Multi-Body Dynamic Model of Single-Wheel Robot stabilized by Reaction Wheel Principle Kadam S.N. #1 and Seth B. *2 Department of Mechanical Department, Indian Institute of Technology Bombay, Mumbai - 400076 India 1 kadamsn5@gmail.com 2 sethbharat@gmail.com ABSTRACT Reactobot is a one-wheel robot developed at IIT Bombay. It is balanced by reaction wheel actuator suspended from the central axis. The Robot can get accelerated in forward or backward direction by pitching the pendulum mass within the wheel in the same direction. Turns can be- executed by tilting robot to right or left while in motion. In this paper, we describe multi-body dynamic model of the Reactobot, which is developed by considering seven generalized variables. The Reactobot is assumed to roll on a flat surface without slipping. The system has two nonholonomic constraints as the direction of the forward velocity of the wheel is constrained to be in the direction of wheel heading. The mathematical model of Reactobot is developed using the Lagrangian constrained generalized formulation. Subsequently, the Lagrangian multipliers are eliminated and the model is converted into 5-DOF normal form. The dynamic model is highly nonlinear. The developed mathematical model is validated by numerical simulation for special cases. The behaviour of the system is along expected lines. In every simulation case, the potential energy decreases while the total energy remains constant over time since the dynamic model is conservative. Keywords - Wheel-robot, reaction wheel stabilization, non linear dynamics, nonholonomic constraints, Lagrange formulation.