Micro air vehicle

Naval Air Weapons Station China Lake, California - A Micro Air Vehicle (MAV) flies over a simulated combat area during an operational test flight. The MAV is in the operational test phase with military Explosive Ordnance Disposal (EOD) teams to evaluate its short-range scouting capabilities.

A simulation screenshot of a "bumblebee-sized" MAV proposed by the U.S. Air Force in 2008

A micro air vehicle (MAV), or micro aerial vehicle (once called by The Washington Post Robobugs^[1]), is a class of unmanned aerial vehicles (UAV) that has a size restriction and may be autonomous. Modern craft can be as small as 15 centimetres. Development is driven by commercial, research, government, and military purposes; with insect-sized aircraft reportedly expected in the future. The small craft allows remote observation of hazardous environments inaccessible to ground vehicles. MAVs have been built for hobby purposes, such as aerial robotics contests and aerial photography.

Configurations

Micro air vehicles are either fixed-wing aircraft, rotary-wing aircraft (helicopter), or flapping-wing (of which the ornithopter is a subset) designs; with each being used for different purposes. Fixed-wing craft require higher, forward flight speeds to stay airborne, and are therefore able to cover longer distances; however they are unable to effectively maneuver inside structures such as buildings. Rotary-wing designs allow the craft to hover and move in any direction, at the cost of requiring closer proximity for launch and recovery.^{[citation needed]} Flapping-wing-powered flight has yet to reach the same level of maturity as fixed-wing and rotary-wing designs. However, flapping-wing designs, if fully realized, would boast a maneuverability that is superior to both fixed- and rotary-wing designs due to the extremely high wing loadings achieved via unsteady aerodynamics.^{[citation needed]}

Aerodynamics/aeroelastic aspect

The range of Reynolds numbers at which MAVs fly is similar to that of an insect or bird (10³ - 10⁵). Thus, some researchers think that understanding bird flight or insect flight will be useful to designing MAVs. The flapping motion used by birds and insects to produce lift involves aeroelasticity, which introduces structural considerations. Unsteady aerodynamics are also present in this type of motion.

Bio-inspiration

A new trend in the MAV community is to take inspiration from flying insects or birds to achieve unprecedented flight capabilities. Biological systems are not only interesting to MAV engineers for their use of unsteady aerodynamics with flapping wings; they are increasingly inspiring engineers for other aspects such as distributed sensing and acting, sensor fusion and information processing. Symposiums bringing together biologists and aerial roboticists have been held in 2007^[2] and some books^[3][4] have recently been published on this topic.

Practical implementations

In January 2010, the Tamkang University (TKU) in Taiwan realized autonomous control of the flight altitude of an 8-gram, 20-centimeter wide, flapping-wing MAV. (See [1] for video download.) The MEMS Lab in the TKU has been developing MAVs for several years, and since 2007 the Space and Flight Dynamics (SFD) Lab has joined the research team for the development of autonomous flight of MAVs. Instead of traditional sensors and computational devices, which are too heavy for most MAVs, the SFD combined a stereo-vision system with a ground station to control the flight altitude,^[5][6] making it the first flapping-wing MAV under 10 grams that realized autonomous flight.

In 2008, the TU Delft University in the Netherlands developed the smallest ornithopter fitted with a camera, the Delfly Micro, the third version of the Delfly project that started in 2005 (see Delfly.nl for photographs). This version measures 10 centimeters and weighs 3 grams, slightly larger (and noisier) than the dragonfly on which it was modeled. The importance of the camera lies in remote control when the Delfly is out of sight. However, this version has not yet been successfully tested outside, although it performs well indoors. Researcher David Lentink of Wageningen University, who participated in the development of previous models, DelFly I and DelFly II, says it will take at least half a century to mimic the capabilities of insects, with their low energy consumption and multitude of sensors—not only eyes, but gyroscopes, wind sensors, and much more. He says fly-size ornithopters should be possible, provided the tail is well designed. Rick Ruijsink of TU Delft cites battery weight as the biggest problem; the lithium-ion battery in the Delfly micro, at one gram, constitutes a third of the weight. Luckily, developments in this area are still going very fast, due to demand in various other commercial fields.

The next goals are an ornithopter weighing just one gram with a size of 3 centimeters, and a helicopter with the same specifications. There are no biological examples of such a design, but Lentink is looking into the flight of hummingbirds.

Ruijsink says the purpose of these craft is to understand insect flight and to provide practical uses, such as flying through cracks in concrete to search for earthquake victims or exploring a radioactivity-contaminated buildings. Spy agencies and the military also see potential for such small vehicles as spies and scouts.^[7]

Robert Wood at Harvard University developed an even smaller ornithopter, at just 3 centimeters, but this craft is not autonomous in that it gets its power through a wire and is led along a rail.

