a robot 100 has a pedestal 1 , upper and lower arms 5 , 12 and a quill 21 . a drive train comprises motors in the pedestal 100 coupled to three concentric drive tubes 3 , 6 , 9 . the outer drive tube is coupled to the upper arm 5 . driven tubes 11 , 14 extend from the upper arm 5 to the lower arm 14 . a twisted belt in the lower arm couples the driven tube 14 to the quill 21 . the articulator 400 has three housings 21 , 44 , 45 ; 61 , 61a each with an axis of rotation transverse to the its neighbors . brakes a , b , and c selectively lock two housing together to rotate the third housing .