Photogrammetry and Optical Scanning The future of small-scale manufacturing
Photogrammetry, can be used for different purposes, but it has quickly realized its application in manufacturing.
In this technique, digital images of an object that have been taken at various angles are used to create a point cloud — or a large collection of points used to create 3D representation of existing structures — from which a computer-aided design (CAD) file can be generated.
The resulting CAD file and subsequent 3D model could then be used to rebuild the part or 3D print it, to its original specifications without using traditional methods, which are both expensive and time-consuming.
If we can take pictures of the parts and use commercial software to create the point cloud file from the images, we can come up with the dimensions within some reasonable amount of accuracy and apply it in industry.
We looked at the variance between the original point cloud files and photogrammetry point clouds to see if there are discrepancies between them and to determine how accurate this technique would be if it were to be used in manufacturing . Photogrammetry has proven to be an accurate approach for applications where tight tolerances are not necessary.
A model generated by photogrammetry of an object with curves and complex features to show the capability of the scanning technology
The objective of the research is to introduce photogrammetry and optical scanning equipment to produce accurate three-dimensional models of the surface of an object. Although this is two items of equipment their use is linked and the full benefits of optical scanning cannot be achieved without using photogrammetry.
Photogrammetry uses a very high-resolution digital camera to capture the image of a large object such as part of an aircraft or car. The image is enhanced by using small circular targets, which are attached to the object before the image is captured. These small circular targets provide a reference system on the object. In the case of a car or aircraft the targets are typically 600 mm apart and 8 mm diameter. In addition to the small circular targets coded targets comprising small uniquely shaped targets are also placed on the object at approximately 1 m intervals.
The optical scanning system is used to capture the actual surface data of the object. The scanning system uses two cameras and a projector to project different fringe patterns on to the surface of the object. The fringe patterns are reflected at different angles dependent upon the distance and angle of the surface of the object.
As the fringes patterns are reflected from the object the cameras use a system of triangulation, similar to finding the position when walking with a map and compass, to accurately calculate the position of any point of the surface of the object. In most cases the accurate positions of approximately 1.3 million points are calculated. Unfortunately, to retain accuracy, the optical scanning system can only work over a small surface area and it is necessary to carry out a number of scans to collect data for a large object such as a car body.
This is where the uniquely shaped targets identified by the photogrammetry system are used. The computer collecting all the point data uses the unique target shapes to stitch all the scanned shapes together and thus produce a digital model of the complete object. The alternative method of doing this would be to measure the object with a small probe continually passing up and down the object.
This method is very slow compared to the use of photogrammetry and digital scanning. The method of accurately calculating the position of any point on the surface of an object can be very useful in a number of areas of manufacturing. A very basic case would be to accurately and quickly measure or produce a three-dimensional model of an object.
This can be useful when examining worn parts where the amount of wear can be calculated by comparing models before and after use. This can be extended to the point where a system can be used to calculate the amount of material that needs to be added to a worn part to enable it to be remachined ready for reuse.
A similar principle can be used to calculate the amount of wear occurring on cutting tools during machining. The system can also be used to compare the shape of an object after manufacture with the desired shape to calculate the amount of deformation produced. The system can also be used to design medical implants. When someone has a deformed jaw, or even if the jaw has been badly damaged in an accident, the dental surgeon can produce a wax model of an implant or replacement.
The wax model can then be scanned using the optical scanning system to produce a three dimensional model. This can then be used to machine a replacement jaw or implant. Other research projects concern the collection of surface data from prototype products and components to design production tooling for a wide variety of manufacturing processes. In all these cases equations need to be determined and built into models to predict the amount of distortion during the manufacturing process and thus design production tooling.
Producing the 3D model using a 3D printer even makes the work easier and cheaper as a future prospect.
In a traditional manufacturing process, large quantities of parts are made in quick succession and then go from the manufacturing line through an inspection process. A quality control engineer or specialist then measures the parts with handheld tools and check for any abnormalities, making sure all of the dimensions of the part are within tolerance so they operate as the part was originally designed.
The ideal application of photogrammetry in the industry setting would be to have a vision system in a manufacturing plant that included cameras fixed on the machines making the parts, taking continuous photos. Live data could be sent back to an engineer or a quality control employee and they could compare the point cloud that has been derived from the digital images to the point cloud of the original file and determine if the part is within tolerance or not.
It has been concluded that photogrammetry has the potential to make the quality control process quicker, less expensive and more efficient for manufacturers. 3D printing hobbyists are using photogrammetry to create models.
We see a future where production of an object can go from scanning to 3D printing without even using CAD.