Design and Manufacturing of a Mobile Rescue Robot

Proceedings of the 2006 IEEE/RSJ International Conference on Intelligent Robots and Systems October 9 - 15, 2006, Beijing, China Design and Manufacturing of a Mobile Rescue Robot S. Ali A. Moosavian1 Hesam Semsarilar2 Arash Kalantari3 Department of Mechanical Engineering K. N. Toosi Univ. of Technology, Tehran, Iran, P.O. Box 16765-3381 Email: moosavian@kntu.ac.ir Abstract. This paper presents design and manufacturing procedure of a tele-operative rescue robot. First, the general task to be performed by such a robot is defined, and variant kinematic mechanisms to form the basic structure of the robot will be discussed. Choosing an appropriate mechanism, geometric dimensions, and mass properties will be detailed to develop a dynamics model for the system. Next, the strength of each component is analyzed to finalize its shape. To complete the design procedure, Patran/Nastran was used to apply the finite element method for strength analysis of complicated parts. Also, ADAMS was used to model the mechanisms, where 3D sketch of each component of the robot was generated by means of Solidworks, and several sets of equations governing the dimensions of system were solved using Matlab. Finally, the components are fabricated and assembled together with controlling hardware. Two main processors are used within the control system of the robot. The operator’s PC as the master processor and the laptop installed on the robot as the slave processor. The performance of the system was demonstrated in Rescue robot league of RoboCup 2005 in Osaka (Japan) and nd achieved the 2 best design award. would be to find victims, determine their situation, and then report back their findings based on a map of the building, [1]. These will immediately be given to human rescue teams preparing to extract all victims that are found. Further expectations of rescue robots such as being able to autonomously negotiate compromised and collapsed structures and provide structural shoring, find victims and ascertain their conditions, deliver sustenance and communications to victims, and emplace sensors (acoustic, thermal, seismic,…) are active research fields. Nevertheless, the basic capability of rescue robots should be their maneuverability in destructed areas which thoroughly depends on their locomotion system and their dimensions. Various rescue robots have been designed and manufactured. For instance, an autonomous Urban Reconnaissance Robot, in the size of 20 Kg, was developed by a research group from University of Southern California, [2]. PolyBot is an example of modular reconfigurable robots, which is a rather small and economic prototype, [3]. CEDRA is also a rescue robot with the ability to adjust its locomotion system with the terrain on which it performs, [4]. Index Terms – Robotics - Tele-operative – Locomotion Mechanisms. This paper presents an illustrative description of the Resquake project at KNTU. Innovative mechanisms and software-based steps of the design procedure are highlighted here. First, the most successful projects in terms of their mechanical structure and locomotion capabilities, regardless of their autonomy and the sensors mounted on them, are studied and concluded to develop new special mechanisms to enhance the maneuverability of a mobile robot while trying to keep its dimensions relevant to the environment in which it performs. After selecting suitable mechanisms, dimensions and parameters of the system are defined. The system dynamics is discussed and the sequence of stress analysis for each member of the mechanism is addressed in order to finalize its shape and to select suitable material for its fabrication. The last phase of the project was to manufacture the parts and assemble the system. Electronic devices and control system of the robot is described in the last section. Resquake is a robot with great capabilities in climbing obstacles in destructed areas, which was participated in Rescue robot league of RoboCup 2005 in Osaka (Japan) and achieved the 2nd best design award. I. INTRODUCTION. Mobile robots whether autonomous or tele-operative play an important role in different fields of human life. Mobile robots are mainly operated for investigating areas in which human health is endangered. Police robots, fire fighter robots and rescue robots are examples of such application. Mobile robots are also used for assisting human forces for doing repeated works such as moving heavy boxes within a defined path in a factory or providing the patients with appropriate medicine on time in a hospital. Earthquake is a natural incident, which threatens human life. Aftershocks occurring a while after the main earthquake cause secondary collapses and take victims from