Leashing UAVs

Unmanned aerial vehicles (UAVs), AKA drones, have been flying for quite some time.  The ease of use and relatively low entry cost have resulted in them being used in a plethora of applications, including military, law enforcement, news provision, science and exploration, traffic reporting and management, as well as a host of others.  Indeed, Amazon has been in the news this week, with the imminent step up of their drone testing bringing them a little closer to fleets of drones delivering small packages directly to shoppers within 30 minutes.

Nevertheless, there are issues, among the most critical of which is that free-flying battery-powered vehicles have limited flight and loiter capabilities.  Not to mention the safety concerns that exist, given the unrestricted nature of the device.

tethered droneA rapidly growing configuration segment is the hovering tethered drone. These aircraft answer the need for greater control and safety as FAA regulations are starting to mandate limited operating envelope capability in certain environments.  At the same time they provide extended flight times, which greatly increase loitering capabilities.

Typically, tethered drones are compact, portable, multi-rotor craft that operate attached to an electrical cable/tether. The tether provides power, telemetry and control signals while physically securing the craft to a ground station. The length of the tether, typically about 500 feet, determines the operating “zone” of the drone and insures it remains under complete control of the operator.

The concept at first look seems simple enough. The drone is designed to carry a specific maximum payload, which will require 1 to “n” vertical lift rotor/motor assemblies to provide sufficient thrust to safely hover and maneuver through a range of operational and environmental conditions. Examining the problem a bit more closely one realizes that sending up 100s of Amps of low voltage DC power is not very practical as the large wire size and weight required considerably limits payload and operational capability (remember, the cable becomes part of the weight carried aloft). Additionally, the increased “sail area” of a hefty cable becomes a huge factor to overcome as a fatter cable develops more lateral forces, which the flight controller must handle.

So, what does one do?

Noting the issues, a high-voltage-based ground supply is desired (higher voltage –> lower the current –> smaller wire –> lower resistive power loss).

If one could convert high voltage DC on board the aircraft to the low voltage needed with little increase in volume consumed and added weight, the resultant decrease in size and weight of the tether would much more than offset any increase in onboard weight. The thinner cable would also dramatically reduce the “sail” effect. Another benefit would be the ground mechanism for storing and playing out the tether would be much more compact and lighter in weight.

Our latest DC-DC Bus Converters, DCM DC-DC Converter Modules and AC-DC PFC modules provide industry-leading efficiency and power density, which allows designers to convert voltages as high as 800V to low voltage high wattage DC for motor control, as well as system voltages for onboard electronics. The resultant highly efficient, compact and light-weight onboard power systems free the designer up for additional payload capability, as well as reducing the bulk and cost of his ground-based system.

Below is a diagram for a 384VDC to multiple 48V 8A motor loads for a total of 3,000W.

drones2

 

If you would like to read more on the topic there is an informative paper, ‘How DCMs Keep Tethered Drones Flying‘ available for download.  Written by two of our applications engineers, the paper draws on their experience of supporting customers developing tethered drones, and details specifically how the compact size and light weight of DCMs make them ideal for these applications.

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