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Journal of Bionic Engineering 3 (2006) 195-208

Rolling in Nature and Robotics:

A

Review

Rhodri H. Arm our, Julian

F.

V. Vincent

Centre or Biomitnetic and Natural Technologies, Department ofMechanica1 Engineering,

University o Bath, Bath BA2 7AY, U K

Abstract

This paper presents a review of recent rolling robots including Rollo from Helsinki University of Technology, Spherical

Mobile Robot from the Politecnico of Bari, Sphericle from the University of Pisa. Spherobo t from Michigan S tate University,

August from A zad University

of

Qazvin and the University of Tehran. Deformab le Robot from Ritsumeijan Un iversity, Kickbot

from the Massachusetts Institute

of

Technology, Gravitational Wheeled Robot from Kinki University, Gyrover from Carnegie

Mellon University, Roball from the Universite de Sherbrooke. and Rotundus from the Angstrom Space Technology Center.

Seven rolling robot design principles are presented and discussed (Sprung central member. Car driven, Mobile masses,

Hemispherical whee ls, Gyroscopic stabilisation. Ballast mass ixed axis, and Ballast mass moving axis). R obots based on

each of the design principles are shown and the performances of the robots are tabulated. An attempt is made to grade the design

principles based on their suitability for movement over an unknown and varied but relatively smooth terrain. The result of this

comparison su ggests that a rolling robot based on a mobile masses principle would be best suited to this specific application.

their own rolling motion or external forces cause their rolling.

Keywords:

Rolling robot, tumbleweed, wheel, ballast, Gryoscope

Copyright

2006, Jilin University.

Published by Science

Press

and Elsevie r Limited.

All

nghts

reserved.

Som e wonderful rolling organisms are introduced and defined as active or passive depending on whether they generate

1

Introduction

It is important to distinguish between a true rolling

robot and one w ith jus t large wheels and a reaction point

with the ground. It seems suitable to define a rolling

robot as one that rolls on its entire outer surface rather

than just external wheels and does not need to react any

of the rotating torque against the ground. Thus they tend

to be spherical or cylindrical in form and have a single

axle (or no axle) and a com pletely active outer surface

i.e. a surface that is completely involved in the move-

ment.

A spherical rolling robot also has the following

advantages over a traditional wheeled robot o r walking

robot:

It

is

possible to enclose the entire robot system

inside the spherical shell and thus provide mechanical

and perhaps even environmental protection to compo-

Correspondingauthor:

Rhodri

H.

Armour

E-mail:

[email protected]

nents and equipment.

There are no body extremities that can hang-up

on obstacles.

The entire outer body is driven (or rotates) help-

ing the device to cover uneven or soft surfaces.

Spherical robots have no side to fall over upon

from which they ca nnot recover.

Spherical robots have a lower ground contact

pressure d ue to a w ider footprint than a simple wheel or

foot

so

they can be used for travelling on soft substrates

such as sand, snow, brush, or vegetation and for paddling

through water.

Rolling spheres can potentially move in any di-

rection and therefore can turn in place when meeting

obstacles over which they cannot roll. This makes their

control relatively simple.

Spherical robots can recover from collisions

easily.


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(3006)

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Significant numbers of rolling robots are under

development but i t is still in the early stage mainly for

the proofs of concepts being undertaken rather than

optimisation for maximum perform ance. The proof of

concept includes the design of the mobility system but

also takes significant consideration of the control s ystem

and path planning issues. Few of the prototypes have a

specific application in mind but there is obviously wide

research interest in rolling robots. Suitable applications

can be found once the rolling and control issues have

been solved.

2

Rolling in nature

By

looking toward nature. perhaps some lessons

regarding rolling can be learned and applied to rolling

robots. However. nature has not adopted a rolling m otion

in

the main and exam ples are rare. This is not to say

there are no rolling organism s, but only a few have been

discovered.

Rolling seems to be a secondary form of motion for

all known rolling organisms, the Tumbleweed plant

being

a

possible exception. There are also two variants

of rolling

in

nature which are here referred to as pas-

sive and activ e. Passive rolling requires extern al

forces, such as wind or gravity. to drive the movement.

Active rolling is where the organism expends its own

energy in achieving the rolling and thus controls its

rolling such that it is able to move

in

a specific direction.

