Low Cost Micro Exploration Robots for Search and Rescue in Rough Terrain

Steven Dubowsky Jean-Sébastien Plante

Massachusetts Institute of Technology 77 Massachusetts Ave. Cambridge, MA, 02139

Professor Penny Boston

New Mexico Institute of Mining and Technology 801 Leroy Place

Socorro, NM 87801

E-mail: dubowsky@mit.edu

Abstract— This paper presents a new concept for search and rescue missions, based on the deployment of a large number of small spherical mobile robots (“microbots”). The concept allows the search for human presence and biological or chemical agents in hard-to-reach places such as collapsed building rubble or caves. A large number (i.e. hundreds or even thousands) of cm-scale, sub-kilogram microbots are deposited over a search site. By using hopping, bouncing, and rolling, they would filter down to search subterranean spaces. The microbots would be powered by high energy-density fuel cells combined with dielectric elastomer actuators. They would be equipped with suites of miniaturized imagers and chemical detection sensors to conduct in situ assessment of biological or chemical presence. Multiple microbots would coordinate to share information and establish a local wireless communication network.

I. INTRODUCTION

Military and civilian search and rescue missions often deal with dangerous highly unstructured environments. For example, survivor search and rescue in collapsed building after earthquakes or terrorist bombing must be done quickly without further endangering the survivors or the searchers. Battlefield reconnaissance missions, such as the search for terrorists or military stores in caves pose similar challenges. Stealth is often a key requirement for military missions. Figure 1 shows two such potential search mission environments.

Current search methods and technology for caves or the urban rubble are limited. Remote imaging techniques to identify subterranean features, including ground penetrating radar, ultrasonic imaging, and resistive imaging, have been developed [1 , 2 ]. However, these methods are limited in resolution and depth due to soil properties. They also cannot detect the presence of military combatants or disaster survivors in difficult to reach locations. The “dog and pole” technique is still the best civilian search technique.

This paper presents a new concept for search and rescue missions using a very large number of low cost small

robotic agents, called microbots, that would penetrate and survey inaccessible refuges, such as urban buildings, rubble, tunnels and caves, including those occupied by enemy forces in military operations. They would greatly aid disaster recovery and rescue missions by pinpointing the location of survivors and victims.

The microbots would be equipped with mission specific

sensor suites, a unique mobility mechanism, and a high-energy micro fuel cell for very long life. A large number (hundreds or even thousands) of these small and inexpensive (largely plastic) ball-like robots could travel through the maze of debris or caves using a combination of bouncing, rolling and hopping. They could also “lie-in-wait” for long periods of time to detect any activity. The balls could be dropped onto the surface or into surface openings by troops, rescue workers, or possibly from the air, such as from a helicopter. They would then move downward through the debris labyrinth, some stopping at various locations. Using their sensors and communication devices, they would detect and send back to the surface signals such as detection of human activities, chemical signatures (e.g. from explosives, ordnance, or Nuclear Chemical Biological substances (NBC)) and their

Fig. 1: Potential environments for search and rescue microbots.

positions. A local area communications network would be established to the surface. Some microbots will intentionally lag behind like a trail of breadcrumbs to be relays in this network.

Having a relatively large number of the low cost and disposable microbots would mitigate the effects of obstacles in the environment on a team’s ability to perform its mission. Having some failure or be trapped would not have a major impact on mission success. This compares favorably to more conventional robot designs that might have more individual capability, but would be subject to mission failure with the loss of just a few agents.

In a situation with chaotic, obstacle-rich terrain such as the rubble of a collapsed building, the microbots could move over unstable areas that would be too hazardous for human rescuers or conventional robots to cross. With their small size, the microbots could enter pockets and crevices where survivors may be trapped, either through their own random motions or by directed motion.

Hopping robots and devices have been considered in the past for both military and space exploration applications [3,4,5]. These works, while interesting, have focused on the development of mechanical designs for mobility. Also, the designs considered were larger, with weights in the kilogram range.

In this paper, the fundamental feasibility of a large team of low-cost microbots for search and rescue mission is explored, including their actuation, power, electronics, sensors, and communication. The results suggest that this approach is very promising compared to more conventional search and rescue technologies.

