Abstract Future Unmanned Aircraft Systems(UASs) are expected to be nearly autonomousand composed of heterogeneous Unmanned Aer-ial Vehicles (UAVs). While most of the currentresearch focuses on UAV avionics and controlalgorithms, ground task automation has come tothe attention of researchers during the past fewyears. Ground task automation not only relieves
This paper is largely reprinted from the paper of thesame title in the Proceedings of the InternationalConference on Unmanned Aircraft Systems 2011(ICUAS‘11), May 2011. The Proof of Theorem 1 andreorganization leading to Section 5 are new.
K. A. O. SuzukiMechanical Engineering Department,Korea Advanced Institute of Science and Technology(KAIST), Daejeon 305-701, Koreae-mail: [email protected]
P. Kemper FilhoElectrical Engineering Department,Korea Advanced Institute of Science and Technology(KAIST), Daejeon 305-701, Koreae-mail: [email protected]
J. R. Morrison (B)Industrial and Systems Engineering Department,Korea Advanced Institute of Science and Technology(KAIST), Daejeon 305-701, Koreae-mail: [email protected]: http://xS3D.kaist.edu
human operators, but may also expand the UASoperation area, improve system coverage and en-able operation in risky environments without pos-ing a threat to humans. We propose a model toevaluate the coverage of a given UAS. We alsocompare different solutions for various modulesof an automated battery replacement system forUAVs. In addition, we propose a ground stationcapable of swapping a UAV’s batteries, followedby a discussion of prototype components and testsof some of the prototype modules. The proposedplatform is well-suited for high-coverage require-ments and is capable of handling a heterogeneousUAV fleet.
The United States Federal Aviation Adminis-tration (US FAA) defines an Unmanned Air-craft System (UAS) as a collection of UnmannedAerial Vehicles (UAVs), control station(s), com-mand and control algorithms and equipment forlaunch, recovery, communication and navigation[1]. While many efforts have been directed tothe study of these components, there has been
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significantly less attention paid to the automationof ground maintenance tasks. While such activitiesmay be relegated to humans, to achieve a UASwith greater range, applicability and autonomy,such efforts must be automated.
Initial ground breaking work in this area hasrecently been conducted. In [2], the first prototypeof a battery recharge platform for UAVs wasdeveloped to support the RAVEN UAS test bed.To our knowledge, the authors in [3] are the firstto study and provide an analytic answer to thequestion of whether a battery charging or replace-ment platform is preferable. They also developedand compared various recharge station designsand proposed a conceptual replacement platform.Simultaneously and independently of [3], authorsof [4] developed the first prototype of a UAVbattery replacement system as part of their ACEtest bed. They also developed management soft-ware to support the process of operating UAVswith such a platform and an off-UAV method todeduce when a battery requires service. In [5],and subsequent work (not yet published), a selfleveling platform that keeps its base parallel tothe ground and centers the UAV was developed.The prototype efforts for UAVs follow relatedwork for ground based robots in [6–12] for batterycharging and [13, 14] for battery replacement.
There has also been some work to auto-mate ground tasks for gas powered UAVs [15],where UAV precision landing capability, auto-mated capturing and centering mechanisms thatmove UAVs to a proper refueling position weretested.
Despite these advances, numerous opportuni-ties remain. First, the analysis of [3] is based onbounds so that there is an opportunity to tightenthe results. Second, while most of the proposedsolutions for ground based service stations relyon homogeneous fleets of UAVs, it is anticipatedthat the UASs of the future will be heterogeneous[16]. Third, service stations should be robust toenvironmental factors that compromise the in-tegrity of UAV positioning systems; existing de-signs are intended primarily for laboratory basedtest beds. Fourth, there is a need to consideradditional design choices for the components ofservice stations to improve system reliability andperformance.
In this paper, we strive to address these needsfor battery operated UAV systems. The contribu-tions and organization are as follows:
– The paper develops Petri net models of bat-tery charging and replacement systems thatenable a tight comparison between them.(Section 2)
– The paper develops and compares design op-tions for the functional components of batteryreplacement service stations. These attempt toaddress the issues of fleet heterogeneity andUAV landing control robustness. (Section 3)
– The paper reviews and conducts operationaltesting of key components of a system proto-type. (Section 4)
– Using estimates of the replacement platformcosts based on the design choices of Section 3,we compare the cost of various platforms andsystems. (Section 5)
Concluding remarks are presented in Section 6and future work is presented in Section 7.
2 Planning UASs
A Petri net can be considered a graphical toolthat may be used to describe distributed, concur-rent, parallel, asynchronous, deterministic and/orstochastic stepwise processes [17]. It is a bipar-tite graph, in which the nodes are divided intotransitions (T, represented by bars) and places (P,represented by circles). The connection betweennodes is made by directed arcs, which connectonly a T to a P, or a P to a T, never P to P or Tto T. Tokens (usually represented by dots) travelthrough the net. Whenever there is a token at theinput of all arcs leading to a transition, the transi-tion “fires”, the tokens at the input are consumedand a token is created at the output of each of theoutgoing arcs. The event of a firing can happenconcurrently and can overlap in time with otherfirings so long there are enough tokens to firea transition. The location and number of tokensat the start of the Petri net evolution is calledthe initial marking. Times may be associated withthe transitions and places. Tokens entering anode must wait for this duration before they arereleased.
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If there is only one output arc and one inputarc at any of the places of a given Petri net,the network is called “decision-free” [18]. Thecycle time of a decision-free Petri net is straight-forward to analyze without much computationaleffort.
Figure 1 depicts Petri nets for a refill/rechargeand a replace system, where TF stands for thetime a UAV spends in the air, TR is the time itspends at the service station and TC is the chargingtime a battery requires to achieve full charge whenfully depleted. TI is the idle time allocated to eachUAV in one operation cycle. During this time,the UAV simply rests and expends no energy.It serves to adjust the system to lower coveragerequirements and can be as low as zero. There isno duration associated with the places.
