Functional Design of a Hybrid Leg-Wheel-Track Ground Mobile Robot

machines

Article Functional Design of a Hybrid Leg-Wheel-Track Ground Mobile Robot

Luca Bruzzone * , Mario Baggetta, Shahab E. Nodehi , Pietro Bilancia and Pietro Fanghella

Department of Mechanical, Energy, Management and Transportation Engineering, University of Genoa, 16145 Genoa, Italy; [email protected] (M.B.); [email protected] (S.E.N.); [email protected] (P.B.); [email protected] (P.F.) * Correspondence: [email protected]; Tel.: +39-010-335-2967

Abstract: This paper presents the conceptual and functional design of a novel hybrid leg-wheel- track ground mobile robot for surveillance and inspection, named WheTLHLoc (Wheel-Track-Leg Hybrid Locomotion). The aim of the work is the development of a general-purpose platform capable of combining tracked locomotion on irregular and yielding terrains, wheeled locomotion with high energy efficiency on flat and compact grounds, and stair climbing/descent ability. The architecture of the hybrid locomotion system is firstly outlined, then the validation of its stair climbing maneuver capabilities by means of multibody simulation is presented. The embodiment design and the internal mechanical layout are then discussed.

Keywords: ground mobile robot; hybrid locomotion; step climbing

1. Introduction In the last decade, the world market of service robotics has increased remarkably, overcoming the market of industrial robotics [1]. In particular, ground mobile robots are the most widespread category of service robots, with important application fields Citation: Bruzzone, L.; Baggetta, M.; such as precision agriculture [2], planetary exploration [3], homeland security [4], surveil- Nodehi, S.E.; Bilancia, P.; Fanghella, P. lance [5], reconnaissance, and intervention in case of terroristic attacks and in the presence Functional Design of a Hybrid of radioactive or chemical contamination [6]. Leg-Wheel-Track Ground Mobile Independently of the robot payload, which depends on the specific application, a Robot. Machines 2021, 9, 10. https:// great research effort has been spent to develop innovative locomotion systems for ground doi.org/10.3390/machines9010010 mobile robots [7–9]. While locomotion on flat and even terrain can be simply and effectively

Received: 10 November 2020 performed by wheels (W) with high speed and energy efficiency [10], in case of unstructured Accepted: 7 January 2021 environments, the use of tracks (T) and legs (L) is also a valuable option. Published: 12 January 2021 Tracked robots can move on uneven and yielding terrains, since their contact surface with the ground is much larger than the one of wheeled robots [11–13]. On the other hand, Publisher’s Note: MDPI stays neu- they usually move slower and with lower energy efficiency. In addition, they are more tral with regard to jurisdictional clai- subject to vibrations with respect to wheeled robots. ms in published maps and institutio- Legged locomotion, which is, differently from wheeled and tracked locomotion, nal affiliations. biologically inspired, is the most effective solution in case of irregular terrains and obstacles, but it is generally slower and less energetically efficient on flat grounds; the main hindrance to the diffusion of legged robots is their cost and complexity, especially for the ones with dynamic gait [14,15]; the higher complexity of the dynamic gait is related not only Copyright: © 2021 by the authors. Li- to control, but also to the mechanical architecture, characterized by a high number of censee MDPI, Basel, Switzerland. actuators. To overcome this issue, at least for small mobile robots, with limited structural This article is an open access article distributed under the terms and con- stresses due to the inertial effects, the complexity of the leg architecture can be simplified: ditions of the Creative Commons At- Examples are robots with rotating and compliant legs [16,17], or hybrid leg–wheel robots tribution (CC BY) license (https:// with stepping triple wheels [18,19]. creativecommons.org/licenses/by/ Hybrid locomotion systems combine the benefits of different locomotion systems. 4.0/). One of the most advanced and impressive hybrid locomotion robots is the legged-wheeled

