Double-track mobile robot for hazardous environment applications

Transcription

1 Advanced Robotics, Vol. 17, No. 5, pp (2003) Ó VSP and Robotics Society of Japan Also available online - Short paper Double-track mobile robot for hazardous environment applications CHEONG HEE LEE 1;, SOO HYUN KIM 1, SUNG CHUL KANG 2, MUN SANG KIM 2 and YOON KEUN KWAK 1 1 Department of Mechanical Engineering, KAIST, Yuseong-gu, Daejon, Korea 2 Advanced Robotics Research Center, KIST, Sungbook-gu, Seoul, Korea Received 4 July 2002; accepted 3 October 2002 Abstract This paper introduces a link-type tracked vehicle, which is developed for potential applications such as re ghting, handicapped assistance and mine detection in various hazardous environments. The vehicle consists of three parts front frame, rear frame and body. The front frame is connected to the rear frame by a rotational passive adaptation mechanism, which is the driving mechanism of the vehicle. This is similar to a link structure such that one frame rotates to the other by external forces between the vehicle and the ground. This passive adaptation mechanism permits good adaptability to uneven terrain including stairs. This link structure also improves energy ef ciency, and makes the vehicle simple and small. The body is a control system for remote control of the vehicle. It communicates visual and distance information to the operator, and commands direction and velocity orders. Keywords: Mobile robot; link structure; passive rotational mechanism; friction coef cient range; stair climbing. 1. INTRODUCTION Robots are no longer just a part of movies. In the near future they will be invaluable to humans, carrying out dangerous work such as re ghting, mine removal and terror suppression. Similarly, they will execute important roles such as building cleaning and assistance for the physically handicapped. In order to use robots in these various elds, off-road navigation ability is necessary because robots have to pass over uneven terrain such as stairs and mountains. The driving mechanism for this ability is a tracked vehicle. A track mechanism is simple to control and mobile on uneven terrain [1]. To whom correspondence should be addressed. kyk@kaist.ac.kr

2 448 C. H. Lee et al. Disadvantages of tracked vehicles are low energy ef ciency and large size. One method to compensate for these problems is a variable con guration tracked vehicle (VCTV), which can change its track con guration according to the ground conditions. Kohler proposed a tracked vehicle using four variable con guration tracks [2]. Maeda proposed a vehicle that added the characteristic of track type to wheel type by using four steering wheels [3]. Iwamoto developed a vehicle wherein track con guration was changed by using a planet wheel [4]. Yoneda proposed a vehicle with a friction coef cient and contact area between the track and stairs that was increased by changing the track material [5]. However, additional motors are needed to change track con gurations for various environments. This is the weak point of previous VCTVs as they are poor in respect of energy ef ciency. This paper proposes a new VCTV that has good ground-adaptable ability and a passive rotational mechanism. The passive mechanism improves the energy ef ciency because it does not need an additional motor to change the track con guration. This passive concept also improves environmental adaptability. 2. DESIGN OF THE DOUBLE TRACK Track types are better than wheel and walking types as a driving mechanism for autonomous navigation vehicles because of high environment adaptability and simple control. However, they also have lower energy ef ciency for navigation and rotation than other driving mechanism types. The VCTV has been proposed to improve these problems in existing tracked vehicles such as tanks. However, previous VCTVs needed additional motors to change the track con guration according to the environment. This paper proposes a new variable con guration tracked vehicle using a passive rotational mechanism. This mechanism is based on passivity and a link structure. The basic concept is shown in Fig. 1. The robot consists of three parts: front frame (Fig. 1a, 1), rear frame (Fig. 1a, 2) and body (Fig. 1a, 3). The rst two parts are the driving mechanism of the robot. Two tracks are used for each frame and they have a link structure in the middle of the robot. (a) Figure 1. Structure of the double-track mechanism. (a) Top view and (b) side view. (b)

3 Double-track mobile robot for hazardous environment 449 Figure 2. Wall climbing for the double track. A passive rotational mechanism needs no additional motor because one frame rotates freely with respect to the other by external forces. This passive link also improves environment adaptability because a double track connected by a link changes its con guration according to the ground conditions. The body is connected to the driving mechanism by a link structure. 3. ANALYSIS OF THE DOUBLE TRACK The robot has to work in various hazardous environments. For ef cient navigation, it is important that small robots can pass high obstacles. Therefore wall-climbing conditions have to be analyzed. Figure 2 shows a situation where a double track meets a wall. F 1 represents friction force on the surface, F 2 is the repulsive force on the wall and F 3 is the friction force on the wall. M 0 indicates the driving moment, M is the mass of the robot, and it is supposed that the mass points of the front and rear bodies are at the middle of each frame. The double-track con guration is based on a passivity concept. This is determined by ground roughness, and the forces generated between the track and ground. Therefore, in order for the robot to overcome the wall, the total moment for point o has to be counterclockwise. This is shown in (1), where c is the distance between wheels and r is the wheel radius: F 1 r C F 3.c C r/ > 2M 0 C Mg 4 c 2 ; (1) F 1, F 2 and F 3 are determined by friction coef cient equation (2) and force equilibrium where ¹ is the friction coef cient, as represented in (3): F 1 D ¹N; F 3 D ¹F 2 ; (2)

