tank-looking machines are effective against bound- ing, tripwire-fused, and simple pressure activated AP mines, but are very expensive and destroy ground and.
Proceedingsof the 2002 IEEE InternationalConference on Robotics & Automation Washington, DC May 2002
Mobile pneumatic robot for demining Lino Marques, Michael Rachkov and Anibal T. de Almeida Abstmct- The enormous amount of anti-personal land mines scattered over many countries worldwide is a major humanitarian problem asking for an efRcient solution at an affordable cost. This article describes a new robust and low cost mobile robot that can transport equipment with about 1.5 its own weight over natural terrains with inclinations superior to 40 degrees. The robot mechanical structure is optimised for precisely scanning large areas of natural environments with land mine detection sensors. The paper also describes the control architecture of the robot with possibility to correct the motion direction. The land mine sensing system of the robot is presented. Keywords- Demining robot, walking robot, pneumatic robot.
I. INTRODUCTION
T
HERE exist more than 100 million active land mines spread over more than 60 countries worldwide. It is also estimated that every 20 min someone is either killed or maimed by a land mine. The cost of an Anti-Personnel (AP) mine is very low ($3 - $30), but the cost of manual removing a single mine is up to $1000, so for every mine cleared, much more are laid. At current demining rate, it would be necessary hundreds of years and more than $57 billion dollars to remove all the existing mines [l]. This situation is particularly serious because mines are laid mainly in developing countries without resources to pay the demining cost. In addition to inflicting physical and psychological damage on civilians, land mines disrupt social services and threaten food security by preventing thousands of hectares of productive land from being farmed. Demining is a dangerous and costly operation that can be carried out by robots. In demining, a robot must pass a mine-detecting sensor over all points of the land mine suspicious region. To do this, the robot must traverse a carefully planned path through the target region. The humanitarian demining mission consists in detecting and removing the land mines. To fulfill these tasks automatically it is necessary to have a mobile platform that can move across the rough terrain with landmine detecting sensors and a manipulator to remove the mines. For higher task security the platform can be tele-operated. The main technologies used for land mine detection The authors are with the Institute of Systems and Robotics, Department of Electrical and Computer Engineering, University of Coimbra, 3030-290 Coimbra, Portugal. E mail: { l i n o , rachkov, adealmeida}@isr.uc.pt
0-7803-7272-710U$17.00 0 2002 IEEE
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are the Metal Detector (MI)), the Ground Penetrating Radar (GPR), Infrared detection (IR), explosive trace/vapour detection and Nuclear Quadrupole Resonance (NQR) [2]. Metal detectors work by measuring the disturbance of an emitted electromagnetic fieId caused by the presence of metallic objects in the soil. Magnetometers are also employed, but almost exclusively for large ferromagnetic objects (e.g. UXO - Unexploded Ordnances). These sensors do not radiate any energy, but only measure the disturbance of the earth’s natural magnetic field. Modern landmines can have almost no metal parts. Although metal detectors can be tuned to be sensitive enough to detect these small metal amounts, this is not always practically feasible, as it will also lead to the detection of smaller debris and increases considerably the false-alarm rate [3]. Ground Penetrating Radar (GPR) systems work by emitting a short electromagnetic pulse into the ground through a wideband antenna. Reflections from the ground are then measured to form a vector. The displacement of the antenna allows to build an image by displaying successive vectors side by side. Any dielectric discontinuity in a propagating media, as for example the presence of a mine, will cause a reflection. Its intensity will be higher with increasing difference between the dielectric coefficients [4]. Mines retain or release heat at another rate than their surrounding, and during natural temperature variations of the environment it is possible, using IR cameras, to measure the thermal contrast between the soil over a buried mine and the soil close to it. Passive infrared imagery can depend quite heavily on the environmental conditions, but there exist some approaches to enhance detection efficiency, namely the analysis of the dynamic behaviour of the scene, the exploitation of the reflected IR radiation polarization and the use multispectral analysis (UV, visible and IR) for mine detection and minefield delineation. The active cooling with water jet sprays [SI or the active heating by microwave radiation can also be used to greatly improve the detection efficiency with IR cameras. It is already possible to use advanced chemical sensors for detection of explosive vapours near a land mine [6]. When gas sensors with enough sensitivity and speed to detect explosive traces in the field become available, it will be possible to use efficient olfaction based navigation algorithms to track an explo-
