By Mark Patrick, Mouser Electronics
There is a markedly rising prevalence in tethered unmanned aerial vehicles (UAVs). These units can be employed in a multitude of potential scenarios where a means of continuously delivering data from an airborne source is required. These can be in relation to public surveillance, border security, emergency response to natural disasters, military reconnaissance, smart agriculture, traffic monitoring, TV broadcasting from sports/music events and various other applications. The fact that they are tethered has several major plus points, with less piloting skill being required and authorisation being easier to obtain (as there is less risk of the UAV drifting into areas of airspace where it is prohibited from entering).
Tethered UAV platforms can be positioned at heights that are several hundred meters above the ground, and in some cases they will need to remain in the air for periods of up to 24 hours. The objective of the following article is to look at the numerous engineering challenges associated with the powering of tethered UAVs – especially as the demands being placed upon them are now starting to ramp up considerably.
Powering Tethered UAVs
Like any item of electronic equipment, a tethered UAV will need to draw electricity in order to operate. This will drive the rotors that keep it in the air, as well powering the on-board data acquisition apparatus (such as HD cameras), plus all the control, positioning and diagnostic hardware. Conventional free-flying UAVs and drones have to carry their primary power reserves (such as a charged bank of batteries). These reserves will become depleted over time, which will thereby limit the period over which the UAV is able to stay airborne. Conversely, tethered UAVs are not subject to such restrictions. They can instead draw their power via cabling from a ground-based source that does not deplete over time (such as a mains supply outlet or a panel of photovoltaic cells). This is why they are proving to be so advantageous in fixed-location applications.
As the amount of functionality incorporated into tethered UAVs continues to increase, so will their overall weight (as more components need to be added to design layouts). The height that tethered UAV platforms will need to attain is also expected to rise. Once again, this will have an impact on how much they weigh, as it will mean that there is a greater quantity of cable harnessing involved (to deliver power, as well as to take care of the transfer of captured data back to the ground). Though lightweight materials can be selected for its construction, the impact that cabling will have on the UAV’s power budget must not be overlooked. It should also be noted that the greater the length of the cable, the larger the transmission losses experienced are going to be.
Reliance on elevated DC voltages means that smaller conductor diameters can be used within the cabling, thereby lowering the power losses that the system will exhibit. 400 VDC transmission is now becoming commonplace in tethered UAVs. The UAV’s on-board DC/DC converters can then be utilized so as to drop this voltage down to the required voltage for particular functions.
Sourcing High Voltage Input DC/DC Converters for Tethered UAV Implementation
If engineers are to meet the requirements of low-loss tethered UAV power systems, they will need access to DC/DC converters that will have a high input voltage rating, along with a suitable broad conversion ratio. For example, Vicor’s BCM6123xD1E5135T01 fixed-ratio DC/DC converter modules (as shown in Figure 1) can support a 400 VDC input voltage and are capable of delivering a 1.75 kW output. They offer a 98% (typical) conversion efficiency, thereby keeping the losses involved to an absolute minimum, and have a 4.24 kV isolation voltage figure for surge protection purposes. Their external dimensions of 63.34 mm x 22.80 mm x 7.21 mm means they are compact enough to make them highly suited to the space constraints that are inherently characteristic of UAV designs.
These DC/DC converters employ a sine amplitude converter (SAC) transformer-based resonant topology that accentuates their conversion efficiency performance. As switching occurs at the zero-crossing points, the related power losses can be mitigated. Not only does this boost the Vicor modules’ conversion efficiencies, but it also suppresses high-order noise harmonics too. Furthermore, SAC converters can run at substantially higher frequencies than standard DC/DC converter units (typically several MHz). This allows miniaturization of their transformers, which brings further size and weight benefits.
Transmission Line Considerations
The circuit parameters relating to the tether cables of these UAVs must also be taken into account when looking to implement an effective power system. Engineers will need to have a good understanding of the resistance (R), capacitance (C) and inductance (L) characteristics involved. Short tethers will act as simple LCR circuits, but things get a little more complicated as tethers start to lengthen. Longer tethers will behave more like transmission lines with distributed LCR attributes, where the presence of transient surges could put hardware at risk of possible damage. Transmission line effects may be dealt with via access to detailed simulations. Furthermore, correct termination of the tether with an appropriate output filter can be pivotal in addressing potential surge issues.
UAV Power System Example
An example of a tethered UAV power system implementation is described in Figure 2. It is based on a 400 VDC supply and features several BCM6123 DC/DC converters. The bus converter module (BCM) delivers the 48 VDC bus supply, along with the necessary isolation. The additional DC/DC converters then provide regulated 24 VDC and 12 VDC rails to power the constituent subsystems. This high power density arrangement takes up minimal board space and does not add excessively to the total weight of the solution. The elevated levels of efficiency that can be achieved have the additional upshots that the space taken up by thermal management and the extra weight this would normally add can both be minimised (with only modest heat sinking being required). This is also beneficial from a commercial perspective, as it will help to keep the bill-of-materials costs down.
Tethered UAVs are going to become increasingly popular in the years ahead, with an array of prospective opportunities opening up for them. With the features and functionality they possess ever expanding and the altitudes at which they will be located rising, higher density power systems exhibiting enhanced efficiency levels will be needed. The use of high voltage, fixed-ratio DC/DC converters presents a way for engineers to address tethered UAV power system designs, while still fully respecting the key operational constraints that are intrinsic to such applications.
As fixed-ratio converters, such as the ones described in this article, do not require inclusion of voltage regulation components, there is scope for them to attain heightened degrees of power efficiency and superior power densities. Consequently, there will be less heat to subsequently dissipate – leading to the thermal management mechanisms that accompany these modules being more streamlined, with space, weight and cost advantages all thus being derived.