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CHINESE JOURNAL OF MECHANICAL ENGINEERING Vol. 20,aNo. 3,a2007 24 DEVELOPMENT AND MOTION ANALYSIS OF MINIATURE WHEEL-TRACK-LEGGED MOBILE ROBOT* DUAN Xingguang School of Mechatronics Engineering, Beijing Institute of Technology, Beijing 100081, China School of Vocational Technology, Hebei Normal University, Shijiazhuang 050031, China HUANG Qiang School of Mechatronics Engineering, Beijing Institute of Technology, Beijing 100081, China XU Yan School of Vocational Technology, Hebei Normal University, Shijiazhuang 050031, China RAHMAN N ZHENG Change School of Mechatronics Engineering, Beijing Institute of Technology, Beijing 100081, China Abstract: A miniature wheel-track-legged mobile robot to carry out military and civilian missions in both indoor and outdoor environments is presented. Firstly, the mechanical design is discussed, which consists of four wheeled and four independently controlled tracked arms, embedded control system and teleoperation. Then the locomotion modes of the mobile robot and motion analysis are analyzed. The mobile robot can move using wheeled, tracked and legged modes, and it has the characteristics of posture-recovering, high mobility, small size and light weight. Finally, the effectiveness of the deve- loped mobile robot is confirmed by experiments such as posture recovering when tipped over, climb- ing stairs and traversing the high step. Key words: Mobile robot Locomotion modes Stair-climbing Posture recovery 0 INTRODUCTION Mobile robots are expected to play an important role in many areas such as radioactive and chemical installations, fire-fighting, anti-terrorist and reconnaissance activities. Such a mobile robot should have the adaptability to maneuver on different types of ter- rains like hills, ditch, steps, stairs or some other unknown obstacles. Basically, there are three types of locomotion mechanisms, wheeled, tracked and legged styles, and many researchers have studied these mechanisms. A lot of wheel type mobile robots were developed such as the omni-directional mobile vehicle 1, space rover 2-3, etc. In order to travel through obstacles and adapt to various ground conditions, different tracked mobile robots have been developed 4-10. For the locomotion in legged type robots, the biped 11, quadruped 12 and others have been presented. The wheel type locomotion mechanism can have high speed with low power consumption but not suitable for the obstacles like ditches, steps, steep stairways, etc. The track type of mobile robot has relatively high adaptability to conditions of the terrains as compared to the wheeled type but it has high energy consumption. The walking type of mechanism seems easier to adapt to various environments, however, its control and mechanism are compli- cated and even the stable high-speed traveling is a complex task. To combine the advantages of different mechanism, some re- searchers have studied the wheel-tracked robots. For emergency pur- pose in nuclear power plants, MF3 vehicle has been developed 13. It consists of four crawlers, trapezium-shaped, connected in pairs to a main body where a manipulator is installed. MAEDA, et al14, pro- posed a moving mechanism that combines merits of wheel and track type robots. The remotec robot (Andros series) 15-16 was designed with auxiliary tracks that are wrapped around a front and a rear mov- able arms, which are coaxial to the main tread sprockets as a surveil- lance robot and as a mobile platform for hazardous duty operations. In order to accomplish tasks in complicated environments such as narrow spaces, stairs, steps, steep terrains and other obsta- cles, the mobile robot must be small-sized, light weight, compact and transportable, and has the ability to recover its posture be- cause there may be a possibility to tip over during its normal op- * This project is supported by National Hi-tech Research and Development Program of China (863 Program, No. 2002AA420110). Received May 25, 2006; received in revised form January 30, 2007; accepted February 7, 2007 eration of climbing stairs or moving in an uneven terrain. In addi- tion, it is also desirable that the mobile robot can move at high speed and has good ability to negotiate obstacles. This paper presents the design of a mobile robot with the characteristics of posture-recovery function and obstacle traverse ability. The robot is equipped with wheels and tracks and inde- pendently rotating four tracked arms. These features allow the robot to move at high speed using wheels as well as capable of posture-recovering and negotiating with different terrains using tracked arms by means of its different locomotion modes. The real time processing, the high performance, small size and compact- ness are also achieved by the use of embedded control system. 1 ROBOTS MECHANICAL DESIGN The mechanism of a mobile robot (Fig. 1) is designed and developed by taking the following things into consideration. Fig. 1 Mechanism of mobile robot (1) Small size, light weight, compact and transportable. (2) High speed locomotion with lower energy consumption while running on a smooth surface. (3) The capability of performing some special functions like steering, pivoting and turning in narrow spaces. (4) The capability of climbing stairs. (5) The capability of posture recovery if the robot tips over. (6) The capability of manoeuvring in natural environment which includes the soft, uneven, marsh and steep terrains. 