V-Stars Measurement of a White Metal Bush Cylinder and the Starboard Kort Nozzle to the Centerline of the Engine Shaft
The primary objective was to determine the center of the cylinder and nozzle relative to the centerline of the engine shaft.
Equipment Used
- V-STARS S Camera System
- Scale Bars
- Retro-reflective targets
- Retro-reflective strip tape
Measurement Objectives & Targeting
In order to meet the measurement objectives outlined earlier it was necessary to target the area. In general, targets are placed on points or surfaces that are of interest. For surfaces, strips of retro-reflective tape of variable pitch and dot size are commonly used. They are relatively cheap, disposable and easy to apply. To coordinate tooling datums such as bushed holes or button datums, tooling targets are used. These come in a variety of shank and dot sizes. They are also available in variable orientations. The remaining key planes and lines were measured using single dot targets with a cross hair or single dot targets.
To automate the measurement process it was necessary to add “coded” targets to the block and the area surrounding it. These targets are automatically detected and help the software determine the location and orientation of the camera at the time the photo was taken. They also help tie the entire object into a uniform coordinate system. The codes were placed along the length of the front of the jig.
The initial coordinates system and scale is determined via the AutoBar. The AutoBar used by the V-STARS system is a fixture with five targets arranged in the form of a cross. The target’s known coordinates are used by the AutoMatch procedure to determine the camera’s orientation relative to the AutoBar. The AutoBar is securely attached on or near the measured object, preferably in a highly visible location. The AutoBar’s default coordinate system has its origin at Target 1 at the bottom of the AutoBar. The positive Z-axis goes through Target 3 at the top of the bar. The positive X-axis is up out of the AutoBar. The diagram on the left shows both the AutoBar and its coordinate system.
Photography
The photography is carried out once the object targeting is completed. Put simply, the aim of the photography is to record each of the targeted points in as many images as possible from as wide a range of angles as possible. To improve the accuracy of the measurement, generally photos are taken both close to the ground and from an elevated position. The number of photos taken depends on the complexity of the measurement and accuracy requirements.
The diagram below illustrates the typical geometry used to create a point cloud.
Processing
Once the photography has been completed the images are transferred to the system laptop. The images are stored on a PCMCIA hard drive and V-STARS accesses these images directly from the drive.
Almost all of the measurement process is automated. The images are processed and the coordinates extracted by the “AutoMeasure” command. A typical AutoMeasure dialog box is shown on the right. The AutoMeasure command will open each of the images, determine the camera location, find new target points and finally adjust all the measurements in the “Bundle Adjustment”.
At the conclusion the user is left with the XYZ coordinates for all the target points in the network. The AutoMeasure procedure is very powerful as it allows the user to continue working while it processes the data. It also means that relatively unskilled workers can be used to process the data.
The AutoMeasure routine will assign random labels to the points it finds. These labels start with the key word “Target” followed by a number. If specific labeling is required the random labels can be easily changed to labels defined by the user. This is possible in both the picture view and the graphical 3D view. For this particular project it was necessary to re-label the points so that analysis could be simplified.
Seen below is a typical measurement image.
Conclusion
The measurement undertaken has shown that the V-STARS S system can be a very powerful measurement tool. The results of the measurement undertaken were very accurate and produced quickly.