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Handle by Boston Dynamics [20], which is a biped with two wheels at the end of the legs; a similar concept has been adopted in the design of the Centauro quadruped, by IIT [21]. At a smaller scale, there are many examples of hybrid robots which combine legs, wheels, and tracks in a simplified manner, with lower cost and consequently a wider range of potential applications: the four possible combinations of L, W and T (LW, LT, WT, LWT) are compared in [8] in terms of maximum speed, obstacle crossing capability, step/stair climbing capability, walking capability of soft terrains and on uneven terrains, energy efficiency, mechanical, and control complexity. Considering locomotion solutions involving tracks (T, WT, LT, LWT), there are many possible layouts. Robots can have non-articulated or articulated tracks with passive relative mobility [22]; if the relative mobility of the articulated tracks is actuated to improve obstacle crossing capability, they can be considered as LT hybrid robots. Another option is to link tracked modules in series to compose snake-like climbing robots [23]. Focusing on LT hybrid robots, there are different ways to combine legs and tracks. Besides the most obvious approach of using them in parallel [24–26], peripheral tracks can be placed on the leg links [27]; this approach is also used for the Kylin robot, with LWT hybrid locomotion [28]. When wheels are present and it is required to use them to increase speed and range on even and compact grounds, they can be activated using many different systems. For example, the contact of the wheels with the terrain can be enabled or disabled by the position of the legs, as in the LWT Kylin robot. Another possibility is to extract the wheels from the robot body to suspend the robot on wheels. This solution has the benefit of enhancing the vehicle maneuverability, which is a drawback of purely tracked robots; an example is the hybrid WT robot presented in [29]. In a previous study [30], a WT robot with two variable-shape tracks and four wheels is proposed; the contact between wheels and terrain can be enabled or disabled by varying the shape of the tracks. The hybrid LWT Azimuth robot [31] is equipped with articulated legs with peripheral tracks and wheels; depending on the leg position, wheels, legs, or tracks can be active. The hybrid WT robot HELIOS-VI [32] has two tracks and two passive wheels placed at the end of a rotating arm, which regulates the contact of the wheels with the terrain to reduce the contact areas of the tracks, to improve maneuverability or to perform stair climbing and descent. In other cases, the tracks can be transformed into wheels changing their outer shape by means of tensioning systems, exploiting the tracks elasticity [33,34]; the transformation of tracks into wheels can also be achieved by adopting complex articulated supports for the tracks [35]. This research is focused on the development of a hybrid LWT ground mobile robot for surveillance and inspection, named WheTLHLoc (Wheel-Track-Leg Hybrid Locomotion). Its locomotion system shows similarities with previously discussed robots, although the proposed solution is new and has unique features with respect to other solutions presented in scientific literature; similarities and differences will be highlighted in Section2. The considered size is small (around 450 × 350 × 130 mm), since only cameras and environmental sensors are to be carried and good maneuverability in restricted spaces is considered a key feature of an inspection robot. Despite its limited dimensions, the combination of all the three locomotion systems allows to perform wheeled locomotion with high speed, energy efficiency, and range on flat and compact grounds, tracked locomotion on yielding and soft terrains, and mixed use of rotating legs, wheels, and tracks to overcome steps, stairs, and other obstacles. The remaining of the paper is organized as follows: Section2 discusses the conceptual design and the main functional features of the WheTLHLoc robot; Section3 illustrates the maneuvers for stair climbing and descent, and the feasibility of the most critical one (climbing) is verified by multibody simulation in Section4; Section5 presents the Machines 2021, 9, x FOR PEER REVIEW 3 of 11

maneuvers for stair climbing and descent, and the feasibility of the most critical one (climbing) is verified by multibody simulation in Section 4; Section 5 presents the embodiment design and the internal layout of the robot, and finally Section 6 outlines conclusions and future work. Machines 2021, 9, 10 3 of 11 2. Conceptual Design of the Hybrid Robot WheTLHLoc The starting point of the conceptual design of the proposed hybrid locomotion robot is the goal of combining tracks for soft and yielding terrains and wheeled locomotion on embodiment design and the internal layout of the robot, and ﬁnally Section6 outlines flat and compact terrains. This combination can be obtained in different ways, as dis- conclusions and future work. cussed in the previous section. The proposed locomotion system (Figure 1) is in some 2.ways Conceptual similar to Design the one of of the HELIOS-VI Hybrid Robot [32], but WheTLHLoc enhances its capabilities thanks to the fol- lowingThe design starting choices: point of the conceptual design of the proposed hybrid locomotion robot is• theInstead goal of of combining a single short tracks rotating for soft arm and placed yielding on terrainsthe front and of the wheeled vehicle, locomotion it makes use on ﬂat andof two compact independently terrains. This actuated combination rotating can legs be (Figure obtained 1, inL); different ways, as discussed in• theThe previous wheels section.at the leg Theends proposed (Figure 1, locomotion W) are not passive, system (Figurebut independently1) is in some actuated ways similarby togearmotors the one of (Figure HELIOS-VI 1, WM [32);], but enhances its capabilities thanks to the following design• The choices: common revolute axis of the two legs (Figure 1, A), is centered with respect to • Insteadthe robot of body, a single and short the legs rotating can perform arm placed complete on the and front continuous of the vehicle, rotations it makes around use ofit; two independently actuated rotating legs (Figure1, L); • • TheTwo wheels omni wheels at the leg(Figure ends 1, (Figure OW) are1, W placed) are not on passive,the rear of but the independently robot, so that actuated the con- bytact gearmotors of the tracks (Figure (Figure1, 1,WM T) );with the terrain is avoided when the robot is supported • Theon the common two wheels revolute and axis on one of the of twothe omni legs (Figure wheels;1, A), is centered with respect to the • robotThe main body, robot and thebody legs (Figure can perform 1, MB) completeis externally and symmetric continuous with rotations respect around to the it; xy • Twoplane. omni wheels (Figure1, OW) are placed on the rear of the robot, so that the contact ofThe the main tracks camera (Figure is 1placed, T) with on thethe terrainfront of is the avoided robot when(Figure the 1, robot FC), but is supported a secondary on camerathe is two placed wheels on andthe rear on one (Figure of the 1, omni RC) since wheels; during step climbing (Section 3) and in •otherThe scenarios main robotit is necessary body (Figure to move1, MBback)wards; is externally additional symmetric environmental with respect sensors to thecan be placedxy plane. inside the robot body, on the basis of the required tasks.

