Optical Measurement

Updates

• 09 Feb 2012

  Compensating for Thermal Expansion – does size matter?

  Compensating for Thermal Expansion – does size matter?
  Posted by Lanny Wilson ⋅ January 30, 2012 ⋅ 1 Comment
  Filed Under Verisurf, Measurement, Dimensional Metrology, Accuracy and precision, Verisurf Software, Alignment, Thermal expansion, Scale factor, Compensation of Thermal Expansion, Scale, Total Stations
  
  In today’s manufacturing world the improvements in technology have allowed us to accurately and precisely design, manufacture and inspect parts and assemblies to tighter tolerances then at anytime in history.
  
  Many applications do not require any Compensation for Thermal Expansion (CTE) due to size, type of material or design/engineering requirements. However, many applications benefit from applying CTE due to the material type and the environment. In general, the larger the part or object being measured the bigger the affect of thermal changes to the material of the part/object.
  
  During Inspection and fabrication (particularly automotive/aerospace tooling), the temperature of the object being measured can have a major impact on the measurement results. Understanding the proper application of the Compensation for Thermal Expansion will result in more repeatable measurements regardless of the objects temperature.
  Measurement Devices and the Environment
  
  Many measurement devices that utilize laser interferometry require accurate environmental measurements of temperature, air pressure and humidity to acquire accurate measurement data. The correction for environmental factors; temperature, air pressure, humidity are not required for the operation of some measurement systems for example; Articulating Arms, Photogrammetry and CMM’s.
  
  When using a measurement system with a laser interferometer or distance meter of some variety for distance measurement (Laser Trackers most common, Total Stations) the temperature and air pressure that is entered for the initialization process of the device is used to correct the instrument measurements back to the standard 68° F or 20° C. This is particularly important to note when point to point distances will be measured on an object that is not 68° F or 20° C.
  
  It is recommended that you refer to the Manufacturers recommendations.
  When to Compensate
  
  The type of measurement task will determine whether Compensation of Thermal Expansion (CTE) must be used and when the CTE should be applied.
  Common Applications
  
  Object is constructed of material that is not thermally stable.
  Tight tolerances in environment where temperature is not controlled
  Large objects
  Establishing Large Measurement Networks on Large Structures
  
  Applying CTE
  
  The actual application of Compensation of Thermal Expansion (CTE) is unique in nearly every Measurement Software on the market. To properly apply CTE in the software that is being used the operator MUST understand the following to be successful:
  
  Type of Material – the exact type of material should be known in order to accurately calculate the CTE of the object.
  Part Temperature – depending on the required accuracy of the measurement task a variety of devices are available to read temperatures. For small parts, one reading is sufficient when the task is Large Volume Measurement a number of readings may be required and an average determined. If the object is tall, readings should be taken at several altitudes.
  What type of ‘Part Alignment’ will be used – the sophisticated measurement software’s on the market provide the power of applying CTE through 7 parameter transformation matrixes. This provides the ability to ‘Align’ to the part using a ‘Best Fit’ algorithm, which fits Translations, Rotations and Scale, based on Nominal data. A keen understanding of how the Nominal data was established is required prior to ‘Scaling’ an Alignment based on the Nominal information. Nominal data that was not properly valued will yield inaccurate to catastrophic results.
  If a ‘minimum fit’ Alignment will be used when should scale be applied – this varies in measurement software’s as some software’s require CTE be applied prior to this type of Alignment, others after the Alignment has been calculated. See Software documentation for best practices.
  Does the CTE need to be applied before or after Data Collection – this is key as some software’s apply the CTE as the data comes in to the software, while others apply it to the measured data during post processing.
  
  Verifying ‘Scale’
  
  When applying a Compensation of Thermal Expansion the ‘Scale’ of the measurement task or dataset is changed. When using a 7-parameter ‘Best Fit’ Transformation this ‘Scale’ factor should be verified in particular on Large Scale measurement tasks. Scale Factors are typically displayed as a ratio of the active Units of Measure, this ratio needs to be considered over the entire Measurement ‘Envelope’, a common mistake is to review ONLY the X, Y, Z, 3D deviations after Scale has been applied.
  
