3D Digital CMM Scanning
3D scanning with a CMM (Coordinate Measuring Machine) is a method used to create a detailed digital representation of a physical object by capturing its geometry. This process combines the precision of a CMM with the capabilities of 3D scanning to provide highly accurate 3D measurements and reverse engineering of highly complex machine’s parts or tools.
3D Scanning Technology
The benefits and limitations of a 3D scanner are typically derived from its positioning method. That’s why it is valuable to take a look at positioning methods within the different 3D scanner categories. The main 3D scanner categories:
- Measuring arms, portable CMM scanners
- Tracked 3D scanners
- Structured-light 3D scanners
- Portable 3D scanners
Measuring arms, portable CMM scanners
CMMs (coordinate measuring machines) and measuring arms can be equipped with either fixed-probe or touch-trigger probe heads. It is also possible to mount a 3D scanning head on a CMM.
POSITIONING METHOD: MECHANICAL ENCODERS
CMMs with portable arms are positioned using the mechanical encoders integrated in the arm.
Advantages
- Many different tools can be mounted on portable CMMs, making it possible to easily integrate scanning and probing in the same project.
Limitations
- Portable CMMs need to be fixed on a surface and use a physical link (arm) as their positioning method. This makes them prone to vibrations and other environmental constraints that can affectthe performance and quality of the result. They also lack flexibility in terms of the locations in which they can be used and the shapeof the objects they can scan.
Tracked 3D scanners
Optical tracking devices can track various types of measurement tools, including the positioning of a3D scanner.
POSITIONING METHOD: EXTERNAL OPTICAL TRACKING DEVICE
These scanners use an external optical tracking device to establish positioning. They usually use markers (such as passive or active targets) that optically bind the tracking device tothe scanner.
Advantages
- Tracked 3D scanners provide very good accuracy and excellent precision throughout the measurement volume. Removing the need for a physical link between the scanner and the object being scanned provides freedom of movement.
Limitations
- The optical link that is a strength of this technology is also one of its limitations. The tracker must always have a clearand direct line of sight to the 3D scanner. Trackers often havea limited working volume. Extending the scanning parameters adds complexity to the process and can induce some additional uncertainty in the measurements. Finally, tracked 3D scanners are usually more expensive than solutions such as portable3D scanners.
Structured-light 3D scanners
These scanners project a pattern of light onto a part and process how the pattern is distorted when light hits the object. Either an LCD projector or a scanned or diffracted laser beam projects the light pattern. One or two (sometimes more) sensors record the projected pattern.
POSITIONING METHOD: OFFLINE TARGET POSITIONING AND GEOMETRY POSITIONING
The scanner can either rely solely on the part geometry to position the data or rely on positioning targets (small stickers provided with the system that can be placed directly on the part) to align 3D data.
If only one camera is used, the position of the projector in relation to the camera must be determined in advance; if two cameras are used, the stereoscopic pair must be calibratedin advance.
Advantages
- High-end structured light scanners generate very high-quality data. They typically deliver excellent resolution, which allows for the smallest features on an object to be captured in the results.
Limitations
- While white-light scanners can acquire large quantities of datain one scan, overall project speed is not always improved by this methodology. Multiple scans are required in most cases to cover all angles on more complex parts, which is very time consuming.
