Automatic Battery Replacement System for UAVs: Analysis and Design

8/10/2019 Automatic Battery Replacement System for UAVs: Analysis and Design

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J Intell Robot Syst (2012) 65:563586

DOI 10.1007/s10846-011-9616-y

Automatic Battery Replacement System for UAVs:

Analysis and DesignKoji A. O. Suzuki Paulo Kemper Filho

James R. Morrison

Received: 15 February 2011 / Accepted: 15 June 2011 / Published online: 9 September 2011 Springer Science+Business Media B.V. 2011

Abstract Future Unmanned Aircraft Systems

(UASs) are expected to be nearly autonomous

and composed of heterogeneous Unmanned Aer-

ial Vehicles (UAVs). While most of the current

research focuses on UAV avionics and control

algorithms, ground task automation has come to

the attention of researchers during the past few

years. 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(ICUAS11), May 2011. The Proof of Theorem 1 andreorganization leading to Section5are 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 UAS

operation area, improve system coverage and en-

able operation in risky environments without pos-

ing a threat to humans. We propose a model to

evaluate the coverage of a given UAS. We also

compare different solutions for various modules

of an automated battery replacement system for

UAVs. In addition, we propose a ground station

capable of swapping a UAVs batteries, followed

by a discussion of prototype components and tests

of some of the prototype modules. The proposedplatform is well-suited for high-coverage require-

ments and is capable of handling a heterogeneous

UAV fleet.

Keywords Unmanned aerial vehicles UAVs

Autonomous consumable replacement

Service station Enabling technologies

Autonomy

1 Introduction

The United States Federal Aviation Adminis-

tration (US FAA) defines an Unmanned Air-

craft System (UAS) as a collection of Unmanned

Aerial Vehicles (UAVs), control station(s), com-

mand and control algorithms and equipment for

launch, recovery, communication and navigation

[1]. While many efforts have been directed to

the study of these components, there has been


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564 J Intell Robot Syst (2012) 65:563586

significantly less attention paid to the automation

of ground maintenance tasks. While such activities

may be relegated to humans, to achieve a UAS

with greater range, applicability and autonomy,

such efforts must be automated.

Initial ground breaking work in this area has

recently been conducted. In [2], the first prototypeof a battery recharge platform for UAVs was

developed to support the RAVEN UAS test bed.

To our knowledge, the authors in [3] are the first

to study and provide an analytic answer to the

question of whether a battery charging or replace-

ment platform is preferable. They also developed

and compared various recharge station designs

and proposed a conceptual replacement platform.

Simultaneously and independently of[3], authors

of [4] developed the first prototype of a UAV

battery replacement system as part of their ACEtest bed. They also developed management soft-

ware to support the process of operating UAVs

with such a platform and an off-UAV method to

deduce when a battery requires service. In [5],

and subsequent work (not yet published), a self

leveling platform that keeps its base parallel to

the ground and centers the UAV was developed.

The prototype efforts for UAVs follow related

work for ground based robots in [612] for battery

charging 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 that

move UAVs to a proper refueling position were

tested.

Despite these advances, numerous opportuni-

ties remain. First, the analysis of[3] is based on

bounds so that there is an opportunity to tighten

the results. Second, while most of the proposed

solutions for ground based service stations rely

on homogeneous fleets of UAVs, it is anticipated

that the UASs of the future will be heterogeneous

[16]. Third, service stations should be robust to

environmental factors that compromise the in-

tegrity of UAV positioning systems; existing de-

signs are intended primarily for laboratory based

test beds. Fourth, there is a need to consider

additional design choices for the components of

service stations to improve system reliability and

performance.

In this paper, we strive to address these needs

for 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 that

enable a tight comparison between them.(Section2)

The paper develops and compares design op-

tions for the functional components of battery

replacement service stations. These attempt to

address the issues of fleet heterogeneity and

UAV landing control robustness. (Section3)

The paper reviews and conducts operational

testing of key components of a system proto-

type. (Section4)

Using estimates of the replacement platform

costs based on the design choices of Section 3,we compare the cost of various platforms and

systems. (Section5)

Concluding remarks are presented in Section6

and future work is presented in Section7.

2 Planning UASs

A Petri net can be considered a graphical tool

that 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 into

transitions (T, represented by bars) and places (P,

represented by circles). The connection between

nodes is made by directed arcs, which connect

only a T to a P, or a P to a T, never P to P or T

to T. Tokens (usually represented by dots) travel

through the net. Whenever there is a token at the

input of all arcs leading to a transition, the transi-

tion fires, the tokens at the input are consumed

and a token is created at the output of each of the

outgoing arcs. The event of a firing can happen

concurrently and can overlap in time with other

firings so long there are enough tokens to fire

a transition. The location and number of tokens

at the start of the Petri net evolution is called

the initial marking. Times may be associated with

the transitions and places. Tokens entering a

node must wait for this duration before they are

released.


