Design and locomotion modes of a small wheel-legged robot

Abstract

Legged robots have superior terrain adaptability comparing to traditional wheeled vehicles. They also offer attractive capabilities in terms of agility and obstacle avoidance. On the other hand, traditional wheeled platforms provide sufficient robustness, mechanical simplicity and energetic performance. They are fast, powerful in terms of load to weight ratio, stable, and easy to control. Hybrid locomotion systems were developed to exploit the terrain adaptability of legs in rough terrain and simpler control as well as high speed associated with wheels. In this paper the design and the locomotion modes of a small wheel-legged robot are presented.

Full text

DESIGN AND LOCOMOTION MODES OF A SMALL WHEEL-
LEGGED ROBOT
IOAN DOROFTEI
Robotics Laboratory, Mechanical Engineering, Mechatronics and Robotics Department,
"Gheorghe Asachi" Technical University of Iasi, B-dul D. Mangeron, 61-63
700050-Iasi, Romania
ION ION
Manipulators and Robots Laboratory, Technology of Manufacturing Department,
POLITEHNICA University of Bucharest, Splaiul Independentei, 313, 060042-Bucharest,
Romania
Legged robots have superior terrain adaptability comparing to traditional wheeled
vehicles. They also offer attractive capabilities in terms of agility and obstacle avoidance.
On the other hand, traditional wheeled platforms provide sufficient robustness,
mechanical simplicity and energetic performance. They are fast, powerful in terms of load
to weight ratio, stable, and easy to control. Hybrid locomotion systems were developed to
exploit the terrain adaptability of legs in rough terrain and simpler control as well as high
speed associated with wheels. In this paper the design and the locomotion modes of a
small wheel-legged robot are presented.
1. Introduction
Walking vehicles have superior terrain adaptability, comparing to other mobile
robots. They also offer attractive capabilities in terms of agility and obstacle
avoidance. Their use is convenient on soft ground or in unstructured
environments, where the performance of wheels and tracks are considerable
reduced. On the other hand, wheeled robots provide sufficient robustness,
mechanical simplicity and energetic performance. They are fast, powerful in
terms of load to weight ratio, stable, and easy to control. Later on, hybrid
locomotion systems were developed to exploit the terrain adaptability of legs in
rough terrain and simpler control as well as high speed associated with wheels.
Many hybrid locomotion systems with different architectures have been
developed since today. In Japan [1] has been developed a vehicle with five
locomotion devices, each device consisting of a wheel and a leg. Another hybrid
robot, called SAPPHYR, with two free rear wheels and two traction legs in the
front, was designed in France [2]. WHEELEG [3], developed in Italy, was a
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CLAWAR 2013
Proceedings of the Sixteenth International Conference on
Climbing and Walking Robots, Sydney, Australia, 14 – 17 July 2013

