Rescue Robot

Research article

Rescue robot module with slidingmembrane locomotionRezia Molfino, Matteo Zoppi and Luca Rimassa

University of Genova, Genova, Italy

AbstractPurpose – The purpose of this paper is to present a cost-effective design for a new rescue robot locomotion module using the principle of a continuoussliding membrane to achieve propulsion ratio (PR) near 1. Such high PR cannot be reached by other locomotion mechanisms that have been proposed.Design/methodology/approach – The paper first introduces the PR as a reference parameter to assess locomotion effectiveness of snake- andworm-like robots. The state-of-the-art is reviewed. A direction to step beyond getting PR near 1 is indicated. The way is by realizing a continuous slidingmembrane. Two solutions in this direction which have been recently proposed are recalled. It is shown that none of them can be practicallyimplemented to realize functioning systems with today’s available technology. A new design with membrane actuation has been identified and it isdescribed in detail. A prototype has been realized and earliest results and evidence of functioning described.Findings – Critical discussion of the concept of locomotion based on a sliding membrane was conducted. A new design for a robot locomotion moduleapplying this concept was presented. Earliest evidence of functioning and effectiveness of the new system proposed was given.Research limitations/implications – A new locomotion principle is shown. The state-of-the-art background is discussed. A design to realize the newsystem in a cost-effective way is described. The research implications lie in the future development of new mobile robots with higher locomotioncapability than today’s available systems. Several future research and development directions are shown.Practical implications – A new generation of more locomotion-effective snake- and worm-robots, especially for rescue application in rubble, isforeseen. The design proposed takes cost-effectiveness and practical realizability into account.Originality/value – The continuous sliding membrane concept had been already proposed but no reasonable realization and actuation solutions hadbeen singled out. The design of the new locomotion system is totally new and contains several breakthrough ideas. A prototype is available provingworthy in concept and functioning. It is cost-effective and this will allow shorter application to real robots.

Keywords Robotics, Search and rescue, Membranes

Paper type Research paper

1. Introduction

The support of mini- and micro-rescue robots to human

rescue teams in calamity response operations for search and

localization of victims in collapsed buildings and disaster sites

can be very relevant. Rescuers cannot access areas that might

further collapse and in many cases debris are so tangled that

humans cannot move inside or under high risk. Where trained

dogs are not available or not enough random excavation is the

only possible way with long time spent.Rubble environments are divided in three classes based on

structure and size of voids available (Murphy, 2004): semi-

structured (partially fallen buildings whose original layout is

globally preserved); confined (fully collapsed constructions

with voids comparable to human size); sub-human confined

(fully collapsed structures bring into very compact debris with

very tight voids).

Confined and sub-human confined spaces can be explored

only using worm rescue robots.Snake- or worm-like architectures with small sectional area

appear the best especially the highest is the compliance of the

robot body to adapt to the geometry of the available paths.

Power and data connection of the robot to a ground station

outside the rubble can be realized by umbilical. Rescuers

can operate the robot to localize people and then plan saving

operations. Robot reconfigurability to adapt to mission/

environment requirements can be obtained by modular

architectures: a suitable sequence – number and assortment

– of modules with same or different sets of functionalities

(locomotion, steering, carrying sensors).One main research issue is locomotion. Several methods

have been proposed to move effectively in rubble

environment. The more the thrusting action is distributed

around the body of the robot, the higher is the overall

advancement capability of the robot and the lower the risk

that it gets stuck because some passive part of its body (not

generating thrust) touches the environment.

The current issue and full text archive of this journal is available at

www.emeraldinsight.com/0143-991X.htm

Industrial Robot: An International Journal

35/3 (2008) 211–216

q Emerald Group Publishing Limited [ISSN 0143-991X]

[DOI 10.1108/01439910810868525]

This paper is an updated and revised version of the paper originallypresented at CLAWAR 2007 10th International Conference on Climbingand Walking Robots and the Supporting Technologies for MobileMachines, Singapore, July 16-18, 2007.

