Modern weaponized unmanned aerial vehicles (UAVs) made headlines in the United States after the first remotely executed air-to-ground missile strike in Afghanistan’s Paktia province in February 2002. Seemingly more mundane, and thus, less covered by the popular press, are the many observational and communication applications for UAVs in support of battlefield operations and for peacetime applications.
UAV applications include battlefield intelligence gathering and targeting, land surveying, and surveillance for contraband interception. UAVs also find use in the search stage of search-and-rescue operations, particularly when seeking subjects in wilderness areas with poor roadway access. Similarly, UAVs provide monitoring vantage points during forest fires that improve both the efficacy and safety of firefighting personnel. In addition to observation, UAVs can provide limited communication services in locales and terrain that otherwise preclude terrestrial links.
UAV manufacturers are increasing functionality and mission-operation time, in part, by reducing vehicle weight. Elements of mechanical design, such as the material selection for frame, fuselage, wing, and control surfaces offer only rare opportunity for weight-to-performance improvement: Advances in materials occur at a rate disassociated with UAV design cycles or, as ex-Secretary of Defense Donald Rumsfeld might have famously said, “You go to flight with the materials you have.” Instead, manufacturers have developed methods to optimize wing, control surface, and propeller designs for the intended operating altitude. This can range from 2,000 feet for hand-held models to well over 30,000 feet for HALE (high-altitude long-range) craft (Figure 1).
Performance improvements to large motion actuators and their drivers, both with respect to weight and energy use, benefit flight- and gimbal-control systems. MEMS-based sensors, using technologies from companies like STMicroelectronics and Analog Devices, form modules such as 10-degree of freedom inertial sensors. These combine a tri-axial accelerometer, tri-axial gyroscope, tri-axial magnetometer, and a pressure sensor in packages smaller and lighter than ring laser gyros. These sensors provide primary data for gimbal stabilization and supplementary data for navigation.
Improvements in vision sensor technologies have improved image resolution while reducing size and weight for both standard image capture and forward-looking infrared (FLIR) systems. In civilian applications, such as surveying, construction site inspection, or geological research, UAVs equipped with advanced 3D HD LIDAR can scan of as many as 1 million points per second. Some limited-range craft operate without GPS—reducing system payload mass. As a result, these small UAVs can operate with smaller energy sources or take advantage of longer maximum flight times.
Depending on their mission, UAVs can exploit either fixed- or rotating-wing designs. Flight times and distances can range from less than an hour and a mile for, say, site inspection, to thousands of miles for ordnance delivery and a full 24 hours for battlefield intelligence, or communication support for ground troops.
Depending on the type of mission a UAV is to execute, its primary energy source can be either petroleum fuel or rechargeable batteries. Either way, virtually all on-board systems operate electrically, and although electric-power management is but an enabling function, its performance and energy density is critical to the UAV’s operating range and duration.
Typical military UAVs develop primary AC electric power from a turbine-based generator. An AC-DC converter provides a distribution-level voltage, which can range from 270 V to 28 V, depending on the design. Distribution at high voltage is becoming more common because it reduces the size of copper feeds—issues for both weight and cost—and reduces I2R losses. The benefit of highly efficient power subsystems extends far beyond issues of fuel use. Power converter’s 1-η losses dissipate as heat, limiting both power density and the power subsystem’s proximity to functional loads.
The demands for high power density and thermal performance in power subsystems are common to many applications outside of aviation. The voltages UAVs commonly use are near enough to non-aviation systems that technologies and topologies can cross from one sector to the other. For example, during a recent conversation, Vicor Vice President Stephen Oliver told me that the company’s factorized topology, which separates the regulation and voltage transformation functions, has application in high-density UAV power subsystems.
The topology implements voltage transformation using the company’s high-frequency Sine Amplitude Converter (SAC), which has the added advantage of reducing the requisite bulk capacitance by the square of the transformation ratio. Such an approach improves power density and reliability over traditional topologies.
Reduction in power-conversion losses simplifies the thermal design, allowing smaller heat sinks and fans. Given the high degree of silicon integration already in use for radios, vision systems, motor and actuator drivers, and computational resources, from a size and weight perspective, careful attention to the power subsystem design may be the single most effective way to reduce total payload.