FigureFigure 1. ConceptualConceptual design design of of the the hybrid hybrid leg-wheel-track leg-wheel-track robo robott Wheel-Track-Leg Wheel-Track-Leg Hybrid Hybrid Lo Locomotioncomotion (WheTLHLoc): (WheTLHLoc): (a) (Fronta) Front view; view; (b) rear (b) rearview view (W: Wheel; (W: Wheel; L: Leg;L :T: Leg; Track;T: Track;MB: MainMB :body; Main WM: body; WheelWM :gearmotor; Wheel gearmotor; OW: OmniOW wheel;: Omni FC: wheel; Front FCcamera;: Front RC: camera; Rear camera;RC: Rear A: camera; Leg axis).A: Leg axis).

The main features camera isof placedthis design on the are front the following: of the robot (Figure1, FC), but a secondary camera is placed on the rear (Figure1, RC) since during step climbing (Section3) and in other scenarios it is necessary to move backwards; additional environmental sensors can be placed inside the robot body, on the basis of the required tasks. The main features of this design are the following: i. It is possible to alternate wheeled locomotion on ﬂat and compact terrains (Figure2a) and tracked locomotion on soft and yielding terrains (Figure2b), both with differential steering; the switch between the locomotion modes is commanded by the rotation of the legs; ii. The legs, longer than the arm of HELIOS-VI, can be used to overcome obstacles, even performing differential steering, if required in case of asymmetrical obstacles; Machines 2021, 9, x FOR PEER REVIEW 4 of 11

i. It is possible to alternate wheeled locomotion on flat and compact terrains (Figure 2a) and tracked locomotion on soft and yielding terrains (Figure 2b), both with differen- Machines 2021, 9, 10 tial steering; the switch between the locomotion modes is commanded by the rotation4 of 11 of the legs; ii. The legs, longer than the arm of HELIOS-VI, can be used to overcome obstacles, even performing differential steering, if required in case of asymmetrical obstacles; iii. iii. WhenWhen the the robot robot is is in in the the wheeled wheeled locomotion locomotion ( (WLWL)) position, position, the the field field of view of the frontfront camera camera is is optimally optimally exploited exploited (Figure (Figure 22a);a); iv. iv. SinceSince the the robot robot main main body body is symmetric is symmetric with with respect respect to the to xy the plane,xy plane, it is fully it is oper- fully ativeoperative even evenafter afteran overturn; an overturn; it is very it is veryunlikely unlikely that thatafter after a fall a fallthe therobot robot remains remains on oneon oneflank, flank, and and even even in this in this case case a rotation a rotation of of the the leg leg in in contact contact with with the the terrain terrain can putput again again the the robot robot on on the the tracks; tracks; v. v. TheThe rotation rotation of of the the legs legs and and the the action action of of the the wheels can be used in case of irregular terrainsterrains and and obstacles, obstacles, i.e., i.e., when when the the use use of of on onlyly tracks tracks is not is not sufficient sufficient to advance; to advance; in thisin this case, case, the the two two legs legs can can be be moved moved in independentlydependently in in presence presence of asymmetric groundground irregularities irregularities (Figure (Figure 22c);c); vi. vi. InIn particular, particular, thethe combinedcombined motion motion of of legs, legs wheels,, wheels, and and tracks tracks can can be usedbe used to overcome to over- comesteps steps and stairs, and stairs, according according to the to sequences the sequences discussed discussed in Section in Section3. 3.