211

The efficiency of a locomotion mechanism can be ranked

based on a propulsion ratio (PR) (Borenstein et al., 2005)

defined as the ratio between external surface providing thrust

and total external boundary surface.No rescue robots developed have PR really near one, which

is the ideal PR realized when all body boundary surface

contributes to locomotion. Some systems with the highest PR

are: Gembu (Kimura and Hirose, 2002) using wheeled

modules; Kohga (Kamegawa et al., 2004) composed of

tracked locomotion modules connected by passive and active

joints; ACM (Mori and Hirose, 2001) merging snake-like

locomotion to traditional locomotion in wheeled joint

modules; MOIRA (Osuka and Kitajima, 2003) and

Omnitread (Borenstein et al., 2005) with tracks distributed

around the body, making the robot indifferent to roll over

which is common to serpentine robots over rugged terrain.The research has moved in the direction of PR ¼ 1 by

progressively increasing the number of tracks around the body

of locomotion modules, Figure 1, top. Such increase cannot

truly lead to unit PR and the design complexity becomes soon

very high. About 5-6 tracks appear a limit difficult to

overcome in small robots.Design canbe simpler using trackswithflexible belts, Figure 1,

bottom.PR ¼ 1 is achieved in principle but, especiallywith long

modules, strips can overlap and part of the frame can contact the

environment with practical reduction of the PR.

2. Continuous sliding membrane locomotion

The real step forward is the realization of a fully moving body

envelope. This is challenging and only two development

experiences can be found in the very recent state-of-the-art.The former (Ingram and Hong, 2005; Ingram and Hong,

2006), more at a conceptual stage, addresses the actuation of

an elastic toroid membrane with oblong section by expanding

and contracting transversal rings integrated in the membrane.

It is proven that locomotion can be generated but no materials

are available today to realize the actuating rings.

The second system developed (Breedveld, 2006) is a rolling

donut with circular toroid section and wire actuation. Wires

are winded around the toroid and make it rolling like belts on

a tri-dimensional toroid pulley. Motors can be used to make

wires moving. A physical prototype has been realized using an

elastic pipe winded to realize the donut and two such donuts

can be mounted together to realize a longer locomotion

module stable during axial movement. An oblong section of

the donut to have an external cylindrical surface (required to

advance effectively in rubble) appears hard to obtain because

along the wire could thrust the membrane only at the round

extremities and a frame inside the membrane would be

necessary to maintain it with the desired shape.We have started from this research background a two-fold

activity aimed at the development of two locomotion modules

with PR ¼ 1 and oblong section for rescue robots: the former is

with rigid frame and uses traditional solutions for the actuation

of the sliding membrane; the second, still under conceptual

development, will be compliant to adapt to the shape of the

channel where it will advance. Several challenging design issues

raise ion both cases. The following presents the main design

steps and the final set up of the locomotion module with rigid

frame, for which a prototype has been realized.

3. Design issues

The locomotion module is designed to be assembled to other

modules to realize a longer snake- or worm-like robot. We can

imagine that, a head module with some conical shape to

penetrate rubble is separately developed and we can focus on

the realization of the module with cylindrical sliding surface.

The PR of a robot composed of the cylindrical surface module

plus the head (and possibly a tail module similar to the head

one) can be near PR ¼ 1 provided that, depending on the

case, the surface of the head gives as well some thrust. This

additional thrust is not really required to have PR ¼ 1 if the

robot can steer to insert in the available openings.Consider now the cylindrical locomotion module.

Figure 1 Increase of PR with the number of tracks (top) and flexible belt track design with PR ¼ 1 (bottom)

n=3

n=8 n=40

n=4 n=5 n=10 n=20 n→∞

n→∞

Pr

Rescue robot module with sliding membrane locomotion

Rezia Molfino, Matteo Zoppi and Luca Rimassa


Volume 35 · Number 3 · 2008 · 211–216

212

It is not reasonable to imagine that the membrane material can

split and recombine during functioning. Therefore, the

membrane (ideally imagined with no thickness) divides twofully separate space domains: the volume inside the membrane

and the volume outside. Two main problems are faced: first,

how to join the group composed of the membrane and its

internal frame to the notch frame of the module (which is therigid frame placed axially inside the locomotion module); and

second, the actuation of the membrane. The two problems are

strictly interrelated and they have been faced in parallel.Actuation splits in two further subproblems: how to make

the membrane moving and were placing the actuators:

provided that a moving mechanism is identified, if it operateson the membrane from inside and actuators are inside as well,

then the problem is how to provide actuation power.

