Cartesian Aerial Manipulator with Compliant Arm

applied sciences Article Cartesian Aerial Manipulator with Compliant Arm Alejandro Suarez * , Manuel Perez, Guillermo Heredia and Anibal Ollero   Citation: Suarez, A.; Perez, M.; Heredia, G.; Ollero, A. Cartesian Aerial Manipulator with Compliant Arm. Appl. Sci. 2021,11, 1001. https://doi.org/10.3390/app11031001 Received: 28 December 2020 Accepted: 18 January 2021 Published: 22 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). GRVC Robotics Labs, University of Seville, 41092 Sevilla, Spain; [email protected] (M.P.); [email protected] (G.H.); [email protected] (A.O.) *Correspondence: asuar[email protected] Abstract: This paper presents an aerial manipulation robot consisting of a hexa-rotor equipped with a 2-DOF (degree of freedom) Cartesian base (XY–axes) that supports a 1-DOF compliant joint arm that integrates a gripper and an elastic linear force sensor. The proposed kinematic configuration improves the positioning accuracy of the end effector with respect to robotic arms with revolute joints, where each coordinate of the Cartesian position depends on all the joint angles. The Cartesian base reduces the inertia of the manipulator and the energy consumption since it does not need to lift its own weight. Consequently, the required torque is lower and, thus, the weight of the actuators. The linear and angular deflection sensors of the arm allow the estimation, monitoring and control of the interaction wrenches exerted in two axes (XZ) at the end effector. The kinematic and dynamic models are derived and compared with respect to a revolute-joint arm, proposing a force-position control scheme for the aerial robot. A battery counterweight mechanism is also incorporated in the X–axis linear guide to partially compensate for the motion of the manipulator. Experimental results indoors and outdoors show the performance of the robot, including object grasping and retrieval, contact force control, and force monitoring in grabbing situations. Keywords: aerial manipulation; Cartesian manipulator; compliance; hexa-rotor 1. Introduction 1.1. Aerial Manipulation Aerial manipulation robots (drones equipped with robotic arms) are extending the potential applications of multirotor platforms, allowing the realization of tasks involving the manipulation of objects and tools in flight, as well as interaction with the environment with a certain level of dexterity and control capability. The development of this technology is motivated by interest in reducing the time, cost and risk for human workers associated with the realization of certain tasks in high altitude or difficult access workspaces such as power lines [ 1 , 2 ], chemical plants [ 3 ], oil and gas refineries [ 4 ], and other infrastructures [5,6] . Recent works in this field have demonstrated the possibility to conduct operations such as object grasping [ 7 – 9 ], valve turning [ 10 ], sensor installation and retrieval [ 2 , 11 ], contactbased inspection [ 3 , 12 , 13 ], insulation of cracks and leaks [ 14 ], or the realization of other tasks with grippers and other tools [ 15 , 16 ]. Several prototypes and morphologies of manipulators have been specifically developed for their integration in multi-rotors, including multi-joint arms [ 17 , 18 ], dual arm systems [ 7 , 10 ], linear actuators [ 11 ], delta manipulators [ 14 ], compliant joint arms [ 2 , 19 , 20 ], long reach aerial manipulators [ 2 , 21 ], or three-arm manipulators used for object grasping and as reconfigurable landing gear [22]. Compliance, either mechanical [ 7 , 19 – 23 ] or at control level [ 11 , 24 – 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 Appl. Sci. 2021,11, 1001. https://doi.org/10.3390/app11031001 https://www.mdpi.com/journal/applsci Appl. Sci. 2021,11, 1001 2 of 20 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. 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. 