Figure 2. ((aa)) Wheeled Wheeled locomotion locomotion on on flat ﬂat and and compact compact surfaces; surfaces; (b) ( btracked) tracked locomotion locomotion on yielding on yielding terrains; terrains; (c) hybrid (c) hybrid leg- leg-wheel-trackwheel-track locomotion locomotion to overcome to overcome obstacles. obstacles.

As previously previously mentioned, the proposed locomo locomotiontion system presents similarities with thethe one one of of HELIOS-VI, HELIOS-VI, though though it it shows shows the the fo followingllowing differences: differences: (i) (i) Independently Independently actu- ac- atedtuated and and longer longer legs, legs, which which can can perform perform complete complete and and continuous continuous rotations rotations in in case of obstacle overcoming and stair climbing; (ii) actuated wheels to perform purely wheeled locomotionlocomotion with differential steering; (iii) capabilitycapability of operating after an overturn, due to thethe main main body symmetry. Regarding thethe wheeledwheeled locomotion, locomotion, there there are are some some similarities similarities with with the the Kylin Kylin robot robot [28]: [28]:Two Two actuated actuated wheels wheels are placed are placed at the atend the en ofd rotating of rotating arms, arms, and and passive passive omni omni wheels wheels are areadopted. adopted. The The advantages advantages of the of WheTLHLocthe WheTLHLo withc with respect respect to the to Kylin the Kylin are: (i) are: The (i) possi- The possibilitybility of using of using the longer the longer legs, alsolegs, in also combination in combination with thewith tracks, the tracks, to overcome to overcome obstacles; obstacles;(ii) the capability(ii) the capability of operating of operating after an overturnafter an overturn due to the due symmetry; to the symmetry; (iii) the most (iii) the compact most compactand robust and design. robust design. The capability ofof operatingoperating afterafter and and overturn overturn is is in in common common with with the the hybrid hybrid WT WT robot ro- botdiscussed discussed in [ 29in], [29], but but in comparison in comparison to this to this one, one, the the following following beneﬁts benefits of WheTLHLoc of WheTLHLoc can canbe outlined: be outlined: (i) The (i) The capability capability of using of using legs, legs, wheels wheels and and tracks tracks in combination in combination to overcome to over- comeobstacles obstacles and climb and stairsclimb (Figurestairs (Figure2c); (ii) the2c); polygon(ii) the polygon of the contact of the points contact for points the wheeled for the locomotion is much larger, allowing higher stability during turns, acceleration and breaking, wheeled locomotion is much larger, allowing higher stability during turns, acceleration and consequently higher motion performance. and breaking, and consequently higher motion performance. 3. Step and Stair Climbing and Descent 3. Step and Stair Climbing and Descent As already stated, the overall size of the robot is quite compact, since the considered payloadAs already is represented stated, the by overall cameras size and of environmentalthe robot is quite sensors compact, for since surveillance the considered and in- payloadspection. is Moreover, represented a small by cameras robot can and travel envi inronmental narrow spaces sensors that for cannot surveillance be explored and in- by spection.humans and Moreover, is much a more small portable. robot can On travel the other in narrow hand, spaces a drawback that cannot of small be robots explored is their by capability of facing stairs. The climbing of steps and stairs can be performed according to the sequence represented in Figure3 (frames a to m). In case of single steps, the sequence a–m is performed once, while it is repeated n times in case of n steps. Machines 2021, 9, x FOR PEER REVIEW 5 of 11

humans and is much more portable. On the other hand, a drawback of small robots is their capability of facing stairs. The climbing of steps and stairs can be performed according to the sequence represented in Figure 3 (frames a to m). In case of single steps, the sequence a–m is performed once, while it is repeated n times in case of n steps. After approaching the step, the rotation of the legs lifts up the robot body (frames a– c); let us note that the robot, which is not fully symmetric with respect to the yz plane, must face the step with the rear side, otherwise in the following phase (frame d) it would not be possible to use the traction of the tracks due to the presence of the omni wheels. Therefore, if the robot approaches the stair moving forward, a 180° pivoting must be performed by differential steering before climbing the first step, imposing an equal speed to the two tracks with opposite directions. This maneuver can be used whenever it is necessary to switch direction. Once the central zone of the tracks is in contact with the step edge (frame d), the legs rotate backwards until they touch the ground behind the robot (frame e) and completely lift the robot over the step (frames f to h); in this phase, both tracks and wheels rotate, pushing the robot forward. Once the robot center of mass is sufficiently beyond the step Machines 2021, 9, 10 edge, the robot moves forward on the tracks while the legs rotate forward (frame i–m) to 5 of 11 face the next step, if present, or to complete the task.