2.1 Passive rolling

Passive rolling organisms include the Tumbleweed

plant, the Web-toed Salamander, Namib Golden Wheel

Spider, and the Woodlouse.

Rolling is a secondary form of movement for all

known passive rollers except the tumbleweed, since the

animals have legs for normal movement such as

searching for food. Rolling motion is only adopted as an

escape during attacks from predators since it allows

much higher speeds for these animals. The web-toed

salamand er curls itself into a hoop sh ape, with its dorsal

side outermost, and rolls down slopes with rocky

surfaces tar faster than would be possible by simply

running down the slope. The Namib golden wheel

spider (Fig. I ) cartwheels down sand dunes when at-

tacked by its nemesis he Poinpilidae tararitirla wasp.

It rolls down slopes at m K

or

20 rotations per sec-

ond. Som e types of woodlouse , curl into an armoured

ball when attacked or startled, and roll passively if on a

sloped surface.

There is very little information regard-

ing the speeds and mechanics

of

these animals rolling

motions.

The tumbleweed is novel i n

its

rolling since it

harnesses the force of the wind to roll over a wide area of

smoo th and flat land. Tum bleweed is a weed that lives

for only one season. Its most recognised form is one

that looks like a rounded skeleton of a normal shrub

(Fig.

2).

However, there was a time in its life when it

was a normal brightly green coloured rooted plant.

During the spring and summer, the plant grows very

much like a normal shrub. How ever.

in

autumn , after the

dry sum mer, a specialised lay er of cells in the plant stem

allows the dried plant to break away from its roots and

begin its wind driven journe y. During rolling, the plant

disperses up to

250

000 seeds over a wide area.

Each

seed is a small coiled embryonic plant wrapped by a

thin skin which is very different from typical hard se eds

Fig. 1 Namib wheeling spider rolling.

Fig. 2 Photograph of tumbleweed.


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Rhodri

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EV. Vincent: Rolling in Nature

and

Robotics:

A

Review

197

containing large nutrient stores. When moisture levels

increase later in the season, the seed germinates and

begins to grow starting the annual cycle.

It is the form of tumbleweed that ensures its ability

to roll. Th e plant is almost spherical in its rolling state

and its large surface area generated by the intricate

branch structure harnesses enough wind force to over-

come the plants weight. The centre of gravity of the

whole plant is not at the centre of the sphere and it is

thought that this causes the plant to bounce as it rolls

encouraging seeds to be released, and improving its

speed by keeping it airborne and removing the rolling

resistance with the ground31.

Tumbleweeding is pursued as a form of locomo-

tion for a range of rolling robots being developed by

NASA

Langle~~.]nd ESA. The direction control of

these devices is not considered in detail but it is felt that

using the wind as a free power source should enable

large unexplored areas to be discovered with very little

energy expended on the movement.

2.2 Active rolling

If developing an autono mous rolling robot, then i t

is probably active rolling that is most interesting since

robots using this sort of movement

are

able to choose in

which direction to travel. After an exten sive literature

search, only two active rolling organisms have been

discovered he caterpillar of the Mothe r-of-pearl Moth

and the stomatopod shrimp Nannosquilla decemspinosa.

2.2.

I

Mother-of-Pearl M oth caterpillar

The caterpillar of the Mother-of-pearl Moth

Pleum0,a rwalis)

when attacked with sufficient ag-

gression, fixes its tail onto a surface and pushes its

foremost segm ents backward quickly with its front legs.

Once the head reaches the tail, the caterpillar rolls into a

wheel shape (Fig.

3)

with its back outermost and con-

tinues to roll for up to five complete revolutions[61.

Speed of around

40

cm.s- has been measured w hich is

about

40

times faster than its normal walking speed.

This means that the caterpillar of 25 mm long will have

moved itself 125 mm in around

0.3 s,

enough to outrun a

predator. This form or retreat obviously surprises a

predator and give s the caterpillar valuable time to escap e.

Fig3 Photographs

of

the M other-of-pearl caterpillar

during rolling[61.

Th e series of rolls are all driven by a single imp ulse so it

cannot be considered a continuous rolling motion,

however, an external force, such as gravity, is not ap-

plied. Table 1 shows the details of the Mother-of-pearl

Moth caterpillar.

