effects of tether on rotor uas flight

Effects of Tether on Rotor UAS Flight

Mathew Walker and David Miller

Abstract In the course of development and deployment of unmanned aerial vehi-cles, there are occasions where it may be necessary to tether the vehicle duringflight. Tethers can help prevent loss of vehicle when using experimental systemsand can also be used to meet current FAA regulations. Outdoor free-flights of UASby research organizations (such as universities) within the US airspace are illegalwithout an FAA Certificate of Authorization (COA) which are difficulty and timeconsuming to acquire and severely limit operations, even when granted. By teth-ering the craft such that it cannot exceed 400 feet (121.9m) above ground level,flights of unmanned vehicles are legal. This paper describes experiments flying atethered hex-copter, SAMRAMP, to better understand the logistics, limitations, andperformance affects of tethered flight.

1 Why Tethers

There are many reasons to use tethers with unmanned aerial vehicles (UAV). Oneof the most common uses in the past has been as safety devices when testing newcontrol systems or experimental UAV designs. In [6], an experimental UAV wassuspended by a tether from a crane when doing autonomous hover tests with a newcontrol system. A small counterweight was used to maintain tension on the tether asthe craft lifted off an proceeded with its hover maneuver. Without the counterweightsystem, there was danger that the tether could become entangled in the rotors, lead-ing to the loss of the vehicle.

Matthew WalkerThe University of Oklahoma, e-mail: [email protected]

Dr. David MillerThe University of Oklahoma e-mail: [email protected]

1

2 Mathew Walker and David Miller

Another reason to tether a UAV is to supply power and have a wired link for com-munication. In [7] a UAV used to assist in environmental monitoring and cleanupis modeled in detail. The UAV is attached through an active winch mounted on asurface ship. The purpose of the tether is both to physically constrain the movementof the UAV and for the purpose of power transmission – so that flight times can beextended indefinitely.

Both of the above papers reference the many difficulties encountered with usingtethers on aerial vehicles. Pratt enumerates these in a paper that describes the use ofa tethered UAV for inspection of a building collapse [8]. That work notes that thereare several difficulties with using tethers, including:

1. entanglement and movement constraints during takeoff;2. limiting rate of descent and horizontal maneuvers in order to allow adequate time

for the tether manager to adjust the tension or take-up the slack.

They also noted that tethered flight required additional crew for tether managementand that coordination between the pilot and tether manager was critical for vehiclesafety.

Given the issues and complications caused by the tether for this application, onemay wonder why a tether was used. The answer given in the paper, and familiar withthose working on UAVs in the USA, is that the FAA severely restricts free flight ofUAVs inside the US national airspace.

The Federal Aviation Agency (FAA) regulates all activities inside the US nationalairspace. Their policies with respect to the operation of unmanned aerial vehicles iscontained primarily within their Memorandum AFS-400 [4]. The essence of thispolicy is that while hobbyists are allowed to fly almost anything at altitudes below400 feet above ground level, commercial and public vehicles are severely limited.Public vehicles, which include anything being operated by a state or federal agency,or any organization supported in part by state or federal agencies (and thus virtuallyall US universities and colleges) are not allowed to free-fly any powered vehicle atany altitude without first acquiring a waiver or certificate of authorization (COA)from the FAA.

Separate COAs are required for each vehicle at each flight location. COAs re-quire pilots and observers to have had formal training and have current flight med-ical certificates. The application and approval process for obtaining a COA can betime consuming (60 business days, not including question and response times). Therequirements for regulation of free flight UAVs within the US are likely to increase,as more incidents such as the hexcopter crash into a crowded grandstand [12], occur.

However, tethering a UAV is a legal way to fly within 400 feet (121.9m) AGLwithout additional authorization from the FAA. The tether used in the system de-scribed in this paper is designed to eliminate, or at least reduce, the many problemsassociated with previous tether designs.

Effects of Tether on Rotor UAS Flight 3

2 Motivation: 3D Mesonet

Located near the center of the United States, Oklahoma is home to servere weatherand has suffered some of the most devestating tornados on record. This is amongthe many reasons why it is home to the National Weather Center and the OklahomaMesonet. A Mesonet is a network of automated environment measuring stations thatare spaced closely enough to monitor environment conditions on the mesoscale (typ-ically <25 miles spacing). The Oklahoma Mesonet consists of 120 stations locatedaround Oklahoma with at least one in each county. Each Mesonet tower measuresenvironmental data such as rainfall, temperature, humidity, windspeed, and soil tem-perature and uploads this to the internet every five minutes.