sive odour plume and localize a specific buried landmine [7]. The utilization of different sensing technologies in the same sensor block with the appropriate sensor fusion methodologies allows reducing the false-alarm rate. There are some known demining mobile robots or vehicles. Mechanical demining systems usually use heavy vehicles with capability to detonate A P landmines remotely and proof areas that have been cleared by means of rotating chains, rollers or ploughs. Those tank-looking machines are effective against bounding, tripwire-fused, and simple pressure activated AP mines, but are very expensive and destroy ground and water systems. Other disadvantages of these machines are the impossibility to detonate all land mines and the risk to become damaged because of detonations [8]. Another possibility is to use a rotating vehicle with hard steel wheels that sweeps some path, covering ground at narrow intervals. It can operate only on open, plane, or moderately sloping ground [9]. Lightweight wheeled vehicles, which are not supposed to trigger the mines they interact, can also be used in demining [lo]. However, these robots are not stable and robust for rough terrains. Combination of wheels and pedipulators were investigated as well 1111. Disadvantages of the design are small scanning possibility and unstable motion in rough terrain because of the wheels. Walking robots are more suitable to overcome obstacles found in natural environments. Some authors propose to use the robot legs also as tools for the demining task. A leg can have a unit to change tools, for example rotating grass cutter, shovel-shaped gripper t o remove the mines, etc [12]. However, such a complicated design is quite expensive and not so robust. The effort devoted to mobile robotic solutions would be more helpful if it were directed to simple equipment and low-cost robotic design [13].It might provide some useful improvements in stability and cost-effectiveness. Not expensive standard pneumatic components can be used for designing of the mobile platform for demining to solve this task [14], [15].If the cost of mine removing will be about the cost of the mine setting, the main advantages of using land mines will disappear.
11. DESIGNOF
THE ROBOT
A design of the demining pneumatic walking robot is intended to transport an on-board sensor block to the working zone and to scan the working zone by means of the sensor block. The sensor block have a weight up to 30 kg so the transport module of the robot should have firm and simple skeleton design to provide reliable motion of such a system along uneven 3509
Table 1. Main specifications of the robot. Specification Overall dimensions length width height Height of obstacles Inclination of the motion surface Weight (without equipment) Payload Number of pedipulators Degrees of freedom Transport speed Searching speed Sensor block
Description 750 mm 750 mm 280 mm up to 150 mm
up to 50 degrees
48 kg over 100 kg 8 12 1.2 m/min up to 0.5 m/s IR detection with microuave heating metal detector
GPR Control system Supply-air pressure DC voltage
On-board processor and PC controller 6 bar
24 V
and slope surfaces. A suitable drive system for this purpose is a pneumatic system that can provide necessary transport force and stiffness of the design at the same time. The design of the transport module of the robot is shown in Fig. 1. It consists of longitudinal pneumatic cylinders 1,2 and latitudinal pneumatic cylinders 3, 4 which bodies are connected symmetrically and have 200 mm stroke. Each pneumatic cylinder has two pedipulators that are fixed at the ends of piston rods. The pedipulator consists of a lifting cylinder 5 of 150 mm stroke and a foot 6 with toothed contact surface. The sensor block 7 is connected to the front part of the cylinder 3. A control unit 8 is placed between the cylinders and is connected to a supply system by means of a flexible tube. The pneumatic cylinders can be actuated in two modes. The first mode is a transport mode. In this case the longitudinal cylinder perform motion of its pedipulators with maximum velocity by full length of the piston rod. During this motion the robot is connected to the ground by means of the latitudinal cylinder pedipulators. They serve as support cylinders during motion in this direction. The longitudinal cylinder pedipulators are lifted. After the first step the longitudinal cylinder pedipulators must be connected to the ground and the latitudinal cylinder pedipulators must be lifted. In such a position, the sensor block can be
Fig. 1. Design of the transport module of the robot.
