206 and placement. This paper discusses advances in laser marking techniques, along with material testing  on those laser marks, as an alternate solution for adding fiducials on the components to be tracked.  

To summarize, traditional DIC methods rely on random contrast fiducials that are either already available  on the tracked component or produced by primitive means. For mechanisms, a highly repeatable process  consisting of laser marking has been developed. Furthermore, the complex geometry  of the mechanisms  drove the development of  an algorithm that implements  conformal type subsets , allowing the tracking of  partially obscured objects .  

DIC for Mechanisms Testing  

DIC, in general, has become a popular means of determining full -field displacements from digital images.  While it has also become a vital component in material characterization applications that use full -field  information as part of a parameter inversion process, the main usage in mechanism testing is rigid body  motion. One commercial package for DIC applications is VIC -2D, from Correlated Solutions. Analyzing  motion in VIC -2D begins by defining a square subset with an odd number of pixels. The central pixe l is  the point of interest, and the surrounding pixels are used as information to help identify the point of interest in the subsequent video frames. This off-the-shelf DIC algorithm operates using the Lucas - Kanade method as a basis. With this, there are underlying challenges for most mechanism testing   applications : 

1. The image subset must have changes in intensity values in each potential axis of motion. With high-precision mechanisms components, striations from machining are often so fine as to be  below the resolution of a high- speed video camera system. This is problematic, as a subset of flat  color could move indeterminately within a larger area. Thus, in order to reliably track a component, intens ity information must be added to the surface.  

2. The second challenge is that motion of the subset must be small between frames – less than a  pixel. VIC -2D interpolates the intensity information within the subset by 10x, in order to provide  higher positional accuracy. The result is that movements less than 1/10th of a pixel are ideal. This  dictates the minimum frame rate for video recording, and helps set camera resolution. For a regularly repeated test  in a production setting, increases in frame rate and resolution increase  computation time for the DIC process and storage space necessary for the resulting videos.  

3. The third challenge is that a subset has a minimum size for effectiveness  (about 9x9 pixels) , and  must be square. The minimum size requirement helps set  the required camera resolution and  lens, as the smallest tracked component must be capable of containing at least a 9x9 pixel  square. With finer resolution, subset size can increase, but the subset must fit on the component.  Thus resolution limits the total amount of information available for correlation.    In an effort to help resolve some of the above deficiencies, the tester scope was expanded to include the development of repeatable fiducials applied via laser marking,  as well as the implementation of  a Sandiadeveloped DIC software known as DICe.  

Laser Marking   After an evaluation of potential methods  of speckle application, laser marking was selected based on it s  existing use in the mechanism  and low risk of creating debris or r esidue.  Laser marking is , however,  not  without its concerns. The most obvious is the potential for etching or ablation of material. To address this concern, laser settings were developed in an attempt to minimize dimensional disruption of the surface.  The settings must account for part material, but also for part geometry, as heat conductivity becomes  more evident in parts at miniature size scales. Avoiding surface melting or ablation is a matter of keeping  instantaneous power delivered to the part below a certain threshold. There are many variables involved,  but a general approach is to use a defocused beam, increasing the area over which the energy is  distributed.