a method and apparatus for use with a computer for tracking the position of a user manipulatable object and for providing forces to lock the apparatus . the manipulatable object is coupled to a mechanical linkage which is supportable on a fixed surface and is tracked by sensors for sensing the location and / or orientation of the object . force generators coupled to the linkage can lock one or more joints of the linkage in position . a multiprocessor system architecture is disclosed wherein a host computer system is interfaced with a dedicated microprocessor which is responsive to the output of the sensors and provides the host computer with information derived from the sensors . the host computer runs an application program which responds to sensor information and can display images in accordance therewith .