Non-contact free-form shape measurement for coordinate measuring machines

• Main Author: Huddart, Yvonne R.
• Other Authors: Moore, Andrew; McKendrick, David
• Published: Heriot-Watt University 2010
• Subjects: 670.42
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.616634

Precision measurement of manufactured parts commonly uses contact measurement methods. A Coordinate Measuring Machine (CMM) mounted probe touches the surface of the part, recording the probe’s tip position at each contact. Recently, devices have been developed that continuously scan the probe tip across the surface, allowing points to be measured more quickly. Contact measurement is accurate and fast for shapes that are easily parameterized such as a sphere or a plane, but is slow and requires considerable user input for more general objects such as those with free-form surfaces. Phase stepping fringe projection and photogrammetry are common non-contact shape measurement methods. Photogrammetry builds a 3D model of feature points from images of an object taken from multiple perspectives. In phase stepping fringe projection a series of sinusoidal patterns, with a phase shift between each, is projected towards an object. A camera records a corresponding series of images. The phase of the pattern at each imaged point is calculated and converted to a 3D representation of the object’s surface. Techniques combining phase stepping fringe projection and photogrammetry were developed and are described here. The eventual aim is to develop an optical probe for a CMM to enable non-contact measurement of objects in an industrial setting. For the CMM to accurately report its position the probe must be small, light, and robust. The methods currently used to provide a phase shift require either an accurately calibrated translation stage to move an internal component, or a programmable projector. Neither of these implementations can be practically mounted on a CMM due to size and weight limits or the delicate parts required. A CMM probe consisting of a single camera and a fringe projector was developed. The fringe projector projects a fixed fringe pattern. Phase steps are created by moving the CMM mounted probe, taking advantage of the geometry of the fringe projection system. New techniques to calculate phase from phase stepped images created by relative motion of probe and object are proposed, mathematically modelled, and tested experimentally. Novel techniques for absolute measurement of surfaces by viewing an object from different perspectives are developed. A prototype probe is used to demonstrate measurements of a variety of objects.