Article Scube—Concept and Implementation of a Self-balancing, Autonomous Mobility Device for Personal

Manfred Klöppel 1,2,* , Felix Römer 1,2 , Michael Wittmann 2 , Bijan Hatam 2, Thomas Herrmann 2, Lee Leng Sim 3, Jun Siang Douglas Lim 3, Yunfan Lu 3, Vladimir Medovy 2, Lukas Merkle 2 , Wy Xin Richmond Ten 3, Aybike Ongel 1 , Yan Jack Jeffrey Hong 3, Heong Wah Ng 3 and Markus Lienkamp 2

1 TUMCREATE, 1 CREATE Way, #10-02, CREATE Tower, Singapore 138602, Singapore; [email protected] (F.R.); [email protected] (A.O.) 2 Technical University Munich, Boltzmannstraße 15, 85748 Garching b. München, Germany; [email protected] (M.W.); [email protected] (B.H.); [email protected] (T.H.); [email protected] (V.M.); [email protected] (L.M.); [email protected] (M.L.) 3 Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore; [email protected] (L.L.S.); [email protected] (J.S.D.L.); [email protected] (Y.L.); [email protected] (W.X.R.T.); [email protected] (Y.J.J.H.); [email protected] (H.W.N.) * Correspondence: [email protected]; Tel.: +49-89-289-10332

 Received: 19 September 2018; Accepted: 20 November 2018; Published: 5 December 2018 

Abstract: Public transportation (PT) systems suffer from disutility compared to private transportation due to the inability to provide passengers with a door-to-door service, referred to as the first/last mile problem. Personal mobility devices (PMDs) are thought to improve PT service quality by closing this first/last mile gap. However, current PMDs are generally driven manually by the rider and require a learning phase for safe operation. Additionally, most PMDs require a standing riding position and are not easily accessible to elderly people or persons with disabilities. In this paper, the concept of an autonomously operating mobility device is introduced. The visionary concept is designed as an on-demand transportation service which people for short to medium distances and increases the accessibility to public transport. The device is envisioned to be operated as a larger fleet and does not belong to an individual person. The vehicle features an electric powertrain and a one-axle self-balancing design with a small footprint. It provides one seat for a passenger and a tilt mechanism that is designed to improve the ride comfort and safety at horizontal curves. An affordable 3D-camera system is used for autonomous localization and navigation. For the evaluation and demonstration of the concept, a functional prototype is implemented.

Keywords: autonomous vehicle; first/last mile; public transport; electric personal mobility device; on-demand; prototyping; localisation; 3D-camera navigation

1. Introduction

1.1. Motivation and State of Art Continuing urbanisation and the world’s growing population pose challenges to the provision of public services, economic opportunity, and maintenance of liveable spaces, particularly for megacities. In 2016, 25 megacities with a population of over 7 million people were housing 453 million people [1]. According to the United Nations [1], the proportion of the world’s population living in urban areas is expected to increase to 66% by 2050, while the number of megacities is projected to reach 42 by

World Journal 2018, 9, 48; doi:10.3390/wevj9040048 www.mdpi.com/journal/wevj World Electric Vehicle Journal 2018, 9, 48 2 of 15 World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 2 of 15

2030.reach One 42 by of 2030. the main One challenges of the main of challenges increasing urbanisationof increasing isurbanisation traffic congestion is traffic due congestion to the increased due to demandthe increased for transportation. demand for transportation. Policies promoting Policies a modal prom shiftoting from a modal private shift from to publicprivate transportation cars to public (PT)transportation have been (PT) a key have area been of interest a key area not onlyof interest to reduce not only externalities to reduce that externalities are caused that by theare caused use of privateby the use cars, of however private cars, also however to reduce also the to land reduce that isthe devoted land that to is private devoted cars. to private However, cars. PT However, systems oftenPT systems suffer fromoften the suffer disutility from ofthe not disutility being able of not to deliverbeing able users to all deliver the way users from all their the way point from of origin their topoint their of destination. origin to their According destination. to Polzin According [2], people to Polzin do not [2], want people to transfer do not modes want to unless transfer necessary: modes theunless out-of-vehicle necessary: timethe out-of-vehicle spent transferring time isspent perceived transferring two or is more perceived times moretwo or unfavourably more times thanmore in-vehicleunfavourably travel than time. in-vehicle Therefore, travel this first/lasttime. Therefor mile probleme, this first/last deters transit mile useproblem even thoughdeters transit a high use PT serviceeven though quality a ishigh provided PT service for the quality majority is pr ofovided the ride for distance.the majority of the ride distance. PersonalPersonal mobilitymobility devicesdevices (PMDs)(PMDs) whichwhich havehave recentlyrecently enteredentered thethe roadroad transporttransport systemsystem cancan provideprovide aa solution solution to to first/last first/last milemile problemsproblems whenwhen coupledcoupled withwith PTPT services.services. However,However, theirtheirusage usage potentialpotential hashas beenbeen limited by their their safety safety and and ease ease of of use. use. Most Most of ofthem them require require learning learning effort effort of the of therider rider and and require require a standing a standing riding riding position position which which makes makes riding riding difficult difficult for elderly for elderly people people or people or peoplewith disabilities. with disabilities. And even And eventhough though most most PMDs PMDs can canbe befolded, folded, carrying carrying the the device device on on board ofof aa crowdedcrowded busbus oror traintrain cancan bebe difficult.difficult. PMDsPMDs withwith seatsseats areare alsoalso available,available, howeverhowever riderrider comfortcomfort isis affectedaffected byby roadroad inclinations.inclinations. PMDsPMDs withwith aa standingstanding ridingriding positionposition allowallow thethe riderrider toto leanlean intointo thethe curvecurve to to compensate compensate for for the the occurring occurring centrifugal centrifugal forces, forces, while while PMDs PMDs with with a seat a need seat toneed reduce to reduce speed duringspeed during turns to turns avoid to discomfort avoid discomfort and the and toppling the topplin of theg vehicle. of the vehicle. Existing Existing tilt mechanisms tilt mechanisms to reduce to occurringreduce occurring forces from forces vehicle from movement vehicle movement only react afteronly an react acceleration after an isacceleration sensed bythe is sensed system whichby the resultssystem in which a latency results between in a latency the occurrence between of the the occu forcesrrence and of the the counter-measures forces and the counter-measures of the system [3–6 of]. Currently,the system most [3–6] PMDs. Currently, are controlled most manuallyPMDs are by thecontrolled rider, however manually the introductionby the rider, of however autonomous the capabilitiesintroduction could of autonomous increase rider capabilities comfort and coul drivingd increase safety. rider comfort and driving safety. PMDsPMDs are are available available in differentin different form form factors factor ands technical and technical implementations. implementations. The Segway The PersonalSegway TransporterPersonal Transporter is an example is an of example a PMD onof a which PMD the on riderwhich is the standing rider is while standing riding. while Figure riding.1a shows Figure the 1a Segwayshows the i2 whichSegway was i2 which introduced was introduced in 2006 [7]. in The 2006 Segway [7]. The weighs Segway 47.3 weighs kg and 47.3 allows kg and people allows to people travel attoa travel speed at of a upspeed to 20 of km/hup to 20 and km/h with and a range with ofa range up to of 39 up km to [ 839]. km Figure [8].1 Figureb pictures 1b pictures the Scewo the Bro,Scewo a self-balancing,Bro, a self-balancing, electric electric wheelchair which provides which prov a seatides for a oneseat person for one and person is able and to climbis able stairs to climb [9]. Thestairs vehicle [9]. The weighs vehicle 120 weighs kg and 120 travels kg and at 10 travels km/h at with 10 km/h a range with of a 25 range km. of 25 km. TheThe AirwheelAirwheel X8X8 isis shownshown inin FigureFigure1 c.1c. It It is is an an electric electric unicycle where where the the rider rider stands stands in in an an uprightupright positionposition andand controlscontrols thethe speedspeed andand directiondirection byby thethe leaningleaning ofof thethe body.body. FigureFigure1 d1d shows shows thethe ZoomZoom Stryder Stryder EX EX electric electric scooter which which is is foldable. foldable. BothBoth thethe AirwheelAirwheel X8X8 unicycleunicycle andand thethe ZoomZoom StryderStryder EX EX electric electric scooter scooter are are targeted targeted for for daily daily commuters commuters who who use use the devicesthe devices to cover to cover the last-mile the last- betweenmile between their hometheir andhome the and station the andstation take and the devicestake the with devices them with onto them onto and trains . and Therefore, buses. theyTherefore, are lightweight they are (11.1lightweight kg; 10.7 (11.1 kg) while kg; 10.7 the Airwheelkg) while X8the offers Airwheel a travel X8 speed offers of a 18travel km/h speed with of a range18 km/h of 23with km a [ 10range], and of the23 km Zoom [10], Stryder and the EX Zoom offering Stryder a travel EX speed offering of 30a travel km/h speed and an of electric 30 km/h range and ofan approximately electric range of 30 approximately km [11]. 30 km [11].