II. MICROBOT SYSTEM OVERVIEW

Each robot will resemble a baseball in size and shape and have a mass on the order of 100 to 200 grams (see Figure 2). The system consists of a mobility system powered by dielectric elastomer actuators (DEAs), position and solid-state sensors, a fuel cell power source, command, communication and control elements. The micro exploration robots would be largely constructed of plastic at very low costs, thus permitting saturation coverage of large areas without post-mission recovery.

Each mobile micro explorer is designed to be simple, inexpensive, disposable, and readily adaptable to a range of sensor suites to meet different mission objectives. They would also be “stealthy,” which is key in many military missions. Their outer shell would be fabricated from plastic that would absorb a significant amount of the noise of their motion. Their fuel cell/DEA polymer actuation is silent and emits no detectable gases. They could lie quietly between motions, detecting any attempt by hostile forces to flee or remain undetected. With their long life (made possible by their micro fuel cell power supplies) they would be micro sentries able to lie in wait in a cave for months, denying its use to enemy forces. Projected microbot performance specifications are given in Table 1.

TABLE 1: ANTICIPATED MICROBOT PERFORMANCE

Mass (total) 100 g Diameter 10 cm Hop height 1.5 m Distance per hop 1.5 m Average hop rate 6 hops/hour Maximum hop rate 60 hops/hour Fuel use 1.5 mg/hop Peak power supply output 1.5 Watt

Figure 3 shows a conceptual model of a microbot. Each

microbot contains a computational and communication module, fuel cell power source, inertial sensors, suite of electro-chemical “sniffers” to sense the environment and detect human presence, and a locomotion module based on a polymer actuator mechanism. The microbots would be able to orient themselves according to sensor inputs or external command. To further enhance the robots mobility and efficiency, the outer skin would be made of polycarbonate or other similarly tough thermoplastic to allow the spherical robots to bounce and roll without breaking between hops.

Fig. 3: Microbot conceptual design.

Fuel Tanks

Communication Antenna

360o Camera

SensorsPowered

Foot

Plastic Shell (polycarbonate)

Mobility System

Fig. 2: Artist’s concept showing of a microbot.

Figure 4 suggests how a micro exploration robot would travel through a complex rubble environment. Some of the microbot might get trapped at some points in the rubble. Again, the low individual cost of the individual microbots makes the deployment sufficient numbers to absorb these losses and to have the team complete its mission. The microbots’ low cost makes their recovery unnecessary.

III. DETAILED SYSTEM DESCRIPTION

A. Mobility The microbot’s mobility is provided by a bi-stable

mechanism activated by a DEA (see Figure 5). Four small orienting actuators located around the main actuator can change the system’s attitude prior to jumping. Following a hop, the Microbots continue to move by bouncing and rolling.

The mobility mechanism is constructed of lightweight polymer materials. The microbot is weighted so that after one locomotion cycle of hopping, rolling and bouncing, it will return to a posture with its “foot” on the ground.

Dielectric Elastomer Actuators (DEA) The mobility system uses DEAs for their large strains,

lightweight, low cost, and inherently simplicity [6,7,8,9]. The operating principle of these actuators is based on the Maxwell (electrostatic) pressure generated by a strong electric field applied across a soft elastomeric material (see Figure 6). The equivalent Maxwell pressure is given by [10,11]:

2

0

−=

uVP d εε (1)

where 0ε is the permittivity of free space (8.85x10-12 F/m); dε is the material dielectric constant; V is the voltage applied across compliant electrodes, and u is the actuated film thickness.

The compressive Maxwell pressure generates expansion in the orthogonal directions. If the film is incorporated into a compliant frame with appropriate preloading, the orthogonal expansion is converted into useful mechanical work. Figure 7 shows a practical diamond shaped DEA [9].

10 cm

Fig. 7: Diamond actuator showing 100% strains.

Fig. 6: Dielectric elastomer principle.

Compliant Electrodes Elastomeric Film

Bistable Device

Main Actuator

Orienting Actuators

Fig. 5: Mobility system.