robot with two pneumatically actuated front legs, each one with three degrees
of freedom (d.o.f.), and two independently actuated rear wheels. A hybrid
wheelchair with two d.o f. planar legs, and four wheels, has been developed at
University of Pennsylvania [4], destined for use on uneven terrain,. In Finland,
Halme et al. [5] designed a hybrid locomotion robot, called WorkPartner, using
four legs equipped with wheels. Each wheel was used to serve as a foot during
walking mode and as a wheel in wheeled mode. Roller-Walker, a hybrid mobile
robot with a special foot mechanism in each leg was developed in Japan [6]. In
Finland, has been developed a hybrid system, called Hybtor [7]. This robot has
four wheeled legs, each consisting of a three d.o f. mammal-type leg and a
rubber wheel as a foot. The rolking mode of locomotion has been introduced.
The advantages of this locomotion mode, comparing to normal walking, are
better speed and stability. At Tohoku University, in Japan, has been developed a
robot with four legs (two in the front and two rear legs) and two wheels in the
middle, to develop a leg-wheel type robot for the exploration [8], [9]. In 2004,
in Thailand, a leg-wheel hybrid robot, with two active two d.o.f. legs in the front
and two passive rear wheels, has been developed [10, 11].
In this paper the design of a small wheel-legged robot and its locomotion
modes will be presented. This report is the result of a research conducted at the
Robotics Laboratory, Mechanical Engineering, Mechatronics and Robotics,
“Gh. Asachi” Technical University of Iasi, Romania.
2. Robot Architecture
The robot developed by our laboratory [12] consists of a chassis, two legs with
two d.o f. each and two passive wheels as feet, in the front, and two rear
actuated/passive wheels, respectively. The rear wheels can be passive or
actuated, thanks to the two electromagnetic clutches that connect these wheels
and the shafts of their driving motors.
The robots using the same architecture (two legs and two wheels),
developed before [3, 10, 11], are able to move using only hybrid locomotion
mode. Our hybrid locomotion robot (see Figure 1) can move using three
locomotion modes: wheeled locomotion mode, using the rear actuated wheels
and the two passive wheels in the feet; the first hybrid locomotion mode, using
the active front legs and the passive rear wheels; the second hybrid locomotion
mode, using the legs and active rear wheels.
Another advantage of this configuration (see Figure 2.b), comparing to
other previous designs kinematics (Figure 2.a) [10, 11], is an improved stability,
when only one leg is in support phase.
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(a) (b)
Figure 1. Hybrid locomotion robot: a) CAD model; b) real prototype
(a) (b)
(c)
Figure 2. Robot stability area and kinematics: a) other solutions; b) our robot – top view; c) our robot
– front view
The prototype developed by us is a small scale hybrid locomotion platform,
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with 230 x 186 x 125 [mm] as overall dimensions (H x l x h). At a real scale, it
may be a robotized chair for people with locomotion disabilities.
3. Locomotion Modes
Three locomotion modes can be implemented on this robot: wheeled locomotion
mode, using the big actuated wheels (acting in the front, for this locomotion
mode) and the two passive wheels in the feet; the first hybrid locomotion mode,
using the active front legs and passive rear wheels; the second hybrid
locomotion mode, using the legs and active rear wheels.
3.1. Wheeled Locomotion Mode
The two actuated wheels (big ones) act in the front of the robot and the small
wheels, as feet of the passive legs, are passive.
One of the small passive wheels, or both of them (depending on the
instantaneously trajectory of the robot), will be in contact with the ground, only
to keep the stability of the robot. We will have two small passive wheels on the
ground when the robot is moving forward/backward on a straight or curved
trajectory (see Figure 3), and a single passive wheel in contact with the ground
for turning on the spot (see Figure 4).
(a) (b)
Figure 3. Wheeled locomotion mode on a straight trajectory: a) robot configuration; b) diagram of
motors parameters (angular position of the two legs servos; angular velocities of rear wheels)
As we may see in Figure 3.b, in this sequence, the two legs are orientated in
the front of the robot, with the feet (small passive wheels) on the ground. Rear
wheels are active and will usually rotate as well as the robot will move
backward. Is preferred this direction because the robot could climb obstacles,
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thanks to the big diameters of the actuated wheels. If the robot moves forward,
the small passive wheels will not be able to climb. Anyway, the robot can move
forward on a flat terrain or for some maneuvers.
(a) (b)
Figure 4. Turning on the spot: a) robot configuration; b) diagram of motors parameters (angular
position of the two legs servos; angular velocities of rear wheels)
This sequence is using to change the robot trajectory direction. In this case,
one leg is touching the ground (both the legs are in the neutral position,
according to the vertical axes) and the rear wheels have opposite rotating
directions.
3.2. Hybrid Locomotion Modes
There are two hybrid locomotion modes that could be implemented on this mall
platform
 First hybrid locomotion mode: the two legs are actuated and the rear
wheels are free (decoupled from their motors shafts,);
 Second hybrid locomotion mode: the legs and the rear wheels are
simultaneously actuated (in order to increase the power of the robot).
The single difference between these two hybrid locomotion modes consists
in the actuation (or not) of the rear wheels. The second solution has the
advantage of improving the traction force of the vehicle, when it moves in an
unstructured terrain. Some intermediary sequences of the robot configuration
during one cycle of the hybrid locomotion mode are shown in Figure 5.
Because the trajectories of the legs are crossing in the support phases, few
precise rules should be established for the case when the robot is using hybrid
locomotion mode:
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 The legs could not be simultaneous in transfer phase (in that case the
robot will fall down);
 The legs could not be simultaneously in support phase otherwise they
will cross each other and they can be destroyed;
 When a leg is in support phase the other one should be in transfer
phase, moving in opposite direction.
3 2 1
6 5 4
(a)
(b)
Figure 5. Hybrid locomotion mode: a) locomotion sequences; b) diagram of motors parameters
(angular position of the two legs servos; angular velocities of rear wheels)
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In Figure 5.a, six sequences of the hybrid locomotion modes are
demonstrated, starting from the first robot configuration (noted with 1) till the
last configuration (noted with 6) of a locomotion cycle (two steps).
Even if the diagram shown in Figure 5.b is identical for both hybrid
locomotion modes, in the first hybrid locomotion mode, when the rear wheels
are passive, their angular velocities, l

and r

, are generated by the
displacement of the support leg.
In order to avoid legs/wheels slippage, for the second hybrid locomotion
mode next relation should be respected:
3med leg
Rl



