An Experimental Characterization of Torveastro, Cable-Driven Astronaut Robot

Article An Experimental Characterization of TORVEastro, Cable-Driven Astronaut Robot

Francesco Samani * and Marco Ceccarelli *

LARM2—Laboratory of Robot Mechatronics, University of Rome Tor Vergata, 00133 Rome, Italy * Correspondence: [email protected] (F.S.); [email protected] (M.C.)

Abstract: TORVEastro robot design is presented with a built prototype in LARM2 (Laboratory of Robot Mechatronics) for testing and characterizing its functionality for service in space stations. Several robot astronauts are designed with bulky human-like structures that cannot be convenient for outdoor space service in monitoring and maintenance of the external structures of orbital stations. The design features of TORVEastro robot are discussed with its peculiar mechanical design with 3 arm-legs as agile service robot astronaut. A lab prototype is used to test the operation performance and the feasibility of its peculiar design. The robot weighs 1 kg, and consists of a central torso, three identical three-degree of freedom (DoF) arm–legs and one vision system. Test results are reported to discuss the operation efﬁciency in terms of motion characteristics and power consumption during lab experiments that nevertheless show the feasibility of the robot for outdoor space applications.

Keywords: service robots; space robotics; experimental robotics; design; testing; astronaut robots

1. Introduction In space, instead of astronauts, it is advisable to use service robots instead of humans and, therefore, such space service robots are increasingly playing a strategic role in in-orbit assistance operations [1]. For more than twenty years, great attention has been paid to Citation: Samani, F.; Ceccarelli, M. service robots, in order to develop new robotic systems for applications as pointed out An Experimental Characterization of for example in [2]. Typical service robots are already developed for medical care, space TORVEastro, Cable-Driven Astronaut exploration, demining operation, surveillance, entertainment, museum guide. Another Robotics 2021 10 Robot. , , 21. https:// future target is related to the increasing exploration of space because there is a growth of doi.org/10.3390/robotics10010021 space debris and defunct spacecraft left in space which will affect the current and future space missions [3]. To avoid the costly and risky tasks manually handled by humans, space Received: 13 November 2020 service robots can automatically perform tasks and services of catching useless debris in Accepted: 21 January 2021 Published: 22 January 2021 space [4]. In some cases, robot designs have come available on the market and considerable literature about service robot is published not only on technical problems [5].

Publisher’s Note: MDPI stays neutral According to the International Federation of Robotics (IFR), “a service robot is a robot, with regard to jurisdictional claims in which operates semi or fully autonomously to perform services useful to the wellbeing of published maps and institutional afﬁl- human and equipment, excluding manufacturing operations” [6]. According to ISO [7], iations. service robots require “a degree of autonomy”, which is the “ability to perform intended tasks that are based on current state and sensing, without human intervention”. Service robots can have a partial autonomy that may include an interaction with the other robots or with human beings. There are also service robots that have full autonomy without those interactions. Service space robotics is considered to be one of the most promising Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. domains for solutions On-Orbit Servicing (OOS) missions such as docking, mooring, refu- This article is an open access article eling, repairing, upgrading, transporting, rescuing up to the removal of orbital debris [8]. distributed under the terms and Characteristics of a space environment are low air pressure, large thermal excursions, high conditions of the Creative Commons solar radiation and microgravity. With these aspects in mind, human beings in space need Attribution (CC BY) license (https:// a system helping in the activities. Consequently, it is important to implement a system creativecommons.org/licenses/by/ capable of controlling the outdoor space environment and a rescue system [9]. The Interna- 4.0/). tional Space Station (ISS) is a space station [10], as an artiﬁcial habitable satellite, in low

Robotics 2021, 10, 21. https://doi.org/10.3390/robotics10010021 https://www.mdpi.com/journal/robotics Robotics 2021, 10, 21 2 of 19

Earth orbits. The ISS is the most complex international scientific and engineering project in the history of humanity and the largest structure ever placed in space [11]. The first ISS component was launched into orbit in 1998 and the first long-term residents arrived in October 2000 [12]. The IIS is a laboratory that deals with research on microgravity and the space environment. In this laboratory, crew members have the opportunity to conduct experiments in biology, physics, astronomy, meteorology and other fields [13]. The IIS has handrails positioned on the outer surface to allow astronauts to move outside. It is very difficult and expensive to transport large or high-mass objects from Earth to a space service station [14]. In this regard, the design of the space robot must have the characteristics to be able to carry out operations in the space station but, at the same time, its mass and volume must be considered with the utmost caution [15]. Space robots play an increasingly important strategic role in assisting human beings in orbit, as service robots as pointed out for example in [16]. In April 1993, in one of the first events in the history of spacecraft, a small-sized multisensory robot was able to perform different tasks inside a spacecraft with different operating modes as reported in [17]. These activities are still carried out today, as operations that are remotely controlled or programmed by astronauts using a stereo video monitor. The robot was remotely controlled by a human operator and by computers using artificial intelligence. In these operating conditions, the robot was able to open and close the connectors (bayonet lock), it was able to assembly structures of individual parts, and it was able to capture floating objects [18]. To date, an astronaut robot in operation is Robonaut from NASA, the National Aero- nautics and Space Administration as a humanoid robot. This robot was designed and developed by the Robotics Laboratory at NASA’s Lyndon B. Johnson Space Center (JSC) in Houston, Texas [19]. Robonaut is able to work with astronauts and can use tools in Space in environments suitable for astronauts. The latest version of Robonaut features a robotic torso designed to assist the crew in EVA (Extra-Vehicular Activity missions) and is able to contain the tools that can be used by the crew. This robot has the ability to grasp and operate a large range of tools. With regard to the evaluation of the grasp quality measurements have been studied in relation on wrench spaces considering the non-uniformity of the wrench space in relation of the dimensions of force and torque [20]. In any case, Robonaut 2 does not have adequate protection to resist outside the space station and improvements are needed in order to allow it to move outside around the station. At NASA/JSC, the EVA Robotic Assistant (ERA) project developed a test-bed for robots in order to explore and understand the problems of astronaut–robot interactions. Together with JSC’s Advanced Spacesuit Lab, the ERA team investigated robotic capabilities and tested them with space-functional test subjects in planetary surface-like situations [21]. Another astronaut service robot is Rollin ‘Justin built by the ESA (European Space Agency) [22]. The robot’s hands consist of four fingers. The mobile base allows the robot to operate autonomously over a long range. This space robot has a system consisting of a light arm. Unstructured, variable and dynamic environments require the space robots to act independently and without human support. Its multiple degrees of freedom allow Rollin ‘Justin to pursue multiple objectives at the same time, while respecting a hierarchy of tasks. Experiments has been followed to perform intelligent robotic coworkers, in particular, Rollin ‘Justin was used as controlled by the astronauts. The system has been tested by AAAI (Association for the Advancement of Artificial Intelligence), which controls Rollin ‘Justin based in Munich, Germany, from Honolulu, Hawaii [23]. Another robot of particular importance in the field of space robotics is Kirobo, the first Japanese astronaut robot that is part of the JAXA (Japan Aerospace eXploration Agency) [24]. Kirobo was developed by the University of Tokyo in collaboration with the International Space Station teams. Kirobo arrived on the ISS on 10 August 2013 on JAXA Kounotori 4’s H-II transfer vehicle. Kirobo has a height of 330.2 mm, a width of 177.8 mm and a depth of approximately 152 mm. The mass is about 0.9 kg, is able to speak Japanese. The robot’s capabilities also include speech recognition, natural language processing, speech synthesis and telecommunications. The implementation of facial recognition of video recording was used in Kirobo [25]. Robotics 2021, 10, 21 3 of 19