Table

1

Performance capability of M other-of-pearl

moth caterpillar

Capability Measurement

Diameter

(m) 0.008

Speed

d s ) 0.4

2.2.2

Stomatopod shrimp annosquilla decemspinosa

The

Nannosquilla decemspinosa

is a type of shrimp

that adopts an active rolling motion when washed up

onto a beach. They spend m ost of their time underwater

in their burrow s where they wait for prey. They swim

only a sma ll distance, up to one body length, out of their

burrows to collect food. Because of their elongated

bodies and short laterally projecting legs, adults cannot

walk when ou t of the water. Their long and low bodies

rest on the substrate and the friction is too large for legs

to drag them along an d the legs are not strong enough to


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Vo1.3 No.4

lift their body from the surface. When a wave washes

them from their burrows onto the beach and out

of

the

water. they perform up to

2

m of backward somersaults

(20 40 rotation s) returning to the The rolls

do not occur uninterrupted since the animal ends up

lying on its back after each cycle and then inputs another

pulse of energy.

A diagram of their rolling motion is

shown in Fig. 4. They are very flat

in

the dorsoventral

plane which imp roves rolling stability and, when rolling.

tend

to

lean downbeach so will eventually reach the

water,

The stability of each roll is good since for m uch of

the time the centre of gravity of the animal is close to the

ground

such

that

it

doesn't topple over. There is a defi-

nite pause after each somersault which also gives a

useful stable resting point. Table 2 shows the details of

the

Nannosquilla

decemspino

su

Table 2 Performance capabilityofNurinosquillu

decenispinosa

Capabil i ty Measurement

Diameter ( n i l 0.007

Length rn) 0

073

Speed d s ) 0-1

Static slope climbing ( ) 10- 30

3

Design principles of rolling robots

The eleven robots reviewed here provide the cur-

rent state-of-art of rolling robots. All are based on the

principle of moving the centre of gravity of a wheel or

sphere outside of the contact area which causes the

wheel or sphere to fall in that direction and thus roll

along. This is best displayed diagram matically as shown

in Fig.

5 .

Of the eleven robots, there are only seven funda-

men tally different design princ iples. It is difficult to

directly compare the individual performances of each

robot since none have been optimised for use specifi-

cally as

an

autonomous mobility platform o factors.

such as speed. obstacle traversabilty, or slope climbing

ability, are far from maximised. A discussion and

evaluation of the seven design principles follows with

details of the robots that have applied them.

3.1 The first design principle prung

central mem ber

The first design uses a single driven wheel on a

sprung member with a passive wheel on the other end

(Fig.

6).

Each wheel makes contact with the inner sur-

face of the sphere at opposite s ide and the sprung central

mem ber applies the contact force. Steering of the device

involves rotating the driven wheel around on its contact

patch within the sphe re using the inertia of a fixed mass

on the member to react the steering torque. The central

member has two motors. One drives the driven wheel

and the other steers the driven wheel.

A s the driven

Stationary

Rolling

\ ontact parch

/

Fig.

5

Rolling by moving centre

of

gravity.

Fig. 6

Sprung central member concept, solid arrow shows

rolling source, open arrow shows steering source.


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Rhodri H.

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Rolling in

Nature

and Robotics: A Review

199

wheel rotates, the drive mechanism moves within the

sphere unbalancing the sph ere and causing it to fall for-

ward and roll. Due to the arrangement of the masses on

the central member, the centre of gravity of the whole

sphere is situated beneath the geometric centre which

ensures stability.

This design should enable the device to stop in one

position and continue in an entirely new direction

without rotating or turning. However, of the two robots

developed,

roll^[^^

is now based on another principle

and the Sph erical Mobile Robot '] has not yet move d

from the existing test cylinder that has a supported cen-

tral axle. It seem s that this design is not popular with

research teams and the one that has pursued it to the

highest level have rejected it in favour of the Ballast

mass moving axis concept (section

3.7).

This is likely to

be due to the difficulties of ensuring a constant force on

the driven wheel whilst keeping the sprung member

running through the geometric centre of the sphere.

Steering may also be a problem as

it

relies on being ab le

to react the torque of the turning motors attempting to

rotate the driven wheel around on its contact patch. This

reaction can only occur through inertia of a mass so will

tend to increase the overall weight of the design. With

optimization it could be m ade to work but there are more

simple solutions.