In the Petri net for the recharge platform on theleft of Fig. 1, the duration TR should be at least aslong as a battery charging time; the UAV remainswith the platform until its battery is fully charged.In the Petri net for the replacement platform onthe right of Fig. 1, TR will be small relative to TC,
since only a battery swap is required during the TR
duration.In the initial marking of the replacement Petri
net, there are NUAV, NPLAT, (NBATT − NUAV),NCGR tokens in the places labeled “Ready to fly”,“Platform ready for UAV”, “Battery charged”,and “Chargers waiting for batteries”, respectively.These represent the number of UAVs (each witha battery), exchange platforms, backup chargedbatteries and battery chargers, respectively. In therefill Petri net, the initial marking is the same asin the replace Petri net for those places shared inboth nets (P1, P2, P3 and P4).
Since the resultant Petri nets are simple, analy-sis is rather straightforward. According to [18], thesystem cycle time for a decision free Petri net can
be obtained as max(
TiNi
), where i ranges over all
loops in the net, Ti is the sum of durations alongthese loops and Ni is the number of tokens inplaces in the loop initial marking. It is equivalentto consider only simple loops.
There are four simple loops in the replacementnet: UAV loop (LUAV), platform loop (LPLAT),
Fig. 1 Petri Net model.The left-hand side modelsa refill/recharge system,and the right-hand sidemodels a replace system
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battery loop (LBATT), and charger loop (LCGR).Defining:
TLUAV = TF + TR + TI
NUAV, (1)
TLPLAT = TR
NPLAT, (2)
TLBATT = TC + TR
NBATT − NUAV, (3)
TLCGR = TC
NCGR, (4)
where NUAV, NPLAT, NBATT and NCGR are thenumber of UAVs, platforms, batteries (total in-cluding those on UAVs and those on the plat-forms) and chargers, respectively.
The minimum cycle time (maximum perfor-mance) [18] of the system is:
TCYC = max(TLUAV , TLPLAT, TLBATT, TLCGR). (5)
Thus each transition fires on average once everyTCYC units of time.
System coverage is defined as the average num-ber of UAVs in the air at a given time. Theorem 1relates system coverage to the Petri net cycle time.
Theorem 1
CSYS = TF
TCYC
= TF
max{
TF+TR+TINUAV
, TRNPLAT
, TC+TRNBATT−NUAV
, TCNCGR
} .
Proof Since we have NUAV UAVs in the system,the system coverage equals:
CSYS = limt→+∞
NUAV∑i=1
1
t
∫ t
0IF
U AVi(t) dt,
where IFUAVi
(t) is the indicator that UAVi is in theair at time t (its value is 0 or 1) and we assume theNUAV UAVs are labelled UAV1, ..., UAVNUAV .
According to [19], decision-free Petri Nets,such as those of Fig. 1, will become periodic withperiod T after an initial transient duration (t0).From [18], each transition fires on average everyTCYC units of time. Thus, one can conclude thatthe period T = K · TCYC, where K is the numberof firings of a particular transition during one fullperiod (any transition will do since they all fireat the same average rate). Let T ′ = (p · K)TCYC,where p is an integer such that p · K = q · NUAV,for q integer. (Clearly, such a p exists since p =NUAV will give p · K as some multiple of NUAV; qtells us exactly how many multiples.)
Now, since T ′ is a multiple of the period T, wecan write
CSYS =NUAV∑i=1
1
T ′
∫ ti+T ′
tiIF
UAVi(t) dt,
where ti ≥ t0 is the time of the firing of tran-sition TF corresponding to the first launch ofUAVi once the periodic regime has been reached.Since T ′ =(p · K)TCYC =(q · NUAV)TCYC, assum-ing FIFO consumption of the tokens from the“Ready to Fly” place, U AVi will start and com-plete exactly q flights in the time interval [ti, ti +T ′]. Thus, the integral for each UAV will giveq · TF . We have:
CSYS =NUAV∑i=1
1
(q · NUAV)TCYC· (q · TF)
= NUAV · TF
NUAV · TCYC= TF
TCYC.
This concludes the proof. �
Usually one of the loops will constrain TCYC,meaning that the resources of the other loopsare under-utilized (on stand-by). If we decide tomake the three loop’s period equal by changingthe variables under our control (e.g. the numberof UAVs, idle time, etc) we can optimize the UASto the desired specifications (or approach thebounds obtained previously). As in [3], Eq. 5 andTheorem 1 can be used to support platform design
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by dictating required battery storage space, num-ber of charge platforms and overall system speed.
Example 1 Consider a UAS consisting of 3 UAVsand one exchange (replacement) platform withTF = 15 min, TR = 1 min and TC = 85 min. SetTI = 0. Let TS denote the duration of a UAVoperation cycle, that is, TS = TR + TF + TI =16 min.
Let NUAV = 3, NBATT = 20, NPLAT = 1 andNCGR = 16. For such a system, the replace-ment Petri net can be used to obtain TLUAV =TF+TR+TI
NUAV= 5.333, TLPLAT = TR
NPLAT= 1, TLBATT =
TC+TRNBATT−NUAV
= 5.059, and TLCGR = TCNCGR
= 5.313.
Thus, TCYC = TLUAV and CSYS = TFTCYC
= 2.8125UAVs/unit time. If we want a lower coveragewith the same system, say 2.5, we can increase
TI . Setting TI = (TF + TR) ·(
NRUAV·CUAV
CTGTSYS
− 1)
= 2
min, we obtain TCYC = 6 min. We then haveCSYS = 2.5 UAVs/unit time. �
3 Design Options
In this section we discuss design options for anautonomous UAV battery replacement systemground station. This station is to automaticallyswap the depleted batteries of a UAV for arecharged battery without human intervention.Here, instead of tackling navigation control algo-rithm problems, we work with design parametersto solve issues such as:
– Guiding the UAV to the battery replacementsite,
– Orienting the UAV in a desired direction,– Locking the UAV position on the station,– UAV-battery connections: extracting and
placing a battery in the UAV,– Battery transportation, and– Battery array recharging.
