Development and Testing of a Control System for the Automatic Flight of Tethered

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Joseph Coleman, Hammad Ahmad, and Daniel Toal Department of Electronic & Computer Engineering, University of Limerick, Limerick, Ireland

Received 30 July 2014; accepted 16 February 2016

This paper presents the design and testing of a control system for the robotic flight of tethered . The use of tethered kites as a prime mover in is undergoing active research in several quarters. There also exist several additional applications for the remote or autonomous control of tethered kites, such as aerial sensor and communications platforms. The system presented is a distributed control system consisting of three primary components: an instrumented tethered , a kite control pod, and a ground control and power takeoff station. A detailed description of these constituent parts is provided, with design considerations and constraints outlined. Flight tests of the system have been carried out, and a range of results and system performance data from these are presented and discussed. C 2016 The Authors Journal of Field Robotics Published by Wiley Periodicals, Inc.

1. INTRODUCTION are performed from the ground. Examples of such systems are Kitenergy (Milanese, Taddei, & Milanese, 2013) and Tethered kites including ram-air inflated parafoils, pneu- EnerKite (Bormann, Maximilian, Kovesdi,¨ Gebhardt, & matically inflated kites (Jehle & Schmehl, 2014), and rigid Skutnik, 2013). These systems have the advantage of wings (Ruiterkamp & Sieberling, 2013) promise low-cost increased system simplicity as the requirement for a kite access to high altitudes above ground with minimal ma- control pod is eliminated, however at least two tethers are terial, civil, and logistics costs. This technology effectively required, which increases the airborne system weight and aims to displace the use of towers to elevate wind energy drag with tether length more rapidly than single tether systems above ground level. The increased wind speeds at systems. elevations up to approximately 1,000 m above ground have KitePower has investigated several kite types and con- spurred the active area of airborne wind energy (AWE) re- trol methods for these. The main demonstrator is a control search (Archer, 2013; Archer & Caldeira, 2009). Other appli- pod actuated tube kite, where the kite geometry is defined cations of such technology also exist, such as low-cost aerial by an inflated tube structure with a stretched membrane platforms for sensors and communications equipment. The skin. The KitePower control pod features two servo actua- replacement of civil structures with smart, airborne systems tors that independently control the steering and depower introduces a challenging embedded systems and control functions of the kite (Fechner & Schmehl, 2012). A kite- problem, namely maintaining the safe, persistent flight of plane hybrid has also been demonstrated by van der Vlugt, the airborne system in a range of flight modes and weather Peschel, and Schmehl (2013). conditions. Solving this challenge is a key step in the de- Controller development and testing for AWE systems velopment of tethered airborne systems. A diverse range can be performed in incremental steps of increasing com- of hardware system architectures have been presented to plexity and precision. As an initial step, pilot in-the-loop date by many researchers and developers. Skysails in Er- control is achieved whereby a remote fly-by-radio system is hard & Strauch, 2013 and Maaß & Erhard, 2013 use an air- implemented. In such systems, a human pilot visually flies borne control pod with a single actuator to fly large the wing through the movement of a joystick or similar in- kites automatically ahead of cargo ships in a towing ap- put device providing remote control to actuators on the kite plication, providing a reduction in ship fuel consumption. or on the kite control pod. A pilot familiar with the manual Skysails in Fritz (2013) outline a 55 kW land-based electri- control of kites can quickly adapt to such a control system, cal power production prototype, which leverages the ship tuning their response to the kite motion, and achieving towing technology in a power-production application. stable flight. With the introduction of automatic controllers, An alternative power production approach utilizes increasing system autonomy can be achieved. As the com- a parafoil kite connected to ground-based actuators in plexity of the control system increases, additional sensors which both the power takeoff and kite-steering functions and processing effort are required compared to human in- Direct correspondence to: [email protected] the-loop operation. Such controllers require high-frequency

Journal of Field Robotics, 1–20 (2016) C 2016 The Authors Journal of Field Robotics Published by Wiley Periodicals, Inc. View this article online at wileyonlinelibrary.com • DOI: 10.1002/rob.21652 This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. 2 • Journal of Field Robotics—2016 estimations of kite position and orientation for the pro- 2. Use of a control-pod actuation method to avoid addi- vision of closed-loop feedback. The ultimate aim of such tional drag of multiple tethers trailing to a ground sta- systems is the demonstration of reliable, persistent, fully tion. autonomous control of tethered wings, and much progress 3. Independently actuated steering implemented on the has been made recently on a variety of hardware platforms. control pod where the steering lines are actuated sep- Leading examples of such systems include Fagiano, arately and together form a longitudinal and lateral con- Zgraggen, Khammash, and Morari (2013a) and Fagiano, trol input, using symmetrical and asymmetrical line dis- Zgraggen, Morari, and Khammash (2013b), where, using placements, respectively. a ground-actuated control system, a number of automatic 4. A pumping-mode winching ground station that uses flight tests are conducted where the wing is maintained in separate, dedicated electrical machines for the power stable figure-of-8 orbits. Good accuracy between a dynamic generation and recovery tasks. Each machine is appro- model of the system and the field test data is shown. Jehle priately sized for the task it performs. and Schmehl (2014) present a tracking controller as applied in field-testing to a prototype 25 m2 leading-edge inflatable Through field tests, the suitability of the flexible kite, (LEI) kite system for pumping-mode AWE power genera- control pod, and distributed control system are examined. tion. A cascaded control system is outlined with a bearing The field-testing is aimed at both the testing of suitable con- controller as an outer loop and an attitude controller as an trol system hardware required to fly tethered kites and the inner loop providing the steering actuator set point. Project- development and testing of estimation and control algo- ing a figure of 8 onto the tether unit sphere, they present ex- rithms for AWE kites. perimental results of the controllers tracking this trajectory. Much of the research is focused on 2. SYSTEM DESCRIPTION power takeoff (electrical or mechanical) applications within Pumping mode airborne wind energy uses a tethered wing the AWE sector. In Argatov and Silvennoinen (2010), the au- to extract power from the wind. Operating in a periodic thors present the formulation of a generic pumping mode pumping cycle, the tethered wing is flown in a high-lift kite system in which practical design considerations such as periodic orbit about the wind vector. This produces high factors of safety for the tether and minimum bend radii are tension in the tether, which pays out from a tether drum. included. Legislative restrictions, in this case U.S. Federal Thus, mechanical power is produced on the ground station Aviation Administration (FAA) regulations, are included. driveshaft. At the maximum tether length, the power phase The analytical modeling of apparent wind load effects on ends and a recovery phase begins where the wing is flown the tether of a pumping-mode AWE system is detailed in in a low-lift configuration and is winched in to the starting Argatov, Rautakorpi, and Silvennonien (2011), which ana- tether length, using a fraction of the previously generated lyzes the degradation of system power output at extended power. This cycle continues, somewhat analogous to a tether lengths where the influence of tether drag becomes slow-moving long-stroke piston engine. At the ground pronounced. station, electrical power takeoff is performed by a generator Having attained long-term stable flight systems and connected via a drivetrain to the tether drum, as shown algorithms, an alternative application also exists: low-cost outlined in Figure 1 (Coleman, Ahmad, Pican, & Toal, aerial sensor platforms. Using kites as aerial imaging 2014). platforms offers low-cost access to altitude, providing A tethered kite control system requires a distributed aerial imagery for a variety of survey and agricultural control system. The control system can be divided into applications (Murray, Neal, & Labrosse, 2013) or geomor- three main subsystems: the ground station, the control pod, phology mapping (Boike & Yoshikawa, 2003). Systems and the wing. The ground station anchors the system and under development within AWE have significantly larger manages the tether while performing the electromechani- payload capacity than the small-scale imaging systems, as cal power takeoff operations, launch, and recovery process. such sensing and imaging payloads may form an auxiliary The control pod houses the sensors, actuators, and proces- application power-generation kite system. sors necessary to control the flight of the kite. The wing is A kite control system design is developed that has some fitted with sensors to enable feedback control. similarities to other approaches, focuses on the best subsys- tem approaches from the literature, and also focuses on 2.1. Parafoil Kite areas that are not directly addressed by other researchers in the literature. The research focus and developed system A parafoil kite is a tethered, ram-air inflated wing whose ge- have the following key features: ometry is defined through the dimensions of its fabric panel elements and through the system of tensioning rigging lines 1. Use of tethered flexible ram-air kites due to cost effi- (Lingard, 1995). The fabric elements form an upper and ciency, physical robustness, and inherent safety in the lower airfoil surface, which is divided into sections along event of a crash compared to rigid wing solutions. the span by ribs. The ribs divide the parafoil into discrete

