# Aerial Physical Interaction in Grabbing Conditions with Lightweight and Compliant Dual Arms
## 1 Introduction  
The reliability in the realization of an aerial manipulation task on flight strongly depends on the positioning accuracy of the aerial robot, which mainly depends on the accuracy of the position sensors, the performance of the multirotor controller, and the effect of endogenous/exogenous forces raised during the execution of the operation. On the one hand, it is desirable that the accuracy in the position estimation of the aerial vehicle is below the 10% of the reach of the manipulator [1], being capable of compensating the undesired deviations while the multirotor hovers within the workspace. Different positioning systems have been employed in the literature. Motion capture systems such as Vicon or Opti-Track have been extensively used in indoor testbeds [2; 3; 4; 5; 6] due to their high accuracy (<1 cm) and high update rates (100-200 Hz), as well as because no additional devices have to be integrated in the aerial platform, but only the passive markers. Similarly, the laser tracking systems used in [7] only require the addition of a reflective marker or a prism to the multirotor, although this solution imposes that the marker is not occluded by any obstacle in the line of the laser. Several on-board perception systems have been developed for multirotor platforms, including optical flow [8], stereo vision [9], live 3D dense reconstruction [10], or laser scanners [11]. However, these solutions reduce the payload capacity of the aerial platform, as additional devices such as cameras and on-board computers have to be added and complicate the system integration. Not only that, but each of these technologies presents certain limitations relative to the operation range, accuracy, reliability, or the update rate. The docking system described in [12] is an alternative solution that exploits the proximity of the aerial platform to the workspace during the manipulation phase, using an articulated link for obtaining the relative position from the encoders of the joints. An aerial manipulation robot operating on flight will be affected by three types of perturbations: the reaction wrenches induced over the multirotor platform due to the motion of the arms [13; 14], the contact forces associated to the physical interactions on flight [15; 16; 17; 4; 6], as well as the aerodynamic effects [7]. As consequence, the realization of certain manipulation tasks requiring the correct positioning of the end effector, such as object grasping [2; 5], valve turning [3], or inspection by contact [7; 18; 15], may be compromised or become unfeasible. In order to overcome these problems, several methods and strategies have been proposed, such as the estimation and control of the external wrenches acting over the aerial platform [19; 20], the development of multilayer control architectures [13], or the design of lightweight and compliant robotic arms [21; 22]. In this sense, it is desirable to improve the accommodation of the aerial platform to the position deviations when it is operating in contact with the environment, exploiting for this purpose the mechanical compliance of the arms [22]. The main contribution of this paper is the design, modelling, and validation of a lightweight and compliant dual arm system that allows for the estimation and control of the position of an aerial robotic manipulator relative to a fixed grabbing point, using one of the arms for grabbing and as position sensor (grabbing arm), while the other is intended to conduct the operation while flying (operation arm). Figure 1 illustrates the application of the dual arm aerial manipulator for the installation of clip-type bird diverters on a power line [23]. Two methods are proposed and evaluated. Firstly, a zero-torque controller was implemented in the grabbing arm, and thus the reaction wrenches induced over the multirotor were relatively low, using the position estimation obtained from the joint servos to control the deviations in the position of the platform with respect to the reference pose.