Abstract
Power tillers are very simple and versatile machines with large scale diffusion in developing countries, where they are commonly used both for agriculture and for transportation purposes.
A new integrated participatory approach that makes use of and improves local end-users knowledge has been used to design a new robotic system for humanitarian demining applications in Sri Lanka, using power tiller as core module. A demining machine composed by a tractor unit, a ground processing tool and a vegetation cutting tool is here presented together with results obtained from the first blast test on the preliminary version of tractor unit armouring. Different breakable connections between wheels and axle have been designed to cause physical detachment and interrupt the transmission of the shock wave released by the explosion of a mine under one wheel. Effects of explosions on different types of wheels and on the chassis have been recorded and commented.
Introduction
Landmines are deployed mainly in poor, developing countries. There, the local population's technical knowledge is limited and access to high technology components is almost nonexistent.
Moreover many developing countries affected by the landmine problem are agricultural countries where most of the people are farmers and have basic knowledge of the technical and operational background of agricultural tools.
Therefore, low complexity, robust, reliable and user safe machines, based on simple agricultural and commercial equipment are considered highly desirable (Bryden, A. & McAslan, A., 2002).
In addition, humanitarian demining is too costly for affected countries to deal with it by themselves. This suggests that this activity is not sustainable for long time. Entirely depending on donors help also puts countries at risk: when attention decreases, funds may end up being moved to the newest, most attractive destination, giving donors higher visibility.
Research and development efforts in the field of technologies for humanitarian demining should aim to improve local capacities to help affected countries to deal with the problem by themselves in the near future.
A field study (Cepolina, E.E.; Bruschini, C. & DeBruyn, K., 2005) shows that high maintenance cost is one of the key factors behind the low adoption of machines by demining organizations. Demining machines currently in use in the field are complex systems that have not been conceived by the deminers who use them; nor have most machines been developed specifically for the environment in which they are being used (Cepolina, E.E, 2006a). These findings support the argument that there is a need to change the approach of designing new machines to one that is participatory and specially tailored to a specific demining environment.
A new participatory integrated approach to design low-cost demining technologies has been implemented and applied to the development of a new machine based on commercially available power tillers, for helping operations in Sri Lanka.
System Design and Preliminary Test
A one-month trip to northeast Sri Lanka was organized, at the beginning of the project. Deminers asked for their preferences during group interviews, expressed a strong desire for new small, light and cheap machines. They want machines to help in the most boring/difficult parts of their job, particularly cutting vegetation and processing the ground, specially the hardest one, currently scarified using a simple rake called heavy rake, to remove the soil hiding mines.
Therefore, the target of the project is to develop a modular system using as core module a power tiller and equipping it with modules specialised for ground processing and vegetation cutting. A special end-effector to process the soil and bring mines up to the soil surface is being designed. Each machine can be considered a semi-autonomous system, helping a single deminer in his work, and a certain number of machines can be controlled automatically to perform area-reduction operations, working as a multi-agent system.
Deminers will always assist machines: once a mine is found and lifted up on the soil surface by the special end-effector, a deminer can remove the mine manually. Manual mine removal has been introduced in order to lower the complexity and cost of the machines, as well as to allow a quicker integration of machines in operational procedures.
Design considerations
A modular top-down design approach was chosen. Starting from the task, defined by deminers, the mechanical modules able to accomplish the work were conceived. The machine exhibits modular structure, as two end-effectors compatible one to each other can be attached to the tractor unit: one is dedicated to ground processing and the other to vegetation cutting.
Therefore, the project involves the mechanical design of three modules:
tractor unit ground processing tool vegetation cutting tool
The project foresees also the implementation of the control of the machine that needs to be operated remotely, when Unexploded Ordnance (UXO) and fragmentation Anti Personnel (AP) mines are known to be present.
The overall essential requirements the machine has to satisfy are:
reliability: 100% clearance safety of operator: 100% depth of demining: 100 mm width of clearance: 1200 mm speed of clearance: higher than manual types of mines: small plastic blast-type AP mines cost: 20.000 € remote control distance: 20 m
Considered mines are small because they are between the less harmful existing ones, containing up to 50g of explosive only. UXO and fragmentation mines are also present, especially Claymore types, but only in certain known minefields. The machine is specifically designed to be proof against AP blast mines and to resist damages caused by fragmentation devices.