Journal of Field Robotics DOI 10.1002/rob Coleman, Ahmad, and Toal: Development & Testing of a Control System for the Automatic Flight • 3

Ram-air kite Instrumented wing

Brake lines Flying lines

Pod Control pod

Block & tackle fairlead

Soil anchor

Main tether Ground control & Low speed tether shaft (N1) Tether power take off station Overrunning Encoder Drum Clutch

Recovery PM DC Clutch Brake Motor Generator High speed recovery shaft (N2) High speed generator shaft (N3) Figure 1. Control system arrangement for pumping-mode airborne wind energy. cells. The fabric is a high-strength, nonporous, rip-stop onto the main tether. The foil is controlled by adjusting nylon weave. The cell divisions are formed from fabric ribs, the relative length of the steering lines, also called brake which act in tension to maintain the airfoil section geometry. lines. The steering lines run to the trailing edge of the foil, The cells may be cross-braced to provide additional rigidity. one on each side, and the relative shortening of these lines The parafoil maintains its aerodynamic shape by ram-air deflects the trailing edge downward on a given side. This pressurization through vents in the leading edge of the foil. has the effect of increasing the camber of the foil locally The oncoming airflow stagnates within the foil, causing where the deflection occurs, changing the local lift and drag the foil to inflate, as internal pressure is now greater than profile of the foil. By the symmetric positive deflection of the local atmospheric pressure. The stagnation pressure of brake lines, the lift of the foil is increased; however, this will the flow is equal to the total pressure (Po)oftheflowasin also increase the drag. Asymmetric deflection of the brake Eq.(1),whereVa is the freestream airspeed, Ps is the static lines will induce a coupled yaw and roll output, enabling pressure of the flow, and ρ is the density of air, steering. In the brake-line zero position, the control lines are very lightly loaded, with increasing tension as the de- 1 2 Po = Ps + ρVa . (1) flection increases. To minimize airborne weight, extremely 2 high strength-to-weight ratio polymers such as Dyneema Cross-port vents in the ribs ensure pressure distribu- braided rope (Bosman, Reid, Vlasblom, & Smeets, 2013) are tion along the span of the wing. The aerodynamic forces used in the kite rigging lines and main tether. Figure 2 il- developed on the foil are distributed through the system of lustrates the basic operation of the brake lines on a parafoil branched lines, which form the rigging of the kite. These kite. lines act in tension to maintain even wing loading and thus the foil geometry. The rigging lines are connected to the 2.2. Kite Sensors lower surface of the parafoil along the ribs, converging be- low the kite to four flying lines: two symmetrical “power” A variety of sensors are required to provide feedback for lines and two steering lines. The power lines carry the force the control system and to measure system parameters for developed by the foil to the control pod, where they merge analysis. The kite is fitted with an inertial measurement

Journal of Field Robotics DOI 10.1002/rob 4 • Journal of Field Robotics—2016

Figure 2. Steering mechanism on during brake line inputs (viewed from below, leading edge forwards). Figure 3. IMU and GPS enclosure mounted on Flysurfer 12 m2 kite upper surface during kite launch. unit (IMU) and global navigation satellite system (GNSS) receiver, which are used to calculate estimates of kite posi- tion, speed, and orientation. The IMU consists of orthogo- The core of this system is a National Instruments single- nally aligned three-axis MEM (microelectromechanical) ac- board RIO (sbRIO) (National Instruments, 2010) real-time celerometers, gyroscopes, and magnetometers and also a (RT) processor, which interfaces to hardware through the barometer and thermometer. The GNSS system is capable IO of a field-programmable gate array (FPGA). The sbRIO of receiving both GPS and GLONASS transmissions. Table I communicates with the kite mounted IMU/GNSS micro- outlines the basic specifications of the IMU sensors. The controller using a serial connection. The servo motor drives IMU is housed in a lightweight enclosure, which is attached are controlled via FPGA generated pulse width modulation to the upper surface of the wing at the estimated aerody- (PWM) signals. Additional sensors interfaced through the namic center (quarter chord, midspan), as can be seen in FPGA include the following: Figure 3. A light cable routed along a rigging line connects the kite sensors to the control pod, providing power and r Load cell on the tether, which determines the tension serial data. r generated by the wing. r Current monitors for each servomotor. 2.3. Control Pod Battery voltage monitors. The central component of the control system is the kite con- trol pod, which ascends with the kite as the tether pays out. The pod also contains an embedded computer (Intel, The pod (Figure 4) is suspended below the kite, rigged such 2012) running a real-time operating system (Phar Lap ETS) that it hangs on the power lines without bearing the tensile (National Instruments, 2004). The real-time PC (RTPC) is load of the tether. The steering lines terminate at the control used to solve complex algorithms with low latency and pod on two steering line pulleys. The control pod houses all a high level of determinism. The RTPC and the sbRIO are the necessary components to measure the orientation and interfaced via an Ethernet switch, which also provides com- position of the kite and to actuate the steering lines of the munications to the ground station via a radio link. Figure 5 kite. The pod also carries batteries required to power the outlines the hardware and communications architecture of control electronics and actuators. the distributed control system.