Fig. 7 Rollo prototype one shown without spherical

enc~osure[~].

3.1.1 The Ro llo robot

Rollo robot was developed by Helsinki University

of Technology

&

Rover Com pany Ltd[ ].

It

is a ball

shaped exploratory robot platform. The initial autono-

mous prototype operated on the sprung central member

concept but this design was rejected in later prototypes

(Fig. 7). A specific explanation of why this occurred has

not been mentioned and no performance details of this

first prototype were presented.

3.1.2The Spherical Mobile Robot (SMR)

The S MR was developed by Politecnico

of B a ~ i ~ ' .

The design of the SMR is based on the sprung central

member concept (Fig.

8).

However, the prototype exists

only as a cylindrical de vice of

160

mmdiameter with the

drive mechanism pivoted around its central axis (Fig. 9).

The drive mechanism uses a DC motor to power two

Fig. 8 SMR design concept '].

Fig. 9 SMR cylindrical prototype ''.


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drive wheels by radio contro l. There is no speed control

since a specific voltage will move the device at

a

quicker

speed

on

a rigid surface than on a soft surface, such as

sand. This feature enables the robot to feel the terrain.

The device achieved a speed of 0.6 m. s but this was to

validate models of it performance so higher speed may

be possible. A complete summary of its capability is

shown in Table

3.

Table

3

Perform ance capability

of

SMR prototype


also has the added effect of smoo thing the surface for the

enclosed car.

The Sph ericle from University of Pisa / Siena is

an example which is an untethered sph erical vehicle that

can roll around on the floor (Fig. 1 1 .

Its movement is

based on the car driven concept with a small wheel

driven unicycle ca r placed

on

the inside surface of the

sphere. The mov ement of the car causes the whole robot

to accelerate, steer and decelerate. There are no details

of its performance presented in the literature.

Diameter (m)

0.16

Speed

d s ) 0.6

Other notes Cylindrical prototype only

3.2

The second design principle ar driven

The second design concept uses a small car

placed within the sphere allowing the whole robot to

move when the car drives around the inside his

is

very similar to the way in which a hamster, placed inside

a ball, moves around a room (F ig.

10).

It allows a device to stop in one position and then

move off in another direction without turning in place

so

long as the car inside can turn

in

place.

There are

drawbacks however.

The car cannot be guaranteed to

remain in contact with the inner surface of the sphere

and if the device were to tumble down a slope or step,

the car could become dislodged and end up resting on its

back within the sphere without being able to turn over

thus stranding the whole device.

When the device is

moving quickly over

a

non-smooth surface, the friction

between the car wheels and the inner surface of the

sphere will vary and perhaps loose contact all together.

This makes control difficult since simp le encoders can-

not be used on the cars wheels and there is no other

stable platform for range mea surements

or

cameras. The

simplicity of this design does enable a very lightweight

solution since the driving mass

of

the sphere is as far

from the geometric centre as possible and can therefore

be made as light as possible. It should allow for the

largest slopes and obstacles to be overcom e for this same

reason although the motion of the car is limited to the

lower half of the sphere. If considering how t he car

alone would move across a rough surface, the sphere

3.3

The third design principle Mobile masses

The third concept uses the movement of masses

along radial axles of

a

sphere to alter the centre of

gravity

of

the sphere such that can achieve movement

(Fig.

12).

The design allows movement

in

any direction

at any time since the centre of gravity of the device can

be located at almost any position within the sphere. Two

Fig. 10 Car driven concep t, solid arrow sho ws rolling

source, open arrows show steering source.

Fig. 11 Close up photograph

of

unicycle car within

Sphericle.


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Nature

and Robotics: A Review

20 1

Fig. 12 Moving masses rolling principle, steering is not shown but can be considered if a 4th radial axis

protrudes

out

of

the drawing.

of the robots based on this design have the same solution

each with four radial axes with masses moving along

them, but the deformab le robot uses a similar principle

of moving the centre of gravity by reducing the length of

shape memory alloy (SM A) actuators.