3.1 UAVs, Batteries and Charging
UASs composed of different types of UAVs areof interest to both civil and military applications[16]. It is desirable that service stations provideservice to as many different kinds of UAVs in
the UAS as possible. This may serve to reducethe number of ground stations and promote aneven spread of the coverage of a given UAS.Since Lithium-polymer (Li-Po) battery-poweredUAVs are popular due to the high energy densityof Li-Po batteries [20], we elected to design asystem that can serve UAVs with two-cell andthree-cell Li-Po batteries. Li-Po batteries requirea balanced charger [20] and many of the com-mercially available hobby chargers are already de-signed to charge two-cell and three-cell batteries.This decision primarily affects the geometry ofthe electric terminals and minimizes charging logiccomplexity.
A popular UAV employing this Li-Po batteryis the Lama V3/4 [21]. It was used in the Au-tonomous Control Environment (ACE) test bedof [4]. The Lama V3/4 is a coaxial, small remotecontrol helicopter designed and manufactured byE-Sky. It is primarily for hobbyists and is easy tofly. The main rotor diameter is 340 mm, the bodyweight is 230 g (with original equipment manufac-turer (OEM) battery) and the OEM power supplyis a 2-cell Lithium Polymer battery pack (7.4V800 mAh). Such UAVs are usually relatively inex-pensive, simple, light and susceptible to weatherconditions.
The HoneyBee King 2/3/4 [22] is an exampleof a UAV using a three-cell Li-Po battery. Fol-lowing a more traditional helicopter design, theHoneyBee King has a main rotor and a tail rotorto control the UAV. The main rotor diameter is600 mm, the body weight is 470 g (with OEMbattery) and the OEM power supply is a 3-cellLithium Polymer battery pack (11.1V 1000 mAh).Such UAVs are typically more robust with greatercapability than their smaller counterparts but re-quire more skill to control.
As seen in [3], systems with expensive UAVs(including attached equipment) are better servedby battery replacement systems. Therefore, sys-tems which are capable of swapping the batteriesof the UAVs are needed for larger, heterogeneousUASs and we will focus on the design of servicestations for this purpose.
Despite very high energy density, Li-Po bat-teries require special care for recharging [20].Cycle aging also reduces the maximum chargecapacity of a battery [20, 23]. These facts to-
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gether demand smart battery chargers to ensurebattery safety and maximal usability through-out the battery lifetime. Therefore, we electedto use smart off-the-shelf battery chargers andrely on the “fully charged” signal from such acharger. The downside is that a small communi-cation network between chargers and the maincontrol is needed. This approach is different thanthe time-estimation approach used by [4], wherethe charger is turned on for a given amount oftime, then turned off regardless of the real statusof the battery. However, the possibility of usingbatteries at their maximum justifies the hardwarecomplexity of a smarter system. Not to mentionthat time-estimation does not address the failureof batteries, whereas a smart charger can identifysuch a situation.
Figure 2 depicts the basic topology of thecommunication network between the main con-trol and chargers. Designing the micro-controllersbehind each charger and the main microcon-troller to have a universal asynchronous re-ceiver/transmitter (UART) or other equivalentserial communication peripheral, one can designa ring network very easily. When a battery isready or faulty, the charger responsible for thatbattery sends such information addressed to themain control. The main control can also requeststatus from any charger by just sending a packetaddress to that charger. When a charger receivesa message which is not addressed to it, it forwardsit to the next charger. The main control neverforwards any message. In this fashion, the burdenof keeping track of the batteries is relieved from
Fig. 2 Communication network topology between charg-ers and main control unit
the main control unit; it can focus on batterychanging tasks and communication with the UASmain control.
3.2 UAV Positioning Methods
Batteries are held in a specific position on theUAV. To be able work with them, one shouldposition the UAV in a specific place after it haslanded or identify its location. Precision landingsystems were required in [2] and [4]. Kemper et al.[3] focused on how to address small errors in land-ing by increasing the UAV capture area duringlanding.
The design options presented in this paper as-sume that the ground stations are outdoors, whereweather conditions cannot be predicted. The focushere is not on landing algorithms, but on howto address UAV positioning after landing withsmall error. While UAV navigation systems areassumed to exist, we will allow landing positionerror. The goal is to allow the UAV to reachthe location where battery swapping will be con-ducted, even if its landing position is non ideal dueto navigation errors, weather conditions, UAVdamage, etc.
The design options related to UAV positioningare depicted in Fig. 3. Figure 3a shows a donutshaped platform that increases the possible cap-ture area of the UAV during landing. If the UAVlands within the boundaries of the donut, it slidesand will be guided towards the center of the plat-form, where the battery change occurs. If it landsoutside, it will slide outward without damaging theUAV.
To increase the area where the UAV canland, we also considered flat surface designs. TheUAV simply lands and is guided toward a specificlocation on the platform where the battery swap-ping occurs [3]. Figure 3b shows a UAV centeringmechanism in which four arms are pinned to fourdifferent points in a flat surface. These arms lie atdifferent heights to avoid collision between eachother and can be actuated individually by one mo-tor each or by one single motor using a pulley andcable actuation to close the arms. To return thearms to their original position, torsional springscan assist. The top part of Fig. 3b shows fourarms in an open/resting state. After the arrival of
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Fig. 3 Design options for UAV positioning system
the UAV, the main controller closes the arms sothat the UAV is pushed towards the center of theplatform as shown in the bottom part of Fig. 3b.A similar, but simpler and perhaps more reliableconcept is proposed in [5].
The design depicted in Fig. 3c is also basedon a flat platform with increased landing area toaccommodate landing errors. The target again isto move the UAV to the center of the platform,where battery swapping will take place. Insteadof solid arms, pulleys and cables are used. Theplatform will have cleft guides radiating from thecenter to the edge of the surface plate, whereslides will be placed (the six pairs of lines leadingfrom the central circle in Fig. 3c). These slidescan be moved radially, by radial actuation. Twopulleys are placed on the interior of each slide andtwo more pulleys are placed on the outward partof the platform (in this case, a hexagon is used,thus 12 pulleys are fixed on the outward part).The cables pass through the pulleys in a way that
one motor can tension the pulleys, forcing them tomove radially towards the center of the platform,while another motor causes cable tension to movethe pulleys outward. The outward resting positioncan be seen on the top part of Fig. 3c. When theUAV lands on the flat surface, the main controllerturns on the motor tensioning the cable of theradially movable pulleys. The pulleys will thenslide towards the center of the platform, draggingthe UAV to the center position.
