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Are you absolutely certain that the fly sniffing around your lunch really is a fly? Or is it really Big Brother in disguise? Outrageous? Impossible? Well, no. It could happen – sooner than you think! By BOB YOUNG Micro Aerial Vehicles I N THE RECENT Bruce Willis’ scifi movie “The Fifth Element”, there is a scene in which the baddies send a remote sensor disguised as a cockroach to check on matters at the nerve centre. This poor little creature is abruptly sent to robotic cockroach heaven as a result of being suddenly swatted – scratch one multi-million dollar hitech toy. It was a funny scene and the audience chuckled at such crazy stuff. But is it so crazy? In 1992 the American Defence Advanced Research Projects Agency (DARPA) held a workshop on future technologies for military operations at Rand Corporation, the initiators of the micro air vehicle (MAV) concept. T h e n - s e n i o r- s c i e n t i s t B r u n o Aug-enstien led a panel on micro 4  Silicon Chip vehicles, including aircraft systems ranging in size from a hummingbird down to less than 1cm in diameter. Yes, 1cm; much smaller than a 5c piece! Rand published a widely-circulated report on the work in 1994. The Lincoln Laboratory was initially sceptical but its own research also concluded that MAVs were becoming feasible. What then is an MAV? DARPA’s own definition alludes to a class of significantly smaller vehicles than the traditional UAV (Unmanned Aerial Vehicle). An arbitrary size limit of 150mm has been imposed and to meet the definition an aircraft must not exceed this limit in length, breadth or height. To fully appreciate the quantum reduction in size, compare the proposed 150mm vehicle to the smallest UAV in current service. This is the US Naval Research Laboratory’s “Sender”, a conventional monoplane with a wing-span of 1.2 metres, weighing 4.5kg and featuring a range of 160 kilometres. From that, an MAV represents a startling step! DARPA held an MAV feasibility workshop in November 1995, a briefing to industry in March 1996 and a user and development workshop in October 1996. These were mainly paper exercises with little to show in the way of hardware. The main thrust of all of this from a military point of view is to provide the individual soldier with battlefield surveillance equipment, far in advance of that which currently exists. These aircraft must fit easily into a soldier’s battlefield pack with little trade-off in food and ammunition and require only one man for launch, control and recovery. Thus the aircraft must be autonomous in operation and hence it will not fly like a model aeroplane. These devices must perform reliably in the hands of unskilled operators under very trying conditions. The last thing a soldier in combat needs is to be totally engrossed in controlling a twitchy little brute of an aircraft. Auto stabilisation is therefore a major consideration in the design. I n 1 9 9 7 , D A R PA s t a r t e d a US$35-million, four-year effort to develop and demonstrate affordable MAVs. The agency wants aircraft with a maximum dimension of 152.4mm (6 inches), range of up to 10km and speeds of up to 80km/h for missions that span from 20 minutes to 2 hours. The development programs are aimed at producing vehicles for operations in three main environments: relatively open terrain, urban areas and jungle. These MAVs are to be deployed by hand, by munitions launch or from larger aircraft. Missions would include reconnaissance, targeting, placing sensors, communications relay and sensing dangerous substances. They are viewed as one-use, one-way missions. Stealth is to be a major factor in the design and thus electric propulsion is favoured in this regard. The hope is that such tiny surveillance vehicles will not attract any attention or better still, be mistaken for birds or insects. In 1997, DARPA’s Tactical Technology Office awarded nine Phase 1, US$100,000 small business innovative research contracts to either pursue system development or a specific technology and in so doing signalled that the chase is on. Are there any readers out there that hold any doubts that once the money starts to flow, answers will soon follow? The awarded contracts cover a variety of projects which include a hovering flying saucer called “Hyperav”, a 1.4f/lb thrust turbine, about 76mm long and 43mm in diameter, possibly for use in “Hyperav”, and a solid oxide fuel cell for MAVs which will provide sufficient energy to power a Photo 1: that’s not a giant butterfly, it’s a regular-sized Monarch with a wingspan of about 70mm or so. In front are the aircraft receiver and processor with the video camera at the bottom. The devices on the right are tiny (3mm diameter) actuator motors. On the left is the propulsion motor. 