Of

all the possi-

ble design conce pts this one is the most difficult to create

linear movement in a plane since the movem ent of mul-

tiple masses need to be controlled simultaneously. In

some ways this can be considered as complex as a

multi-legged robot with numerous actuators per leg.

However, once suitable algorithms have been developed,

remote control should be easy but autonomy would be

complex due to the absence

of

fixed points for sensors.

Using moveable masses does en able the centre of grav-

ity to be located at the geometric centre of the sphere

when required and this could be useful if a wind driven

tumbleweeding motion is also desired.

3.3.

I The Spherobot

The Spherobot from Michigan State Univer~ity ~'

is a spherical omni-directional robot designed using the

mobile mass concept. In this conceptual robot four

radial spokes are fixed within a hollow sphere and

positioned w ith equal angles between them in 3D space

(Fig. 13). Along each spoke is a mass which can be

moved from one extremity (centre sphere) to the other

(sphere shell). Moving the masses radially along each of

their spokes in a systematic way would enable the sphere

to accelerate, roll at a constant speed, in a straight or

curved path or stop at a specific point.

No

working

prototype has yet been produced using this principle but

a significant amount of work has been done on the mo-

tion planning.

Fig. 13 Spherobo t design concept 31.

3.3.2 The August robot

The August robot by Azad University of Qazvin

and University of Tehran 41 s another example based on

the moving mass concept which has been prototyped

(Fig.

14).

It is autonomous via a radio link and control

base station observing the sphere from the outside. The

centre of mass of the device with the moving m asses in

equally spaced locations is exactly at the geometric

centre of the sphere thus the robot tends not to tip over

on flat surfaces when stopped. The four radial spokes

are located at 109.47 to one another. Each spoke has a

1.125 kg weight positioned along it that can be moved

using a stepper motor with 200 steps per revolution. An

external camera is used as feedback for the positioning

and control system. Th e device has successfully moved

across a surface in straight and curved lines and a com-

plete summary of its capability is shown in Table

4.


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Journal

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Fig.

14 August design concept .

Table 4 Performance capability of August robot

~

Capabilit) Measurement

for the SM A spokes is via

a

cable tether

so

it is not truly

autonom ous. The robot has successfully crawled up a

20 incline and jumped 80 mm vertically. The maxi-

mum speed is 260 mm over 10

s.

Possibly the most

interesting aspect of this design is that i t is also able to

jump meaning that with further development and an

on-board power supply, it could make an ideal all-terrain

robot. complete summ ary of its capability

is

shown in

Table

5 .

Table 5 Perform ance capability of Deformab le

robot


Mass

(kg) 0.003

Diameter

(m)

0.04

Speed

(

d s 0.026

Static slope climbing ( ) 20

Other Notes: Can jump

0.08

m (twice diameter)

Mass

(kg ) 12.13

Diameter m i 0.29

Speed

(m/s

(but

this is not maximum)

3.3.3 The Deformable robot

The Deformable robot was developed by Rit-

sumeikan University' I This 40 mm diameter tethered

deformable robot can roll and jum p using shape mem-

ory alloy (SMA) spokes within

a

soft rubber shell

(Fig

.15).

Both cylindrical and spherical versions exist

and are both based upon the moving mass design con-

cept. Although these prototypes do not have additional

masses as such. when a voltage is applied to the SMA

spokes they contract. moving the centre

of

mass of the

robot toward the rubber hoop.

By controlling which

S M A actuators con tract, the entire robot is able to move

along. The robot weighs only 3g but the v o h g e supply

3.4

The fourth design principle emispherical

wheels

The fourth design type uses the two hemispheres as

large all-encompassing wheels each driven individually

from their centres (Fig.

16).

This is perhaps the simplest

form of rolling robot.

It has a single axle and uses two

motors driving each wheel to control acceleration, de-

celeration and steering. Th e torque of

the

driving motors

is resisted by the pendulum mass of the enclosed chassis.

Steering is achieved by rotating each w heel at

a

different

speed

or

by counter rotating them for turning on the spot.

Whe n stationary, there is

a

known fixed vertical hanging

orientation of the central chassis allowing for the known

location of sensors. This design does have

a

number of

drawback s however. Each side of the sphere needs to be

able to rotate independently of the other so there cannot

Fig. 15 Photograph s of the Deformab le robot climbing a slope .


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203

Side Front

Fig.