3.3 Skid Changes or Add Ons
In order to swap a UAV battery, there is a needfor a more dynamic battery-UAV coupling sys-tem, a fast battery capturing device as well as asimple UAV positioning system. To achieve theserequirements, changes to the UAV are neces-sary, as original equipment manufacturer (OEM)skids and chassis do not comply with the systemneeds. For example, lets say that the UAV is
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being guided by four arms symmetrically pushingthe UAV towards a center platform as shownin Fig. 3b. It will be more convenient for ourdesign if the geometric center of the UAV skidsand battery are aligned. There are two options ifthe OEM UAV skids or chassis are not suitable.Either new skids can be designed to fit our neces-sities or add on parts to the skids/UAV chassis canbe used. New skids may reduce weight, but maycost more than adding extra parts. Using add ons,the weight of the UAV will increase, but cost maybe lower. To change the skids or to make add onsfor it, certain issues should be considered:
– The skid must support the UAV’s weight andkeep the UAV balanced,
– Skid’s modifications or add ons do not affectthe UAV payload significantly,
– Skids must allow battery removal from theUAV,
– Skids must allow UAV to be locked in placefor battery removal/placement, and
– Skid geometry must comply with the UAVpositioning method described in Section 3.
Figure 4 depicts three different skid configu-rations that can be used to guide the UAV to-
wards the battery swapping site. On the left, anadd-on to the existing skids is proposed. It is adisc of plastic material concentric to the geometriccenter of the battery case. It can be used for posi-tioning systems such as the ones shown in Fig. 3band c, since the actuation of the UAV occurs bypushing the add on disc part towards the center ofthe ground station. The center skid modificationin Fig. 4 is a square configuration of legs that hassides symmetric to the geometric center of thebattery case. The positioning system can use fourarms as proposed in Section 3.2 in Fig. 3b in a waythat the skids can be guided to the center of theground platform. In the case of the hexagonal skidshapes, it can use the positioning system shownin Fig. 3c, because the cables will tend to matchthe sides of the skids, as the flat platform itself ishexagonal. As in the previous case, the sides of thepolygon are symmetric to the battery housing inorder to guide the UAV to the desired centeringwith the battery coupling system.
Focusing on the square skids case, we proposea battery housing, which will provide a securelocation for the battery and avoid safety issues dueto mechanical damage. This can facilitate batterytransportation and provides easy modifications.It is also a good modular design for prototyping(it can be changed and redesigned very easily).
Fig. 4 Skid designs and add ons
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The battery housing has terminals that can cou-ple with the UAV in order to power it and alsohas connections to match with the charger in theground station. A possible design of a batterycase is depicted in Fig. 5. Projection views of abattery case are shown on the left and the iso-metric view of a possible skid assembly can beseen at the right. At the bottom and at the rightviews of the battery case, ferromagnetic plates areplaced with the intent of using an electromag-net to move it in other systems (see Sections 3.6and 3.7).
3.4 Battery and UAV Connecting System
Normally, batteries are connected to UAVs viawire plugs and placed inside the UAV, thereby,ensuring terminal connection and battery stability.However, in a platform that has to rapidly swap aUAV’s depleted battery for a recharged one, sucha connection/holding system is not suitable. Themethod by which the battery is secured and physi-cally connected to the UAV is of great importancebecause it will effect the repeatability, complexity,and the time that the ground station will take toswap batteries. Also, the added weight will effectthe UAV’s payload and flight time.
In order to create the interface between UAVand platform, mechanical and magnetic couplersare considered and devices that can easily holdand release batteries while ensuring terminal con-nections in a UAV are proposed.
3.5 Mechanical and Magnetic Couplers
A battery holding system in a UAV must be lowin weight, securely hold the battery during flight,resist small impacts (such as small drops), main-tain the battery and UAV terminal connections,and allow easy insertion and extraction of thebattery when necessary. To do so with mechanicalcomponents, a simple model can be employed.
Figure 6a depicts a mechanical coupler modelin which a battery is forced upward against thelower angled surface of spring assisted latch de-vices (the symmetric arrow-shaped pins). Thisactuation generates a force in the latch device,whose horizontal component will contract thespring allowing the battery to pass through themechanism. When the battery finally reaches theend, it does not compress the springs anymore andthe spring of the latch device will force the batteryupward, guaranteeing the electric contact of UAVand battery terminals during flight.
Fig. 5 Battery case design for magnetic couplers
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Fig. 6 Model of batteryinsertion in mechanicaland magnetic coupler
Figure 7 shows an example mechanical couplerfor a battery case to match the UAV. It consistsof leaf spring latches that have angles on the ends.When actuated upward, the latches bend, allow-ing the battery to pass through the spring. Sincethe battery case has matching indentations on itssides, it allows the spring to lock the battery casein place.
Instead of using moving parts as in a me-chanical lock, neodymium magnets can be used(Fig. 6b). These magnets can provide similar hold-ing characteristics as a mechanical coupler. If themagnets are used as interface terminals betweenthe UAV and battery, then it becomes a more ver-satile plug-and-play terminal auto-guiding system.This approach was used in [4], where neodymium
magnets were set in a fiberglass battery casing andin an ESky LAMA V4 helicopter’s belly to estab-lish terminal match and coupling. In that system,a servo motor rotates the battery case in orderto cause shear in the magnets and allow batterydisengagement from the UAV. While there canbe concern about interaction between the magnetsand the UAV gyroscope, [4] reported no suchcomplications.
Figure 6b shows a straightforward procedure ofcoupling and terminal matching. By simply mov-ing the battery toward the UAV, which both pos-sess magnet terminals, the fixed UAV will attractthe battery to it, forcing terminal match. Magnetpolarity on the terminals can be inverted to ensureonly one locking position.