4 450 C. H. Lee et al. Figure 3. Double track on even terrain. F 1 D F 2 D ¹Mg 2.1 C ¹ 2 / ; F 3 D (3) ¹2 Mg 2.1 C ¹ 2 / : M 0 is obtained from the condition that the robot can move without slipping on a plane as shown in Fig. 3. The driving force F is obtained by force and moment equilibrium (4) and rolling condition, x D rµ. Then the driving moment is determined by substituting F into non-sliding condition (5) as represented in (6). Wheels are supposed to be of cylinder type for mass moment inertia, J. M Rx 2F D 0; J Rµ C F r D 2M 0 ; (4) F 6 ¹N; (5) M 0 6 ¹rMg : (6) 4 From (1), (3) and (6), the relation between the track speci cation and friction coef cient to climb the wall is obtained as follows: r c > 1 3¹2 4¹ 2.1 ¹/ : (7) The wall-climbing range of track speci cations r and c according to friction coef cient ¹ is shown in Fig. 4. The graph shows the minimum values of r=c to climb the wall and the dotted area represents the wall-climbing range without considering geometric limitations. The geometric limitation is that the diameter cannot be larger than the distance between the wheels, c. This means r=c has to be below 0.5. Finally, the wallclimbing range is represented by the slashed area. Thus, the friction coef cient ¹ has to be above 0.5 to climb the wall. In order to compare the result with one body type, the wall-climbing conditions of a tank are obtained as follows under the same conditions:

5 Double-track mobile robot for hazardous environment 451 Figure 4. Wall-climbing range. Figure 5. Wall-climbing range for one-body type. r c > 1 ¹ 2 2¹ 2.1 ¹/ : (8) Similarly, the relation between ¹ and r=c is shown in Fig. 5. There is no slashed area. This means that r=c does not satisfy the geometric condition in the whole friction coef cient range. Therefore, in order to climb the stairs as one body type, the wheel radius has to be larger than the height of the stairs. To overcome this problem, previous VCTVs used additional motors for frame rotation to achieve the attack angle. However, this induces another disadvantage in terms of energy ef ciency. In this paper, a link-type tracked vehicle is developed to solve these energy and size problems. 4. FRAME DESIGN OF THE DOUBLE TRACK The wall-climbing range of the friction coef cient for an I-type link track is above 0.5, which is analyzed in Section 3. This can be a constraint for the navigational

6 452 C. H. Lee et al. Figure 6. Triangle-frame double track. Figure 7. Relationship of ¹; and r=c for the triangle track. environment of the robot. Therefore the frame has to be designed so as to increase this range. A triangular frame designed to increase the wall-climbable friction coef cient range is shown in Fig. 6. The frame increases the effect of the repulsive force by the attack angle. Similarly, the wall-climbable condition is shown in (9), and r=c is related to ¹ and as shown in Fig. 7: r c > 1 2¹2 2¹ sin 2¹ 2 cos : (9) 2¹ 2.1 ¹/ Figure 7 shows that the triangle-frame double track can climb the wall if the friction coef cient is above When ¹ is equal to 0.4, for example, the attack angle has to be above 20 ± to climb the wall, as represented by the slashed area in Fig. 7. Thus, the wall-climbing range is increased. Another advantage of the triangle track is shown in Fig. 8. If motors on both sides are driven in the inverse direction, the frame con guration changes as in Fig. 8. This reduces the energy consumption when the robot rotates or navigates on a plane.

7 Double-track mobile robot for hazardous environment 453 Figure 8. Con guration change of the triangle track. 5. CONSTRUCTION OF THE ROBOT Figure 9 shows the structure of the robot. The robot consists of a driving mechanism and body. The body consists of a control block and camera, and the driving mechanism is comprised of front and rear frames thus it is called a double track. The body is connected with the double track by four joints, as shown in Fig. 10. It is designed for the body to be inclined according to the change of the relative angle between the front and rear frames. A hydraulic damper is attached on each side of the body in order to absorb impacts from rough landforms. Figure 11 shows the implemented double-track mobile robot. Eight ultrasonic sensors, four in front of the robot and two on each side, are used so the operator can feel the force re ection that the remote robot sends. A pan tilt stereo camera is employed to control the robot in remote areas, where the camera monitors the surrounding. In the control block, controllers to the control amp and motors are inserted in the middle of the block where the pan tilt camera is attached. A power control system is established on the front where an emergency switch is also attached. It can turned on or off with a RF remote controller. There are also a small LCD PC established in the robot that can monitor a PC and a fan for cooling. The batteries that supply power are located on the bottom of each frame. The speci cations of the robot are described in Table 1. The system has major features such as force re ection, stereoscopic monitoring and pan tilt operation according to the position detection sensor. Ultrasonic sensors are used for the force re ection. The distance to an obstacle is measured and sent to the remote operation PC. Then the operator feels the force in the opposite direction of the obstacle. This enables the robot to avoid the obstacle. The stereoscopic monitoring system enables the operator to observe the surroundings of the robot in a remote area by the attachment of a stereo camera on the pan tilt mechanism. The video information is sent to the operator through an independent RF channel to reduce the data communication load. Also, the pan tilt mechanism is designed to rotate according to the position and movement of the operator s head because it is dif cult for the operator to control the camera while controlling the motion of the robot. The control system of the robot consists of one PC, three sub-controllers and a remote PC. The sub-controllers have microprocessors and each controls two motors. They communicate the speed data of the motors and sensor information to the PC