FigureFigure 1.1.Examples Examples ofof variousvarious PMDsPMDs (Personal(Personal mobilitymobility devices):devices): ((aa)) SegwaySegway i2i2 PersonalPersonal Transporter;Transporter; (b(b)) Scewo Scewo Bro Bro [ 12[12];]; ( c(c)) Airwheel Airwheel X8; X8; ( d(d)) Zoom Zoom Stryder Stryder EX EX [ 11[11].].

AnAn example example of of an an autonomous autonomous personal personal mobility mobility device device is presentedis presented in [ 13in ].[13]. The The base base platform platform for theirfor their scooter scooter is the S19is the from S19 Heartway from Heartway Medical whichMedical can which carry onecan seated carry passenger.one seated The passenger. autonomous The autonomous scooter uses two SICK LMS 151 LIDAR devices for navigation and obstacle recognition. The vehicle pose is estimated by the adoption of a Monte Carlo Localization (MCL) scheme after an World Electric Vehicle Journal 2018, 9, 48 3 of 15 scooter uses two SICK LMS 151 LIDAR devices for navigation and obstacle recognition. The vehicle pose is estimated by the adoption of a Monte Carlo Localization (MCL) scheme after an initial manual mapping process of the environment [13]. LIDAR (light detection and ranging) systems offer a high resolution and long view range, however they require high computational power and are still expensive.

1.2. Problem Statement and Concept Description Based on the state of the art, different problems of current PMDs are identified. Available PMDs require manual control by the driver and, thus, require learning before the safe operation of the PMD is possible. Further, some PMD concepts require active body control and balancing of the user, e.g. the Airwheel X8, which makes control of the PMD more difficult and prevents the use by elderly or disabled persons. PMDs are already used for the first/last mile to reach PT, however bringing a PMD into a or during peak hours is inconvenient due to the limited space, especially on days with precipitation when the PMD is wet and dirty. Larger sized PMDs offer space for seating, however they are even more inconvenient to bring into crowded PT systems. Further, while a seated position offers increased comfort, the passenger needs to shift their weight during turns to compensate for lateral forces. Current solutions which have been developed to counter lateral forces during turns can only react after sensing a force, reducing the effectiveness of the force compensation. To address the identified problems, we present an autonomous on-demand mobility device as a first/last mile solution which is developed to augment and improve the accessibility of existing public transport systems. The device does not belong to an individual person, however it is operated in a larger fleet. Passengers request a trip via a mobile application by entering their current location and the desired destination and then wait for pickup, omitting the need to carry the device on board of PT. The vehicle—dubbed Scube—is electrically driven, steers and navigates autonomously, and requires no control input by the passenger. A 3D-camera system in conjunction with local odometry is used for localisation to enable indoor-navigation. A camera-based system is used rather than a LIDAR device to reduce the costs. The vehicle features a seat for one person with no restraint system (e.g. seatbelt) for quick and easy accessibility. In order for the passengers to feel safe and comfortable during a ride on the device, a tilt-mechanism is developed to provide an active vehicle and passenger stabilisation. Contrary to state of the art solutions, the autonomous vehicle that is proposed in this paper determines the upcoming route itself and, therefore, can calculate the required tilt angles proactively. Further, the tilting allows the passenger to anticipate the path of the vehicle. A self-balancing vehicle design is chosen to maintain an upright sitting position for the passenger irrespective of the inclination of the to increase rider comfort as well as to reduce the size and footprint of the vehicle.

2. Methodology After the introduction of the vehicle concept, a prototype of the vehicle was built. The primary purpose of the prototype was to demonstrate the general functionality of the concept. This section describes the design of the vehicle structure and tilt-mechanism of the computing system and the localisation system.

2.1. Ergonomic Design The dimensions of humans of the German population [14] and a study about the anthropometry of the Singaporean population [15] were taken into account for the ergonomic design of the vehicle. The respective body dimensions of a 5th-percentile Singaporean female and the 95th-percentile German male are shown in Figure2 and were used to calculate the dimensions of the vehicle. The vehicle dimensions were further validated using the virtual manikins provided by the CATIA CAD software. The seating geometry is fixed and cannot be changed by the passenger. World Electric Vehicle Journal 2018, 9, 48 4 of 15 World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 4 of 15

Measurements in cm Percentile Singapore German Male Female 95% 5% 13. Distance elbow—seat 19.0 28.5 surface 14. Length of lower leg 38.0 49.0 16. Seat depth 42.0 54.0 21. Hip width sitting 32.0 42.0

FigureFigure 2. 2. BodyBody measurements measurements according according to to reference reference [14,15]. [14,15].