Figure 3. Stair-climbingFigure 3. Stair-climbing sequence: thesequence: legs lift the up legs the lift robot up the body robot (a –bodyc), then (a–c rotate), then backwardrotate backward to complete to the step climbing (d–hcomplete); then the the robot step goes climbing forward (d–h by); tracksthen the (i– robotl) to face goes the forward next step by (tracksm,n). (i–l) to face the next step (m–n). After approaching the step, the rotation of the legs lifts up the robot body (frames a–c); The successfullet us noteexecution that theof this robot, sequence which is is not not assured fully symmetricin any dynamic with and respect friction to the yz plane, condition, sincemust the face friction the stepof tracks with and the rearwheels side, with otherwise the ground in the is followingnecessary phaseto complete (frame d) it would the maneuvernot in the be possiblephase corresponding to use the traction to the offrames the tracks d to g; due therefore, to the the presence motion of strat- the omni wheels. ◦ egy must be plannedTherefore, and if verified the robot by approachesanalytical approach the stair or moving numerically. forward, In Section a 180 4,pivoting the must be multibody simulationperformed of by the differential stair climbing steering maneuver before is climbing discussed. the first step, imposing an equal speed The stairto descent the two can tracks be executed with opposite inverting directions. the stair climbing This maneuver sequence, can as be shown used in whenever it is Figure 4. In thenecessary descent, to the switch successful direction. execution of the maneuver is less critical, since it is not necessary to overcomeOnce the the central gravity zone force of the by tracksmeans is of in the contact traction with of thewheels step and edge tracks. (frame d), the legs rotate backwards until they touch the ground behind the robot (frame e) and completely lift the robot over the step (frames f to h); in this phase, both tracks and wheels rotate, pushing the robot forward. Once the robot center of mass is sufficiently beyond the step edge, the robot moves forward on the tracks while the legs rotate forward (frame i–m) to face the next step, if present, or to complete the task. The successful execution of this sequence is not assured in any dynamic and friction condition, since the friction of tracks and wheels with the ground is necessary to complete the maneuver in the phase corresponding to the frames d to g; therefore, the motion strategy must be planned and verified by analytical approach or numerically. In Section4, the multibody simulation of the stair climbing maneuver is discussed. The stair descent can be executed inverting the stair climbing sequence, as shown in Figure4. In the descent, the successful execution of the maneuver is less critical, since it is not necessary to overcome the gravity force by means of the traction of wheels and tracks. Machines 2021, 9, x FOR PEER REVIEW 6 of 11