the search and rescue personnel. In order to minimize the risks for rescuers, while increasing victim survival rates, fielding teams of collaborative robots is a good alternative. The mission for the robots and their operators 1- Associate Professor 2- B.Sc. Student 3- Graduate Student 1-4244-0259-X/06/$20.00 ©2006 IEEE 3982 II. MECHANISM DESIGN There are three major categories of rescue and search robots in terms of locomotion system, i.e. wheeled, tracked, and legged robots as shown in Fig. 1. The simplest are wheeled robots, while tracked robots are used because of their ability to move on uneven terrains and their inevitable traction. Legged robots can be the most maneuverable, due to their high degree of freedom. The number of actuators and sensors is relatively high which makes their dynamic analysis and modeling more complicated than the former types. Consequently, stable control of such systems is more difficult, and eventually more expensive. environment. Whether destructed or not a rescue robot should have the ability to climb and move down stairways in order to cover the whole area. In order to compromise between the two contradictory aspects of providing a small robot with maneuverability of a larger locomotion system, an exceptional mechanism has been developed. This mechanism, which includes a base with two expandable tracks (arms), enables the robot to resize depending on the situation that encounters. Accordingly, these tracks should have a minimum length to prevent loosing its balance and having a steady movement without extra vibrations, Fig. 3. However, such lengthy tracks will need a wide area for turning, which is rarely reachable in a destructed environment, and a compromise between these two aspects is also required. Fig. 3. Minimum length for tracks of the robot A. Expandable tracks (Arms) Fig. 1. Three major categories of rescue and search robots Wheeled robots could be considered as the lowest-priced system to implement for searching flat areas, while developing the autonomy of such systems is easier due to its simple dynamics. Of course wheeled robots are capable of climbing up obstacles depending on the diameter of the wheels while a relatively small tracked robot has the same capability. The sketch shown in Fig. 2 compares the two systems encountering the same obstacle to climb. Weeled platform tracked platform Fig. 2. Two types of locomotion encountering the same obstacle As well as the type of locomotion, the size of a mobile rescue robot is also an important issue. In a destructed indoor field there may exist some obstacles that can not be passed by any system such as when the walls or the ceiling collapses. At this situation, the robot should search for a bypass or a way between the obstacles rather than climbing over them, which definitely depends on its size. A relatively small robot can easily pass a narrow aisle and continue its search. It should be noted that stair way is an inseparable part of an indoor The kinematic structure shown in fig. 1 (top right) enables the robot to expand its tracks when it needs to bypass obstacles. Like wise, when the robot is going through narrow passages and needs to be rather small, the front tracks can be closed. This was the original idea, developed to overcome the contradiction. This concept is improved to a system with two pairs of arms at both sides of the vehicle, That would reduce the length of the robot with closed arms while the expanded length remains relevant. Another advantage would be the symmetry of the plan, which enables the robot to approach in both directions. This arrangement remains operative in turning in a confined space. So far, the length of the robot with closed arms has become shorter than its width. The next improvement is to make the arms collinear where the main tracks are located at each side,. The last improvement is adding another joint to each arm in order to use an extra area between the arms when they are closed. The tracks on each side of the robot are also separated into three parallel planes, which provides more efficient traction. Top view of the designed locomotion system is shown in Fig. 4 . However, it should be noted that adding four independent joints to the system would increase the complexity of its dynamic model as well as the number of actuators and the total price of the system. Therefore, a planetary gear chain is substituted to simply transmit the power of the main joint of each arm to its second joint. In this case, the rotation of the two parts for each arm will not be independent. Thus, two desirable positions of the arms are considered, and the gear 3983 ratio is obtained such that the arms move based on a path between two main positions, Fig. 5. It should be noted that while