2.1.1.2.1.1. Vehicle Vehicle Geometry Geometry AfterAfter the the dimensions dimensions of of the the ergonomics ergonomics of of the the passenger passenger seat seat are are defined, defined, the the position position of of the the wheelwheel and thethe ground ground clearance clearance were were set. Theset. deviceThe device tilts forward tilts andforward backwards and backwards during longitudinal during longitudinalacceleration acceleration and deceleration. and deceleration. According Accordin to experimentsg to experiments conducted conducted by Hatam by et Hatam al. [16 et], al. a slight[16], abackward slight backward tilt is more tilt comfortable is more comfortable for the passenger for the than passenger a forward than tilt fora forward travel at tilt a continuous for travel speed.at a continuousTherefore, thespeed. Therefore, is placed the directly wheel underis placed the di centrerectly of under gravity the (CG) centre of of the gravity passenger (CG) with of the the passengerlargest size with (95th-percentile the largest size German (95th-percentile male) sitting German on the vehicle.male) sitting For smaller on the passengers, vehicle. For the smaller device passengers,consequently the tilts device backwards consequently as the CG tilts of backwards smaller sized as the people CG wouldof smaller move sized to the people front would of the vehicle.move to the Thefront of the vehicle. can be retracted to lower the vehicle for comfortable mounting by the passenger. WhenThe the wheels wheels can are be completely retracted to retracted, lower the the vehicl devicee for rests comfortable on the flat mounting bottom surface by the for passenger. increased Whenstability. the Accordingwheels are tocompletely DIN 18040-1 retracted, [17], ramps the device in public rests spaces on the should flat bottom have surface a maximum for increased gradient ◦ stability.of 6 per According cent or 3.43 to. DIN The Singaporean18040-1 [17], ramps Code onin public Accessibility spaces inshould the Built have Environment a maximum gradient [18] states of a ◦ 6maximum per cent or gradient 3.43°. The of 8.3 Singaporean per cent or 4.75Codefor on rampsAccessibility with a verticalin the Built rise overEnvironment 200 mm. Therefore,[18] states the a ◦ maximummaximum gradient gradient of for 8.3 the per vehicle cent or is 4.75° set to for 5 .ramps The ground with a clearancevertical rise is setover to 200 70 mmmm. which Therefore, results the in ◦ maximuman approach gradient angle offor approximately the vehicle is 11set towhich 5°. The provides ground sufficient clearance clearance, is set to even70 mm during which acceleration. results in ◦ anDue approach to the short angle rear of overhang, approximately a departure 11° anglewhich of provides approximately sufficient 24 isclearance, achieved whicheven during enables acceleration.high decelerations. Due to the short rear overhang, a departure angle of approximately 24° is achieved which enables high decelerations. 2.1.2. Suspension/Tilt Mechanism 2.1.2. Suspension/TiltThe tilt mechanism Mechanism of the vehicle is integrated into the suspension, as shown in Figure3. Two independent linear actuators allow each wheel to be retracted and extended vertically. Each The tilt mechanism of the vehicle is integrated into the suspension, as shown in Figure 3. Two electric wheel hub motor is fixed to a mounting plate which is connected to the linear actuator. The independent linear actuators allow each wheel to be retracted and extended vertically. Each electric mounting plates are attached to the vehicle chassis with four slide bearings which are guided by two wheel hub motor is fixed to a mounting plate which is connected to the linear actuator. The mounting guide rails. plates are attached to the vehicle chassis with four slide bearings which are guided by two guide rails. For the dimensioning of the actuators, the abstract model of the vehicle that is shown in Figure4 For the dimensioning of the actuators, the abstract model of the vehicle that is shown in Figure was used. For simplification, the mass of the vehicle and the passenger were summarised as a point 4 was used. For simplification, the mass of the vehicle and the passenger were summarised as a point mass m in the CG. The device has a track width t and is tilted by the angle Φ to the vertical axis. mass m in the CG. The device has a track width t and is tilted by the angle Φ to the vertical axis. To To tilt the device, both actuators move the same travel ∆st/2 in opposite directions. In this case, the tilt the device, both actuators move the same travel Δ𝑠⁄2 in opposite directions. In this case, the instantaneous centre of rotation M is at street level, centric between the wheels. The constant distance instantaneous centre of rotation M is at street level, centric between the wheels. The constant distance between the centre of rotation M and the CG is hCG. To fulfil the requirement that the passenger is free between the centre of rotation M and the CG is hCG. To fulfil the requirement that the passenger is free of lateral forces, the resultant force FR needs to point towards the centre of rotation M and is calculated of lateral forces, the resultant force FR needs to point towards the centre of rotation M and is calculated by the superposition of the centripetal force FC and the weight force FG. by the superposition of the centripetal force 𝐹 and the weight force 𝐹. World Electric Vehicle Journal 2018, 9, 48 5 of 15 World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 5 of 15

World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 5 of 15

FigureFigure 3. 3. DisplayDisplay of of the the suspension suspension and and actuator system.

The static force FS,t perFigure side 3. isDisplay calculated of theas: suspension and actuator system. The static force 𝐹, per side is calculated as: → → → The static force 𝐹 per side is calculated as:𝐹 ⃗ 𝐹⃗ 𝐹⃗ , 𝐹 FG + FC FR (1) Fs,t ,= 2 = 2 (1) 𝐹2⃗ 𝐹⃗ 2𝐹⃗ (1) The dynamic forces 𝐹, were calculated𝐹, according to Equation 2 with the angular acceleration The dynamic forces Facc,t were calculated according2 to Equation2 (2) with the angular acceleration Φ.. and the inertia 𝐽 described with the mass 𝑚 and ℎ: Φ andThe the dynamic inertia J forcesdescribed 𝐹, with were the calculated mass m and accordinghCG: to Equation 2 with the angular acceleration Φ and the inertia 𝐽 described with the mass 𝑚 and𝐽 ℎ 𝑚ℎ: 𝐹 Φ 2 Φ (2) , J 𝑡.. mh 𝑡 .. F = ± = ± CG acc,t Φ𝐽 𝑚ℎΦ (2) 𝐹 t Φ t Φ (2) The resulting total force for the retracting, actuator𝑡 𝐹,𝑡 and the extending actuator 𝐹, were calculatedThe resulting as: total force for the retracting actuator F and the extending actuator F were The resulting total force for the retracting actuator 𝐹t,r and the extending actuator 𝐹t,e were calculated as: , , calculated as: 𝐹, 𝐹, 𝐹, (3) Ft,r = Fs,t − Facc,t (3) 𝐹, 𝐹, 𝐹, (3) 𝐹, 𝐹, 𝐹, (4)

Ft,e = Fs,t + Facc,t (4) 𝐹, 𝐹, 𝐹, (4)

