A Small-Scale Prototype to Study the Take-Off of Tethered RigidAircrafts for Airborne Wind Energy

Lorenzo Fagiano, Eric Nguyen-Van, Felix Rager, Stephan Schnez, and Christian Ohler∗†

AbstractThe design of a prototype to carry out take-off and flighttests with tethered aircrafts is presented. The system fea-tures a ground station equipped with a winch and a linearmotion system. The motion of these two components is reg-ulated by an automatic control system, whose goal is to ac-celerate a tethered aircraft to take-off speed using the linearmotion system, while reeling-out the tether from the winchwith low pulling force and avoiding entanglement. The me-chanical, electrical, measurement and control aspects of theprototype are described in detail. Experimental results witha manually-piloted aircraft are presented, showing a goodmatching with previous theoretical findings.

1 IntroductionThe term Airborne Wind Energy (AWE) refers to a series oftechnologies to convert wind energy into electricity by us-ing aircrafts tethered to the ground, [2, 11]. The claimed ad-vantages of AWE systems over conventional wind turbinesare, for the same rated power, a lower capital cost, sinceno expensive structure like the tower has to be built, anda higher energy yield, thanks to the possibility to harvestwind energy at higher altitude (usually in the range 200-400m above ground), where the time-share of above-rated windspeeds is larger. The main disadvantages are a higher com-plexity, due to the heavy reliance on active control systemsto achieve stable and optimal operation, and higher opera-tion and maintenance costs, caused by the use of materials,in particular for the tether, which are not as durable as thoseof conventional wind generators. AWE concepts have beenfirst envisioned in the late ’70s [23, 22], however their tech-nical development started only in the early 2000s and hasseen a quite steady progress in the last decade. Today, be-sides few notable exceptions [32], most of the approachesunder development exploit the so-called crosswind motion,i.e. a flight trajectory roughly perpendicular to the wind per-formed at relatively large speed, and either onboard powerconversion [31] or ground-based one [26, 4, 30, 10].

In the scientific literature, systems with ground-basedgeneration and soft kites are by far the ones that receivedthe largest attention, with several contributions concernedwith aerodynamics [7, 5, 20, 28, 6] and controls [8, 19, 13,∗This is the pre-print of a paper submitted for publication. The authors

are with ABB Switzerland Ltd., Corporate Research, 5405 Baden-Dättwil- Switzerland. E-mail addresses: {lorenzo.fagiano | eric-nguyen-van | fe-lix.rager | stephan.schnez | christian.ohler}@ch.abb.com.†Corresponding author: Lorenzo Fagiano.

33, 9, 10]. On the other hand, fewer results have been pub-lished pertaining to systems with ground-based electricitygeneration and rigid aircrafts[26, 27], and even fewer forsystem with onboard generators [31]. One cause of such adisparity is the fact that concepts with rigid aircrafts are in-herently more complex than those based on soft kites, hencemore difficult to study within academia, especially when itcomes to experiments. In fact, with power kites several de-grees of freedom are constrained by the bridle design, sothat two control inputs, usually a steering deviation and theforce exerted on the tether, are enough to obtain stable flightpatterns and produce energy. When two or more tethers arepresent, control can even be achieved using ground-basedactuators and sensors only [13, 33]. With rigid aircrafts,on the contrary, an onboard autopilot has to be developed,which is able to coordinate with the ground station in orderto actively stabilize all of the degrees of freedom of the sys-tem. Considering a standard aircraft design, like a propelledglider, this entails the coordination of five onboard controlinputs (the aerodynamic control surfaces and the propeller)and one on the ground (the tether force). Moreover, the su-perior aerodynamic performance of rigid aircrafts leads tofaster system’s dynamics, which render the control problemmore challenging, in particular for what concerns the in-teraction with the tether. Finally, soft kites are much moreresilient to impacts and easier to repair than rigid aircrafts,hence making experimental tests with the latter even morechallenging and time-consuming.

With the aim of easing the mentioned difficulties, thefirst contribution of this paper is the description of a small-scale, low-cost prototype, which can be used to study ex-perimentally some relevant aspects of AWE systems withground-based generation and rigid aircrafts. Similar in spiritto [14], concerned with a small-scale prototype to controlsoft kites, we present the design of the hardware, softwareand operation of the prototype with enough details to allowother teams of researchers to replicate and/or improve thesetup.

The operation of any AWE system can be divided inthree main phases, which shall be carried out autonomously:take-off, power generation, and landing. Considering thecurrent development status of AWE systems with ground-based generation, autonomous power generation has beenassessed both theoretically and experimentally and severalrelated contributions are available in the scientific literature,see e.g. [10, 26, 24, 29]. The same cannot be said about au-tonomous take-off and landing of the aircraft. These twotopics have been explored by few papers in the scientificliterature, mainly with theoretical and numerical analyses

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Figure 1: Rendering of the small-scale prototype built at ABB Corporate Research. The numbers in the picture indicate:1. the winch, 2. the tether connected to the aircraft (dashed line), 3. the spooling mechanism, 4. the series of pulleys thatredirect the tether from the winch to the aircraft, 5. the slide, 6. the aircraft, 7. the rails, 8. the tether used to pull the slidein backward/forward directions (“slide tether”, dash-dotted line), 9. the drum hosting the slide tether, 10. the pulleys thatredirect the slide tether, 11. the mass-spring system, 12. the frame. As a reference, the rails’ length in this rendering is5.2 m.

only, and demonstrated by AWE companies only to a lim-ited extent. Actually, the capability to carry out take-offand landing in compact space and in an economical wayis one of the main technical challenges (hence risks) stillstanding in AWE development. Again, this holds partic-ularly for systems using rigid aircrafts, for which there isevidence of autonomous take-off [1], however by using awinch launch that requires a significant space in all direc-tions in order to adapt to different wind directions. In thescientific literature, [17] presents a simulation study for arotational take-off, while in [3] an analysis of several ap-proaches is carried out and three alternatives are deemed themost promising: buoyant systems, linear ground accelera-tion plus on-board propeller, and rotational take-off. Then,the rotational take-off is examined in more detail by meansof numerical simulations. In [12], a theoretical analysis ispresented, which shows how a linear take-off approach ap-pears to be the most viable one, according to different per-formance criteria. About the landing, to the best of the au-thors’ knowledge there is only one contribution in the liter-ature [25], which studies this aspect through numerical sim-ulations after presenting a possible control approach for theaircraft to realize a cycle of tethered take-off, low-tensionflight and landing.

