Is it a bird? Is it a plane? In the case of micro air vehicles, tiny, self-piloted flying machines, it is a little of both with some insect and robot characteristics thrown in. While not quite able to leap over a building in a single bound, microflyers will have some fairly astounding characteristics. At the Georgia Institute of Technology, engineers envision that these 6in. machines will be able to fly to a target and feed back, or retrieve, information in a variety of forms, including visual, chemical and biological. Adding to their complexity, microflyers will be able to accomplish their tasks by themselves: they will not rely on humans to provide remote control.
The initial applications for the microflyers will be for the military. However, civilians, such as police and fire officials, scientists and farmers, could be the ultimate users of this technology. But as useful as these flyers may be in the future, there are some immediate problems that have to be solved before microflyers become a reality.
It is not possible, for example, to simply shrink a 747 proportionally down to six inches and expect it to fly. Wind, rain, and even air itself, pose different problems to a 6in. flyer than they do to a large jet.
The smaller a system is, the more skittish it acts. Therefore humans – even if they are looking through a camera – will not be able to react in time to control a machine this tiny. Proposed means of making the microflyer autonomous include using a geographic information system (GIS) to provide a map of the terrain or a global positioning system (GPS), which employs a satellite to map the location of the flyer. However, GIS cannot include power lines or cars and GPS is useful only outdoors.
Traditional planes use heavy motors and hydraulic machinery to manoeuvre control surfaces in the wings and tail. Altered wing design and channelled exhaust may help provide control without extra weight and allow the small aircraft to fly under control at very low speeds not possible with conventional wings and control surfaces.
Since a drop of petrol has more energy potential than current batteries of the same size, early models of the microflyers will probably use fossil fuels. GTRI researchers have proposed a micropulse jet engine which is not as efficient as a traditional jet turbine engine, but is much simpler. All of the components, including the payload, must weigh less than about four ounces.
THE AIRBORNE MICROFLYER
The initial goal of the microflyer program is to design a machine that can take off, land and follow simple instructions while aloft. Robert Englar, a principal research engineer at GTRI, is working on a fixed- wing model for the microflyer. By combining an altered wing design that has a rounded trailing edge and channelling the engine exhaust out of the wings through tiny slots, Englar’s model employs what is known as the Coanda effect to greatly augment the wing’s lift and control without any external moving parts.
This results in an aircraft’s ability to lift, land and turn, all at very low speeds without the need for complex control devices.
‘Traditional planes rely on fast wind speeds to generate lift over their wings and allow them to stay aloft. Our blown model will allow the microflyer to take off and land at much slower speeds and turn while airborne,’ explains Englar. This would allow these very small aircraft to fly within buildings, for example.
To change directions, Englar’s model uses fluidics devices to control the stream of exhaust. To turn to the left, the amount of exhaust is increased out of the right wing or reduced from the left. To lift or lower the craft, the quantity of exhaust is manipulated. More exhaust exiting equally through both wings makes the aircraft fly much slower.
Robert Michelson, also a GTRI principal research engineer, was inspired by insects for his design. He envisions the microflyer as a multi-mode vehicle – capable of flying, crawling, and perhaps, swimming. To fulfil that vision, he has developed a reciprocating chemical muscle (RCM) which uses a monopropellant fuel to generate a reciprocating, or up and down, motion, such as beating wings or scurrying feet. As an added plus, the RCM can generate electricity, which could be used to power sensors for directional or mission purposes.
Resembling a metallic wasp with about a 10in. wingspan, his prototype (dubbed the entomopter in reference to its insect-like characteristics) flaps its wings as the fuel is injected into the body, enabling it to fly forward. Gas generated as a byproduct of the RCM can be used to change the lift on one wing, or the other, to allow the otherwise autonomic symetric wing beating to result in ‘rolling’ of the device so it can turn right or left.
The next step is to shrink the RCM device down to insect size. Near future goals for this model include trimmed, or directed, flight; multi-mode locomotion; and sensors which will enable it to perform simple homing activities.
At such a small scale, every piece of the microflyer has to perform double, or even triple, duty. For example, a radio antenna could also be used as a stabilizer for navigation while the legs could double as receptacles for fuel storage and for adjusting the entomopter’s weight and balance during flight. It is believed that even a ‘self-consuming’ system is possible, in which the microflyer would consume itself to generate energy as it flies. Alternatively, the RCM concept is even amenable to conversion of biomass into useable fuel reactions, so future entomopters may be able to ‘eat on the run’ to extend their mission endurance.
To be useful machines, microflyers will have to carry payloads ranging from cameras to chemical sensors. Therefore, other scientists on the Georgia Tech team are examining how to make miniature sensor systems.
The prototype chemical and biologic sensors are basically small chips of glass with optical wave guides fabricated on their surfaces which can trap and manipulate light. On the most basic level, the sensor would have two channels: sensing and reference. When a laser beam is passed under the strips, the phase of the light contained in the guides is altered by the change in refractive index that occurs when the sensing channel interacts with the chemical or biological species that it is designed to measure.
The information contained in the light is read after the laser beams passing under the sensing and reference channels are combined to generate a unique interference fringe pattern, which moves past a solid-state detector array in proportion to the phase change that has been caused by the sensing interaction.
With this technology, each sensing/reference pair can be designed to respond to a particular type of analyte. In some cases, it is possible to design a sensing channel that will respond uniquely to an analyte that needs to be detected. In other cases, where unique sensing devices are not available, multiple sensing/reference pairs can be designed on the same optical chip and pattern-recognition techniques used to sort out which analyte is being measured. Up to two dozen channels can be placed on a sensor chip to determine what the microflyer is flying through.
Already small (about 1cm by 2cm), the sensors will need to be further reduced for the microflyer. For example, the laser and detectors, which currently feed the information to circuit boards in laptop computers, are external to the sensors. Researchers are currently working on integrating all of these components, including the signal processing electronics, into one small unit.
SENSORS MAKE IT HAPPEN
An important feature of these sensors is the ability to fabricate thin films of polymers, antibodies, or other chemical reagents on top of the waveguides. These thin films are the key to gathering chemical or biological information in a tiny space. With these integrated-optic sensors, chemical reactions, adsorption into polymers and other means can be used to gather analytical information. Combined, these features make the integrated-optic sensors more specific and sensitive than other monitoring techniques.
A group of researchers in Georgia Tech’s School of Electrical and Computer Engineering department is working on other types of sensors – those that would supply visual images and those necessary for communication.
The visual sensors could use active pixel arrays already used in the nose cones of missiles to allow for real time processing of images. The trick is, of course, to make the technology tiny enough.
Researchers at Georgia Tech are also examining how to communicate with the microflyers once they are airborne, how to transfer information that the microflyer gathers and how to keep the information and communications secure – an important consideration in wartime use. Therefore, the microflyers will not use a standard cellular frequency band, since that is easily jammed or crowded, but will use a higher frequency, which allows a smaller antenna to be used.
WHAT THE FUTURE HOLDS
As the initial problems are solved, other specialists will begin work on the design of human/machine interfaces. This area is important because even though the microflyers must operate autonomously after they have been given a task, humans must still play a large role in their missions, including identifying tasks for the microflyers, such as specifying the mission objective and the information to be obtained; servicing the machines by providing logistics support in refuelling and repair; and, receiving and interpreting the information obtained from the aircraft.
In spite of the many challenges that remain in order to get a microflyer in the air, researchers hope to have a machine flying in three years. Cost of the prototype has been estimated at over $10 million.
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