U.P.B. Sci. Bull., Series D, Vol. 76, Iss. 1, 2014 ISSN 1454-2358 INVERSE DYNAMICS OF COMPASS ROBOT ARM USING PRINCIPLE OF VIRTUAL WORK Stefan STAICU 1 , Ion STROE 2 , Andrei CRAIFALEANU 3 The principle of virtual work is used to study the motion of a system under the action of external and internal forces. General kinematics problem of systems of rigid bodies with constraints are presented in first part of the paper. If an internal joint force has to be found, a virtual displacement is considered in the system. Based on the principle of virtual work, the elaborated method solves the problem of calculus of input forces and joint forces of a compass robot arm, sketched as a serial chain. Keywords: Dynamics, Joint force, Kinematics, Serial robot 1 Introduction Considering the gravitational effects, the relevant objective of the multi-bodies dynamics is to determine the input torques or forces and the external and internal joint forces. Several methods have been applied to formulate the dynamics, which could provide the same results concerning these torques or forces. The first one is using the Newton-Euler procedure, the second one applies the Lagrange formalism with its multipliers and the third one is based on the principle of virtual work [1], [2], [3]. Difficulties commonly encountered in dynamics of multi-bodies systems include problematic issues such as: complex spatial kinematical structure with possess a large number of passive degrees of freedom, dominance of inertial forces over the frictional and gravitational components and the problem linked to the real-time control and the solution of inverse dynamics. In the present paper, a recursive matrix method, already implemented in the inverse kinematics and dynamics of robots, is applied to the analysis of a serial mechanism. It has been proved that the number of equations and computational operations reduces significantly by using a set of matrices for dynamics modelling. The general problem of kinematics for rigid body systems with constraints is first presented in that follows. In the second part of the paper, the application of 1 Professor; Department of Mechanics, University POLITEHNICA of Bucharest, ROMANIA, e-mail: staicunstefan@yahoo.com 2 Professor; Department of Mechanics, University POLITEHNICA of Bucharest, ROMANIA 3 Associate Professor, Department of Mechanics, University POLITEHNICA of Bucharest, ROMANIA