Small robotic cell with Real Time Position Correction

Robotic multipurpose cell with real time position correction that is working in CSEM facilities, one of the parters of MEGAROB Project. The realtime correction aims to eliminate both the intrinsic robot positioning error as well as dynamic errors of the system. Off-the-shelf industrial robots today present high repeatability, but low absolute accuracy. This means that the robot will have a variable positioning error in a fixed reference frame, which depends on the pose of the robot. Furthermore, robots have low stiffness, which results in an inability to reject disturbances in terms of processes forces. Machining processes require the robot to exert a process specific force, which, however, can vary during the process. As the robot present low stiffness, it will comply with the forces and deviate from the programmed path, intensifying the low accuracy of the robot (resulting in poor geometrical tolerance and quality surface in case of milling) as well as lead to damage of the machine, workpiece, reduced tool lifetime, among other effects. This will ultimately inhibit the robot to comply with the design tolerances of a part. The realtime correction is performed in the level of robot motion control. Traditionally, an industrial robot controls its position by controlling the position of its joints. In such approach, all sources of positioning error in the system will have cumulative effect. Furthermore, such scheme does not allow the robot to take into account the structural flexibility of the kinematic chain nor the intrinsic positioning error of the robot structure. A form to overcome this problem is to control the position of the robot with an absolute position of the tool of the robot. This can be done using the absolute position measuring system, to read the real position of the robot and on realtime correct the robot position to the planned position. The compensation is performed continuously in a closed loop at a frequency of 1ms and, although it is performed in the level of the robot motors, the controlled variable is the robot cartesian position. The implementation of the concept developed in MEGAROB Project allows the robot in this cell to obtain a positioning with very high accuracy values.