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Figure 4. Stair-descent sequence: the robot approaches the descent by tracks (a–b) and uses the Figure 4. Stair-descentlegs to come sequence: down the gradually robot approaches without shocks the descent until bythetracks tracks ( atouch,b) and the uses next the step legs (c to–f); come then downthe gradually Figurelegs 4.rotate Stair-descent backwards sequence: (g) to complete the robot the approaches step descent the ( hdescent–i). by tracks (a–b) and uses the without shockslegs until to come thetracks down touch gradually the next without step( shocksc–f); then until the the legs tracks rotate touch backwards the next (g) step to complete (c–f); then the the step descent (h,i). legs rotate backwards (g) to complete the step descent (h–i). 4. Multibody4. Simulation Multibody Simulationof Stair Climbing of Stair Climbing 4. MultibodyThe feasibility SimulationThe of feasibility the of stair Stair climbing of Climbing the stair maneuv climbinger discussed maneuver in discussed Section 3 inhas Section been checked3 has been checked with a dynamic model implemented in Recurdyn, a multibody software commonly used The feasibilitywith of a dynamic the stair climbing model implemented maneuver discussed in Recurdyn, in Section a multibody 3 has been software checked commonly used to simulate complex mechanical systems involving a large number of components and to with a dynamicto model simulate implemented complex mechanical in Recurdyn, systems a multibody involving software a large commonly number of used components and perform efficientto perform contact efficient analysis contact by means analysis of penalty by means algorithms of penalty [36]. In algorithms particular, [36 these]. In particular, to simulate complex mechanical systems involving a large number of components and to features canthese be exploited features to can simulate be exploited the co tomplex simulate dynamic the complex behavior dynamic of a tracked behavior system, of a tracked sys- perform efficient contact analysis by means of penalty algorithms [36]. In particular, these composed oftem, a high composed number of of a bodies high number (sprocke ofts, bodies track (sprockets,links, and rollers) track links, in contact and rollers) with in contact features can be exploited to simulate the complex dynamic behavior of a tracked system, each other andwith with each the other ground and with in configurat the groundions, in which configurations, are highly which variable are highlyduring variable the during composed of a high number of bodies (sprockets, track links, and rollers) in contact with vehicle motitheon vehicle[37]. motion [37]. each other and with the ground in configurations, which are highly variable during the Figures 5–7 Figuresrepresent5– 7simulation represent results simulation related results to a stair related with to 300 a stair mm withof run 300 and mm 160 of run and vehicle motion [37]. mm of rise 160(standard mm of risesize (standardfor buildings). size for Figure buildings). 5 shows Figure the 5angular shows theposition angular vs. position time vs. time Figures 5–7 represent simulation results related to a stair with 300 mm of run and 160 graph for wheelsgraph (red), for wheels tracks (red),(black) tracks and legs (black) (green). and These legs (green). motion laws These have motion been laws de- have been mm of rise (standard size for buildings). Figure 5 shows the angular position vs. time signed to realizedesigned the climbing to realize sequence the climbing of Figure sequence 3 and are of Figureimposed3 and to the are actuators imposed in to the the actuators graph for wheels (red), tracks (black) and legs (green). These motion laws have been de- simulation. inThe the laws simulation. are characterized The laws by are co characterizednstant speed by phases constant and speedstop phases. phases Since and stop phases. signed to realize the climbing sequence of Figure 3 and are imposed to the actuators in the instantaneousSince speed instantaneous variations are speed not variations feasible (they arenot would feasible require (they infinite would acceleration), require infinite accelera- simulation. The laws are characterized by constant speed phases and stop phases. Since constant speedtion), phases constant and speed stop phases phases have and stopbeen phaseslinked haveby cubic been splines linked to by obtain cubic twice splines to obtain instantaneous speed variations are not feasible (they would require infinite acceleration), continuouslytwice differentiable continuously motion differentiable laws without motion acceleration laws without peaks. acceleration peaks. constant speed phases and stop phases have been linked by cubic splines to obtain twice continuously differentiable motion laws without acceleration peaks.

Figure 5. Motion laws of tracks, wheels, and legs (angular positions vs. time) imposed in the multibody simulation to Figure 5. Motion laws of tracks, wheels, and legs (angular positions vs. time) imposed in the realize the stairmultibody climbing sequencesimulation of to Figure realize3; stop the stair phases climbing and constant sequence speed of Figure phases 3; are stop linked phases by cubicand constant splines to avoid Figure 5. Motion laws of tracks, wheels, and legs (angular positions vs. time) imposed in the acceleration peaks.speed phases are linked by cubic splines to avoid acceleration peaks. multibody simulation to realize the stair climbing sequence of Figure 3; stop phases and constant speed phases are linked by cubic splines to avoid acceleration peaks.

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Figure 6. Multibody simulation results of the stair climbing sequence of Figure 3: Horizontal (x) and vertical (z) displacements of the reference frame origin of the robot main body (Figure 1) and pitch angle θ of the robot main body.

Figure 6 represents the horizontal and vertical displacements of the robot reference Figure 6. MultibodyFigureframe 6. originMultibody simulation (Figure simulation results 1) with of the results respect stair of climbing the to staira fix sequence climbinged reference of sequence Figure frame3 :of Horizontal Figurelocated 3: atHorizontal ( xthe) and lower vertical (x )edge (z) displacements of theandof reference verticalthe first frame (z step) displacements origin (Figure of the7), of robottogether the mainreference with body framethe (Figure robot origin1) pitch and of pitchthe angle robot angle θ main. θ of body the robot (Figure main 1) and body. pitch angle θ of the robot main body.

Figure 6 represents the horizontal and vertical displacements of the robot reference frame origin (Figure 1) with respect to a fixed reference frame located at the lower edge of the first step (Figure 7), together with the robot pitch angle θ.

