Compliance provided by the elastic joints will also be exploited for stable object grasping and for collision detection. The development of a specific electronics for finger control instead of employing conventional servos make possible the design and application of a wider range of control strategies, including position, velocity or open-loop force control. The proposed design is validated through different grasping and control experiments.

Analysis of Perturbations in Trajectory Control Using Visual Estimation in Multiple Quadrotor Systems

This paper describes the trajectory control of a quadrotor using external position estimation obtained from visual tracking in a scenario with multiple quadrotors and a camera mounted in the base of two or more UAVs, with the images being transmitted through a radio link. Applications where visual tracking can be used include fault detection and recovery of internal sensors, formation flying and autonomous aerial refueling. The dynamic model of the quadrotor and its trajectory control scheme is described along with the model of perturbations considered for the external position estimation. Graphical and numerical results are presented in different conditions, commenting separately the effect of each identified perturbation over the trajectory control. This study is done in simulation as previous step before testing quadrotor trajectory control in real conditions due to the high risk of accidents and damages on the vehicle.

Cooperative Virtual Sensor for Fault Detection and Identification in Multi-UAV Applications

This paper considers the problem of fault detection and identification (FDI) in applications carried out by a group of unmanned aerial vehicles (UAVs) with visual cameras. In many cases, the UAVs have cameras mounted onboard for other applications, and these cameras can be used as bearing-only sensors to estimate the relative orientation of another UAV. The idea is to exploit the redundant information provided by these sensors onboard each of the UAVs to increase safety and reliability, detecting faults on UAV internal sensors that cannot be detected by the UAVs themselves. Fault detection is based on the generation of residuals which compare the expected position of a UAV, considered as target, with the measurements taken by one or more UAVs acting as observers that are tracking the target UAV with their cameras. Depending on the available number of observers and the way they are used, a set of strategies and policies for fault detection are defined. When the target UAV is being visually tracked by two or more observers, it is possible to obtain an estimation of its 3D position that could replace damaged sensors. Accuracy and reliability of this vision-based cooperative virtual sensor (CVS) have been evaluated experimentally in a multivehicle indoor testbed with quadrotors, injecting faults on data to validate the proposed fault detection methods.

Vision-Based Deflection Estimation in an Anthropomorphic, Compliant and Lightweight Dual Arm

This paper proposes the application of a stereo vision system for estimating and controlling the Cartesian and joint deflection in an anthropomorphic, compliant and ultra-lightweight dual arm designed for aerial manipulation. Each arm provides four degrees of freedom (DOF) for end-effector positioning in a human-like kinematic configuration. A simple and compact spring-lever mechanism introduced in all joints provides mechanical compliance to the arms. A color marker attached at the end effector of the arms is visually tracked by a stereo pair installed over the shoulders. The Cartesian position and velocity of the markers is estimated with an Extended Kalman Filter (EKF), while the corresponding points in an equivalent stiff-joint manipulator are obtained from the kinematic model and the position of the servos. The Cartesian deflection is defined as the difference between these two measurements, obtaining the joint deflection from the inverse kinematics. The vision-based deflection estimator is validated in test bench experiments: position estimation accuracy, impact response, passive/active compliance and contact force control.

Design of a High Performance Dual Arm Aerial Manipulator

This paper presents the design of a dual arm aerial manipulation robot consisting of a customized hexarotor platform equipped with a lightweight dual arm manipulator. The proposed design is intended to integrate multiple devices required for building a complete aerial manipulation system, including vision and range sensors, on-board computers, communication devices, navigation systems, along with the manipulator. The developed platform will provide optimum performance in terms of flight time and payload taking into account the current technology available for building these kind of aircrafts. The design of the platform also considers vibration isolation, control and stability, and extended workspace for the manipulator. A lightweight (1.8 kg) and human-size dual arm manipulator has been integrated in the developed platform. Each arm provides 5 degrees of freedom (DOF) for end effector positioning and orientation. The arms are built using smart servo actuators and a frame structure manufactured in anodized aluminum. The design is validated through rigidity and modal analysis using finite element methods (FEM). The developed platform has been tested in outdoor flights, evaluating the influence of arms motion over the stability of the platform.