8 454 C. H. Lee et al. Figure 9. Structure of the robot. Figure 10. Connection of the body and double track. Table 1. Speci cations of the robot Item Size (w h l) Weight (battery included) Maximum velocity Power source Speci cations mm 50 kg 2 m/s Ni Cd rechargeable battery through a CAN bus. A wireless LAN is used for communication between the robot and the remote PC. The robot sends various sensor information to the remote PC and then the PC commands the robot to move in a direction with a certain speed. Figure 12 shows a block diagram of the control system of the robot.

9 Double-track mobile robot for hazardous environment 455 Figure 11. Double-track mobile robot. Figure 12. Control system diagram. 6. EXPERIMENTS Stairs are one of the most dif cult obstacles for a robot to overcome in a building. Figure 13 shows the stair-climbing experiment for the constructed robot. It is

10 456 C. H. Lee et al. (a) (b) (c) Figure 13. Stair climbing experiment: steps (a) 1, (b) 2 and (c) 3. veri ed that the passive mechanism has good adaptability for the environment. The maximum climbable stair angle is 40 ±. Figure 14 shows an uneven terrain navigation experiment. The terrain has irregular rounding and 31 ± of slope. The robot also displays good movement over this environment.

11 Double-track mobile robot for hazardous environment 457 (a) Figure 14. Uneven terrain navigation experiment: steps (a) 1 and (b) CONCLUSIONS (b) This paper describes the design of a double-track mobile robot based on a link mechanism and passivity. It improved on previous robots required to change the track con guration to t the environment. The passive rotational mechanism made the vehicle not only highly adaptable to different environments, but also more ef cient because an additional motor for con guration change is not needed. The wall-climbable condition was obtained as a function of track speci cation and friction coef cient. In order to increase the wall-climbable friction coef cient range, a triangle frame was designed. In addition, the robot was controlled by stereovision monitoring and PC-based sensor technology. The robot was tested on stairs and uneven terrain designed for the test. A remote PC and joystick were used to drive the robot. The results showed good environment adaptability and demonstrated the ef ciency of the new design concept. REFERENCES 1. T. Iwamoto and H. Yamamoto, Mechanical design of variable con guration tracked vehicle, J. Mech. Des. 112, (1990).

12 458 C. H. Lee et al. 2. G. W. Kohler, M. Selig and M. Salaske, Manipulator vehicle of the nuclear emergency brigade in the Federal Republic of Germany, in: Proc. 24th Conf. on Remote Systems Technology, pp (1976). 3. Y. Maeda, S. Tsutani and S. Hagihara, Prototype of multifunctional robot vehicle, in: Proc. Int. Conf. on Advanced Robotics, Japan, pp (1985). 4. T. Iwamoto and H. Yamamoto, Stairway travel of a mobile robot with terrain-adaptable crawler mechanism, J. Robotic Syst. 2 (1), (1985). 5. K. Yoneda, Y. Ota and S. Hirose, Development of hi-grip crawler using a deformation of powder, J. Robotics Soc., Japan 15 (8), (1997). ABOUT THE AUTHORS Cheong Hee Lee graduated in the Department of Mechanical Engineering of KAIST in He is currently working on the development of mobile robots based on adaptability and passivity for his PhD. Soo Hyun Kim received his PhD in 1991 on a subject concerning parameter identi cation and model-based control of direct drive robots from Imperial College and since 2001 he has been a Professor at KAIST. His current research interests include mili-machine systems, micro-manipulation and a new actuating mechanism using piezoelectric materials. Sung Chul Kang graduated in the Department of Mechanical Design and Production Engineering of Seoul National University in 1989 and he received his PhD from the same university in During , he worked in the Mechanical Engineering Laboratory in Japan. His research activity deals with robot control, mobile manipulation, humanoids and haptics. Mun Sang Kim graduated in the Department of Mechanical Design and Production Engineering of Seoul National University in He received his PhD from the Technical University of Berlin in Since 2001 he has been Head of the Advanced Robotics Research Center in KIST. His main eld of activity is advanced robotics system integration and humanoid robotics.

13 Double-track mobile robot for hazardous environment 459 Yoon Keun Kwak was born in During he worked at the University of Texas. For his PhD he carried out research investigating the dynamics of rubber-tired automated guideway transit vehicles. Since 1985 he has been a Professor at KAIST. He has done extensive research in the eld of design, modeling and simulation of direct drive SCARA robots and low-cost, free-ranging AGVs.