The European Space Agency (ESA) and the Italian Space Agency (ASI) have reached an agreement to develop a space robotic system called Jerico, this system will be installed in the SPEKTR module of the MIR station. Jerico is a robot that has seven axes with a sensorized end-effector [26]. Currently, many strategic technologies have been developed for space robot technology. Rotex is a project for the realization of space automation to robotics. Rotex makes use of multisensory clamp technology, uses local sensory feedback control concepts in which a powerful 3D graphic simulation with delay compensation (predictive simulation) has been implemented in the telerobotic ground station [27]. The Mobile Servicing System (MSS) is a robotic system developed for the International Space Station. The system plays a key role in the construction and maintenance of the space station by moving equipment and structures around the station, assisting astronauts with Extra-Vehicle Activities (EVA) and performing other operations outside the station. The system consists of Canadarm2 which was successfully installed in-orbit by the Canadian Space Agency in April 2001, and performed its first task of assembling the Space Station during STS-104 in July 2001 [28]. The microgravity facilitates movement but makes the stability of mechanisms vul- nerable even to small vibrations. Furthermore, space radiations can be dangerous for the actuators and for many other components: controllers; driver; sensors; electronic devices as pointed out in [29]. In space, there are strong changes in temperature and thermal gradients. Furthermore, in orbital stations, the energy source is very limited since energy transport is expensive and therefore the power consumption must be managed very carefully [30]. It is necessary to consider the difficulty in carrying objects: a robot should be as light and small as possible, as pointed out in [31]. Considering that space operations by astronauts are both dangerous and expensive, service robots are being used more and more often either as assistants or as substitutes in order to reduce risk and cost. Space robots can install devices while maintaining the space station and performing experiments in space [32]. Experimental characterization of TORVEastro is presented in this article. TORVEastro is a service space robot with multiple functionalities. In particular, the use of TORVEastro will be useful to repair mechanical parts in ISS. The application of a service robot in a space orbital station needs to consider the spatial characteristics as outlined in [33]. The paper reports result to analyzes a CAD modeling and a performance evaluation for design feasibility. Finite Element Analysis (FEA) is a useful numerical technique that has been used in this paper for modeling and simulating various thicknesses of link design to achieve the best compromise in terms of weight and resistance. A feasibility study is discussed through performance evaluation using kinematics and dynamic simulation results as reported in [34]. A TORVEastro robot prototype was first designed and then built in LARM2, in Rome. Three arm–legs are used both for locomotion and grasping. The robot design with three arm–legs has been conceived as inspired by a chameleon structure to have robot limbs available both for grasping and locomotion tasks, as outlined in [19]. With the proposed structure the space TORVEastro robot can move in most of the places of a space station by using rods and handrails. In particular, assuming to have more arm–legs is not suitable because the weight and the transmission complexity. Assuming that having fewer arm–legs is not suitable for locomotion and grasping because of the reduced possibility of doing multiple tasks simultaneously. TORVEastro is designed to repair mechanical parts of the ISS. During the testing activities, the validity of the robot design was verified and the performance was characterized. The CAD model was also used for 3D printing of the components. The built prototype was tested in order to verify the operational efficiency and to evaluate the performance characteristics during the basic operational activities.

2. TORVEastro Design TORVEastro space robot has a cylindrical body design with three legs, each of which is made of three links [35]. The symmetrical assembly of the arm–legs makes them in- terchangeable and gives the possibility of adopting a structure with multi-functional end-effectors. The conceptual design in Figure1 shows the kinematic structure with design Robotics 2021, 10, x FOR PEER REVIEW 4 of 19

printing of the components. The built prototype was tested in order to verify the operational efficiency and to evaluate the performance characteristics during the basic operational activities.

2. TORVEastro Design Robotics 2021, 10, 21 TORVEastro space robot has a cylindrical body design with three legs, each of which4 of 19 is made of three links [35]. The symmetrical assembly of the arm–legs makes them inter- changeable and gives the possibility of adopting a structure with multi-functional end- effectors. The conceptual design in Figure 1 shows the kinematic structure with design and operation parameters. In particular, αij is the joint angle of the i-th joint in the j-th and operation parameters. In particular, αij is the joint angle of the i-th joint in the j-th arm– arm–leg. The parameter wij is the corresponding angular velocity. The parameter θij is the leg. The parameter wij is the corresponding angular velocity. The parameter θij is the cor- corresponding joint angular acceleration of the i-th joint in j-th arm–leg. Lij is the link body responding joint angular acceleration of the i-th joint in j-th arm–leg. Lij is the link body of of TORVEastro. Lij vector refers to the length of the i-th link in the j-th arm–leg. Rij is the TORVEastro. Lij vector refers to the length of the i-th link in the j-th arm–leg. Rij is the reaction force vector. Aj is the shoulder point, Kj is the elbow point and Pj is the extremity reaction force vector. Aj is the shoulder point, Kj is the elbow point and Pj is the extremity point of j-th arm–leg. Sij is the servomotor for the i-th link of the j-th arm–leg. point of j-th arm–leg. Sij is the servomotor for the i-th link of the j-th arm–leg.

Figure 1. A model for the mechanical design of TORVEastro withwith parameters.parameters.

The central body body of of the the robot robot has has a acylindrical cylindrical surface surface design design and and the the central central body body of ofthe the robot robot has has a cylindrical a cylindrical surface surface and and is provided is provided of three of three arm–leg arm–leg limbs limbs to perform to perform lo- locomotioncomotion and and grasp grasp on ondemand. demand. The Theservice service robot robot can move can move along along on the on rods the and rods hand- and handrails.rails. The three The threearm–legs arm–legs have three have threedegreees degreees of freedom of freedom (DoFs) (DoFs) with three with revolute three revolute joints joints per arm–leg allow the rotation α , α and α (n = 1,2,3), Figure1. Three arm–legs per arm–leg allow the rotation αn1, αn2n1 andn2 αn3 (n n3= 1,2,3), Figure 1. Three arm–legs are areused used both both for forlocomotion locomotion and and grasping. grasping. Arm–legs Arm–legs are are three three because because one one is is necessary toto grasp handrail,handrail, thethe secondsecond is is necessary necessary to to grasp grasp the the second second handrail handrail to to have have the the motion motion or a static posture, the third can be used for manipulation. With the proposed structure, the or a static posture, the third can be used for manipulation. With the proposed structure, space TORVEastro robot can move in most of the places of a space orbital station by using the space TORVEastro robot can move in most of the places of a space orbital station by rods and handrails. There are six identical links L11,L12,L21,L22,L31,L32 that define six using rods and handrails. There are six identical links L11, L12, L21, L22, L31, L32 that define correspondent vectors L11,L12,L21,L22,L31,L32 as related to arm–leg structures. There are six correspondent vectors L11, L12, L21, L22, L31, L32 as related to arm–leg structures. There other three identical links L10,L20 and L30 that connect the limbs to the central body. Each link has one DoF and, in total, there are nine revolute joints that are controlled by nine servomotors (S10,S11,S12,S20,S21,S22,S30,S31,S32). Each limb consists of three links as for example L10,L11 and L21 for the first arm–leg. The first DoF gives the possibility to rotate the angle α10 and it has an angular velocity direction orthogonal to the lateral surface of the central body. The second DoF gives rotation α11 angle as shoulder rotation. The third DoF gives the possibility of an elbow rotation. Servomotors are placed inside the main body and can actuate the links by tensioned cables and limbs after transmission with spur gears. In this configuration links L11,L12,L21,L22,L31 and L32 are moved by tensioned cables. Cable Robotics 2021, 10, x FOR PEER REVIEW 5 of 19