(1)
where
2
rl
med





(2)
3
l is the horizontal projection of the leg length, l

and r

are the angular
speeds of the left and right rear wheels, leg

is the angular speed of the support
leg, R is the radius of rear wheels (see Figure 2.a). In order to simplify the
control algorithm, the robot has been designed as well as 3
lR

.
4. Conclusion
The testing gave a qualitative view of the system’s mobility performance. All
the locomotion modes have been tested, using: forward, backward, turning right
on the spot and turning left on the spot sequences, curved trajectory. All the
tests have been done in the laboratory. We should also test the robot on a soft
ground. The accuracy of direction and movement of the mobile robot is
improved thanks to the small passive wheels used as feet. These wheels avoid
the legs slippage. An open loop control was enough to test the mobility of the
vehicle and no sensors have been used till now. A closed loop control should be
implemented in the future, using encoders for the wheels and legs, touch sensors
for the feet and range sensors for the robot.
References
1. Y. Ichikawa, N. Ozaki and K. Sadakane, A hybrid locomotion vehicle for
nuclear power plants. IEEE Transactions Systems, Man and Cybernetics
13:(6), pp. 1089–1093 (1983).
615

2. M. Guihard, P. Gorce and J.G. Fontaine, Sapphyr: Legs to pull a wheel
structure. Proc IEEE Int Conf on Robotics and Automation, Nagoya, Japan,
pp. 1303–1308 (1995).
3. G. Muscato and G. Nunnari, Legs or wheels? Wheeleg - a hybrid solution.
Proceedings of the 1st International Conference on Climbing and Walking
Robots - CLAWAR-99 (editors G.S. Virk, M. Randall, and D. Howard),
Portsmouth, UK. Professional Engineering Publishing, pp. 173–180 (1999).
4. V. Krovi and V. Kumar, Modeling and control of a hybrid locomotion
system, ASME J Mech Des 121:(3), 448–455 (1999).
5. A. Halme, L. Leppanen and S. Salmi, Development of workpartner robot—
design of actuating and motion control system. Proceedings of the 2nd
International Conference on Climbing and Walking Robots - CLAWAR-99
(editors G.S. Virk, M. Randall, and D. Howard), Portsmouth, UK,
Professional Engineering Publishing, pp. 657–665 (1999).
6. S. Hirose, and H. Takeuchi, Study on roller-walk (basic characteristics and
its control), in Proc IEEE International Conference on Robotics and
Automation, pp. 3265–3270 (1996).
7. A. Halme, L. Leppanen, S. Salmi and S. Ylonen, Hybrid locomotion of a
wheel-legged machine. Proceedings of the Third International Conference
on Climbing and Walking Robots - CLAWAR-2000 (editors M. Armada
and P. Gonzalez de Santos), Madrid, Spain, Professional Engineering
Publishing, pp. 167–173 (2000).
8. Y.J. Dai, E. Nakano, T. Takahashi and H. Ookubo, H., Motion control of
leg-wheel robot for an unexplored outdoor environment. Proceedings of the
1996 IEEE/RSJ International Conference on Intelligent Robots and
Systems, vol. 2, Nov. 4-8, pp. 402-409 (1996).
9. N. Eiji and N. Sei, Leg-wheel robot: a futuristic mobile platform for
forestry industry. IEEE/Tsukuba International Workshop: Can Robots
Contribute to Preventing Environmental Deterioration?, Nov. 8-9, pp. 109-
112 (1993).
10. K. Suwannasit and S. Laksanacharoen, A BIO-Inspired Hybrid Leg-Wheel
robot. TENCON 2004, (2004 IEEE Region 10 Conference), Proceedings
Analog and Digital Techniques in Electrical Engineering, 21-24 Nov.,
Chiang Mai, Thailand (2004).
11. K. Suwannasit and S. Laksanacharoen, A Hybrid Leg-Wheel Robot. The
18th Conference of Mechanical Engineering Network of Thailand,
KhonKan, Thailand, October 18-20 (2004).
12. I. Doroftei, C. Marta, C.O. Hamat, L. Suciu, G. Prisacaru, A Hybrid Wheel-
Leg Mobile Robot, Annals of DAAAM for 2008, ISSN 1726-9679 (2008).
616