Fig. 4: Microbot working its way into a collapsed building.

Linear strains of approximately 200% are possible with the diamond actuator design [12]. When compared to conventional DC motor/gearhead combinations, DEAs contain 10 to 100 times fewer parts. Since they are all plastic and do not require transmissions, they are also much lighter and less expensive than conventional actuators. Recent research has showed how the reliability problems of early actuator designs can be avoided [9,13,14].

Bistability The deformation process that gives DEA its great

simplicity has the disadvantages of showing large viscous impedance at high speeds. Hence, current actuators show a limit at about 5-10 mm/s when using currently available elastomer materials, such as VHB 4905/4910 [9]. For this reason, DEA cannot directly provide the jumping power of microbots and a storage/release strategy based on bistability is proposed.

In the proposed strategy, jumping energy is pumped into an asymmetric bistable device by the main actuator (see Figure 5). At the end of the actuator opening stroke, the bistable device reaches its stable high-energy state. The actuator is shut down and, at the end of its return stroke, trigs the bistable device out of its high energy state, providing a fast, efficient energy release. The sequence is entirely controlled by the main actuator and doesn’t require additional actuators to latch or unlatch the bistable device, making for a very simple and robust mechanical design. A working prototype of the bi-stable jumping mechanism has been developed and is shown performing jumps of 10 cm in Figure 8.

B. Power

Power is critical role for long duration missions. The proposed microbots uses miniature fuel cells such as those shown in Figure 9 for power generation [15]. The use of bi-stable mechanisms with DEAs lowers the peak power consumption necessary for hopping, which in turn enables the use of high efficiency/low power devices such as fuel cells.

Analyses have shown that fuel cells powered microbots

offer significant mass reduction for long range missions over similar battery powered units. Figure 10 shows the ratio between the mass of a fuel cell system and the mass of a lithium-polymer battery system (200 Whr/Kg [16]) as a function of microbot lifetime. The system lifetime is expressed in the total number of hops a microbot can make before depleting its energy reserves. The results show that for lifetimes of less than about 100 hops, the weight ratio is greater than one, indicating that batteries would be a lighter energy storage mode for a short mission. However, for search and rescue missions in which microbots can perform more than 1000 hops, a fuel cell power system would have considerably lower weight than batteries.

The mobility mechanism, sensor suites,

communications electronics, and system microcomputers will all draw significant power. The mobility mechanism is estimated to draw a peak power of 0.2 W. The power required for the other subsystems will be on the same order of magnitude. Thus a power supply with peak output of 1.5 W could run these systems with intelligent power management (i.e. not all systems would run simultaneously). Current state-of-the art miniature fuel cells (see Figure 9) can generate 450 mW/cm3 of power continuously and would easily fit inside microbots [15].

0.1

1.0

10.0

10 100 1000 10000 100000

Number of Hopping Cycles

Fuel

Cel

l Sys

. Mas

s B

atte

ry S

ys. M

ass

Fig. 10: Performance of fuel cells vs. batteries.

10 cm

Fig. 9: Printed circuit board fuel cell.

10 cm

Fig. 8: Hopper prototype.

10 cm

C. Electronics Each microbot maybe be required to handle up to

several megabytes of data per day. In addition, microbots will need to relay data and command information from other team members and the central command center. Considering an entire team of microbots, this could become a very large volume of information. Therefore, it is crucial for microbots to possess on-board data processing and data reduction capabilities to minimize the amount of information exchanged.

Example of current miniaturized on-board data processing systems indicates that several megabytes per second of data can be processed within a volume of 12 cm3, mass of 10 g, and power of 500 mW, see Figure 11a [17]. Also, a Toshiba 4GB disk the size of a US quarter is shown in Figure 11b. It is reasonable to believe that next generation devices could further improve these numbers.

D. Sensors Microbots would possess a number of sensors, including imagers, chemical “sniffers” and microphones. The data recorded by these sensors could be combined to identify a human presence or a chemical substances such as an explosive. Miniaturized microphones are currently available and could be used to detect distress signals or specific sounds. Possible strategies to achieve miniature imagers and chemical detectors are discussed below.