Fig. 1 Oklahoma Mesonet Environmental Tower Locations. As of writing, there are 120 locationswith at least one in each of the 77 counties.

While the Oklahoma Mesonet is one of the largest in the country, it and all otherMesonets are limited in that they are only two dimensional. The Mesonet could bemade considerably more effective if it were possible to take atmospheric data such astemperature, pressure, and humidity readings at multiple heights in the atmosphereabove each tower. This data could help separate the thunderstorms that may spawntornados from those that will only deliver wind and rain. This data will also be usedto increase the resolution and quality of future weather forecasts.

Releasing weather balloons every hour is one option that has been considered.Weather balloons are currently released from approximately 100 NOAA (NationalOceanic and Atmospheric Administration) locations around the United States twicea day. However, with the number of stations in the Oklahoma Mesonet and the de-sired frequency of data, it would be prohibitively expensive since each balloon anddata acquisition payload costs over $250 and is usually not recoverable and there-fore can only be used once. In addition, since weather balloons are not powered,


they are at the mercy of the wind and cannot guarantee properly spaced data. Theyalso rise through the lower atmosphere quickly, which limits the amount and typeof data that can be acquired.

Planes, whether autonomous or piloted, are another option that have successfullybeen used in the past to gather atmospheric data [1],[9]. However, these have thedistinct disadvantage of only being able to fly above 500 feet because of FAA (Fed-eral Aviation Administration) regulations. Even without the FAA regulations, onewould not want to fly them much lower due to safety concerns. Some of the mostinteresting and important data, especially regarding boundary layer phenomena, oc-curs closer to the earth, which reduces the usefulness of planes. [5]

3 SAMRAMP

SAMRAMP started with a stock Mikrokopter Hexakopter XL with gps. Becausethe Hexakopter is already a very capable vehicle, very little modification had to bedone to adapt it. Stronger legs were created, the battery compartment was extendedto allow for multiple batteries to be used in parallel, and extensions were added toeach arm to ensure the safety of both the propellers and any nearby objects/people.

The next step involved creating platforms at multiple testing heights above theHexakopter for the microprocessor and atmospheric sensors. The platform must bestrong enough to survive a small crash yet light enough that it does not negativelyimpact the performance of the craft. Light weight aluminum 6061-T6 was used forthe main supports and was connected on each end with 3D printed brackets made ofABS plastic.

The processor of the sensing package (ATMEGA 2560) communicates with theHexakopter over a serial connection with a baud rate of 57600. The Hexakopter pro-vides information from its accelerometers, gyroscopes, GPS, barometer, and motorcontrollers. The processor also receives data from a SHT15 temperature and humid-ity sensor on the platform. (Temperature: 0.01C resolution, 0.5C accuracy, RelativeHumidity: 0.05% resolution, 2% accuracy)[10]. This data is then stored locally on amicro SD card and transmitted back to the ground station via a 900MHz Xbee Promodule[2]. Custom software running on the ground station computer displays thisdata in an easy to read format and saves it locally in case there is an accident andthe sd card is lost. The ground station can also input GPS target coordinates whichare transferred to the processor and then onto the Hexakopter. Later, multiple windsensors will be added to measure vertical and horizontal wind speeds.

The final piece of the sensor package is a tension sensor. One end of a string isattached to a spring which is attached to a mount that will be clipped to the bottom ofthe hexakopter while the other is attached to a tether on the ground. Between them,a loop of the string is wound around a linear slide resistor such that when there istension on the line the slide moves and the resistance of the sensor is changed. Thecurrent sensor has a range of 0 - 3.0 pounds force with an accuracy of 10%.


Fig. 2 SAMRAMP with lowest sensor platform, sensor package, and GoPro Camera.

Fig. 3 Tension sensor. 0 - 3 lb force range with an accuracy of 10%.

4 Tether Construction

Because of the FAA regulations, a tether was created so that the aircraft would al-ways be anchored to the ground. The original design used clear fishing line with a50 lb breaking strength and 4.5 gram weights clamped every ten feet. To avoid prob-lems discussed in Pratt [8] and insure that the light line did not blow into the rotors,additional weights were clamped every foot for the ten feet closest to SAMRAMP.On the ground the tether was attached to a fifteen pound steel weight to ensure thatthey craft couldn’t fly away.

