Nonsingular Hand-Eye and Robot-World Calibration for SCARA-Type Robots: A Comparative Study

Four degree-of-freedom selective compliance assembly robot arm (SCARA) robots are increasingly used in industry due to their unique advantages in speed and accuracy. When integrated into visual-guided systems with cameras, SCARA robots require hand-eye and robot-world (HE&RW) calibration to establish the system's geometric relationships. The conventional HE&RW calibration methods are pose-based or point-based, which have been widely validated on full degree-of-freedom articulated robots. However, these methods may appear singular and unusable due to SCARA's restricted movement. Besides, some calibration methods designed for this situation still impose additional requirements or lack in-depth singularity analysis. Inspired by this, we conduct a thorough study on HE&RW calibration for SCARA robots. First, we analyze the reasons for the SCARA's singularity of conventional methods from the perspective of nonlinear least squares. Then, we redefine parameters with clear geometric interpretation and propose two nonsingular HE&RW calibration methods. Note that comparative studies of pose-based and point-based methods on both articulated and SCARA robots are carried out. Finally, the effectiveness, adaptability, and generality of the proposed methods are validated on both synthetic and real data.