Multivariate control charts for monitoring internal cameraparameters in digital photogrammetry for LSDM (Large-ScaleDimensional Metrology) applications

Industrial non-contact dimensional measurements using photogrammetry rely critically upon stabilityin time of camera calibration. This is particularly relevant for multi-camera systems employed for con-tinuous and/or long term monitoring of some dimensional process, e.g. dimensional checks of the samemanufactured component as it comes off the production line. In most of these cases, camera calibration isupdated regularly to ensure optimal accuracy. Specifically, the use of photogrammetric systems requiresthe knowledge of both internal and external camera parameters estimated by calibration. Constancy ofboth sets is required during use. Internal parameters, pertaining to camera-specific properties, requirestability over the operational lifespan of the system, while external parameters, concerning location andorientation, may change between calibrations. A diagnostic method for internal parameters based onmultivariate control charts is proposed. The purpose of this method is to provide a comprehensive stabil-ity control over all the performed calibrations, especially for those systems used for regular monitoring ofproduction lines. By integrating chart building into calibration software, no additional steps are added tothe operator’s workload for the calibration process. A practical application of the described methodologyis presented at the end of the paper.