Frequency Domain Identification of an Industrial Delta Robot and Motion Correction

Delta robots are prominent examples of agile parallel kinematic machines (PKMs) designed for highly dynamic pick-and-place tasks. Optimized minimum time trajectories lead to dynamic load cycles, induce vibrations, and cause overshooting of the end effector (EE) due to the flexibility of the PKM. Crucial to mitigate these effects by means of model-based control is a dynamics model that accounts for the principal elastic compliance, such as gear stiffness and structural elasticities. However, robot manufacturers do not provide data on the structural stiffness. Also, established dynamics identification methods cannot determine stiffness and damping parameters. In this article, a two-step frequency domain identification method is proposed to identify elastic properties by examples of an industrial Delta robot. As a peculiarity of the Delta PKM, the identification is carried out when the platform is removed and for the complete PKM. This allows distinguishing elasticities of the gear-drive units and of the struts. The identified parameters are employed for motion correction to avoid overshooting. This correction does not interfere with the original planning and control function of the industrial robot. Three motion correction schemes (preloading of drives, quasistatic correction, flatness based) are compared. Laser tracker measurements of the EE confirm a drastic reduction of overshooting and thus an increase in the overall tracking accuracy.