Aerial Physical Interaction in Grabbing Conditions with Lightweight and Compliant Dual Arms

Abstract

This paper considers the problem of performing bimanual aerial manipulation tasks in grabbing conditions, with one of the arms grabbed to a fixed point (grabbing arm) while the other conducts the task (operation arm). The goal was to evaluate the positioning accuracy of the aerial platform and the end effector when the grabbing arm is used as position sensor, as well as to analyze the behavior of the robot during the aerial physical interaction on flight. The paper proposed a control scheme that exploits the information provided by the joint sensors of the grabbing arm for estimating the relative position of the aerial platform w.r.t. (with respect to) the grabbing point. A deflection-based Cartesian impedance control was designed for the compliant arm, allowing the generation of forces that help the aerial platform to maintain the reference position when it is disturbed due to external forces. The proposed methods were validated in an indoor testbed with a lightweight and compliant dual arm aerial manipulation robot.

Cartesian Aerial Manipulator with Compliant Arm

“…Positioning accuracy is one of the most relevant and challenging requirements for an aerial manipulator operating on flight since it determines the reliability and success rate in the realization of tasks like grasping [7] or the installation of devices on power lines [1,26], especially outdoors where Vicon or OptiTrack systems are not available, whereas sensors like 3D laser scanners [28] and vision-based systems [29] are more suited. This feature is directly related to the performance of the multirotor position controller [30,31], affected by the endogenous and exogenous wrenches exerted through the manipulator [25,27].…”

“…The compliant arm depicted in Figure 5 employs a Herkulex DRS-0101 servo and a spring-lever transmission mechanism (1.4 Nm/rad stiffness) that allows the estimation and control of the torque from the deflection measurement [19], integrating an AS5048 magnetic encoder to obtain the deflection angle at 500 Hz with 0.2 • accuracy. The estimator described in Section 4.2 employs this signal for collision detection and controls the torque/force/impedance [26] exerted at the end effector. The link of the arm consists of a NS-01-17-200 linear guide and a NW-02-17 carriage that supports the displacement of the end effector (±30 mm range), using a pair of extension springs (3.2 N/mm stiffness) and a linear potentiometer to estimate the pushing/pulling forces in the XZ-axes in grabbing conditions, as will be explained in Section 4.2.…”

“…Compliance, either mechanical [7,[19][20][21][22][23] or at control level [11,[24][25][26], is a highly desirable feature for an aerial manipulation robot operating in contact with the environment since the stability of the aerial platform may be compromised due to the interaction wrenches exerted on flight [24,25,27]. The design and development of compliant manipulators aim to increase safety by exploiting the energy storage capacity and passivity properties of springs [7,19,26] and elastomers [23] while protecting the aerial robot against impacts and overloads [23]. Unlike most industrial manipulators that use expensive forcetorque sensors in the joints or in the end effector, the servo actuators (Herkulex, Dynamixel) typically employed for building lightweight robotic arms for aerial manipulation do not provide a direct measurement of the torque; or this is based on the current, so its accuracy is low due to the friction of the gearbox.…”

“…Unlike most industrial manipulators that use expensive forcetorque sensors in the joints or in the end effector, the servo actuators (Herkulex, Dynamixel) typically employed for building lightweight robotic arms for aerial manipulation do not provide a direct measurement of the torque; or this is based on the current, so its accuracy is low due to the friction of the gearbox. This has motivated the development of methods for estimating and controlling the forces and torques from the measurement of the deflection in the elastic element, either at joint level [7,19] or in the Cartesian space [26]. Despite the evident benefits of mechanical compliance during the physical interactions of the aerial robot while flying, the main drawback of these mechanisms is the reduction in position accuracy due to the deflection of the joints [19] or links [2], a consequence of the second order dynamics associated with the mass-spring-damper system [26].…”

“…This has motivated the development of methods for estimating and controlling the forces and torques from the measurement of the deflection in the elastic element, either at joint level [7,19] or in the Cartesian space [26]. Despite the evident benefits of mechanical compliance during the physical interactions of the aerial robot while flying, the main drawback of these mechanisms is the reduction in position accuracy due to the deflection of the joints [19] or links [2], a consequence of the second order dynamics associated with the mass-spring-damper system [26]. As a result, the realization of tasks requiring accurate positioning tends to become more difficult.…”