In early 2008 the United States company Honeywell received FAA approval to operate its MAV, designated as gMAV in the national airspace on an experimental basis. The gMAV is the fourth MAV to receive such approval. The Honeywell gMAV uses ducted thrust for lift, allowing it to takeoff and land vertically and to hover. It is also capable of "high-speed" forward flight, according to the company, but no performance figures have been released. The company also states that the machine is light enough to be carried by a man. It was originally developed as part of a DARPA program, and its initial application is expected to be with the police department of Miami-Dade County, Florida.^[8]

Flapping Wing MAV

Wright State University (WSU) in Dayton, Ohio is home of the Center for Micro Air Vehicle Studies (CMAVS) ^[9], an Ohio Center of Excellence. WSU has developed multiple Flapping Wing Micro Air Vehicles (FWMAV). The most stable platforms are 4-wing clap-and-fling models similar to those produced by DELFLY. These models are controllable enough to land and take-off from a person's hand while carrying a 1.5g color camera. Videos can be found on YouTube.com search WSU MAV. In addition to the stable 4-wing platforms they are also producing 2-wing, tailless, models which more closely mimic the flight characteristics of a hummingbird.

Practical Limitations

Although there are currently no true MAVs (i.e., truly micro scaled flyers) in existence, DARPA has attempted a program to develop even smaller Nano Air Vehicles (NAVs) with a wingspan of 7.5 centimeters. However, no NAVs meeting DARPA's original program specification were forthcoming until 2009. AeroVironment has recently demonstrated a controlled hovering of DARPA's flapping-wing NAV.^[10]

Beyond the difficulties in developing MAVs, few designs adequately address control issues. The MAVs' small size makes teleoperation impractical because a ground station pilot cannot see it beyond 100 meters. An onboard camera allowing the ground pilot to stabilize and navigate the craft was first demonstrated in the Aerovironment Black Widow, but truly micro air vehicles cannot carry onboard transmitters powerful enough to allow for teleoperation. For this reason, some researchers have focused on fully autonomous MAV flight. One such device, which has been designed from its inception as a fully autonomous MAV, is the Entomopter originally developed at the Georgia Institute of Technology under a DARPA contract by Robert C. Michelson.

Given that MAVs can be controlled by autonomous means, significant test and evaluation issues continue to exist.^[11]

See also

• Aerial reconnaissance
• Entomopter
• History of unmanned aerial vehicles
• Hybrid Insect Micro-Electro-Mechanical Systems
• Miniature helicopter
• Miniature UAVs
• Model aircraft
• Surveillance
• Unmanned aerial vehicle (UAV)

References

1. ^ Weiss, Rick (2007-10-11). "Dragonfly or Insect Spy? Scientists at Work on Robobugs.". Washington Post. http://www.washingtonpost.com/wp-dyn/content/article/2007/10/08/AR2007100801434_pf.html. 
2. ^ International Symposium on Flying Insects and Robots, Monte Verità, Switzerland, http://fir.epfl.ch
3. ^ Zufferey, J.-C. (2008). Bio-inspired Flying Robots: Experimental Synthesis of Autonomous Indoor Flyers. EPFL/CRC Press. ISBN 9781420066845. http://book.zuff.info. 
4. ^ Floreano, D. and Zufferey, J.-C. and Srinivasan, M.V. and Ellington, C., ed (2009). Flying Insects and Robots. Springer-Verlag. ISBN 978-3-540-89392-9. http://www.springer.com/computer/artificial/book/978-3-540-89392-9. 
5. ^ Cheng-Lin Chen and Fu-Yuen Hsiao*, Attitude Acquisition Using Stereo-Vision Methodology, presented as Paper VIIP 652-108 at the 2009 IASTED Conference, Cambridge, UK, Jul. 13-15, 2009
6. ^ Sen-Huang Lin, Fu-Yuen Hsiao*, and Cheng-Lin Chen, Trajectory Control of Flapping-wing MAV Using Vision-Based Navigation, accepted to present at the 2010 American Control Conference, Baltimore, Maryland, USA, Jun. 30 - Jul. 2, 2010
7. ^ Bug-sized spies: US develops tiny flying robots
8. ^ Honeywell Wins FAA Approval for MAV, Flying Magazine, Vol. 135., No. 5, May 2008, p. 24
9. ^ WSU CMAVS: Wright State University Center of Excellence
10. ^ Benchergui, Dyna, “The Year in Review: Aircraft Design,” Aerospace America, December 2009, Volume 47, Number 11, American Institute of Aeronautics and Astronautics, p. 17
11. ^ Michelson, R.C., “Test and Evaluation for Fully Autonomous Micro Air Vehicles,” The ITEA Journal, December 2008, Volume 29, Number 4, ISSN 1054-0229 International Test and Evaluation Association, pp. 367-374

Further reading

• Thomas J. Mueller (Ed.), ed (2002). Fixed and Flapping Wing Aerodynamics for Micro Air Vehicle Applications. AIAA. ISBN 1-56347-517-0. http://www.amazon.com/dp/1563475170/. 
• Peter Forbes, The Gecko's Foot: How Scientists are Taking a Leaf from Nature's Book, Harper Perennial, 2006, pp. 161–179.