Variable Geometry Single-Tracked Mechanism for a Rescue Robot
Sung Kyun Lim Mechanical Engineering Korea Advanced Institute of
Science and Technology 373-1 Guseong-dong, Yuseong-gu,
Daejeon 305-701, Korea Email: isk@kaist.ac.kr Telephone: +82-42-869-3252
Fax : +82-42-869-5201 Byung-Soo Kim Hanool Robotics, Corp.
461-68 Jeonmin-dong, Yuseong-gu Daejeon, Korea 305-811 Telephone: +82-42-478-9090
Fax: +82-42-478-9094
Dong Il Park Mechanical Engineering Korea Advanced Institute of
Science and Technology 373-1 Guseong-dong, Yuseong-gu,
Daejeon 305-701, Korea Email: parkstar@kaist.ac.kr Telephone: +82-42-869-3252
Fax : +82-42-869-5201 Sang-Won Jeon Hanool Robotics, Corp.
461-68 Jeonmin-dong, Yuseong-gu Daejeon, Korea 305-811 Telephone: +82-42-478-9090
Fax: +82-42-478-9094
Yoon Keun Kwak Mechanical Engineering Korea Advanced Institute of
Science and Technology 373-1 Guseong-dong, Yuseong-gu,
Daejeon 305-701, Korea Email: ykkwak@kaist.ac.kr Telephone: +82-42-869-3212
Fax : +82-42-869-5201
Abstract — There have been numerous studies directed toward the development of driving mechanisms for off-road mobility and various utilities of rescue robots. To achieve objectives such as surveillance, reconnaissance, and rescue, it is necessary to develop a driving mechanism that can handle off-road environments. We propose a new type of driving mechanism for a rescue robot that has a variable geometry single-track. This mechanism has a symmetric configuration so that the robot advances in dual directions and prepares against overturning. By using transformation, it can reduce the energy consumption in steering and rotating and also maximize the ability to overcome obstacles such as stairs. It is also designed with consideration of compact size and low center of gravity for driving on stairs. In this paper, we analyzed the design parameters for 4 phases of climbing stairs and determined the specifications of the robot to enhance adaptability to stairs.
I. INTRODUCTION
To date, massive losses of life and property have been caused by accidents and disasters such as building collapses, fires, earthquakes, terrorism, and wars. And they are tending to increase gradually day by day. To reduce the extent of such damage and loss, robots, in the place of humans, can play key roles in rescue operations in dangerous areas such as in building fires and regions where there is a threat of additional collapse or explosion, as well as in scouting unknown terrain. As such, the development of an efficient rescue robot has become an important goal.
Dating back to the 1970s, there has been a wealth of studies on driving mechanisms for overcoming steep paths as well as on various utilities of rescue robots. First, VCTV- 1(Variable Configuration Tracked Vehicle - 1) presented by Kohler[1] and VCTV-2 proposed by
Maeda[2] were designed to maximize adaptability to steep paths and minimize resistance of rotation by transforming four tracks. Iwamoto’s VCTV-3[3], the first single tracked mechanism with planet wheels, showed good adaptability to stairs. Also, Martens’ Andros[4] had a similar mechanism to that of VCTV-1 and controlled the tensions of tracks by using idlers. Xevious, developed by Yoneda[5], made tracks using powder packs to increase the friction coefficient of the tracks. Pandora[6] used a double track mechanism with dual attack angles by Hagen.
Since the beginning of the 21st century, the functions of the rescue robot as well as the driving mechanism have been studied, such as in Micro-VGTV of Inuktun Corp., Packbot of iRobot Corp.[7] and DT-2 of KAIST MSD-lab[8].
This paper introduces a new type of rescue robot driving mechanism with improved performance in terms of overcoming stairs and energy efficiency. The proposed robot also represents an improvement in weight. We confirm the viability of the proposed driving mechanism through various experiments.
II. DRIVINGMECHANISM
A. Design Concept
In this section, the main requirements of the driving mechanism of a rescue robot are presented and the basic structure of the mechanism corresponding to these requirements is shown in Fig. 1.
First, a track mechanism is more stable and adaptable to various conditions of ground than wheeled mechanisms or legged mechanisms. That is, a tracked system is practicable to overcome unexpected obstacles and steep paths including stairs. By transforming the geometrical
Proceedings of the 2005 IEEE
International Workshop on Safety, Security and Rescue Robotics Kobe, Japan, June 2005
structure of tracks, it is possible to maximize the energy efficiency and prepare for overturning. In particular, a single tracked system is suitable for simple control and repair under urgent rescue circumstances and is feasible means to minimize the robot size for passing through small spaces and ease of transportation.
Therefore, a VGST (Variable Geometry Single Tracked) driving mechanism is proposed in this paper.
Driving Mechanism of Variable Geometry Single Track 1. Off-Road Mobility
2. Energy Efficiency 3. Control, Repair 4. Miniaturization
Tracked System
Variable Shape
Single Track
Driving Mechanism of Variable Geometry Single Track 1. Off-Road Mobility
2. Energy Efficiency 3. Control, Repair 4. Miniaturization
Tracked System
Variable Shape
Single Track
Fig. 1. Design concept
The driving mechanism must entail the following features.