are other three identical links L10, L20 and L30 that connect the limbs to the central body. Each link has one DoF and, in total, there are nine revolute joints that are controlled by nine servomotors (S10, S11, S12, S20, S21, S22, S30, S31, S32). Each limb consists of three links as for example L10, L11 and L21 for the first arm–leg. The first DoF gives the possibility to rotate the angle α10 and it has an angular velocity direction orthogonal to the lateral surface of the central body. The second DoF gives rotation α11 angle as shoulder rotation. The third Robotics 2021, 10, 21 5 of 19 DoF gives the possibility of an elbow rotation. Servomotors are placed inside the main body and can actuate the links by tensioned cables and limbs after transmission with spur gears. In this configuration links L11, L12, L21, L22, L31 and L32 are moved by tensioned cables. tensionCable tension in static in mode static ismode designed is designed of about of 10 about N. To 10 have N. To low have inertia low and inertia not and to expose not to theexpose engines the toengines harsh spaceto harsh conditions, space conditions the motors, the are motors located are inside located the centralinside the body central and theybody move and they the linksmove by the cables. links by Three cables. IMU Th sensorsree IMU (inertial sensors measurement (inertial measurement unit) are used unit) toare monitor used to the monitor position, the velocity position, and velocity acceleration and acceleration of robot links. of robot In Figure links.2 Ina kinematic Figure 2 a functionalkinematic schemefunctional of TORVEastroscheme of TORVEastro is presented is whenpresented Zi,j axis when coincides Zi,j axis with coincides the joint with axis, the ijoint represents axis, i therepresents number the of arm–legnumber andof arm–leg j represents and thej represents number ofthe links number per arm–leg.of links per In thearm–leg. revolute In jointsthe revolute 1–2–3 ofjoints the first1–2–3 arm–leg of the first being arm–leg Z10 perpendicular being Z10 perpendicular to Z11, Z12 and to toZ11, the Z12 baseand to of the base central of the body central cylinder. body At cylinder. the same At time the same revolute time joints revolute 4–5–6 joints and 4–5–6 7–8–9 and have 7– a8–9 position have a thatposition is a centralthat is a symmetry central symmetry with respect with torespect the center to the ofcent theer centralof the central body withbody jointswith joints 1–2–3. 1–2–3. Each linkEach is link made is ofmade a hollow of a hollow cross-section cross-section of elliptical of elliptical shape with shape a curved with a ◦ structurecurved structure of 100 cm of radius 100 cm for radius an extension for an extension of 80 , Figure of 80°,3. TheFigure design 3. The is developed design is devel- to fit withoped the to shapefit with of the centralshape of body the incentral the home body configuration. in the home Theconfiguration. internal volume The internal of the linkvolume can beof usedthe link for can electrical be used cables for electrical wiring the cables components. wiring the components.

Robotics 2021, 10, x FOR PEER REVIEW 6 of 19

FigureFigure 2.2.A A kinematickinematic functionalfunctional schemescheme ofof thethe designdesign inin FigureFigure1 .1.

(a) (b)

Figure 3. Mechanical design of a link of TORVEastro: (a) shape of L11; (b) cross-section. Figure 3. Mechanical design of a link of TORVEastro: (a) shape of L11;(b) cross-section.

3. Performance3. SimulationPerformance Simulation TORVEastro CADTORVEastro model is CAD designed model considering is designed the considering space environment. the space Kinematicenvironment. Kinematic and dynamic simulationand dynamic results simulation are used results to check are the used feasibility to check of athe prototype feasibility design. of a FEAprototype design. analysis resultsFEA and analysis reaction results forces showand reaction the TORVEastro forces sh robotow the operation TORVEastro peculiarities. robot operation Fig- peculiari- ure4 summarizesties. the Figure TORVEastro 4 summarizes simulation the TORVEastro scheme considering simulation peculiarities scheme considering in terms of peculiarities in terms of interactions with the space environment. A consideration of the environment includes also how a service robot affects or is affected by the environment, by analyzing and designing the variety of feasible conditions and situations. A dynamic simulation according to FEA analysis and kinematic analysis gives fundamental aspects for performance evaluation.

Figure 4. A scheme of simulation of TORVEastro.

TORVEastro robot has been simulated with a payload of 100 N. The direction of the load in the static condition is the same as the gravity acceleration. The mesh radiates from vertices to edges, from edges to faces, from faces to components, and from a component to connected components as in Figure 5a that shows link joint connection and in Figure 5b that shows joint assembly. Results of FEA analysis are reported in Figure 6 and the red arrow represents the yield strength of the material after which the deformation begins to become plastic. The stress is calculated according to Von Mises criterion, which is a scalar value of pressure (Pa) that can be computed from the Cauchy stress tensor that completely defines the state of stress of the material. The deformation in Figure 6 is highlighted by a multiplication factor of 100 times. In consideration of several computations by FEA, the final design consists of thick of 2.5 mm for L11 and of 1.0 mm for L12. In this way there is a proper compromise between resistance and mass (a low thick value limits the weight of

Robotics 2021, 10, x FOR PEER REVIEW 6 of 19

(a) (b)

Figure 3. Mechanical design of a link of TORVEastro: (a) shape of L11; (b) cross-section.

3. Performance Simulation TORVEastro CAD model is designed considering the space environment. Kinematic Robotics 2021, 10, 21 and dynamic simulation results are used to check the feasibility of a prototype design.6 of 19 FEA analysis results and reaction forces show the TORVEastro robot operation peculiarities. Figure 4 summarizes the TORVEastro simulation scheme considering peculiarities in terms of interactions with the space environment. A consideration of the environment in- interactionscludes also how with a the service space robot environment. affects or Ais considerationaffected by the of environment, the environment by analyzing includes alsoand howdesigning a service the robotvariety affects of feasible or is affected conditions by theand environment, situations. A bydynamic analyzing simulation and designing accord- theing varietyto FEA ofanalysis feasible and conditions kinematic and analysis situations. gives A dynamicfundamental simulation aspects according for performance to FEA analysisevaluation. and kinematic analysis gives fundamental aspects for performance evaluation.

Figure 4.4. A scheme of simulation ofof TORVEastro.TORVEastro.

TORVEastro robotrobot hashas beenbeen simulatedsimulated withwith aa payloadpayload ofof 100100 N.N. TheThe directiondirection ofof thethe loadload inin thethe staticstatic conditioncondition isis thethe samesame asas thethe gravitygravity acceleration.acceleration. TheThe meshmesh radiatesradiates fromfrom verticesvertices toto edges,edges, fromfrom edges edges to to faces, faces, from from faces faces to to components, components, and and from from a component a component to connectedto connected components components as inas Figurein Figure5a that5a that shows shows link link joint joint connection connection and and in Figurein Figure5b that5b that shows shows joint joint assembly. assembly. Results Results of of FEA FEA analysis analysis are are reported reported in in Figure Figure6 6and and the the red red arrow representsrepresents thethe yieldyield strengthstrength ofof thethe materialmaterial afterafter whichwhich thethe deformationdeformation beginsbegins toto become plastic.plastic. The stress is calculated according to Von MisesMises criterion,criterion, whichwhich isis aa scalarscalar valuevalue ofof pressure (Pa)(Pa) thatthat cancan be computed from the Cauchy stress tensor that completely Robotics 2021, 10, x FOR PEER REVIEWdefines the state of stress of the material. The deformation in Figure6 is highlighted7 by of 19 a defines the state of stress of the material. The deformation in Figure 6 is highlighted by a multiplication factor of 100100 times.times. InIn considerationconsideration ofof severalseveral computationscomputations byby FEA,FEA, thethe final design consists of thick of 2.5 mm for L and of 1.0 mm for L . In this way there is a final design consists of thick of 2.5 mm for L1111 and of 1.0 mm for L1212. In this way there is a proper compromise between resistance and mass (a low thick value limits the weight of theproper L12 and compromise thus the stress between on the resistance L11) and, and as shown mass (a in low Figure thick 6, thisvalue configuration limits the weight has the of possibilitythe L12 and to thus support the stress the load on the of 100 L11) N and, with as suitable shown inconfiguration. Figure6, this configuration has the possibility to support the load of 100 N with suitable configuration.

(a) (b)

Figure 5. A mesh design for Finite Element Analysis (FEA) analysis of link design for: ( a)) link joint connection; ( (b)) joint assemblyassembly..

(a) (b)

Figure 6. Results of FEA analysis (red line is the yield strength): (a) with tick of L11 and L12 of 2.5 mm; (b) with tick of L11 is 2.5 mm and tick of L12 of 1.0 mm.