Locomotion by anything embedded in the membrane bodywas not taken into account at this stage because of complexity.The connection of the membrane-frame group to the notch

frame is necessary to transmit the traction force from the

membrane to the rest of the robot. Independently from the

membranemovingmechanism adopted, nomaterial bonding is

possible to transmit this traction force. Two ways can beidentified instead: a coupling that can generate bonding forces

through themembrane and geometric bonding with distributed

contact forces. Both directions have been investigated.For the former, we have studied combinations of magnetic

fields generating repulsion forces and a stable equilibrium

configuration for the notch frame inside the membrane-frameassembly. Strong Neodymium magnets are cheap and can be

used to generate the magnetic fields; the considered magnets

layouts are shown in Figure 2. The major drawback of thissolution is the quantity of ferromagnetic dust, always present

in soil and construction material, that would enter and lock

the locomotion module very soon in real application.Then we decided to realize a geometric coupling by suitably

shaping the extremities of the frame inside the membrane.

The design, is further explained in Section 5.

4. Membrane actuation

Different principles and related mechanisms have been devised

and investigated to actuate the continuous membrane. Themain ones are briefly presented in the following. Complexity

and cost have been taken into account as steering criteria.

4.1 Worm gears

Two worm gear designs have been considered with barrel

(Figure 2(a)) and cylindrical (Figure 2(b)) gear geometry. In

both cases, a screw with rotation axis coincident withthe central axis of the locomotion module is adopted. The

membrane drag force generates at the interface between the

membrane and the worm of the screw.With barrel gear the membrane frame has a curved design

mating the rounded gear shape. The positioning of the notchframe in the locomotion module is provided by shape coupling.

The height of the worm increases from the sides to the center to

make gearing progressive. Worm rounding is studied to limit

local stress in the membrane and avoid scrapping. The largecontact areaalongall thebarrel gear results in safeanddistributed

drag pressures which also limit the friction force at the boundary

between membrane internal side and membrane frame.The direction of motion is easily inverted by inverting the

direction of rotation of the worm gear. A dead rotation angle

is present during which the torsional deformation in the

membrane is recovered and changes in direction before gear

rotation becomes again effective to generate drag.With cylindrical gear the membrane drag force is generated

in the same way as with barrel gear but there is no forcecomponent to constraint the internal frame axially inside

the locomotion module. The advantage is lower pressure onthe membrane and then lower stress but a different system for

axial internal frame constrain is required such as driving

wheels or shape coupling.The main drawback of gear actuation is the presence of a

membrane-worm friction force component orthogonal to thedesired drag direction which causes membrane torsion and

stress. This torsion is compensated by the friction betweenmembrane and membrane-frame in the case of barrel worm

gear and grooves in the frames can be adopted to guide the

membrane by shape coupling. Still the desired drag forcecomponent is small compared to the tangent component

mainly due to friction. (In the ideal case with no friction themembrane-worm force is orthogonal to the worm and the

major component is in the desired drag direction).A counter rotating worm gear can be added (Figure 2(b)) to

balance most of the coupling torque but this transforms the

problem rather than solving it because membrane stress in theregion between the two gears increases dramatically.To reduce both stress induced by friction and counter

torque a new design has been studied where the worm gear is

replaced by a crown of rolling elements, Figure 2(c). Grooves

on the notch frame balance membrane torsion, which is muchlower than with rigid gear but still present.Drawbacks of this alternative design are complexity (due to

the high number of components) and the small contact area

between rolling elements and membrane with risk of damage.