Table I. Inertial measurement unit and global navigation satellite system sensor specifications.

Sensor Model Range Resolution Sampling rate (Hz)

Accelerometers MPU-6000 ±8 g 2.44 × 10−4 g 100 Gyroscopes MPU-6000 ±1,000 deg/s 0.0305 deg/s 100 Magnetometer HMC5883L ±8 G (G) 5 mG 100 Barometer MS5611 10–1,200 mbar 0.012 mbar 100 Sensor Model Accuracy Output rate (Hz) GNSS LEA-6H Horizontal accuracy (SBAS aided): 2 m 5 Course accuracy: 0.5 deg Velocity accuracy: 0.1 m/s

Journal of Field Robotics DOI 10.1002/rob Coleman, Ahmad, and Toal: Development & Testing of a Control System for the Automatic Flight • 5

Figure 4. Control pod subsystem.

2.3.1. Steering Actuators Nylon spools attached to each gearhead output form small The kite is steered by two geared servomotors. The motors winches for each steering line, which control the length of are brushed dc machines, powered by a dual-channel motor the left and right steering lines. Position feedback and speed drive. A reduction gearhead is fitted to the motor output. estimation are provided by incremental encoders fitted to

Serial Wing IMU/GNSS

Control Pod

Motor Drive DC Motor 1 Ethernet CH1 Switch CH2 DC Motor 2 RT RTPC FPGA Processor Current sensing Voltage sensing Power Supply GPS/IMU input 2x LiPo Battery Ethernet Load Cell Radio link Encoder inputs

Ground Station

Radio link Current sensing Modbus Ethernet ng Ground Voltage sensing Recovery Switch RS232 Winch Motor Encoder input PLC Drive PC1 PC2 Relays Barometer Joystick USB

Figure 5. Functional diagram of the distributed control system architecture.

Journal of Field Robotics DOI 10.1002/rob 6 • Journal of Field Robotics—2016

effective inertia seen by the motor shaft. The motor speed, ωr (rad/s), is determined by the voltage induced in the winding (Vind) and the speed constant kω: = ωr Vind kω. (6)

Similarly, the mechanical torque developed by the mo- tor is determined by the torque constant kT and the armature current:

Tm = Ia kT . (7)

The key motor properties are given in Table II from the manufacturer data (Maxon Motor, 2012). A disadvantage of the implemented arrangement is that the motors draw current to hold position in the pres- Figure 6. Steering line actuator fairlead and passive level ence of an opposing load. An alternative arrangement us- winding. ing fast-acting electromagnetic brakes on each motor would possibly reduce the motor power consumption, but only when no position change is demanded. Other actuation ar- each motor shaft. Bidirectional current is measured on each rangements have been developed, notably that of van der motor input to enable closed-loop torque control and anal- Vlugt et al. (2013), where the steering function is provided ysis of the loads and power consumption during various by one motor and the longitudinal input is provided by a flight maneuvers. Consideration must be made to the suit- second motor. This configuration may allow for reductions able routing and guidance of the steering lines on the ac- in actuator power consumption if the longitudinal motor tuator reel. A simple fairlead with offset attachment to the could largely remain in a braked, low-power condition. actuator reel provides a simple passive level winding effect The advantage of the implemented configuration is that of the steering lines, as shown in Figure 6. the steering line lengths can be directly and independently The governing equations for the servomotors are given controlled, which to the authors’ knowledge is novel. Other as follows: control-pod arrangements either do not provide longitudi- Pe = VI = ω τm + τf , (2) nal depower control (Fritz, 2013), or they provide longitudi- nal depower and lateral steering control through individual Pm servos and thus cannot actuate the steering line lengths in- η = , (3) Pe dependently (van der Vlugt et al., 2013). τl T = J ω˙ = τ − − τ , (4) r m G f 2.3.2. Control Mixing As the same two control surfaces and actuators must T = F r, (5) l l perform two separate control actions, a suitable method where τm is the mechanical torque developed by the motor, of mixing these signals is required. A software control τl is the torque on the pulley from the actuation load, G the mixing function performs this task by superimposing the gear ratio of the gearhead, τf is the total torque produced by symmetric longitudinal (δe) offset value on the asymmetric frictional effects (Coulomb & viscous damping), and J is the lateral-directional control input (δa ) and converting them

Table II. Maxon (RE 40) graphite brushed motor with gearhead (GP42) and encoder.

Power nominal Pnom 150 W Torque constant kT 0.0302 N m/A Voltage nominal Vnom 24 V Speed constant kw 33.196 (rad/s)/V Speed no load nnl 7,580 RPM Speed constant kn 317 RPM/V Torque nominal Tnom 0.177 N m Terminal resistance t 0.299  Torque stall Tstall 2.42 N m Terminal inductance Lt 0.0823 mH Current nominal Inom 6 A Motor efficiency 91% Motor mass 480 g Gearhead efficiency 81% Gearhead mass 360 g Gearhead backlash 0.8° Gearhead reduction 91/6 (15:1) Encoder resolution 1,024 CPT

Journal of Field Robotics DOI 10.1002/rob Coleman, Ahmad, and Toal: Development & Testing of a Control System for the Automatic Flight • 7 to nondimensional left and right motor position set points (δbl,δbr ):

δbl,br = δe + δa. (8) These commands are then dimensionalized to degrees of gearhead output by a steering line displacement variable (δbmax) and sent to each servo position controller as a position set point.