Figure 7. MultibodyFigure 7. simulation Multibody results simulation of the results stair climbing of the stair sequence climbing of Figure sequence3: Trajectory of Figure of 3: reference Trajectory frame of origin of the robot main bodyreference (Figure frame1). origin of the robot main body (Figure 1).

Figure 7 showsFigure the trajectory6 represents of the the robot horizontal body frame and vertical origin with displacements respect to ofthe the fixed robot reference reference frameframe and origin six significant (Figure1) positions with respect of th toe aclimbing fixed reference process: frame c, e, h, located m, n, ate’. theThe lower edge of Figuresymbols 7. Multibody of suchthe simulation firstsix positions step (Figure results correspond 7of), the together stair to climbing the with frame the sequence robot symbols pitch of Figure of angle the 3: stairTrajectoryθ. climbing of se- referencequence frameof Figure origin 3.Figure ofThe the position robot7 shows main e’ the isbody similar trajectory (Figure to e of1). after the robotclimbing body one frame step, origin so the with cycle respect e–h– to the fixed m–n–e’ can bereference repeated frame to climb and all six the significant stair. The positionstime values of corresponding the climbing process: to these c,six e, h, m, n, e’. Figure 7 shows the trajectory of the robot body frame origin with respect to the fixed positions are markedThe symbols in the of graphs such sixof Figure positionss 5 and correspond 6. The simulations to the frame have symbols validated of the the stair climbing reference frame and six significant positions of the climbing process: c, e, h, m, n, e’. The feasibility of thesequence maneuver of Figure and can3. Thebe used position to tune e’ is the similar climbing to e strategyafter climbing as a function one step, of so the cycle symbols of such six positions correspond to the frame symbols of the stair climbing se- the step geometry.e–h–m–n–e’ can be repeated to climb all the stair. The time values corresponding to these quence of Figuresix 3. positions The position are marked e’ is similar in the to graphse after ofclimbing Figures one5 and step,6. The so the simulations cycle e–h– have validated m–n–e’5. Internal can be Layout repeatedthe feasibility and toEmbodiment climb of the all maneuver the Design stair. andThe cantime be values used tocorresponding tune the climbing to these strategy six as a function positions are markedof the stepin the geometry. graphs of Figures 5 and 6. The simulations have validated the In this section, the embodiment design of the robot is discussed. Figure 8 shows the feasibility of the maneuver and can be used to tune the climbing strategy as a function of internal layout of the robot body; the labels of the main components are listed in Table 1. the step geometry.5. Internal Layout and Embodiment Design In this section, the embodiment design of the robot is discussed. Figure8 shows the 5. Internal Layoutinternal and Embodiment layout of the robotDesign body; the labels of the main components are listed in Table1. In this section, the embodiment design of the robot is discussed. Figure 8 shows the internal layout of the robot body; the labels of the main components are listed in Table 1.

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Figure 8. InternalFigure layout 8. Internal of the layout robot: of ( athe) Section robot: of(a) the Section main of body the main along body the xz alongplane; the ( bxz) upperplane; view(b) upper (see the list of the main componentsview (see in Table the list1). of the main components in Table 1).