the main part of the arm rotates π/2 rad, the second part should rotate more than π rad. The gear chain with such performance should be a planetary gearbox. The main part of the first arm play the role of the arm in the planetary chain, which is directly powered by a motor. The sun gear should be attached to the body of the robot and the planet gear should be a attached to the second part of the arm. A pair of medium gears are placed between the sun and the planet so that the diameter of gears does not exceed a reasonable length (the diameter of the main wheels of the tracks), Fig. 6. Another advantage of this mechanism is that the distance between the centers of the two joints of the arm will remain constant during its rotaion. This enables us to fill the gap between the main track and the arm's with another track, Fig. 4. This track can also be used to transmit power from the main part of the tracks to the latest part of the arm. B. Tracks The traction of the locomotion system strongly depends on the friction between the track peaces and the surface on which the robot moves. That is why the material and the shape of the track pieces are of great importance. On the other hand, the tracks should also bear a reasonable tension. Thus, the tracks are designed to be assembled using two components. A basis of chain-sprocket provides the system with sufficient tensile strength and tooth shaped pieces made of latex filled the gap between the chain and surface to create required friction. Figure 8 shows how latex pieces are attached to the chains. Arm’s motor Robot’s Arm Fig. 7. Final structure designed for the planetary gearbox Attached to the main body Medium tracks Fig. 4. Final mechanism chosen for the tracks Fig. 8. Latex pieces fixed on the chain C. Suspension system Two major advantages are obtained by including a suspension mechanism. - Increasing the flexibility and stability of the system on bumpy surfaces; - Damping any shock to the system caused by collisions. Fig. 5. The path for motion of the arms Gear4 Planet Gear Gear3&Gear2 Medium Gears Gear2 Sun Gear The suspension system was designed by separating the two sides from the main body and then attaching them by a revolute joint.. A pair of linear springs limits the angle of rotation and makes the system remain at a single position when no extra forces are applied. It should be mentioned that using dampers was not needed, because the friction of the plain bearings used as the so-called joints was enough to limit any extra shaking of the springs. Robot’s Arm l4 l3 l2 l1 Fig. 6. Planetary gear-chain Helical gears are chosen for the planetary gears due to their small backlash and also higher strength of gear tooth comparing with spur gears, [5-6]. The angular velocity of the arm should be less than 2-4 rpm while the motor's output velocity is normally around 3000 rpm. Hence the transfer ratio between the arm motor and the link should be around 1000. Therefore, a combination of a 3 stage planetary gearbox , (constructed right at the motor shaft where the angular velocity is relatively high) with a ratio of 3:1 at each stage, and a wormgear set with ratio of 30:1 (gear is attached to the arm's link) provides the desirable ratio within a compact space, Fig. 7. As shown in Fig. 7, the tracks at each side of the robot are powered by a DC motor. Finishing the design of locomotion mechanisms, now it is time to determine the dimentions. Some of the components for building the mechanisms are available as standard parts, so we should select other dimensions to match their counterparts. Besides, the overall size of the robot and the formulas on the gear chains must be considered in our calculations. Since there are numerous equations governing these factors, an optimised solution is not reachable by manual calculations. Thus, Matlab is used to extract desired values from a set of equations. III. DYNAMICS Analysis A. Locomotion system 3984 There are some important points in robot motion: 1- Convenient control over the robot’s motion by the operator; 2- Stability of the robot while maneuvering; 3- Reducing the risk of collisions. Using the expressions for the kinetic and potential energy, and applying Lagrange’s equations for a constrained or unconstrained mobile robotic system, the dynamics model can be obtained as presented in [7-8]. Here, avoiding a cumbersome analysis, in order to determine the torques needed to actuate the locomotion system and arms, it is reasonable to choose the input torques such that they suffice the highest torques assumed to be applicable on the system. Thus, considering a simplified model as shown in Fig. 15, the minimum torque required for climbing a surface with slope of 35˚ is calculated. To this end, Newton's 2nd law can be