4.2 Spur gears

To overcome the drawbacks of worm gears a design with spur

gears has been developed, Figure 2(d), with gear rotation axesorthogonal to the longitudinal axis of the module. The teeth

of each gear penetrate in the membrane and pull it providing

drag force in the desired sliding direction with no membranetorsion. Drag is mostly generated by shape coupling instead of

tangential friction, which plays now a negligible role inmembrane actuation.Differently from the case with worm gear, the drag forces

are now applied at the spur gears contact areas only. Themembrane, which is continuous, distributes drag between the

gears so that at a certain short distance from the contact areasthe stress in the membrane becomes homogeneous. The spur

gears are actuated using one single motor and a long wormgear coaxial to the notch frame, which can transmit motion to

all spur gears at the same time.The main design issue is the distribution of the gears on the

notch frame to guarantee the most homogeneous distribution

of dragging stress in the membrane. Design parameters arenumber, radius and diameter of the gears and design is mainly

targeted to find a distribution of gears with no interferences.There are limitations to radius and thickness of the gears to

avoid membrane damage. This results in a bottom threshold

to the minimum angular spacing of gears on any sametransversal section of the notch and then a limitation in the

maximum number of gears that can be placed in the notchframe. This maximum depends on the notch frame radius as

well. To better the distribution of dragging forces and reduce




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membrane stress it is possible to use more sets of gears at

different transversal sections with an angular offset between

gears on different sets, Figure 3(c).

5. Final design with spur gears

The final design, shown in the detailed view of Figure 4,

groups the advantages of spur gears and geometric coupling

between membrane-frame and notch frame. Details of the

design solutions and the assembly sequence are shown in the

figure. First the notch frame is inserted in the membrane

frame (1); the axial worm gear to actuate the spur gears is

already assembled inside the frame. Then the spur gears are

inserted at the extremities with their axes positioned inside

half cylindrical grooves in the notch frame (2). The

membrane frame is cylindrical with two inward lobes at the

extremities; The gears are inserted oblique and then turned

and positioned below the lobes thanks to the play available

Figure 2 (a) Actuation with worm barrel gear; (b) cylindrical gears; (c) rollers; (d) spur gears

(a) (b)

(c) (d)

Worm barrel gear

Internal frame Internal frame

Internal frame

Internal frame

Membrane Membrane

Membrane

Membrane

Notch frame Notch frame

Notch frameNotch frame

Note: Magnetic notch coupling in all designs

Crowns of magnetsCrowns of magnets

Crowns of magnetsCrowns of magnets

Worm gears

Torque balancing groovesGear with helicoids

Block with spur gears

Figure 3 (a) Membrane assembly; (b) over stress and cumulation with wrong gear-membrane mating; (c) design with spur gears and end wheels

(a)

(b)

(c)

Block with spur gearsInternal frame

Membrane

Membrane

Membrane frame

Sliding wheels

Notch frameBonding




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between membrane and notch frames because the membrane

has not been assembled yet. A cover is fixed at each side of the

notch frame to lock the spur gears and the two half shells

composing the notch frame (3). Two additional covers

complete the assembly (4). The membrane is first winded and

bonded to realize a tube which is inserted in the module using

the spur gears to drag (5). Finally, the tube is winded around

the membrane frame, slightly stretched and bonded to get the

final membrane toroid shape (6).A polychlorophene micro foam is selected as membrane

material after an attentive evaluation of alternative elastomers

with suitable softness, compliance and mechanical and

fretting resistance characteristics. It is a quite common

material very low cost and easy to find in variable thickness

and density. Its more common use is for diving wetsuit and

thermal insulation. Structural glues and sealing chemicals

exist for bonding and sealing and it is possible to collapse the

foam structure at the surface of the material and for a

controlled thickness to improve surface resistance and better

the geometric coupling to the gear teeth. By stretching the

foam before surface collapsing treatment it is also easily

possible to realize layered membranes with pre-stressed foam

layer relaxing when winding the membrane around the

membrane frame.In order to improve locomotion energy efficiency the

friction factor between membrane and membrane frame

should be the minimum possible. With this aim, a nylon

fabric sheet is glued to the inner side of the membrane before

assembly with the weaving direction oriented along the sliding

direction to further reduce friction and a small amount of

siliconic lubricant is injected inside the membrane after

closing. The membrane frame is realized using Teflon.The notch frame, realized in PVC, embeds two sets of five

gears at each side. It is composed of four parts (two shells and

two covers) plus gears, transmissions and motor. The motor is

in the center with double shaft and worm gears at both sides

each moving one set of spur gears.After insertion and bonding of the membrane the play