moved for one step towards the working zone and so on. The robot can change a motion direction to 90" by actuating the latitudinal cylinders as transport ones instead of the longitudinal cylinders. The rotation of the robot can be carried out by means of simultaneous motion of longitudinal or/and latitudinal cylinders in opposite directions while contacting with all their feet to a motion surface. The second mode is a searching mode. During this mode the sensor block must carry out searching functions and be moved along a scanning trajectory. This trajectory is performed by means of latitudinal and longitudinal cylinders, which are actuated with nominal searching velocity. A value of the searching velocity is determinated by characteristics of
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I i I I
the sensor block. The main specifications of the robot
I-
are shown in Table 1. 111. CONTROL SYSTEM OF
of a zone of danger. The central computer analyzes the motion and sensor information. An operator can change a mode of motion according to a situation. The transport mode algorithm carries out by means of a comljination of longitudinal, latitudinal and rotation motion from an initial position. It is possible to perform three different kinds of rotation of the robot. All of them are based on a frictional principle. A supported rotation is carried out by means of motion of the feet of one of the transport cylinder respective the parallel cylinder fixed feet. This mode is intended for small correction in motion direction. An accelerated rotation its fulfilled by means of motion of the feet of the both parallel transport cylinder in opposite directions at the same time. This mode permits to perform relatively rapid rotation of the robot body. A safe rotation is carried out by means of the all transport cylinder in opposite directions in low position of all pedipulators at the same time. Such position of the pedipulators allows to provide a safe range of pedipulator lateral forces during rotation for all types of motion surfaces because a lever of the piston rod is minimal. The searching mode is carried out along scanning trajectories, which are combined of latitudinal and longitudinal motion with the searching velocity. Transport and scanning trajectories of the robot are shown in Fig. 3.
-i
1
I 4
THE ROBOT
The control system of the robot (Fig. 2) consists of an on-board computer, which is connected to a central computer and by means of an interface to the two valves V1 of the longitudinal cylinders LNC, two valves V2 of the latitudinal cylinders LTC, four valves V3 of the longitudinal lifting cylinders LLN and four valves V4 of the latitudinal lifting cylinders LLT. All 5/3-way valves have a closed neutral position and two flow positions. The flow positions provide motion of the piston rod in opposite directions and the closed neutral position is a "Stop" position, which locks a piston rod of a cylinder in a desirable point according to a control signal. All valves are supplied by compressed air. The on-board computer fulfills control algorithms of the robot motion and receives information from the sensor block of detection equipment and from positional sensors installed on each cylinder of the drive system. The on-board computer is controlled by means of a central computer that is placed outside
Fig. 3. Transport and scanning trajectories of the robot.
The shown scanning trajectory of the robot allows scanning all surface of the minefield without rotation motion if there are no essential deviations from the set trajectory. If it is necessary t o correct a trajectory, the rotation motion can be used. To overcome obstacles while motion along a rough terrain a feedback control is used for positioning of the pedipulators. A diagram of the adaptive pedipulators positioning is shown in 3510
Demining
sensor block
controller
n
Fig. 2. Control system of the robot.
Rotation angle, grad
0
Stroke, number 0
Fig. 4. Diagram of adaptive pedipulators positioning.
1
2
3
4
5
Fig. 5 . Experimental characteristics for the supported rotation. 1 - hard ground and stone, 2 - soft ground and sand, 3 - ground with vegetation.