First, the system prepares for overturning caused by unknown terrain conditions or unexpected disturbances. It can move in both directions without a distinction between the front and the rear for quick changes of directions.
G G Second, the system minimizes contact length with the ground to reduce energy consumption in the cases of high-speed moving, turning, and steering on an even surface. On the other hand, it maximizes contact length with the ground for adaptability to off-road environments and stairs, and has a lower center of gravity for system stability.
G G Third, this system generates various attack angles to cope with diverse obstacles such as stairs.
B. Design Proposal
Fig. 2 shows the proposed driving mechanism, which has a single track with a variable configuration.
Mode 1 in Fig. 2(a) has characteristics including a low center of gravity and long contact length with the ground so that the robot is capable of climbing stairs. The attack angle and short contact length shown in mode 2 are advantageous to overcome obstacles and minimize energy consumption during steering. Also, this mechanism has many symmetrical configurations such as a rectangle, trapezoids, and inverse-trapezoids besides the aforementioned two modes, and generates diverse attack angles from 0 to 90 degrees. During the transformation, total track length does not change in order to allow synchronized symmetrical transformation of arms.
Although there are various ground conditions over which this system must travel, it is assumed that standard stairs are the typical surface. Also, various variables and measurements of the system are designed from constraints for overcoming stairs.
(a) mode 1 (b) mode 2 Fig. 2. Proposed design
III. SPECIFICDESIGN
Fig. 3 shows four phases of climbing stairs. First, the robot climbs the first step utilizing an attack angle in mode 2. Then, it transforms to mode 1 on the first step and climbs the second step in mode 1. Finally, it ascends the remaining steps as if it were proceeding along a normal slope of 34 degrees.
G G In this section, performance indices to overcome the stairs and some constraints are analyzed for each phase.
Fig. 3. Four phases for climbing stairs
A. Performance Index: Phase 1
Fig. 4 shows a free body diagram containing the forces acting on the robot and the design parameters in phase 1.
The force equilibrium equation is shown in (1) and the moment for the rear contact point is represented as (2). Since the robot has to be rotated to overcome the first step after contact with this step, the moment must have a positive value.
M0
Fig. 4. Free-body diagram in the phase 1
2 2 1 1
2 1 1
1 1 2
0 cos
sin :
0 sin cos :
N F N F where
mg N
N F F
N F
F F
y x
P P
T T
T T
(1)
) 2 cos sin
(
cos ) tan 18 . tan 0
cos 18
. sin 0 (
1
1
0
F amg r
r a
r N r r
a M
T T
T T T
T T
(2)
The performance index can be expressed as (3), which is induced from (1) and (2).
T P T
T P
T T P P P
T P
P
2 2
2 2
2 0
sin 18 . ) 0 sin
) cos 1 ( sin
) 1 ( (cos
2 ) 1 (tan
1) (
u
r a
M mg (3)
Finally, using 45 degrees for the attack angle, the performance index can be obtained as (4).
0 36 . 0 ) ( 414 . 0 2 )
( 1 2
2
!
P P P P
P a r (4)
(a) Friction coefficient and wheel radius
(b) Friction coefficient and length a Fig. 5. Performance index 1
The moment is expressed as a function of the wheel radius r and the friction coefficient P in Fig. 5(a) and as a function of the contact length and the friction coefficient in Fig. 5(b). From Fig. 5, it can be seen that overcoming the first step becomes easier with a decrease of contact length and an increase of wheel radius, and the robot can ascend the first step if the friction coefficient is more than 0.3.
a
B. Performance Index: Phase 3
After ascending the first step, the robot transforms its configuration to mode 1 and proceeds up the stairs. In phase 3, it is possible to overcome the second step when
the robot contacts the wall of the second step. Fig. 6 shows a free body diagram containing the forces acting on the robot and the design parameters in phase 3. The moment for the rear contact point is represented as (5) in the same manner as for phase 1.
Fig. 6. Free body diagram in phase 3
) ( ) 1 2 ( 2cos
sin 2 2
0 TPd T P rP P
d
M (5)
The moment is expressed as a function of the body orientation angle T and the friction coefficient P in Fig.
7(a). With ascension of the second step, the body orientation angle T changes from 22 degrees to 34 degrees; however 22 degrees, which is the worst condition, is set for T since an increase of T facilitates easier step climbing. From Fig. 7, it can be seen that the robot can ascend the second step if the friction coefficient is more than 0.45 and phase 3 is more dominant than phase 1.
(a) Friction coefficient and body orientation
(b) Friction coefficient and wheel radius Fig. 7. Performance index 2
C. Performance Index: Phase 4
In phase 4, the robot, which has been transformed to mode 1, ascends the stairs. To ascend the stairs stably as in the normal slope, the robot has to contact the stairs with at least three edges and the minimum contact length is expressed as (6). Here, the steepest standard step whose angle of inclination is 34 degrees is applied and the ground contact length is selected as the minimum value for the system minimization.
d
T
D
E
T
D
E
Fig. 8. Contact length in phase 4
m d b a d
65 . 0
0.18m b 0.27m, a
where
2 2 2
t
t (6)
G
D. Design Parameters
In the previous sections, performance indices to overcome the stairs are analyzed for each phase. The main design parameters are the ground contact length ( ), the length between the centers of the wheels ( ) and the wheel radius (
d c
r).