4. Prototype and Testing Modes Figure 7 shows the prototype of TORVEastro that was built at LARM2 at the Univer- sity of Rome Tor Vergata. The structure of the robot was 3D printed in PLA filament (Pol- ylactic Acid). The central body of the robot had cylindrical dimensions of 12 cm of radius and 9 cm height. The total mass of the space robot was less than 2 kg and its robot arm length was 60 cm.

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the L12 and thus the stress on the L11) and, as shown in Figure 6, this configuration has the possibility to support the load of 100 N with suitable configuration.

(a) (b) Robotics 2021, 10, 21 7 of 19 Figure 5. A mesh design for Finite Element Analysis (FEA) analysis of link design for: (a) link joint connection; (b) joint assembly.

(a) (b)

11 12 11 Figure 6.6. Results ofof FEAFEA analysisanalysis (red(red lineline is is the the yield yield strength): strength): ( a()a) with with tick tick of of L 11L andand L L12 of 2.5 mm;mm; ((bb)) withwith ticktick ofof LL11 isis 2.5 mm and tick of L12 of 1.0 mm. 2.5 mm and tick of L12 of 1.0 mm.

4. Prototype and Testing Modes 4. PrototypeFigure 7 andshows Testing the prototype Modes of TORVEastro that was built at LARM2 at the Univer- sity ofFigure Rome7 showsTor Vergata. the prototype The structure of TORVEastro of the robot that was was 3D built printed at LARM2 in PLA at the filament University (Pol- ofylactic Rome Acid). Tor Vergata. The central The body structure of the of therobot robot had was cylindrical 3D printed dimensions in PLA ﬁlament of 12 cm (Polylactic of radius Acid).and 9 cm The height. central The body total of mass the robot of the had space cylindrical robot was dimensions less than of2 kg 12 and cm ofits radius robot andarm Robotics 2021, 10, x FOR PEER REVIEW 8 of 19 9length cm height. was 60 The cm. total mass of the space robot was less than 2 kg and its robot arm length was 60 cm.

Figure 7. AA built built prototype prototype of TORVEastro at LARM2 Rome.Rome.

The inside cross-section of the link allows the use of cables for sensors and actuators. The assembly process followed the following steps: (1) fixing the battery in the center of the central body of the robot; (2) fixing the toothed wheels; (3) fixing of links 1,2 and 3; (4) fixing the servomotors; (5) fixing the transmissions; (6) tension testing of steel cables. Ser- vomotors S10, S20 and S30 were fixed with external teeth spur gears with module 1 and 19 teeth, Figure 8a, and external teeth spur gears transmit motion to internal teeth spur gears that have the same module 1 and 45 teeth. Actuation of links L12 and L13 is performed by tensioned cables (grey color), Figure 8b. Servomotors S11, S12, S21, S22, S31 and S33 are fixed inside the central body of the space robot. Servomotors actuate links L11, L12, L21, L22, L31, L32 by tensioned metal cables. The LiPo battery had 7,4 V and 6000 mAh for an operation estimated duration of 4–6 h. A webcam vision system was utilized to have visual information of the surroundings and link motion in real-time. A servomotor was mounted over the central body and webcam system is mounted on the servomotor to visualize the surrounding environment with the possibility of rotating the visual system by 360°. Using Arduino, it was possible to control properly the movement of the links using the testing layout in Figure 9. A LiPo battery gave power to the servomotors linked to Arduino by electric cables. A capacitor was used to reduce electrical noise and to stabilize the voltage. A capacitor with 1 μF was used to decouple servomotors of electrical network circuit from other parts. Noise by other circuit elements was shunted through a capacitor. A breadboard was used to host all the devices together. IMUs sensors (Inertial Measure- ment Unit) have been used and chosen according to the functional characteristics and the cost. GY-BMI 160 has a size of 13 mm × 18 mm [36]. GY-BMI 160 IMU sensors were positioned at the center of mass of each monitored link. Each IMU consisted of a three-axis gyroscope sensing acceleration with its cartesian components. The angles were calculated by integrating the velocity into the time variable, repeating the process ten times and averaging it. The current sensor was used to measure power consumption, which is very important data in space operation considering the difficulty to obtain energy in space. Servomotors (MG995*) have 57 g of weight, like the one in Figure 9, were used to move

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The inside cross-section of the link allows the use of cables for sensors and actuators. The assembly process followed the following steps: (1) fixing the battery in the center of the central body of the robot; (2) fixing the toothed wheels; (3) fixing of links 1, 2 and 3; (4) fixing the servomotors; (5) fixing the transmissions; (6) tension testing of steel cables. Servomotors S10,S20 and S30 were fixed with external teeth spur gears with module 1 and 19 teeth, Figure8a, and external teeth spur gears transmit motion to internal teeth spur gears that have the same module 1 and 45 teeth. Actuation of links L12 and L13 is performed by tensioned cables (grey color), Figure8b. Servomotors S 11,S12,S21,S22,S31 and S33 are fixed inside the central body of the space robot. Servomotors actuate links L11, Robotics 2021, 10, x FOR PEER REVIEWL12 ,L21,L22,L31,L32 by tensioned metal cables. The LiPo battery had 7,4 V and 60009 mAh of 19 for an operation estimated duration of 4–6 h. A webcam vision system was utilized to have visual information of the surroundings and link motion in real-time. A servomotor was mounted over the central body and webcam system is mounted on the servomotor to the nine links of the robot at operating speed with these characteristics: 0.13 s/60 degrees visualize the surrounding environment with the possibility of rotating the visual system maximum speed; maximum torque 13 kg/cm (7.2 V). by 360◦.

(a) (b)

FigureFigure 8. TORVEastro8. TORVEastro mechanical mechanical transmission transmission in: in: (a) ( designa) design solution; solution; (b) ( ab particular) a particular of tensionedof the prototype cables andin Figure spur gears.10.

Using Arduino, it was possible to control properly the movement of the links using the testing layout in Figure9. A LiPo battery gave power to the servomotors linked to Arduino by electric cables. A capacitor was used to reduce electrical noise and to stabilize the voltage. A capacitor with 1 µF was used to decouple servomotors of electrical network circuit from other parts. Noise by other circuit elements was shunted through a capacitor. A breadboard was used to host all the devices together. IMUs sensors (Inertial Measurement Unit) have been used and chosen according to the functional characteristics and the cost. GY-BMI 160 has a size of 13 mm × 18 mm [36]. GY-BMI 160 IMU sensors were positioned at the center of mass of each monitored link. Each IMU consisted of a three-axis gyroscope sensing acceleration with its cartesian components. The angles were calculated by integrating the velocity into the time variable, repeating the process ten times and averaging it. The current sensor was used to measure power consumption, which is very important data in space operation considering the difﬁculty to obtain energy in space. Servomotors MG995 [37] have 57 g of weight, like the one in Figure 10, were used to move the nine links of the robot at operating speed with these characteristics: 0.13 s/60 degrees maximum speed; maximum torque 13 kg/cm (7.2 V).

Figure 9. A testing layout of TORVEastro prototype at LARM2 Rome.

Figure 10. MG995* servomotor [37] with its geometry and dimensions in mm.

Robotics 2021, 10, x FOR PEER REVIEW 9 of 19

the nine links of the robot at operating speed with these characteristics: 0.13 s/60 degrees maximum speed; maximum torque 13 kg/cm (7.2 V).

(a) (b) Figure 8. TORVEastro mechanical transmission in: (a) design solution; (b) a particular of the prototype in Figure 10. Robotics 2021, 10, 21 9 of 19

Figure 9. A Figuretesting 9. layoutA testing of layout TORVEastro of TORVEastro prototype prototype at LARM2 at LARM2 Rome Rome..

Figure 10.Figure MG995* 10. MG995 servomotor servomotor [37] with with its geometryits geometry and dimensions and dimensions in mm. in mm.