1.1 Driving wheel and track design The main mechanism of the developed differential drive mo- bile robot is composed of four wheels, four tracks and the body. Every wheel and track forms a locomotion unit which has the uniform structure and dimensions. The symmetrical layout is de- CHINESE JOURNAL OF MECHANICAL ENGINEERING 25 signed by locating the four units at front and rear, in both left and right side of the vehicle body. The main driving wheels are the rear wheels and each one is controlled to rotate independently by the respective motor. Every motor integrates with a planetary gear reducer in output shaft and an encoder at the rear. The four wheels have the same diameter of 200 mm, and the width of 80 mm. The concavo-convex shape is given to the wheel surface, so that it can offer more friction between wheels and ground. In order to reduce the weight of the whole vehicle, the wheels are made hollow. The track mechanism located inside the wheels can move as well as having the ability to rotate. In order to enhance its agility and ability to traverse wide range of obstacles, the four tracks can be actuated independently to rotate even a full circle around the pivot point. According to these locomotion requirements, the de- grees of freedom (DOF) are: 2-DOF for rear wheels; 4-DOF for every track movement and 4-DOF for every track rotation. Thus the total DOFs of the robot could be 10. The overall DOFs have been reduced to 8 by putting the motion of rear wheel and track together without affecting the functionality of the mobile robot. 1.2 Actuator and reducer The wheel and track system is provided with a mechanism that independently rotates the wheel track system and the arms shown in Fig. 2. Motor 2 provides the inside pivot for wheels driving power to operate the wheel mode locomotion through the integration of planetary reducer and a pair of end reducer gears. Motor 1 offers the track rotating power by driving the outside pivot that shares the same center line with the inside axis. The outside axis flange is connected with the track arm to achieve the track rotating locomotion. There are two pairs of bearings placed between the inside and outside axes, so the inner hole of the out- side axis holding the bearing outer ring and the inside shaft is attached with the bearing inside ring. In the outside driving sys- tem big reducer ratio is chosen to provide a low swing speed and big torque of the track when supporting the vehicle to lift up the body for the legged mode locomotion. In the driving system de- sign, the gear transmission and roll bearings are used because of their high transmission efficiency and lower frictioncompared with the worm gear style. Arrangement for a pair of pinion and gearwheels are used to reduce the width of the vehicle as well as to reduce the impact of force on the motor when it is actuated. Fig. 2 Wheel and track driving mechanism 2 CONTROLS AND TELEOPERATION 2.1 Hardware Fig. 3 shows the block diagram of the hardware being used by the mobile robot. Hardware includes control system, teleopera- tion system and environment sensing system. The primary control system includes 8 TMS320F2812 processor boards on which the PID controllers are implemented and 8 encoders for feedback. These TMS320F2812 processor boards are connected to an em- bedded controller ARM7 through controller area network (CAN) bus. The teleoperation is done by programming RS232 bus on both ARM7 and teleoperated PC using wireless serial communi- cation module Friendcom FC-201/s4325. The environment sens- ing system is a part of future work which will include ultrasonic sensors, CCD based vision system, bumper sensors, etc. Fig. 3 Block diagram of hardware 2.2 Software The software includes two communication protocols, i.e. the communication between PC-ARM7 and ARM7-DSP and three application programs on PC, ARM7 and DSP board side. (1) Application programs for PC, ARM7 and DSP processors. The system can operate both in teleportation and semi autono- mous modes, therefore both ARM7 and the PC side are pro- grammed for teleoperation and semi autonomous modes. The semi-autonomous mode only includes the command signal from PC to ARM7 for different tasks that are programmed on the ARM7. The teleoperation mode includes the calculation of the trajectories and their transmission to the ARM7 to perform dif- ferent tasks. In both the teleoperation and the semi autonomous mode the TMS320F2812 processor boards are programmed to implement PID controller. (2) Communication protocol between PC and ARM7. The communication between ARM7 and teleoperation PC includes a 19 bytes frame of data. The transfer of data initiates with start byte followed by the data to select the mode. The modes include the wheel motor program mode, track data program mode, emergency break mode, read belt angle mode and communication state checking mode. (3) Communication between ARM7 and DSP boards. The communication comprises of motor control mode, communication test mode and arm angle monitor mode. The motor control mode includes the transmission of data from the ARM7 to DSP control cards. The communication mode frame includes the two ways transmission, i.e. broadcast from ARM7 and receiving from the DSP cards. Arm angle monitor mode also includes the two way communications, i.e. the request for the angle for a particular arm from ARM7 and in return the frame with the current angle of the requested arm. 3 LOCOMOTION MODES The wheel-track-legged style robot can have many kinds of locomotion modes. The obstacle negotiation can be considered as the basis for selection of the locomotion modes. Due to the 4 inde- pendent track arms, different obstacle postures can be achieved by controlling different arm joint angle. The basic locomotion mode and its advantages are as follows. CDUAN Xingguang, et al: Development and motion analysis of miniature wheel-track-legged mobile robotC 26 3.1 Wheeled mode Fig. 4a shows the wheeled mode where tracked arms of the robot rotate up and 4 wheels touch the ground. This mode is used to make the robot move forward, backward and turn round at high velocity with minimum power consumption. 