Figure 4. Abstract model of the vehicle for the calculations of occurring forces on the actuators. WithFigure an assumed4. Abstract total model mass of the of vehicle 155 kg for (80 the kg calculations for the passenger, of occurring 75 forces kg for on the the vehicle), actuators. a track 2 width t of 0.49 m, a height ℎ of 0.7 m, and a maximum angular acceleration Φ of 50°/s , a With an assumed total mass of 155 kg (80 kg for the passenger, 75 kg for the vehicle), a track maximum actuator force of 901 N was obtained. .. width ttof of 0.49 0.49 m, m, a height a heighth ofℎ 0.7 of m, 0.7 and m, a maximumand a maximum angular accelerationangular accelerationΦ of 50◦/s Φ2, aof maximum 50°/s2, a Based on the calculations,CG two Thomson Electrak Pro linear actuators (PR2402-2A65) with Acme actuatormaximum force actuator of 901 force N was of obtained.901 N was obtained. screw drives, 24 V electric motors, and 100 mm travel were chosen [19]. The linear actuators are Based on the calculations, two Thomson Electrak Pro linear actuators (PR2402-2A65) with Acme capable of carrying loads up to 1125 N and have a maximum speed of 50 mm/s. They are equipped screw drives, 24 V electric motors, and 100 mm travel were chosen [19]. The linear actuators are capable of carrying loads up to 1125 N and have a maximum speed of 50 mm/s. They are equipped World Electric Vehicle Journal 2018, 9, 48 6 of 15

Based on the calculations, two Thomson Electrak Pro linear actuators (PR2402-2A65) with Acme screw drives, 24 V electric motors, and 100 mm travel were chosen [19]. The linear actuators are World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 6 of 15 capable of carrying loads up to 1125 N and have a maximum speed of 50 mm/s. They are equipped withwith built-inbuilt-in rotary encoders whichwhich provide relative information about the position of the actuator. TheThe upperupper partpart ofof thethe actuatorsactuators is fixedfixed toto thethe strutstrut bearings,bearings, whilewhile thethe lowerlower rodsrods areare connectedconnected toto thethe wheelwheel mountingmounting plates.plates. This enables the relativerelative motion between the wheel and the frameframe whenwhen thethe actuatoractuator isis operated.operated. The self-locking actuatorsactuators with Acme screw drives were chosenchosen toto preventprevent unintended loweringlowering ofof thethe devicedevice whenwhen thethe actuatorsactuators areare notnot powered.powered.

2.1.3. System Processing Units The vehiclevehicle consists of oneone mainmain computercomputer andand severalseveral micro-controllersmicro-controllers to controlcontrol differentdifferent functionalitiesfunctionalities of the vehicle. Figure 55 illustratesillustrates thethe connectionconnection diagramdiagram ofof thethe systemsystem components.components. TheThe main processing processing unit unit is is the the “UP-Board” “UP-Board” [20]. [20 This]. This microcomputer microcomputer is equipped is equipped with an with Intel an Atom Intel AtomX5 processor X5 processor and 4 and GB 4 of GB RAM of RAM [20]. [20 Ubuntu]. Ubuntu 16.10 16.10 is isinstalled installed as as the the operating operating system, system, and the “Libmraa”“Libmraa” librarylibrary [[21]21] isis usedused forfor accessaccess toto thethe hardwarehardware I/O. I/O.

Figure 5. Overview of the system components.

Computationally intensive tasks of thethe autonomousautonomous positioning,positioning, navigation, and control of movement werewere processed processed by by the the UP-Board. UP-Board. Directly Directly connected connected to the UP-Boardto the UP-Board are the Intel are Realsensethe Intel R200Realsense 3D-camera R200 [3D-camera22], the ultrasound [22], the distance-measuring ultrasound distance-measuring sensors, an Arduino sensors, Due an [ 23Arduino] microcontroller Due [23] formicrocontroller the control of for the the powertrain, control of an the Arduino powertrain, Leonardo an Arduino [24] for theLeonardo control of[24] the for linear the control actuators, of the receiverlinear actuators, of the wireless the receiver remote, of andthe wireless a Wi-Fi accessremote point, and fora Wi-Fi the remote access accesspoint for of thethe UP-Board.remote access of the UP-Board.The long-term release “Kinetic Kame” of the open source meta-operating system Robot Operating SystemThe (ROS) long-term [25] runs release on the “Kinetic UP-Board Kame” to enable of thethe communicationopen source meta-operating between the various system functions Robot andOperating components System of (ROS) the vehicle. [25] runs on the UP-Board to enable the communication between the various functions and components of the vehicle. 2.1.4. Powertrain 2.1.4.Two Powertrain brushless-DC (BLDC) motors SYM-36-50ZE are selected for the powertrain of the vehicle [26]. The motorsTwo brushless-DC have a single-sided (BLDC) shaft motors and SYM-36-50ZE a maximum are selected output for of 28the Nm powertrain with a nominal of the vehicle power output[26]. The of motors 500 W, whereashave a single-sided the peak power shaft output and a reaches maximum 700 Wtorque and enablesoutput aof maximum 28 Nm with vehicle a nominal speed ofpower approximately output of 20500 km/h. W, whereas The maximum the peak required power output current reaches is 25 A. 700 W and enables a maximum vehicleThe speed vehicle of approximately is designed to have20 km/h. one electricThe maximum motor onrequired each side. current The is motor 25 A. controller RoboteQ FBL2360The wasvehicle selected is designed as it is ato dual have channel one electric controller motor that on supportseach side. BLDC The motor machines controller and sinusoidal RoboteQ controlFBL2360 of was the selected motors [ 27as ].it Sinusoidalis a dual channel excitation cont isroller smoother that supports and results BLDC in amachines quieter operation and sinusoidal and a morecontrol precise of the control motors as [27]. well Sinusoidal as a higher excitation efficiency is duesmoother to less and dissipation results in of a heat. quieter A serial operation connection and a allowsmore precise sending control commands as well andas a receivinghigher efficiency data from due the to less inverter. dissipation The motor of heat. controller A serial supportsconnection a continuousallows sending maximum commands current and of receiving 40 A, a voltage data from of 60 the V perinverter. channel, The and motor the recuperationcontroller supports of brake a energy.continuous It can maximum estimate thecurrent state of charge40 A, a (SOC)voltage of of the 60 battery V per andchannel, uses theand hall thesensors recuperation of the motorof brake to provideenergy. It odometry can estimate data. the state of charge (SOC) of the battery and uses the hall sensors of the motor to provideA 36 V, odometry 12s3p-battery data. pack with a capacity of 297 Wh was selected for the prototype. The cells were of theA type 36 V, ANR26650M1B 12s3p-battery withpack awith LiFePO a capacity4 chemistry of 297 and Wh support was selected C-rates for up the to 28prototype. [28]. The The capacity cells waswere sufficient of the type for testANR26650M1B drives of about with 3 kma LiFePO with an4 chemistry average speed and support of 11.5 km/h. C-rates up to 28 [28]. The capacity was sufficient for test drives of about 3 km with an average speed of 11.5 km/h. The control algorithm of the powertrain and self-balancing was executed on the Arduino Due and requires the lateral velocity of the vehicle and the balancing angle as inputs. Vehicle speed was obtained by the wheel odometry via the motor controller. The balancing angle was measured with World Electric Vehicle Journal 2018, 9, 48 7 of 15

The control algorithm of the powertrain and self-balancing was executed on the Arduino Due and requires the lateral velocity of the vehicle and the balancing angle as inputs. Vehicle speed was obtained by the wheel odometry via the motor controller. The balancing angle was measured with an inertial measurement unit (IMU) of an Arduino 101 [29] which includes a gyroscope and an accelerometer.

