2009 IEEE International Conference on Robotics and Automation
Kobe International Conference Center
Kobe, Japan, May 12-17, 2009

HELIOS Carrier: Tail-like Mechanism and Control Algorithm for
Stable Motion in Unknown Environments
M. Guarnieri, P. Debenest, T. Inoh, K. Takita, H. Masuda, R. Kurazume, E. Fukushima and S. Hirose
Abstract— Mobile platforms when negotiating steps andstairs should be able to control theirs posture in order toavoid sudden tilting or falls. In particular, when considering
applications for search and rescue operations where users havea very limited time of operation, the motion on stairs should
be automated as much as possible. In this way operators
can concentrate on their tasks (i.e. search of survivors and/or
exploration of dangerous environments) rather than having tofocus on the stability of the vehicle. A simple but very effective
mechanism called ”tail” is introduced. The mechanical design
and its control method is presented together with several tests
and experiments carried out with a simple tracked vehicle inreal environments.

I. INTRODUCTION
Robotic platforms for Urban Search and Rescue Operations (USAR) must easily adapt to unstructured environmentsthat consist of stairs, random steps and rubble. For this kindof applications the control of the CoG of the vehicle allows tonegotiate several type of obstacles. To improve the vehicle
terrain adaptability different mechanical solutions, together
with the use of several sensors, have been so far proposed.
Auxiliary tracked ﬂippers are often utilized as in [11],[5],[10]. In other type of robotic platforms also an extra linkage
with one active dof has been also utilized as in [1] and [2].
Also reconﬁgurable track proﬁles are adopted for obstacle
negotiation [9],[8],[4],[3]. In many cases these solutions arecharacterized by complex hardware designs such as the useof extra treads and/or wheels together with sensors for theircontrol. This might lead also to extra power consumption
and to complex system architectures for the control of all the
degrees of freedom. Thus if the dimensions of the platform
are not increased, there is a limited space for the housing of
sensor devices to be utilized for search and rescue operations.
In [6] the authors presented HELIOS VIII tracked vehicle
equipped with a manipulator. The arm was proved to bevery effective to assist the platform motion (i.e. control ofthe vehicle stability) when utilized as an extra linkage. The
manipulator is utilized also for the connection of the vehicle
and of an auxiliary simple tracked carrier to obtain articulated mobile conﬁgurations with high terrain adaptability.
However, when operating in real scenarios, it is not always
possible to make use of a unit equipped with a manipulator.
Guarnieri, Hirose, Debenest are with Faculty of Mechanical andAerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan

guarnieri@robotics.mes.titech.ac.jp

Inoh and Takita are with HiBot Corporation, Kamata 2-10-1, Ohta, Tokyo
144-0052, JAPAN, Japan takita@hibot.co.jp
Masuda and Kurazume are with the Graduate School of Information
Science and Electrical Engineering, Kyushu University, Tokyo, Japan

978-1-4244-2789-5/09/$25.00 ©2009 IEEE1851

For this reason carriers must be capable up to a certain degree
to move separately over stairs or steps.
In this research a simple and compact tail like mechanism,
consisting of a ﬂexible rubber pipe attached at the rear sideof a basic tracked platform, is proposed. The tail permits
the automatic motion support of the vehicle on steps andstairs by utilizing it to counteract the vehicle eventual tilting
behavior. Nevertheless because of the particular mechanicaldesign, plenty of space on the top of the platform is left
for auxiliary batteries and array of sensors. Figure 1 shows
one of the three carrier vehicles utilized for USAR missions.
The addition of a tail improves signiﬁcantly the mobility of
crawler robots, while adding very little mass to the total massof the robot. Compared to a normal extra rigid linkage with
one dof, the use of ﬂexible material for the tail permits toabsorb eventual impacts without damaging the structure. In
addition, a ﬂexible tail would be less likely to become stuck
among obstacles when the robot is moving in unstructured
environments.

Fig. 1.

