Unmanned aircraft load up on compact sensors

Significant advances in the design of sensor systems for unmanned aerial vehicles enable a single payload to carry multiple sensors, which cuts weight and increases mission time.

As the number of unmanned aircraft systems continues to grow, the volume of collected data is expanding at a far higher rate. More sensors are being carried aloft as users clamor for multispectral data that shows more than images alone, prompting designers to build up the infrastructure that supports these sensors.

As the Defense Department continues driving to more knowledge-based strategies, unmanned aerial vehicles (UAVs) are expected to continue their rapid growth. The Teal Group estimates that UAV spending worldwide is expected to almost double during this decade from $5.9 billion annually to $11.3 billion in 2020.

These vehicles are carrying an increasing number of sensors, augmenting cameras with radars, infrared sensors and other technologies. Teal’s 2011 study predicts even faster growth for payloads, including sensors and support electronics. Although sensor costs will go down thanks to the ongoing decline in the cost of semiconductors, payload revenues should rise from $2.6 billion in 2011 to $5.6 billion in 2020.

Shrinking electronics let developers put two sensors in spaces previously used for one. At the same time, these sensors are providing improved capabilities, giving users higher resolution.

“Integration of visible imagers and thermal imagers into a single sensor package has removed two single purpose sensors from the payload,” said Curtis Reichenfeld, chief technology officer at Curtiss-Wright Controls Electronic Systems. “Multispectral imagers are further integrating a larger spectrum to further reduce the number of specialty sensors.”

Supporting the many sensors squeezed onto each platform requires a solid infrastructure. Mundane items such as heat sinks and cables are also evolving as engineers strive to reduce weight and power consumption so aircraft can stay aloft longer. Cameras and radar get the attention, but the gimbals that position them and the systems that move data are equally important.

The systems that focus sensors on targets and keep them precisely aimed as the aircraft moves are critical elements when high resolution imagers zero in on targets that are miles away. The minute vibrations that occur when ailerons are adjusted or winds buffet the aircraft cannot disturb these gimbals. Equipment suppliers continue to devise new techniques for improving performance.

“Our MTS-B [Multi-spectral Targeting System] has a highly stable gimbal,” said Neil Peterson, business development director at Raytheon Intelligence, Surveillance and Reconnaissance Systems. “One aspect of that is a mechanical construction that isolates the gimbal from the platform and reduces the impact of drive motors in the gimbal.”

Managing the motors that control this positioning is often handled by dedicated chips, which perform this complex task more efficiently than microprocessors or digital signal processors. GE Intelligent Platforms recently unveiled an image stabilizer that weighs only six grams and takes little more board space than a microcontroller. The chip also includes serial communications so it can communicate directly with motors and other controls, eliminating an external communications component.

Fire and ice

The increasing density of electronics provides many benefits, but it creates more problems for thermal engineers. UAVs present unusual conditions. High density electronics generate a lot of heat that must be removed, but motors and other parts must be heated so they can operate when aircraft are flying in sub-zero temperatures.

“Very dense electronics like radar generate enough heat, but actuators and other electronics away from the high density in the center need to be warmed when you’re up where the temperature is -50 C,” said Gene Fraser, senior vice president for aerospace engineering at Northrop Grumman.

Many designers are employing liquid cooling techniques to transfer heat from highly integrated modules to remote components that need more heat. Thin pipes that carry liquids with rapid heat transfer characteristics are typically lighter than copper heat sinks and they eliminate the need for heaters, further trimming overall heft.

Although heat can be beneficial, generating heat means wasted energy. System designers are reducing heat generation by moving to lower power electronic devices, moving from five volts to under two volts for many chips. Low voltage chips require less power so they can run longer on existing power supplies or let designers use fewer batteries or smaller power generators.

UAV infrastructures also are moving away from electromechanical devices. As flash memory capacities have moved upward while prices shrink, flash is rapidly replacing hard drives.

“Using storage without moving parts eliminates a lot of heat,” said Robert Robinson, senior program manager for airborne reconnaissance systems at Lockheed Martin IS&GS-Defense.

On-board storage systems record every minute bit of data generated by each of the on-board sensors. This native data is typically used when users want higher resolution imagery than they get when compressed data is transmitted directly to ground stations. Data compression techniques provide high quality images with minimal loss, but analysts looking at photos taken from 50,000 feet through hazy skies often want to view uncompressed imagery.

Can you see me now?

Sending these compressed video images to the ground is a complex challenge for both mechanical and electrical engineers. The on-board infrastructure for UAVs includes the antennas that link them to ground stations or satellites. While UAVs often communicate directly to ground stations, direct links typically work only when they have line of sight to the receiving terminal.

Line of sight isn’t practical in mountainous regions or when aircraft fly at low elevations. When flying elevations are below 50,000 feet, earth stations must be in 100 miles to receive line-of-sight transmissions.

Carrying both types of antennas improves communications capabilities, but it also has a big effect on mission duration. Antennas add bulk and require a clear exit path from the vehicle, so openings or bubble nodes are needed. “We can’t have a lot of apertures open, that adds drag and impacts your radar footprint,” Fraser said.

Developers are responding with a time-tested technique: shrinking footprints. As with all types of satellite antennas, there’s a push to make them smaller and lighter. Satellite communications providers are deploying new bands and squeezing more power into existing bands so antennas can be smaller.

“To minimize the impact of weight and drag on each UAV, both of which affect mission duration, the Satcom terminals they carry need to be small and easily integrated into the airframes,” said Peter Hadinger, president of Inmarsat Government Service.

The effort to build a sensor infrastructure that’s lightweight, fast and requires little power extends well beyond antennas and gimbals.  The cables that link sensors, storage systems and communication technologies are also undergoing a transformation. Shielded copper wires are bulky and heavy, so wires bundled into harnesses are being replaced whenever possible.

“Wiring harnesses have always been a major contributor to the overall weight of UAVs,” Reichenfeld said. “Interconnect weight has been greatly reduced with the use of fiber optics and high speed serial buses to transmit data between sensors and electronics.”

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