Fig. 7 Example of mechanical coupler with matching battery case
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Magnets are very versatile, but they have dis-engagement problems against shearing forces andimpact. Also, if they are used as terminals, theymay attract metallic bodies that may cause short-circuits. Adding magnets to all batteries in a sys-tem is a good idea to ensure terminal connectionand locking. However, it may cause problems inbattery transportation because the batteries mayattract each other and cause unwanted terminalcontact (which can be very undesirable for Li-
Po batteries). Another issue is the total numberof magnets in the system; too many may increaseoverall system cost. In addition, neodymium mag-nets are very brittle, and if they are releasedto match from a distance, they may break uponcollision.
Although magnets on both the UAV and bat-tery can enable easy matching, there are disad-vantages as discussed previously. An option toreduce the use of magnets is to use them only in
Fig. 8 Battery case and UAV module assembly in UAV skids
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the UAVs and replace the ones in the battery casewith sheet metal ferromagnetic terminals.
To steer clear of the possible short-circuit is-sues, a male-female plug connection is proposed.This type of connection allows a physical “wall”to stay between the terminals, and even if metal-lic parts are attracted to them, the probabilityof short-circuits is reduced. This design optionis depicted in Fig. 8. The top left figure showsmale connectors in the front view of the batterycase, while the top center figure provides a cutview of the UAV module to show the femaleplugs and terminals. The terminals plus (+) andminus (−) in the battery case are the terminalsthat will power the UAV when connected to it,while all four of them (given a 3 cell battery) willbe used to recharge the battery when it reachesthe recharging platform. The terminals plus (+)and minus (−) in the UAV module are the UAVterminals, while 1 and 2 are only magnets that helpto secure the battery case in the correct position.
3.6 Battery Capturing System
Another important system in the design of an au-tomatic battery swapping machine is the methodto secure a battery for transportation and how tosafely insert and extract batteries from the UAVs.The required systems are a battery capturing de-vice and a battery transportation method fromUAV to swapping site. We consider only mechan-ical and electrical options.
The battery capturing system has only one func-tion: prevent the battery from movement in anydirection relative to the capture device or “grab-ber”. We considered a mechanical claw, servo-assisted magnets and electromagnets. Mechanicaldevices require guidance and positioning in orderto clamp the battery and pull it out of the UAV.This sort of system demands use of physical spaceto open and close arms, has many moving parts(is thus more prone to failure) and may requiretight tolerances to ensure assembly and guidemovements. Electromagnets could also be used asa capturing system. They are easy to control andhave high repeatability, although a ferromagneticplate must be attached to the battery case, increas-ing on board weight. A servo-assisted permanentmagnet was used in [4] to extract the battery fromthe UAV. It is also an option to use magnets onboth sides of the UAV/battery interface. Such asystem does not use a servo-motor at all times,and so, small amounts of current are required.However, it has moving parts and magnets mayattract foreign bodies, causing short-circuits in thesystem.
We propose a capturing system with one elec-tromagnet to remove the battery case from theUAV. Such a system must be tested to ensure nointeraction with UAV guidance systems.
In Fig. 9a, the UAV is assumed to have reacheda specific position. At the bottom of the UAV,there is a battery case, which is being held byeither mechanical coupling or by magnets, as de-
Fig. 9 In a, an electromagnet is moved up. In b, the electromagnet reaches the battery case and is turned on, capturing thebattery from the UAV. In c, the electromagnet is pulled downward and the battery case is extracted from the UAV
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scribed in the previous chapter. At the bottom ofthe battery case, there is a ferromagnetic metallicplate. The capturing system moves vertically viaan elevator and the electromagnet is activatedwhen necessary. The system moves upward untilit reaches the bottom of the battery case metallicplate. As shown in Fig. 9b, the electromagnet isthen turned on and thereby securely attaches tothe battery case. As seen in Fig. 9c, the elevatorpulls the electromagnet down, resulting in batterycase extraction from the UAV. This approach isdifferent from the one in [4], that instead usesshearing forces to separate the magnets. Due toour approach, either mechanical or magnetic cou-plers can be used to secure the battery case to theUAV.
3.7 Battery Transportation Method from UAVto Swapping Site and Battery Storage System
As seen in Section 3.6, there is a need to transportthe depleted battery and case to the swapping site,where a recharged battery can be obtained. Inorder to charge the depleted batteries, a chargerarray is required. In our proposed design, eachcharger has a housing that matches the batterycase geometry. It must have terminal interfacesthat can connect the battery case terminals withthe charger ones. The chosen design for the eleva-tor was a vertical scissor elevator that can extractthe battery and the case from the UAV, deliverthe battery to the battery supply site, and returnthe case and recharged battery back to the UAV.
Fig. 10 In a, swapping system components are shown. Inb, the rack and pinion actuation system places the depletedbattery in the buffer zone, while releasing the chargedbattery in the swapping zone. In c, the charged battery is
actuated to the UAV, while the rack retrieves the depletedbattery from the buffer and places it inside the chargermagazine
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The charger array design chosen was a circulararray of chargers that feed the elevator: a batterymagazine. To connect these two systems, a rackand pinion system is proposed for the battery andcase transportation that removes the battery fromthe charger array and delivers it to the elevatorand vice versa. The rack has another electromag-net on the actuating end. It will hold the batterycase while being transported.
The systems described in this section are de-picted in Fig. 10. Figure 10a provides informationabout the system parts. Initially, the UAV is tobe guided to a fixed place, where the elevator canaccess the battery case. In Fig. 10b, the depletedbattery is assumed to have reached the swappingsite, at the center of the circular platform. Thebattery is being held on the top of the elevatorby an electromagnet (as seen in Section 3.6). Thecharger array positions a magazine cell containinga charged battery in alignment with the rack andpinion system. The electromagnet at the tip of therack will turn on and the rack will be actuated. Thecharged battery is pushed toward the depletedbattery by the rack, and together they move untilthe used battery reaches the buffer zone, markedwith the sign “−”. The rack then retracts (leavingthe old battery in the “−” position until the fullycharged battery and case are positioned on theelevator. The rack electromagnet is turned off,thereby releasing the new battery and case. Therack retracts further to clear the elevator opera-tion area. In Fig. 10c, while the elevator raises the
new battery to the UAV, the rack system capturesthe depleted battery from the buffer and places itin the charger magazine, initiating charging of thedepleted battery.
