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Category Details
Category Name
HDI
Category Created
Thu, 27th Feb 2014
Last Article Update
Wed, 2nd Apr 2014
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   HDI

Accuracy of scan data 

Our accuracy specifications are given on the product data sheets. Links to those data sheets are provided below. It is important, when comparing accuracy data, to know how these specifications were measured. LMI uses a VDI/VDE-approved ball-bar test to determine the accuracy of the sensor throughout its entire FOV. The specifications given on the data sheet represent the worst-case measurements at the extreme limits of our scanners FOV. In many cases the accuracies obtained within a smaller portion of the FOV are significantly better than these figures.

We have found that the best way to determine if the systems will meet your required accuracies is to do a benchmark scan on a representative sample object. Over the years we have found our data has been comparable to that of scanners costing many thousands of dollars more for many applications.

You can find the product datasheets online in the following locations:

Advance (adjustable FOV): http://downloads.lmi3d.com/system/files/HDI/documents/HDI%20Advance/DATASHEET_HDI_ADVANCE_X_1.0_WEB.pdf

HDI 100 series (fixed FOV): http://downloads.lmi3d.com/system/files/HDI/documents/HDI%20100%20Series/DATASHEET_HDI_100.pdf

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Accuracy testing standards used 

LMI uses a VDI/VDE ball-bar test to determine the accuracy of its scanners. For this test two spheres are mounted on a ball bar of know length. Scans are taken of the spheres as the bar is positioned in various orientations throughout the scanner's field of view (FOV). For each scan the distance between the spheres, calculated based on the scan data, is compared with the nominal value. The largest error at any location said to be the accuracy of the scanner.

Note: HDI Advance scanners are field-calibrated, so the scan data accuracy will depend on following good calibration technique.

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Measurement uncertainty of scan data 

Measurement uncertainty is very often related to the type of geometry being measured. When using a high-density measuring technique, like scanning, a larger feature defined using many points will always provide less uncertainty than a small feature defined by fewer points.

The good news is, since scanning is so fast, it is very easy to do a quick repeatability study on the features of interest to you. Our applications engineers are available to assist you. Please contact your sales representative for more information.

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Necessity of reflective stickers (photogrammetry stickers) 

Reflective stickers (also called photogrammetry stickers) are a convenient way to align multiple scans to a global coordinate system. However, they are not required. Scan data can also be aligned using a best fit of overlapping geometry between scans or using an automated turntable.

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Necessity of spray painting target 

Sometimes. Extremely dark, translucent, or shiny parts must be painted in order for the projected pattern to be detected by the scanner's cameras. It is always a good idea to take a sample scan on a representative part to determine whether or not you need to spray paint the target.

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Operating principle of HDI scanners 

All of our HDI sensors use structured light projection and stereo triangulation. A brief description of the technology and the basic workflows can be found at http://www.lmi3d.com/assets/resources/understanding-3D/ebooks/3D_scanning_101.pdf.

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Ways that environmental variables affect scan data quality 

Let’s take each variable one at a time.

Vibration: Structured light scanners, such as the HDI series, must be relatively still during the sequence of frames required to capture a scan image. This means that vibration and relative movement between the subject and a scanner will have a significantly adverse effect on the scan data.

Dust: Obviously, significant build-up on the scanner's lenses would be an issue for any scanner. However, the HDI 120 is the ONLY scanner that is IP67 rated for use in dusty environments. It is truly one of the few optical scanners designed and built for the factory floor.

Temperatures changes: Both the HDI advance and the HDI 120 have been designed to minimize the impact of temperature changes on the scan data accuracy. Development is ongoing in this area, so please contact your sales representative to get the latest information.

Lighting Conditions: All HDI scanners have both manual and automatic exposure control settings to help compensate for environmental lighting conditions. However, some care needs to be taken to control lighting conditions for optimum scanning. Moving shadows and changing lighting conditions during a scan should be avoided. Scanning in direct sunlight is not possible, as the projection light source is not bright enough for such conditions.

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