Fig. 4. A lifting cylinder 1 actuates a piston-rod 2 connected to a foot 3 towards a motion surface. When a footplate 4 reaches the surface at any point of the plate, it contacts t o a position sensor’5. The sensor generates a control signal that is used by the local microcontroller to actuate a valve 6 to block the pistonrod in a level corresponding to a surface level under the foot. As result all of the feet can have different level according to surface roughness and obstacles to keep the sensor block on the same nominal level. To increase climbing possibilities of the robot, metal teeth 7 can be mounted to the footplate, if necessary. A protection screen can be installed on the footplate against soiling of the sensor during motion. The averaged characteristics for the supported rotation on different surfaces are shown in Fig. 5.
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A value of the rotation angle for one stroke has deviation about 5% for different strokes but averaged characteristics for all kinds of surfaces are linear. The angle value is proportional to a friction coefficient between pedipulators and motion surface. For example, the robot can be rotated through 45” on the soft ground and sand in four strokes. The averaged characteristics for the accelerated rotation are shown in Fig. 6. Accelerated rotation is more effective from the viewpoint of rotation speed. But this mode has a stroke angle deviation about 10%. The robot can be rotated through 90” on the ground with vegetation in three strokes. To perform small angle rotation it is possible to use a part of the stroke according to the experimental characteristics. It is necessary to notice that
Rotation angle, grad
Stroke, number 0
1
2
3
4
5
Fig. 6. Experimental characteristics for the accelerated rotation. 1 - hard ground and stone, 2 - soft ground and sand, 3 ground with vegetation.
Fig. 7. Temperature distribution in the cut of a volume of soil containing a landmine after 30 s exposure to microwave radiation.
sometimes the rotation on the ground with vegetation cannot be completed because of pedipulator wedging. In such cases, the safe rotation mode is more preferable. The rotation characteristics of the safe rotation mode are about the same as supported rotation characteristics. IV. LANDMINE SENSING BLOCK
A . Microwave heating and infrared detection A new sensor for detection of plastic or metallic landmines is being developed t o be integrated in the robot. The sensor is based on infrared image analysis during microwave soil heating and posterior cooling by convection [lS]. The current prototype uses a microwave klysiron emitting 1 kW power at the frequency of 2.45 G H z and two infrared detectors sensitive in the range of 8 - 14 p m (see Fig. 9). The use of two different IR sensors gives a possibility of data fusion in order t o eliminate the error from different surface radiation properties. Depending on the soil dielectric properties, the emitted radiation will be either absorbed, reflected or transmitted through. Common plastic material is transmissive, metals reflect the microwaves and wet soil absorb and convert the radiation to heat. Using this sensor it is possible to image thermal gradients in the soil surface and detect different rates of temperature changing depending on the soil content. The soil heating while exposed to microwave radiation was modeled with finite elements. Fig. 7 and 8 show the temperature inside a volume composed by sand with 15% humidity and a plastic volume (landmine) with 5 m diameter and 3 cm height buried at 1 c m depth from the soil surface. Fig. 7 shows a temperature increase in the soil surface of about 4 degrees after 30 s exposure to microwaves. The landmine is transmissive to the microwave, so it does not becomes hotter. Fig. 8 represents the temperature distribution
Fig. 8. Temperature distribution in the soil volume after 60 s diffusion.
60 s after the heating period (diffusion period). During this period it is possible to observe a temperature gradient in the soil surface superior to 2°C being the coolest region right above the landmine. Fig. 9 shows the experimental prototype assembled on a precision linear positioner for testing. It is possible to construct an infrared image by scanning the system over the target surface. Fig. 10 shows the output of each IR sensor for a line scan over an area with a plastic cylinder simulating a buried landmine. As can be seen in the picture, the ground surface just above the buried object has a temperature about 2°C less than an average temperature of the other parts of the surface. V. CONCLUSIONS The pneumatic robot was designed t o provide a lowcost solution for the demining problem. It is robust and stable in motion along uneven and slope terrain. The robot design implements a simple solution t o fulfill the scanning trajectories along minefields with the possibility to correct the motion direction in small an-
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Fig. 9. Active IR sensing system installed on a precision cartesian positioner for experimental testing. I
,
,
,
,
,
,
,
[ V I
Fig. 11. Pneumatic platform walking along a sloppy terrain.
