Modelling and Calibration of a Structured-Light Optical CMM for Dimensional Measurement of Sheet Metal Parts and Dies

The accurate measurement of die and sheet metal surfaces has always been a tedious and time consuming task using mechanical coordinate measuring machines (CMMs). Optical CMMs employing single-beam laser probes, while faster than their contact probe equivalents, suffer similar throughput limitations and often have difficulty in accurately locating certain features, particularly edges & holes. Thus, the development of techniques for high-throughput measurement of dies and sheet metal parts becomes necessary. In this paper, a new tetrahedron-target-based approach is presented for the extrinsic calibration of a non-contact “light-striping” (structured light) optical CMMs. The procedure makes automated on-line calibration possible. The modeling of the scanning mechanism is based on a unique skewed frame representation without the use of pin-hole camera model assumption. It is demonstrated that the extrinsic calibration matrix can be decomposed into two classes of transformations, one homogeneous and the other nonhomogeneous. The nonhomogeneous transformation between a Cartesian world frame and the non-Cartesian skewed sensor frame is studied. Experimental studies show that a micron level calibration accuracy can be achieved.