Practical Guide to Machine Vision Software: An Introduction with LabVIEW

8Dimension Measurement Using Coordinate System

To measure the dimensions of an imaged object, a ROI is used to define the measurement location as discussed in previous chapters. In most cases, the object under inspection may shift and/or rotate in the viewing field of the video image. If a static ROI does not incorporate the entire object under investigation, the dimension may not be measured. For this case, the ROI location will need to be shifted and rotated according to the objects location. To enable this dynamic ROI location, a reference coordinate system can be used to define the measurement area around the object relative to the reference location of the object. The reference coordinate system is based on a characteristic feature of the object. To locate a reference coordinate system relative to the object, pattern matching or edge detection can be used. The ROI defined with respect to the reference coordinate system is then used to measure the dimension of a part of the object.

The example using a reference coordinate system can be found from the following folder:

C:\Program Files\National Instruments\LabVIEW 2013\Examples\Vision\Vision Assistant Express VI\Battery Clamp Inspection (Express).vi

As seen in Figure 8.1, a reference coordinate can be located irrespective of any shift and/or rotation of the object in an image via pattern matching. From the reference coordinate, the ROI used for measurement can be automatically located on a part of the object that is ...