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If there is only one output arc and one input

arc at any of the places of a given Petri net,

the network is called decision-free [18]. The

cycle time of a decision-free Petri net is straight-

forward to analyze without much computational

effort.

Figure1depicts Petri nets for a refill/rechargeand a replace system, where TF stands for the

time a UAV spends in the air, TR is the time it

spends at the service station and TCis the charging

time a battery requires to achieve full charge when

fully depleted.TIis the idle time allocated to each

UAV in one operation cycle. During this time,

the UAV simply rests and expends no energy.

It serves to adjust the system to lower coverage

requirements and can be as low as zero. There is

no duration associated with the places.

In the Petri net for the recharge platform on theleft of Fig.1, the duration TRshould be at least as

long as a battery charging time; the UAV remains

with the platform until its battery is fully charged.

In the Petri net for the replacement platform on

the right of Fig.1,TRwill be small relative to TC,

since only a battery swap is required during the TRduration.

In the initial marking of the replacement Petrinet, there are NUAV, NPLAT, (NBATT NUAV),NCGRtokens 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 with

a battery), exchange platforms, backup charged

batteries and battery chargers, respectively. In the

refill Petri net, the initial marking is the same as

in the replace Petri net for those places shared in

both nets (P1, P2, P3 and P4).Since the resultant Petri nets are simple, analy-

sis is rather straightforward. According to [18], the

system cycle time for a decision free Petri net can

be obtained as max TiNi , where i ranges over allloops in the net, Ti is the sum of durations along

these loops and Ni is the number of tokens in

places in the loop initial marking. It is equivalent

to consider only simple loops.There are four simple loops in the replacement

net: UAV loop (LUAV), platform loop (LPLAT),

Fig. 1 Petri Net model.The left-hand side models

a 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 the

number 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 every

TCYCunits 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+TRNBATTNUAV

, TCNCGR

.

ProofSince we have NUAVUAVs in the system,

the system coverage equals:

CSYS= limt+

NUAVi=1

1

t

t0

IFU AVi(t) dt,

whereIFUAVi(t)is the indicator that UAViis in the

air at time t (its value is 0 or 1) and we assume the

NUAVUAVs are labelled UAV1, ..., UAVNUAV .

According to [19], decision-free Petri Nets,

such as those of Fig.1, will become periodic with

period T after an initial transient duration (t0).

From [18], each transition fires on average every

TCYC units of time. Thus, one can conclude that

the period T= K TCYC, where Kis the number

of firings of a particular transition during one fullperiod (any transition will do since they all fire

at the same average rate). Let T = (p K)TCYC,

where pis an integer such that p K= q NUAV,

for q integer. (Clearly, such a p exists since p =

NUAVwill give p Kas some multiple ofNUAV;q

tells us exactly how many multiples.)

Now, sinceT is a multiple of the period T, we

can write

CSYS=

NUAVi=1

1T

ti+Tti

IFUAVi(t) dt,

where ti t0 is the time of the firing of tran-

sition TF corresponding to the first launch of

UAVionce 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 =

NUAVi=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 loops

are under-utilized (on stand-by). If we decide to

make the three loops period equal by changing

the variables under our control (e.g. the number

of UAVs, idle time, etc) we can optimize the UAS

to the desired specifications (or approach the

bounds obtained previously). As in [3], Eq.5 and

Theorem 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 UAVs

and one exchange (replacement) platform with

TF= 15 min, TR = 1 min and TC= 85 min. Set

TI= 0. Let TS denote the duration of a UAVoperation cycle, that is, TS =TR + TF+ TI=

16min.

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 =

TRNPLAT

= 1, TLBATT =TC+TR

NBATTNUAV= 5.059, and TLCGR =

TCNCGR

= 5.313.

Thus, TCYC = TLUAV and CSYS = TFTCYC

= 2.8125

UAVs/unit time. If we want a lower coverage

with the same system, say 2.5, we can increaseTI. Setting TI= (TF+ TR)

NRUAVCUAV

CTGTSYS 1

= 2

min, we obtain TCYC = 6 min. We then have

CSYS= 2.5UAVs/unit time.

