Scanners Advance Portable Data Acquisition

December 21, 2009
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In the portable metrology field, OEMs serving this industry have focused their best engineering minds and invested millions of dollars into the research and development of scanning technology.





In the portable metrology field, OEMs serving this industry have focused their best engineering minds and invested millions of dollars into the research and development of scanning technology. Second- and third-generation products are being launched into the marketplace with groundbreaking features. Third-generation scanners have introduced a doubled data acquisition rate of 20 kHz, which means large surfaces can now be captured in half the time compared to the previous versions. These optical scanners are suited for capturing complex freeform surfaces where the shape is not easily represented by simple prismatic entities. Examples include cars, aircraft, motorcycle bodies, trim and seats, consumer products, medical components and turbine blade shapes used in power generation.

Portable coordinate measuring machines (CMMs) are different from traditional CMMs as they most commonly take the form of an articulated arm, laser tracker, laser radar, photogrammetric camera or other mobile device used for the in-place measurement of a part or assembly. In 1995, data acquisition was notably enhanced with the first integration of a hand scanner with a portable arm. In 2005, the hand scanner was incorporated with a laser tracker.



Flying Dot Technology

Scanners most often are used to acquire coordinate data from large industrial parts and assemblies; soft or delicate parts made from clay, foam or textiles; or sensitive surfaces that cannot be touched such as a telescope lens or a museum artifact. Each application presents a host of issues for the metrologist-various surfaces of an object, reflectivity, differing lighting scenarios in the measurement environment.

Based on these challenges, the most significant new scanning advancement is the breakthrough flying dot technology. Laser line scanners employ this new feature to automatically adjust the laser intensity to obtain the best measurement of an object’s exterior, such as shiny metallic or dark surfaces.

The secondary reflections on concave parts of an object, such as mirror-like surfaces on machined parts, get reflected back into the imager and are very difficult to filter out. Here is an example of a poor quality scan from a laser line scanner caused by spurious reflections from a machined surface. Source: Hexagon Metrology

Triangulation Approach to Scanning

The triangulation laser scanner is an active sensor that actively projects a light pattern to probe the environment. The triangulation laser shines a collimated laser beam on the subject and exploits a camera to look for the location of the laser spot. Depending on how far away the laser strikes a surface, the laser dot appears at different places in the camera’s field of view. This technique is called triangulation because the laser dot, the camera and the laser emitter form a triangle.

The length of one side of the triangle, the distance between the camera and the laser emitter, is known. The angle of the laser emitter corner also is known. The angle of the camera corner can be determined by looking at the location of the laser dot in the camera’s field of view. These three pieces of information fully determine the shape and size of the triangle and give the location of the laser dot corner of the triangle.

The use of laser light is advantageous for three reasons:

1) its source, a laser diode, is compact and generates little heat;

2) the laser light is monochromatic, thus it is easy to bring to a focus; and

3) ambient light (the light appearing in the environment) is easy to filter out from the image.

The single spot scanner can be extended to a laser line system by extending the above system into the plane. This is achieved by extending the image sensor from a linear 1-D image sensor to a 2-D array image sensor, and the laser from a single point laser to a line laser. This duplicates the single point sensor typically 1,000 times over resulting in 1,000 measured points in each measuring cycle.

However, the basic extension from a single spot device to a line device has three significant drawbacks. First, the secondary reflections on concave parts of an object, such as mirror-like surfaces on machined parts, get reflected back into the imager and are very difficult to filter out.

Also, objects with highly contrasting surface finish, for example, a casting with some machined faces, are very hard to measure accurately.

And lastly, if the scanner has automatic signal level control, a complete camera image must be analyzed before the system can feedback any changes to the laser power to maintain an optimal level for the surface being measured. 

Due to these issues, a basic laser line triangulation sensor needs to have several parameters adjusted depending on the part being measuring. So, while these basic devices can capture data quickly, it is often at the expense of accuracy, reproducibility and reliability.

Figure 1
The single spot scanner can be extended to a laser line system by extending the above system into the plane. This is achieved by extending the image sensor from linear 1-D image sensor to a 2-D array image sensor, and the laser from a single point laser to a line laser. This duplicates the single point sensor typically 1,000 times over resulting in 1,000 measured points in each measuring cycle. Source: Hexagon Metrology

How Does the Technology Work?

Today’s sophisticated laser scanners employ a flying dot approach to solve the surface variation and reflectivity problems. Measurements are performed by using the same basic system show in Figure 1, but forming a laser line by sweeping the laser spot over an angle using a rotating polygon mirror. (See Figure 2).

Since it is still a single spot device, the intensity of the laser light can still be continuously adapted for each point on the line, in real time, by feedback to the optimal intensity for the surface of the part.  And because the image sensor is only 1-D, the camera speed is much faster-at 20,000 Hz, almost 500 times than of a 2-D image sensor. The result is a more accurate measuring system that eliminates the fine tuning of the scanner to the object being measured, removes the need for surface treatment such as powdering and does not require photogrammetric targets.

When a flying dot scanner is linked to a laser tracker, the measuring process can be conducted within large distances to microns of accuracy. This system works through the use of extremely accurate distance measurements using the tracker’s laser and a digital camera system that tracks infrared light- emitting diodes (LEDs) mounted on the scanner’s housing.

This integration of long distance location and orientation measurement together with a laser line scanner enables an operator to scan objects, such as aircrafts, in a very large volume up to 98 feet, a distance nearly 10 times larger compared to a standard hand scanner coupled with a portable arm.

Figure 2
A polygon mirror makes a laser “fly” across the surface to form a line. Source: Hexagon Metrology

New Applications

Presently, flying dot scanners with laser trackers are available as a manual device or robot-mounted for fully automated, repeatable path scanning. Because flying dot technology can automatically scan and reliably handle a diverse range of surfaces, there is now demand from the field for a highly flexible scanning probe to be mounted on portable arms and DCC CMMs. This will require more research and development from industry players.

While it is anticipated that the basic laser line triangulation scanners will proliferate at the lower end of the market, flying dot technology will dominate in the areas where there is a customer need for reproducible, high accuracy solutions that work reliably without the need for time- consuming operator input during the setup and/or measurement process. Q

Tech Tips

  • Flying dot technology can automatically scan and reliably handle a diverse range of surfaces.
  • The integration of long distance location and orientation measurement together with a laser line scanner enables an operator to scan objects in a very large volume up to 98 feet.
  • Presently, flying dot scanners with laser trackers are available as a manual device or robot-mounted for fully automated, repeatable path scanning.

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