The simplistic definition of UAV reads like this:
“An unmanned aerial vehicle (UAV), colloquially known as a drone, is an aircraft without a human pilot on board. Its flight is controlled either autonomously by computers in the vehicle, or under the remote control of a pilot on the ground or in another vehicle.”
While accurate, this definition does not adequately convey the technological sophistication of the UAV or its importance in the theater of war.
The Department of Defense, Department of Homeland Security and U.S. intelligence agencies are increasingly using UAVs for everything from battlefield surveillance to remote-controlled strikes against terrorists. And it isn't just the United States that has shown such dramatic interest in these aircraft. An article in the May 6, 2013 edition of The Guardian notes that the British military has amassed at least 500 drones as part of an ambitious plan whereby a third of the Royal Air Force will consist of remotely piloted aircraft by the year 2030.
As noted above, these a maz ing devices are controlled and monitored through a variety of methods; however, the most prevalent are the sophisticated UAV ground control stations that can either be located in the immediate vicinity of the UAV’s mission or thousands of miles away from where the aircraft is flying. A typical UAV ground control station has two consoles – one for the aircraft operator and one for the payload operator, working in tandem. Utilizing the skills of each operator, in combination with the sophistication of the control station's technology, the UAV can achieve significant military objectives without endangering live military personnel.
Of course, despite its technological intricacy, the UAV ground control station requires something that even the most primitive electrical device requires – power. For the stations established at United States military bases, a reliable electrical supply generally presents no problem. Conversely, in the foreign battlefield areas where many of the ground control stations are deployed, a dependable electric grid is a rarity. To begin with, in the more remote areas of the world, there may be no electrical grid at all. And even where there is a grid, the unreliability (due to constant outages, surges, and other fluctuations) precludes their use, given the critical nature of the typical military UAV mission, which requires 100% up-time.
Consequently, the military has had to turn to alternative energy sources to power these ground control stations. This has primarily meant the use of diesel-fueled generators — not surprising, given their availability and low cost to produce AC or DC power. Additionally, diesel fuel is readily available in large supplies virtually everywhere in the world.
As with most military applications, UAV ground control stations use two generators in an A/B switchover mode (a redundancy system like N+1). In the event of a failure by generator A, the secondary generator B comes online.
While generators have become more reliable over the years, some issues remain. Today’s primary con cern is the downtime during the switchover from generator A to B. This results in a system that, for some period of time, however limited, is without power. As a result, the UAV can, in effect, be "flying blind."
A significant issue with generators is the ongoing downtime needed to regularly refuel and check other fluid levels as well as routine maintenance. In the case of a soldier who forgets, or is unable to refuel a generator due to combat conditions, the generator could shut down. This could cause the UAV to lose power and also go down. While a scheduled and, therefore, controlled shutdown can reduce lag time, even those few seconds can be critical, depending on the UAV's mission and the situation in which it is being deployed.
Yet another drawback of generators is the quality of power produced. As they run best when a constant load is applied, the generators can sag and transient spike the output voltage if they must compensate for load variations. This fluctuation can cause sensitive electronic equipment to become damaged or shut down. Because most computerized equipment cannot withstand a power interruption greater than 8 milliseconds, a sag in power could destroy an entire mission and/or the UAV.
This is further complicated with threephase generators, as each of the three phases needs to be balanced with similar power draw on each phase. If the load or power draw is significantly changed on a phase, the output can become unbalanced and, again, this can cause voltage sags and transient spikes.
Due to the reliability issues surrounding generators, the military began using uninterruptible power supplies (UPS) to support generators in the late 1980s. With the advent of more sensitive electronic equipment in the mid ‘90s, the use of UPS devices became more widespread. The primary purpose of a UPS is twofold. First, to clean “dirty” power to ensure sensitive electronics are not damaged, and second, to provide back up power through short power outages or sags in power. The best type of UPS to support military applications is a true online double conversion UPS with Power Factor Correction (PFC).
Designed to continuously convert the input power source, which may be AC or DC, into a high voltage DC bus, the UPS then converts the high voltage DC bus to virtually any AC or DC output required. This double conversion creates very high galvanic isolation between the input and output which helps keep sensitive equipment isolated from poor quality input power.
Power Factor Correction provides the highest efficiency of conversion, which ensures maximum power is delivered to the system by compensating for reactive power disturbances in the system and/or load equipment. Reactive disturbances create power loss, generate more heat, and decrease the life of electronic equipment.
In a number of military applications, the UPS is counted on not only to supply power, but to withstand the rigors of abusive and harsh environments. Ruggedized military UPS systems will provide clean AC or DC power while protecting against the most harmful elements, including dust, rain, wind, heat, cold and blowing sand. What's more, they are designed to withstand the vibration and shock that are familiar problems in virtually any military scenario. The key elements of a ruggedized UPS include: a lightweight and rugged aluminum chassis; conduction cooled electrical components that are surface- mounted; and a heatsink tunnel, which prevents dust, debris and blown precipitation from entering the system. Only purpose-built, rugged, military UPS devices meet the newly upgraded MILSTD 461-F. This military standard changed to ensure that magnetic fields and interference don’t compromise the performance of digital mission-critical devices. It ensures all electronic equipment used for military applications complies with acceptable levels of radiated and conducted emissions as well as radiated emissions and susceptibility.
The way generators and UPS devices are deployed in connection with UAV ground control stations creates even harsher environments than the same devices used in connection with other electronic equipment. UAV control stations or command centers are typically used in a mobile environment; that is, they are moved from place to place, depending on the location of the UAV’s mission. The generators, as well as the UPS, are moved along with the stations and the UAV itself, often in an armored vehicle.
The generators and UPS devices are usually placed in a wide-open area, constantly being set up and taken down. As they do not remain in a permanent location, they are often sitting directly on the ground or stuck on an open pallet.
This means that the UPS, as well as the generators, will be in prime position to be affected by any harmful weather- or environment-related conditions. This is just more evidence that UAV missions should include fully ruggedized, military UPS systems.
Mission crucial equipment tends to require 1kVA to 5kVA of back-up power. Top UPS manufactures provide a modular solution so that users can purchase one system and use it in a myriad of applications. For example, a unit offering 1.25kVA can be employed for loads in that range. For a 5kVA load, a user can parallel four 1.25kVA units (or five in an N+1 configuration) to procure the necessary power. This model of modularity offers convenience for procurement and logistics as it can scale to meet requirements.
Despite their effectiveness in UAV ground control stations, UPS devices are not always used. Given the potential failure of generators in many mission-critical situations, it would seem that the use of a military UPS would be an easy decision as even a fully-ruggedized UPS system is typically less than 1% of the costs of the UAV, not to mention the added costs of the ground station it is providing power to. While the devices can save millions of dollars by protecting the UAV as well as the sensitive ground station electronic equipment, the cost for a UPS may still be viewed as an unnecessary investment. Considering the many ways a rugged UPS supports UAV missions, not procuring one can only be categorized as, to coin an old phrase, “being a penny wise and a pound foolish.”
This article was written by Jeff Boudreau, Sustaining Engineering Manager, Acumentrics Corp. (Westwood, MA). For more information, Click Here .