16 Spherical wheels concept, solid arrow shows rolling

source, open arrow s show steering source.

be an effective environmental seal between them allowing

moisture and debris into contact w ith the control circuits.

There is a specific axis through the d evice which m eans

som e areas of its surface is not active and the device

could become stuck if resting on those axle end points.

The ends of the axle do however provide an excellent

location for external sensors.

3.4.1 The Kickbot

Kickbot by Massachusetts Institute of Technol-

~ g y ' ~ ~ 's an autonomous robot which rolls around its

environmen t and invited passers-by to kick it as a play-

thing (Fig. 17). The project had three objectives:

Build an autonomous robot from scratch, focus-

sing on a new control system rather than an off the shelf

one.

Create a robot w here falling over is not a failure

Experiment with various emergent behaviours.

The prototype is based on a hamster ball and a

polystyrene cha ssis. The two hem i-spheres of the ham-

ster ball act as large wheels and sit either side of a central

disc containing the control circuits, motors and sensors.

The central chassis also includes a cou nterweight which

is critical to the movement of the device, since as the

counterweight is raised by the motors, the centre of

gravity of the robot moves in front of its contact patch

and the robot falls forward. The two hem ispheres d o not

meet when assembled

so

the ball does not have a con-

tinuous rolling surface although it is able to roll in all

directions when acted upon externally or example

after a luck from a bystander. Each hemisphere is driven

mode (either self-orienting or orientless).

Fig. 17 Photographof Kickbot 61.

by a dedicated motor allowing a form of differential

steering so is able to turn in place. There are no details

of performance presented but given the objectives above

the project was con sidered a success by its inventors.

3.4.2 The Gravitational wheeled robot

The Gravitational w heeled robot by K inki Univer-

sity'17] is anothe r exam ple using this design principle.

Although this device has two wheels, it does not react

the torque of the two driving motors against the ground,

rather it uses a hanging pendulum mass on the central

chassis. The prototype has successfully climbed up

slopes and over obstacles (Fig. 18). It does not comply

with the entire definition of a rolling robot a bove but if

the wheels were replaced by half spheres then it would

have an entirely active outer surface. A summary of its

capability is shown in Table 6 and Table 7.

Fig.

18

Photograph of Gravitational wheeled r obot[171.


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Table 6 Performance capability of Kickbot


Diameter

m)

0.28

Other N otes: 0.03 m wheelbase

Tahle

7

Performance capability

of

Gravitational

Wheeled Robot

Caoabi i

t v

Measurement

~~

D i amet e r i m) 0.28

Mass ( k p )

Speed

rid\) 0.45

2 Wheels @ 1.13kg

Static slope climbing (') 10

Static obstacle climbing

m)

0.0

3.5

The fifth design principle yroscopic

stabilisation

The

f if th

design concept uses a gyroscope to stabi-

lise the rolling axis of a wheel and a separate motor for

forward movement (Fig. 19). An internal gyro spins in

the same plane and direction as the direction of travel but

when its axis is twisted to be non-parallel with the axis

of the wheel,

i t

causes the wheel to lean over and so

allows radiused turns. A separate motor rotates the outer

wheel around the chassis and the gyro using the chassis'

mass as a pendulum to react the torque and produce

acceleration and braking.

A

design based on this prin-

ciple cannot turn

in

place due to the fixed rotation ax is so

turns can only be made when moving along. The inte-

gral function of the gyroscope means that the device is

unlikely to be light weight and the stabilisation and

steering gyroscope require energy thus competing with

the energy required to achieve motion. Again the device

has a fixed axle through it mean ing that there are som e

Sidc

Front

Fig. 19 Gyroscopic stabilisation concept. solid a rrow shows

rolling source, open arrows

show

steering source.

non-active parts of its surface but the gyroscope should

be able to rotate the device off those points

if

required.

Gyrover by C arnegie M ellon University 81 is a

single wheel robot which is steered and balanced by a

single gyroscope (Fig. 20). It is not completely spherical

but the designers argue that its form gives better steering

response and highe r speed d ue to its smaller frontal area.

It has no problem recovering after falling on its side

since rotating the axis of the g yro self-rights the device.

Two radio controlled prototypes have been produced.

