Aerial platform
The aerial platform of choice for the UltraSwarm project is the co-axial rotor helicopter. Recent advances in battery technology in particular the use of lithium polymer materials have driven the emergence of new rotary wing designs, especially in the toy and model markets. The demands of the marketplace also mean that these models are cheap, and very robust. Some additional advantages offered by the rotary wing solution are the ability to manoeuvre in three dimensions (very helpful for keeping position in a flock), and the ability to hover (very useful when there is a need to stay in place, e.g. for a surveillance task).
A disadvantage of using helicopters for the aerial platform is that they are inherently unstable in flight. Stability has therefore been a priority when deciding on the model of helicopter to use. With this criterion in mind two helicopters have been evaluated both of which employ a naturally stable system of two counter-rotating rotors that cancel out each others yaw effect.
The first helicopter evaluated was the proxflyer, a relatively simply model weighing just 45g and having fixed pitch rotors. The proxflyer was fitted with Gumstix onboard computer and a miniature wireless video camera. To compensate for the additional weight of the electronics it was necessary to upgrade the motors and batteries.
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| The proxflyer Model | |
Although the initial experiments with the proxflyer proved successful is was clear that the relatively simple design of the vehicle was being pushed close to its limits and a larger model would therefore be required. The next model considered was the Hirobo XRB-SR Lama.
The Hirobo XRB, weighing 195g, is a much more substantial machine than the proxflyer. The pitch of the lower rotor can be altered to control the lateral movement of the helicopter and the upper rotor is attached to a stabilising bar which is very effective in its purpose. Continuous flight of up to 15 minutes and excellent controllability make the Hirobo a good choice of model for the project
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| The Hirobo XRB-SR Model |
The Hirobo XRB series easily fulfils our requirements in terms of size, payload, and endurance.
Miniature Electronics
The second substantial component in our system is the onboard electronics which will perform sensing and computation. Miniaturisation, low weight, and low power consumption are fundamental requirements for this part of the system.
The chosen computational platform is the Gumstix Basix400, which has a 400MHz Intel XScale processor with 64MB of SDRAM and 4MB flash providing all the basic components of a computer on a single board. It is extremely small in size (8cm x 2cm x 0.63cm), and has a correspondingly low weight of only 8g. The board comes with a pre-installed version of the Linux operating system, which enables fast development of the on-board software.
Sensors are important components of every autonomous vehicle. In particular helicopters have poor stability qualities and an autonomous controller requires inertial sensing for the purpose of stability control. It is only recently that MEMS technology (Micro- Electro-Mechanical Systems) has made available small, light and relatively inexpensive inertial sensors - A complete 6 DoF IMU (3 axial accelerometers and 3 gyros) is now available in a self contained package with a typical volume of a few cubic centimetres, and weighing from 5 to 20 grams. These sensors make it possible to implement a stability system using inertial measurements on a model helicopter.
Navigation and other high level tasks demand an additional global localisation system. A passive infra-red tracking system installed in our test arena will be used. Each of the helicopters will be equipped with infra-red markers that will be simultaneously tracked by the system. This will provide each of the flock members with absolute position data.
Good quality colour single chip cameras with plastic lenses are nowadays available with weights as low as 5 grams. Unfortunately, the current version of the Gumstix computer board does not allow for real time image processing, and so the video stream must be relayed wirelessly to a ground based computer that will perform the necessary computations. The camera information will be used for high level tasks (e.g. target reconnaissance) where the communication problem can be tolerated.
Wireless connectivity will be provided between the flock members (and the ground station) to allow for data exchange and distributed computation. One choice for such communication is the 802.11b/g standard which allows for point to point connections at speeds up to 54Mbit/s. A small and light (5g) SDIO wireless LAN module can be interfaced with the Gumstix system, although it substantially increases the power requirements. An alternative temporary solution is provided by the built-in Bluetooth module present in the Gumstix board. The bandwidth of 723.2 Kbit/s is considerably lower than 802.11, but the low power consumption and the lack of weight penalties make this possibility attractive. The information exchange between the flock members consists essentially of positional data, and the bandwidth allowed by the Bluetooth standard is considered sufficient for this purpose.