2.2. Localisation Scube is equipped with various sensors to determine its location. While local positioning is used to measure the relative movement of the vehicle, global positioning determines the absolute position on a 2D-plane. The local positioning is conducted by in-vehicle odometry for dead reckoning and ultrasound-sensors and does not rely on external sources of information. The two wheels of Scube are arranged on a single axis with a differential steering system. The odometry relies on the revolution speed, the difference of the revolution speed of the two wheels, the wheel radius, and the distance between the two wheels, the so-called track-width. In the case of Scube, wheel revolutions are measured by the motor controller which evaluates the information of the hall sensors that are built into the motors. For the detection of objects in the near proximity of the vehicle, two rear- and two front-facing ultrasound-sensors of the type HC-SR04 are built into the Scube [30]. The sensors have a range of maximum 4 m and have a measuring angle of 15◦. For the global positioning, a landmark-based approach with an Intel Realsense R200 3D-camera [22] was implemented. The Intel Realsense R200 camera is equipped with two infrared (IR) cameras to capture a 3D-image with a resolution of 640 × 480 pixels at a rate of 60 Hz. An additional RGB-camera provides a colour-image with a resolution of 1920 × 1080 pixels. The integrated IR-projector is designed for a range of up to 2.8 m, however longer ranges are possible with increased noise and inaccuracies. The camera is mounted on the front side of the vehicle below the footrest. The imaging algorithms are able to function despite the shift of the camera tilt during vehicle movement. Landmarks are any objects in the environment of which the global position is known [31]. For this experimental setup, QR-codes were used as landmarks. By recognising and measuring the distance to the landmarks, the position was calculated by trilateration. Trilateration requires three known points in space, however for the presented use case, the recognition of two points is sufficient for the positioning on a 2D-plane which results in two possible locations. Figure6a illustrates how the correct location is determined by a logical check if the calculated location is within the perimeter of the map and by comparing the relative position of the two landmarks from the video stream of the camera. If more than two landmarks are visible to the camera at the same time, an algorithm decides which two landmarks are most suitable for positioning. The decision is based on the distance between the landmark and the vehicle as well as the distance between the landmarks to avoid unfavourable angles for 2D-trilateration. Besides the position, the heading of the vehicle is required. The orientation is calculated by using the pinhole-camera model [32] which is shown in Figure6b. The heading is described by the angle α between the x-axis of the local coordinate system to the x-axis xG of the global coordinate system. The camera only returns the orthogonal distance to the plane of the focal point. With the

→ → specifications of the focal length and the camera image size, the distances PS and PS are 1 2 0 0 determined. The landmarks S1 and S2 are projected to the plane of the camera image (S1 and S2) and → → are set in relation to the mid-point H’ of the camera image to determine S1 H, S2 H and the point H. → The obtained vector PH represents the local x-axis of the vehicle and is used to calculate the vehicle heading α. World Electric Vehicle Journal 2018, 9, 48 8 of 15 World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 8 of 15

Figure 6. (a) Localization by recognition of landmarks; (b) Calculation of heading by the pinhole- Figure 6. (a) Localization by recognition of landmarks; (b) Calculation of heading by the camera model. pinhole-camera model.

3. Results

3.1. Structure/Body The load-bearingload-bearing structurestructure ofof the the device device is is mainly mainly made made of of square square aluminium aluminium profiles profiles (Figure (Figure7). The7). The profiles profiles are joinedare joined in various in various positions positions and anglesand angles by connectors by connectors which which can endure can endure tensile tensile forces offorces up toof 10up kNto 10 [33 kN]. The[33]. mainThe main frame frame is made is made of 40 of× 4040 × 40 mm mm and and 30 30× ×30 30 mmmm aluminium profiles. profiles. The strutstrut bearingsbearings taketake thethe loadload of of the the actuator actuator forces forces and and are are milled milled out out of of aluminium. aluminium. The The backrest backrest is madeis made from from a 12 a mm12 mm aluminium aluminium rod torod provide to provide both strengthboth strength and a curvatureand a curvature to increase to increase the comfort the ofcomfort the backrest. of the backrest. The outer outer shell shell of of the the vehicle vehicle is made is made from from 3D-printed 3D-printed parts parts with witha maximum a maximum thickness thickness of 2 mm. of 2A mm.hexagon A hexagon reinforcement reinforcement structure structure was incorporated was incorporated into the into 3D-printed the 3D-printed parts partsto increase to increase their theirstiffness. stiffness.

Figure 7. (a) Mainframe of Scube; (b) Scube with attached shell.shell.

Figure 8 displays the final dimensions of Scube. The rigidity of the aluminium frame is sufficient to carry a person, while the lowering mechanism allows for improved accessibility. When the vehicle World Electric Vehicle Journal 2018, 9, 48 9 of 15

World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 9 of 15 Figure8 displays the final dimensions of Scube. The rigidity of the aluminium frame is sufficient World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 9 of 15 to carry a person, while the lowering mechanism allows for improved accessibility. When the vehicle is in the lowered position, the whole bottom of the vehicle rests on the ground to provide a stable is in the lowered position, the whole bottom of the vehicle rests on the ground to provide a stable isplatform in the lowered for ingress position, and egress the whole that is bottom comparable of the to vehicle a wheelchair. rests on The the reduction ground to of provide the thickness a stable of platform for ingress and egress that is comparable to a wheelchair. The reduction of the thickness of platformthe shell forto 2 ingress mm reduced and egress the coststhat is for comparable 3D-printing, to aand wheelchair. the hexagon The reductionstructure providesof the thickness sufficient of the shell to 2 mm reduced the costs for 3D-printing, and the hexagon structure provides sufficient thestrength, shell toeven 2 mm for thereduced larger the parts, costs e.g. for fender 3D-printing,s. The total and weight the hexagon of the prototype structure isprovides 74 kg. sufficient strength, even for the larger parts, e.g. fender fenders.s. The total weight of the prototype is 74 kg.