The first is able to traverse laterally ac ross a slope of 45

and has moved through a pile of gravel. The second

prototype has much improved electrical efficiency

mainly from improvements to the design of the gyro

e.g. it is now enclosed in a vacuum. The second proto-

type has also successfully moved across water by in-

flating a large tyre and paddling over the surface.

Summaries of the two Gyrover prototypes are shown in

Table

8

and Table

9.

Fig. 20 Photograph of Gyrover in action 81.

Table

8

Performance capab ility

of

Gyrover one


Diameter

(m) 0.29

Mass (kg) 2

Static

slope

climbing ( )

14

Speed d s ) 2.78

Traversed a 45 ramp moved

through a pile of gravel

ther Notes:


11/14

Rhodri H.

A m o u r ,

Julian F.V. Vincent: Rolling in Nature and Robotics:

A

Review

205

Table

9


of

Gyrover two


Diameter (m) 0.34

Mass (kg) 2

400%

improved battery life from redes-

igned gyro. Pneumatic outer tyre. In-

flatable tyre for movement on water.

Other Notes:

3.6 The sixth design principle allast mass

fixed axis

The sixth design principle rotates an off-axis mass

around within a sphere for linear rolling. Moving the

same mass toward one end of the rotation axis, either

linearly or by swinging, causes the device to lean and

therefore steer around a comer (Fig.

21).

It is not pos-

sible to turn in place w ith such a design sin ce the rotation

axis is fixed within the sphere. There fore, if the device

stops at a point, it must continue in that initial heading

whilst making a radiused turn. This solution has a dis-

tinct axis through the device and so some non-active

surface points.

3.6.1 The Roball

The Roball by UniversitC de Sherbrooke is an

autonomo us indoor mobile robotic toy (Fig. 22). It has

been specifically designed to engage children in their

play and has a sophisticated control system that deter-

mines its behaviou r. It is the mobility platform that is

interesting from a rolling point of view since it adopts a

rolling motion when moving across flat surfaces. It

moves by rotating its spherical shell around its internal

components on an axis that travels through the device.

Swinging a ballast mass from side to side enables the

robot to steer a path and for it to fall onto one side with

its face upperm ost for interaction with the childre n.

Fig.

22

Photograph of R~ball ~'.

The almo st spherical shell has a small flat band area

around it which ensures straight line movement. It has

been tested only on hard surfaces but is able to

roll

up a

slope of

8

(Fig. 23) and climb over a

0.79

mm obstacle.

Its speed has not been recorded in the literature but from

videos it has been estimated as

0.5

m 4 ' .

A

complete

summ ary of its capability is show n in Table 10.

Fig.

23

Photograph

of

Roball climbing a ~lop e ~' .

Table 10 Performance capability

of

Roball

Caoabilitv Measurement

Diameter (m) 0.15

Speed d s ) 0.5

Static slope climbing

( )

8

Mass

(kg)

1.81

Static obstacle climbing

(m) 0.00079

Side

Front

Fig. 21 Ballast mass fixed axis concept, solid arrow shows rolling source, open arrows show steering source.


12/14

206

Journal

of

Bionic Engineering

(2006)

Vo1.3

No.4

3.6.2 The Kotundux

Rotundus (Fig.

24)

was developed at the Angstrom

Space Technology Center but is now produced by a

Swedish company and is marketed as a autonomous

rolling spherical robot for security applications specifi-

cally in hazird ous environm ents. The robot is based on

the ballast mass fixed axis concept but details

of

its in-

ternal construction. power supply and control system are

axis concept (Fig. 26). The central axis is supported on a

track around the equator of the sphere allowing the bal-

last mass to rotate

in

any vertical plane. This prototype

has been tested on grass and sandy surfaces and works

well. It is able to climb slopes and roll over small ob-

stacles as predicted by design calculations although the

exact size of these has

not

been mentioned. A summary

of its capability is shown in Table 12.

held confidential.

It

has been tested on hard surfac es and

on snow and is completely sealed from the environm ent.

The device can travel at up to 15mph but no detailed

information is presented

in

the public literature. sum -

mary

of

its published capability

is

shown

in

Table

11.

Side Frotit

Fig. 25 Ballast mass moving axis concept,

solid

arrow shows

rolling source, open arrows show steering source.

Fig.

