**5.2 Extendibility of the add-on module**

Under the condition that the add-on module should have a much higher bandwidth than that of a companion main robot, the add-on module can be designed to be more than two DOFs. Specifically, it is suitable to adopt the parallel mechanism for an add-on module with more than three DOFs to assure motion accuracy. The high-speed sensory feedback for detection of on-line accumulated error can be high-speed vision with 2D/3D configuration or other forms of sensors depending on a specific task.