2.3.3. Lateral-directional Control Lateral-directional control authority is provided by the asymmetric displacement δa of the steering lines. This com- mand ranges from −1to+1, where right is positive. The shortened line deflects one side of the trailing edge pro- Figure 7. Joystick control inputs during manual flight. ducing asymmetric lift across the wing, inducing a roll-yaw coupled moment. During manual control, moving the joy- stick between the left and right limits provides the lateral tether breaking. Voltage monitoring of the batteries is im- input. A trim setting enables the lateral input to be offset, plemented to estimate the remaining battery charge and which is useful in trimming out small asymmetries in the endurance of the system; this is also required for accurate rigging setup without requiring a continuous stick deflec- estimation of the servo motor power consumption. The con- tion. trol electronics have been measured to draw a continuous current of 0.7 A at 24 V (16.8 W). Due to the varied loads experienced by the actuators, the power draw is best mea- 2.3.4. Longitudinal/Depower Control sured in terms of a peak and an average during flight time. Symmetric deflections of the steering lines produce a sym- The 3.3 A·h battery was found to provide an actuator en- metric deflection of the trailing edge region of the wing; this durance of approximately 1-h flight time. Instantaneous ac- pitches the wing upward while also changing the coefficient tuator current is measured and logged with peak values of of lift through airfoil changes similar to the action of an air- up to approximately 10 A occurring in each motor. The mo- craft plane flap. The longitudinal depower inputs are made tor current control loop constrains the maximum current to using the joystick throttle setting, as this input is generally 10 A, while a resettable fuse will activate if a sustained cur- held constant with infrequent changes. rent at greater than 10 A occurs to prevent motor winding burnout.

2.3.5. Communications 2.4. Ground Control System The RT processors are interconnected on an Ethernet net- work. An Ethernet switch in the control pod connects the 2.4.1. Control PC RT processor and RTPC and allows additional components The control PC provides the link between the airborne com- to utilize the network, e.g., camera systems or additional ponents of the system and the ground system. The con- sensors. The link to the ground station is made using a trol PC is connected to the control network via an Ether- 2.4 GHz IEEE 802.11 wireless point-to-point (PtP) transmis- net switch. Custom LabVIEW virtual instruments provide sion. Similarly, an Ethernet switch at the ground station pro- display and human interface consoles. Audible tones are vides access to the network for all necessary components, generated by the PC to alert the operator to any caution or such as the winch, control PC, display, and visualization. alarm conditions. Further audible tones are used to provide simple feedback to the operator, without breaking eye con- tact with the wing. A data log of the network data stream is 2.3.6. Power Supply recorded onto the control PC hard drive. The control pod is powered by two 24 V (nominal) lithium polymer batteries, which enable operation for approxi- mately 2 h. The batteries are positioned such that they can 2.4.2. Joystick Input be exchanged easily in the field. Future developments of A USB joystick connected to the ground control PC is used the system may incorporate a dc supply through the tether, by the operator manually to control the kite and to enable although smaller batteries would be retained to provide and disable automatic controllers. During manual control, an uninterruptable supply sufficient to land the system in the longitudinal and lateral inputs are provided by the hu- the event of a power loss, e.g., due to a conductor in the man pilot. As shown in Figure 7, the left to right movement

Journal of Field Robotics DOI 10.1002/rob 8 • Journal of Field Robotics—2016

Figure 8. Winch control electronics front panel and mechanical assembly.

of the joystick provides the manual lateral input (δa) while Table III. Induction motor specifications. the longitudinal input (δe) is provided by the “throttle” lever of the joystick, as this input is not frequently changed. Fine Motor power 2.2 kW Number of poles 4 adjustment to the lateral trim (δa trim) is provided by the joy- Current nominal 8.1 A Speed nominal 1,445 RPM stick hat switch. Automatic controllers can be overridden by (50 Hz) pulling the joystick trigger, reverting to manual control. Winding Delta Power factor (cosϕ)0.77 connection Voltage nominal 230 V Number of phases 3 2.4.3. Ground Winch Station The ground winch controls the tether length and speed and performs the mechanical to electrical power conversion. The Modbus port. An encoder is fitted to the low-speed shaft tether enters the winch through a fairlead, which ensures providing a measurement of tether length and speed. The correct alignment with the tether drum. The tether drum encoder is also required to provide closed-loop position and holds the excess tether, and its radius provides a lever arm, speed control in the tether recovery phase. An electromag- converting the tether tension into mechanical torque on the netic brake provides an emergency stop function and en- driveshaft. The winch prototype (Figure 8) has three drive- ables the kite to be flown on fixed tether lengths. The winch shafts: the main low-speed shaft where the tether drum is is powered by a single-phase 230 V ac supply, which can be mounted, a high-speed recovery and brake shaft, and a fur- provided from a mains connection or a portable generator. ther high-speed power takeoff shaft, which terminates at the Appropriate electrical safety devices are installed to ensure generator. A direct drive power takeoff topology has also the safe operation of the electrical system. The winch front been proposed in Coleman et al., (2014), however due to panel provides a low-level human interface to the winch budgetary constraints a geared solution with higher-speed system. An emergency stop button when activated rapidly electrical machines was required in the prototype. stops the recovery motor drive, applies the electromechani- The winch is controlled by a programmable logic con- cal brake, and switches in the full generator dump load. The troller (PLC) with human interface and access to the dis- tether recovery function is performed by a dedicated three- tributed control variables provided by the ground control phase induction motor connected to a variable frequency PC. The PLC receives inputs from various switches and sen- drive (VFD), as detailed in Table III. sors on the winch, such as the low-speed shaft position and The mechanical power is delivered through the drive- speed, the generator current and voltage, and a barometer train for electrical power takeoff by a dedicated permanent enabling the electronic altimeter on the parafoil to be refer- magnet dc generator, detailed in Table IV. The generator enced to ambient ground barometric conditions (see Section dump load provides a power sink circuit for the generator. 2.4.4). PLC outputs control various elements of the winch, Three 2.2 kW 1.2  resistors are connected to the genera- such as the electromagnetic clutch, brake, dump load re- tor via electromagnetic relays. These relays are controlled lays, and the recovery motor drive communications over a by the PLC. The dump resistors are switched parallel such

Journal of Field Robotics DOI 10.1002/rob Coleman, Ahmad, and Toal: Development & Testing of a Control System for the Automatic Flight • 9

