Dragonfly Pictures (DPI) has developed a new type of drone, the Tethered Hover Drone. This new vertical take-off drone is weatherproof and can stay in the air for long periods of time. Drones use flex cables as permanent physical links to provide power and communications as well as mechanical support.

Unlike battery-powered multi-rotor drones that require battery replacement every 20 minutes or fast charging, tethered drones receive power via a cable that connects to a base station. This allows them to keep operating at altitudes up to 500 feet for more than 400 hours – thanks to the constant flow of energy through the tether.

Tethered Military/Industrial Unmanned Multirotor Aerial Relay (UMAR) UAVs are designed to track maritime and land-based platforms such as ships, boats, trucks and other vehicles, in addition to their classic use as naval base communications.

tethered drone

Increased demand for long-range communications and intelligence, surveillance, and reconnaissance (ISR) has driven interest in UAVs; however, previous generations of UAVs have encountered technical difficulties and limitations, especially in flight control. High-speed flying fixed-wing UAVs for logistics applications do not have on-board surveillance and communication capabilities. Untethered drones experience stability issues in adverse conditions such as strong winds and heavy rain. To overcome these difficulties, Dragonfly has developed a new type of drone, the tethered hovering drone.

Compared to classic drones, these drones offer vertical take-off and landing capabilities. No runway, launch or recovery equipment is required. They also provide good performance in turbulent weather, rain, snow, dust and high temperatures, and are optimized for saltwater marine environments. The tether eliminates the danger of electronic interference, thus making the system immune to GPS and radio frequencies.

DPI's tethered drones are currently eligible for use by the U.S. Navy for ISR, communications and video applications in a maritime/marine environment.

Drones typically offer operational altitudes of up to a few hundred meters, with the limiting factor being the drone’s ability to support the weight of the tether. Tethers can be made of aramid or other lightweight synthetic materials for strength, copper or copper plated for energy transfer, and fiber optics for data and communications. The system may be ground based or vehicle based.

Tethered drones hovering hundreds of feet above the ship overcome the challenges of ship communication systems, which are limited by the curvature of the Earth and can only communicate with the horizon within line of sight. Tethered Unmanned Aerial Systems (UAS) can provide continuous communications beyond line of sight. The tether provides continuous power and command/control to the UAS, thereby greatly increasing the duration of the mission. Connected UAS systems reduce operator workload and mission planning, allowing longer communication between host ship and offboard resources.

"UMAR can host a variety of sensors, including cameras, radar, and weather data, that can be collected at high altitudes for extended periods of time," said Joe Pawelczyk, vice president of operations at DPI. "DPI is already onboard 45-foot U.S. Coast Guard vessels, Its tethered UAS was tested on U.S. Navy ships and U.S. Army ground platforms."


Figure 1: Unmanned Multi-Rotor Aerial Relay (UMAR) (Source: Dragonfly)

power challenge

The design challenges for tethered architectures are primarily for power systems. Power must be delivered from the host ship to the multi-rotor drone at very high voltage and low current to allow the use of thinner, lighter hardware resources, allowing the drone to be more maneuverable and more efficient in the air load.

The weight, size and electrical characteristics of the tether directly affect the performance of the tethered drone. In turn, the power delivery network has a direct impact on tether design. To achieve thinner and lighter tethers, power must be delivered from the host to the drone at very high voltages and low currents. This allows for greater maneuverability and heavier aircraft payloads, resulting in improved communications and surveillance

Power levels range from 8 to 10 kW; in these conditions, the UMAR drone is very powerful and robust, allowing it to maintain a persistent stationary position in turbulent conditions that affect the positioning of the host vessel.

Inside a multi-rotor drone, high-voltage conversion must take place with the smallest possible footprint and form factor that reduces space and thermal effects.

To address these tough power challenges, DPI has implemented low profile Vicor high voltage BCM VIA modules in its UMAR for high efficiency (98%) conversion with only 2% loss and heat over the 800 V to 50 V range.

To power the eight independent drone rotors, DPI uses eight Vicor high-voltage BCM4414 fixed-ratio DC/DC converters. These modules have inherently low EMI and few high frequency harmonics. The integrated EMI filter in the Vicor BCM helps minimize EMI noise by adding less size and weight than traditional DC/DC converters.

"Using the Vicor power module, we were able to reduce the weight of all components on the drone, thereby increasing altitude and airspeed while carrying the required mission payload," Pawelczyk said.


Figure 2: Eight Vicor HV BCMs power eight independent UMAR DPI rotors. (Source: Vic)

UMAR could be a useful tool for communicating with unmanned surface ships during defense and commercial operations. UMAR can also serve as a backup for telecommunications, as a backhaul method for maritime mobile communications. Its potential applications include communication base stations in offshore oil and gas and renewable energy operations. For longer distances, multiple daisy-chained drones can be used.

Reviewing Editor Huang Haoyu

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