Figure 11 shows an electrical scheme for an arm–leg actuation in which connections are represented for the LiPo battery, the servomotor that actuates the link, the force sensor that acquires the value of contact force, the IMU (inertial measurement unit) that measures the values of angular position, velocity and acceleration of links of the robot, the current sensor that monitors values of current and power for operational tasks.

Robotics 2021, 10, x FOR PEER REVIEW 10 of 19

Figure 11 shows an electrical scheme for an arm–leg actuation in which connections are represented for the LiPo battery, the servomotor that actuates the link, the force sensor that acquires the value of contact force, the IMU (inertial measurement unit) that measures the values of angular position, velocity and acceleration of links of the robot, the current Robotics 2021, 10, 21 sensor that monitors values of current and power for operational tasks. Figure 11 10shows of 19 the prototype built by 3D printer with PLA material using the same geometry of 3D CAD model used in simulations.

FigureFigure 11.11. TheThe electricalelectrical designdesign ofof aa singlesingle arm–legarm–leg actuation.actuation.

5. Experimental Results 5. Experimental Results Experimental tests of performance evaluation of the built prototype shown in Figure Experimental tests of performance evaluation of the built prototype shown in Figure 11 11 were repeated three times to characterize the movement repeatability. The experi- were repeated three times to characterize the movement repeatability. The experimental mental results were acquired by using IMU sensors in terms of link angular position, ve- results were acquired by using IMU sensors in terms of link angular position, velocity and locity and acceleration to check TORVEastro space robot feasibility in basic operations. acceleration to check TORVEastro space robot feasibility in basic operations. GY-BMI 160 GY-BMI 160 IMU sensors were positioned at the center of mass of each monitored link. IMU sensors were positioned at the center of mass of each monitored link. Links moved Links moved from start-point to end-point using different angular velocities for the three from start-point to end-point using different angular velocities for the three servomotors. servomotors. 5.1. Experimental Test 1 5.1. Experimental Test 1 Figure 12 shows a robot snapshot motion of the ﬁrst experimental test in which only one servomotorFigure 12 shows S10 (see a robot Figure snapshot1) is used motion at 10% of ofthe its first max experimental speed. The test maximum in which angle only rangeone servomotor in terms of S rotation10 (see Figure is 25–35–12-degrees 1) is used at 10% (respectively of its max considering speed. The rotation maximum around angle x, yrange and z-axes),in terms as of is rotation reported is in25–35–12-degrees Figure 13. The angle (respectively values vary considering almost linearly rotation resulting around inx, y a and uniform z-axes), speed as is motion. reported An in importantFigure 13. The consideration angle values is vary that inalmost a space linearly microgravity resulting environment,in a uniform speed transmission motion. mechanisms An important are consideration less stressed is because that in they a space are smallermicrogravity with respectenvironment, to test conditionstransmission on mechanisms the ground. Inare this less test, stressed maximal because angular they velocity are smaller measured with 1.0respect rad/s to andtest itsconditions time evolution on the ground. had comparable In this test, values maximal in the angular module velocity in the measured outward and1.0 rad/s return and motion its time phases, evolution as represented had comparable in Figure values 14 .in Maximum the module angular in the outward acceleration and 2 isreturn 1 rad/s motionas shownphases, in as Figures represented 15 and in 16 Fi showsgure 14. the Maximum angular positionangular ofacceleration link L12 with is 1 characteristicrad/s2 as shown values in Figures of 48; 12; 15 5and deg 16 as shows maximum the angular (respectively position considering of link L12 rotation with character- around x,istic y andvalues z-axis) of 48; and 12; 5; 5 8; deg−3 as deg maximum as a minimum. (respectively Angular considering velocity variation rotation isaround shown x, in y Figureand z-axis) 17 and and its 5; value8; −3 deg gives as ana minimum. indication Angular of a smooth velocity motion variation with is a behaviorshown in thatFigure is characterized17 and its value by gives a continuous an indication motion of with a smooth periodic motion motion with following a behavior the imposedthat is character- motion. Maximumized by a continuous angular velocity motion valuewith periodic measured motion 1.0 rad/s following and itsthe value imposed shows motion. a periodic Maxi- trendmum andangular has oscillationsvelocity value especially measured when 1.0 rad/s the robot and armits value is at theshows end a positions.periodic trend Angular and accelerationhas oscillations variation especially in function when the of robot time isarm shown is at the inFigure end positions. 18 with Angular the maximum acceleration value 2 ofvariation 1.0 rad/s in functionand its trendof time shows is shown oscillations in Figure for 18 vibrating with the motion.maximum Power value consumption of 1.0 rad/s2 with a mean value of 5 W is shown in Figure 19 and by integrating the value it is possible to estimate an operation duration of 5 h. The mean current value is 0.17 A. Experimental results of this test in terms of power consumption show no-linear variation because the

servomotor continuously runs to move according to the set velocity and because the cables are always tensioned. Robotics 2021, 10, x FOR PEER REVIEW 11 of 19

and its trend shows oscillations for vibrating motion. Power consumption with a mean value of 5 W is shown in Figure 19 and by integrating the value it is possible to estimate an operation duration of 5 h. The mean current value is 0.17 A. Experimental results of this test in terms of power consumption show no-linear variation because the servomotor continuously runs to move according to the set velocity and because the cables are always Robotics 2021, 10, 21 11 of 19 tensioned.

5.2. Experimental Test 2 5.2. ExperimentalFigures 19–22 Test show 2 a snapshot and trends are shown for the angular position, velocity andFigures acceleration 19–22 show in a asecond snapshot experimental and trends test are shownin which for three the angular actuators position, in an arm–leg velocity workand acceleration simultaneously. in a second The input experimental angular velocity test in which of servomotors three actuators S10, S11 in and an S arm–leg12 is imposed work atsimultaneously. 6% of their maximum The input speed. angular Th velocitye angular of position servomotors of link S10 ,SL1111 inand Figure S12 is 20 imposed shows an at angle6% of theirrange maximum in terms of speed. rotation The in angular about position10-4-13 deg of link (respectively L11 in Figure considering 20 shows anrotation angle aroundrange in x, terms y and of rotationz-axes). inThe about result 10-4-13 outlines deg that (respectively in the three-repetition considering rotation test, values around are x, ysatisfactorily and z-axes). repeatable The result giving outlines a proper that in lin thek three-repetitionmotion considering test, data values input are and satisfactorily data output.repeatable The maximum giving a an propergular linkvelocity motion value considering is 0.8 rad/s, data Figure input 21. and Angular data output. acceleration The showsmaximum a strong angular variation velocity as a value function is 0.8 of rad/s,time and Figure its maximum 21. Angular value acceleration is 1.2 rad/s shows2, Figure a 22.strong The variationpower consumption as a function of ofthe time robot, and as its shown maximum in Figure value 23, is is 1.2 less rad/s than2 ,theFigure sum 22of. Thetotal powerpower consumptionconsumption ofof thesingle robot, servomotor as showns inbecause Figure it 23 does, is lessnot thanhappen the that sum all of three total powermotors consumptionconsume the of maximum single servomotors energy at becausethe same it doestime, notshowing happen a thatsmall all difference three motors be- tweenconsume thethe use maximum of one or energythree servomotors at the same si time,multaneously. showing aBy small integrating difference the betweenvalue of powerthe use consumption, of one or three it is servomotors possible to estimate simultaneously. an operation By integrating duration of the 7 h value as related of power to a meanconsumption, value of it9 isW possible and by tointegrating estimate anthe operation value it is duration possible of to 7 hestimate as related an tooperation a mean valueduration of 9 of W 2.5 and h. by integrating the value it is possible to estimate an operation duration of 2.5 h.

(a) (b) (c)

10 RoboticsFigure 2021, 10 12., x A FOR snapshot PEER REVIEW during testing of the TORVEastro prototypeprototype ofof FigureFigure 11 11 actuating actuating SS10:: in (a–c) is shown the motion12 of 19 of links L10, L11 and L12 of the first arm–leg limb. of links L10,L11 and L12 of the ﬁrst arm–leg limb.