The upper and lower internalTable 1. platesMain robot(UP components.and LP) constitute the structural frame of the robot, on which the supports of the transmission components (gearmotors, axes, bear- Component Symbol in Figure8 Characteristics ings, and gears of tracks and wheels) are fixed. The plates support also the other devices Outerhosted Case in the robot body: Controller, OC battery, PWM drives, 3D and printed video kevlar-reinforced cameras. Free nylonspace Upper structural plate UP 3D printed micro-carbon-fiber-filled nylon Lower structuralis available plate in the front and rear LP sides to place additional 3D components printed micro-carbon-fiber-filled (e.g., environmental nylon Leg gearmotorsensors, extra batteries) based LM on the required robot task. Micromotors The structural RH-158-2s-12V, components 26 rpm Track gearmotor(plates and supports) are 3DTM printed in micro carbon fiber Micromotors filled nylon, RH-158-2s-12V, while the 33upper rpm Wheel gearmotorand lower external cases are WM3D printed in Kevlar-reinforced nylon EMG 30,for12v, higher 170 rpmimpact re- Motor driversistance. (PWM) PD L298N Dual H-Bridge motor controller Leg axis LA Stainless steel tube (∅10 mm, 1 mm wall) Track axis TA High carbon steel tube (C45, ∅6 mm) Table 1. Main robot components. Leg gear LG 20:14, M2 Track gearComponent Symbol TG in Figure 8 Characteristics 18:12, M1.5 Omni wheels groupOuter Case OW OC 3D printed Rotacasterkevlar-reinforced 95A-50 mmnylon Controller C National Instruments MyRio 1900 Upper structural plate UP 3D printed micro-carbon-fiber-filled nylon Battery B HRB 50C-4S-RC-LiPo, 14.8V, 2200 mAh Lower structural plate LP 3D printed micro-carbon-fiber-filled nylon Front camera FC Yosoo OV7670-300KP-VGA Rear cameraLeg gearmotor RC LM Micromotors Yosoo RH-158-2s-12V, OV7670-300KP-VGA 26 rpm Slip ringTrack gearmotor SR TM Micromotors 12 mm-12wires RH-158-2s-12V, miniature 33 sliprpm ring Wheel gearmotor WM EMG 30, 12v, 170 rpm Motor driver (PWM)The upper and lower PD internal plates L298N ( UPDualand H-BridgeLP) constitute motor controller the structural frame of ∅ Leg axisthe robot, on which the LA supports of the Stainless transmission steel tube components( 10 mm, 1 mm (gearmotors, wall) axes, bear- ∅ Track ings,axis and gears of tracks TA and wheels) Highare fixed. carbon The steel plates tube (C45, support 6 mm) also the other devices Leg gearhosted in the robot body: LG Controller, battery, PWM 20:14, drives, M2 and video cameras. Free space Track gearis available in the front TG and rear sides to place additional 18:12, M1.5 components (e.g., environmen- Omni wheelstal group sensors, extra batteries) OW based on the required Rotacaster robot 95A-50 task. mm The structural components Controller(plates and supports) are C 3D printed in micro National carbon Instruments fiber filled MyRio nylon, 1900 while the upper and Battery B HRB 50C-4S-RC-LiPo, 14.8V, 2200 mAh lower external cases are 3D printed in Kevlar-reinforced nylon for higher impact resistance. Front camera FC Yosoo OV7670-300KP-VGA The wheel gearmotors (Figure8, WM) are directly connected to the wheels, and the Rear camera RC Yosoo OV7670-300KP-VGA necessary power supply and encoder signals are transmitted by slip rings (Figure8, SR), Slip ring SR 12 mm-12wires miniature slip ring allowing continuous rotation, if necessary. The MyRio-1900 board by National Instruments (USA) has been selected as the control system, since it can properly manage the motion The wheel gearmotors (Figure 8, WM) are directly connected to the wheels, and the control of the six axes with encoders (two legs, two tracks, and two wheels), the video necessary power supply and encoder signals are transmitted by slip rings (Figure 8, SR),

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camera signals, and the WiFi connection with a host computer; moreover, it is equipped with a three-axis accelerometer to acquire data regarding the robot motion, and can manage an Inertial Measurement Unit.

6. Conclusions and Future Work In this paper, the conceptual design of a novel small-scale leg-wheel-track hybrid locomotion robot, named WheTLHLoc, is presented. A comparison with respect to other hybrid mobile robots is outlined in Section2, highlighting similarities and differences. The mechanical architecture of WheTLHLoc has been designed to combine the advantages of tracked locomotion on soft and yielding terrains and the ones of wheeled locomotion on flat and compact grounds. The switch between these two modes is realized by means of rotating legs carrying driving wheels. The robot is capable of performing step and stair climbing and descent by moving in proper combination legs, wheels, and tracks. The effectiveness of the climbing maneuver is not obvious due to the limited dimension of the robot with respect to step height. Therefore, the feasibility has been verified by numerical simulation using the multibody software Recurdyn. Variations of this maneuver can be used to overcome obstacles with different shape, not squared, using the same principle of lifting the side of the robot close to the obstacle by the legs, then advance using the tracks, and finally completing the robot lifting by rotating backwards the legs. The detailed embodiment design has been completed, as discussed in Section5. In the following of the research, the first prototype of the robot will be realized for an experimental validation of the proposed hybrid locomotion system and its test in real surveillance and inspection tasks.

Author Contributions: L.B. conceived the locomotion system and the robot architecture; L.B., S.E.N., P.B., and M.B. developed the detailed embodiment design; P.F. supervised the scientific methodology for the functional design and the multibody simulations; M.B. performed the multibody simulations; all authors have prepared the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest.

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