Traceability of measurements in optical coordinate measuring machines
Date
2020-12
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Optical coordinate measurement systems (photogrammetry or fringe projection)are gaining acceptance in the measurement industry. Their portability, scalability and ability to scan large areas instantaneously make them very attractive to the industry.
Measurements on these systems can be taken with any type of camera, from a compact cell phone camera to high-end digital cameras. Basic triangulation can be accomplished with simple programs and there are many open-source packages available. For high-quality measurements, the cost of such a system can be very significant.
Despite the growing use of these optical systems in various industry sectors, there is currently no internationally accepted standard to establish their traceability to the length standard.TheVDI/VDE(German standard)standard is widely used for verification of these systems using non-complex artefacts that are easy to measure and have already been calibrated with other traceable measuring machines,e.g. the sphere, gauge-blocks, etc.
Achieving traceability of measurements in optical systems is a very big challenge.This is because camera-based measurements are fraught with many challenges,e.g.changing camera settings or moving cameras relative to each other completely changes the configuration of the system and normally requires an entirely new calibration. Changes in light conditions can also affect the measurements over and above the normal ambient effects. Based on a survey and literature review, the state of the art of optical coordinate measurement systems and the challenges to establishing a traceability standard were investigated in this research. The urgent need for such a standard in the South African industry was also demonstrated by this research.
Owing to the wide range of uncertainty contributors, it is a time consuming and difficult task to set up a robust uncertainty analysis for optical CMMs. The Virtual CMM technique offers a better solution to such a problem. The Virtual CMM technique has only been applied to tactile CMMs and articulated arm optical CMMs, also known as discrete point systems since they measure one point at a time. The development and application of the Virtual CMM technology to stereo vision scanning optical CMMs was demonstrated in this research. Finally, successful measurements that conform with other measurement systems were taken with the developed stereo vision system using the Virtual CMM technique.
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
Description
Thesis (MEng)--Stellenbosch University, 2020.
Keywords
Photogrammetry, Coordinate measuring machines, Photogrammetry -- Digital techniques, UCTD, Digital photogrammetric systems