The second contribution of this paper is to demonstratethe take-off in a compact space and the subsequent flightof a tethered aircraft, using the described small-scale pro-totype. In particular, the aircraft is manually piloted, whilethe ground station is fully autonomous. We describe theautomatic control system for the ground station, which reg-ulates the linear acceleration of the aircraft up to take-offspeed and the tether reeling during take-off and flight. Wepresent experimental results collected by a series of sensorson the ground and onboard, and comment on the match-ing between such results and the theoretical ones derived in[12].

Together, the two mentioned contributions (description

of the prototype and demonstration of tethered take-off andflight) make it possible for other research teams to realize asimilar system and use it to investigate a number of possibleresearch topics, from aircraft design and control to filteringand state estimation. As an example, we used this prototypeto develop an onboard control system for the aircraft, ableto achieve fully autonomous take-off and flight. A movieof the related experimental results is available online [15].The system modeling, identification and controller designaspects related to such an autopilot, as well as the obtainedexperimental results, fall beyond the scope of the presentpaper, whose focus is on the prototype hardware and groundstation control. The autonomous take-off and flight of theaircraft are therefore subject of a separate contribution [16].

The paper is organized as follows. Section 2 describesthe layout of the prototype and its operation. Section 3provides details on the design of the prototype, with par-ticular care to the most critical components. Section 4 isconcerned with the automatic control design of the groundstation. Section 5 presents the experimental results and con-cluding remarks are given in Section 6.

2 Prototype layout and principle of op-eration

A rendering of our small-scale prototype is shown in Fig.1, highlighting all the main mechanical components. More-over, Fig. 2 presents a picture of the realized prototype,and Fig. 3 a conceptual layout of the various subsystemsand their mechanical and electrical links, the latter dividedinto power and signal. The system under consideration iscomposed of two main sub-systems, the ground station andthe aircraft, connected by a tether. The ground station in-cludes a mechanical frame supporting a number of com-ponents needed to achieve two main tasks: accelerating theaircraft from standstill to take-off speed, and controlling the

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tether reeling in order to limit the pulling force while, atthe same time, avoiding entanglement and excessive sag.In particular, referring to Figs. 1-2, the tether is coiled

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Figure 2: Picture of the small-scale prototype built at ABBCorporate Research. The numbers 1. to 12. correspondto the components described in the caption of Fig. 1. Inaddition, the picture shows: 13. the winch motor, 14. theslide motor, 15. the real-time machine, 16. the human-machine interface, 17. the metal enclosure containing thepower supply and motor drives.

around a winch and passes through a series of pulleys be-fore attaching to the aircraft. The winch is connected toa spooling system that translates backward/forward as thetether is reeled in/out, in order to distribute the latter evenlyalong the winch’s axial length. One of the pulleys which thetether passes through is installed on a moving plate, con-nected to a spring. This component is used to reduce thestiffness of the link between the ground station and the air-craft and to control the winch speed, as described in section4. Before the take-off, the aircraft is installed on a slide,able to move on rails and controlled by a linear motion sys-tem, which in our prototype is composed of a second tetherwound around a drum (“slide drum”) and connected to bothends of the slide via another series of pulleys. The winchand the slide drum are each connected to an electric mo-tor, controlled by a drive. A supply module provides theelectrical power required by the drives and by the controlhardware. The latter acquires the motors’ position measure-ments from the drives and the spring’s compression froma linear potentiometer and computes the reference positionand speed values for the slide and winch motors, respec-tively (see Fig. 3). A human-machine interface (HMI) al-lows the operator to interact with the ground station. Re-garding the aircraft, we consider a conventional airplane de-sign whose main functional components, for the sake of ourapplication, are a tether attachment and release mechanism,a front propeller, the typical control surfaces (ailerons, ele-vator, rudder and flaps), an onboard electronic control unit,a radio receiver linked to a remote controller, employed by

a pilot on ground, finally onboard position, inertial, attitudeand airspeed sensors.

The operation of the described system is quite straight-forward: when the control system of the ground station re-ceives a “take-off” command (in our prototype issued by ahuman operator, in an eventual final application originatedby a supervisory controller e.g. on the basis of the windconditions), it accelerates the aircraft up to take-off speedusing about half of the rails’ length, the second half beingneeded to brake the slide. At the same time, the winch ac-celerates in a synchronized way with the slide, in order toreel-out the tether at just the right speed to avoid pullingon the aircraft (which would quickly cause a stall condi-tion) while also avoiding to entangle the tether due to anexcessive reel-out with no pulling force. This is achievedby a feedforward/feedback control strategy described in de-tail in section 4. The aircraft has also to coordinate withthe ground station, in such a way that the onboard motoris ramped up to full power in order to quickly climb to asafe altitude. As anticipated, in this paper we assume that ahuman pilot controls the aircraft and carries out such a ma-neuver. In [16] we describe an automatic controller of theaircraft, which synchronizes with the ground station usingthe longitudinal acceleration measured onboard. After thetake-off, the pilot is in charge of maneuvering the aircraftto fly in a bounded region relatively close to the ground sta-tion, in order not to exceed the total length of tether installedon the winch. Typically flown patterns are figures-of-eightor ellipsoidal trajectories roughly above the ground station.In this phase, the winch controller has to still regulate thereeling speed to fulfil the same two conflicting objectives(low pulling force and low tether sag) while the aircraft pe-riodically flies closer and farther from the ground station.

In the next section, we provide more details about eachof the mentioned sub-systems and components, as well asdesign guidelines and the specific design choices we madefor our small-scale prototype.