  To properly evaluate the CTE a calibrated ‘artifact’ made of the same type of material must be used, the ‘artifact’ must also be the same temperature as the part being measured, for these reason’s many facilities are unable to properly evaluate the application of CTE.
  Using a Certified Artifact
  
  One method for verifying Scale is to use some type of ‘Artifact’ this is typically a ‘Scale Bar’ or ‘Ball Bar’, the artifact must be the same type of material as the part being measured, and be the same temperature.
  
  Certified Ball Bars
  
  Scale Bars/Ball Bars can be made from a wide variety of materials with the most common available are Aluminum, Steel, Composite and Invar.
  
  Ball Bars have highly accurate spheres fixed to each end. These Ball Bars typically have a calibrated distance between the two spheres.
  
  Scale Bars typically have magnetic mounts at each end to accept Laser Tracker Spherically Mounted Reflectors (SMR’s) or highly accurate Sphere’s. The distance between the points is calibrated by using highly accurate Laser Interferometers.
  
  Certified Scale Bars
  
  How to Verify Scale – When an artifact of the same type of material is available, it must be thermally ‘soaked’ to the same temperature as the object being measured. After applying the CTE within the Measurement Software application the operator can measure Points or Spheres (depending on artifact) and simply check the point to point distance. Dependent on the accuracy of the measurement system being used, the result should typically be within 0.0020 inches or 0.050 mm. The results will vary depending on the device being used, and the accuracy of the artifact being used.
  
  Analyzing the Results – when point to point distances vary from the Calibrated Artifact significantly, the operator must determine what the overall impact to the measurement task will be. If the artifact is significantly bigger than the part will the scale impact be acceptable within the parts envelope? If using a 7 parameter ‘Best Fit’ Transformation is the ‘nominal’ information accurate? If the ‘nominal’ information was derived from a previous measurement; was CTE accounted for correctly?
  
  The use and application of CTE can improve the accuracy and repeatability of nearly every measurement task when applied correctly. Refer to your Metrology Software package documentation for details of use within that software.
  
  Found at JUSTMETROLOGY.WORLDPRESS.COM
  
  by Gerd Schwaderer
  

  0 Comments
• 26 Jan 2012

  Hexagon acquires surveying, mapping software firm MicroSurvey

  Hexagon acquires surveying, mapping software firm MicroSurvey
  01.19.12.microsurvey
  by Sam Pfeifle | January 19, 2012
  
  NACKA STRAND, Sweden - Hexagon, parent company of active 3D firms like Leica Geosystems, Intergraph, and Erdas, announced today it has entered into an agreement to acquire all outstanding shares of MicroSurvey Software Inc., a "Canadian-based developer of surveying and mapping software for the land surveying, construction, and forensic markets." Financial terms were not released, but MicroSurvey reportedly employs 30 people and had 2010 revenues of roughly $2.5 million. The company does business in more than 100 countries around the world.
  
  "The acquisition of MicroSurvey notably expands Hexagon's product offerings and software development capabilities for several of our key markets including land surveying, construction and public safety," said Ola Rollen, CEO and president of Hexagon AB, in a statement. "Additionally, both MicroSurvey's software products and Hexagon's instrument products enjoy strong leadership positions in these markets. The ability to offer such comprehensive, market-leading and innovative solutions will undoubtedly benefit both current and future customers of Hexagon."
  
  MicroSurvey struck a deal with Leica in 2010 for the laser scanner manufacturer to distribute MicroSurvey's PointCloud CAD, a software package that uses Leica's point cloud engine technology "to efficiently manage large laser scan data sets from any scanner or aerial Lidar as users process point cloud data into mapping elements in the software’s final deliverables."
  