3.8 Battery Storage System Optionsand Consequences of Design Choices
The place where the battery will be stored forcharging is of great importance for the system. It isdirectly related to the way that the battery will betransported back to the UAV as well as how thechargers will be connected to the power line. Theoptions proposed are:
– One or more circular arrays of batteries to beused as a magazine,
– X–Y axis array of batteries, and– X–Z axis array of batteries.
The circular array, depicted in Fig. 11 is notspace efficient; the center part is not used. Thereare complications in connecting the chargers,since the disc will spin, and simple wire con-nections may be caught in the assembly. As thebatteries are arranged in a circular array, accessto batteries is simple and may be more energyefficient. There is a need only for one actuator tospin the disc, and another to swap batteries fromthe assembly. As the disc only spins, it is easier tocontrol when compared to a two axis system.
The linear arrays are very space efficient, be-cause a grid of battery chargers can be organized
Fig. 11 Magazine of batteries in a circular array of chargers actuated by rack and pinion system
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Fig. 12 Batteries in a matrix array of chargers accessed by a small gantry crane
easily and side-by-side. Since it is a static system,wiring is possible, making it easy to make chargerconnections with the station. There is a need forprecise actuation and linear guides in at least twoaxes, in order to find the correct battery positionin the charger array. This will increase controlalgorithm complexity, when compared to the cir-cular array design. Figure 12 depicts one possibleimplementation of such a linear array in an X–Yorientation. A rectangular X–Z array can also besimilarly conceived.
3.9 Elevator Design Option for UAV VerticalTransportation
There are other options to deliver the batterycase to the swapping site, other than moving thebattery after extracting it from the UAV. Oneis to lower the entire UAV, and at the swap-ping site, exchange batteries with the UAV, asdepicted in Fig. 13. Such a system may reduce
complexity of the battery holding device, allowbattery extraction from the front part of the UAVand minimize UAV modifications. As a con, thenumber of floors of the circular battery array discscannot be increased easily, since the UAV must beable to drop low enough to reach each array oncea more complicated system is required. Also, theamount of force required from the elevator will beincreased, since the entire UAV is elevated.
4 Prototype
We next discuss module prototypes developedto learn the failure modes and measure robust-ness to continuous operation. Considering thata deployed station is expected to operate with-out human assistance for a long time, we de-signed many of the parts to operate under loosetolerance specifications to account for eventualwearing. Most of the parts were manufactured
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Fig. 13 In a, UAV is lowered to the swapping site. In b, the depleted battery is extracted straight out of the UAV. In c, anew battery is actuated in the UAV after magazine actuation
with laser-cut acrylic to simulate loose toleranceand unpredicted wearing. Successful operation ofthe system under loose tolerance conditions sug-gests that the system also works well with tighttolerances. A full CAD prototype is shown inFig. 14.
Figure 15 depicts the block diagram of ourground station. The dark blocks are the mod-ules which we prototyped and tested. Some ofthe blocks are complementary to each other. Forexample, to lock and unlock the UAV in theplatform, the opposite actuation is used, althoughother methods and mechanisms could be applied.
The following sections will briefly introduce themodule prototypes.
4.1 Orientation-fixing Module
Outdoor conditions may cause difficulties for thenavigation system. Therefore, to require the UAVto land in a very specific position with a veryspecific orientation may not only be unattainablebut also risky; the UAV already has low batterypower when seeking the platform. Therefore, ac-cepting as many different landing positions as pos-sible enhances the chances of keeping the UAS
Fig. 14 Full groundstation CAD prototype
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StartUAV hasarrived?
No
Yes Center UAV
Unlock UAVInform control
That UAV is ready
No
Has the UAVdeparted?
Prepare systemfor next UAV
End
Yes
Extract batteryfrom UAV
Insert batteryback in UAV
Bring battery toexchange zone
Swap battery witha charged one
Place depletedbattery into charger
Place nextcharged battery in
position
Move UAV to rightorientation
Lock UAV
Fig. 15 Block diagram of the complete system for swapping batteries
fully operational for a longer time. Assuming thatthe position was already corrected by the position-fixing module and the UAV is centered but withunknown orientation, the orientation fixing mod-ule fixes the UAV at its center of mass and spins ituntil it finds the correct orientation. The correctorientation is detected by an IR sensor. Whenthe UAV reaches the correct position, light froman IR LED reflects from the skid into the IRsensor. To prevent outside sources (e.g. the sun)from interfering with the reading, the IR is sent inpulses at the frequency of 250 Hz, and a straightsequence of 15 matching pulses (checking on highand low) determines a positive orientation match.Since we are using a step motor to spin the UAV,and the step motor is also driven by the same codewhich drives the IR pulses, 15 matching pulses isabout what is needed to certify a correct orien-
tation without clocking the step motor one moretime, which would then add some error. The blockdiagram can be seen in Fig. 16.
The prototype of this module is shown inFig. 17. The round white plate in Fig. 17a liftsand rotates the UAV, while the black dots in theforeground of the black colored base plate arethe IR sensor. Lift is done manually by rotatinga lever at the side of the prototype. After findingthe correct position, the step motor stops spin-ning and the round plate can be lowered. Successrate of this assembly, indoors, was 100% in 100trials.
4.2 UAV Locking/unlocking Module
Assuming that the UAV is already in the correctposition with the correct orientation, the UAV
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Move UAV to rightorientation
Start
Lift UAV
Turn off Step MotorControl
Turn on Step MotorControl
Lower UAV
End
Rotate UAV 1 step
Is the UAV on theright orientation?