Figure 8. Exterior dimensions of the vehicle. Figure 8 8.. ExteriorExterior dimensions of the vehicle. 3.2. Simulation of the Tilt Mechanism 3.2. Simulation Simulation of of the the Tilt Mechanism A multi-body simulation of the vehicle concept was conducted to evaluate the influence of the tilt mechanismA multi-body on simulationsimulation driving behaviour. ofof thethe vehicle vehicle The concept conceptmulti-body was was conducted conductedsimulation to to evaluatewas evaluate set theup the influenceto influence determine of theof thethe tilt tiltmechanismmaximum mechanism achievable on drivingon driving lateral behaviour. behaviour. accele Theration multi-body The during multi-body a simulation turn. Asimulation predefined was set upwas route to set determine was up generatedto determine the maximum with the an maximumachievableincreasingly achievable lateral reduced acceleration radiuslateral (Figureaccele duringration 9a). a turn. Theduring Avehicl predefined a turn.e was A simulated routepredefined was with generated route a constant was with generated speed an increasingly of with 2 m/s. an increasinglyreducedThe simulation radius reduced (Figurewas run radius9a). until The (Figure the vehicle vehicle 9a). was The started simulated vehicl toe deviatewas with simulated a constantfrom the with speed route a ofconstant due 2 m/s. to slipping speed The simulation of or 2 afterm/s. Thewastumbling. runsimulation until Figure the was vehicle9b showsrun starteduntil that the the to vehicle deviatevehicle started fromwithou the tot the route deviate tilt due mechanism from to slipping the enabledroute or afterdue can tumbling.to reach slipping a maximum Figure or after9b tumbling.showslateral thatacceleration Figure the vehicle 9b ofshows 2.5 without m/s that2 before the vehicle tilt toppling mechanism withou over.t enabledthe Figure tilt mechanism can 9c displays reach a enabled maximum the result can lateralof reach the vehiclea acceleration maximum with 2 lateralofthe 2.5 tiltingm/s acceleration enabled.before toppling ofA 2.5maximum m/s over.2 before acceleration Figure toppling9c displays of over. 6.8 m/s theFigure2 result was 9c achieved ofdisplays the vehicle withoutthe result with the theof vehiclethe tilting vehicle tumbling enabled. with 2 theAover. maximum tilting enabled. acceleration A maximum of 6.8 m/s accelerationwas achieved of 6.8 withoutm/s2 was the achieved vehicle without tumbling the over. vehicle tumbling over.

Figure 9. Simulation of occurred accelerationacceleration during a defineddefined turn (a) withoutwithout tilting (b) andand withwith Figuretilting (9.c). Simulation of occurred acceleration during a defined turn (a) without tilting (b) and with tilting (c). 3.3. Vehicle Performance 3.3. Vehicle Performance 3.3. VehicleAfter acquiringPerformance map and location data, the vehicle waits for user input about the next destination. After acquiring map and location data, the vehicle waits for user input about the next Once the destination is entered via a website-based user interface, the path from the current position destination.After acquiring Once the mapdestination and location is entered data, via the a website-basedvehicle waits userfor interface,user input the about path thefrom next the is calculated and the vehicle turns on the spot into the direction of the first waypoint. After the destination.current position Once is thecalculated destination and the is enteredvehicle turnsvia a on website-based the spot into theuser direction interface, of thethe firstpath waypoint. from the correct initial heading is reached, the vehicle smoothly accelerates to a configurable driving speed and currentAfter the position correct is initial calculated heading and is the reached, vehicle the turns vehicle on the smoothly spot into accelerates the direction to a ofconfigurable the first waypoint. driving Afterspeed the and correct follows initial the pre-calculatedheading is reached, path. theAt thevehicle destination smoothly point, accelerates the vehicle to a configurableis stopped and driving waits speedfor the and next follows destination the pre-calculated input. path. At the destination point, the vehicle is stopped and waits for the next destination input. World Electric Vehicle Journal 2018, 9, 48 10 of 15 follows the pre-calculated path. At the destination point, the vehicle is stopped and waits for the next destination input. If any obstacle is detected within approximately 0.5 m, the vehicle stops. After a waiting period, the vehicle starts to turn on the spot until it can pass the obstacle. If turning is not sufficient, the vehicle aborts the current route. During travel, the expected current position provided by the local odometry is constantly compared with the actual measurement by the global localisation system. If the difference increases to over 0.4 m, a recalibration is executed where the vehicle comes to a stop. In order to prevent an oscillating driving course, a heading deviation of +/− 0.2 rad (approximately +/− 11.5◦) is allowed. Autonomous navigation and driving were conducted in a test area of 12 m × 5 m with three different destinations. The vehicle speed in the autonomous mode is limited to 0.3 m/s due to safety measures. The maximum rotational speed is set to 0.5π rad/s. The minimum rotation speed is 0.6 rad/s in order to overcome initial rolling resistance. Within the test area, the vehicle was able to acquire its position by landmark recognition and responded to passenger transport requests. Obstacle detection (e.g. people standing in the pathway) proved to be successful.

3.4. Localisation Results The results of the localisation system are presented to demonstrate the achieved accuracy with the RealSense R200 camera system and the local odometry. The effect of the size of the QR-code is studied, then the accuracy of the global localisation is measured. Further, the accuracy of the vehicle odometry is tested with a pre-defined path. Finally, the results of an autonomously followed path are shown. The QR-codes that were utilised for landmark localisation were printed full-page on different paper sizes ranging from DIN A1 (594 mm × 841 mm) to DIN A4 (210 mm × 297 mm). Due to the limited resolution of the camera system of 640 × 480 pixels, identification and recognition of the landmarks are only possible within a certain range to the landmark depending on the size of the QR-code, as shown in Table1. Landmarks printed in DIN A1 size are recognisable up to 7–8 m, while landmarks printed on DIN A4 can only be recognised up to 2 m.

Table 1. Influence of QR-code landmark size on recognition distance.

Size of QR-Code Maximum Distance for Recognition DIN A1 7–8 m DIN A2 5–7 m DIN A3 3–4 m DIN A4 2 m

Besides the recognition of the landmarks, the resulting accuracy of the localisation was measured and is shown in Figure 10. The setup that is displayed in Figure 10 contains five QR-code-landmarks which are aligned on the same plane and facing the same direction. The two outermost QR-code-landmarks (code 14 & code 15) have the size DIN A1, while the QR-code-landmarks in the middle (code 1, code 8, code 11) are printed in DIN A3 size. The distance between the landmarks and the vehicle was increased from 1 m to 4 m in 0.5 m increments. Two further measurement runs were conducted at a distance of 5 m and 6 m. The red crosses indicate the true position of the vehicle. At each position, 100 localisation runs were conducted (blue and green marks) by the vehicle. The black cross indicates the averaged measured position. The results in Figure 10 show that up to a distance of 2 m, the accuracy in both the x- and the y-axis is high, and the standard deviation in x- and y-direction is less than 2 cm and 1 cm, respectively. With increasing distance, the accuracy decreases. At a distance of 6 m, the standard deviation in the x-direction is 9 cm while it is 2 cm in the y-direction. Overall, the accuracy of the distance measurement (y-axis) is distinctively higher than the measurement in the x-direction because the distance is natively World Electric Vehicle Journal 2018, 9, 48 11 of 15

World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 11 of 15 measured by the 3D-camera, while the lateral position is calculated by triangulation and is more stronglyWorld influencedElectric Vehicle Journal by measuring 2018, 9, x FOR errors. PEER REVIEW 11 of 15

