Tom Shelley reports on the capabilities of robots
measuring less than 150mm across, the influence of nature and
some spin-off technologies
It is a well-known fact (based on scientific theory) that bumblebees cannot fly. But, as it turns out, not only can they fly, but they do so much more efficiently than any aeroplane or helicopter. Their secrets are efficient control, very smart materials, advanced aerodynamics and fast actuators with elastic return mechanisms. It seems that science still has some way to go to match the capabilities of insects, but attempts to emulate them in machine form are yielding many new technologies.
>A few years ago, the idea got around, mainly
from the US, that it might be possible to make micro air vehicles
with flapping wings, about 150mm across. The intention was and is
to make machines that could perform surveillance, move around
unobtrusively and hover with a minimum of effort - on the
battlefield or within buildings and malfunctioning chemical
plants. A swarm of such machines could constitute a formidable
weapon or force for construction.
Artificial insects for covert observation are still some way in the future, but more immediate expected benefits of this technology are expected to include artificial hearts similar to natural ones, and a host of new materials and micro-engineering techniques.
At a recent conference, Micro-robotics - can technology outsmart nature?, at the Institute of Materials in London, it became apparent that there has been a considerable amount of research effort aimed at making such machines work, including study of what might be learned from insects.
Serious work began with the passing out of small bottles containing live samples of what engineers might wish to emulate, in the form of Encarsia Formosa, a small parasitic wasp used in whitefly control, and only 0.6mm long.
The flight of the bumblebee
Professor Charlie Ellington from the Department of Zoology in Cambridge revealed that the bumblebee, and most other insects and hummingbirds, move their wings rapidly back and forth, twisting them by 120 degrees at the end of each stroke, so that they move through the air with an approximately 30 degree angle of attack. This generates a vortex at the leading edge that spirals out towards the wing extremity, increasing as it does so.
It is this vortex that creates suction and provides most of the lift - two or three times that which can be obtained by any kind of steady, forward motion. It works well for insects, because they only need to fly at up to 10mph, requiring an energy consumption of around 30 to 70W/g, as opposed to 150W/kg for a conventional aeroplane or the even larger figures associated with helicopters.
The problem with applying the technique to conventional aeroplanes is that the vortices create a lot of drag, but small machines can take great benefit. Conventional model aircraft are at the mercy of gusts of wind, while insects, with their smart, conforming wing shapes, can exploit them.
Insects show way to smart materials
Dr Robin Wootton of the University of Exeter explained how nature, rather than using metals to construct insects, instead invented advanced composites. One of the consequences is that insect legs and wings have mechanical properties that vary widely and can endure very large deformations. The hind wing of a locust has Young's Moduli varying from 0.3 to 9.5GPa at different points, allowing it to deform in an optimum manner. Similarly, spiders get around with very long spindly legs that act as shock absorbers and conformable actuators, yet rarely break. He also demonstrated a model insect thorax, a deformable monocoque, which allowed attached wings to be flapped using a single acting actuator between the front and back ends, although a real insect would use several muscles, not least because it needs to work its wings independently in order to manoeuvre.
Artificially reproducing such materials is difficult, but Dr Eoin O'Keefe of DERA Farnborough described how a very promising substitute can be made by reverse emulsion polymerisation. The process was originally patented by Unilever back in the 1980s, and uses water droplets dispersed in a special oil. When the oil is polymerised, the resulting structure consists of interconnected voids in a foam, with pore sizes as small as 100 microns across. When fibre-reinforced, the final structure closely resembles balsa wood, but can be made much stronger and non-biodegradable. The finest pore metal foams can be made using silica micro balloons 100 or 30 microns across in aluminium silicon alloys.
Dr O'Keefe also said that some of the most promising designs for micro surveillance vehicles under consideration by DERA neither fly, nor owe anything to insects or conventional vehicles.
As a demonstration, he set off a small, Chinese-made toy that uses an electric motor with a eccentric weight inside a hollow ball. The effect is to make the ball roll around, searching for ways to go further. It is droppable, can climb steps up to about one quarter its diameter, cross water or mud, and is much less prone to becoming stuck than either wheeled or legged vehicles of the same size.
A marine analogue, he said, might be an underwater flying wing able to change its buoyancy. Negatively buoyant, it could be made to glide downwards and, positively buoyant, it could glide upwards. Turning could be achieved by banking, also using buoyancy changes. Such an approach would do away with any need for small propellers or hydrovanes, both liable to become entangled on a really small AUV.
Artificial muscles have heart
Two of the big issues with micro robots are how to best make their actuators and how to power them.
Dr Anthony Kucernak of Imperial College presented a poster about his artificial muscles. These are made of membranes of Nafion, a flexible cation exchange resin based on perfluorsulphonic acid. When a voltage is applied between electrodes deposited on the two sides, negative cations move from one side to the other, dragging water molecules with them. The membrane then swells on one side and contracts on the other, bending as it does so. An actuator 12mm long, 2mm wide and 0.44mm thick produces a force of about 0.02N. Response time is about 2s, unless the device is excited at its mechanical natural frequency, in which case it can be made to vibrate at 100Hz or so depending on dimensions.
As well as being a candidate to flap wings in
micro air vehicles, it is also being seriously considered as a
basis for artificial hearts and other prosthetic devices,
directly replacing natural muscle and performing in a similar
Powering micro robots remains a problem. Dr Kevin Green from DERA Haslar revealed that his organisation is looking into methods of storing hydrogen for fuel cells for applications both large and small. The two most promising methods appear to be thermal decomposition of ammonia borane, which has a theoretical capacity of nearly 20%, and storage by carbon nanofibres and nanotubes. Articles on the Internet claim storage capacities of from 5 to 300%, according to whom the reader cares to believe. A previous paper on the subject given by DERA's Dr Darren Browning is available at www.h2net.org.uk/PDFs/Stor2000/H2nettalk_Nov00.pdf .
* Insects fly using much more efficient methods than
conventional aircraft, which poses considerable challenges to the
design of practicable micro air vehicles
* Smart materials can be made with properties approaching those used in the insect world by a combination of fibre reinforcement and reverse emulsion polymerisation
* Totally new approaches may produce the best results for micro land and marine vehicles, drawing on neither natural nor conventional engineering
* Novel actuation techniques for micro robots, such as
chemical muscles, may have considerable usefulness in medical
* Suitable power supplies could well be hydrogen fuel cells, awaiting the expected successful conclusion of the current world-wide research effort into finding better methods of hydrogen storage
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