3 Prototype design

3.1 Ground station3.1.1 Mechanics

The mechanical frame has to support all other componentsand withstand the forces exerted by the aircraft through thetether, as well as the torques (motor, inertia and friction)acting on the winch and on the slide drums. Equations tocompute the tether’s force as a function of the aircraft’s pa-rameters during crosswind flight can be found in several ref-erences [21, 2, 11, 14]. Dimensioning the frame accordingto these equations provides large enough resistance for alltypes of loads that can be expected during take-off and low-tension flight, since during these maneuvers the tether forceis much lower than in crosswind conditions. In our design,we actually dimensioned all of the components such that theweakest element in the mechanical system is the aircraft,which is also the cheapest component since we employedcommercially available model gliders made of styrofoam.

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Aircraft

Tether

Powersupply

Real-timeControl

hardwareHMI

Mechanicalframe and linearmotion system

IMU

Radio

Referenceinputs

Manual controlData logging

GroundUnit

sensors

GU positionTensioningsystemposition

Signal

Electrical power

Mechanical link

Ground station

Winchdrive

JoystickLaptop

12V batteries in parallel+ inverter

Motor

Aircraftremote

Slidedrive

WinchMotor

SlideMotor

Motors’positions

Pilot

Figure 3: Conceptual layout of the prototype showing the various sub-systems and their mechanical and electrical (powerand signal) interconnections.

Other important aspects in the frame design for a researchprototype are transportability, modularity and the possibil-ity to achieve a roughly horizontal orientation of the railsalso on uneven terrain. In our prototype, we employed T-slotted aluminum framing profiles forming a structure thatcan be easily modified. The frame is divided in two halves,which we loaded side by side on a trailer for transportation,and bolted head-to-head for testing. We used four extensi-ble profiles to support the part of the frame overhanging thetrailer and to easily adapt to rough terrains, see Fig. 2. Thefinal dimensions of the frame we built are approximately1m×1m×3m (width-height-length) for the part bolted tothe trailer, and 0.6m×1m×2.4m for the overhung part, thusproviding a total rail length of about 5.4 m. The frame ismade of 0.04×0.04 aluminum profiles and it features three“layers”, see Fig. 1: the lower one for the slide drum andtether, the middle one for the winch, and the upper one forthe mass-spring system. Finally, the rails are mounted ontop. We adopted such a layered arrangement to provideenough room for modifications during the experimental ac-tivities, in order to cope with the uncertainty associated withthe research activity. A much more compact arrangementcan be easily achieved in an eventual final design, for ex-ample with all the components on just one layer and packedmuch closer one to the other.

For the rails, we employed L-shaped aluminum profilesbolted to the frame, on which the slide can roll thanks tofour rubber wheels, see Fig. 4. The slide was designed inorder to easily change the initial aircraft pitch. In partic-ular, we employed two wooden bars mounted on the slidefloor via adjustable shafts are used to support the plane andcan be tuned in height and distance, in order to set the start-ing pitch angle. The wooden supports feature plastic rollerswith bearings, which allow the tether to slide with verysmall friction. We arranged four such rollers in order tocope with all the possible directions that the tether can takeduring flight, i.e. 360◦. The tether passes through the slidefloor by means of an in-line exit block with two low-frictionnylon pulleys, of the kind used for sailing boats, see e.g.

[18]. Finally, in order to avoid lateral oscillations duringthe take-off, we installed four vertical-axis rubber wheelsat the slide’s corners. To limit its mass, the slide floor hasto be as thin as possible compatibly with the need to sup-port all the mentioned components. In our design we useda 0.4m×0.6m×0.004m aluminum plate. The slide’s lengthhas to be subtracted from the rail’s length when comput-ing the distance available for the take-off, which in our casereduces to about 4.8 m. The slide floor, wooden supports,rollers and vertical-axis wheels are visible in Fig. 4 as well.The final mass of the slide in our design is about 10 kg.

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Figure 4: Snapshot of the rails, slide and aircraft shortlyafter take-off. The picture shows: 1. the frame, 2. therails, 3. the slide floor, 4. the slide wheels, 5. the vertical-axis wheels, 6. adjustable steel shafts, 7. wooden supportsfor the aircraft, 8. plastic rollers, 9. bumpers, 10. tetherconnected to the aircraft, 11. slide tether, 12. aircraft.

As mentioned, in our prototype the slide is pulled inboth directions by means of a tether attached to the twosides of the slide’s floor, see again Fig. 4. We employed

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a 0.003m-diameter ultra-high-molecular-weight polyethy-lene (UHMWPE) tether for this purpose, with a minimumbreaking load of 104 N which is much larger than the max-imum force acting on the slide tether, of the order of fewhundreds of N. The slide drum shall be as light as possi-ble and be able to reel-in the slide tether on one side whileat the same time reeling-out on the other side. To achievethese features, in our design we used three aluminum disksconnected by steel rods bolted with regular spacing arounda circular pattern with 0.1m of radius, see Fig. 5. The alu-minum disks create two half-drums, each one 0.1m-wide.The drum is linked to the slide motor via a joint that wesized according to the maximum motor torque of 26 Nm.

Figure 5: Picture of the slide drum with the tether installedand of the slide motor.

The aircraft’s tether shall be chosen with the smallest di-ameter possible compatibly with the expected loads, in or-der to limit the tether drag. The material employed by mostresearch groups and companies in AWE is UHMWPE, forits large minimum breaking load and lightweight. We em-ployed a 0.002m-diameter UHMWPE tether for the proto-type, with a total length of 150m. The minimum breakingload is about 4,500N, way above the breaking load of the at-tachment mechanism on the aircraft. Regarding the winch,its main task is to withstand the tether force and to reel-out/in the tether fast enough to coordinate with the aircraftmovement, while avoiding tether entanglement. In our pro-totype we manufactured a rather bulky drum of aluminumwith a thickness of 0.01 m, a radius of 0.1 m and an axiallength of 0.6 m. Considering the 0.002 m of tether diame-ter, this corresponds to about 190 m of tether that could bestored in a single layer. To control the tether spooling with-out the need of additional servomotors, we designed a lin-ear motion system composed by a leadscrew driven by thewinch through a belt transmission system, see Fig. 6. Therotation of the leadscrew translates a small carriage wherewe mounted an organizer block, composed of two nylonpulleys [18] which the tether passes through. We sized theleadscrew and the transmission ratio of the belt such thata full revolution of the winch corresponds to 0.002 m (i.e.equal to the tether diameter) of translation of the spoolingblock, hence theoretically leaving no gap between two con-secutive turns of the tether on the drum.