40 α11x ]

° α11y 30 α11z

10 angular position [ angular

0 0 5 10 15 20 25 30 -10 time [s]

Figure 13. Experimental results of a test like in Figure 12 in terms of angle link L11 of the first arm–leg limb whit only S10 Figure 13. Experimental results of a test like in Figure 12 in terms of angle link L11 of the ﬁrst arm–leg limb whit only S10 actuator moves. actuator moves.

0.8 w11x 0.6 w11y w11z 0.4

0.2

0 0 5 10 15 20 25 30 -0.2

angular velocity [rad/s] angular -0.4

-0.6

-0.8 time [s]

Figure 14. Experimental results of a test like in Figure 12 in terms of the angular velocity of arm–leg L11 of the first arm– leg whit only S10 actuator moves.

Robotics 2021, 10, x FOR PEER REVIEW 12 of 19

40 α11x ]

° α11y 30 α11z

10 angular position [ angular

0 0 5 10 15 20 25 30 -10 time [s]

Robotics 2021, 10, 21 12 of 19

Figure 13. Experimental results of a test like in Figure 12 in terms of angle link L11 of the first arm–leg limb whit only S10 actuator moves.

0.8 w11x 0.6 w11y w11z 0.4

0.2

0 0 5 10 15 20 25 30 -0.2

angular velocity [rad/s] angular -0.4

-0.6

-0.8 time [s]

Figure 14. Experimental results of a test like in Figure 12 in terms of the angular velocity of arm–leg L11 of the first arm– Robotics 2021Figure, 10, 14.x FORExperimental PEER REVIEW results of a test like in Figure 12 in terms of the angular velocity of arm–leg L11 of the ﬁrst arm–leg 13 of 19 leg whit only S10 actuator moves. whit only S10 actuator moves.

0.8 θ11x 0.6 θ11y 0.4 θ11z 0.2

0 0 5 10 15 20 25 30 -0.2

-0.4 angular acceleration [rad/s^2] acceleration angular

-0.6

-0.8 time [s]

Figure 15. Experimental results of a test like in Figure 12 in terms of the angular acceleration of arm–leg L11 of the first Figure 15. Experimental results of a test like in Figure 12 in terms of the angular acceleration of arm–leg L11 of the ﬁrst arm–leg whit only S10 actuator moves. arm–leg whit only S10 actuator moves.

60 α12x α12y α12z 50 ]

° 40

10 angular position [ angular

0 0 5 10 15 20 25 30 -10 time [s]

Figure 16. Experimental results of a test like in Figure 12 in terms of the orientation angle of arm–leg L12 of the first arm– leg whit only S10 actuator moves.

1 w12x w12y w12z 0.8 0.6 0.4 0.2 0 -0.2 0 5 10 15 20 25 30 -0.4

angular velocity [rad/s] angular -0.6 -0.8 -1 time [s]

Figure 17. Experimental results of a test like in Figure 12 in terms of the angular velocity of arm–leg L12 of the first arm– leg whit only S10 actuator moves.

Robotics 2021, 10, x FOR PEER REVIEW 13 of 19

1 1 0.8 0.8 θ11x 0.6 θ11x 0.6 θ11y 0.4 θ11yθ11z 0.4 θ11z 0.2 0.2 0 0 0 5 10 15 20 25 30 -0.2 0 5 10 15 20 25 30 -0.2 -0.4 angular acceleration [rad/s^2] acceleration angular -0.4 angular acceleration [rad/s^2] acceleration angular -0.6 -0.6 -0.8 time [s] Robotics 2021, 10, 21 -0.8 13 of 19 time [s]

Figure 15. Experimental results of a test like in Figure 12 in terms of the angular acceleration of arm–leg L11 of the first Figurearm–leg 15. whit Experimental only S10 actuator results moves. of a test like in Figure 12 in terms of the angular acceleration of arm–leg L11 of the first arm–leg whit only S10 actuator moves. 60 60 α12x α12y α12z 50 α12x α12y α12z 50 ]

° 40 ]

° 40 30 30 20 20 10 angular position [ angular 10 angular position [ angular 0 0 0 5 10 15 20 25 30 -10 0 5 10 15 20 25 30 -10 time [s] time [s]

Figure 16. Experimental results of a test like in Figure 12 in terms of the orientation angle of arm–leg L12 of the first arm– Figure 16. Experimental results of a test like in Figure 12 in terms of the orientation angle of arm–leg L12 of the ﬁrst arm–leg Figureleg whit 16. only Experimental S10 actuator results moves. of a test like in Figure 12 in terms of the orientation angle of arm–leg L12 of the first arm– whit only S10 actuator moves. leg whit only S10 actuator moves.

1 w12x w12y w12z 0.81 w12x w12y w12z 0.80.6 0.60.4 0.40.2 0.20 -0.20 0 5 10 15 20 25 30 0 5 10 15 20 25 30 -0.2-0.4 -0.4

angular velocity [rad/s] angular -0.6

angular velocity [rad/s] angular -0.6-0.8 -0.8-1 -1 time [s] time [s]

Figure 17. Experimental results of a test like in Figure 12 in terms of the angular velocity of arm–leg L12 of the first arm– Figureleg whit 17. only Experimental S10 actuator results moves. of a test like in Figure 12 in terms of the angular velocity of arm–leg L12 of the first arm– RoboticsFigure 2021 17. ,Experimental 10, x FOR PEER resultsREVIEW of a test like in Figure 12 in terms of the angular velocity of arm–leg L12 of the ﬁrst arm–leg14 of 19 leg whit only S10 actuator moves. whit only S10 actuator moves.

0.8 θ12x 0.6 θ12y 0.4 θ12z 0.2 0 -0.2 0 5 10 15 20 25 30 -0.4 -0.6

angular acceleration [rad/s^2] acceleration angular -0.8 -1 time [s]

Figure 18. Experimental results of a test like in Figure 12 in terms of the angular acceleration of arm–leg L12 of the first Figure 18. Experimental results of a test like in Figure 12 in terms of the angular acceleration of arm–leg L12 of the ﬁrst arm–leg whit only S10 actuator moves. arm–leg whit only S10 actuator moves.

Power [W] 4

0 0 5 10 15 20 25 30 -2 time [s]

Figure 19. Experimental results of a test like in Figure 12 in terms of utilized power consumption for the first arm–leg whith only S10 actuator moves.

(a) (b) (c)

Figure 20. A snapshot during testing of TORVEastro prototype of Figure 11 actuating S10, S11 and S12: in (a–c) are shown the motions of links L10, L11 and L12 of the first arm–leg, respectivley.

Robotics 2021, 10, x FOR PEER REVIEW 14 of 19

1 0.8 1 θ12x 0.60.8 θ12xθ12y 0.40.6 θ12yθ12z 0.2 0.4 θ12z 00.2 -0.2 0 5 10 15 20 25 30 0 5 10 15 20 25 30 -0.4-0.2 -0.6-0.4

angular acceleration [rad/s^2] acceleration angular -0.8-0.6 angular acceleration [rad/s^2] acceleration angular -1-0.8 -1 time [s] time [s]

Robotics 2021, 10, 21 14 of 19 Figure 18. Experimental results of a test like in Figure 12 in terms of the angular acceleration of arm–leg L12 of the first Figure 18. Experimental results of a test like in Figure 12 in terms of the angular acceleration of arm–leg L12 of the first arm–leg whit only S10 actuator moves. arm–leg whit only S10 actuator moves.

14 14 12 12

10 10

8 8

6 6 Power [W] Power [W] 4 4

2 2

0 0 00 5 5 10 10 15 20 20 25 25 30 30 -2 -2 time [s][s]

Figure 19. Experimental results of a test like in Figure 12 in terms of utilized power consumption for the first arm–leg Figure 19. ExperimentalExperimental resultsresults of of a a test test like like in in Figure Figure 12 in12 termsin terms of utilized of utilized power power consumption consumption for the for ﬁrst the arm–leg first arm–leg whith whith only S10 actuator moves. whithonly S 10onlyactuator S10 actuator moves. moves.