1.2. Positioning Accuracy 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 ]. On the one hand, tilted-rotor hexa-rotors, also known as fully actuated aerial platforms [32,33] , allow the translation of the platform as well as the application of wrenches [ 34 , 35 ] without changing the orientation, which contributes to increasing the accuracy in the realization of manipulation tasks compared to coplanar multi-rotors. On the other hand, the error propagation at the end effector is closely related with the kinematic configuration and the physical construction of the manipulator. This is, in most cases, a multi-link arm [7,8,17,18,24] with revolute joints. However, this configuration present two main drawbacks: (1) the joint limits (especially in the elbow joint) constraint or reduce the effective reach and workspace of the arm, (2) a significant amount of torque and energy is needed to lift the weight of the manipulator itself (forearm-upper arm configuration), requiring high torque actuators that tend to increase the weight. Alternatively, other works propose the use of simple manipulators [ 3 , 5 , 11 , 14 , 15 ] in order to reduce the weight, at expenses of reducing the reach and dexterity. 1.3. Contribution of This Work The main contribution of this paper is the design, development and experimental validation of a new morphology of aerial manipulation robot consisting of a hexa-rotor platform integrating a 2-DOF (degree of freedom) Cartesian base (XY–axes) that carries a 1-DOF compliant joint arm equipped with a gripper and a deflection-based force-torque sensor. Figure 1shows a picture of the prototype. The proposed morphology is intended to improve the positioning accuracy of the end effector while reducing the inertia, torque and weight of the actuators, since the Cartesian base supports most of the weight of the robotic manipulator (0.85 kg weight, 1 m reach, and 10 cm/s speed). The elastic deflection sensors integrated in the compliant joint and in the fore-arm link are used to estimate and monitor the force (XZ–axes) and torque (pitch) exerted by the manipulator over the aerial platform. This allows the realization of a number of tasks such as sensor installation with contact force control, force monitoring in grabbing situations, or object grasping and retrieval. The kinematic and dynamic models of the aerial manipulator are derived, while analyzing in more detail the error propagation at the end effector from the Jacobian, comparing the proposed morphology with respect to that of a revolute-joint arm in terms of positioning accuracy, reach, and workspace. This paper also proposes a force-position control scheme for the aerial robot that takes into account the wrench estimation provided by the arm. Appl. Sci. 2021,11, 1001 3 of 20 Experimental results conducted in test-bench as well as indoor/outdoor flight tests validate the functionalities and capabilities of the developed Cartesian aerial robot. Figure 1. Cartesian manipulator (left) integrated in S550 hexarotor platform (right). The rest of the paper is organized as follows. Section 2presents the motivation for and describes the prototype of a Cartesian aerial manipulator with compliant joint arm. Section 3 presents the kinematic and dynamic models of the aerial robot, analyzing and comparing the features of the Cartesian manipulator with respect to a revolute-joint arm. Section 4covers the control of the aerial robot, the experimental results are shown in Section 5, and the conclusions in Section 6. 2. System Description 2.1. Motivation The Cartesian manipulator aims to avoid three inconveniences of the human-like robotic arms developed in our previous work [ 7 , 19 ]. First of all, several manipulation tasks like grasping or peg-in-hole are defined in the Cartesian space rather than in the joint space, so it is more convenient that the mapping between both spaces is straightforward in order to improve the positioning accuracy. According to the forward kinematic model, the XYZ position of the end effector depends simultaneously on the rotation angle of the three [ 19 ] or four [ 7 ] joints of the manipulator (shoulder and elbow). Therefore, the joint position errors, as well as errors associated with the manufacturing and assembly of the arm (clearance, misalignment), will be propagated to the three Cartesian coordinates of the end effector. Additionally, the mechanical deflection of the joints in a compliant arm [ 7 ] may cause deviations of several centimeters in the position of the end effector with respect to an equivalent stiff-joint arm. In a Cartesian manipulator, instead, the position of the end effector on each axis is controlled with its own actuator independently from the other joints, so the position errors are decoupled and the trajectory control in the task space is simplified. Secondly, the effective reach of a human-like manipulator (~50 cm reach) is