# Aerial Tele-Manipulation with Passive Tool via Parallel Position/Force Control +
### Related Works and Contribution  
There has been a growing interest in aerial robots with physical interaction in the past few years, and many research projects such as [12] have focused on this context. In the following, we try to concisely provide a general overview of the state of the art of this broad topic, focusing then on the works more closely related to the one presented in this paper. Aerial physical interaction with the environment can be macro-categorized as:  
1. _using active manipulators_; and
2. _using passive tools_;  
and naturally, different mechanical solutions demand different controllers. To mention some examples belonging to the first group, in [13] the authors designed and installed a small parallel manipulator on one side of a VTOL, while the use of one and two serial manipulators to grasp objects was proposed in [14] and in [15], respectively. In these works, the authors implemented and validated different instances of hybrid force control. A visual servoing approach to control a quadrotor equipped with a serial manipulator is suggested by [16], while a passivity-based adaptive controller, which can be applied  
Figure 1: A conceptual example of aerial tele-manipulation with a passive tool: an aerial robot equipped with a lightweight passive tool tasked to press an emergency shut down push button. Figure 2: The passive lightweight tool with compliant end-effector used to convert a normal VTOL UAV to an aerial manipulator. to both position and velocity control to guide a quadrotor aerial manipulator, is utilized by [17]. Furthermore, the behavioral control of an aerial manipulator is presented in [18]. In the multi-robot scenario, a team of quadrotors, equipped with serial manipulators, controlled by a visual servoing technique, is demonstrated in [19]. Differently from these works, we tackle the scenario of aerial tele-manipulation with haptic feedback of an object of interest via parallel/force control, using a passive tool. In the second group, which encompasses the research presented in this paper, different works have focused on solutions to applications requiring less dexterous manipulation capabilities, but with the benefits of being more cost-effective, versatile, and lightweight, thus allowing for longer operational flight time. In [20], a quadrotor UAV equipped with a rigid tool, controlled based on a mapping between the desired vehicle attitude and the commanded force, is used to establish contacts with the environment. The control strategy therein is designed as a variation of near-hovering control, and does not take into account the friction cone constraints that allow maintenance of the contact between the robot tooltip and the environment, while instead our strategy does. The authors of [21] propose instead an interesting combination of both mechanical design and control strategy to handle collisions and interaction in a more compliant way, without focusing on direct force control. A hybrid position/force control framework for a quadrotor is presented in [22], which allows the exertion of forces with the quadrotor airframe, without any tool, on the environment. A similar control approach is also adopted in [23] for the very relevant task of tool operation with quadrotors. Planning and control for an aerial robot in contact with its environment, based again on a hybrid position/force switching controller, is presented in [24], where obstacle avoidance is also performed. The control schemes of [22; 23; 24] are based on the decoupling of axes of the applied force and motion; that is, force is applied in the motion constrained axes while on the other axes the motion is controlled. Differently from [22; 23; 24] and other similar approaches, in this work, we do consider friction constraints in a compliant uni-lateral contact to avoid slipping of the tool in the non-constrained axes of motion and to maintain the contact, which allows us to generate 3D force vectors.