3 Design Options

In this section we discuss design options for an

autonomous UAV battery replacement system

ground station. This station is to automatically

swap 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 parameters

to solve issues such as:

Guiding the UAV to the battery replacement

site,

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 are

of interest to both civil and military applications

[16]. It is desirable that service stations provide

service to as many different kinds of UAVs in

the UAS as possible. This may serve to reduce

the number of ground stations and promote an

even spread of the coverage of a given UAS.

Since Lithium-polymer (Li-Po) battery-powered

UAVs are popular due to the high energy density

of Li-Po batteries [20], we elected to design a

system that can serve UAVs with two-cell andthree-cell Li-Po batteries. Li-Po batteries require

a 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 of

the electric terminals and minimizes charging logic

complexity.

A popular UAV employing this Li-Po battery

is the Lama V3/4 [21]. It was used in the Au-

tonomous Control Environment (ACE) test bed

of [4]. The Lama V3/4 is a coaxial, small remotecontrol helicopter designed and manufactured by

E-Sky. It is primarily for hobbyists and is easy to

fly. The main rotor diameter is 340 mm, the body

weight is 230 g (with original equipment manufac-

turer (OEM) battery) and the OEM power supply

is a 2-cell Lithium Polymer battery pack (7.4V

800 mAh). Such UAVs are usually relatively inex-

pensive, simple, light and susceptible to weather

conditions.

The HoneyBee King 2/3/4 [22] is an example

of a UAV using a three-cell Li-Po battery. Fol-lowing a more traditional helicopter design, the

HoneyBee King has a main rotor and a tail rotor

to control the UAV. The main rotor diameter is

600 mm, the body weight is 470 g (with OEM

battery) and the OEM power supply is a 3-cell

Lithium Polymer battery pack (11.1V 1000 mAh).

Such UAVs are typically more robust with greater

capability than their smaller counterparts but re-

quire more skill to control.

As seen in [3], systems with expensive UAVs

(including attached equipment) are better served

by battery replacement systems. Therefore, sys-

tems which are capable of swapping the batteries

of the UAVs are needed for larger, heterogeneous

UASs and we will focus on the design of service

stations for this purpose.

Despite very high energy density, Li-Po bat-

teries require special care for recharging [20].

Cycle aging also reduces the maximum charge

capacity of a battery [20, 23]. These facts to-


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gether demand smart battery chargers to ensure

battery safety and maximal usability through-

out the battery lifetime. Therefore, we elected

to use smart off-the-shelf battery chargers and

rely on the fully charged signal from such a

charger. The downside is that a small communi-

cation network between chargers and the maincontrol is needed. This approach is different than

the time-estimation approach used by [4], where

the charger is turned on for a given amount of

time, then turned off regardless of the real status

of the battery. However, the possibility of using

batteries at their maximum justifies the hardware

complexity of a smarter system. Not to mention

that time-estimation does not address the failure

of batteries, whereas a smart charger can identify

such a situation.

Figure 2 depicts the basic topology of thecommunication network between the main con-

trol and chargers. Designing the micro-controllers

behind each charger and the main microcon-

troller to have a universal asynchronous re-

ceiver/transmitter (UART) or other equivalent

serial communication peripheral, one can design

a ring network very easily. When a battery is

ready or faulty, the charger responsible for that

battery sends such information addressed to the

main control. The main control can also request

status from any charger by just sending a packetaddress to that charger. When a charger receives

a message which is not addressed to it, it forwards

it to the next charger. The main control never

forwards any message. In this fashion, the burden

of 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 battery

changing tasks and communication with the UAS

main control.

3.2 UAV Positioning Methods

Batteries are held in a specific position on theUAV. To be able work with them, one should

position the UAV in a specific place after it has

landed or identify its location. Precision landing

systems 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 during

landing.

The design options presented in this paper as-

sume that the ground stations are outdoors, where

weather conditions cannot be predicted. The focus

here is not on landing algorithms, but on howto address UAV positioning after landing with

small error. While UAV navigation systems are

assumed to exist, we will allow landing position

error. The goal is to allow the UAV to reach

the location where battery swapping will be con-

ducted, even if its landing position is non ideal due

to navigation errors, weather conditions, UAV

damage, etc.

The design options related to UAV positioning

are depicted in Fig. 3. Figure 3a shows a donut

shaped platform that increases the possible cap-ture area of the UAV during landing. If the UAV

lands within the boundaries of the donut, it slides

and will be guided towards the center of the plat-

form, where the battery change occurs. If it lands

outside, it will slide outward without damaging the

UAV.