24

Photograph

of

Rotundu s moving across a

snow covered surface'20'.

Table

11


of

Rotundus


Diameter (m )

Speed

( i d s ) 6.7

Other Notes:

0.5 (estimated from photographs)

Completely sealed

from

the environment.

A

product intended

for

security applica-

tions

3.7

The seventh design principle allast

mass

moving

axis

The seventh solution is sim ilar to the sixth solution,

except that the axis

is

capable of being moved w ithin the

sphere and this achieves a change

in

direction (Fig. 25).

This enables the device to move off

in

any direction after

stopping but it is not possible to make radiused turns at

all since the direction of travel is always perpe ndicular to

the rotation axle.

The most recent prototype of the Rollo spherical

robot by Helsinki University

of

Technology

&

Rover

Com pany Ltd ). is based on the ballast inass moving

Fig.

26 Photographs

of

Rollo

prototype


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Rhodri H. Armou r, Julian F.V. Vincent: Rolling in Na ture and Robotics:

A

Review

207

Table 12 Performance capability of Rollo prototype three


Diameter (m) 0.24

Mass (kg) 3

Speed

d s )

0.5

Other Notes: Peak current consumDtion

0.8

A

4 Conclusion

From these different designs, it can be seen that

there are numerous solutions to the problem of rolling

and the most suitable design solution depends on the

specific application and requirements chosen by the

developers. If autonom y is impo rtant, then the most

robust solution is desirable and that is likely to be the

third or seventh solutions, Mobile masses or Ballast

mass on a moving axis. If movement in a confined space

is required then the first (Sprung central member), third

(Mobile masses) or seventh (ballast mass moving axis)

designs are suitable since they can turn in place rather

than having to make a curved turn. If wind driven

tumbleweeding is required as another mobility solu-

tion, then only the third design (Moving masses) is

suitable since the centre of gravity can lie at the centre

of the devic e. If move ment in wide open and flat

areas is required, then a more simple solution can be

used such as the second (C ar driven), fifth (Gyroscopic

stabilisatio n) and sixth (Ballast mass fixed axis) types. If

mobility across water is important then only enclosed

solutions can be used which rejects the fourth type

(Hemispherical wheels).

If each of the designs is scored on a scale of 1 to 3

(l=poor, 2=average, 3=good) against a series of pa-

rameters defining the suitability for use in an unknow n

and varied but relatively smooth terrain such as the

surface of Mars (Fig. 27) it is possible to determine the

best overall design for that specific application. The list

of parameters is not comprehensive and the scoring

system is somewhat subjective

so

re-evaluation may be

required once a working environment is selected.

Al-

though adding the scores may not give the perfect design

for a specific application, it does give a good overview

of the potentially best all-rounders. The scores are

shown in Talbe 13.

Fig.

27

Surface

of

Mars.

Table 13 Design principle scoring

Parameter

1

Design

2ld

Design

3rd

Design

4Ih

Design 5 I h Design 61hDesign 7IhDesign

Mobile . Spherical Gyroscopic Ballast mass Ballast mass

wheels stabilisation fixed axis moving axis

prung Car driven

central axis

Obstacle Avoidance

Simplicity

of

design

Easy of autonomy

Weight

Solid or flexible roll-

ing surface

Tumble-weeding

Slope Climbing

Obstacle Climbing

Movement in enclosed

areas

Solid

No

2

2

3

1

2

3

Flexible

possible

No

3

3

2

Flexible

Yes

3

3

3

Solid

No

1

1

2

Flexible

N o

2

2

1

Flexible

Possible

2

2

I

Solid

Possible

2

-

3

Total 14 15 17 13 14 I 16


14/14

208

Journal of Bionic Engineering (200 6) Vo1.3 No.4

From this summary.

it

can be determined that the

mobile masses solution perhaps offers the most flexi-

bility for its use. Given that the deform able robot pre-

sented above (section 3 . 3 . 3 is also able to jump, then

using this type of solution offers an interesting all-terrain

mobility system

so

long

as

the energy supply situation

can be resolved.

This work has been conducted as part of the de-

velopment of a rough terrain mobility system based on

rolling and jumping movements at the University of

Bath.

Acknowledgement

Thanks to Professor Julian Vincent for his guidance

throughout the progression

of

this work.

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