Influence of vibration and out-of-plane motion on ballbar measurement accuracy

Robotic machining systems provide flexibility for complex operations but often suffer from trajectory deviations due to structural compliance and dynamic disturbances. Conventional ballbar diagnostics, originally designed for rigid CNC platforms, may not be suitable for evaluating circular trajectories in such non-rigid environments. This study investigates how out-of-plane motion and spindle-induced vibration affect ballbar-based assessments on a hexapod robot using a QC20-W ballbar. Experiments were conducted under controlled vertical displacements up to 800 µm and spindle speeds up to 12,000 RPM. Results show that orthogonal disturbances can inflate circular deviation (CD) values by up to 20% and distort squareness and straightness parameters. Direction-dependent errors between CW and CCW paths also emerged under vibration. A simplified mathematical model based on sinusoidal projection and Taylor expansion was developed to explain the observed distortions, and vibration amplitudes were estimated via signal processing with < 5% error compared to accelerometer measurements. A major challenge addressed is the inability of planar sensors to decouple non-planar disturbances. The proposed approach provides a validated framework that combines modeling, experiment, and signal-based quantification, offering insights into trajectory distortions in flexible robotic machining.