Tracked platform equipped with tail like mechanism

The tail system consists of one dof. However, as explainedin the experiments section, also the passive tail type with noactuation was proved to be very effective.
The paper is organized as follows: in the next sectiona basic geometric analysis of the proposed mechanism isintroduced. Afterward the mechanical design and the control
algorithm are explained in details. The effectiveness ofthe tail like solution is then validated throughout several
experiments on prototypes and in real urban areas.
II. P ROBLEM A NALYSISThe following analysis has been carried out considering
as a model for the tail a rigid ﬁxed linkage. Further analysis considering the type of material and therefore the tailﬂexibility can be carried out. However this ﬁrst basic study
serves to give an idea of the dimensions and position of theextra linkage for stairs climbing.
For a crawler robot to climb stairs in a stable way, theprojection of its center of mass must be always ahead of thepivoting point. The pivoting point is the edge of the stairaround which the robot would rotate if its center of mass
were located in the extreme back of the crawlers. Whileclimbing stairs, the pivoting point corresponds to the lowest
edge the crawlers are touching. Figure 2 shows the pivot
point P at the most critical moment of stair climbing, when
the projection of the center of gravity (CG) of the robot
is exactly over the pivot point. If the projection of the CG
falls slightly to the left, the robot will lose balance and tilt
backwards. However, if the projection of the CG is on theright of the pivot point, the robot will be in a statically stableposture.

tail once the projection of its center of gravity reaches thepivot point P. Until that moment, an additional support isnecessary. This support is provided by the tail mechanism,
which extends until the edge of the previous step.

Fig. 3.

Stair-climbing supported by tail mechanism

The distance between the center of the crawler and the
projection of the center of gravity measured along the baseof the crawler, d, is given by the following equation:tan(α) =

d
b
=
a
h

(4)

bh
a

(5)

Thus
d=
Since
Fig. 2.

Boundary condition for stable climbing of stairs

With considering Figure 2, for the projection of the centerof mass to be always ahead of the pivoting point:Bcos(α) − h sin(α) ≥ a
2Thus, the two following relations are obtained:2bh
B > 2 a2 + b2 +
a

B
− d)
2
the length of the tail t is given by:
c=t+(

(1)

(2)

a B  2(3)
( − a + b2 )b 2Making use of equations 2 and 3, the minimum necessary
length of the crawler body for given heights of the center ofmass to perform stable climbing of the stairs can be obtained.

(6)

B
)
(7)
2
Even if the projection of the center of gravity falls behindthe edge of the lower edge, as shown in ﬁgure 4, the same
equation is valid, resulting in a longer tail that touches theedge of the previous step (i.e. previous to the projection of
the center of gravity).
t = c + (d −h<A. Tracked Robot equipped with TAIL mechanism
If the crawler robot is equipped with a tail-like structure,
it may be able to climb stairs and other obstacles thatwere previously inaccessible. The tail would function as anextension of the crawlers, but without adding much mass
to the robot. In addition, if the tail can be rotated around its
base in the pitch direction, it may help the crawler to achieve
stable climbing on stairs that were previously impossible toclimb.
To estimate the necessary length of the tail mechanism,
consider the variables depicted in ﬁgure 3. The crawler will
become able to climb the steps without the support of the1852

Fig. 4.

Extend tail supports the climbing crawler even with high CG

According to ﬁgure 4, if d < B2 − c , the crawler is always
stable while climbing the stairs, and no tail is required.

Table I shows possible conﬁgurations of crawler (withlength B and height of center of gravity h) and tail mechanism (of length t) for stable climbing of standard stairs.
TABLE IN ECESSARY LENGTH AND ANGLE OF TAIL FOR PERFORMING STABLE
CLIMBING

B (mm)
395
395
395
395
395

h (mm)
87.5
175
262.5
350
437.5

t (mm)
171.6
222.5
273.4
324.3
375.2

Note that all the crawler conﬁgurations presented in table
I were not able to perform stable stair-climbing, as shownin the previous section. However, with the addition of acontrolled tail, the robots become able to overcome the steps.
B. Theoretical Comparison between standard and tailequipped crawler robots
Given the equations of the previous section, ﬁgure 5 shows
the relation between the height of the center of gravity of
the robot, h, and the necessary length of the crawler, B, withdifferent lengths of tail, t. The physical meaning of theseparameters is displayed in ﬁgure 4.