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Figure 6: Picture of 1. the winch, 2. the belt transmissionsystem, 3. the leadscrew and 4. the moving organizer block.

Regarding the mass-spring system, Fig. 7 provides apicture and description of the solution we built in our pro-totype. The main task of this system is to limit the pullingforce exerted by the tether on the aircraft, providing enoughtime for the winch control system to react and increase thereel-out speed until the tether is not taut anymore. To achievethis goal, the spring stiffness and travel need to be properlydesigned, moreover the spring position is measured and em-ployed as feedback variable by the winch controller. Thelatter is described in detail in section 4, while in the fol-lowing we introduce an approach to size the spring traveland stiffness as a function of the aircraft and winch fea-tures, by means of a simple numerical simulation. Indeed,the mass-spring system proved to be the most critical com-ponent in the whole mechanical design of the prototype. Ifthe travel and stiffness of the spring are not well-designed,tethered flight might become impossible because when thetether is taut and the spring is fully compressed the pullingforce increases very quickly, due to the high stiffness ofUHMWPE, and there is not enough time for the winch toreact before stalling the aircraft. This holds in particularwhen the tether is roughly aligned with the aircraft’s lon-gitudinal body axis and pulls backwards, for example rightafter take-off (while on the contrary in crosswind conditionsthe tether does not represent a problem, rather it’s requiredto achieve high speed and generate power according to thewell-known equations developed in [21]).

We consider a simplified setup shown in Fig. 8, wherethe aircraft moves along one dimension and the tether forceis aligned with the drag force. We consider that no windis present, that the angle of attack of the aircraft is constant(hence the drag coefficientCD is fixed) and that the onboardpropeller is providing the maximum thrust, T . Finally weassume that the tether behaves like a spring whose stiffnessKt(t) depends on the tether length l(t), where t is the con-tinuous time variable:

Kt(t) =F t

εt l(t), (1)

where F t is the breaking load and εt the corresponding elon-gation. The equation of motion of the aircraft in such a sim-

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Figure 7: Picture of the mass-spring system. The tether 2.passes through a stand-up pulley 4., attached to a mobileplate 1. which can move on steel shafts, each one support-ing four springs in series, 5.. A linear potentiometer 6. islinked to the mobile plate and measures its displacement,i.e. the spring compression. One of the fixed stand-up pul-leys 3. used to re-direct the aircraft tether is also shown.

TetherMoving mass

Spring

Aircraft

Winch

Figure 8: Simple model to study the interactions betweenaircraft, mass-spring system and winch in order to designthe spring’s travel and stiffness.

plified setup is then:

x(t) = T − ρCD A x2(t)

2m− sat (Kt(t)(x(t)− l(t)), 0)

m,

(2)where x is the aircraft position (x .

= dxdt ), x(t) − l(t) is

the tether elongation, ρ is the air density, A is the aircraft’seffective area, m its mass and sat (F, 0) is a saturation func-tion that returns 0 if F < 0, otherwise F . The tetherlength, for the sake of computing its elongation, is the sumof two contributions: the amount that was reeled-out fromthe winch, and the one yielded by the spring’s compression:

l(t) = Rwθw(t) + 2xs(t) (3)

where Rw is the winch radius, θw its angular position, andxs(t) is the spring displacement. The dynamic behavior ofthe latter is governed by the following equation:

xs =2sat (Kt(t)(x(t)− l(t)))

ms− βs(xs(t))xs(t)

ms− Ksxs

ms,

(4)where Ks is the spring’s stiffness, ms the mass of the mov-ing plate carrying the tether’s pulley (we neglect the spring’smass) and βs(t) is the viscous friction coefficient of themass-spring system. The latter is a function of xs to ac-

count for the end of travel of the spring in both directions:

βs(t) =

β

s(xs) if δs ≤ xs ≤ xs − δs

γsβsif 0 ≤ xs ≤ δs and xs < 0

or xs > xs − δ and xs > 0

,

(5)where xs is the maximum spring travel, β

sis the friction

coefficient when the spring is free to move (typically a verysmall value) and γs � 1 is a coefficient accounting for theincrease of friction when the spring is close to its travel lim-its, e.g. due to the presence of rubber bumpers. The travellimits are given by a small value δs > 0 when the springis uncompressed, and xs − δ when it’s fully compressed.Finally, we assume that the winch reels-out with the maxi-mum available motor torque, Tw. The equation of motionof the winch is:

θw =Tw

Jw+Rw sat (Kt(t)(x(t)− l(t)))

Jw− βwθw(t)

Jw, (6)

with Jw being the winch moment of inertia and βw its rota-tional viscous friction coefficient. In order to simulate themodel equations (1)-(6), initial conditions for the states ofthe aircraft, spring and winch have to be set. In particular,we parameterize such initial condition as:

x(0) = x0; x(0) = x0 (7a)xs(0) = 0; xs(0) = 0 (7b)

θw(0) =x0Rw

; θw(0) =x(0)−∆x

Rw(7c)

(7d)

where x0, x0 are the initial position and speed of the air-craft, and ∆x an initial difference of speed between the air-craft and the winch. The rationale behind (7) is the follow-ing: we assume that the aircraft is flying with a certain ini-tial speed x0 and that the initial tether length is just enoughto provide no tether force with the spring being at rest (com-pare equations (2), (3) and (7b)-(7c)), however with thewinch speed lower than the aircraft’s one by a quantity ∆x.When simulating this model, such a speed difference willinduce a positive tether force which will slow down theaircraft and compress the spring, see eqs. (2) and (4). Atthe same time, the winch will accelerate due to the appliedtorque and the tether pull, until at some time instant t∗ thetether length will be large enough to eventually bring thepulling force down to zero, this time with a winch speedequal or larger than the aircraft’s. Then, we can evaluatewhether the aircraft’s speed in the time interval [0, t∗] is al-ways larger than the minimum cruise speed x, i.e. whetherthe aircraft is able to stay airborne. This test allows oneto judge whether a given combination of spring’s stiffnessand maximum travel is suitable for the considered aircraft(mass, available thrust, drag coefficient, area), winch (iner-tia and available torque), and tether (stiffness) and for theselected initial aircraft speed x0 and speed difference ∆x.Such a simple simulation study can then be used to evaluatedifferent design alternatives and to select one. Note that thedescribed approach considers a worst-case scenario wherethe tether is perfectly aligned with the airspeed (hence with

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the drag force), while in reality the tether is pulling with anangle such that not all of its force adds to the drag.