This initial design accomplished the goal of ensuring that the craft could not flyaway and that the flight was legal with the FAA but it had several problems. The


first problem was that the clear color made it almost impossible to see the line eitherfrom the ground or from SAMRAMP. This was a problem both for safety issuesand because it made it harder to monitor line tension. Another problem was that theline tangled very easily and required copious amounts of time spent untying knots.Finally, although the line was rated for 50 pounds, it started stretching at muchlower weights, especially in the hot afternoons. Since part of the reason for havinga tether is to ensure it stays below a given height, this stretching was a significantissue. Additionally, once a portion of the line was stretched, it would become muchweaker and increase the possibility of breaking.

Version two of the line switched to a bright orange twisted nylon with a safeload limit of 113 lb. Every twenty feet, the line was cut and a spinning linkagewas inserted so that no amount of spinning would twist the line. Two weights wereclamped around these linkages and an additional weight was clamped halfway be-tween each section. As in the first version, weights were clamped every foot for theten feet closest to SAMRAMP.

Fig. 4 A comparison betweenthe first and second version ofthe tether. Version 1 is clearfishing line with 50lb limit.Version 2 is orange twistednylon with a 113lb limit.

5 Testing

In order to test the effects of the tether on flight, a flightpath was created that wouldproduce several different levels of tension in the tether. We began by flying straightup 26 meters then up to 53 meters. Next we flew south at this height for 26 metersthen descended to 26 meters and continued south another 27 meters. We then flewsuch that we were directly west of the starting location and repeated the pattern inreverse. At each point, SAMRAMP paused for thirty seconds to allow the craft andtether to stabilize and ensure enough data was collected to reduce error from wind.In addition, at several points along the path, SAMRAMP flew either up or out untilthe tether had the maximum readable tension. Later, oscillation tests were performedabove the starting point at 10 meters and 50 meters.


Fig. 5 Flightpath for SAMRAMP tether test.

6 Analysis of Tethered Flight

6.1 Tension

One of the more important indications of a tether’s effect on a UAV can be seen inthe tension in that tether. A higher tension increases the downward and side forceon the UAV, which increases the power needed for the vehicle to hover and move aswell as limits the vehicle’s range of movement.

The tension on a tether used to constrain the flight of an aerial vehicle bearssome similarity to the effect that tension has on the shape of electrical power cablesor chains. In typical cable problems, effective straight line distance between theattachment points is a function of the cable’s physical length, weight per unit length,the tension, and the angle of the cable at the attachment points (the more horizontal,the larger the difference between cable and straight line distance for a given tension– due to the weight of the cable contributing to both the vertical and horizontal


components of tension. The values are typically solved using the caternary curveequation, or a numerical approximation.[3]

However, for a UAS, finding the shape of the tether using these methods is notpossible because of the many unknowns of the tether. Tension for an aerial vehicleis a function, not only of the cable weight and the angles at the attachment points,but also of wind and the previous motions. If the vehicle is sufficiently low andclose the anchor point, then a portion of the tether is lying on the ground, whichmakes it unpredictable for the purpose of these calculations. The amount that is onthe ground is affected by where the cable was previously – the amount of tether inthe air may just be the vertical distance, or the it could be off the ground at a highangle starting at the anchor point, or even beyond the anchor point due where it hasbeen laid down (see Figure 6).

Fig. 6 The tether (highlightedby hand with a yellow line)is completely off the groundand is displaced both by thewind and the movements ofthe vehicle.

Figure 7 is a plot over the duration of the test flight showing tension and straight-line distance to the anchor point. This straight line distance was found using thegreat circle distance and the haversine function [11] shown in equation 1, where Ris the radius of the Earth in meters and Latitude and Longitude are both given inradians. This was used rather than other methods such as the law of cosines becauseit is better at the smaller distances used when tethered.

d = 2Rsin−1

(√sin2

(∆Lat

2

)+ cos(Lat1)cos(Lat2)sin2

(∆Long

2

))(1)

The tension remains almost negligible at a force below 0.5 lb (2.22 N) except forcases where the end of the tether is approached and then increases rapidly. In figure7 each spike of the tension meter lines up nicely with an increase in distance fromthe anchor point. The important lesson to take from this data is that until you reachthe end of the tether, the tether has a negligible effect on flight.


Fig. 7 A time series of tension and straight line distance. Note that tension jumps significantlywhenever distance approaches the actual tether length.

6.2 Power Usage

Another concern of using a tether is reduced flying time because of the increasedpower of carrying the tether. Figure 8 shows both power and altitude for the firstpart of the test flight as SAMRAMP ascends to 26m, 53m, and finally pulls againstthe tether before backing off. There are four areas of note in this figure. Area 1shows a sharp increase in power consumption as SAMRAMP takes off. As shouldbe expected, this is the highest power usage during the flight. Area 2 shows SAM-RAMP settling at 26m and Area 3 shows SAMRAMP settling at 53m. Area 4 showsSAMRAMP pulling against the tether.