Send IR burst
Count receivedpulses
No
Yes
Fig. 16 Block diagram of the orientation connectionmethod
locking system ensures that the process of extract-ing the old battery and inserting a new one will notaffect the position and orientation of the landedUAV. This is required since the battery swap-ping system assumes the UAV to be in a knownposition to perform its tasks. The module wasimplemented as a pair of servomotors each with a
plastic arm designed to match the UAV landingskid at the given position. This implementation al-lows for some position correction based on the an-gular motion of the servo, which can displace theUAV sideways in case the orientation-fixing mod-ule had affected the position alignment. Althoughthis system works just fine, it still allows somefore/aft translation of the UAV skids. Adding twoplastic arms per skid (instead of just one) allowsthe system to lock on the skid support structure aswell, preventing not only sideways translations butalso fore/aft translations. The white plastic lockscan be seen in Fig. 17a, and a locked UAV can beseen in Fig. 17b.
4.3 Battery Extraction Module
Assuming that the UAV is already in the right po-sition, with desired orientation, and secured, thissystem extracts the old battery from the landedUAV. There is an electromagnet at the centerof the orientation fixing module. At the bottomof the battery case there is a steel plate. Theelectromagnet secures the battery case by thisplate. Once the electromagnet has attached tothe steel plate at the bottom of the battery case,the structure holding the electromagnet descends.The force generated by the electromagnet exceedsthat of the battery case magnets holding the caseto the UAV. Thus, the battery case is separatedfrom the UAV. In the prototype, this downwardmotion is done by hand, since the weight of theorientation fixing module is not enough to de-tach the battery. The complete implementation,however, relies on the elevator to provide enoughforce to remove the battery and case. The electro-magnet can be seen on Fig. 17a, at the center.
The battery case and matching plate on theunderbelly of the UAV add about 20 g to theoverall UAV mass.
4.4 Battery Swap Module
Assuming that the battery was successfully re-moved from the UAV, brought to the exchangezone, and that there is a charged battery alreadyin position to be exchanged, the battery swapsystem exchanges the old battery for a new one.The battery swap module is also responsible for
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Fig. 17 Prototype for the lock module, orientation-fixing module and battery extraction module. In a it is possible to see theelements of each module. b shows a UAV with the orientation already fixed and locked into the platform
placing the old battery inside a charger. Figure 18ashows the basic components of the module. Thebatteries are moved by a rack and pinion system.There is a step motor driving the pinion. At one ofthe ends of the rack there is an electromagnet. Theset of step motor, rack, pinion and electromagnetwe call “battery actuator”. The battery case hasa steel plate on its side, which is used by thebattery actuator to secure it during movement.The zone marked with an “X” is the exchangezone, it indicates the location where the batteryextraction module rests the battery after it hascompleted removal of the battery and case fromthe UAV.
The zone marked with a minus is a buffer.Figure 18b shows the situation where the usedbattery and case have been lowered to the swapmodule; a fresh battery and case is to the imme-diate left of the old battery. In Fig. 18c, the rackhas pushed the new battery to the “X” position,displacing the old battery to the buffer (marked“−” (minus)). The rack electromagnet releasesthe new battery and it is delivered to the UAV.After, in Fig. 18d, the rack extends, attaches to theold battery case and pulls it out of the “−” (minus)location. That battery would then be delivered tocharging module. This setup went through a reli-ability test session, successfully completing 2,500trials without any errors.
The final location of the used battery is shownin Fig. 11 in the context of the larger system. It will
rotate away from the rack and a new battery willreplace it.
4.5 Estimated Swap Time of One Ground Station
To evaluate the performance of a full-scale pro-totype, we used data from the prototyped mod-ules and estimations from our CAD models. Theprototyped modules were extensively tested andimproved until 100% success rate was obtainedat loose tolerances. The worst-case execution timewas then measured. The non-prototyped moduleswere assumed to work also at 100% success rate,and the worst-case execution time was estimatedusing motor data sheets, eventual reductions ap-plied to the motor and the moment of inertia fromthe CAD models. Table 1 summarizes the data.
With this data in hand, we could then define amaximum size and maximum coverage of a givenground station (see Section 2). Of course, by try-ing to make all the terms of Eq. 5 equal, some de-sign adjustments may affect the time estimationsin Table 1, so some iteration may be required.Assuming that our designed platform swappingtime is within 1 min (the 47.5 s estimated plussome extra time), and charge time and flight timeare 85 min and 15 min, respectively, the maximumnumber of UAVs that this service station cansupport is 16 UAVs (forcing TLUAV = TLPLAT ), andthe minimum number of batteries to support theUAVs continuously is 103 batteries (16 on the
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Fig. 18 a shows the components of the system, whileb–e demonstrate the swap process. On b we see the oldbattery at the exchange zone and c shows the old batteryat the buffer and the new one at the exchange zone. At d,
the new battery was carried to the UAV and the batteryactuator reaches the old battery while in e the old batteryis brought to the charger
UAVs, 87 on the service station). The maximumcoverage which one of these stations can produceis then CACH
SYS = 15, so if higher coverage is re-
quired, either the modules and/or processes onthe ground station must work faster, or an extraground station should be employed.
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Table 1 Estimated swap time, detailed by each moduleexecution time
Module Time (s) Notes
Position fixing 15.0*Orientation fixing 13.0 Worst case (full turn)UAV lock 1.0 To be counted twiceElevator 6.5* To be counted twiceBattery swap 4.5Total 47.5
Estimated times are marked with an asterisk (*)
5 UAS Cost Comparison
To determine the cost of a UAS requires an esti-mate of the number of components in the system.From Theorem 1 we can directly obtain the num-ber of components to achieve a target coverageCTGT
SYS .
Corollary 1 For a recharge platform, the minimumnumber of components that will guarantee a targetcoverage of CTGT
SYS is:
NUAV =⌈
CTGTSYS
TF· (TF + TR + TI)
⌉, (6)
NPLAT =⌈
CTGTSYS
TF· TR
⌉. (7)
Proof By Theorem 1, CTGTSYS ≤ CSYS = TF/TCYC.
Equivalently, TF / CTGTSYS ≥ max
(TLUAV , TLPLAT
)or TF / CTGT
SYS ≥ TLUAV and TF / CTGTSYS ≥ TLPLAT .