50 gram MAV for several hours as well as providing power for the payload. Ornithopters (flapping wing aircraft) are included in the list, as the problems of Reynolds numbers in extremely small aircraft steer development in that direction. The Reynolds number is an expression of the wing chord (width) to airspeed over the wing. Reynolds numbers reduce as the size goes down. Readers who have followed previous radio control articles will understand only too well just how large a part Reynolds numbers play in successful operation of aerodynamic devices. Flapping wings allow an increase in Reynolds numbers without increasing the size of the vehicle. The faster the wings flap, the higher the Reynolds numbers. The 150mm disc shown in Photo 2 has a Reynolds number of approximately 100,000. Urban and jungle areas tend to require hovering aircraft and orni-thopters are one way of achieving this result. Flapping wings also add to the stealth of the aircraft, as they become more difficult to distinguish from birds. Investigations are also under way into the use of piezoelectric transducers that would resonate thin metallic structures that will actuate the wings in ornithopters. Miniature engines (both internal combustion and turbine) and waste heat recovery devices are also included in the list of Phase 1 grants. There is absolutely no room for waste or inefficiency in these machines. An interesting project grant is for the development of a shirt-button size turbine that will be made of ceramic and produce 13 grams of thrust. The turbine in the engine will spin at 1,000,000 rpm! This will power a 50-gram MAV. One of the most serious problems facing the ultra-miniature aircraft is that of video power. High resolution and frame rate make it easier for an unskilled operator to fly the aircraft but that requires more power for the greater bandwidth. As the size of the aircraft shrinks, propulsive power and hence battery capacities go down but the video power required remains the same. Ultimately, continuing the shrinkJULY 1999  5 age means that all that is left is the video power source, a ridiculous situation. Yet even here, experimental work is already being undertaken into beaming microwave power into the vehicle. However, all of the foregoing is in the future. Let us now look at some of the more practical considerations affecting MAVs. Practical MAVs AeroVironment Inc has made the most hardware progress to date, with one 6-inch disc achieving 22-minute flights. Their Phase 1 study concluded that a disc was the best configuration for the open terrain option since it gave the most wing area and a relatively good lift to drag ratio. At first AeroVironment were achieving only a few seconds for each flight, then 10 seconds, a minute and finally, using NiCd cells, 2.5 minutes. The 22-minute flights were achieved using experimental high-energy lithium batteries costing US$200 each. Photo 2 shows one of the AeroVironment discs whilst Photo 3 shows a mock-up of a projected disc several years from realisation. A novel approach to control actuation is with the use of electrostrictive polymer artificial muscles. These would actuate the controls directly and change their length in direct proportion to the applied voltage. The disc shown in photo 2 uses a simple UHF receiver to give a small antenna size and the combined weight of the receiver with command processor and four actuators is under 3 grams. However, in a disc only three channels are required (throttle, pitch and roll) and this installation comes in at 2 grams. The actuators use “smoovy” motors made by RMB Miniature Bearings in Switzerland and are amongst the world’s smallest. They weigh a mere 0.35 grams each and a 25:1 reduction gearbox is available which boosts the weight by 0.5 gram! Sufficient power is available from the geared motor to drive the control surfaces directly via pushrods. The actuators measure 10.16mm x 3.05mm and are brushless. They move the controls in about 60 discrete steps via the command processor. The discs shown in Photos 2 and 3 use direct drive motors in which the 6  Silicon Chip motor shaft is connected directly to the propeller but this is a very inefficient method. By adding a reduction gearbox to the motor, larger diameter propellers with better Reynolds numbers may be fitted and the motor runs faster and uses less current. This also means that the efficiency of the whole system is better as the endurance of any given size of battery is improved as the current is reduced. However MAV designers are not as concerned with endurance as they are with control, stabilisation and navigation. AeroVironment has built a complete navigation package that weighs 4.5 grams and consumes negligible power. It consists of two gyros (1.8g), a compass (2g) and an anemometer (0.5g). The black & white camera is less than 25mm long and weighs 2.2g. Such is the state of the art at present. What