An Investigation Into the Compliance of SCARA Robots. Part I: Analytical Model

A special class of robots suited for assembly tasks called SCARA (Selective Compliance Assembly Robot Arm) provides a degree of built-in flexibility due to robot structure. In such robots there are three revolute joints and a prismatic joint. They offer four degrees of freedom consisting of rotation about two vertical and parallel axes at the revolute joints, and translation and rotation about the tool axis. Some models offer additional degrees of freedom at the end effector. Structural compliance can arise due to the stiffness of the robot links, drive system, grippers as well as the assembled parts. The largest effect is due to the drive torsional stiffness followed by the grippers, workpieces and the robot tool link. Knowledge of the inherent flexibility is extremely useful in designing tooling and fixtures, in laying out the assembly work cell according to the amount of compliance available in various regions of the robot work envelope, in guarding against wedging and jamming and in specifying external Remote Centre Compliance devices (RCC) if necessary. In this paper the various sources of compliance built into a SCARA robot system are outlined together with their relative significance. A mathematical model which expresses the end effector deflection as a function of the robot Jacobian and the drive compliance parameters in Cartesian coordinates has been developed. The modified generalized assembly force model developed for the Selective Compliance Assembly Robot Arms (SCARA), used in this investigation, is described. Constraints required to prevent jamming and wedging of parts during assembly are outlined. The application of this compliance model for both rotational and prismatic part insertion is described. The conditions required to obtain true or semi-compliance centres for the SCARA robot end effector are derived and discussed.