To increase the area where the UAV can

land, we also considered flat surface designs. The

UAV simply lands and is guided toward a specific

location on the platform where the battery swap-

ping occurs[3]. Figure3b shows a UAV centering

mechanism in which four arms are pinned to four

different points in a flat surface. These arms lie at

different heights to avoid collision between each

other and can be actuated individually by one mo-

tor each or by one single motor using a pulley and

cable actuation to close the arms. To return the

arms to their original position, torsional springs

can assist. The top part of Fig. 3b shows four

arms 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 the

platform as shown in the bottom part of Fig. 3b.

A similar, but simpler and perhaps more reliable

concept is proposed in [5].

The design depicted in Fig. 3c is also based

on a flat platform with increased landing area to

accommodate landing errors. The target again is

to move the UAV to the center of the platform,

where battery swapping will take place. Instead

of solid arms, pulleys and cables are used. The

platform will have cleft guides radiating from thecenter to the edge of the surface plate, where

slides will be placed (the six pairs of lines leading

from the central circle in Fig. 3c). These slides

can be moved radially, by radial actuation. Two

pulleys are placed on the interior of each slide and

two more pulleys are placed on the outward part

of 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 move

the pulleys outward. The outward resting position

can be seen on the top part of Fig. 3c. When the

UAV lands on the flat surface, the main controller

turns on the motor tensioning the cable of the

radially movable pulleys. The pulleys will then

slide towards the center of the platform, dragging

the UAV to the center position.

3.3 Skid Changes or Add Ons

In order to swap a UAV battery, there is a need

for a more dynamic battery-UAV coupling sys-

tem, a fast battery capturing device as well as a

simple UAV positioning system. To achieve these

requirements, changes to the UAV are neces-

sary, as original equipment manufacturer (OEM)

skids and chassis do not comply with the system

needs. For example, lets say that the UAV is


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being guided by four arms symmetrically pushing

the UAV towards a center platform as shown

in Fig. 3b. It will be more convenient for our

design if the geometric center of the UAV skids

and battery are aligned. There are two options if

the 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 can

be used. New skids may reduce weight, but may

cost more than adding extra parts. Using add ons,

the weight of the UAV will increase, but cost may

be lower. To change the skids or to make add ons

for it, certain issues should be considered:

The skid must support the UAVs weight and

keep the UAV balanced,

Skids modifications or add ons do not affectthe UAV payload significantly,

Skids must allow battery removal from the

UAV,

Skids must allow UAV to be locked in place

for battery removal/placement, and

Skid geometry must comply with the UAV

positioning method described in Section3.

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, an

add-on to the existing skids is proposed. It is a

disc of plastic material concentric to the geometric

center of the battery case. It can be used for posi-

tioning systems such as the ones shown in Fig. 3b

and c, since the actuation of the UAV occurs by

pushing the add on disc part towards the center ofthe ground station. The center skid modification

in Fig.4 is a square configuration of legs that has

sides symmetric to the geometric center of the

battery case. The positioning system can use four

arms as proposed in Section3.2in Fig.3b in a way

that the skids can be guided to the center of the

ground platform. In the case of the hexagonal skid

shapes, it can use the positioning system shown

in Fig.3c, because the cables will tend to match

the sides of the skids, as the flat platform itself is

hexagonal. As in the previous case, the sides of thepolygon are symmetric to the battery housing in

order to guide the UAV to the desired centering

with the battery coupling system.

Focusing on the square skids case, we propose

a battery housing, which will provide a secure

location for the battery and avoid safety issues due

to mechanical damage. This can facilitate battery

transportation 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 also

has connections to match with the charger in the

ground station. A possible design of a battery

case is depicted in Fig. 5. Projection views of a

battery 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 right

views of the battery case, ferromagnetic plates are

placed with the intent of using an electromag-

net to move it in other systems (see Sections 3.6

and3.7).

3.4 Battery and UAV Connecting System

Normally, batteries are connected to UAVs via

wire plugs and placed inside the UAV, thereby,ensuring terminal connection and battery stability.

However, in a platform that has to rapidly swap a

UAVs depleted battery for a recharged one, such

a connection/holding system is not suitable. The

method by which the battery is secured and physi-

cally connected to the UAV is of great importance

because it will effect the repeatability, complexity,

and the time that the ground station will take to

swap batteries. Also, the added weight will effect

the UAVs payload and flight time.

In order to create the interface between UAV

and platform, mechanical and magnetic couplers

are considered and devices that can easily hold

and 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 low

in 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 the

battery when necessary. To do so with mechanical

components, a simple model can be employed.