An example of the mentioned design procedure pertain-ing to our small scale prototype is presented in Fig. 9,where four different values of maximum spring compres-sion (0.05m, 0.2m, and 0.35m) are compared, with all theother parameters reported in Table 1.

Table 1: Parameters of the simplified model used to designthe mass-spring system

Aircraft and ambient Mass-spring systemA 0.3 m2 ms 2 kgCD 0.05 - Ks 70 N mm 1.2 kg β

s10−4 kg s−1

T 10 N γs 106 -x 7 m s−1 δs 0.001 mx0 20 m Winchx0 10 m s−1 Rw 0.1 mρ 1.2 kg m−3 Tw 13 N mTether Jw 0.1 kg m2

F t 4500 N βw 0.01 kg m2 s−1

εt 0.02 - ∆x 4 m s−1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.85

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Figure 9: Simulation results obtained with the simple modelof interaction between the winch, the mass-spring system,the tether and the aircraft. Comparison of aircraft speed (up-per plot) and spring compression with xs = 0.35 m (solidline), 0.2 m (dashed) and 0.05 m (dash-dot). Dotted line:minimal allowed aircraft speed.

It can be noted that the spring with the smallest maxi-mum travel does not provide enough buffer to keep the air-craft’s speed above the minimum one. As the spring lengthincreases, more than one cycle of compression and exten-sion emerge and the minimum aircraft speed improves. More-over, the time at which the minimum speed occurs is shiftedfarther in time as the available spring travel increases.

In our prototype, we used the described approach to de-sign our spring system, whose maximum displacement is

equal to 0.35 m and all other parameters are equal to thoseshown in Table 1.

Finally, regarding the pulleys that redirect the tether fromthe winch to the aircraft, these have to be strong enough towithstand the involved forces, oppose little friction and beable to slightly adjust their orientation to cope with smallmisalignments and with the tether spooling. The pulleys’diameter shall be at least 20-30 times larger than the tetherdiameter to reduce wear. In our design we employed nylonstand-up pulleys used for sailing boats, see Fig. 7, with adiameter of 0.045 m.

3.1.2 Power supply, motors and drives, measurementand control hardware and human-machine inter-face (HMI)

To maximize flexibility we designed a modular power sup-ply, where the motor drives can be either connected to anexternal single-phase 220VAC source or to a 3kW inverterconnected to 12V batteries with a total capacity of 260 Ah(see Fig. 3). We used the latter configuration in our outdoortests. We sized the batteries to provide enough energy forat least one full day of testing, as well as large enough peakcurrent. The latter can be estimated from the peak torqueof the motors and their current/torque constant, consideringthe ratio between the voltage on the battery side and the oneon the drive side.

Regarding the motors, since power production was notwithin the goals of our research we chose not to install alarge machine for the winch. Rather we employed the samemotor model, an ABB BSM90N-3150 permanent magnetservomotor, for both the slide and the winch. Each motoris equipped with incremental and Hall-effect encoders andit is connected to an ABB MicroFlexr E150 servodrive.We chose the motor/drive combination according to quitestandard design considerations as briefly outlined in the fol-lowing. The motor rated torque is equal to 13 Nm, corre-sponding to a current of 9 A, up to the rated speed of about2000 rpm, which corresponds to about 21 m/s of slide/tetherspeed, considering that the radius of both the slide drumand the winch is 0.1 m. This value provides enough marginwith respect to the take-off speed of the aircraft, approxi-mately equal to 9 m/s. For short time intervals, the drivescan provide twice the rated current to the motor (i.e. twicethe torque): we employed this functionality for the slidemotor in the very first acceleration at take-off. In particular,considering that the total mass of the slide drum, the slideand the aircraft is about 11.2 kg and that the slide drum’sradius is 0.1 m, the peak torque of 26 Nm can accelerate theaircraft from zero to take-off speed in about 0.38 s using1.7 m of rails. This leaves enough room for the slide motorto break. The energy produced during the breaking of theslide and of the winch is dissipated in resistors connected tothe drives, even though in principle one could recover it. Weprogrammed low-level speed and position control loops onthe drives (see section 4) using an ABB basic-like languagecalled Mintr. We installed all the power supply compo-nents (batteries, inverter, a battery charger) and the drivesin a metal box to protect them during road transportation.

7

As shown in Fig. 3, the drives send their position mea-surements to a real-time machine, which processes themtogether with the inputs received from the HMI and fromthe potentiometer measuring the spring’s compression. Thelatter is a conductive plastic linear sensor (model LP-400F)manufactured by Midori Corp., with a maximum travel of0.4 m, which provides enough bandwidth and high resis-tance to outdoor environments (dust and water). As real-time machine we employed a Speedgoatr performance ma-chine in which we installed a National Instruments PCI-6221 data acquisition board. The real-time machine canbe conveniently programmed in Matlabr/Simulinkr usingrapid prototyping tools (Simulink Realtimer). Its main tasksare to compute the reference position and speed to be sentto the drives, and to log all the data related to the groundstation.

Besides the development laptop used to modify the con-trol programs running on the real-time machines and thedrives, the HMI consists of a joystick to issue manually aposition (resp. speed) reference to the slide (resp. winch)motor, and of switches used to enable/disable the drives andcommence a take-off procedure.