Average Standard Distance Averagedeviation [m] Standarddeviation [m] Distance[m] deviation(x-axis /[m] y-axis) deviation(x-axis [m] / y -axis) [m] 1.0 (x-axis 0.01 / y-axis) / 0.00 (x-axis 0.010/ y-axis) / 0.001 1.01.5 0.01 0.02 / 0.00 / 0.04 0.010 0.020/ 0.001 / 0.004 1.52.0 0.02 0.02 / 0.04 / 0.02 0.020 0.020/ 0.004 / 0.004 2.02.5 0.02 0.10 / 0.02 / 0.01 0.020 0.040/ 0.004 / 0.010 2.53.0 0.10 0.11 / 0.01 / 0.02 0.040 0.030/ 0.010 / 0.010 3.03.5 0.11 0.21 / 0.02 / 0.02 0.030 0.070/ 0.010 / 0.010 3.5 0.21 / 0.02 0.070 / 0.010 4.0 0.19 / 0.06 0.080 / 0.010 4.0 0.19 / 0.06 0.080 / 0.010 5.05.0 0.10 0.10 / 0.18 / 0.18 0.090 0.090/ 0.010 / 0.010 6.06.0 0.22 0.22 / 0.22 / 0.22 0.090 0.090/ 0.020 / 0.020

FigureFigure 10. 10. Distribution Distribution of of localisation localisation measurements measurements using usingusing multiple multiplemultiple landmarks. landmarks.landmarks.

TheTheThe overalloverall overall accuracyaccuracy accuracy ofof of thethe the locallocal local odometryodometryodometry sy systemsystemstem was waswas examined examinedexamined by bytrackingby trackingtracking the predefined thethe predefinedpredefined route shown in Figure 11a. The vehicle was manually moved on a 4 m × 4 m box-shaped route (black) routeroute shownshown inin FigureFigure 11 11a.a. TheThe vehiclevehicle waswas manuallymanually movedmovedon ona a4 4 mm× × 44 m m box-shaped box-shaped route (black) and the data of the internal odometry is plotted (blue). Figure 11b shows the course of the measured and the data of the internal odometry is plotted (blue). Figure 11b shows the course of the measured and heading.the data Inof thethe final internal position, odometry a deviation is plotted of 0.1 m(b lue).(x-axis) Figure and 0.2 11b m shows (y-axis) the is measured. course of Thethe measuredfinal heading.heading.heading In theerror finalfinal is 0.037 position,position, rad, respectively a deviationdeviation 2.1°. of 0.10.1 mm ((xx-axis)-axis) andand 0.20.2 mm ((yy-axis)-axis) isis measured.measured. TheThe finalfinal headingheading errorerror isis 0.0370.037 rad,rad, respectivelyrespectively 2.12.1°.◦.

Figure 11. (a) Comparison of the predefined driving route (black) with data recorded by onboard odometry (blue); (b) Recorded heading during travel over the predefined route.

FigureFigureFinally, 11.11. ( athe) ComparisonComparison overall functionality of the predefinedpredefined of autonomous drivingdriving driving routeroute (black)(black) was evaluated. withwith datadata Figure recordedrecorded 12a bybyshows onboardonboard the outlineodometryodometry of the (blue);(blue); testing ((bb)) RecordedRecordedarea with headingheadingthe dimensions duringduring traveltravelof approximately overover thethe predefinedpredefined 5 m × 12.5 route.route. m. For this evaluation, the initial position of Scube is measured (green cross) and a destination is transmitted to the vehicle (orangeFinally,Finally, cross). thethe overalloverall The calculated functionalityfunctionality track is of ofa straight autonomousautonomous line (black drivingdriving dotted waswas line), evaluated.evaluated. which helps Figure to detect 1212aa course showsshows thethe outlineoutlinedeviations ofof thethe testingfortesting this evaluation. areaarea withwith theFigurethe dimensionsdimensions 12b displays ofof an approximatelyapproximately enlarged view of 55 mthem× ×test 12.5 track m. and For includes this evaluation, the thethe initialinitialwaypoints positionposition (black ofof crosses) ScubeScube isiscalculated measuredmeasured by (green(greenthe vehicle. cross)cross) As andand the a avehicle destinationdestination follows isis the transmittedtransmitted calculated to totrack, thethe vehiclevehicleit (orange(orangecontinuously cross).cross). TheThe tracks calculatedcalculated its position tracktrack (blue isis aline).a straightstraight In the linelin beginning,e (black(black dottedadotted course line),line), deviation whichwhich can helpshelps be seen, toto detectwhichdetect is coursecourse deviationsdeviationsbecause for of thethis intentionally evaluation. evaluation. allowed Figure Figure course12b 12 bdisplays displaystoleranc ane anto enlarged avoid enlarged an oscillatingview view of ofthe driving the test test track course. track and The and includes track includes the correction occurring at the position (2 m|3.5 m) is explained by a recalibration of the localisation thewaypoints waypoints (black (black crosses) crosses) calculated calculated by bythe the vehicle. vehicle. As As the the vehicle vehicle follows follows the the calculated calculated track, itit system after it detected a deviation between the positions provided by local odometry and the global continuouslycontinuouslylocalisation. trackstracks Afterwards, itsits positionposition the vehicle (blue(blue line).followsline). InIn the thethe ca beginning,beginning,lculated track aa coursecourse until the deviationdeviation destination. cancan It bebe stops seen,seen, short whichwhich isis becausebecauseof the ofof destination thethe intentionally point to allowedallowedavoid excessive coursecourse turning tolerancetoleranc one the toto avoidavoidspot, which anan oscillatingoscillating would cause drivingdriving discomfort course.course. for The the track correctioncorrectionpassenger. occurringoccurring The final at position thethe positionposition of the device (2(2 m|3.5m|3.5 is meas m) ured isis explainedexplained manually byby(red aa cross). recalibrationrecalibration The deviation ofof thethe between localisationlocalisation systemsystem afterafter itit detecteddetected aa deviationdeviation betweenbetween thethe positionspositions providedprovided byby locallocal odometryodometry andand thethe globalglobal localisation. Afterwards, the vehicle follows the calculated track until the destination. It stops short of the destination point to avoid excessive turning on the spot, which would cause discomfort for the passenger. The final position of the device is measured manually (red cross). The deviation between World Electric Vehicle Journal 2018, 9, 48 12 of 15 localisation. Afterwards, the vehicle follows the calculated track until the destination. It stops short of the destination point to avoid excessive turning on the spot, which would cause discomfort for the World Electric Vehicle Journal 2018, 9, x FOR PEER REVIEW 12 of 15 passenger. The final position of the device is measured manually (red cross). The deviation between thethe calculatedcalculated andand thethe manuallymanually measuredmeasured finalfinal positionposition isis −−0.040.04 m in the x-direction and 0.46 mm inin thethe y-direction.-direction.