Figure6a depicts a mechanical coupler model

in which a battery is forced upward against thelower angled surface of spring assisted latch de-

vices (the symmetric arrow-shaped pins). This

actuation generates a force in the latch device,

whose horizontal component will contract the

spring allowing the battery to pass through the

mechanism. When the battery finally reaches the

end, it does not compress the springs anymore and

the spring of the latch device will force the battery

upward, guaranteeing the electric contact of UAV

and 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

Figure7shows an example mechanical coupler

for a battery case to match the UAV. It consists

of leaf spring latches that have angles on the ends.

When actuated upward, the latches bend, allow-

ing the battery to pass through the spring. Since

the battery case has matching indentations on its

sides, it allows the spring to lock the battery case

in 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 the

magnets are used as interface terminals between

the 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 and

in an ESky LAMA V4 helicopters belly to estab-

lish terminal match and coupling. In that system,

a servo motor rotates the battery case in order

to cause shear in the magnets and allow battery

disengagement from the UAV. While there can

be concern about interaction between the magnets

and the UAV gyroscope, [4] reported no such

complications.

Figure6b shows a straightforward procedure of

coupling and terminal matching. By simply mov-

ing the battery toward the UAV, which both pos-

sess magnet terminals, the fixed UAV will attract

the battery to it, forcing terminal match. Magnet

polarity on the terminals can be inverted to ensure

only one locking position.

Fig. 7 Example of mechanical coupler with matching battery case


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the UAVs and replace the ones in the battery case

with 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 option

is depicted in Fig. 8. The top left figure shows

male connectors in the front view of the battery

case, while the top center figure provides a cut

view of the UAV module to show the female

plugs and terminals. The terminals plus (+) and

minus () in the battery case are the terminals

that will power the UAV when connected to it,

while all four of them (given a 3 cell battery) will

be used to recharge the battery when it reaches

the recharging platform. The terminals plus (+)and minus () in the UAV module are the UAV

terminals, while 1 and 2 are only magnets that help

to 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 method

to secure a battery for transportation and how to

safely insert and extract batteries from the UAVs.

The required systems are a battery capturing de-

vice and a battery transportation method from

UAV 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 any

direction relative to the capture device or grab-

ber. We considered a mechanical claw, servo-

assisted magnets and electromagnets. Mechanical

devices require guidance and positioning in order

to clamp the battery and pull it out of the UAV.This sort of system demands use of physical space

to open and close arms, has many moving parts

(is thus more prone to failure) and may require

tight tolerances to ensure assembly and guide

movements. Electromagnets could also be used as

a capturing system. They are easy to control and

have high repeatability, although a ferromagnetic

plate must be attached to the battery case, increas-

ing on board weight. A servo-assisted permanent

magnet was used in[4] to extract the battery from

the UAV. It is also an option to use magnets onboth sides of the UAV/battery interface. Such a

system does not use a servo-motor at all times,

and so, small amounts of current are required.

However, it has moving parts and magnets may

attract foreign bodies, causing short-circuits in the

system.

We propose a capturing system with one elec-

tromagnet to remove the battery case from the

UAV. Such a system must be tested to ensure no

interaction 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 by

either mechanical coupling or by magnets, as de-

Fig. 9 Ina, an electromagnet is moved up. Inb, the electromagnet reaches the battery case and is turned on, capturing thebattery from the UAV. Inc, 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 of

the battery case, there is a ferromagnetic metallic

plate. The capturing system moves vertically via

an elevator and the electromagnet is activated

when necessary. The system moves upward until

it reaches the bottom of the battery case metallic

plate. As shown in Fig. 9b, the electromagnet isthen turned on and thereby securely attaches to

the battery case. As seen in Fig. 9c, the elevator

pulls the electromagnet down, resulting in battery

case extraction from the UAV. This approach is

different from the one in [4], that instead uses

shearing forces to separate the magnets. Due to

our approach, either mechanical or magnetic cou-

plers can be used to secure the battery case to the

UAV.

3.7 Battery Transportation Method from UAV

to Swapping Site and Battery Storage System

As seen in Section3.6,there is a need to transport

the depleted battery and case to the swapping site,

where a recharged battery can be obtained. In

order to charge the depleted batteries, a chargerarray is required. In our proposed design, each

charger has a housing that matches the battery

case geometry. It must have terminal interfaces

that can connect the battery case terminals with

the charger ones. The chosen design for the eleva-

tor was a vertical scissor elevator that can extract

the battery and the case from the UAV, deliver

the battery to the battery supply site, and return

the 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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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 charger

connections with the station. There is a need for

precise actuation and linear guides in at least two

axes, in order to find the correct battery position

in the charger array. This will increase control

algorithm complexity, when compared to the cir-

cular array design. Figure12 depicts one possible

implementation of such a linear array in an XY

orientation. A rectangular XZ array can also be

similarly conceived.