3.2 Aircraft3.2.1 Hardware and communication

We started from a commercially available glider that wemodified for our needs, see Fig. 10. In particular, we em-ployed a model glider made out of styrofoam, whose ad-vantages are resilience to impacts, low cost and the ease ofstructure modification to include additional hardware, whilethe main disadvantages are relatively large deformations ofthe structure during flight and high skin drag, which limitthe flight speed. The chosen model glider also offers a largeenough volume in its fuselage to host all the onboard com-ponents (sensors and control hardware). The geometricalcharacteristics of the glider are resumed in Table 2 and thoseof the propulsion system in Table 3.

3

1

24

5 67

8

Figure 10: Model glider used for the experiments. 1.ailerons; 2. flaps; 3. rudder; 4. elevator; 5. release mech-anism; 6. right-angle cut to engage with the slide; 7. pro-peller; 8. fuselage containing the measurement and controlhardware (see Fig. 11).

In order to connect the glider to the tether and be able

to detach it during flight, we installed a release mechanismcontrolled by a servomotor and re-enforced the body withfiber glass and plywood. Such a release mechanism is meantto be used to tow model gliders using a winch placed onground, hence it is already designed to be mounted underthe fuselage (like in our case) and not in the nose. Usuallyemployed in competitions, these mechanisms can supporthigh forces. We chose a model milled in aluminum and in-tegrated it in the body close to the center of gravity.

To engage the glider on the slide, we cut the fuselagewith a right angle, similarly to a hydroplane. In this man-ner, we obtain a horizontal surface, used to place the glideron the slide, and a vertical surface against which the slidepushes during the initial acceleration at take-off. In order toland the glider in a compact area, we added flaps.

Table 2: Main features of the employed model gliderWingspan 1.68 mWing surface area 0.3174 m2

Aspect ratio 8.89 -Mean aerodynamic chord (MAC) 0.194 mMass 1.2 kg

For the communication between the glider and the pilotwe used as radio link a 2.4GHz Futaba system composed ofa T14SG remote control and an 6308SBT receiver. The re-mote control can transmit 14 channels and the receiver cancontrol up to 12 servomotors using the S-bus protocol. Thisprotocol is able to transmit all channels digitally throughone cable instead of several dedicated pulse width modula-tion (PWM) lines.

Table 3: Main features of the glider propulsion systemBrushless Motor Driver Propeller Lithium Polymer

Model Kv A batteryD2836 1100 20 30A 9"×6" 14.4V, 1300mAH

Regarding the servomotors, we employed 9g-class onesfor all control surfaces (rudder, ailerons, elevator and flaps).In addition, the servomotor chosen to drive the release mech-anism is a HS-82 MG with higher torque and reinforcedgears. All servomotors have a travel of ±90◦ and are con-trolled in position by a PWM signal. The width of the pulseis generally between 1000µs and 2000µs and the frequencyis 50 Hz. The other features of the servos are resumed in Ta-ble 4.

Table 4: Performance of the employed servomotors.Servo Type 9g HS-82 MGWeight (10−3 kg) 9 19Torque (10−1 Nm) 15 29Positioning time (s/60◦) 0.15 0.12Gears Plastic Metal

8

3.2.2 Measurement and control

We installed the measurement and control hardware insidethe fuselage, except for the airspeed sensor which we at-tached to one of the wings. Fig. 11 shows the main com-ponents. The conceptual layout of the glider’s measure-ment and control system is depicted in Fig. 12. As mainmicro-controller, we employed an Arduinor Mega 2560.The ease of programming, high number of input-output in-terfaces and the 5VDC operating voltage are the main rea-sons for this choice. The board acquires all signals fromthe sensors and pilot inputs, issues the control inputs to theactuators (control surfaces, release mechanism and motor)and logs data. We added an extension board (also shownin Fig. 11) to interface with the sensors and servomotors,supplying both power and control signals. The two boardstogether occupy a relatively large space and weigh about0.2 kg. Together with the sensors and the power sources,the overall weight of the measurement and control hardwareis almost 0.3 kg, which is a large share of the total weight(about 1.2 kg) for such a small-scale, light glider.

2

1

34

5 6

7

Figure 11: Onboard measurement and control hardware forthe glider. 1. DC-DC power converter; 2. motor driver; 3.inertial measurement unit; 4. GPS antenna; 5. Arduinor

Mega 2560 embedded platform; 6. extension board for RS-232 and S-bus interfaces; 7. RC receiver. The LiPo batteryis not shown in the figure.

The attitude, angular velocity vector, accelerations, andinertial position and velocity vectors of the glider are mea-sured by a commercial inertial measurement unit (IMU) bySBG Systems. The latter integrates inertial sensors (gy-ros and accelerometers), magnetometers, a barometer, anda GPS receiver. The IMU carries out sensor fusion withan extended kalman filter and provides a full state estima-tion at constant sampling frequency, 50 Hz in our case. Theunit weighs about 0.05 kg plus about 0.02 kg of the GPSantenna, which we installed on top of the fuselage, see Fig.11. The antenna is a TW1421 model from Tallysman witha gain of 28dB. For the airspeed sensor we employed aPitot probe, installed on the wing far away from the mo-tor, and a digital transducer (MS4525DO) that provides dif-ferential pressure measurements and temperature readingsthrough an I2C protocol. The differential pressure has arange 6894.76 Pa with an accuracy of 0.84Pa and is ac-quired with a precision of 14 bits. The temperature sensorhas a range of -50◦ C to 150◦ C, with an accuracy of 1.5◦ Cand a precision of 11 bits. The airspeed is then computed

Arduino Mega +expansion modules

ON-BOARD

AccelerationsAngular ratesAttitudePosition wrt groundVelocity wrt ground

Control inputs fromhuman pilot

SBG InertialMeasurement Unit

Servomotors

Propeller

Receiver

RudderElevatorAileronsFlaps

Thrust

Human pilot

ON-GROUND

Pilot inputs Remotecontroller

SD-Card

Radio link

Wind speed probeAirspeed in bodyx-axis direction

Log

Figure 12: Layout of the measurement and control systemfor the glider.

from the differential pressure and temperature readings, us-ing the absolute pressure measurement given by the IMU.

