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What is Photogrammetry?

Photogrammetry is the process of collecting the precise 3D coordinates of reference points using a digital camera. The points are either measured directly, like in this example, or can be used to increase the accuracy of scanning for larger objects. Capture 3D's photogrammetry system, known as TRITOP, is extremely compact and portable and is not limited to a fixed stand like a traditional CMM. TRITOP also doesn't need to undergo complex procedures for repositioning to measure larger objects. To complete measurement, simply walk around the subject and take pictures of it. One common use case for standalone photogrammetry is the measurement of production machining and welding fixtures. Since the TRITOP photogrammetry system can be set up and torn down very quickly, it allows for the measurement of many fixtures while the team is on break or between shifts, so there's no need to build up backfill stock to keep the line running.

How to Perform Basic Inspection of Photogrammetry Data Using Free GOM Inspect software

To begin inspection in the free version of GOM Inspect, you'll need photogrammetry data collected using either TRITOP or ATOS Professional Software. To focus exclusively on the inspection, import and align the CAD data to the reference points already collected. Click here to learn more about alignments.

This example uses a file from a photogrammetry shoot on the GOM training object. The collected points are in green, the CAD is in blue, and the pictures taken to collect this data are also visible. At this point, it's best to hide the images, so it's easier to focus solely on the inspection. The photos can be hidden by navigating to the bottom toolbar, selecting the image mapping button, and turning image mapping off, leaving just the CAD data and the collected reference points visible.

How to Create a Surface Comparison with Photogrammetry Data Points

The first step is to perform a surface comparison to understand how the measured part compares to CAD. In the inspection workplace, surface comparison can be accessed using the surface comparison menu on the toolbar. This example begins with a surface comparison on actual.

First, utilize the default naming and select GOM block for the nominal element. An exclamation point will appear explaining that the conditions required for creating the element have not been fulfilled completely, and an area on the mesh or point cloud must be selected. All of the collected points will be utilized for this surface comparison, so choose "Select All" from the selection toolbar. The surface comparison can now be created.

When the surface comparison is first created and fully computed, the points will be quite small and difficult to see. To make them easier to see, navigate to the Explorer, find the surface comparison in the tree, and open the Properties window by either clicking on the double arrow icon on the right-hand side of the screen or by using the Tab key.

Next, navigate to the display section and change the display size from small to large for better visualization. Surface comparison, the distance between the CAD and the collected points, is measured. If the point is above the CAD's surface, it will trend towards yellow, and if it's very far above the surface, it will trend towards red. If the point is below the surface of the CAD, it will trend towards blue. Points that are very close to the surface of the CAD will show in green. There is a legend on the right-hand side of the display which correlates the colors to the values. In this case, a bright red is 0.285 millimeters above the surface. To better understand each of these points' deviation to the CAD, apply deviation labels using the Pointwise Inspection menu on the toolbar, and select deviation labels.

Measuring the Overall Length of the Part

Next, there will be a dialog in the main 3D view that shows how to create deviation labels. The dialog explains that holding the Ctrl key shows a preview of the deviation label, and Ctrl plus left click creates an element. To exit the function, hit the Escape key or use the right mouse button. Apply deviation labels by holding the Ctrl key left-clicking on a few of them.

Some red labels, blue labels, yellow labels, and green labels will appear throughout the part. Some red and orange markers will appear on the left-hand side of this part, and there are green markers on the right-hand side of the part, which may indicate an issue with the alignment, in which case these reference points can be moved to bring both dimensions within specification. Or it could suggest that the part is too large physically. This issue can be checked by measuring the part's width by going to the Construct menu, choosing Distance, and using the outer disc caliper tool.

This function works best after the dialog is filled out, so do that by hitting Ctrl plus left click to select the first point. Select a point on the part's left-hand side and then a point on the right-hand side part. Then select a direction. In this case, the distance that needs to be measured is along the X-axis. Select the X-axis from the drop-down menu. A graphic will appear that explains how this function works. Create two discs with a radius of 210 millimeters. These will be some distance away from the part, and they'll move towards the part along the X-axis and stop when they contact the first piece of data. At this point, the shortest distance between these two discs will be used to measure the part's width.

The final step before creating the outer disk caliber is selecting points for measurement. Since the software is measuring the part's entire width, it makes sense to select all of the points. Now that the distance is constructed, its value compared to the nominal must be checked. Begin by opening the I-Inspect wheel by either clicking the magnifying glass in the main toolbar or by holding the Ctrl key and right-clicking anywhere in the 3D workspace.

Next, use the check function to check the distance. Since the nominal value for this part is 450 millimeters, and the tolerance is 0.1 millimeters for the overall width, the part is too large with an overall width of 450.376 millimeters, which is too large for the plus or minus 0.1 millimeters tolerance. Clicking off the inspection result hides what it's inspecting and points to the middle of the part, making it a little bit difficult to show which distances are measured in the resulting inspection report. To make it easier to visualize what the 450 is referring to, click on the actual inspection item (the length item), and a small blue box will appear in the middle. Grab that blue box and pull it up using the left mouse button and drag that inspection result outside of the part.

Since the dialog now contains no useful information, hide it by clicking on its eyeball in the Chosen Elements list or by hitting the "I" key (invisible shortcut) on the keyboard. The inspection result is now outside of the part and more directly represents what's being measured. Also, since the distances are pulled out on theater lines, it now looks like a standard engineering drawing.

Measuring the Diameter of a Cylinder

Next, measure the diameter of any cylinder on the part by going to the Construct menu, hit Construct Cylinder, and then Auto Cylinder Nominal. Then hit Ctrl plus left click on the geometry to create the cylinder, and then select Create and Close.

There will be a red indicator on the cylinder, a red indicator in the label, and a red section in the status Explorer, which indicates that the measuring principle is missing, prompting a possible action to apply a measuring principle. Apply a fitting element measuring principle, which tells the software exactly how to measure this nominal cylinder and what data is to be used to measure this cylinder's features. Use the collected reference points and a Gaussian best fit. Input plus or minus three sigmas, which represents six standard deviations for the data that will be used. Then hit OK. The red is now gone.

The software knows exactly how to measure this cylinder. Apply a check again, using the I-Inspect wheel, and check the diameter. Since a nominal element was constructed, the software knows the nominal value of 60 millimeters for the cylinder, so it measures the reference points cloud's diameter around that cylinder and finds that it's 60.021 millimeters across.

How to Create an Inspection Report Page

The final step in this inspection is creating a report page. Begin by selecting the Create Report page icon in the top toolbar, and title the report page. Next, edit the layout of the page. For example, the page can be arranged so the focus is just on the top view with the calipers well placed near the diameter call out and keep all of the deviation labels visible. Then hit OK and OK again to create that report page.

How to Get Started with Photogrammetry

Though this tutorial covered a lot, this is just a basic inspection of photogrammetry data using the free version of GOM Inspect software. GOM's software features a fully compliant GD&T inspection suite with many capabilities, including the ability to measure the deformation of parts and assemblies under static load. If you would like to learn more about TRITOP photogrammetry, contact a Capture 3D team member. If you'd like to see how photogrammetry can benefit your specific application, schedule a one on one with our team.

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