Cost is one of the main points of the project. The cheapest machine for humanitarian demining applications available on the international market, the Tempest which is classified as mini-flail, is used for cutting vegetation and costs US$120,000 (Koppetch, K., 2006). Although entirely produced in Cambodia, it is entirely specifically designed for demining purposes and therefore is inherently expensive.
Cost has to be contained as much as possible being one of the major causes of poor adoption of machines into demining programmes. The price of €20.000 does not include the research and developing cost and is equally divided between the modules and the control.
Attention is being paid to keep the machine extremely simple, avoiding adding complex modifications or tools to a very simple basic unit such as the power tiller. Therefore, parameters have been identified that contribute to achieve extreme simplicity and effectiveness, referred to as Simpleffectiveness.
The features the tractor unit has to present in order to be Simpleffective are:
forward/backward motion (traction) steering: 1m curve radius energy supply to end-effectors stability assessment of ground processing depth mine disposal safety of operator shock wave protection for machine (and operator, in manual use)
While the vegetation cutting tool is placed on the front of the power tiller to allow the machine to pass through thick vegetation, two different possible working configurations for the ground processing tool, designed to rake the ground and bring mines up on soil surface, have been identified: one with the ground processing tool at the back (G-P-B) and one with it on the front (G-P-F). The choice is subject to the evaluation of Simpleffective parameters values in the two cases.
A way to protect the tractor unit from shock waves caused by the explosion of a landmine under a wheel was researched first.
Following the idea of modifying the power tiller as little as possible, it was chosen to protect the tractor unit by allowing the wheel to drop off when explosion occurred. The idea was that if physical connection was interrupted, the shock wave transmission to the axle and to the power tiller drive train should be significantly reduced. Moreover we believed that damages to the wheel should be reduced as well.
Therefore, a breakable connection to be placed between the wheel and the axle was designed for withstanding normal working load conditions while breaking when an explosion occurred. The idea was to interrupt the transmission of the shock wave from the wheel touching the ground to the power tiller axle and subsequently to the gear train.
To connect modern wheels to the axle of the old power tiller it was necessary to design a supporting flange. The breakable connection is therefore designed to be fastened to the flange connected to the axle on one side and to the wheel on the other side (see Fig. 1).
The breakable connection (see Fig. 1) presents 8 holes, of which 4 are cone shaped to host counter sunk head M12 screws, used to fasten the connection to the flange, and the other 4 are simple holes hosting hexagonal head M12 bolts, used to fasten the connection to the wheel. The connection is 8mm thick and is shaped like a ring. In order to weaken the structure to favour failure in case of explosion, material between the holes has been reduced.
Assembly of wheel, breakable ring and flange and technical drawing of breakable ring.
In case of explosion and fracture of the ring, most likely to happen along the bridges, a new ring could be replaced simply by screwing it into the flange first and into the wheel later.
The breakable ring is made of cast iron ASTM 30 class.
A blast test was organized in May 2006. It took place under the supervision of Danilo Coppe, one of the major explosive experts in Italy, in an open air cave near to Fornovo, Parma.
The aim was to verify the suitability of the preliminary version of the tractor unit armouring, based on the concept of the breakable connection, to resist detonation of AP mines.
Together with the breakable ring, two other breakable connections were tested, simply constituted by two sets of screws, fastening the wheel to the flange, presenting double and single cuts along their body (see Fig. 2).
Technical drawing of single and double cut screws.
Wheels tested were common agricultural pneumatic wheels, filled in with two different types of elastic gums: GoFill tyre filler and GoSeal tyre sealer, kindly provided by GoGomma, who has also agreed to provide a workshop in the field in Sri Lanka with second hand machines necessary to fill in every kind of wheel with those special elastic gums, in case of positive test result. Five explosions took place, with landmines respectively underneath:
wheel filled in with GoFill wheel connected to the axle with breakable ring wheel connected to the axle with screws with double cut wheel connected to the axle with screws with single cut wheel filled in with GoSeal
As proper mine casings were not readily available, small plastic screw top containers were used instead. The containers were approximately the same size as real mines generally found in the Vanni region: P4Mark1 and Type72, 70 mm in diameter and 40 mm high.
