Tethered Drones for Unlimited Flight Time

Tethered drones and UAV (unmanned aerial vehicle) technology are part of a market that is growing at a significant rate. Technavio, a market analysis company, has estimated a CAGR of over 70% in the coming years. Growth drivers include communications and security where drones can be rapidly deployed to establish robust channels while also being able to cover broad areas.

Flyfocus is a company specialized in the design and manufacture of ground-connected unmanned aerial vehicles. The latest developments in the field of power electronics have enabled the company to optimize their latest tethered drone systems to improve payloads, flight times and safety. Tethered drones have special features such as fast data transmission, the possibility of continuous charging of the battery, greater control and maximum safety. Flyfocus’s new CableGuard can soar to altitudes up to 70 meters in tethered mode.

In an interview, a Flyfocus spokesperson pointed out that applications also include security and surveillance. Their ability to be ‘connected’ in extended flight provides an emergency telecommunications network in special needs situations. As applications need to find more and more interest, design requirements must meet additional features such as operational extension and, therefore, more weather resistance but, above all, an adequate power source and security systems to keep the drone safe from hacking.

Why a tethered drone?

Technological developments in tethered UAVs have opened up new possibilities for border patrols, live streaming events, temporary communication towers, low-altitude satellite surveillance, and much more. But many more applications are definitely on their way.

The advent of tethered drones began in 2016 as they were involved in the military system. A tethered drone is an unmanned aerial vehicle tethered to the ground. It consists of a base station on the ground and the drone, which is connected to the station through the tether (cable). The drone can be operated remotely to perform specific tasks. Tethered drones can be used for various applications, although most of them are focused on aerial observation and telecommunications. In particular, tethered UAVs are perfect for data capture and telemetry due to the reliability of tethered connections.

By starting with the tethered drone and following the cable connection backward, anyone, at any time, can have an idea of who the owner is. On the surface, this may seem secondary to security, but the image of the leash itself, combined with the certainty of the owner, can be reassuring to people nearby. However, the connecting cable also has advantages for the owner: if the flight system suddenly goes haywire, the tethered drone will never be able to wander off and get lost.

Power management

One of the major challenges for a tethered drone is power management involving the rotor drive motors and control electronics. Having power flowing through a long cable introduces transmission losses that must be compensated. The objective of the cable (transmission line) is to keep the drone at a predefined height and to guarantee power and data transmission. Constraints on thickness and length are also linked to the application. Each transmission line can be considered as composed of infinite elements of infinite length ∆x, each equivalent to a circuit containing an infinite inductance, capacity, resistance, and conductance. A correct simulation of the transmission line allows the parameters to be defined. The phenomena of electromagnetic field propagation in a conductor become significant at frequencies for which the wavelength is comparable with the dimensions of the conductor. At these frequencies, electrical connections between two points can no longer be described by a concentrated parameter system but must be described using a distributed parameter approach. We can see the voltage and current along a transmission line as consisting of a pair of waves traveling in opposite directions.

Thinking the way Tesla did, the possibility of offering low losses is to employ a high voltage that can travel over the cable and then perform power conversion at the desired levels inside the UAV. A high DC voltage lowers the current flowing in the tether. Obviously, weight is a critical element in the choice of DC/DC components.

Fixed-ratio bus converter modules

Flyfocus uses Vicor’s converter technology, in particular BCM modules that typically provide 98% efficiency capable of delivering 1,750 W at 48V on an input voltage of 400 VDC. BCMs are available in ChiP or a Vicor Integrated Adapter (VIA) package, which simplifies cooling as well as providing integrated PMBus control, EMI filtering, and transient protection. High efficiency is ensured by the fact that these converters are fixed-ratio devices or referred to as bus converters and do not incorporate output voltage regulation. These types of converters are suitable for bidirectional power conversion.

Vicor’s BMC family employs the sinusoidal amplitude converter (SAC) topology to provide high levels of conversion efficiency. While most DC/DC converter topologies employ PWM techniques to control the secondary power output, the SAC provides an elegant method of solving switching loss challenges that limit PWM frequency. The goal of SAC is to reduce the generation of noise harmonics, thereby improving efficiency through lighter and smaller components. The low EMI reduces the use of filtering components. This type of converter uses a square wave signal in the order of MHz, much higher than classic PWM, and with amplitude to match the output loads.



Figure 1: A simplified layout of a SAC converter (Source: Vicor)


Figure 2: block diagram of the power system for tethered drones (Source: Vicor)


The Flyfocus spokesperson highlighted how a power management configuration is important to the success of the project. “Figure 2 shows our configuration using Vicor modules: BCM6123 to provide isolation and a 48 VDC bus supply from 400 VDC for the drone’s internal electronics. By using this technology, we were able to achieve a total weight of the power converter part in the order of 200 grams.”


Figure 3: Ground control system in rugged plastic case up to 6h of work on inbuilt battery (Source: Flyfocus)


Tethered drone technology is gaining momentum, as a power configuration that delivers high efficiency with low weight allows UAV designers to maximize payload capacity and reduce tether drag. Tethered drones are finding application in the market for public service missions and carrying out 24/7 surveillance and monitoring in different areas.