III. HELIOS C ARRIER
The mobile platform introduced hereafter and equippedwith the tail like mechanism, is part of a special robot teamproposed for USAR operations. In particular it represents
the ﬁrst stage of a Japanese national founded project whichaims to the development of a robotic system to be utilized in
extreme environments such as rescue operations in heavily
stricken urban areas. As presented in [7] the solution provided by the authors consists of ﬁve tracked platforms (twoHELIOS IX vehicles and three simple carriers). These mobile
bases are utilized to take sensors and measuring devices into
heavily stricken areas for the realization of 3D maps andfor collecting information on the explored areas. Becauseof weight and occupancy of the devices mounted on thevehicles, tilting motions must be addressed when moving
over stairs and unstructured environments.
Figure 1 shows one of the three carriers equipped with thetail mechanism introduced earlier. The tail is composed of acylinder, and it is made of a ﬂexible material like rubber. By
adjusting the ﬂexibility of the material, one can determine
how much impact can be absorbed by it, and how muchthe robot will bounce while climbing stairs. As presented
in the test and experiments section, also with the simple
ﬁxed tail mechanism the tracked robot presents very goodstability. However in order to utilize the mechanism also
when descending the stairs and in case of sudden tilts, it
is important to have a counter reaction. This can be obtainedby supplying one dof (PITCH axis) to the mechanism.
As explained in the next section, using an attitude sensor
installed inside the body and two distance sensors (connectedto the front and to the back of the mobile base) it is possible
to automatically trigger the tail actuation. Besides when the
tail is not utilized, it can be set vertically with respect of therobot base in order to not obstruct pivot rotations in limited
spaces. Eventually a protected camera (with 180◦ view) canbe installed at the tip of the linkage and aligned to the tailaxis for driving the vehicle. For this reason a hollow pipe
was utilized as tail in order to allow the passage of wires
and the antenna.
A. Mechanical design

Fig. 5. Necessary length of crawler (B) against height of center of gravityof the robot (h) for different lengths of tail mechanism (t)Note that if no tail is employed, a robot with center ofgravity 350mm above the ﬂoor needs a crawler of at least
1044mm of length to be able to climb steps. However, if atail of 325mm of length is added, the necessary length of thecrawler is reduced to 395mm, without affecting its mobility
when climbing stairs.
Since the mass of the crawlers per unit of length is muchlarger than that of the tail, it becomes evident that the useof a tail structure leads to lighter, more compact robots withhigh mobility on uneven terrain.

1853

The base of the tail is connected to a worm gear, androtates around two bearings connected to the base of therobot, as shown in ﬁgure 6. The worm gear is driven by aworm screw, which is actuated by a motor installed on thebase of the robot. Because of the worm gear, the tail cannot
be back-driven, and is able to keep its position even when
external forces are applied, with no additional consumptionof power by the driving motor. By changing the angle ofthe tail, it can be lifted in order to make the body of therobot more compact, or to avoid obstacles. In addition, bycontrolling the angle, traversing obstacles becomes smootherand more controllable.
Figure 7 shows sequences of crawler robots climbing
stairs. The robot on the left (a-b) has no tail, while the robot
on the right (c-d) is equipped with the tail mechanism.

Fig. 8.

Fig. 6.

Section view of the tail mechanism

Fig. 7.

Effect of the tail in ascending stairs

Effect of the tail in descending stairs

Fig. 9.

Figure 8 shows the robot descending a ﬂight of stairs. The
robot without the tail falls suddenly once its center of mass
is over the pivot edge, causing a loss of stability. The robot
equipped with the proposed mechanism is able to change its
pitch angle continuously with the help of the tail, and thus
perform a controlled descent of the steps.
B. Architecture and control method
Figure 9 depicts the system architecture of the tracked
platform. In the robot one main HiBot SH2 Tiny microcontroller is utilized for the reading of the attitude sensors
data and for the communication to the external computer.
Another SH2 Tiny is in charge for the control of the motorof the tail mechanism.
This microcontroller is also constantly acquiring the distance from the ground for both the front and rear side of
the mobile platform with respect of the motion direction.
According to the data received by CAN bus of the robot
posture, this microcontroller enables and controls the actuation of the tail.
The tail is always actuated in position control and its target
angle position is deﬁned according to the distance sensors
and the vehicle attitude readings.
1) descending stairs: Figure 10-a illustrates the case of
the vehicle descending stairs.

