Power tether for long duration multi-copter flight

Abstract

The possibilities and promises of unmanned aerial vehicles (UAVs) for smart agriculture, 5G cellular integration, package delivery, and persistent surveillance are but a few of the active drivers for advancing UAV technology and systems. The UAVs' dependence on battery power represents a key limitation for practical deployment. As every remote pilot understands, even a modest research payload can limit a UAV's endurance to under 20 min. When horizontal maneuverability is not required, a power tether from the ground can provide near infinite flight time. Commercial tethers exist but can be prohibitively expensive or underpowered for research payloads. This paper describes the detailed design, construction, and operation of a relatively inexpensive open-source alternative. The designed and prototyped tether system delivers 1 kW of power at the tether base which, on an efficient UAV, corresponds to a payload of approximately 4.75 kg. We discuss the tradeoffs, design choices, best practices, and customization options, and provide empirical data for characterizing the power-payload relationship. The power and payload are scalable thanks to the modular design and the tools presented in this paper. The very low cost compared to commercial heavy-lift tether systems and the open-source design enable reproducibility and widespread use for supporting research, development, and emerging services/applications.

Keywords: Arduino; Power tether; Ultra-long duration flight; Unmanned aerial system; Unmanned aerial vehicle.

Graphical abstract

Fig. 1

Open-source UAV power tether system. Base box (left top) is connected to a generator or extension cord. Power converter module (left bottom) is affixed to the UAV for practically unlimited flight time and a payload over 4 kg.

Fig. 2

Overview of UAV tether system design. Line voltage (left) is converted to high voltage, low current DC and transmitted up the tether to the DC-DC converters (center). Their output supplies the UAV and the payload. An emergency battery can rescue the UAV if the tether fails.

Fig. 3

Tether bottom circuit diagram. An AC-DC converter (U2) takes household line voltage (J1) and outputs 360 VDC. An Arduino Uno (U3) with motor controller shield with built in Xbee radio (U4) monitors voltage, amperage, and the converter’s enable output, as well as controls a LCD, spool drive motor, and servo powered clutch for that motor.

Fig. 4

Tether bottom custom PCB for power conversion with monitoring.. The large space in the middle is for the AC-DC power converter (TDK-Lambda PF-1000A) which converts 120–240 VAC line power to 360 VDC power for transmission up the tether. Bottom right corner includes the current and voltage measuring components which are passed to the Arduino Uno for digitization and communication to the user.

Fig. 5

Top power module circuit diagram. 360 VDC power from the tether (J8, upper left) is fused and filtered before supplying 1, 2, or 3 DC-DC converters (U7-9, center) which output 28 VDC (nominal). A switching diode D1 passes the greater of that voltage or the rescue battery’s 24 V (J9) to the UAV. Payload power (J11) can only be supplied by the tether which saves the rescue battery for the emergency landing. The mass of the power converter, excluding the rescue battery, is 480 g.

Fig. 6

Custom PCB for the power module at the top of the tether: 360 VDC enters on the left and is filtered and fused before entering 1, 2, or 3 Vicor DCM4623TD2K31E0T00 DC-DC power converters. The output is filtered and the greater of this or the rescue battery voltage is passed to the UAV. Payload power connector is included but can only be powered by tether. The large pad in the center, with multiple vias to an identical pad on the bottom, enables heat dispersal from the bottom of the DC-DC converters with the help of fans (one fan per converter). All traces are double thickness for high current requirements.

Fig. 7

Base PCB with components soldered in place: Line in and intermediate connections for external filter are on the left. 360 VDC output is at the rear right. M3 mounting sockets are visible on the power converter.

Fig. 8

Components in place. From upper left- Main Power switch, tether power switch. Mid left- line filter, fan, 12 V supply. Bottom left- Line -in receptacle, PCB with AC-DC converter and heatsink.

Fig. 9

Front and rear views show inlet vent, line-in receptacle, and fan exhaust. Screws on the right center secure the line filter.

Fig. 10

Spool motor switch wiring and Arduino and Servo power.

Fig. 11

Illustration of a convenient pin cluster on the motor shield that makes LCD and PCB connections easier. Multiple Gnd and 5 V pins are available as well. Note- the LCD SCL and SDA pins are reversed on the data sheet. This diagram is correct.

Fig. 12

Final connections to Arduino/Motomama shield with Xbee radio installed.

Fig. 13

Heat dissipating stack: PCB, heat spreader, DC-DC converter IC, heat sink, and fans with heat paste between each layer. The 24 V cooling fan is zip tied to the heat sink through drilled holes that match holes in heat spreader and PCB. The diode switch D1 also uses a small heatsink.

Fig. 14

Completed top power module with room for third DC-DC converter. Lid screws in place.

Fig. 15

Spool dimensions. Electricity is passed into the rotating center tube (3) via brushes (10) and (9) in contact with the left side contact plate 5 and the right side flange (7).

Fig. 16

The electrical contacts that connect to the tether on the spool: Left image shows the spring contacts that rub on the rotating doughnut shaped connector (5) in Fig. 15. Middle image is the rear of those contacts. Right image is the simpler negative contact that pushes back against the springs. This contact touches the flange on the right side of the spool.

Fig. 17

Spool secured in place with previously described electrical connections on left and right sides visible.

Fig. 18

Before and after views of the motor shaft. The right side square profile will match the ¼ in socket.

Fig. 19

Overhead view and closeup of lever arm, yoke, servo, and manual actuator. The manual actuator is mounted in a small block on the switch panel.

Fig. 20

Clutch parts removed for clarity. L-R, socket with flange, lever arm, manual switching yoke, yoke housing.

Fig. 21

Closeup of servo arm showing #10 washer (bent and drilled) on end of lever arm and small tension spring. The remainder of parts (plastic arms, screws, and grommets) are from the servo package. This flexible arm accommodates the occasional misalignment of the bolt head and socket. If the socket cannot slide into position easily, the spring allows the servo arm to rotate anyway. As the drive motor turns, the socket will eventually slide onto the bolt head.

Fig. 22

Top power module affixed to UAV with low voltage connections made.

Fig. 23

Extending cable manually with master power switch off and spool clutch in coast position.

Fig. 24

Tether electrically connected to top power module (right) and mechanically to UAV frame (bottom).

Fig. 25

Start up of tetherGUI.py by selecting the correct USB Serial Port on the opening screen and verifying power readout and Freewheel/Manual button is active on the following screen.

Fig. 26

Rescue battery supplies UAV when tether power is off (left), tether power is supplying UAV (right) when on and functioning properly.

Fig. 27

Observe tether power increases to nominal 360 V and enable (En) is true.

Fig. 28

Initial takeoff with excess cable extended from spool and gathered on ground (left). Climb out with UAV pulling tether from freewheeling spool (right).

Fig. 29

Tether powered UAV lifting bundle of hand weights for payload vs power measurement.

Fig. 30

Power required and payload lifted. Multiple payloads are flown by an efficient UAV with power measurements taken at the base and at the UAV. The difference is displayed as transmission loss due to tether resistance and DC-DC converter inefficiency. Maximum power at the base of 1030 W delivers 830 W to the UAV for a maximum payload of 4778 g, including tether weight.