in practice relatively small, around 30 cm, taking into account that the nominal operation position of the arm is typically an L-shaped configuration (forearm lifted 90 ◦ ) and due to the limit in the rotation angle of the elbow joint. Note that humans extend the reach of the hands with the torso and legs, whereas in an aerial manipulator the motion of the arms is usually constrained by the landing gear and the perimeter of the propellers. In this sense, the Cartesian base avoids dead-zones in its workspace due to joint limits, allowing access to points close to the multirotor base, which can be useful, for example, in object retrieval and Appl. Sci. 2021,11, 1001 4 of 20 storage. Finally, a two link manipulator in the upper arm-forearm morphology typically requires a significant amount of torque and energy to lift its own weight, whereas in the developed prototype the Cartesian manipulator only has to compensate for the inertia and friction of the carriage, reducing in this way the required torque of the actuators, and thus, their weight. Not only that, but the linear guide also isolates the actuators from impacts and loads, avoiding the design of bearing-based protection mechanisms, as in [7]. The Cartesian manipulator shown in Figure 1implements a Prismatic-Prismatic- Revolute (PPR) configuration, introducing a revolute joint attached at the Cartesian base, since the landing gear and central body of the hexa-rotor platform do not allow the placement of a linear actuator in the vertical axis, and because this configuration extends the reach of the end effector in the X–axis (forward direction). No additional degrees of freedom are considered for end effector positioning due to the convenience of reducing the weight, complexity and size of the manipulator, and because the capabilities of the robot can be extended with the additional degrees of freedom provided by the aerial platform [13,32]. 2.2. Mechanical Design The Cartesian aerial manipulator shown in Figure 1consists of two systems. The aerial platform is built from a Banggood S550 hexa-rotor (similar to the DJI F550) that integrates the DJI 2312E brushless motors with 9 × 4.5 inch propellers, and DJI 430 LITE electronic speed controllers (ESC). The onboard controller comprises a Pixhawk autopilot and a Raspberry Pi model 3B+ that implements the software architecture described in the next subsection. The Cartesian manipulator is divided into the five parts identified in Figure 2: X–axis linear guide (base), Y–axis linear guide, servo actuator, compliant arm and battery counterweight mechanism. The mechanical specifications of the manipulator are indicated in Table 1, representing in Table 2the mass and inertia parameters obtained from the 3D model. Figure 2. Components of the Cartesian manipulator, indicating the center of mass of each structure (XYZ axes). Appl. Sci. 2021,11, 1001 5 of 20 Table 1. Specifications of the Cartesian manipulator and the compliant joint arm. Cartesian Manipulator Compliant Joint Arm Actuator features Stall torque: 0.3 Nm Mass: 10 g, 120 RPM Actuator features Stall torque: 1.17 Nm Stiffness: 1.4 Nm/rad Linear range X–axis: 500 mm Rotation range ±120◦ Y–axis: 250 mm Joint deflection 30◦ Max. speed 100 mm/s Link length 250 mm Total mass 0.85 kg (including battery–2S LiPo, 1500 mAh) Table 2. Mass and inertia parameters of the hexa-rotor platform and the different structures of the Cartesian manipulator relative to the center of mass. The cross inertial terms (I xy , I xz , I yz ) are omitted since their value is negligible. Structure Mass [kg] Ixx [kg·m2] Iyy [kg·m2] Izz [kg·m2] Hexa-rotor 1.27 1.9 ×10−21.8 ×10−23.0 ×10−2 X–axis 0.2 7.9 ×10−56.8 ×10−36.8 ×10−3 Y–axis 0.14 1.0 ×10−32.1 ×10−51.0 ×10−3 Servo 0.08 3.4 ×10−53.0 ×10−51.0 ×10−5 Link 0.09 6.0 ×10−46.0 ×10−41.1 ×10−5 The 2-DOF Cartesian base is built from the igus ® NS-01-17-600 (X–axis) and NS- 01-17-300 (Y–axis) linear guides, using three NW-02-17 carriages to support the mobile parts. Two 22 mm Ø timing pulleys are placed at the tip of both linear guides to transmit the motion of the Pololu micro metal gear motors (10 g weight, 250:1 reduction ratio, 2.7 kg·cm stall torque) to the timing belts, measuring the rotation angle with a Murata SV01A potentiometer and with the magnetic encoder of the micro-motors. The linear displacement of the Cartesian base relative to an initial position is obtained from the