Besides the performance indices shown in the previous sections, there are some constraints such as restriction of body size, clearance depth between the body and the ground, and minimum gap between the wheels. Also, the robot height in the mode 1 configuration has to be more than 18cm. These constraint equations are as follows.
c r
c r
r c
r
d
d
d d d
254 . 0 414 . 1 , 03 . 0 2
1 . 0 06 . 0 , 68 . 0 ) (
2 (7)
The contact length is selected as the minimum value in phase 4 and r and , which make the robot climb the stairs, are selected using the performance indices for each phase and the constraint equations.
c
IV. EXPERIMENT
A. Specification
The system is manufactured on the basis of design values calculated in the previous section, as shown in Fig. 9.
Table 1 shows the detailed dimensions and measurements of the system.
Fig. 9. Manufactured robot system Table 1 Specifications
Specifications Body Size 550(L) x 400(W) x 90(H) [mm]
Total Size 800(L) x 560(W) x 150(H) [mm]
Mass 34 kg Motor for
Driving 90W x 2 (43:1 gear ratio) Motor for
Transformation 150W (353:1 gear ratio) Materials Body : Al, Track : Rubber z Even : 1.5m/s, Step : 0.3m/s
B. Experimental Results
Experiments of transformation, turning, and climbing stairs were executed to assess the performance of the manufactured system.
1) Transformation
Fig. 10. Transformation
When the front arm is rotated, the rear arm is symmetrically rotated in the opposite direction, because it is mechanically synchronized. As shown in Fig. 10, the system can generate various geometrical configurations.
On account of the mass of the system, at least 8.4Nm is needed to transform, and the angular velocity of the rotating arms is approximately 0.7rad/sec. Therefore, it is possible to transform the mechanism to a well-matched configuration with the specific ground conditions.
2) Turning
(a) Mode 1 (b) Mode 2 Fig.11. Turning
For the evaluation of turning performance, experiments are carried out in two modes, as shown in Fig. 11. The contact length is 0.68m in mode 1 and 0.28m in mode 2.
Theoretically, the resistance moment in mode 1 is 2.4 times larger than that in mode 2. For the difference of resistance moments of turning, the system turns about 1.5 times slower in mode 1 than in mode 2.
3) Overcoming Stairs
Fig. 12. Climbing failure
Fig. 13. Climbing Stairs
In mode 1, the system slips and it is impossible to climb the first step, which is in agreement with the theoretical evaluation.
Therefore, as noted with regard to the phases of climbing stairs in Section 3, the system approaches the first step in mode 2. After it climbs the first step, it is transformed into mode 1. It then ascends the remaining stairs as if proceeding up a normal slope. Fig. 13 shows the phases of overcoming the stairs in sequence. The system traveled with a velocity of about 0.3m/sec as predicted.
The experiment of descending stairs is carried out in mode 1. When the robot descends without lock-up between the arms and the body, it exhibits better adaptability to the stairs and the degree of impact also decreases.
V. CONCLUSIONS
We proposed a new type of variable geometry single-track driving mechanism for a rescue robot. The performance indices were analyzed for the transformation and overcoming stairs and the design variables were evaluated. This mechanism has a symmetric configuration so that the robot advances in dual directions and prepares against overturning. Using transformation, it can reduce the energy consumption in steering and rotating while maximizing the capacity to overcome stairs. We have confirmed the feasibility of the proposed driving mechanism for a rescue robot through various experiments.
Although the task of overcoming stairs was emphasized in this paper, improvement of the system will be accomplished by optimization of energy efficiency and additional analyses of maneuvering over various off-road terrains in future work. Space utilizations for cameras, microphones, manipulators, lights, antennas, and so forth should be considered to achieve the objectives of the rescue robot.
REFERENCES
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[3] Taro Iwamoto and Hiroshi Yamamoto, Mechanical Design of Variable Configuration Tracked Vehicle, J. of Mechanical Design, Vol. 112, 289-294, 1990.
[4] J. D. Martens and W. S. Newman, Stabilization of a Mobile robot Climbing Stairs, Proceeding of the IEEE International Conference on Robotics and Automation, Vol. 3, 2501-2507, 1994.
[5] Kan Yoneda, Yusuke Ota and Shigeo Hirose, Development of Hi-Grip Crawler using a Deformation of Powder, JSRJ, Vol.15, 1188-1193, 1997.
[6] Hagen Schempf and E. Mutschler, Pandora : Autonomous Urban Robotic Reconnaissance System, IEEE, 2315-2321, 1999.
[7] Binoy Shah and Howie Choset, Survey on Urban Search and Rescue Robotics, CMU, Pittsburgh, PA 15213, 2003.
[8] Cheong Hee Lee and Yoon Keun Kwak, Double-track mobile robot for hazardous environment applications, Advanced Robotics, vol. 17, No.
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