3.9 Elevator Design Option for UAV Vertical

Transportation

There are other options to deliver the battery

case to the swapping site, other than moving the

battery after extracting it from the UAV. One

is to lower the entire UAV, and at the swap-

ping site, exchange batteries with the UAV, as

depicted in Fig. 13. Such a system may reduce

complexity of the battery holding device, allowbattery extraction from the front part of the UAV

and minimize UAV modifications. As a con, the

number of floors of the circular battery array discs

cannot be increased easily, since the UAV must be

able to drop low enough to reach each array once

a more complicated system is required. Also, the

amount of force required from the elevator will be

increased, since the entire UAV is elevated.

4 Prototype

We next discuss module prototypes developed

to learn the failure modes and measure robust-

ness to continuous operation. Considering that

a deployed station is expected to operate with-

out human assistance for a long time, we de-

signed many of the parts to operate under loose

tolerance specifications to account for eventual

wearing. Most of the parts were manufactured


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Fig. 13 Ina, 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 tolerance

and unpredicted wearing. Successful operation of

the system under loose tolerance conditions sug-

gests that the system also works well with tight

tolerances. A full CAD prototype is shown in

Fig.14.

Figure 15 depicts the block diagram of our

ground station. The dark blocks are the mod-

ules which we prototyped and tested. Some of

the blocks are complementary to each other. For

example, to lock and unlock the UAV in the

platform, the opposite actuation is used, althoughother methods and mechanisms could be applied.

The following sections will briefly introduce the

module prototypes.

4.1 Orientation-fixing Module

Outdoor conditions may cause difficulties for the

navigation system. Therefore, to require the UAV

to land in a very specific position with a very

specific orientation may not only be unattainable

but also risky; the UAV already has low battery

power 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 has

arrived?

No

Yes Center UAV

Unlock UAVInform control

That UAV is ready

No

Has the UAV

departed?

Prepare system

for next UAVEnd

Yes

Extract battery

from UAV

Insert battery

back in UAV

Bring battery to

exchange zone

Swap battery with

a charged one

Place depleted

battery into charger

Place next

charged battery in

position

Move UAV to right

orientation

Lock UAV

Fig. 15 Block diagram of the complete system for swapping batteries

fully operational for a longer time. Assuming that

the position was already corrected by the position-

fixing module and the UAV is centered but with

unknown orientation, the orientation fixing mod-

ule fixes the UAV at its center of mass and spins it

until it finds the correct orientation. The correct

orientation is detected by an IR sensor. When

the UAV reaches the correct position, light from

an 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 in

pulses at the frequency of 250 Hz, and a straight

sequence of 15 matching pulses (checking on high

and 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 code

which drives the IR pulses, 15 matching pulses is

about what is needed to certify a correct orien-

tation without clocking the step motor one more

time, which would then add some error. The block

diagram can be seen in Fig.16.

The prototype of this module is shown in

Fig. 17. The round white plate in Fig. 17a lifts

and rotates the UAV, while the black dots in the

foreground of the black colored base plate are

the IR sensor. Lift is done manually by rotating

a lever at the side of the prototype. After findingthe correct position, the step motor stops spin-

ning and the round plate can be lowered. Success

rate of this assembly, indoors, was 100% in 100

trials.

4.2 UAV Locking/unlocking Module

Assuming that the UAV is already in the correct

position with the correct orientation, the UAV


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Move UAV to right

orientationStart

Lift UAV

Turn off Step Motor

Control

Turn on Step Motor

Control

Lower UAV

End

Rotate UAV 1 step

Is the UAV on the

right orientation?

Send IR burst

Count received

pulses

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 not

affect the position and orientation of the landed

UAV. This is required since the battery swap-

ping system assumes the UAV to be in a known

position to perform its tasks. The module was

implemented as a pair of servomotors each with a

plastic arm designed to match the UAV landing

skid at the given position. This implementation al-

lows for some position correction based on the an-

gular motion of the servo, which can displace the

UAV sideways in case the orientation-fixing mod-

ule had affected the position alignment. Although

this system works just fine, it still allows somefore/aft translation of the UAV skids. Adding two

plastic arms per skid (instead of just one) allows

the system to lock on the skid support structure as

well, preventing not only sideways translations but

also fore/aft translations. The white plastic locks

can be seen in Fig.17a, and a locked UAV can be

seen in Fig.17b.