rotation angle and the number of turns given by the encoder (6 pulses per revolution at the micro motor shaft, 1500 pulses per turn at the output shaft of the gearbox). Figure 3 illustrates the timing pulley-belt actuator in the X and Y linear guides. The battery counterweight is supported by a C-shaped aluminum frame that can slide along the X–axis linear guide and move in synchronization with the Y–axis structure, but in the opposite direction when it is attached to the X–axis timing belt (see Section 5.2). The integration of the Cartesian manipulator in the hexa-rotor platform and the battery counter-weight mechanism are shown in Figure 4. The X–axis linear guide is supported by the carbon fiber tubes attached to the multirotor base through the two U-shaped aluminum frames identified in Figure 2. As can be seen in the front view of Figure 4, the legs of the landing gear should be separated to avoid collision of the Y–axis linear guide. However, this additional separation is not needed if the battery counter-weight mechanism is enabled. As depicted in Figure 4, this mechanism consists of a C–shaped aluminum frame that supports the 2S LiPo battery and is actuated by the X–axis timing pulley, but in the opposite direction to the Y–axis linear guide. Since both structures share the same linear guide, the effective range is reduced by half. The counterweight mechanism can be set to a fixed position by simply removing the zip tie that connects the timing belt with the linear carriage. Appl. Sci. 2021,11, 1001 6 of 20 Figure 3. Pololu micro metal gear motors employed in the XY axes’ timing pulley-belt transmission. Figure 4. Side (left) and front (right) views of the Cartesian aerial manipulator, indicating the components. The compliant arm depicted in Figure 5employs 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. Appl. Sci. 2021,11, 1001 7 of 20 Figure 5. Magnetic gripper supported by the compliant linear joint (left). Compliant joint actuator (right). The end effector of the arm can be reconfigured according to the intended task and type of object to be manipulated. This can be a simple hook to retrieve a device with handles, a magnetic gripper for catching ferromagnetic objects as hand tools, or a servo-gripper for grabbing objects of different shapes. As indicated in the previous subsection, one of the main benefits of the Cartesian morphology with respect to the human-like dual arm systems developed in our previous work [ 7 , 26 ] is that the manipulator wastes lesser energy in lifting its own weight or reaching any point in its workspace, as the displacement of the end effector along the XY–axes is supported by the linear guide system. Note that in a forearm-upper arm configuration, the shoulder actuators must lift a 50 cm length frame structure, including the servo actuators placed in the links. Thus, the proposed solution is more energy efficient in this sense. Finally, it is necessary to remark that the purpose of the Cartesian manipulator and the compliant arm is to provide a certain level of dexterity (3–DOFs) and sense capability ( Fx , Fz , τθ ) to the aerial platform, sufficient to implement a number of functionalities and tasks without unnecessarily increasing the weight with additional sensors or actuators. 2.3. Hardware Architecture The components and architecture of the Cartesian aerial manipulator are represented in Figure 6. The aerial platform consists of the S550 hexa-rotor integrating the NAVIO autopilot and a Raspberry Pi 3B+ board running the PX4 firmware [ 36 ] as well as the control program of the manipulator, developed in C/C++. This program is built around the Task Manager class [ 2 ], which implements the different functionalities used in the experiments: go to rest position, go to operation position, teleoperation, force control and monitoring, and Cartesian base calibration. The manipulator is interfaced through a USB– to–USART (Universal Synchronous-Asynchronous Receiver Transmitter) device connecting the Raspberry Pi board with the STM32F303K8 Nucleo microcontroller that takes care of the low level control of the XY–axes’ micro motors and reads the measurements from the three sensors: •The two potentiometers used to estimate the displacement of the linear base. •The magnetic encoder attached to the servo that measures the joint deflection. •The linear potentiometer in the link used to estimate the pushing/pulling