available between membrane and notch frame is fully

recovered. The teeth of the spur gears slightly sink in the

membrane and the lobes in the membrane frame guide the

membrane along the profile of the spur gears. This geometric

coupling provides also to lock axial movements of the notch

frame.When the spur gears rotate the notch frame tends to

translate in one direction. The pressure between gear teeth

and membrane increases at that side of the locomotion

module and at the same time the membrane pulling force

increases due to the presence of the lobe in the membrane

frame. Finally, the external resistance to advancement of

the module is overcome and the module starts moving. Themechanism is conceptually similar to the one of traditional

tracks. The higher is the resistance of the membrane to move

(related to the force required to the locomotion module to

advance in the environment) the more the teeth of the spurgears penetrate in the membrane and increase membrane

thrust.Shape of lobes and number, thickness and radius of the

spur gears limit the stress in the membrane to a safe value also

when the motor applies maximum torque. The membrane is

guided to avoid local buckling and cumulation, shown inFigure 3(b).Electric power supply and signal bus cables run in the notch

frame and between the spur gears. Interfaces are available atboth module sides for interfacing to other modules.Because, the membrane is compliant no specific tolerances

are necessary in the manufacturing of membrane and notchframes. Cheap standard techniques like moulding can be

adopted. In order to simplify prototyping all parts are

designed for manufacturing with rapid prototypingequipment. For minimum cost, gears, some axes and other

small components are commercial LEGO parts.A prototype of module has been realized. The final size

is 110mm length and 50mm diameter plus membrane. Mass

is about 200 g. Testing has started with the aim to assess

locomotion force, actuation torque and efficiency. An

important part of the earliest testing campaign is dedicatedto the verification of membrane life and resistance.

6. Modular robot with unitary PR

The locomotion module presented can be used to assembleworm robots with various architectures. Distributed steering

for small bending radius of the worm and high-steering

capability can be realized with short locomotion modules and

steering modules between every two of them.A compact design variant also suitable for inspection of

pipes and structured channels is with one crown of gearsinstead of two on the notch frame and with the membrane

frame internally rounded as if the two lips described in

Section 5 had moved and become tangent. This rounded

shape matches the profile of the gears with minimumlongitudinal backlash when the direction of motion is

inverted.Externally the membrane is straight and the module has

oblong section although shorter than in the version with more

crowns of gears, as required to generate effective locomotion.Membrane pulling is not as well distributed as in the case of

multiple crowns of spur gears, but sliding friction is much

lower due to the shorter membrane frame.Owing to the compact shape, the motor cannot be

integrated in the module anymore. One motor for all

modules is placed at one side of the robot and a flexible

shaft is used to transmit motion to each and every module.The smartest solution appears using short rigid shaft in each

locomotion module and short segments of compliant pipe to

connect such shafts.For pipe inspection, the steering modules can be passive

and the robot is guided along the pipe by the pipe walls.

Smaller bending radii are then achieved.

Figure 4 Final design and details of the assembly sequence

12 3

4 5 6




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The same robot architecture with some re-design of themodules can be adapted to intestinal inspection. The mainchange should be in the membrane, to be replaced or coveredso that the external surface is rough and with good grip on theintestinal walls.

7. Conclusions

A locomotion mechanism to realize snake- or worm-likerobots with PR ¼ 1 has been developed and is presented indetail. It realizes the principle of continuous slidingmembrane to provide uniform thrust along all the robot body.The focus is on a locomotion module using this locomotion

mechanism. It is shown that conventional actuation solutionscan be used to move the continuous membrane and solutionsfor joining the different parts of the locomotion module(membrane, frame inside the membrane and main moduleframe) are singled out overcoming the issue of the twocompletely separate volumes divided by the membrane.Tests in simulation and preliminary tests of a physical

prototype are confirming the effectiveness of the designproposed.This work belongs to a larger research frame whose final

goal is a complete sliding membrane rescue robot withcompliant body for which a compliant locomotion module isunder development. The design will extend to the steeringmodules and the head and tail modules, necessary to obtainan overall PR ¼ 1. One challenging problem is to designsteering modules which do not interrupt thrust continuity byopening lateral windows in the robot body when changing theangle between two following modules. This would reduce theoverall PR.A further research subject addressed is the realization of a

long sliding membrane enveloping the whole robot body,including all locomotion and steering modules.The locomotion system has been patented.

References

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Corresponding author

Rezia Molfino can be contacted at: [email protected]




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