Table IV. Permanent magnet dc generator specifications. and yaw Euler angles are positive when clockwise about the x, y,andz axes, respectively, observing the common Power nominal 7.22 kW Torque constant 4.88 A/Nm aeronautical convention (Stevens & Lewis, 2003). Current max 110 A Resistance at 0.016  Fixed Earth axes (XE , YE , ZE ) are defined with the origin terminals fixed to the tether anchor position, i.e., the point at which Voltage constant 0.22 V/RPM Inductance at 0.019 mH the tether exits all fairleads and guides. The Earth-fixed axes terminals have the x axis aligned with magnetic north, the y axis with east, and the z axis down along the Earth spheroid normal. From the magnetic heading, a wind axes system is defined that when an additional resistor is activated, it is switched in by rotating the ZE axis by the wind direction w, such that parallel with the existing load, which increases the electrical XE aligns with the wind vector Vw. load. 2.5.2. Roll and Pitch Estimation 2.5. Estimation and Control Algorithms A complementary filter adapted from Lai, Jan, & Hsiao 2.5.1. Definition of Axes and Coordinate System (2010) and Dongwon & Tsiotras (2007) is used to provide A right-handed, Cartesian, body-fixed coordinate system is roll and pitch estimation from the accelerometer and gy- defined to describe the airborne system orientation. For the roscope data. The complementary filter first estimates the purposes of orientation and position definition, the parafoil roll ( ) and pitch ( ) angles based on accelerometer data is assumed rigid with six degrees of freedom. Three body- (xacc,yacc,zacc): fixed axes (Xp,Yp,Zp) are defined with the origin on the = yacc estimated aerodynamic center (midspan, quarter chord) of acc atan2 , (9) zacc the parafoil. As illustrated in Figure 9, the x axis is positive forward through the leading edge, the y axis is positive moving out the starboard wing, and the z axis is on the plane = xacc acc atan2 . (10) 2 + 2 of symmetry of the wing, downward positive. Roll, pitch, yacc zacc

Figure 9. Coordinate system definitions.

Journal of Field Robotics DOI 10.1002/rob 10 • Journal of Field Robotics—2016

The gyroscope data are used to provide an estimate of 2.5.4. Altitude Estimation the roll and pitch angular rates using the previous iteration To determine altitude from barometric pressure, two pres- estimate of roll and pitch and the current iteration gyroscope sure measurements are required, one at the altitude to be data: measured and a second reference pressure that can be sea level (QNH) or at a field elevation (QFE). During short du- ˙ est 1sin esttan est cos esttan est = ration flights, the pressure can be measured by the wing- ˙ est 0cos est sin est ⎛ ⎞ mounted barometer while it is still on the ground. This xgyro measurement can be stored as the reference pressure (QFE) ⎝ ⎠ × ygyro , (11) and used to perform altitude calculations. However, if the zgyro atmospheric conditions at ground level change, the altitude determined using an invalid stored reference would be in- where the estimated roll and pitch angles are given by accurate. Traditionally in aviation, this problem is overcome by verbal and information system radio signals, communi- − + ˙ est (k) = ( est (k 1) estdt acc − + cating the updated reference pressure for the field. Locating est (k) ( est (k 1) ˙ estdt acc a barometer at the winching station provides measurements a of the local atmospheric pressure at the field elevation (QFE) × . (12) 1 − a at a high rate, and this can be transmitted via the commu- nications network to enable live updates of the reference pressure. Thus, the altitude can be accurately measured In Eq. (12), a tunes the complementary filter with a filter even during changes in local atmospheric conditions. Al- period, t : f titude (h) is determined from two static pressures according

tf to the following equation (Kayton & Fried, 1997): a = . (13) tf + dt = p0 T p0 h log10 , (16) ρ0g log10 (e) T0 p where T ,p,ρ are the reference temperature, pressure, 2.5.3. Heading Estimation 0 0 0 and density at sea level, respectively. Using the estimated roll and pitch angles, the magnetic The measurement of altitude above a reference point heading ( Mest) can be estimated using the three-axis mag- is calculated as a function of the ratio of the reference pres- netometer data (xmag,ymag,zmag) by Lai et al. (2010): sure to the observed pressure at altitude. Aircraft altimeters require the reference pressure to be manually inputted; dif- M = est ferent reference pressures are used based on the current z sin − x cos flight segment and the local aviation regulations. For the mag est mag est . atan2 + + AWE application, the altitude most useful for control sys- xmagcos est sin est ymagsin est zmagcos est tems is the altitude above ground level at the tether anchor (14) point.

This provides a heading from a magnetic north estimate 2.5.5. Wind Profile Estimation in the range −180° to +180°, which is then wrapped between A log law estimating the wind velocity at the wing altitude 0° and 360°. The magnetometer derived heading is naturally is implemented. The law outlined by Archer (2013) provides relative to magnetic north while GNSS system bearings are estimates of wind speed at a given altitude based on a ref- referenced to true (geographic) north (Veremeenko, 2013). erence wind speed (Vw,href) measured at a reference height True and magnetic north are offset by the magnetic decli- (href): nation (alternatively, variation) (δmag), with the conversion h between true ( T ) and magnetic headings ( M )definedby log = z0 = − Vw,h Vw,href , (17) T M δmag. (15) h log ref z The declination angle is defined as positive if the mag- 0 netic north is east of the true north. Magnetic declination where Vw,h is the wind at altitude h, Vw,href is a wind speed varies with location and time with the local value and an- measurement at reference altitude href,andz0 is the surface nual change available in print on maps and navigational roughness length, which accounts for the impact of local charts or from numerical models such as the World Mag- terrain features and obstacles. In Archer (2013), open farm- netic Model (Maus et al., 2010). land with windbreaks more than 1 km apart has a roughness

Journal of Field Robotics DOI 10.1002/rob Coleman, Ahmad, and Toal: Development & Testing of a Control System for the Automatic Flight • 11 length of 0.1 m, while farmland with many windbreaks has a value of 0.4 m. An intermediate value of 0.2 m is imple- mented, as sites with 1 km between windbreaks are not common in the testing region.