3.2 Legged mode This mode can realize obstacle negotiation locomotion of the robot in the special legged pattern in an unstructured environment as shown in Fig. 4b. By controlling the joint angle of the tracked arm the robot can step obstacles like quadruped. The obstacle negotia- tion capability is tremendously increased by the increase in the chassis height from the ground. Fig. 4 Locomotion mode 3.3 Tracked mode This locomotion mode primarily uses the 4 tracks for provid- ing the driving power. The independent rotating arms increase the obstacle negotiating capability. Different obstacles will make mo- bile robot respond in different sets of sequences. Followings are some basic obstacle negotiation methods(Fig. 5). Fig. 5 Track mode obstacle negotiation method (1) Fig. 5a shows that all the 4 tracks are placed parallel to the ground and the lower portion of the track has a full contact with the ground. This allows the robot to maneuver in natural environments such as soft, marsh and uneven terrain. (2) Fig. 5b shows the step climbing sequence in which the robot extends its front arms at an attack angle with the ground and can climb a step having a height even two times the height of the wheels diameter. (3) The special locomotion method shown in Fig. 5c allows the robot to climb and descend the standard step size stairs despite the short length of the tracks. The last finishing sequence for de- scending the robot along the stair is done smoothly by the appro- priate positioning of the rear arms as shown in Fig. 5d. (4) The large angled slope can be climbed easily because of the extra power of 4 actuators provided with each track as shown in Fig. 5e. (5) The robot stabilization is done by the independently con- trolled arms while moving laterally along the slope as shown in Fig. 5f. 4 MOTION ANALYSIS In this section, we mainly discuss the motion procedure of the mobile robot for some typical situations such as posture re- covery, stair-climbing and obstacle negotiation. 4.1 Posture recovery There may be a possibility of the mobile robot to tip over during its normal operation of climbing stairs or moving in an uneven terrain. This situation will stop the robot to perform its normal tasks. To rectify such situation the robot is provided with posture recovering locomotion scheme. The sequences of phases of the scheme are depicted in Fig. 6. Fig. 6 Motion analysis for posture recovery Phase 1: Fig. 6a shows the initial posture of the robot when tipped over. Rotate down F1 and F2 (denoted with F for front arms) to uplift the front side of the vehicle body (denoted with B for body) and gradually incline it to the position shown in Fig. 6b. Phase 2: Rotate the two arms R1 and R2 (denoted with R for rear arms) in the counter clockwise direction to the position shown in Fig. 6c. This provides support area close to the centre of gravity (COG) of the robot. Phase 3: The further actuation in the arms R1 and R2 will lift the body B in the clockwise direction and during the course of time move the arms F1 and F2 in counter clockwise direction as shown in Fig. 6d. Phase 4: Actuate arm R2 to rotate clockwise to touch the ground on the other side as shown in Fig. 6e. Phase 5: Actuate both the right side arm R1 and R2 in counter clockwise direction equal angle so the COG of the body shifts towards right as shown in Fig. 6f. Phase 6: Once the shifting of the COG is finished, actuate the arm R1 clockwise to touch the ground on the other side as shown in Fig. 6g. Phase 7: Actuate both the arms R1 and R2 counter clockwise slowly to lower the body to touch the ground and adjust the pos- ture of the robot to the final position as shown in Fig. 6h. 4.2 Stair-climbing Among the artificial structures stairs are one of the most dif- ficult situation for a mobile robot to traverse. The four independ- ently controlled arm structures are developed to climb the stairs successfully. Initially our mobile robot is standing in front of the CHINESE JOURNAL OF MECHANICAL ENGINEERING 27 stair with its both arms making an angle of 60 with the body shown in Fig. 4a. The stair climbing method can be divided into sequentially control action or phases (Fig. 7). Fig. 7 Motion analysis for climbing stairs Phase 1: The vehicle locates itself in front of the stairs and ro- tates its front arm F towards the stair making an angle of 55 with the ground and the rear arm R is rotated to be parallel with ground. Phase 2: The mobile robot starts to move forward with a lower velocity until the front track F hits the leading upper edge of the first step as shown in Fig. 7a. Phase 3: The third phase starts with the rotation of the tracks of all the arms and lower the front arms slowly to lift the robots body till the arm F and the body are aligned. This makes the angle of the robot with the ground the same as the stair as shown in Fig. 7b. At this stage the mobile robot has two contact points with the stairs and one with the floor. The small wheel of the front arm touches the leading edge of second step and the rear wheel of the front arm touches the leading edge of the first step. Phase 4: Once the three parts of the whole robot are inline on the stairs, it can climb on the stairs robustly by using the torque of all the four tracks until the arrival of the last step of the stairs with- out any change in the robots posture as shown in Figs. 7c, 7d. Phase 5: The fifth phase starts with the arrival of the last step where the rear wheel of the front arm F is on the leading edge of the last step and the front wheel of the front arm is in the air since no steps ahead as shown in Fig. 7e. While the robot moves forward it lowers F by an angle equal to the stair inclination angle to make it parallel with the ground. Simultaneously, its rear arms are raised to avoid uplifting the robots body as shown in Figs. 7f, 7g. Phase 6: The final phase includes the repositioning of the robot to its initial track mode posture shown in Fig. 7h. 4.3 Climbing high steps By using its tracked arms, the robot can easily traverse normal dimension steps. The developed design allows climbing even a higher dimension steps. The sequences of phases of the scheme are depicted