Since TLUAV = (TR + TF + TI) /NUAV and TLPLAT =TR/NUAV, it is equivalent to require NUAV ≥CTGT
SYS · (TR + TF + TI) /TF and NPLAT ≥ CTGTSYS ·
TR/TF . The result follows. ��
Similarly, for replacement we obtain thefollowing.
Corollary 2 For a replacement platform, the mini-mum number of components that will guarantee asystem coverage is (6) and (7),
NBATT =⌈
CTGTSYS
TF· (TC + TR) − NUAV
⌉, (8)
NCGR =⌈
CTGTSYS
TF· TC
⌉. (9)
The proof follows the same method as inCorollary 1.
For a charging platform the total componentcost of the UAS (including the platforms, charg-ers, batteries and UAVs) is
VRECHTOT = NPLAT · VRECH
PLAT + NUAV · VUAV + NCGR
·VCGR + NBATT · VBATT,
and similarly, the cost of a replacement system is
VREPTOT = NPLAT · VREP
PLAT + NUAV · VUAV + NCGR
·VCGR + NBATT · VBATT,
where VRECHPLAT , VREP
PLAT, VUAV, VBATT and VCGR arethe costs of a recharge platform, a replace plat-form, UAV, battery, and charger, respectively.
The UASs described here are composed of sev-eral modules that result in the final cost estimationof this ground automation system. The cost analy-sis is a rough estimate. We focus our attentionon parameters specifically related to the systemsdesigned and shown throughout the developmentof this work, which is the energy replenishmentof the UAV itself. Parameters such as the cost ofthe navigation system, the external power and thehuman resources are not used in the estimation.We consider
– Platform basic cost: estimated from the com-plexity of the system,
– Battery: number of batteries in the system,– Charger: number of chargers present in the
system,– UAVs: number of UAVs the system needs to
reach the required coverage.
Here, in the case of refill platforms, the basiccost is estimated according to UAV complexity.This is a rough estimate based on a system thathas a fair level of electronic complexity (related tonumber of LiPo battery cells) and UAV-platform-connection interfaces (related to how many bat-tery terminal connections are needed). In the caseof battery replacement platforms, this basic costis estimated to be 1500 USD, since moving partsare added in order to transport batteries in thesystem.
Refill platforms work with one charger eachand each UAV carries its own battery. Replace-ment systems require fewer UAVs to obtain the
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Table 2 Data used toplot the cost comparisongraph
Cheap Mid-range Expensive
Cost of (USD)Battery 5 20 100UAV 30 500 1,500Charger 10 15 50Refill platform (raw cost—same for all) 20Replace platform (raw cost—same for all) 1,500
Time of (min)Flight 10 15 30Charge 25 85 85Replace 1 1 1
Maximum values per replacement platformMaximum coverage 10 15 30Minimum chargers 27 87 87
same coverage, because of the relatively fast en-ergy replenishment time (See Section 4.5).
The information obtained from the Petri nets(see Section 2) and the experiments mentioned inprevious sections are summarized in Table 2, forthree types of UAVs.
The “Cheap” UAV is in the micro aerial ve-hicle (MAV) range, which can be a very smallUAV with the size of an insect or a UAV forhobby purposes with estimated flight time of 10min and battery recharge time of 25 min. Theirbattery has fewer cells, thus less cost (battery andcharger costs are estimated as 5 and 30 USD
respectively). The cost for a platform designedto fit this model of UAV is 20 and 1,500 USDfor refill and replacement systems respectively.The refill platform cost is very low compared tothe replacement one. This happens because therefill case only needs a few electronic componentswhile the replacement platform requires batteryreplacement (therefore, guides, actuators and pre-cise movements are needed).
Based on the information of the previous sec-tions, we draw a cost comparison graph. Sincethe current focus of this paper is on replacementplatforms, in Fig. 19 we plot the cost advantage
Fig. 19 Cost comparisonbetween batteryreplacement and batteryrecharging ground servicestations
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of a UAS using replacement platforms against anequivalent UAS based on refill (recharge) plat-forms. We define cost advantage at a given cov-erage C as:
Adv(C) = VRECHTOT (C) − VREP
TOT(C). (10)
Positive values of Adv(C) imply that the re-placement system is economically more viablethan a refill system. Figure 19 shows that thepoints where replacement systems become moreefficient are C = 1.23 and C = 1.33 for mid-range and expensive UAVs, respectively. On theother hand, a replacement UAS never becomeseconomically better than a refill UAS for cheapUAVs. However, it is important to notice that thenumber of UAVs required to fulfill a given targetcoverage is usually greater in a refill UAS than in areplacement UAS. This analysis does not includecontrol algorithm complexity nor the availabilityof control channels, therefore replacement UASmay still be better.
6 Concluding Remarks
In order to plan a new UAS or to analyze an exist-ing UAS, we developed a Petri net model whichallows us to calculate the system coverage basedon the component parameters. UAS expansionor resource management can be guided and/oroptimized with the aid of this model.
We also developed several module prototypes,both conceptually and physically, which togethercomprise one service station to swap batterieswith high success rates. The complete stationis able to compensate for orientation and posi-tioning errors. It addresses navigation impreci-sion, weather conditions and/or UAV damage,which are common issues in outdoor missions andcombat environments. The ground station is alsocapable of handling heterogeneous UAV fleetswith not only different shapes and sizes, but alsodifferent number of battery cells per battery pack.Further development of such replacement stationsand replenishment stations is essential to reducethe limitations caused by having humans in theloop.
7 Future Work
As future work, we will conduct further test-ing with prototype modules of UAV positioningsystems (see Section 3.2), new skid designs (seeSection 4) and battery transportation systems(see Section 3.7). After the prototype modulesare fully operational and reliable, the next in-tended step is to make an entire integrated sys-tem that can replace UAV depleted batteries fornew recharged ones autonomously upon UAVlanding.
Other future work is related to gasoline refu-eling ground stations, where studies will be per-formed with the goal of having fully autonomousground service stations for heterogeneous fleets.
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