Panoramic Imager Panoramic imagers would be used principally for

navigation and for visual identification of human presence. They would cover the range of visible to near infrared spectra (approx 400 nm to 1100 nm) with appropriate filters. Miniaturized prototypes of such cameras already exist, see Figure 11c [18]. Such CMOS image sensors have achieved a volume of 0.27 cm3, power consumption of 30 mW and weight of approximately 0.3 grams. Further advancements should lead to improved pixel resolution. The microbot concept might accommodate two such cameras mounted with a baseline spacing in the range of 70 mm to 90 mm (approximate human interocular distance = 70 mm) to yield stereo-based range images. This baseline spacing would be useful for close-range navigation in tight spaces. Range images might also be obtained with simpler monocular approaches such as optical flow.

Mass Spectrometer Mass spectrometers are primary instruments for

chemical characterizations. Conventional mass spectrometers use both magnetic and electric field properties to identify ionized molecules. The precision of these instruments relies on the measurement accuracy and stability of these fields. Conventional laboratory spectrometers characterize both solid and gaseous compounds with high precision. For a miniaturized system, spectrometers that use a radiation source to create the electric field (an ion mobility spectrometer – IMS) are most promising [19] (see Figure 11d). The spectrometer total volume is 0.6 cm3 and achieves the precision of parts

per billion. The use of this type of instrument is not simple. It requires sample preparation, such as a laser source to vaporize the sample, and a means to ingest the resulting gases. Research is currently underway to develop lab-on-chip micro gas analyzers with MEMS size dimensions and power consumption in the order of few milliwatts [20].

E. Communication

The main communications challenge is to establish reliable communications from subsurface to surface. Due to radio wave absorption by rock and/or debris, high power and very low frequency is required to communicate directly from subsurface to surface. A very large antenna would be needed for low frequencies, which makes this solution impractical.

At high frequencies the distance for reliable non-line-of-sight communication is small, preventing direct subsurface-to-surface communications via the cave entrance. A solution to this problem is to use the microbot units as communications network to relay information back to a central unit on the surface via the cave entrance, where it could be relayed back to command center (see Figure 12).

a) b)

c) d) Fig. 11: Example of miniaturized electronics components

and sensors.

Fig. 12: Illustration of subsurface communication.

Base Relay

Microbots

Microbots

Some microbots would be programmed to stop at various penetration distances into the cave and act as nodes in a communication network. Based on experimental results in terrestrial caves, non-line-of-sight wireless communication at a bandwidth of 2.4 GHz is possible up to distances of approximately 20 meters [ 21 ]. A simulation-based analysis of microbot subsurface communications showed that approximately 50 microbots acting as relay nodes could gain 1 km penetration distance while maintaining communication with the surface.

IV. FUTURE WORK

A first-generation microbot prototype is being developed to demonstrate autonomous hopping. The prototype will first use Lithium-Polymer batteries as an energy source and will later switch to miniaturized fuel cells. Preliminary estimations suggest that the 60 grams spherical robot (φ11 cm) will hop about 30 cm high. Future research progress should enable 1 to 1.5 meter high jumps.

In addition to this prototype, advanced simulations of microbots performance will be conducted. These simulations will study soil-robot interactions using terramechanics models, the robot’s mobility in rough terrain, and how the group behavior of a team of these robots can be optimized for the greatest search and rescue mission effectiveness.

V. CONCLUSION

Microbots use hopping, bouncing and rolling as a mode of locomotion to overcome obstacles in very rough terrains that are typical of disaster sites or combat zones. Their low costs enable them to be used in large numbers. Thus they could search substantial sites in relatively short times compared to conventional robotics technology that would use only one or two large robots. Because search time is critical, microbots could potentially become a powerful search and rescue tool. With current technological development rates, microbots search and rescue mission appear feasible within just a few years.

ACKNOWLEDGEMENT

The support of the Cambridge-MIT Institute (CMI) and the NASA Institute for Advanced Concept (NIAC) is acknowledged. The important assistance of Sauro Liberatore and Samuel Kesner is also acknowledged.

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