smc rover : planetary rover with transformable wheels · confirmed that uni-rover can climb over...
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SMC Rover : Planetary Rover with transformable wheels
ATSUSHI KAWAKAMI, AKINORI TORII, KAZUHIRO MOTOMURA, and
SHIGEO HIROSE
Department of Mechano-Aerospace Engineering , Tokyo Institute of Technology,
2-12-1 Oh-okayama, Meguro-ku, Tokyo, 152-8552, Japan
Abstract: We are proposing a new planetary rover system called “SMC rover”. This sys-
tem consists of one main body and some wheel units, which are detachable and able to
work as child rovers, called “Uni-Rover”. Trial models of “Uni-Rover” and “SMC rover”
have been designed. This paper explains the mechanisms of this rover, and also the results
of basic experiments on locomotion, manipulation and transformation.
Keywords: Super-Mechano-Colony, Planetary Rover, Arm Equipped Single Wheel Rover
1. Introduction
Planetary rover systems are expected to undergo various missions on other planets
employing various different abilities. In the construction of large-scale facilities
and detailed investigation at wide areas, rovers have an advantage over conven-
tional probes and landing. For such missions, several abilities are required, such as
high mobility on rough terrain, position identifying ability and special devices for
each mission.
However, severe financial restrictions must be taken into account when transport-
ing probes and rovers to a target planet. It is undesirable to make rovers large and
heavy when trying to accomplish these abilities. Therefore, we have proposed a
new concept of planetary rover system called SMC rover [1][2]. This paper ex-
plains the mechanisms of the trial model and shows results of basic experiments of
the trial model.
Child rovers
Main body
Fig. 1. Overall view of SMC-Rover
2. The design of SMC Rover
2.1 The concept of SMC Rover
SMC-Rover consists of one main body and multiple child units as shown in Fig.1.
The main body has solar battery cell, communication devices, sample analyzer,
battery chargers and tool changers for the child rovers. Each child rover has a
wheel for locomotion and an arm for manipulation. This system has the following
characteristics:
1. The main body cannot move by itself, but the child rovers hold the main body
of SMC rover by their manipulators and act as active wheels of the main body.
If it is necessary, each child rover can separate from the main body and move
around to undergo separate missions.
2. Each child rover is composed of a single wheel and a single arm. When the
wheel unit is in “locomotion mode” as shown in Fig.2a, with extended arm, it
(a) locomotion mode� (b) manipulation mode�
caster arm wheel
connecter
grippermanipulator
(arm)
base(wheel)
Fig. 2. Child rover
(a) transform into manipulation mode� (b) transform into locomotion mode�
Fig. 3. Transformation of child rover
� can move around on the ground with high mobility. The arm of the child rover
has a caster around the wrist. By changing the orientation (yaw angle) of this
caster with the wrist motion, it is possible to steer the child rover.
3. The child rover can be converted into the “manipulation mode” by adopting the
posture as shown in Fig.2b, and acts as a manipulator with a gripper.
4. The child rover can change between the locomotion mode and the manipulation
mode by its arm motion, shown in Fig.3.
5. Still in the manipulation mode, the child rover can change its position by “pivot
turn” motion of Fig.4a, pushing the ground with its arm.
6. By grasping another child rover, child rovers can take the shape of a multi-
ple-wheel rover to go over rough terrain, as shown in Fig.4b.
7. Using multiple child rovers, SMC rover achieves fault tolerance and high reli-
ability. Adopting heterogeneous, hierarchical system, it also achieves high
controllability.
MoveDirection
WheelLotation
(a) pivot turn (b) combined locomotion mode
Fig. 4. Various locomotion modes of the child rover
8. Using the wheels of child rovers as wheels of the main body and the arms of
child rovers as connecters between them and the main body, the SMC rover can
cut down total weight and size of the system.
9. Each child rover has only one wheel because its diameter can be larger than in
the case of a rover with multiple wheels. With larger wheels, this system
achieves high mobility on rough terrain. Moreover, in this propulsive motion,
the arm sustains the reaction moment, generated by pressing the wrist caster
against the ground while the wheel rotates.
10.Child rovers cannot equip a large solar battery cell, but they can be supplied
electric energy from the solar battery cell on the main body, while they are con-
nected to the main body.
2.2 Previous research
There has been increasing research interest in multiple autonomous robot system
with cooperative behavior, focusing on fault tolerance, reliability, and scalability
[3]. Such research interest is not restricted to decentralized homogeneous system
[4][5][6], but also includes heterogeneous parent-children type systems to achieve
the advantages of well-regulated coordination control [7][8].
However, the parent-children type rover system with detachable wheels described
in this paper is very different. The child units are not simply carried by the mother
unit, but they act as propulsion mechanisms of the main unit, characteristics men-
tioned in item 8 of Sect. 2.1. This characteristic is very new and there are no pre-
vious studies about it.
On the other hand, there have been several works about single-wheel type loco-
motive robots [9][10][11]. A single-wheel shaped robot with no arm can generate
limited propulsion torque, so the locomotion ability on rough terrain of this kind
of robot is very limited. But the child unit of the system discussed in this paper is
shaped with a single wheel and single arm, and it can generate
θ
1
2
3
4
RC Servo motor
DC motor (wheel)Battery
θ
θ
θ
Fig. 5. Mechanisms of Uni-Rover
enough propulsion torque even on rough terrain, as mentioned in items 2 and 9 of
Sect. 2.1. This concept is also very new.
