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The concept is one of a vehicle capable of traversing an anti-personnel minefield carrying mine detecting sensors or video cameras. The vehicle must be able to traverse rugged terrain and degrade gracefully in the event of damage.
The proposal is for an eight legged vehicle with emergent walking behaviour using pneumatic actuators and local materials where possible. These factors contribute to the simplicity of the basic vehicle and low cost if destroyed. The vehicle is designed to protect the mine detecting sensors at the expense of the vehicle if necessary. As a result, only this detection equipment is placed in protected spaces, and the legs in particular are sacrificial.
A fully equipped mine-detector vehicle needs to be able to complete a variety of possible missions...
Thus the vehicle part of the project has the following requirements...
For reconnaissance missions the navigation and other operating procedures are dependent on the terrain and available sensors and so will not be discussed further here.
For clearance missions the search pattern has a large impact upon the vehicle design.
The actuators are adapted 30 mm. Shadow Air Muscles which are both light and cheap and simple enough to be low technology manufactured (there is no machining, but they need labour). Four muscles control each leg arranged in opposing pairs in the manner used in the Zephyrus robot.
It is probable that a large number of muscles will suffer shredding in the event of an explosion. The muscles are designed to be easy to attach and replace, however.
This is one of the main research areas associated with the vehicle. The vehicle has to produce a walking gait in rough terrain. In addition it has to develop a gait with one or more legs missing.
The initial prototypes and computer simulations will have eight legs. The highest speed will see one leg on each side in the air at any one time. With 5 kg load allowed for each leg, this gives a maximum weight of 30 kg for the whole vehicle.
The length of the legs imposes restraints on the type of terrain. Some manoeuvring will be required for trees and hollows. It is not necessary for the vehicle to move very quickly. Although the legs are long, the actual steps will be short to prevent leg interference. The requirement of at least 1 kilometre per hour for reconnaissance imposes a movement rate of 1.1m per second per leg-tip at full speed. In search mode covering a square metre in 90 mins. (Approx.) gives a half hour for 1 metre of forward movement. The steps would be much shorter in this case; this also permits the leg to be rested on an obstacle, and shorter steps taken. Typically ten steps could be taken to cover one metre with a speed of 1-4 mm. per sec. of forward movement for those legs on the ground.
Following the classical subsumption architecture for the design of autonomous vehicles, we divide the control electronics up into layers in this way:
Every opportunity will be taken to integrate these components with the specific mine-detecting systems available. Thus these systems must be flexible and able to receive outside stimuli.
Other constraints on the electronics include survivability against severe shock, simple maintenance and resistance to high temperatures and humidities. The vehicle must also protect sensing equipment as far as possible from fragments and rain.
The main energy source will be either a petrol or diesel engine. This will eventually supply a combined compressor and generator topping up batteries. The initial prototypes will be built from standard components.
There are two options for the fuel tanks. Either to mount them in the protected spaces or to mount them sacrificially. The sacrificial arrangement (to be experimented with) is to mount two tanks on poles, forward and aft, a metre clear of the vehicle and the sensor probes. Fuel is gravity fed down fire retardant hoses wrapped around the poles. The tanks can be cheap plastic containers replaceable locally. It is probable that both tanks will be ruptured in the event of an explosion. Hopefully this will result in the fuel tank being torn from its mounting and thrown clear, taking the flammable fuel clear of the vehicle sensors. There should be enough fuel in the fuel lines for the vehicle to move clear of any burning wreckage assuming that the robot is still mobile.
The compressor and generator need to be of simple design fuelled by petrol or diesel. Ease of field maintenance and repair is vital. The compressor does not need to supply a large air reservoir, nor does it need to provide a high pressure; therefore it can be reasonably light. If possible components will be duplicated for survivability. All reservoirs and casings will be plastic, where possible, to reduce weight and to prevent metallic fragments contaminating the minefield.
Electricity has to be supplied to the mine detection sensors, navigation electronics, radio gear and air valves. Truck batteries are too heavy, so 6 V. motorcycle batteries (lead-acid) will have to be used. It is unlikely that anything else will be available locally.
The three main defences are:
There are two navigation problems: local whilst searching, and long range. The following are statements of design goals. Intermediate prototypes will have simpler systems.
Local navigation will be by ground pointing camera or sonar. Sonar is preferable as this also tells the vehicle how high off of the ground the sensors are, but a video device is more robust. The ground pointing sensor thus gives the same information as odometry and should be adequate for the last ten to fifteen metres of travel.
Long Range Navigation
Long range navigation could be by differential GPS, which is accurate to 1 to 3 metres resolution. Otherwise the operator has to walk 100 metres behind the vehicle using radio control.
There are several options as the vehicle is designed to be a general platform for a wide range of sensors many of which are not yet available. The vehicle body can be positioned at any height from 10 cm. to 60 cm. or more and makes available a 40 cm. wide, 1 metre long platform for sensor placement.
Possibilities include: Metal detector,The Alberta probe and Ground penetrating radar.
Many thanks to those companies
and organisations that have helped the project get off the ground. Many
thanks to Cambridge Adaptive for the supply of controllers.
© Marcus Baker and The Shadow Robot Company 1999
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