Real mines contain 50g of TNT, but the explosive inside is compressed. To have similar blast effects, our explosive expert suggested the use of slightly higher quantity TNT for each mine, 55 g in each, and to fill in the remaining space with sand. The mines were prepared by placing the plastic bags containing 55 g of TNT inside the casing, after having cut a small hole in the bag to allocate the detonator. Before closing the casing and wrapping around adhesive tape, sand was added to fill the remaining space, approximately 20 mm high.
The soil was wet grey clay; temperature was around 25°C. Data from tests was recorded with two sensors: an accelerometer, able to measure accelerations within the range of −500g, +500g, placed on the axle under which explosions took place and a video camera placed approximately 7m away from the power tiller.
Data recorded with the accelerometer, through a laptop containing an acquisition card connected to the accelerometer through a long cable, was not synchronized with the triggering of the explosive charge nor with data recorded by the camera, as it was impossible to actuate all systems exactly at the same time manually.
Unfortunately, results from tests were not as expected and the preliminary armouring is not currently suitable to protect the power tiller from damages caused by the detonation of AP landmines under wheels.
In fact, the principle adopted into the design of the breakable connections proved not to work; although the wheel connected through the breakable connection dropped off during two of the three explosions, the physical detachment happened after the blast wave was already passed through the connection and entered the power tiller structure, as it can be seen from measures recorded by the accelerometer placed on the axle (Fig. 3). The blast wave travelling at supersonic speed passed through the medium before it had the time to deform and break. Nevertheless, the breakable connection worked at reducing the total energy transmitted by the blast wave to the power tiller, by filtering waves at high frequency. Therefore, the breakable connection designed acted more like a high frequency filter than a low frequency filter, as expected.
Test n04: zoom on the acceleration [m/s2] versus time [s] during time between 0,747s (when explosion started) and 0,9s (when the event ended); power tiller after explosion.
The wheel rims got damaged almost in the same manner after each explosion except for the second, when the mine was not placed directly underneath but slightly internally. Therefore, no advantages of using the breakable connection are visible on wheel deformations. The reason for this could be that they remain physically connected to the power tiller until the breakable connection breaks, absorbing the same low frequency waves of the blast wave that pass through the power tiller structure.
Even if not damaged too much by each explosion, the wheel filled in with GoFill could not keep on working normally after the fourth explosion. The wheel filled in with GoSeal could definitely not keep on working after the first explosion.
The energy released by the blast wave and passed through the power tiller during the fifth explosion was 5 times less than the energy passing through the structure during the first explosion and 3 times less than the energy passing through the structure when the breakable connection was in place, see Fig. 4. (Cepolina, E.E., 2006b). This suggests that pneumatic wheels filled in with air and liquid, in this case GoSealer, work at reducing the total energy transmitted to the structure better than solid wheels, filled in with GoFill. This result is in accordance with results obtained by Defence R&D Canada (Hlady, S. 2006) who conclude that detonations under water-filled tires transferred more kinetic energy upwards than either the standard or runflat tires.
Energy[J] versus frequency[Hz] passing through the power tiller in the five explosions (data1 refers to first explosion, data5 to the fifth).
Upon these test results, we have decided to place the ground processing tool on the front of the machine.
This lowers the possibility to have an explosion under the machine as landmines should have already been removed when the machine passes. In order to increase traction, rubber tracks will be added connecting driven wheels to additional smaller wheels supporting the ground processing tool on the front. Remote control of the tractor unit will then be possible by actuating the two semi-axles through brakes, to be added. Moreover, we believe that tracks could absorb part of the energy of an accidental explosion underneath by deforming flexibly.
Therefore, the system will work in the configuration shown in Fig. 5, with both the end-effectors on the front, the vegetation cutting tool supported by the same frame supporting the ground-processing tool.
Digital mock-up of power tiller with tracks.