written for the motion direction: ∑F = ma x x (1) Finally, the appropriate speed, by operating the robot within a variety of situations was obtained around 18~20 cm/s. B. Arms The rotational speed of arms is chosen in range of 3~5 rpm. The exact speed will be specified after evaluating the power of actuators. In order to obtain the appropriate torque applied on the arms, we should consider the most critical case that the system may encounter, which is assumed as shown in Fig. 10, and the motor torque is determined such that the robot bears its own weight. Fig. 10. Arms bearing the weight of the robot over a hole The torque of the arm’s motors is calculated by considering the free body diagram of the gear chain within each arm, Fig.6. Solving the moment equation yields: N Ft T = (4) where W=30 kg, la = 106 mm, l1 = 44 mm, l2 = 43 mm, l3 = 40.5 mm, and dG4 = 39 mm. Mg y W .l a .(l1 + l 3 − l 2 ) + 0.25W (l1 + l 2 + l 3 ) 8d G 4 By replacing appropriate values, the required torque on each arm is obtained as: x y Fig. 9. Free body diagram of the robot over a 35o slop Since the final speed of robot is to be rather small and constant, we could assume the linear acceleration and thus the rotational acceleration of the wheels to be zero: ax ≈ 0 Ft − mg sin θ = 0 ⇒ Ft = mg sin θ (2) So, an estimation of the robot’s mass is considered as m=30 kg. This estimation should be made conservatively because the whole calculations should be repeated if the robot appeared to be heavier when it was completely designed: T= 26.5 N.m Simulating a model of the arms using ADAMS, the same result is abstained, which confirms the above approximations. By specifying the speed and power needed we can choose the motors. Assuming the power losses due to friction in chains and other components to be less than 50% of the maximum power, 7.5 Nm output torque would be sufficient for each side. Specifications of the selected motors are listed in Table I. Table I Specifications of motors ⇒ Ft = 163.25 N Writing the moment equation with respect to the center of one of the wheels, the desired torque is determined as follows: ∑M = Iα α ≈ 0 ⇒ T t = Ft .r = 9.5N .m o (3) Torque Rotary speed Nominal Voltage Nominal Current Gear ratio Locomotion motor 10 N.m 14.75 Rpm 24 V 1.92 A 16:12 1.5 N.m 120 Rpm 27.5V 1.5 A 1:30 Arms motor As mentioned in section 1.3, the locomotion system consists of two separate sides connected to the main body while a motor independently actuates the track at each side. Consequently, the desired torque of each motor is obtained by dividing the T t by 2: Tmotor ≈ 5 N.m Motor specification Arm’s motor is coupled to the arm of the planetary gear-set with a 1:30 worm gear set, resulting in output torque of 30 N.m (considering losses) and maximum speed of 4 rpm. IV. STRESS ANALYSIS The aim of such analysis is a minimal weight design, to reduce on-board energy consumption. The results lead us to redesign critical parts such that undesired deformation and stresses are prevented. There are two methods available: 3985 1. Classic methods; 2. Numerical methods. Using classic methods, the stresses in all gears and bearings can be calculated and properly designed. Using this method is not applicable to some parts, e.g. the motor base structure or the gear-set basis, where equations became really complex and unsolvable. In such cases, the finite element method using Patran/Nastran is applied. For instance, the part shown in fig. 11, is considered as the arm for the planetary gear-set, considering the following roles defined for it: - This part acts as the arm of the planetary gearbox. - It must stand all the force and moments applied to robot’s arm, which are transferred to this part via Gear 4 of planetary system. - It should form the cover of planetary gearbox. By applying the loads and constrains and assuming that this part is made of aluminum the result of analysis was obtained as shown in fig. 11. According to this analysis, the maximum pitting stress is 78 Mpa. By ignoring the effect of pitting stress, the maximum stress is 35 Mpa which comparing to the yield strength of aluminum is acceptable. The hardware of the control system consists various devices: 1. An operator desktop PC (A P4 2.4 MHz with 256 Mgb sdRam) 2. An on-board Laptop (HP Model: nc4000 with 12.1” LCD,Wireless LAN 802.11 a/b/g, 512 MB Ram, CPU: 1.6 Mobile) 3. Micro controllers (mostly ATMega 128 with 53 I/Os, 8 A/D, 2 serial ports, Machine frequency of 16 MHz, 128 KB of program memory and 4 KB Ram), 4. Sensors: (pyrometer, potentiometer and cameras), electronic circuits (for Interface, power and drivers), 9 lithium-polymer on-board batteries. As shown in Fig. 13, the operator’s desktop PC is used as the master processor and the laptop installed on robot as the slave