(a) (b) (c)

Figure 20. A snapshot(a) during testing of TORVEastro prototype(b) of Figure 11 actuating S10, S11 and(c S) 12: in (a–c) are shown the motions of links L10, L11 and L12 of the first arm–leg, respectivley. RoboticsFigure 2021, 10 20., x A FOR snapshot PEER REVIEW during testing ofof TORVEastroTORVEastro prototypeprototype ofof FigureFigure 11 11 actuating actuating S S10,S, S11 and SS12:: in in ( (a–c)) are shown15 of 19 10 11 12 the motions of links L10, L11 and L12L12 of the ﬁrstfirst arm–leg, respectivley.respectivley.

15 α11x 10 α11y 5 α11z ] ° 0 0 5 10 15 20 25 30 35 -5 degree [ -10 -15 -20 time [s]

Figure 21. Experimental results of a test like in Figure 20 in terms of the orientation angle of arm–leg L11 of the first arm– Figure 21. Experimental results of a test like in Figure 20 in terms of the orientation angle of arm–leg L11 of the ﬁrst arm–leg leg whit S10, S11, S12 actuators. whit S10,S11,S12 actuators.

0.7 0.6 0.5 0.4 0.3 0.2 w11x 0.1 w11y 0 w11z -0.1 0 5 10 15 20 25 30 35 angular velocity [rad/s] velocity angular -0.2 -0.3 -0.4 time [s]

Figure 22. Experimental results of a test like in Figure 20 in terms of the angular velocity of arm–leg L11 of the first arm– leg whit S10, S11, S12 actuators.

1.5 θ12x 1 θ12y

0.5 θ12z

0 0 5 10 15 20 25 30 35 -0.5

-1 angular acceleration [rad/s^2] acceleration angular

-1.5 time [s]

Figure 23. Experimental results of a test like in Figure 20 in terms of the angular acceleration of arm–leg L11 of the first arm–leg whit S10, S11, S12 actuators.

Robotics 2021, 10, x FOR PEER REVIEW 15 of 19

15 15 α11x 10 α11yα11x 10 α11y 5 α11z

] 5 α11z °

] 0 ° 0 0 5 10 15 20 25 30 35 -5 0 5 10 15 20 25 30 35 degree [ -5 degree [ -10 -10 -15 -15 -20 -20 time [s]

Robotics 2021, 10, 21 time [s] 15 of 19 Figure 21. Experimental results of a test like in Figure 20 in terms of the orientation angle of arm–leg L11 of the first arm– Figureleg whit 21. S10 Experimental, S11, S12 actuators results. of a test like in Figure 20 in terms of the orientation angle of arm–leg L11 of the first arm– leg whit S10, S11, S12 actuators. 0.7 0.60.7 0.50.6 0.40.5 0.30.4 0.20.3 w11x 0.10.2 w11yw11x 0.1 0 w11zw11y -0.10 0 5 10 15 20 25 30w11z 35 angular velocity [rad/s] velocity angular 0 5 10 15 20 25 30 35 -0.2-0.1 angular velocity [rad/s] velocity angular -0.3-0.2 -0.4-0.3 -0.4 time [s] time [s]

Figure 22. Experimental results of a test like in Figure 20 in terms of the angular velocity of arm–leg L11 of the first arm– Figure 22. Experimental results of a test like in Figure 20 in terms of the angular velocity of arm–leg L11 of the ﬁrst arm–leg Figureleg whit 22. S10 Experimental, S11, S12 actuators results. of a test like in Figure 20 in terms of the angular velocity of arm–leg L11 of the first arm– whit S10,S11,S12 actuators. leg whit S10, S11, S12 actuators. 1.5 1.5 θ12x 1 θ12yθ12x 1 θ12y 0.5 θ12z 0.5 θ12z 0 0 0 5 10 15 20 25 30 35 -0.5 0 5 10 15 20 25 30 35 -0.5 -1 angular acceleration [rad/s^2] acceleration angular -1 angular acceleration [rad/s^2] acceleration angular -1.5 time [s]

-1.5 time [s] Figure 23. Experimental results of a test like in Figure 20 in terms of the angular acceleration of arm–leg L11 of the first Figurearm–leg 23. whit Experimental S10, S11, S12 actuatorsresults of. a test like in Figure 20 in terms of the angular acceleration of arm–leg L11 of the first Figure 23. Experimental results of a test like in Figure 20 in terms of the angular acceleration of arm–leg L11 of the ﬁrst arm–leg whit S10, S11, S12 actuators. arm–leg whit S10,S11,S12 actuators.

Tables1 and2 show a summary of input and output test results. The results show that the robot can be considered feasible for practical implementation in service space tasks since the tested TORVEastro motions and actions have been shown proper performance.

Table 1. A summary of experimental results of tests in Figures 12–19 with servomotor S10 imposed motion at 10% of its maximum speed.

Link L11 L12 Ref. Fig. axis x y z x y z α range [◦] 1; 25 10; 46 −1; 12 4; 48 8; 11 −4; 6 13,16 w range [rad/s] −0.4; 0.4 −0.7; 0.8 −0.2; 0.3 −0.8; 0.8 −0.2; 0.4 −0.2; 0.2 14,17 θ range [rad/sˆ2] −0.4; 0.3 −0.7; 0.8 −0.2; 0.2 −0.8; 0.8 −0.6; 0.8 −0.2; 0.2 15,18 Robotics 2021, 10, x FOR PEER REVIEW 16 of 19 Robotics 2021, 10, x FOR PEER REVIEW 16 of 19

Tables 1 and 2 show a summary of input and output test results. The results show that theTables robot 1 and can 2be show considered a summary feasible of input for practical and output implementation test results. The in service results spaceshow tasksthat the since robot the cantested be TORVEastroconsidered feasible motions fo anr dpractical actions implementationhave been shown in proper service perfor- space mance.tasks since the tested TORVEastro motions and actions have been shown proper performance.

Table 1. A summary of experimental results of tests in Figures 12–19 with servomotor S10 imposed motion at 10% of its maximumTable 1. A speed summary. of experimental results of tests in Figures 12–19 with servomotor S10 imposed motion at 10% of its maximum speed. Link L11 L12 Ref. Fig. Link L11 L12 Ref. Fig. Robotics 2021axis, 10, 21 x y z x y z 16 of 19 α rangeaxis [°] 1; x25 10;y 46 −1;z 12 4; x48 8; y11 −4;z 6 13,16 w αrange range [rad/s] [°] −0.4;1; 25 0.4 −0.7; 10; 460.8 −0.2;−1; 120.3 −0.8; 4; 48 0.8 −0.2; 8; 11 0.4 −0.2;−4; 0.26 14,1713,16 θw range range [rad/s^2] [rad/s] −0.4; 0.30.4 −0.7; 0.8 −0.2; 0.20.3 −0.8; 0.8 −0.6;0.2; 0.80.4 −0.2; 0.2 15,1814,17 θ Tablerange 2. [rad/s^2]A summary of experimental−0.4; 0.3 results−0.7; of 0.8 tests in Figures−0.2; 200.2– 27, with−0.8; servomotors 0.8 S−100.6;,S 110.8and S12 imposed−0.2; 0.2 motion 15,18 at 6% of their maximum speed. Table 2. A summary of experimental results of tests in Figures 20–27, with servomotors S10, S11 and S12 imposed motion at Table 2. A summary of experimental results of tests in Figures 20–27, with servomotors S10, S11 and S12 imposed motion at 6% of Linktheir maximum speed. L L Ref. Fig. 6% of their maximum speed. 11 12 Linkaxis xL11 y z x yL12 z Ref. Fig. Link L11 L12 Ref. Fig. α axisrange [◦] − x15; −2 8;y 14 0;z 14 −5;x 4 10; 18y −3; 25z 21,24 wα rangerangeaxis [rad/s] [°] −15;− x0.2; −2 0.2 − 8;0.1; y14 0.1 −0.3; 0; z14 0.6 −0.4;−5;x 0.3 4 −0.3; 10; 0.4y 18 −0.3;−3; 0.8z 25 22,25 21,24 θ − − − − − − w αrangerange range [rad/sˆ2][rad/s] [°] −−0.2;15;0.6; 0.2−2 0.6 −0.1; 8;0.3; 14 0.1 0.3 −0.3; 0;1.1; 14 0.6 1 −0.5;0.4;−5; 0.5 0.34 0.4;−0.3; 10; 0.3 180.4 −1;0.3;−3; 1 250.8 23,26 22,2521,24 θw range range [rad/s^2] [rad/s] −0.6;0.2; 0.60.2 −0.3;0.1; 0.30.1 −−0.3;1.1; 0.6 1 −0.5;0.4; 0.50.3 −0.4;0.3; 0.30.4 −0.3;−1; 10.8 23,2622,25 θ range [rad/s^2] −0.6; 0.6 −0.3; 0.3 −1.1; 1 −0.5; 0.5 −0.4; 0.3 −1; 1 23,26