of the future? Future developments By March 2000, the disc is to carry a colour camera, operate at 3km, have a 20-minute endurance and perform automatic flight to way points with dead reckoning navigation using airspeed and magnetic compass. This requires about 10 times more effective TV transmitter power to triple the video range. Various tricks will be used to compensate for this increase in power by using lower frame rates and resolution when possible, steering the ground antenna for higher gain and commanding less power if there is excess signal strength at the TV receiver. Assuming the MAV quest is successful and there is every reason to believe that it will be, we could shortly see the following scenario played out: “The small speck in the sky approaches in virtual silence, unnoticed by the large gathering of soldiers below. In flight, its tiny size and considerable agility evade all but happen-stance recognition. After hovering for a few short seconds, it perches on a fifth floor window sill, observing the flow of men and machines on the streets below. “Several kilometres away, the platoon leader watches the action on his wrist monitor. He sees his target and sends the signal. The tiny craft swoops down on the vehicle, alighting momentarily on the roof. It senses the trace of a suspected chemical agent and deploys a small tagging Photo 2: this disc plane is about 150mm in diamater and can carry a television camera aloft with a link back to earth. Power is required not only to launch and keep the craft flying but also to keep the video system working. Table 1: AeroVironment Proposed 150mm Disc Micro Air Vehicle • • • Line-of-sight operation within 1km radius 10 minute duration Black & white video payload Aircraft Subsystem Weight Peak Power (grams) (mW) Lithium battery 25 0 Propulsion motor 7 4000 Gearbox 1 0 Propeller 2 0 Airframe 4 0 Control actuators 1 200 Receiver & CPU 1 50 Downlink transmitter 3 1200 B&W video camera 2 150 Interface electronics 1 50 Roll rate gyro 1 60 Magnetic compass 1 180       TOTAL 50 5890 device, attaching it to the vehicle. Just seconds later it is back in the sky, vanishing down a narrow alley. Mission accomplished....” (From the introduction to the DARPA web site, www.darpa.mil/tto/mav/ mav_auvsi.html) This is not science fiction but a serious military aim being pursued with relentless determination by groups scattered all over the world. Before scoffing too loudly, spare a thought for the dreamers who have given us notebook computers more powerful than mainframes of 10 years ago or the dreamers who put a model size aeroplane on a solo flight across the Atlantic, as featured in SILICON CHIP last month. We dream and so it will be! The predicted range of 21st century conflict has influenced and motivated the new development. The shift toward a more diverse array of military operations, often involving small teams of soldiers operating in non-traditional environments (eg, urban centres) is already more than evident in post cold war experience. MAVs are envisioned as an asset at the platoon level or below. They will give the individual soldier on-demand information about his surroundings, resulting in greater effectiveness and fewer casualties. Probably the most commonly identified scenario for this type of vehicle is the classic over-the-hill reconnais- Average Power (mW) 0 4000 0 0 0 200 50 300 50 50 60 180 2890 sance mission in which the MAV ranges out some 10km, loiters for an hour or so and sends back real-time images of the terrain below and all of the surprises it may or may not hold in store. Allied to this type of mission is the use by road transport in which an MAV is sent along the road ahead to locate an ambush, downed bridges or road-blocks. However the most dangerous of all conflicts are the house-to-house fighting undertaken in urban situations. Here the MAV will come into its own. While the previous missions could (and actually are) undertaken by more conventional sized UAVs, only a hovering MAV could scout ahead in urban canyons or more demanding still, enter buildings to give the individual soldier a look at what is inside. The savings in casualties could be enormous. Thus there are great benefits to be derived from the quest for SC the successful MAV. Acknowledgments: AeroVironment Inc. (web site www.aerovironment.com) Aviation Week and Space Technology. June 8th 1998. DARPA web site. www.darpa.mil NICAD BOOST BATTERY GPS RECEIVER AND "X" ANTENNA ELEVON ACTUATOR RECEIVE/TRANSMIT CIRCUITRY AND ANTENNAS AIRSPEED AIRSPEED SENSOR SENSOR X,Y,Z, X,Y,Z, AXIS AXIS MAGNETOMETERS MAGNETOMETERS LITHIUM LITHIUM BATTERIES BATTERIES PITCH, PITCH, ROLL ROLL PIEZO PIEZO GYROS GYROS Photo 3: a mock-up of an autonomous MAV fitted with a video camera and downlink. The theory is fine but this MAV is several years from reality. JULY 1999  7