Regarding the actuators, the servomotors are controlledby a PWM signal which we generate with the Adafruit ServoDriver Board based on a PCA9685 chip. The board com-municates via I2C with Arduino and can command up to 16servos. It has a 12 bit precision which translates into a min-imum step of 5.85µs in the signal sent to the servo. Sincemost analog servos have a precision of 10µs to 8µs, onecan obtain a very smooth control without jittering. Duringdynamic maneuvers, the servomotors can draw a significantamount of current. For this reason we connected the PWMboard to two separated power supplies, one for the chip andthe other for the servomotors: the chip is supplied by themain power source of the Arduino board, a DC-DC con-verter providing a regulated 5V, while the servos are sup-plied by the same DC-DC converter as the motor controller.

To log data, we used a micro-SD card slot from Mikro-Electronika interfacing via SPI with Arduino. We used anopen-source library to rapidly write binary log on the SD-card. The logs can be decoded after each test session usinge.g. Matlab. The logging function programmed on the Ar-duino board is synchronized with the IMU to log at 50Hz.

When armed, the embedded controller always reads theincoming data from all the onboard sensors and the inputsfrom pilot through the receiver, it generates the control sig-nals for the servos and the motor, and it logs data. A switchoperated by the pilot changes the flight mode, from com-pletely manual (i.e. the pilot inputs are fed directly to theactuators), to fly-by-wire (with low-level controllers thatstabilize the glider pitch and roll, and the pilot providingreferences for these angles), to fully autonomous. The mod-eling and automatic control of the glider is described in de-tail in a separate contribution [16].

9

4 Ground station control designThe control structure for the ground station is hierarchi-cal: inner feedback control loops programmed on the drivestrack references of position (for the slide) and speed (forthe winch), which are computed by an outer control loopprogrammed on the real-time machine. In the following wedescribe in detail the control algorithms, while we providethe numerical values of the involved parameters in section5. All control loops are implemented in discrete time withsampling period Ts; we denote with k ∈ Z the discrete timeinstants.

For the inner loops we employ static state-feedback con-trollers designed via pole-placement, using a standard lin-ear model of the motors which takes into account inertiaand viscous friction. Denoting with the subscript “s” and“w” quantities related to the slide and the winch motors, re-spectively, the corresponding control laws are:

us(k) = Kθ,s(θref,s(k)− θs(k))−Kθ,s θs(k) (8a)

uw(k) = Kθ,w(θref,w(k)− θw(k)) (8b)

(8c)

where us, uw are the commanded torques, θs, θw the an-gular positions of the motors, θref,s the reference positionfor the slide motor, θref,w the reference speed for the winchmotor, and Kθ,s, Kθ,s, Kθ,w the feedback gains. The statevariables θs, θs, θw, θw are measured accurately with theencoders (incremental and Hall effect) mounted on the mo-tors and connected to the drives. The torques are saturatedto their maximum limits ±T s, ±Tw.

For the outer loops, we implemented two modes: man-ual operation and automatic one. In the first mode, the ref-erences θref,s, θref,w are issued by a human operator usingthe joystick in the HMI. This operation mode is used fordebugging and to collect data for parameter identification(e.g. to estimate the inertia of the slide motion system andof the winch).To carry-out a take-off maneuver, the second operating modeis used instead. The starting conditions are with the gliderinstalled on the slide, and the latter positioned at one end ofthe rails. Without loss of generality let us assume that k = 0when the take-off maneuver starts. Then, a step of positionreference θref,s(k) = L/Rs, ∀k > 0 is issued, where L isthe desired slide travel used for take-off and Rs the radiusof the slide drum. As a consequence, the inner controllerfor the slide will move it as fast as possible to the new ref-erence position and during this motion the take-off speedof the aircraft is reached. The winch has to coordinate itsmotion with the slide and always achieve a good tradeoffbetween limiting the pulling force exerted by the tether, andavoiding tether entanglement, which quickly occurs whenreeling-out without any load. In order to achieve this goalthroughout the take-off and during flight, we employ a com-bined feedforward/feedback approach to compute the refer-ence winch speed. The feedforward contribution latches thewinch speed to the slide speed:

θffwdref,w(k) = γθs(k) (9)

where γ > 0 is a scalar that can be tuned to achieve agood coordination between the two drums. In our proto-type, since the presence of the glider does not affect muchthe inertia of the slide and of the winch (due to their rela-tively much larger mass), the tuning of γ could be carriedout in preliminary laboratory tests without the aircraft. Thefeedback contribution exploits the measure of the springtravel xs(k). In particular, we set two threshold values,xIs , x

IIs , which divide the available spring travel in three

zones:

• Zone a (0 ≤ xs(k) ≤ xIs ): the spring is practicallyuncompressed, the winch shall decrease speed andeventually reel-in;

• Zone b (xIs < xs(k) < xIIs ): the spring is subject tolow force, the winch shall be held in place (constanttether length);

• Zone c: (xIIs ≤ xs(k) ≤ xs): the spring is subjectto relatively large force, the winch shall increase itsspeed and reel-out to release the tether.

Then, the feedback contribution to the reference winch speedis computed according to the following strategy (see Fig.13):

If 0 ≤ xs(k) < xIs (Zone a)

xs(k) =xs(k) − xIs

xI,as − xIsθfbck

ref,w(k) = min(

0,max(θ

fbckref,w,

(θfbck

ref,w(k − 1) + Tsθaref,wxs(k)

)))Else if xIs ≤ xs(k) < xIIs (Zone b)

θfbckref,w(k) = θfbck

ref,w(k − 1)

Else if xIIs ≤ xs(k) ≤ xs (Zone c)

xs(k) =xs(k) − xIIs

xII,cs − xIIs

θfbckref,w(k) = max

(0,min

(θ

fbck

ref,w,(θfbck

ref,w(k − 1) + Tsθcref,wxs(k)

)))(10)

where θfbckref,w, θ

fbck

ref,w are the desired minimum and maximumreference speed values that can be issued, and θa

ref,w, θcref,w

are the desired angular accelerations for the reference speed.Such values are scaled according to the position of the po-tentiometer relative to the values xI,as < xIs and xII,cs >xIIs , respectively for zones a and c, which are design param-eters as well. Equation (10) represents in practice an inte-gral controller, where the integrated quantity is the distanceof the spring position xs(k) from the interval (xIs , x

IIs ) (i.e.

zone b) and the gain is piecewise constant, since it is dif-ferent in zones a and c. Moreover, a saturation of the inte-grated variable to negative (resp. positive) values is oper-ated whenever the spring enters zone a (resp. c), in order toquickly start to reel-in (resp. reel-out) when the tether is re-leased (resp. pulled). As shown in the next section, a sensi-ble choice for the involved design parameters is θ

fbckref,w, θ

aref,w <

0, θfbck

ref,w, θcref,w > 0, xI,cs ≈ xIs /2 and xI,cs ≈ (xs + xIIs )/2.