30 30 25 25 20 20 ]

° 15

] α12x

° 15 10 α12x 10 α12y

degree [ degree 5 α12y α12z degree [ degree 5 0 α12z 0 0 5 10 15 20 25 30 35 -5 0 5 10 15 20 25 30 35 -5 -10 -10 time [s] time [s]

Figure 24. Experimental results of a test like in Figure 20 in terms of the angular position of arm–leg L12 of the first arm– Figure 24. Experimental results of a test like in Figure 20 in terms of the angular position of arm–leg L12 of the ﬁrst arm–leg legFigure whit 24. S10 Experimental, S11, S12 actuators results. of a test like in Figure 20 in terms of the angular position of arm–leg L12 of the first arm– whit S10,S11,S12 actuators. leg whit S10, S11, S12 actuators.

1 1 0.8 0.8 w12x 0.6 w12yw12x 0.6 w12y 0.4 w12z 0.4 w12z 0.2 0.2 0 0 0 5 10 15 20 25 30 35 -0.2 0 5 10 15 20 25 30 35 -0.2 -0.4 angular velocity [rad/s] velocity angular -0.4 angular velocity [rad/s] velocity angular -0.6 -0.6 time [s] time [s]

Figure 25. Experimental results of a test like in Figure 20 in terms of the angular velocity of arm–leg L12 of the ﬁrst arm–leg whit S10,S11,S12 actuators.

Robotics 2021, 10, x FOR PEER REVIEW 17 of 19 Robotics 2021, 10, x FOR PEER REVIEW 17 of 19

Robotics 2021, 10, 21 17 of 19

Figure 25. Experimental results of a test like in Figure 20 in terms of the angular velocity of arm–leg L12 of the first arm– legFigure whit 25. S10 Experimental, S11, S12 actuators results. of a test like in Figure 20 in terms of the angular velocity of arm–leg L12 of the first arm– leg whit S10, S11, S12 actuators. 1.5 1.5 θ12x θ12y θ12z 1 θ12x θ12y θ12z 1

0.5 0.5

0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 -0.5 -0.5

angular acceleration [rad/s^2] acceleration angular -1

-1.5 -1.5 time [s] time [s]

Figure 26. Experimental results of a test like in Figure 20 in terms of the angular acceleration of arm–leg L12 of the first Figure 26. Experimental results of a test like in Figure 20 in terms of the angular acceleration of arm–leg L12 of the ﬁrst arm–legFigure 26. whit Experimental S10, S11, S12 actuators results of. a test like in Figure 20 in terms of the angular acceleration of arm–leg L12 of the first arm–leg whit S10,S11,S12 actuators. arm–leg whit S10, S11, S12 actuators.

25 25 20 20 15 15 10 10

Power [W] 5 Power [W] 5 0 0 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30 -5 time [s] time [s]

Figure 27. Experimental results of a test like in Figure 20 in terms of utilized power consumption for the first arm–leg whit SFigure10, S11, 27.S12 actuatorsExperimental. results of a test like in Figure 20 in terms of utilized power consumption for the first ﬁrst arm–leg whit S10, S11, S12 actuators. S10,S11,S12 actuators. 6. Conclusions 6. Conclusions The TORVEastro robot design is presented with experimental characterization of a 6. Conclusions lab prototypeThe TORVEastro looking atrobot its basic design performance. is presented Design with experimentalissues and solutions characterization are discussed of a tolab illustrate prototypeThe TORVEastro the looking built prototype robotat its designbasic that performance. is has presented been used withDesign for experimental performanceissues and characterizationsolutions characterization. are discussed of aThe lab builttoprototype illustrate prototype looking the isbuilt designed at prototype its basic with performance. that low-cost has b eensolutions Design used for for issues performance ground and solutionstesting characterization. to arecheck discussed the feasi- The to bilitybuiltillustrate prototypeof the the arm–leg built is prototypedesigned operation. with that Tests haslow-cost been are reported usedsolutions for performancefor for the ground basic characterization.testingoperation to checkof one the Thearm–leg feasi- built prototype is designed with low-cost solutions for ground testing to check the feasibility of limbbility when of the actuated arm–leg by operation. one and threeTests servomotorare reporteds. forResults the basic of experimental operation of tests one show arm–leg the the arm–leg operation. Tests are reported for the basic operation of one arm–leg limb when suitablelimb when capability actuated of by arm–leg one and motion three servomotor both in termss. Results of motion of experimental performance tests and show power the actuated by one and three servomotors. Results of experimental tests show the suitable consumptionsuitable capability as feasible of arm–leg for an motion astronaut both robot in terms in monitoring of motion and performance maintenance and tasks power in capability of arm–leg motion both in terms of motion performance and power consumption outdoorconsumption space as of feasible the orbital for stations.an astronaut In pa robotrticular, in monitoringprototype testing and maintenance has shown motionstasks in as feasible for an astronaut robot in monitoring and maintenance tasks in outdoor space ofoutdoor arm–leg space at an of angularthe orbital velocity stations. of 1In rad/s particular, with an prototype extremity testing point hasacceleration shown motions of 0.54 of arm–leg the2 orbital at stations.an angular In velocity particular, of prototype1 rad/s with testing an extremity has shown point motions acceleration of arm–leg of 0.54 at m/s for an estimated autonomy of 1.5 h using all nine servomotors. 2 m/san angular2 for an estimated velocity of autonomy 1 rad/s with of 1.5 an h extremityusing all nine point servomotors. acceleration of 0.54 m/s for an estimated autonomy of 1.5 h using all nine servomotors. Author Contributions: Data curation, M.C.; Formal analysis, M.C.; Methodology, M.C.; Project administration,Author Contributions: M.C.; Supervision, Data curation, M.C.; Writing—originM.C.; Formal analysis,al draft, M.C.; F.S.; Writing—reviewMethodology, M.C.; & editing, Project F.S.ad- Author Contributions: Data curation, M.C.; Formal analysis, M.C.; Methodology, M.C.; Project Allministration, authors have M.C.; read Supervision, and agreed M.C.; to th Writing—origine published versional draft, of the F.S.; manuscript. Writing—review & editing, F.S. Alladministration, authors have M.C.; read Supervision,and agreed to M.C.; the published Writing—original version draft,of the F.S.; manuscript. Writing—review & editing, F.S. All authors have read and agreed to the published version of the manuscript.

Robotics 2021, 10, 21 18 of 19

Funding: The financial support of the Italian National Project ISAF, Integrated Smart Assembly Factory is acknowledged within grant n. ARS01_01188. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Acknowledgments: Italian National Project ISAF, Integrated Smart Assembly Factory. Conflicts of Interest: The authors declare no conflict of interest.

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