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Sensitivity Control Method for the Measurement  Systems

Defne Mühendislik Ltd. Şti. has been performing projects in the areas of  measurement , reverse engineering and product development  for more than six years.  As a result of experience gained on the subject of usage area ,sensitivities and capacities of 3-D instrumentation systems; Defne Mühendislik specialized on the measurement and  methodology of detailed geometry,very small and large-scale parts. This study provides the technical details for the measurements  of small-scaled(0.01 to 0.05 mm) parts Nowadays, these details are an important step for lighting, defense and medical sectors in production control  as well as new designs for the development of new products.

Down below,presented a study on the method of measurement accuracy and control for the measurement systems that use optical scanning principle with a measuring fringe projection method. These systems reflect shadows of the fringes on the product to be measured , 2-dimensional images  that are taken by the camera or cameras of the system,  turned into a in 3-D point cloud 'data. (Figure 1)

Figure 1. Schematic image of the fringe projection of 3-dimensional measurement system (Ref. 1)

2-dimensional images are converted into 3-dimensional measuring points by using phase difference algorithm. In order to eliminate the errors that  occurs  during the level crossing and the presence of sharp edges and to get a clearer picture of this type of regions different processing algorithms with(image processing) method are used  to generate 3-dimensional data.

The mathematical approach will cause roundings on the transition stage and flat surfaces to ensure   flatter or a sharp transition . These roundings are identified with a parameter which is defined as noise level of the systems. Noise level is given as 10-50 µm (0:01 to 0:05 mm) for the measurement systems with fringe projection.  Figure 2

Figure 2: measurement error - the noise graph (ref: 1-2)

In order to get the 3-dimensional data of a product into the computer environment  the camera view of the product should be taken from more than one location. The number of images can be from 3 to 100 to get all the features including the rear regions. Data taken only from one angle for measurement will not reflect actual value as in actual practice.

Each measurement taken, and their merger will caused the joining and accumulation of the errors occured.   Standards and calibration sensitivities of fringe projection measurement systems are defined in VDI / VDE Guideline 2634 part1 and 3, which appeared in 2002-2008 and 2011. Noise, sensitivity, calibration issues,applicable measurement method and defining the values are identified in these standards. How many measurement to be taken from different angles are stated in these standards.  Unlike these standards, various scientific studies are conducted with different mathematical algorithms or fringe projection methods to get more efficient results from the systems . (Reference: 1 - 2 - 3).

A few tests were performed to obtain the performance of 3-dimensional measurement systems in real applications as well as high precision and detailed geometry .  Steinbichler COMET 5 4MP optical measurement system was used. In the system software, parameters are defined which are suitable for the product as a result of the trial of parameters which are in control of user.  Test part was prepared to have detailes like 10, 50 and 100 µm (0.01, 0.05 and 0.1 mm). Figure 3

This part was measured from 3 different angles. Obtained point cloud ,as a result of combining the measurements taken , was compared with the other measuring methods.

Figure 3: Schematic view of test piece

As a result of the measurement data obtained from the point cloud shown in Figure 4. 3 different stages are clearly seen in measurement.

When the process is started, in order to get detailed measurement of the stages part was cleaned with a special solution, paint or any other operation was not performed on the part. Transition zones in the to each other are achieved in full. 3 stages wich are the physical portion of the part, captured  clearly as a result of the measurement.

Figure 4: Point cloud data obtained by the fringe projection method

Marked reference area in other wards zero was selected  on obtained data  . Figure 5

Figure 5 :  Reference area

A planar geometry (plane) is defined to reference area. average error of the defined plane is appeared to be  0.0003. Figure 6

Figure 6: The definition of a reference area plane

Singular measurements values of the stages were taken from the reference plane. The measurement values obtained from the stages in 0.01, 0.05 and 0.1 mm ​​are given in Figure 7.

Measurements taken are the distances to the plane and the points are chosen at random. Error quantity of the singular points  can be seen in detail.

Figure 7:  Measurement results of the stages

The regional and overall errors are shown in Figure 8. as a result of comparing the obtained measurement result with the mathematical (CAD) model. Maximum local error -0004 mm is seen in 0.1 mm  region. Errors in other region are in the average of -0,002 mm.

Figure 8: Obtained  regional and overall errors a result of the comparison with CAD model.

If  we gather these errors  in a report, results can be seen as in Figure 9.

Figure 9: Quality control report

Data obtained with different measurement systems were control ,in order to control and verify the measurement results of the test piece.  Measurements made with 2 different measuring systems. The first measurement is done with Nikon VMA-2520 video measuring system which is defined  as optical (video measuring). This system in principle, is measuring the distance by sharpening the video image which is taken without touching to the product. The second control measurement system, the CMM (coordinate measuring machine) is used. Measurements were taken by touching on the product. Point measurement method were preferred for both systems . In other words, spot measurements were taken from the regions where the stages are introduced as plane of "zero" level which is accepted as the reference of the product.  Picture of the works done  are shown Figure 10.

Figure 10: CMM and  stages of the video measurement system.

Figure 11: Video streaming and CMM measurement report

The purpose of this study ; identifying the location of the start and end of a 0.01 mm detail in fringe projection method and  indicating the possibility of determining  the boundary of this type of detail measurement and size of the value precisely. These type of studies are needed intensively in mechanical valve systems, prothesis used in the medical sector, diversified fields as lighting,product development  and control.   Products in particular shaped surfaces ,stage and the differences  for 0.01 mm and under can be obtained with only fringe projection method.

Fringe projection systems,equipped with advanced technology such as mechanical and thermal stability control systems can manage such a work. This type of details in the use of measurment systems can be overcome with only experience and expertise.

Defne Mühendislik; having the domination of different brands and systems, with experience gained across many industries and dealing different types of parts ,has been continuing to improve experties in measurement methodology and quality control methods to provide complete solutions to it's customers from design to production.

References:  :

1-  Calibration of 3D surface profilometry using digital fringe projection;

MEASUREMENT SCIENCE AND TECHNOLOGY; Meas. Sci. Technol. 16 (2005) 1554–1566; Liang-Chia Chen1 and Chu-Chin Liao

2- Neural network applied to reconstruction of complex objects

based on fringe projection; Optics Communications 278 (2007) 274–278; Tangy Yan, Chen Wen-jing , Su Xian-yu, Xiang Li-qun

3- Skin topography measurement by interference fringe

projection: a technical validation; Skin Research and Technohgy 2001; 7: 332-121; J. M. Lagarde, C. Rouvrais, D. Black, S. DiridoUou and Y. Gall

4- http://www.defnee.com/tr/hakkimizda/teknik-yazi-ve-makaleler.