Abstract: he normal and tangential directions of the unknown environment at the contact point with the robot are determined by the contact forces acquired from a force sensor mounted on the distal link of the robot under consideration of friction. A constraint coordinate system at the tip of the robot is established by using these two directions. Position control is performed along the tangential direction and force control is executed along the normal direction in this coordinate system,then automatic unknown contour following procedure can be implemented.The unknown contour is modeled gemetrically using the tangent and position information obtained at each contact point during the tracking motion. Finally,the traeking strategy has been implemented on a PUMA 562 robot successfully,the geometric model of the unknown environment is given.