3. Development of the trial model of Uni-Rover
3.1 Composition of new trial model of Uni-Rover
We have designed and developed a trial model of “Uni-Rover”, a child unit of
SMC-Rover. The trial model consists of a single wheel (diameter: 190mm, width:
140mm) and a single manipulator with three joints (length of upper arm and lower
arm: 170mm) as shown in Fig.5. At the end of the manipulator there is a caster
and a gripper with four fingers. One DC motor for the rotation of the wheel is lo-
cated inside the wheel. The controller circuits, transmitter and batteries for motors
are also located inside the wheel. To actuate the manipulator, one RC-servo motor
is located in each of the four joints of the manipulator. The total weight of the
rover is 3.2kg. The trial model is driven with a wireless manual controller.
3.2 Improvement of new trial model of Uni-Rover
We have set the mass balance of the mechanism inside the wheel with a wide ec-
centricity between the center of mass and the axis of rotation of the wheel. Be-
cause of this mass property, Uni-Rover is very stable in manipulation mode
θ 2
Stable side Unstable side(Good at manipulation)
(Easy to fall down)Center of massin wheel
Fig. 6. Stability of Uni-Rover
when the arm takes the position of the stable side, and it is also easy to bring it
down during transformation, when the arm takes the position of the unstable side
as shown in Fig.6.
4.Development of the trial model of SMC rover
We have also designed and developed a trial model of SMC rover to be used with
the trial models of Uni-Rover, as shown in Fig.7a.
The trial model of SMC rover has a main body, four dummy wheel units and two
connecters for Uni-Rover. The dummy wheels can rotate freely without motors,
but they have been designed in a way that they can be motorized and replaced by
real Uni-Rovers with the connecter in future works.
The connecters for Uni-Rover consists of a handle for the gripper and two sup-
porting plates to hold the manipulator of Uni-Rover as shown in Fig.7b.
Connecters
Child rovers (Dummy)
SupportingPlates
Handle
Solar battery cell(Dummy)
(a) Overall view of main body (b) Connecter
Fig. 7. Mechanisms of main body
5.Experiments of Uni-Rover and SMC rover
5.1 Experiment of Uni-Rover in locomotion mode
On a flat surface, we carried out propulsion and steering experiment with the rover
in locomotion posture as shown in Fig.2a. The orientation of propulsion is con-
trolled by changing the orientation of the caster with the joints of the manipulator
as shown in Fig.8.
Moreover, we carried out experiments of climbing over small obstacles. It was
confirmed that Uni-Rover can climb over gaps of 5cm of height, but there were
problems at the control of the orientation of Uni-Rover on rough terrain.
5.2 Experiment of Uni-Rover in manipulation mode
With the rover in the manipulation mode, experiments of the manipulator were
carried out, as shown in Fig.2b.
With those experiments, it was possible to confirm that each joint is controllable
by the wireless controller. It was verified that the manipulator has enough ability
to actually hold and transport various objects in several shapes (a pen cap, a small
bottle, a grip of a driver, and so on), as shown in Fig.9.
Turn directionTurn
direction(a) posture of right turn (b) posture of left turn
Fig. 8. Steering motion of Uni-Rover
driver tape box stone
Fig. 9. Handling various objects
Furthermore, we carried out experiments of “pivot turn”, which is the locomotion
method in manipulation mode, as shown in Fig.4a. It was confirmed that the
Uni-Rover can change its position by this “pivot turn” motion while in the ma-
nipulation mode.
5.3 Experiment of transformation of Uni-Rover
We also carried out experiments on the transformation sequence of Uni-Rover.
Initially, we measured the N.E.S.M. (Normalized Energy Stability Margin) [12] of
Uni-Rover by changing its shoulder angle, as shown in Fig.10. According to this
graph, it was confirmed that the N.E.S.M of the stable side is higher than the
N.E.S.M of the unstable side, so Uni-Rover achieves both high stability in ma-
nipulation and high transformability.
Secondly, experiments of transformation from locomotion mode to manipulation
mode were carried out, and it was confirmed that the Uni-Rover can stand up by
-90 -45 0 45 900
10
20
30
Shoulder Angle θ2 [deg]
N.E
.S.M
.[mm
]
Unstable
S t a b l e
:result of experiment
Fig. 10. N.E.S.M. of Uni-Rover in manipulation mode
Fig. 11. Detaching wheel units
using its manipulator, as shown in Fig.3a. Experiments of its transformation from
manipulation mode to locomotion mode were also performed as shown in Fig.3b,
and we verified that the Uni-Rover can fall down into manipulation mode by using
the kinetic effect of the manipulator motion in unstable position.
5.4 Experiment of SMC rover with Uni-Rovers
We carried out experiments of SMC rover with Uni-Rovers as detachable wheel
units, as shown in Fig.11.
It was confirmed that the Uni-Rovers can transport the trial model of SMC rover
and can detach from the connecters of SMC rover.
6.Conclusion and future work
In this paper, the design and development of the trial models of Uni-Rover and
SMC rover have been explained. The results of experiments in locomotion and
manipulation mode and transformation experiments have also been mentioned.
In the future, propulsion experiments over rough terrain and docking experiments
will be carried out, and the control system of the unit will be improved. Besides,
combined locomotion and cooperative work by multiple units will be achieved.
Acknowledgment
This research is supported by the grant-in-aid for COE Research Project of Su-
per-Mechano-Systems by The Ministry of Education, Culture, Sports, Science and
Technology of Japan.
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