  You can see how the software manages point clouds in the following video:
  
  
  
  MicroSurvey also makes MapScenes, a popular software package used to process 3D data in the forensics field.
  
  "MicroSurvey has grown very rapidly in the last two years," said Darcy Detlor, president of MicroSurvey Software, in a statement, "and the acquisition by Hexagon gives us the means to grow even faster and keep pace with the ever-growing customer demand. Our team is extremely excited about this acquisition as it provides us the vehicle for further investment in innovation and greater access to the market." According to a press release, Detlor will continue as president of MicroSurvey.
  
  SPAR Point Group continues to report on this story.
  
  found by Geomagic GmbH
  Sales Director Europe, Middle East and Africa
  Gerd Schwaderer
  Stuttgart, Germany
  Tel. +49 178 77 67 887
  http://www.geomagic.com
  europe@geomagic.com
  XING group optical measurement
  https://www.xing.com/net/dssp
  
  

  0 Comments
• 19 Jan 2012

  Smartphone Scanner?

  Depth acquisition, in your pocket, soon
  Sam Pfeifle from Spar Point
  1/16/2012 5:21:25 PM
  
  
  
  How's this for a paragraph to get you excited?
  
  Now imagine a device that provides more-accurate depth information than the Kinect, has a greater range and works under all lighting conditions — but is so small, cheap and power-efficient that it could be incorporated into a cellphone at very little extra cost. That’s the promise of recent work by Vivek Goyal, the Esther and Harold E. Edgerton Associate Professor of Electrical Engineering, and his group at MIT’s Research Lab of Electronics.
  
  A laser scanner in my cell phone? Sign me up!
  
  Unfortunately, that paragraph might be over-reaching just a bit (we writers are shameless!). But this new technique of creating depth maps, called CoDAC (short for "Compressive Depth Acquisition Camera"), is intriguing nonetheless.
  
  I came across the above paragraph in an article put together MIT's news arm, and it does a great job of getting you excited about this new technology's capabilities and possibilities. Essentially, using commercial-off-the-shelf technology, these MIT researchers have built upon the time-of-flight principle of 3D data acquisition and created a very inexpensive way to get depth information. Sort of.
  
  When you navigate to the team's home page (they're the Signal Transformation and Information Representation Group), you'll find lots of great information about what they're up to, including this part of the FAQ:
  
  7. What are the challenges in making CoDAC work?
  
  There are several challenges to making CoDAC work. The most important challenge comes from the fact that measurements do not give linear combinations of scene depths. For this reason, standard compressed sensing techniques do not apply. The light signal measured at the photodetector is a superposition of the time-shifted and attenuated returns corresponding to the different points in the scene. However, extracting the quantities of interest (distances to various scene points) is difficult because the measured signal parameters nonlinearly encode the scene depths. Since we integrate all the reflected light from the scene, this nonlinearity worsens with the number of scene points that are simultaneously illuminated. Without a novel approach to interpreting and processing the measurements, little useful information can be extracted from the measurements. The superposition of scene returns at the single detector results in complete loss of spatial resolution.
  
  Hmmm. "Little useful information can be extracted from the measurements." That would seem to be a hurdle for most of you working with commercial applications of acquiring 3D data. They also can only currently create depth maps for scenes where all the objects are basically flat, which is again quite a limiter.
  
  Still, they're not saying it's a finished product by any means and the size and cost efficiencies they've created are incredibly tantalizing. This is certainly important work worthy of following.
  
  How does it all work? Best to let the MIT brain do the explaining:
  
  http://www.youtube.com/watch?v=amTMqxX0T_U&feature=playe...
  
  Found at SPAR by
  
  Geomagic GmbH
  Sales Director Europe, Middle East and Africa
  Gerd Schwaderer
  Stuttgart, Germany
  Tel. +49 178 77 67 887
  http://www.geomagic.com
  europe@geomagic.com
  XING group optical measurement
  https://www.xing.com/net/dssp
  
  

  0 Comments
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