27,
related t o humanitarian demining technology, products and practice,” Tech. Rep., EU EUDEM Project Report, July 1999.
T. Weiser,
“Minedetection and vehicle based widearea minedetection with metal sensors,” in First Int. Workshop on Robotics for Humanitarian Demining, 1998, pp.
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P.D. Gader, J.M. Keller, and B.N. Nelson, “Recognition technology for the detection of buried land mines,” IEEE Pans. on Fuzzy Systems, vol. 9, no. 1, pp. 31-43, Feb 2001. K. Wasaki, N. Shimoi, Y . Takita, and P.N. Kawamoto, “A smart sensing method for mine detection using time difference IR images,” in Proc. Int. Conf. on Multisensor Fusion and Integration for Intelligent Systems, 2001, pp.
Displacement [cm]
Fig. 10. Experimental results with the temperature measured by the two IR sensors during a line scan over a landmine.
133-139.
M. Krausa and K. Schorb, “Detection of TNT and chemical warfare agents by electrochemical methods,” in First Int. Workshop on Robotics for Humanitarian Demining, 1998. L. Marques and A. T. de Almeida, “Electronic nose-based odour source localization,” in Proc. 6th Int. Workshop on Advanced Motion Control, 2000, pp. 36-40. J.D. Nicoud, “Vehicles and robots for humanitarian demining,” Industrial Robot, vol. 24, no. 2, pp. 164-168, 1997. S.H. Salter, “Cartesian and hyperbolic conversion for Dervish decca control in command mode from CAD based minefield maps,” in First Int. Workshop on Robotics for Humanitarian Demining, 1998, pp. 89-108. J.D. Nicoud, “Robots for humanitarian demining: Plenty of solutions, easy to get funding, but are the deminers ready to use them?,” in IEEE Int. Conf. on Robotics and Automation, 1998, Workshop WS9: Robotics for humanitarian de-mining. G. Muscat0 and G. Nunnari, “Legs or wheels? WHEELEG - a hybrid solution,” in Int. Conf. on Climbing and Walking Robots (CLAWARB9), 1999. K. Kat0 and S. Hirose, “Proposition and basic experiments of shape feedback master-slave arm: On the application for the demining robots,” in IEEE Int. Conf. on Robotics and Automation, 2000, pp. 2334-2339. A.T. de Almeida and 0. Khatib, Autonomous Robotic Systems, Springer, 1998. Fr.-W. Bach, M. Rachkov, J . Seevers, and M. Hahn, “High tractive power wall-climbing robot,” Automation in Construction, vol. 4, no. 3, 1995. M. Rachkov, “Walking robot for slope surface motion,’’ SU patent 1706155, 1992. Lino Marques, Michael Rachkov, and Anibal T. de Almeida, “Detec$io e localizxlo de minas antipessoais utilizando robas mbeis,” Robdtica, ,no. 43, pp. 9-13,2001, (in Portuguese).
gular range. It can carry a landmine sensor block up to 30 kg weight. In the present implementation the robot was used with an active infrared landmine detector. The general view of the robot with the sensor block tested in natural environments is shown in Fig. 11. In the future, the landmine sensor block should be completed by means of two additional different sensors: a metal detector and a ground penetrating radar. At that time the local processing capabilities will be increased by means of an embedded PC with capabilities to grab images from an additional infrared camera and process information from the different landmine sensors. This fused information will increase the reliability of landmine detection.
ACKNOWLEDGMENTS This work was partially supported by the Portuguese Science and Technology Foundation (FCT/MCT) by project DEMINE, contract POSI/
36498/SlU/2000. REFERENCES [l] Landmines, “Mine action news,” Tech. Rep. 3.2, United Nations, 4th quarter 1998. [2] C. Bruschini et al., “Study on the state of the art in the EU
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