Figure 12. (a) Overview of the testing area andand the testtest track;track; ((b)) Enlarged detaildetail ofof thethe plannedplanned tracktrack (black(black dotteddotted line),line), plannedplanned waypointswaypoints (black(black crosses),crosses), startstart position (green cross), destination (orange cross), internallyinternally computedcomputed track (blue(blue line),line), computedcomputed destinationdestination position (blue cross),cross), manuallymanually measured finalfinal positionposition (red(red cross).cross).

4. ConclusionsConclusion and and Outlook Outlook InIn thisthis paper,paper, thethe development development and and implementation implementation of of an an autonomous autonomous mobility mobility device device concept concept is presented.is presented. The The electric electric vehicle vehicle is is designed designed to to be be part part of of a a fleet fleet which whichcan can bebe orderedordered on-demandon-demand by customers viavia aa mobilemobile applicationapplication forfor last-mile/shortlast-mile/short distance transport to improve connectivity toto publicpublic transporttransport systems. The system reduces the reliance onon privateprivate vehiclesvehicles andand privatelyprivately ownedowned PMDs and increases thethe convenienceconvenience ofof thethe commutingcommuting population.population. The vehiclevehicle is designeddesigned asas aa self-balancing,self-balancing, two-wheeltwo-wheel device which can carrycarry oneone personperson comfortably in aa seatedseated position.position. The size of thethe vehicle is determineddetermined basedbased onon anthropometricanthropometric measures ofof thethe EuropeanEuropean andand SingaporeanSingaporean population.population. The two-wheeltwo-wheel design reduces the footprint of thethe vehiclevehicle toto enable enable travel travel on on narrow narrow roads and an ind in congested congested areas. areas. Driving Driving safety safety and and comfort comfort are increasedare increased by a by tilt a mechanismtilt mechanism which which leans leans the vehiclethe vehicle into into the the turn turn and and reduces reduces lateral lateral forces. forces. The The tilt mechanismtilt mechanism consists consists of twoof two independent independent linear linear actuators actuators for for a a swift swift change change of of the the tilt tiltof of thethe vehiclevehicle and additionally acts acts as as a a means means to to lower lower the the vehicle vehicle to tothe the ground ground for foreasy easy mounting/alighting. mounting/alighting. The Theeffectiveness effectiveness of the of tilt the mechanism tilt mechanism on the on dyna themic dynamic driving driving behaviour behaviour is demonstrated is demonstrated by a multi- by a multi-bodybody simulation simulation and substantially and substantially increases increases the vehicle the vehicle stability stability during during turns. turns. Driving Driving comfort comfort is isincreased increased by by the the self-balancing self-balancing system system of of the the dev deviceice because because it it keeps keeps the the passenger passenger in an uprightupright position,position, even during during travel travel over over inclined inclined surfaces. surfaces. Additionally, Additionally, the the calculation calculation of the of therequired required tilt- tilt-angleangle based based on the on routing the routing of the of vehicle the vehicle can potent can potentiallyially counter counter lateral lateral forces forces on the on passenger the passenger more moreeffectively effectively and with and reduced with reduced latency latency compared compared to the to existing the existing tilting-devices tilting-devices which which only onlyreact reactafter afterthe occurring the occurring acceleration acceleration has been has measured. been measured. In successive In successive studies, studies, a user study a user with study the with vehicle the should be conducted to evaluate the driving comfort, the driving speed, and the safety from a passenger’s perspective. To enable autonomous navigation, a landmark-based localisation system based on a 3D-camera system was implemented which works in conjunction with the local odometry system. The 3D- camera system proved to be a feasible and affordable solution for navigation and localisation in situations where no localisation by GPS is possible, e.g. indoors. The accuracy is sufficient for the World Electric Vehicle Journal 2018, 9, 48 13 of 15 vehicle should be conducted to evaluate the driving comfort, the driving speed, and the safety from a passenger’s perspective. To enable autonomous navigation, a landmark-based localisation system based on a 3D-camera system was implemented which works in conjunction with the local odometry system. The 3D-camera system proved to be a feasible and affordable solution for navigation and localisation in situations where no localisation by GPS is possible, e.g. indoors. The accuracy is sufficient for the demonstrated purposes, however the occurrence of occlusion of the landmarks can impact the localisation accuracy. For the application of higher vehicle speeds, the resolution of the camera system should be increased to enable a greater viewing range. QR-codes were used as landmarks for the presented indoor-evaluation which could be replaced by natural landmarks with an improved camera system and also to reduce the occurrence of occlusion of landmarks. The algorithms are able to detect obstacles in front of the vehicle by analysis of the camera image and the information from the ultrasound sensors to prevent collisions with any objects. This contributes to increased safety in comparison to existing PMDs without any object detection systems. The vehicle was realized as a functional prototype for demonstration and testing purposes. The selected construction with aluminium profiles and a 3D-printed shell fulfils the requirements regarding stability, the time required for the built-up, and outer appearance. The size of the vehicle footprint is similar to other PMDs with a one-axle, two-wheels setup, however it offers increased comfort because of the seated position for the passenger. The weight of the prototype is between the weight of the presented Segway and the Scewo PMD. For the realization of the prototype in mass production, a more integrated structure should be considered to reduce the overall weight of the vehicle. The selected components for the control of the system allowed for flexible integration of the conceived functionalities. The principal functionality of the autonomous operation was demonstrated, however the real-life usage is still limited by technical constraints of the self-balancing, the electrical actuators, and the accuracy of the localization system. Further development of the vehicle functions needs to be carried out to increase the operating speed and the overall stability of the autonomous navigation functions. Going forward, the presented concept shows a possible solution for the first/last mile problem and can contribute to improving the ridership of public transport to reduce the reliance on private for transport in megacities. Besides closing the gap for the first/last mile, the small footprint of the vehicle enables the vehicle to operate in other public areas, including airports and shopping malls, where longer walking distances could be reduced.

Author Contributions: Conceptualization, M.K., F.R., M.W., B.H., T.H., L.L.S., J.S.D.L., Y.L., V.M., M.L., W.X.R.T., and M.L.; Investigation, B.H., T.H., V.M., and M.L.; Methodology, B.H., T.H., L.L.S., J.S.D.L., Y.L., V.M., M.L., and W.X.R.T.; Project administration, M.K., F.R., and M.W.; Resources, M.K., F.R., Michael Wittmann, Y.J.J.H., and H.W.N.; Supervision, M.K., F.R., M.W., Y.J.J.H., and H.W.N.; Visualization, L.L.S. and J.S.D.L.; Writing—original draft, M.K., B.H., T.H., V.M., and M.L.; Writing—review and editing, F.R., M.W., A.O., and M.L. Funding: This work was financially supported by the Singapore National Research Foundation under its Campus for Research Excellence and Technological Enterprise (CREATE) programme and by the German Research Foundation (DFG) and the Technical University of Munich (TUM) in the framework of the Open Access Publishing Program. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript, or in the decision to publish the results.

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