In this paper, we propose a new closed-form dynamic model of interaction forces between two interacting robotic modules and a methodology to control the interaction forces by using inherent kinematic redundancy of robotic systems. To show the usefulness of controlling interaction forces, two exemplary cases are introduced. The first example investigates the interaction force between a mobile platform and a serial robot mounted on the platform. The interaction force between the two modules is controlled so that the motion of the mobile platform can be controlled in a desired way. The second example deals with interaction between a serial robot and a human operator. An interaction force is controlled so that the human operator does not feel uncomfortable when the operator performs a designed trajectory motion in cooperation with the robot