LETTERS International Journal of Recent Trends in Engineering, Vol 2, No. 2, November 2009 146 Computer Aided Dynamic Simulation of Six- Legged Robot Abhijit Mahapatra 1 , and Shibendu Shekhar Roy 2 1 Virtual Prototyping & Immersive Visualization Laboratory, Central Mechanical Engineering Research Institute , Durgapur 713209, (CSIR), India. Email: abhi_mahapatra@yahoo.co.in 2 Department of Mechanical Engineering, National Institute of Technology, Durgapur 713209, India. Email: ssroy99@yahoo.com Abstract—In this study, kinematics and dynamic simulation of a six-legged robot is performed based on virtual prototyping technology. Modeling and simulation of six- legged robot with three joint legs are carried out with CATIA solid modeler, SimDesigner and ADAMS multi- body dynamic solver. Tripod periodic gait pattern was generated varying the cycle time as well as stroke of the swing of the legs. Variation of joint torques and aggregate center of mass of the robot during periodic gait were analysed. The simulation result provides theoretical basis for developing algorithm for robot motion control. Index Terms—Legged robot, simulation, dynamic modeling, torque, tripod gait I. INTRODUCTION Multi-legged robot locomotion has been an area of keen interest to the researchers over the years because of the advantages of the superior mobility in irregular terrain and the less hazardous influences on environment comparing with the wheeled robots. On the other hand, the requirements for leg coordination and control impose difficulties beyond those encountered in wheeled robots [1]. The dynamic properties of a six-legged robot can significantly influence control response. Such systems are characterized by complex dynamic interactions between individual members connected by actuated joints, including trunk body and relatively less weight legs. Moreover, a six-legged robot in motion is at any one moment a complex combination of open and closed kinematic chains. Legs contacting the ground form closed loops with the trunk body and ground. Non-contacting legs represent individual, branching open kinematic chains. Several algorithms exist for legged robot open and closed chain simulation. The algorithm of Rodriguez and Kreutz [2] uses linear operator methods to derive simple forms of dynamic equations. Another method, that of Lilly and Orin [3] treats a legged robot as a system of multiple manipulators (i.e. legs) contacting an object (i.e. trunk body), with ground contact modeled as a manipulator joint. However, both the models are approximate model of the complex legged robot. In order to design efficient control algorithm for six-legged robot, it is very much essential to develop more accurate dynamic model of real legged robots. In this connection, work of Song and Waldron [1], Shih et al. [4], Pfeiffer et al. [5], Lin, and Song [6], and Kimura et al. [7] are important to mention. Barreto et al. [8] developed the free body diagram method for kinematic and dynamic modeling of a six-legged machine. Erden [9] investigated the dynamics of a hexapod walking robot in a level tripod gait based on Newton-Euler formulation. Koo and Yoon [10] obtained a mathematical model for quadruped walking robot to investigate the dynamics after considering all the inertial effects in the system. Most of the previous researcher developed analytical dynamic model using number of approximation in mechanical structure of legs as well as foot/ground interaction model. Therefore, developed dynamic model was far from actual dynamic behavior. In the present work, an attempt has been made to develop the more accurate dynamic model of the six legged robot using Lagrange-Euler formulation as well as simulate the Computer Aided Design (CAD) model of the robot in ADAMS (Automatic Dynamic Analysis of Mechanical Systems) solver using the concept of rigid multibody dynamics and analyze its dynamic performance. II. MODELING OF THE SIX-LEGGED ROBOT A three dimensional CAD model [11] of six-legged robot has been developed in Computer Aided Three Dimensional Interactive Application (CATIA) as shown in Fig. 1. The robot trunk body is 495 mm in length, 205 mm in width and 89 mm in height and made of aluminium alloy. The legs are identical and symmetrically distributed around the body on both sides. Each leg consists of three links, namely link 1(tibia), link 2 (femur) and link 3 (coxa) with effective lengths 83.5mm, 119mm, 98mm respectively. All the joints are motorized revolute joint with rotational axis configuration Z-X-X for the three joints respectively. Total number of DOF of the system is 24 (6 DOF of the trunk body and 18 DOF of the legs). The robot consists of 19 main parts along with 18 servomotors. The three main steps followed to develop virtual prototype of the robot are given below. Step 1 : Part modeling of the different parts of the robot in CATIA V5 Step 2: Assembling of the parts in CATIA V5 © 2009 ACADEMY PUBLISHER