1854

Carrier robot architecture

If rd > T h (where rd is the rear ground distance and T his the track height) the tail is set downward of an angle β inorder to contact or at least decrease the distance between
the robot and the ﬁrst stair step. While the robot moves
backward the base starts to tilt, a sudden α tilting would
increase the tail angle of the same attitude α. When the
robot is moving downward rd ≤ T h thus the portion β isremoved. Afterwards the tail conﬁguration is set only by thepitch attitude motions.
2) ascending stairs: Figure 10-b presents the case of thevehicle ascending stairs:
When the robot starts to move over the ﬁrst step the only
data added to the posture of the tail is the pitch attitude (ifhigher than a certain α degree) of the main body. This doesn’t
obstruct the climbing motion as the ﬂexibility of the robot
tail allows the treads to climb over the ﬁrst step. Afterwards
the motion continues over the stairs. When the vehicle isapproaching the top, f d > T h (case shown in ﬁgure 10-b),
this enables a further rotation of the tail posture of β in orderto obtain a smooth motion without tilting.
While moving along the stairs if sudden Pitch accelerations occur, the tail is actuated in order to counter react tothem and to maintain the stability of the robot. The simple
control method is illustrated in the ﬂow-chart of Figure 11.
α and β must be set according to the type of stairs. DueFig. 12.

Fig. 10.

Data reading for the tail control

Fig. 13.

Fig. 11.

First prototype with ﬁxed tailFlow chart of the simple tail control method

to the ﬂexibility of the tail, it is not necessary to set precise
values for the above parameters, however to extend thiscontrol method, data acquired realtime from the environment
should be included. In the experiments presented in the next
section, α and β were set in order to follow the inclinationof the stairs proﬁle at about 36◦ .

Motion on stairs with and without tailthe distance sensor and the consecutive actuation of the tailtoward the stair proﬁle.
Due to the weight of the sensor device installed on theplatform, (see frame number 3 and 4) the tail is widely
bending. However, as this caused an increment of the vehicle
inclination, by using the attitude sensor data, it was further
rotated downward in order to compensate this issue. Anotherexperiment is presented in ﬁgure 15; the tail bending isclearly visible when ascending and descending the stairs.

IV. T ESTS
Several tests have been carried out. Figure 12 shows theﬁrst validation test with using a non actuated tail system.
The motion was successfully carried out however it was
conﬁrmed that when starting to descend, on the top and along
the stairs it is necessary to actuated the tail for a stable andsmooth motion.
The experiment presented in ﬁgure 13 was carried out withand without the proposed tail like mechanism. It is clear fromthe left side of the ﬁgure as a normal basic crawler vehicle is
not capable to avoid tilting and to overcome simple obstacles.
In ﬁgure 14 the CoG of the robot is higher than in theprevious experiments. Making use of the tail mechanism thecarrier could ascend and descend the stairs without fails
and/or tilting. Figure 14-1 shows the triggering made by

1855

Fig. 14.

1

2

3

4

Stair descending with high CoG

The mobile platform with the tail like mechanism was also
tested during real experiments in one of the Tokyo subway
stations. Figure 16 illustrates three frames of the motion inV. C ONCLUSION
A new simple tail-like mechanism has been presented forthe stability control of simple tracked mobile platforms. A
basic model analysis has been carried out and the proposedsolution has been applied to a team of tracked robots forsearch and rescue operations in urban areas. The effectiveness of the tail has been proved throughout several tests andexperiments in simulated and real environments.
In future works, further analysis will be carried out inorder to deﬁne a precise dynamic model of the system. Alsoa detailed study of the tail ﬂexibility versus the speciﬁcationof the mobile platform will be considered.
VI. ACKNOWLEDGMENTS
Fig. 15.

The authors acknowledge the contribution of NEDO Japan
who is sponsoring this research.