force. The microcontroller generates as output the PWM (Pulse Width Modulation) signals taken as input by the DRV 8833 dual H-bridge that controls the micro motors (Section 4.1) , providing another PWM signal to control a gripper servo. The Herkulex servo of the compliant joint is controlled in position through a dedicated USART interface [ 7 , 26 ]. The control references generated by the Task Manager and the feedback measurements collected by the microcontroller are sent at 100 Hz through the USART as data packets codified as C-type data structures. Appl. Sci. 2021,11, 1001 8 of 20 Figure 6. Components and architecture of the Cartesian aerial manipulator with ground control station (GCS). The Cartesian aerial manipulator is operated from the Ground Control Station (GCS) through a wireless link, typically a Wi–Fi network, that supports the SSH (Secure Shell) session used to launch and manage remotely the aerial manipulator control programs. The commands introduced by the operator to select the particular task to be executed or to tele-operate the manipulator using a joystick are sent as C–type data packets through UDP (User Datagram Protocol) sockets. 3. Modelling 3.1. Kinematic Model In the definition of an aerial manipulation task, three reference frames can be identified: the Earth fixed frame {E} (inertial), the multirotor base frame {B} , and the manipulator base frame {0} . The position and orientation of the multirotor, ErB=[x,y,z]T and EηB= [φ,θ,ψ]T , are referred to the Earth fixed frame, whereas the position of the tool center point (TCP) of the manipulator, 0rTCP =[xTCP,yTCP,zTCP ]T , is expressed in {0} for convenience, considering that the onboard perception system usually gives the reference positions in the local axes. Figure 7depicts the reference frames, position vectors and the joint variables in the Cartesian aerial manipulator. Here d1 and d2 denote the displacement of the Cartesian base along the X0Y0 axes, respectively, q3 is the rotation angle of the joint in the pitch angle, whereas D1,D2and Lare the corresponding link lengths. Figure 7. Kinematic model of the Cartesian aerial manipulator. Appl. Sci. 2021,11, 1001 9 of 20 The position of the tool center point referred to {0} can be computed easily from the joint variables applying the forward kinematic model FK :<3→ <3: 0rTCP =  xTCP yTCP zTCP  =FK   d1 d2 q3  =  d1+L·sin(q3) d2 −L·cos(q3) (1) Note that xTCP depends on two joint variables ( d1 and q3 ), although q3 is determined by zTCP . The inverse kinematic model, IK :<3→ <3 , provides two analytical solutions for q3 if |xTCP|<D1/ 2, and only one solution if |xTCP|≥D1/ 2. The double solution is determined by the sign of q3 , and indicates the pointing direction of the link, either forwards or backwards:   d1 d2 q3 =IK   xTCP yTCP zTCP   =  xTCP ∓L·sin(α) yTCP ±α (2) where angle αis defined in the following way: α=cos−1zTCP L(3) 3.2. Positioning Accuracy and Workspace It is interesting to compare the workspace, joint limits and position errors of the proposed Cartesian manipulator with compliant joint arm with respect to other kinematic configurations, like the 3–DOF robotic arm described in [ 19 ], where the TCP position is given by: 0r∗ TCP =  r(q2,q3)·cos(q1) r(q2,q3)·sin(q1) L1cos(q2)+L2cos(q2+q3) (4) Here L1and L2are the upper arm and forearm link lengths, respectively, and: r(q2,q3)=L1sin(q2)+L2sin(q2+q3)(5) As stated in the introduction and in Section 2.1, the Cartesian base reduces the propagation of positioning errors in the TCP due to angular deviations (joint deflection) or manufacturing errors (misalignments, clearance). This can be quantitatively evaluated through the Jacobian matrix: εTCP =J(q)·εq(6) where εTCP is the position deviation of the TCP in the XYZ axes due to a variation in the vector of joint variables εq , which depends on the Jacobian of the arm particularized to the nominal pose, J(q) . Although it is omitted for space reasons, the Jacobian of the manipulator described by Equation (4) introduces coupled terms in the XYZ axes, i.e., each of the Cartesian coordinates depends on the three joint variables simultaneously, whereas in the kinematic configuration considered in this paper, the Jacobian matrix is significantly simplified, resulting in the following expression. As can be seen, the positioning accuracy in the X–axis depends on the accuracy of the X–axis actuator and on the compliant