10

Integrator saturation in zones a and c

× ,

,

,

a cb

Figure 13: Block-diagram of the feedback contribution tothe reference winch speed.

Finally, the reference speed actually issued to the low-level controller is computed as:

If θs(k) > 0

θref,w(k) = max(θffwd

ref,w(k), θfbckref,w(k)

)Else

θref,w(k) = θfbckref,w(k)

(11)

According to (11), the feedforward contribution is used onlyif larger than the feedback one, and only if the speed of theslide is positive, i.e. during take-off.

5 Experimental resultsTable 5 shows the values of the controller parameters thatwe used in our experimental tests. The system parameters(ground station and glider) are the ones reported in Tables1-4 and introduced throughout the paper. The chosen springhas a maximum travel xs = 0.35 m.

Table 5: Control parameters used for the experimentsTs 0.001 s xIIs 0.1 mKθ,s 14 Nm rad−1 xI,cs 0.025 mKθ,s 2.5 Nm s rad−1 xII,cs 0.2 m

Kθ,w 1 Nm s rad−1 θfbckref,w −10 rad s−1

T s 26 Nm θfbck

ref,w 120 rad s−1

Tw 13 Nm θaref,w −100 rad s−2

γ 1.2 - θcref,w 30 rad s−2

xIs 0.05 m L 3.7 m

We present here the typical results obtained during atake-off test with manually piloted glider and autonomousground station. Fig. 14 shows the elevation of the gliderabove ground during the take-off, together with its distancefrom the ground station, the tether length measured betweenthe slide and the glider (i.e. taking into account also theslide motion), and the spring compression. The corre-sponding average climb angle up to 20 m of elevation isabout 30◦. It can be noted that a very good agreement existbetween the spring movement and the difference betweenthe glider distance from the ground station, measured bythe onboard IMU, and the tether length, measured with the

0 2 4 6 8 10 12 14 16 18 200

10

20

30

40

50

60

70

Time (s)

Leng

th(m

),sp

ring

com

pres

sion

(10-2

m)

Figure 14: Experimental results. Courses of the glider ele-vation (thick dotted line), its distance from the ground unit(solid line), the tether length between the glider and theslide (dash-dot), and the spring compression (dashed). Thelatter is expressed in 10−2 m for the sake of readability, allother quantities are in m.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

2

4

6

8

10

12

Time (s)

Leng

th(m

),Sp

eed

(ms-1

),Sp

ring

com

pres

sion

(10-1

m)

Figure 15: Experimental results. Courses of the slideposition (thick solid line), winch position (thick dash-dotted line) in m, spring compression (thick dashed line) in10−1 m, and of the slide speed (solid line) and winch speed(dash-dotted line) in m s−1.

winch motor encoder. Indeed, these two measurements al-low one to obtain a quite accurate estimate of the lengthof slack tether. The behavior of the slide and of the winchduring the first few seconds of the same test is presented inFig. 15, where all quantities are expressed in m and m/sfor the sake of comparison. The lag between the speed ofthe winch and that of the slide is due to the higher inertiaof the former (and lower applied maximum torque, see Ta-ble 5); we compensated the resulting position difference byleaving a small initial slack line length, visible in Fig. 15in terms of non-zero winch position at time t = 0. Fig.15 also illustrates the typical behavior of the control strat-egy given by (9)-(11): during the very first time instantsthe winch speed is increasing even if the spring is uncom-pressed, since the feedforward contribution is larger thanthe feedback one, while after about 0.5 s the feedback strat-

11

0 1 2 3 4 5 6 7 8 9 10-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Time (s)

Pow

er(k

W)

Figure 16: Experimental results. Courses of the power con-sumed by the slide motor (solid line) and by the winch mo-tor (dash-dotted line).

egy based on the spring compression is used. Overall, thedescribed hardware solution and the corresponding controlapproach achieve the desired, conflicting goals of limitingthe tether pull while avoiding an excessive reel-out of theline, which would lead to entanglement very quickly. Amovie showing the winch behavior (in combination withthe glider autopilot described in detail in [16]) is availableon-line [15].

Finally, Fig. 16 shows the courses of the power con-sumed by the winch and the slide during take-off. The neg-ative power (i.e. power required to brake the motors) isdissipated in the braking resistors connected to the drives inour prototype, while in a final solution they could be recov-ered with storage or fed back to the grid. The peak powervalues are 2.18 kW and 1.34 kW respectively for the slideand winch motor. The theoretical analysis of [12] predicts,for the same glider and ground station features, values of2.11 and 1.26 kW, very close to the ones obtained in ourexperiments, hence confirming the validity of the results in[12] pertaining to this take-off approach.

6 ConclusionsThe paper presented in detail the design of a small-scaleprototype to study the take-off of tethered aircrafts. Such adesign can be easily replicated and improved by researchersand developers working on airborne wind energy systemswith ground-based generation and rigid wings. The reportedexperimental results show that a quite effective take-off ma-neuver can be achieved in compact space, with power re-quirements in line with previous theoretical findings. Thesensible next steps along this line of research are a simi-lar study for other types of take-off strategies, e.g. usingvertical-axis propellers, in order to further validate the ex-isting theoretical analyses [12], the development of land-ing strategies (including their experimental validation) withsimilarly effective performance in terms of low cost andcompactness, finally the study of full operational cycles oftake-off, power generation, and landing.

AcknowledgmentThe authors would like to thank Alessandro Lauriola andStefan Schmidt for their helpful contributions during theproject.

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