4.3 Battery Extraction Module

Assuming that the UAV is already in the right po-sition, with desired orientation, and secured, this

system extracts the old battery from the landed

UAV. There is an electromagnet at the center

of the orientation fixing module. At the bottom

of the battery case there is a steel plate. The

electromagnet secures the battery case by this

plate. Once the electromagnet has attached to

the steel plate at the bottom of the battery case,

the structure holding the electromagnet descends.

The force generated by the electromagnet exceeds

that of the battery case magnets holding the case

to the UAV. Thus, the battery case is separated

from the UAV. In the prototype, this downward

motion is done by hand, since the weight of the

orientation fixing module is not enough to de-

tach the battery. The complete implementation,

however, relies on the elevator to provide enough

force 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 the

underbelly of the UAV add about 20 g to the

overall UAV mass.

4.4 Battery Swap Module

Assuming that the battery was successfully re-

moved from the UAV, brought to the exchange

zone, and that there is a charged battery already

in position to be exchanged, the battery swap

system exchanges the old battery for a new one.

The battery swap module is also responsible for


19/24


Fig. 17 Prototype for the lock module, orientation-fixing module and battery extraction module. Inait is possible to see theelements of each module.bshows a UAV with the orientation already fixed and locked into the platform

placing the old battery inside a charger. Figure18a

shows the basic components of the module. The

batteries are moved by a rack and pinion system.

There is a step motor driving the pinion. At one of

the ends of the rack there is an electromagnet. The

set of step motor, rack, pinion and electromagnet

we call battery actuator. The battery case has

a steel plate on its side, which is used by the

battery actuator to secure it during movement.

The zone marked with an X is the exchange

zone, it indicates the location where the batteryextraction module rests the battery after it has

completed removal of the battery and case from

the UAV.

The zone marked with a minus is a buffer.

Figure 18b shows the situation where the used

battery and case have been lowered to the swap

module; a fresh battery and case is to the imme-

diate left of the old battery. In Fig. 18c, the rack

has 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 the

old battery case and pulls it out of the (minus)

location. That battery would then be delivered to

charging module. This setup went through a reli-

ability test session, successfully completing 2,500

trials without any errors.

The final location of the used battery is shown

in Fig.11in the context of the larger system. It will

rotate away from the rack and a new battery will

replace 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. The

prototyped modules were extensively tested and

improved until 100% success rate was obtained

at loose tolerances. The worst-case execution timewas then measured. The non-prototyped modules

were assumed to work also at 100% success rate,

and the worst-case execution time was estimated

using motor data sheets, eventual reductions ap-

plied to the motor and the moment of inertia from

the CAD models. Table1summarizes the data.

With this data in hand, we could then define a

maximum size and maximum coverage of a given

ground station (see Section2). Of course, by try-

ing to make all the terms of Eq.5equal, some de-

sign adjustments may affect the time estimations

in Table 1, so some iteration may be required.

Assuming that our designed platform swapping

time is within 1 min (the 47.5 s estimated plus

some extra time), and charge time and flight time

are 85min and15min, respectively, the maximum

number of UAVs that this service station can

support is16UAVs (forcingTLUAV =TLPLAT ), and

the minimum number of batteries to support the

UAVs continuously is 103 batteries (16 on the


20/24


Fig. 18 a shows the components of the system, whilebe 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, 87on the service station). The maximum

coverage which one of these stations can produce

is then CACHSYS = 15, so if higher coverage is re-

quired, either the modules and/or processes on

the ground station must work faster, or an extra

ground station should be employed.


21/24


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 twice

Elevator 6.5* To be counted twice

Battery 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 coverage

CTGTSYS.

Corollary 1 For a recharge platform, the minimum

number of components that will guarantee a target

coverage of CTGTSYS is:

NUAV =

CTGTSYS

TF (TF+ TR + TI)

, (6)

NPLAT = CTGTSYS

TF TR . (7)

ProofBy Theorem 1, CTGTSYS CSYS= TF/TCYC.

Equivalently, TF/ CTGTSYS max

TLUAV, TLPLAT

or TF/ C

TGTSYS TLUAV and TF/ C

TGTSYS TLPLAT .

SinceTLUAV = (TR + TF+ TI) /NUAVandTLPLAT =

TR/NUAV, it is equivalent to require NUAV

CTGTSYS (TR + TF+ TI)/TF and NPLAT CTGTSYS

TR/TF. The result follows.

Similarly, for replacement we obtain the

following.

Corollary 2 For a replacement platform, the mini-

mum number of components that will guarantee a

system coverage is(6) and(7),

NBATT =

CTGTSYS

TF (TC+ TR) NUAV

, (8)

NCGR =

CTGTSYS

TF TC

. (9)

The proof follows the same method as in

Corollary 1.