2.5.6. Downwind and Crosswind Position Estimation The ground station is set up facing into the wind with the soil anchor forming the origin of a wind axis. The down- wind and crosswind position of the kite are calculated rel- ative to the position of the tether anchor. The position of Figure 10. Kite to wind angle and crosswind and downwind the tether anchor is determined by GPS and stored in the position definitions. control system as a reference value. In general terms, the distance between two sets of coordinates (lat1, lon1)and (lat2, lon2) can be determined using the haversine method (Sinnott, 1984). The first step of this method requires that the difference between the longitudes and the latitudes is calculated, and then intermediate values a and c are cal- culated. Finally, from the intermediate values the distance between the points, d, is calculated. For use in the following formulaz, the longitudes and latitudes must be converted to radians. = − δlat lat2 lat1, (18) = − δlon lon2 lon1. (19) In the tethered wing case, the anchor reference position = is inserted into lat2 and lon2, such that lat2 latref and Figure 11. Tethered wing azimuth φ and elevation angles θ in lon = lon , the quarter sphere wind window. 2 ref δlat δlon a = sin2 + cos (lat ) cos (lat ) sin2 , (20) 1 2 = 2 2 ddown d cos (γKW ) . (25) √ a c = 2 atan2 √ , (21) 1 − a 2.5.7. Azimuth and Elevation Estimate from Cartesian Position Estimates d = Rc, (22) Using the altitude estimate (h) and the crosswind and where R equals 6,371 km, the mean radius of Earth for downwind distances, azimuth (φ) and elevation (θ) angles the WGS84 ellipsoid. The bearing between the points between the wing and the anchor point, as defined in (lat1, lon1)and(lat2, lon2) is given by Figure 11, are estimated as follows: BT = h φ = atan2 , (26) sin (δlon) cos (lat ) ddown 2 , atan2 − cos (lat1) sin (lat2) sin (lat1) cos (lat2) cos (δlon) = dcross (23) θ atan2 . (27) ddown where BT is the bearing from true north to the kite about the anchor point in a range of −180° to +180°. As illustrated in The wind window Figure 10, the angle between the bearing (B ) and the wind T Human kite pilots naturally reference the kite position in a direction (B ) is defined as the kite to wind angle (γ ) W KW qualitative spherical coordinate system known as “the wind and can be used to calculate the crosswind and downwind window” shown in Figure 11. The wind window 12 o’clock distances of the kite from the anchor position. The crosswind position is aligned with the wind direction, with the 9 and and downwind distances to the wing from the anchor in a 3 o’clock positions forming the left and right constraints of Cartesian frame are given by the wing to wind angle and where a wind-powered tethered wing can be flown. Low el- the distance d calculated in Eq. (22), evation and azimuth angles maximize the crosswind flight = dcross d sin (γKW ) , (24) power of the wing (Loyd, 1980), resulting in a high tether

Journal of Field Robotics DOI 10.1002/rob 12 • Journal of Field Robotics—2016

2.5.9. Data Stream and Logging Parameters Wing centreline Data logging is an important part of the system. Post-test data processing can be performed to evaluate the system performance and the impact of iterative design changes. The data logging function is performed by the ground-based control PC. An application on this PC receives data from the control pod and the ground winch and saves this to file Left heading target Right heading target at 15 ms intervals. The data are transferred using buffered network streams, ensuring lossless transfer. Table V out- lines the parameters recorded by the data logging program. The data are stored in a human readable (.txt) format, using approximately 36 kB per second of operation. This is nonop- timal from a data storage efficiency perspective; however, it is a convenient human readable format for a quick overview Wind direction and plotting of results.

Figure 12. Plan view of symmetric point-to-point heading 3. FIELD SETUP AND TESTS controller. The control system detailed herein was deployed in numer- ous field trials in southwest Ireland during 2013 and 2014. tension and mechanical power, yielding a region of the wind The field trials were carried out in a range of wind con- window known as the “power zone.” At larger angles of el- ditions at several different sites. To facilitate rapid deploy- evation and wind speed, dynamic crosswind maneuvers are ment with minimal logistical requirements, the system has not possible and so the wing airspeed and hence tether ten- been designed to operate without the electrical winch when sion is lower. The zenith position over the anchor point is a necessary. No modifications are required to facilitate the useful positon in which to “park” the wing, facing into wind changeover between winched operations and static tether with little or no groundspeed. This is also a useful positon operations. The typical setup for flight-testing without the in which to initialize and handover to automatic controllers power takeoff winch is displayed in Figure 13. Such an in a relatively steady condition compared to other positions arrangement is suitable for much of the initial controller during a dynamic maneuver. testing and system analysis, and for what is essentially a special case of winched operations where the tether veloc- ity is constrained to zero and operation is with short tether lengths (20–35 m). A shorter tether length also represent a 2.5.8. Point-to-point Heading Controller more challenging flight control problem, as with a shorter A simple point-to-point heading controller has been de- tether, the period of maneuvers about the wind decreases, signed with the objective of flying an oscillating pattern requiring more frequent changes from the control system. about the wind vector with a fixed magnitude oscillating The setup during field tests consists of the following steering input. Figure 12 outlines the operation of this con- steps: troller. The controller is initialized with the wing steady and r level on the “into-wind” heading. When the controller is ac- Wind speed and direction are measured and recorded. tivated, a constant steering input is applied until the first Wind direction determines the layout of the test setup, “out-of-wind” heading is reached. Upon reaching the first r such that the anchor aligns with the wind direction. heading target, the negated constant steering input is ap- A secure soil anchor is established that removes the ver- plied until the negated out-of-wind angle is reached. Thus, tical force components from the tether. Anchor position the wing flies in an oscillating pattern about the wind head- r is recorded in the control system. ing, bounded by the symmetric out-of-wind heading angles. A winching anchor is established that counters the ten- This controller uses a single output from the orientation esti- sion in the tether without slippage or lifting of the anchor mation algorithm: the magnetic heading. Three data inputs station. This is typically several meters upwind of the soil are required prior to activation: the wind direction, the de- r anchor position. sired out-of-wind angle, and the steering input magnitude. A tether is deployed with appropriate routing through In the example, flight data presented that the wind was fair leads; secure connection of tether to control pod ten- from the south (183°) with the out-of-wind angle set point r sion connection. at 50°. A steering input of nondimensional magnitude 0.8 r Kite and kite control lines are deployed. was used. Power and test control system.

Journal of Field Robotics DOI 10.1002/rob Coleman, Ahmad, and Toal: Development & Testing of a Control System for the Automatic Flight • 13

Table V. Parameters recorded by the data logging program every 15 ms (66.66 Hz).

