Hierarchical Whole-Body Control of the Cable-Suspended Aerial Manipulator Endowed with Winch-Based Actuation

During operation, aerial manipulation systems are affected by various disturbances. Among them is a gravitational torque caused by the weight of the robotic arm. Common propeller-based actuation is ineffective against such disturbances because of possible overheating and high power consumption. To overcome this issue, in this paper we propose a winch- based actuation for the crane-stationed cable-suspended aerial manipulator. Three winch-controlled suspension rigging cables produce a desired cable tension distribution to generate a wrench that reduces the effect of gravitational torque. In order to coordinate the robotic arm and the winch-based actuation, a model-based hierarchical whole-body controller is adapted. It resolves two tasks: keeping the robotic arm end-effector at the desired pose and shifting the system center of mass in the location with zero gravitational torque. The performance of the introduced actuation system as well as control strategy is validated through experimental studies. I. I N T RO D U C T I O N In the past decade, the aerial manipulation (AM) field has attracted intense interest among researchers due to numerous prospective industrial applications [1]–[3]. In general, the aerial manipulator can be defined as the aerial base with attached manipulation device, e.g., robotic arm. Due to the physics of the coupled system, various disturbances affect the aerial manipulator during operation. Rapidly evolving perturbations, e.g., caused by wind, can be efficiently handled by use of high bandwidth actuation, namely, propeller propulsion. However, quasistatic disturbances such as internal displacement of the system center of mass (COM) due to robotic arm weight cannot be compensated in the same manner because of risk of actuation overheating and high power consumption [4], [5]. Nevertheless, this type of perturbation significantly complicates the interaction tasks by affecting AM performance and onboard sensor (IMU, camera) measurements. As a solution, researchers investigated the direct weight sliding at the aerial base, e.g., the motion of additional masses/battery [6]–[9]. In pursuit of increased performance and safety in the AM field, cable-suspended aerial manipulators have been recently 1The authors are with Institute of Robotics and Mechatronics, German Aerospace Center (DLR), Wessling, Germany. 2The author is with Robotics and Mechatronics Group, University of Twente, Enschede, The Netherlands. 3The author is with Dextrous Robotics Inc., Memphis, United States. 4The author is with Instituto Tecnol ́ogico de Aeron ́autica, S ̃ao Jos ́e dos Campos, Brazil. 5The author is with Intelligent Robotic Systems Lab, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea. 6The author is with Automation and Control Institute, TU Wien, Vienna, Austria. e-mail: ysarkisov90@gmail.com Fig. 1: Cable-suspended aerial manipulator SAM. proposed [10]–[12]. One example is the crane-stationed cable- Suspended Aerial Manipulator SAM developed in DLR [13], see Fig. 1. External crane suspension allows the platform to be compact and to utilize new approaches in order to deal with the displacement of COM. In the scope of this paper, we introduce a novel winch- based actuation integrated to the SAM and inspired by the cable-driven robotics [14]–[16]. By regulating the length of three suspension rigging cables connecting the SAM with the crane’s hook, the winch-based actuation compensates for the gravitational torque generated by robotic arm weight. In order to design a controller for this kinematically-redundant system, a number of challenges should be resolved. First of all, a closed-chain winch cabling in pair with attached robotic arm is modeled by Lagrangian constrained dynamics and mapped to the equal serial-chain coordinates describing the translational motion of the SAM base [17]–[19]. Secondly, the complex dynamics is simplified in order to find a reasonable trade-off between the system behavior description and equations that can be efficiently utilized in the controller. Further, a hierarchical whole-body controller with integrated admittance interface is adapted to the resulted kinematically redundant system extending our previous contribution [20] by adding capability of aerial base’s translation motion in the null space. The main aim of the controller is to perform two objectives with different priorities defined through the null-space: regulation of the robotic arm toward desired configuration and shifting system COM to the location with zero torque due to gravity. Both tasks are regulated under impedance-based control. Benefits and performance investigation of the introduced winch-based actuation under the designed control strategy are studied through experiments. 2023 IEEE International Conference on Robotics and Automation (ICRA 2023) May 29 - June 2, 2023. London, UK 979-8-3503-2365-8/23/$31.00 ©2023 IEEE 5366