Stair climbing

R EFERENCES
which one of the carriers was taken by the user from theﬁrst ﬂoor of the station down to the fourth ﬂoor of the train
platform. As explained in section II, the middle frame shows
clearly the carrier in its critical point; however because of thetail a good robot stability was successfully achieved. During
this test the operator was concentrating only on the datareading on the environment supplying to the microcontroller
only a simple command of moving forward or backward.
The architecture presented earlier was in parallel taking careof the stability control along the stairs.

Fig. 16.

Mobility experiment inside a Tokyo subway station

1856[1] Y. Chiu,N. Shiroma,H. Igarashi,N. Sato,M. Inami and F. Matsuno,”FUMA: Environment Information Gathering Wheeled Rescue Robot
with One-DOF Arm”, Proceedings of the 2005 IEEE International
Workshop on Safety, Security and Rescue Robotics, Kobe, Japan, 2005,
pp 81-86.
[2] A. Birk,K. Pathak,S. Schwertfeger,W. Chonnaparamutt, ”The IUB
Rugbot: an intelligent, rugged mobile robot for search and rescue
operations”, International Workshop on Safety, Security, and Rescue
Robotics (SSRR) IEEE press, 2006.
[3] P. Ben-Tzvi,A.A. Goldenberg and J.W. Zu, ”Design, simulations
and optimization of a tracked mobile robot manipulator with hybrid
locomotion and manipulation capabilities”,Proceedings of the 2008
IEEE/RSJ Conference on Intelligent Robots and Systems, pages 23072312, Pasadena, CA, USA, May 19-23, 2008[4] J. Kyungmin et all, ”A Remotely Operated Robotic System for Urban
Search and Rescue”, SICE-ICASE International Joint Conference
2006, Oct. 18-21, 2006 Bexco, Busan, Korea[5] M.C. McHenry, K.Matthies, D.M.Hlmick, S.I.Roumeliotis, ”Multisensor, high speed autonomous stari climbing”, Proceedings of 2000
IEEE/RSJ International Conference on Intelligent Robots and Systems,
pages 733–742, Lausanne, Switzerland, October 2002.
[6] M. Guarnieri and I. Takao and E. F. Fukushima and S. Hirose, ” HELIOS VIII search and rescue robot: Design of an adaptive gripper andsystem improvements”, Proceedings. 2007 IEEE/RSJ International
Conference on Intelligent Robots and Systems, San Diego, USA, Nov
2007, pp. 1775-1780.
[7] M. Guarnieri and I. Takao and P. Debenest and K. Takita and E. F.
Fukushima and S. Hirose, ” HELIOS IX Tracked Vehicle for Urban
Search and Rescue Operations: Mechanical Design and First Tests”,
Proceedings. 2008 IEEE/RSJ International Conference on Intelligent
Robots and Systems, Nice, France, Sep 2008.
[8] Y. Hosoda, H. Yamamoto, M. Hattori, H. Sakiri, T. Iwamoto, M.
Oowada, A. Kanno, Y.Saitou, ”Swan: a robot for nuclear disaster prevention support”, Journal of Advanced Robotics, volume 16, pp.485488, 2002.
[9] J. Liu, Y. Wang, S. Ma, B. Li, ”Analysis of stairs-climbing ability
for a tracked reconﬁgurable modular robot”, Proceedings of the 2005
IEEE International Workshop on Safety, Security and Rescue Robots,pages 36–41, Kobe, Japan, June 2005.
[10] C. Marques, J. Cristovao, P. Lima, J. Frazao, I. Riberio, R. Ventura,”Raposa: Semi-autonomous robot for rescue operations”, Proceedingsof the 2006 IEEE/RSJ Conference on Intelligent Robots and Systems,
pages 3988–3993, Beijing, China, October 2006.
[11] L. Woosub, K. Sungchul, K. Munsang, S. Kyungchul, ” Rough Terrain
Negotiable Mobile Platform with Passively Adaptive Double-Tracks
and Its Application to Rescue Missions”, Proceedings. 2005 IEEE
International Conference on Robotics and Automation, San Diego,
USA, April 2005, pp. 1591-1596.