1. System time 2. X acceleration (m/s2) 3. Y acceleration (m/s2) 4. Z acceleration (m/s2) 5. X gyroscope (deg/s) 6. Y gyroscope (deg/s) 7. Z gyroscope (deg/s) 8. X magnetometer (mG) 9. Y magnetometer (mG) 10. Z magnetometer (mG) 11. Barometer (mbar) 12. Altitude (estimate) (m) 13. Vertical speed (estimate) 14. Air temperature (K) 15. Latitude (GPS) (deg) 16. Longitude (GPS) (deg) (m/s) 17. Ground speed (GPS) 18. Course (GPS) (deg) 19. Altitude (GPS) (m) 20. GPS time (m/s) 21. Motor 1 set point (deg) 22. Motor 1 position (deg) 23. Motor 2 set point 24. Motor 2 position (deg) 25. Lateral input (δa) 26. Longitudinal input (δe) 27. Roll (estimate) (deg) 28. Pitch (estimate) (deg) 29. Heading (estimate) 30. Controller active (enum) 31. Tether speed (m/s) 32. Tether length (m) (deg) 33. Tether tension (kN) 34. Motor 1 speed (deg/s) 35. Motor 2 speed (deg/s) 36. Motor 1 current (mA) 37. Motor 2 current (mA) 38. Crosswind distance 39. Downwind distance 40. Distance over ground to kite (estimate) (m) (estimate) (m) (m) 41. Bearing to kite (deg) 42. Tether drum position 43. Tether drum speed (RPM) 44. Recovery motor current (A) (rev) 45. Recovery motor power 46. Generator current (A) 47. Generator voltage (V) 48. Generator power (kW) (kW) 49. Barometer on ground 50. Winch clutch state 51. Winch brake state 52. Load relay 1 state (mbar) 53. Load relay 2 state 54. Load relay 3 state 55. Tether azimuth (estimate) 56. Tether elevation (estimate) (deg) (deg)

As the testing system was designed to be mobile but the tether tension. The anchoring arrangement resolves the not necessarily vehicle-mounted, as is the case with many tether forces into the vertical and horizontal components AWE prototypes, a two-part anchoring arrangement is de- and anchors these separately. This arrangement alleviates ployed that eliminates the vertical force component from the requirement of the winch to counter the vertical force

Figure 13. Flight-testing setup in the field with kite in zenith position.

Journal of Field Robotics DOI 10.1002/rob 14 • Journal of Field Robotics—2016

during non-power-generating periods or for the recovery of the system. This position hold capability is also useful for al- ternative or complementary applications of tethered wings, such as commutations relays or sensor platform. Such ap- plications are not likely to be feasible if the system is rapidly maneuvering, as is required for power production.

Figure 14. Portable two-part anchoring solution for field setup. 3.1.2. Dynamic Maneuvers The maximum tether tension is produced when the kite flies in a crosswind motion, which increases the airspeed over the component by passing the tether through a pulley attached foil, thus increasing lift. The tension profile produced by the to the vertical soil anchor, as shown in Figure 14. This re- wing depends largely upon the trajectory followed during solves the tether tension into a horizontal vector, requiring the dynamic maneuvers. The trajectory is centered about the the winch anchor to counter this horizontal force only,rather wind vector and typically traces either a figure-of-8 orbit or than a large lifting component. An opposing pair of meter- a circular orbit. If the tether is unwound, such as during long steel spikes are driven into the ground, as illustrated. power production, these trajectories result in helical flight These spikes form the vertical anchor to which the tether paths. Figure 15 displays an example of a crosswind flight pulley block is attached. This pulley constrains the tether maneuver where the wing is flown perpendicular to the tension into the horizontal axis, which is anchored by addi- wind direction, which results in a large airspeed and tether tional steel pegs passing through the winch baseplate. tension, which is required for power takeoff applications. 3.1. Flight Maneuvers 4. RESULTS AND PERFORMANCE 3.1.1. Zenith Position Hold 4.1. Manual Control It is possible to hold the wing steady facing into the wind, with as little departure from the zenith position as possible. The manual flight of the kite using the human operated This position is a low-lift configuration compared to when joystick established the performance of the system and the kite is undergoing dynamic maneuvers. This position enabled a shakedown of the system over several test days, hold is useful as a method of “parking” the kite overhead where issues such as the best rigging arrangements of

Figure 15. Image of crosswind flight maneuver.

Journal of Field Robotics DOI 10.1002/rob Coleman, Ahmad, and Toal: Development & Testing of a Control System for the Automatic Flight • 15

Figure 16. Manual control of 12 m2 kite in field tests. the steering and power line attachment to the control bearing, kite, and kite sensors were found to perform pod were determined. Through iterative adjustments reliably. and flight-testing, the required lengths of the control-pod The following figures illustrate a subset of the data from power-line attachment line and the length of the steering one of these short trial flights. In Figure 17, the servo actual lines were found. Figure 16 provides video capture from position is seen to closely track the set point except during two of the many such test days. large actuation loads exceeding the torque capability of the As the steering lines also provide the longitudinal input servomotor. This condition occurs during tight turns when that controls the kite geometry and hence the lift coefficient, the foil is flying at a large airspeed. Large airspeeds require the steering lines must be sufficiently long such that at the increased actuator force to displace the control surface of the zero deflection position they are just slack, in a “depower” kite due to the increased aerodynamic forces on the control condition. In this condition, the kite is at minimum lift (pos- surface. The servo position errors can be seen where the cur- sibly zero lift, depending on kite design). The servomotor rent loop of the servo position controller reaches the max- maximum deflection range was set such that with a full lon- imum allowable current (10A) and saturates at that level. gitudinal input, the kite was brought to a stall. The required Recalling that the servomotor is a brushed dc machine, the displacement for this was found to be 2,750 deg or 1,056 mm current is directly proportional to the torque output [see for the 12 m2 kite employed. It was thought initially that the Eq. (7)]. This results in a steady-state actuator position error pulley passive winding might not function well at full servo until the actuator load reduces. The maximum allowable speeds, particularly during reel out where the steering line current must be traded off against the requirements to meet may not have sufficient tension to clear the released line the actuator load with the current limitations of the mo- from the pulley spool and fairlead. The initial field tests tor windings. Sustained currents above the rated maximum found that the aerodynamic drag acting on the steering line will overheat the motor with possible winding burnout oc- was indeed sufficient to increase the servo speed to the max- curring. imum possible speed (i.e., motor-rated speed). In addition to the line displacement range and max- 4.2. Point-to-point Heading Controller imum speed, the behavior of the servomotors is modi- fied by tuning the PID gains of the motor position and The sequence begins with the wing under manual control, current control loops. These were set in the laboratory as can be seen by the initial varying steering input with the initially and adjusted in the field to provide a behavior wing heading approximately southerly.When the controller similar to a human pilot, providing rapid set-point track- is activated, the initial constant steering input (−0.8) is ap- ing for large position errors without being overly stiff plied until the southwesterly (233° = 183°+50°) heading tar- during small position errors. Bracing lines, which balance get is crossed. The opposite steering input (+0.8) is applied the orientation of the control pod, were also adjusted to by the controller until the southeasterly (133° = 183°−50°) minimize control-pod twisting about the tether axis. Fol- heading target is crossed. As the heading controller requires lowing these tests and adjustments, the radio communi- the target heading to be crossed, an overshoot occurs; it can cation, data logging program, servo performance, anchor- be seen in Figure 18 that the overshoot is approximately ing and rigging arrangement, control-pod mechanical load 20°–30° with a maximum of 70°. The wing position during

Journal of Field Robotics DOI 10.1002/rob 16 • Journal of Field Robotics—2016

Figure 17. (a) Left actuator positioning and loading during manual maneuvers. (b) Right actuator positioning and loading during manual maneuvers.