processor. The robot is remotely operated using either keyboard or joysticks, by taking advantage of the wireless connection between the computers via LAN-Cards. Operator Joystick Operator’s PC Video User interface Sensor data Wireless LAN Wireless LAN On-Board Following a complete stress analysis of all different parts, and redesigning of those required, the last step is to fabricate the designed parts and assemble those together with the available standard elements to achieve the robot as shown in Fig. 12. Camera 1 USB Fig. 11. Results of FEM analysis for the arm USB Operator’s commands Laptop Camera 2 Serial Sensors Battery Interface circuit Actuators . Power circuit Fig. 13. Correlation of the main parts Fig. 12. Resquake on a sample wooden step field Next, a brief review of the control system and performance characteristics of the robot will be discussed. V. CONTROL SYSTEM The data acquired by sensors and video streams captured from the two cameras are collected in the laptop, and then sent to the operator’s PC. Operator commands received by the laptop are transmitted to the actuators through a micro processor (ATMEGA128L AVR). Angular positioning sensor (potentiometer) of each arm is also connected to the micro processor where its data is processed in order to set the arms at the desirable position. Power circuits consisted of 3986 LM2576HV-ADJ switching regulators; distribute the voltage proportionally to driver circuits and other electrical devices. It should be mentioned that powered by the battery cells with a full charge (4.2 volts, 10 A.H.), the whole system operates more than 4 hours. Resquake works as a tele-operative mobile robot with the following capabilities: 1. Surmounting uneven terrains with a relatively high stability as shown in Fig. 12; 2. Climbing stairs and slopes up to 35o as shown in Fig. 14; 3. Searching unreachable environments within a maximum distance of 50 meters from the operator station; 4. Broadcasting thermal and visual data from the area; 5. High reliability of hardware and electronic devices. Other specifications of the robot are listed in Table II. Fig. 14. Resquake climbing up the stairs REFERENCES [1] Adam Jacoff, Elena Messina, Brian A. Weiss, Satoshi Tadakoro, Yuki Nakagava, "Test Arenas and Performance Metrics for Urban Search and Rescue Robots", IEEE/RSJ International Conference on Intelligent Robots and systems, 2003. [2] L. Matthies, Y. Xiong, R. Hogg, . Zhu, A. Rankin, B. Kennedy, "A Portable, Autonomous, Urban Reconnaissance Robot', California Institute of Technology, Jet Propulsion Laboratory, Pasadena CA, 91109, 2000. [3] Mark Yim, David G. Duff and Kimon Roufas, "Modular Reconfigurable Robots, An Approach To Urban Search and Rescue", Xerox Palo Alto Research Center, 2000. [4] A. Meghdari, S. H. Mahboobi, and A. L. Gaskarimahalle, "Dynamics modeling of “cedra” rescue robot on uneven terrains", ASME International Mechanical Engineering Congress, 2004. Joseoh E. Shigley, and Charles R. Mischke, “Mechanical Engineering design”, Sixth edition, McGraw-Hill, 2003. Table II Main specifications of Resquake Overall weight 25 Kg Length with expanded arms 80 cm Length with closed arms 41 cm Minimum height 26 cm Width 40 cm Maximum velocity 19 cm/sec Arms maximum angular velocity 4 rpm Number of ball bearings 82 Number of plain bearings 32 Total number of parts 688 [5] VI. CONCLUSION This paper presented an illustrative description of the Resquake project at KNTU. Innovative mechanisms and software-based steps of the design procedure were highlighted here. First, a new special mechanism was developed to enhance the maneuverability of a mobile robot while trying to keep its dimensions relevant to the environment in which it performs. After selecting suitable mechanisms, dimensions and parameters of the system were defined. The system dynamics and the sequence of stress analysis for different parts were addressed. Electronic devices and control system of the robot was briefly described in the last section. The outcome is Resquake, which is a mobile robot with great capabilities in climbing obstacles in destructed areas, and was participated in Rescue robot league of RoboCup 2005 in Osaka (Japan) where achieved the 2nd best design award. [6] George Henry Martin, “Kinematics and dynamics of Machines”, 1917. [7] Moosavian, S. Ali. A. and Papadopoulos, E., “Explicit Dynamics of Space Free-Flyers with Multiple Manipulators via SPACEMAPL,” Journal of Advanced Robotics, Vol. 18, No. 2, pp 223-244, March 2004. Rastegari, R., and Moosavian, S. Ali. A., “Multiple Impedance Control of Mobile Robotic Systems,” Proc. of the ISME Int. Conf. On Mechanical Engineering, Isfahan, May 2005. [8] [9] 3987 Reutlingen and Ulrich Fischer, “Metals handbook” Third edition, Tarrah, 2004.