Determination of a reference model for controlling the deformation of an industrial robot. Application to riveting in aeronautics

Concerns the control of the deformation of a robot manipulator exerting large forces and moments on its environment. This was motivated by a real application consisting in inserting rivets with a two-arm robot (to assemble a wing or a fuselage on an airplane structure). The solution consists in performing the complete task rivet after rivet. To do so, it is necessary that the two parts be compressed together to avoid any introduction of turnings between them during drilling. Therefore, the forces that the arms have to exert exceed 1000 N. With such constraints, a position controlled robot deforms and the end-effecters are not orthogonal to the parts anymore, which is not acceptable. These deformations cannot be detected by the optical encoders mounted on the motors shafts, because they are due mainly to the flexibility of the joints. We have developed a force control scheme with explicit force feedback to compensate for these deformations by servoing the forces and moments at the level of the end-effector: the desired values are zero except for the force orthogonal to the parts being assembled. To avoid using force sensors in a production line, we propose to use a force control scheme to derive a reference model. This model furnishes at every point the position increments to be applied to the robot in order to compensate for its nondesired deformations