For a charging platform the total component

cost of the UAS (including the platforms, charg-

ers, batteries and UAVs) is

V

RECH

TOT = NPLAT V

RECH

PLAT + NUAV VUAV + NCGRVCGR + NBATT VBATT,

and similarly, the cost of a replacement system is

VREPTOT = NPLAT VREPPLAT + NUAV VUAV + NCGR

VCGR + NBATT VBATT,

whereVRECHPLAT, VREPPLAT,VUAV,VBATTand VCGRare

the 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 attention

on parameters specifically related to the systems

designed and shown throughout the development

of this work, which is the energy replenishment

of the UAV itself. Parameters such as the cost of

the navigation system, the external power and the

human 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 basic

cost is estimated according to UAV complexity.

This is a rough estimate based on a system that

has a fair level of electronic complexity (related to

number of LiPo battery cells) and UAV-platform-

connection interfaces (related to how many bat-

tery terminal connections are needed). In the case

of battery replacement platforms, this basic cost

is estimated to be 1500 USD, since moving parts

are added in order to transport batteries in the

system.

Refill platforms work with one charger each

and each UAV carries its own battery. Replace-

ment systems require fewer UAVs to obtain the


22/24


Table 2 Data used toplot the cost comparisongraph

Cheap Mid-range Expensive

Cost of (USD)

Battery 5 20 100

UAV 30 500 1,500

Charger 10 15 50

Refill platform (raw costsame for all) 20

Replace platform (raw costsame for all) 1,500

Time of (min)Flight 10 15 30

Charge 25 85 85

Replace 1 1 1

Maximum values per replacement platform

Maximum coverage 10 15 30

Minimum chargers 27 87 87

same coverage, because of the relatively fast en-

ergy replenishment time (See Section4.5).The information obtained from the Petri nets

(see Section2) and the experiments mentioned in

previous sections are summarized in Table2, for

three types of UAVs.

The Cheap UAV is in the micro aerial ve-

hicle (MAV) range, which can be a very small

UAV with the size of an insect or a UAV for

hobby purposes with estimated flight time of 10

min and battery recharge time of 25 min. Their

battery has fewer cells, thus less cost (battery and

charger costs are estimated as 5 and 30 USD

respectively). The cost for a platform designed

to fit this model of UAV is 20 and 1,500 USDfor refill and replacement systems respectively.

The refill platform cost is very low compared to

the replacement one. This happens because the

refill case only needs a few electronic components

while the replacement platform requires battery

replacement (therefore, guides, actuators and pre-

cise movements are needed).

Based on the information of the previous sec-

tions, we draw a cost comparison graph. Since

the current focus of this paper is on replacement

platforms, in Fig. 19 we plot the cost advantage

Fig. 19 Cost comparisonbetween batteryreplacement and batteryrecharging ground servicestations


23/24


of a UAS using replacement platforms against an

equivalent UAS based on refill (recharge) plat-

forms. We define cost advantage at a given cov-

erage C as:

Adv(C) =VRECHTOT (C) VREPTOT(C). (10)

Positive values of Adv(C) imply that the re-

placement system is economically more viable

than a refill system. Figure 19 shows that the

points where replacement systems become more

efficient are C = 1.23 and C = 1.33 for mid-

range and expensive UAVs, respectively. On the

other hand, a replacement UAS never becomes

economically better than a refill UAS for cheap

UAVs. However, it is important to notice that the

number of UAVs required to fulfill a given target

coverage is usually greater in a refill UAS than in areplacement UAS. This analysis does not include

control algorithm complexity nor the availability

of control channels, therefore replacement UAS

may 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 which

allows us to calculate the system coverage basedon the component parameters. UAS expansion

or resource management can be guided and/or

optimized with the aid of this model.

We also developed several module prototypes,

both conceptually and physically, which together

comprise one service station to swap batteries

with high success rates. The complete station

is 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 and

combat environments. The ground station is also

capable of handling heterogeneous UAV fleets

with not only different shapes and sizes, but also

different number of battery cells per battery pack.

Further development of such replacement stations

and replenishment stations is essential to reduce

the limitations caused by having humans in the

loop.

7 Future Work

As future work, we will conduct further test-

ing with prototype modules of UAV positioning

systems (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 for

new recharged ones autonomously upon UAV

landing.

Other future work is related to gasoline refu-

eling ground stations, where studies will be per-

formed with the goal of having fully autonomous

ground service stations for heterogeneous fleets.

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Automatic Battery Replacement System for UAVs: Analysis and Design