Robotic arm for maintaining a tool in a desired orientation

An improved robotic arm system (1) for servicing the tubesheet (11) located within the channel head (13) of a nuclear steam generator. The system (1) includes a robotic arm (3) whose shoulder and elbow joints (42, 44, 45, 47, 48, 49) are rotatable only in the plane parallel to the tubesheet (11) in order to eliminate the imposition of cantileverly-induced torques on the electric motors driving these joints, as well as to minimize the possibility of mechanical interference between the arm (3) and the walls 15 of the channel head. Each of the motorized joint assemblies (42, 44, 45, 47, 48, 49) of the arm (3) includes resolvers (100, 102) that move the joint and drive drive train (81). The control circuit assembly (7) of the robotic arm system (1) includes a control processing unit (CPU) (273) capable of simultaneously driving joint assemblies (42, 44, 45, 47, 48, 49) and any computer-operated mechanisms associated with end effectors (5, 27) delivered by arm (3).