Figure 18. Heading and roll response to steering inputs under the point-to-point controller. this test in Figure 19 is shown to remain centered on the surement at ground level is indicative of the wind condition wind vector and at a high elevation angle close to the zenith at the wing. Figure 20 shows the correlation between wing position. groundspeed and tether tension. The optimization of tether tension and hence mechanical power production, through the control of flight path trajectories and hence airspeed, is 4.3. Dynamic Maneuver Data: Speed and Tension a key step to facilitate airborne wind energy as a practical During the dynamic maneuvering of the wing, the tension power-generation method. produced by the wing varies according to the airspeed. While a direct measurement of airspeed has not been im- 4.4. Steering Actuator Performance plemented, the GPS ground speed of the wing, in addition to the measured wind speed, can provide an approxima- The performance of the steering motors during the con- tion of the airspeed, especially when using short tether troller test flight is shown in Figure 21. The alternating se- lengths and hence lower altitudes, where the wind mea- quence of steering inputs can be seen as the square wave of

Journal of Field Robotics DOI 10.1002/rob Coleman, Ahmad, and Toal: Development & Testing of a Control System for the Automatic Flight • 17

Figure 19. (a) 3D kite position in wind axes during test sequence. (b) (i) Orthogonal views of kite position in wind axes [i.e., from (a)]; (ii) orthogonal views of kite position in wind axes; (iii) orthogonal views of kite position in wind axes.

position set points, dimensionalized here from the nondi- force estimate when the position set point has been reached. mensional input δa to meters of steering line displacement. In this condition, the motor is at low or zero speed and the The directly measured current drawn by each motor is over- current is proportional to the torque required to counter the laid. Both left and right motors experienced approximately steering line force and pulley torque (recalling that the steer- equal loads during this sequence, which is to be expected ing motors are brushed dc machines). The force estimate is from a roughly symmetrical flight about a steady wind vec- expressed in kilograms equivalent (kgf), as this allows for tor. From the current measurements, the torque as seen by an intuitive comparison of the line loads for human kite the steering line pulley can be estimated. pilots. For example, at time 783 s, motor 2 is seen to have a The steering line forces are not directly measured, but steady state current of 1.4 A at the position set point, which an equivalent force to that of the pulley radius is estimated, equates to a pulley torque of 0.6 N m and a steering line however it should be noted that this estimate is only valid force of 3 kgf. when the motor is in a steady state, when the current drawn is only that needed to counter the pulley torque. The large force spikes seen are caused by inrush current during the 5. CONCLUSION step change in input to the position controller, and they are The robotic system developed to facilitate research in the au- not steering line forces; however, it is useful to observe the tomated flight of a tethered parafoil has been outlined with

Journal of Field Robotics DOI 10.1002/rob 18 • Journal of Field Robotics—2016

Tension and Ground Speed 1 20 Tension Speed

0.5 10 Tension (kN) GPS Ground Speed (m/s)

0 0 295 300 305 310 315 320 325 330 335 340 345 Time (s) Figure 20. Tether tension (kN) and kite ground speed (m/s).

a focus on suitability to the field of airborne wind energy. weight ratio is a most important consideration in design- Further, potentially complementary applications of the ing kite robotic systems. While the installed motors have system are possible, such as the addition of airborne sensors been capable of flying the 12 m2 kite, there have been brief and communications systems, which leverage the low-cost motor current saturations beyond the design rating of the access to altitude afforded by wind power aerial platforms. motors. Larger kites may require a motor upgrade, includ- The prototype development and early testing has been ing the weight and battery endurance penalty that would detailed, including an in-depth description of the key sub- result. systems, components, and the implemented estimation and A simple point-to-point heading controller maintained control methods. The system has been tested under a vari- the wing in a stable oscillating flight about the wind vector, ety of wind conditions, and such tests are continuing with emulating the action of a human pilot flying the kite in a gen- future work focused on reducing the human operator in- tle pattern about the wind, near the zenith position. The test volvement by implementing automatic control algorithms, flight data collected will enable the identification of various and a greater focus on the power-producing operation of the system properties, such as roll and heading responses to the system. The two- and three-dimensional plot displays give steering input, which will be used in control design tech- an overview of the flight trajectory of the kite. The develop- niques and controller development, and they will remain ment of an augmented reality tool with three-dimensional the focus of further work. Data presented, such as actuator position and orientation overlay is another focus of future position ranges, current and torque loadings experienced work. This provides a useful operator display with the full by the motor, kite speed and tension profiles, and the wing position and orientation data incorporated, and it provides turn rate response, offer valuable insight into the require- a visual method for the replay of logged data for postflight ments of a kite control system, and they will be beneficial to analysis. The flight-test results verify the performance and future kite-control system designs. suitability of the system parameters and components, such as the joystick to servo control mapping, actuation load and handling, kite lines to control pod interaction and rigging, ACKNOWLEDGMENTS wireless communications range and interference, battery endurance under real operating conditions, and actuator This research is funded by the Irish Research Council Post- loads. graduate Enterprise Partnership Scheme [RS/2010/2632] Challenges identified during the testing program have with Bord Gais´ as an enterprise partner and the Irish been discussed, including the solutions resolving these is- Research Council Postdoctoral Enterprise Partnership sues, which may be beneficial during future kite robot de- [EPSPD/2011/56] with Analog Devices as an enterprise sign and test cycles. One key challenge identified during the partner. The experimental and testing work has also been testing was the actuator forces required to fly large ram-air supported in part by MMRRC, UL support and by Science kites. A tradeoff exists between the torque capabilities of Foundation Ireland MaREI (Marine Renewable Energy Ire- motors and their weight. Thus, the servomotor torque-to- land Research Centre) funding [12/RC/2302].

Journal of Field Robotics DOI 10.1002/rob Coleman, Ahmad, and Toal: Development & Testing of a Control System for the Automatic Flight • 19

Figure 21. (a) Left steering actuator performance data during heading controller flight. (b) Right steering actuator performance data during heading controller flight.

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Journal of Field Robotics DOI 10.1002/rob