The development of a non-contact co-ordinate measurement machine

Master Thesis


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University of Cape Town

The Department of Surveying and Geodetic Engineering at the University of Cape Town, in conjunction with the Department of Mechanical Engineering at the University of Cape Town have developed a non-contact co-ordinate measurement machine in a project called MILIMAP. The project had the following objectives : 1. To determine unique surface co-ordinates for continuous, complex objects with submillimetre accuracy. 2. The representation of the co-ordinates was to be in a format that could be utilised by a computer numerically controlled (CNC) milling machine in a computer aided design/ computer aided manufacture (CADCAM) environment. 3. The device had to use a non-contact method for data capture. The MILIMAP project was undertaken because there is a demand for co-ordinate measurement machines in industry for the inspection of objects for quality control purposes. Conventional Co-ordinate Measurement Machines (CMMs) are expensive and use a contact probe to measure the object. The contact probe measurement technique is unsuitable for the measurement of non-rigid objects such as shoes and automobile seat padding. The MILIMAP system provides a noncontact measurement technique that can be applied to non-rigid as well as rigid objects. Additional applications in the archaeological field exist for the non-contact measurement of sensitive, historical artefacts. A digital photogrammetric system was developed to measure the position of a laser dot projected onto the surface of the measurement object. This measurement system satisfied the criteria of a non-contact measurement method required for the project. The system utilised three digital CCD cameras to capture images of the laser dot projected onto the object. Image processing software, developed from existing software within the Department of Surveying and Geodetic Engineering, was used to photogrammetrically determine the co-ordinates of the laser dot to sub-millimetre accuracy on the surface of the object. A mechanical device was designed and constructed by the Department of Mechanical Engineering in order to move the laser over the surface of the object, and to rotate the object. The entire surface of the object could be measured by the system using these operations.

Bibliography: pages 113-117.