joint, whereas the accuracy on the YZ axes dep εTCP =  εY εY εZ =  1 0 L·cos(q3) 0 1 0 0 0 L·sin(q3) ·  εd1 εd2 εq3  =  εd1+L·cos(q3)·εq3 εd2 L·sin(q3)·εq3  (7) ends on the accuracy of the d2and q3coordinates. The volume of operation of the Cartesian manipulator is a parallelepiped of length D1= 0.5 [m] , depth D2= 0.2 [m] , and height L= 0.2 [m] , with two additional circular Appl. Sci. 2021,11, 1001 16 of 20 Figure 12. Multirotor position deviation when the manipulator moves in the forward direction ( d1 , q3 ). Figure 13. Variables of interest ( left ) and sequence of images ( right ) in the force monitoring experiment. Appl. Sci. 2021,11, 1001 17 of 20 Figure 14. Sequence of images from the contact force control experiment. Figure 15. Contact force control experiment. Force reference and estimation ( up, left ), servo position and PI correction terms ( up, right ), multirotor position, velocity, orientation and angular rate ( down ). Appl. Sci. 2020, 10, x 17 of 20 Figure 14. Sequence of images from the contact force control experiment. Figure 15. Contact force control experiment. Force reference and estimation (up, left), servo position and PI correction terms (up, right), multirotor position, velocity, orientation and angular rate (down). . Figure 16. Contact force control experiment. Second test. Figure 16. Contact force control experiment. Second test. Appl. Sci. 2021,11, 1001 18 of 20 6. Conclusions This paper described an aerial manipulator consisting of a multirotor equipped with a 2–DOF Cartesian base (XY–axes) and a compliant arm with angular and linear deflection sensors that allow the estimation, monitoring and control of the interaction wrenches exerted on flight, improving safety in grabbing conditions. This new morphology presents several benefits with respect to revolute joint manipulators in terms of positioning accuracy, energy consumption, and effective workspace, thanks to mechanical construction with two transversal linear guide systems that support the translation of the compliant arm. The forward kinematic model shows that the map between the joint space and the Cartesian space, in which the intended task is usually defined, is almost straightforward, so the Cartesian coordinates (XYZ) can be controlled with the corresponding actuators, reducing in this way the kinematic coupling between the joints and, with it, the propagation of errors at the end effector. The lack of dead-zones around the base of the manipulator extends the effective workspace, particularly useful for object retrieval and store operations. In this sense, the experiments presented in the paper showed how the linear deflection sensor can be used to detect overloads when the arm tries to grab an object too heavy to be lifted, moving the Cartesian base to release it and preventing the stability of the aerial platform from being compromised. The experiments also show the possibility of exerting sustained contact forces with the compliant joint arm, and the effect of the battery counterweight over positioning accuracy (at the expense of reducing the motion range in the X–axis by half). Although the mechanism is able to compensate partially for the reaction wrenches caused by the displacement of the Y–axis structure in the pitch angle, its effectiveness is reduced when the compliant arm is rotated, so this may not be practical in some cases taking into account the limitation it imposes on the range of motion of the X–axis structure. Author Contributions: Conceptualization, A.S.; validation, A.S and M.P.; writing—original draft preparation, A.S.; writing—review and editing, A.S. and G.H.; supervision, G.H. and A.O.; project administration, A.O.; funding acquisition, A.O. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the AERIAL-CORE (H2020-2019-871479) and HYFLIERS (H2020- ICT-25-2017-779411) projects funded by the European Commission; and the ARTIC (RTI2018-102224- B-I00) projects funded by the Spanish Ministerio de Economia, Industria, y Competitividad. Informed Consent Statement: Not applicable. Acknowledgments: Authors want to thank Antonio Enrique Jimenez-Cano for his support in the realization of the experiments. Conflicts of Interest: The authors declare no conflict of interest. References 1. The AERIAL-CORE Project. Available online: https://aerial-core.eu/ (accessed on 28 December 2020). 2. 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