It is important to note that power usage during both area two and three are thesame despite the fact that area three is 27 meters higher and therefore has the lengthof tether to support. This demonstrates that the small weight of the tether is notenough to affect power usage during flight as long as the last couple of meters oftether are avoided. Battery life and flight time will therefore be unaffected by theusage of a tether within this range.

6.3 Oscillation

The final concern one might have is the tether’s effect on movement, particularlythat a swinging weight might cause increased oscillations. Small oscillations willoccur when the UAV is stopping abruptly in either the horizontal or vertical direc-tion. By comparing how these naturally occurring oscillations are changed by theof additional tether weight, we can draw conclusions about the overall effect of thetether on producing unwanted oscillations. To test this, SAMRAMP performed os-


Fig. 8 Power usage and altitude over the first section of the test flight showing takeoff (Area 1),hovering at 26m (Area 2), hovering at 53m (Area 3), and pulling against the tether (Area 4)

cillation tests at both 10m and 50m. Each time, it started several dozen meters to theleft of the starting location and flew to the right as fast as possible. When directlyover the center point, it attempts to stop which causes oscillation. Figure 9 showsthe angle of SAMRAMP as it settles after this abrupt stop. The oscillations at 50mare larger; however, they settle in the same amount of time. This is consistent withseveral oscillation tests performed in the past several months. This leads to the con-clusion that while the presence of the tether may have a small impact on mobility, itdoes not significantly reduce performance.

7 Conclusions and Future Work

A simple tether system has been designed to allow testing of a hexcopter for gath-ering meteorological data. The tether allows full functionality of the aerial system.By using a light line with integrated swivels and distributed weights, the tether hasminimal effects on the aircraft performance within well over 90% of the volumeof the workspace defined by the tether. Furthermore, the weighted line keeps thetether from floating and clear of the rotors, even during severe and sudden changesin course and attitude.

The tether system has also had serendipitous effects. During one tethered flight,a piece of the instrumentation payload broke free and impacted against the #4 rotor,causing the blade to fail, and the craft to lose stability. Figure 10 shows three framesfrom the onboard video system, as SAMRAMP passed through the far edge of onlytree within the tether workspace. The tether secured the craft two meters above theground until it was recovered by the authors.


Fig. 9 Oscillation of SAMRAMP at both 10m and 50m showing negligible differences in settlingtime.

Future work will include several additional tests to determine the exact effectof the tether on UAV performance. One of these tests will attempt to determinehow the location accuracy of the UAV is affected by the tether. SAMRAMP will beflown at both 10 meters and 60 meters above the ground and directly over the tethermounting point while the GPS records location data. From this data, we should beable to see if the wind blowing the tether will pull the UAV off course. Because ofthe inaccuracy of GPS, this experiment should be performed for at least five minutesat each height in order to reduce the error. Another potentially important test willfly the UAV at a low height but off to the side of the anchor point such that most ofthe tether is in the air and pulling the UAV to the side, even when the tension is low.How much power does the craft need to maintain a steady position with this weakbut constant force?

Testing will also be done using longer tethers and a lighter UAV to allow theeffects of the tether to be seen more easily. Additional testing will be done usingdifferent weighting configurations to determine what, if any, effect that will haveon performance. Finally, although a tether that also supplies power is not ultimatelyuseful to the 3D Mesonet project because of the required heights, it would be inter-esting to see how performance changes because of the added cable weight.

A final additional project combines the tether with testing wind speed. One ofthe methods being examined for measuring local winds is to use a tethered capsulecontaining an IMU and gathering motion data from the capsule while the craft isactively holding position. Managing two tethers, and keeping them from interferingis probably not feasible so we will be looking into the possibility of combining bothfunctions into a single tether.


References

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Fig. 10 By serendipitously threading the tether through a tree, a potentially damaging impact wasavoided.


9. Veerabhadran Ramanathan, Muvva V Ramana, Gregory Roberts, Dohyeong Kim, Craig Cor-rigan, Chul Chung, and David Winker. Warming trends in asia amplified by brown cloud solarabsorption. Nature, 448(7153):575–578, 2007.

10. Sensirion. Sht15 - digital humidity sensor. http://bit.ly/16kL32c, August 2013.11. R.W. Sinnott. Virtues of the haversine, 1984.12. Martin Weil. Drone crashes into virginia bull run crowd. The Washington Post, August 25

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effects of tether on rotor uas flight

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