Quality Scan: Give Some Thought to 3D Scanning
Significant advances in 3-D scanning technology have been driven by customer demand and the increased processing power of today's computers. Entire airplanes and automobiles can now be scanned with more precision in less time than previously thought possible.
When compared to only two or three years ago, portable scanning solutions are very much a reality today. Using mobile scanners, operators can capture surface data from a part or tool on the shop floor, and get immediate visual feedback about the area of interest. The resulting point cloud can be processed by many specialized software packages to create inspection data, generate reverse engineering data, and much more.
Today's scanning sensor technology can be broken down into three main categories: Structured light scanners project an array of different lines onto the surface in a grid-like pattern, and use cameras to determine each 3-D point location in a large patch of data. Line scanners project a laser line onto the surface and measure the profile of that line on an image sensor to determine the specific 3-D coordinates for each point on the surface. Laser-dot scanners project single laser points in rapid succession onto the surface, and calculate each point's location as its reflection returns to the sensor.
Each type of scanner has its strengths and weaknesses. For instance, some devices can measure more points in a shorter period of time. Other systems can measure many different surface types without any special surface preparation. But from a user's perspective, the most important difference between all of these technologies is how all of the collected data is held together.
Structured light scanners measure points in patches, line scanners measure points on a line, and dot scanners measure individual points. So how do all of these patches, lines, and points combine into a single coordinate system?
Looking at structured light scanners first, these systems require a series of reflective targets to be placed on the surface of the object to be measured. Enough dots need to be used so that the scanner can see multiple common dots between scan patches. These dots are then surveyed with a photogrammetry system to locate their position relative to the part's coordinate system. Not until all of the previous steps have been completed (and possible problem surfaces have been sprayed with a white powder to allow a homogeneous light return) can the scanning begin.
On the other hand, line scanners can only determine each point on each line, but cannot associate each line to the next. There must be another system involved that can associate each line to the other, then relate all of them to the part's own coordinate system. An articulated arm is typically used for this procedure, but the scan volume is limited to the reach of the arm. The arm can be moved to locations around the measured object using common points, but this procedure drastically reduces the system's achievable accuracy.
Dot scanners have a similar limitation. All measured dots are relative to the sensor, but the sensor still needs to be located in a common coordinate system as it is moved around the part.
Technology has advanced to solve these challenges. Major laser tracker manufacturers have entered the scanning world with new solutions that use the laser tracker to do what it does best: locate an object with excellent accuracy in a very large volume. Some manufacturers have started to use their laser trackers in combination with an articulated arm. The tracker locates the arm, the arm locates the scanner, and the operator can reposition the arm without losing accuracy.
Another innovation is the 6DoF (Six Degrees of Freedom) laser tracker that can track a scanner body directly.This capability allows the operator to scan an entire part without the need to reposition an intermediate device such as an articulated arm. In addition, this laser tracker allows a measurement probe to be used in conjunction with a scanner. The combination enables the tactile probing data and noncontact scanning data to be combined in a single coordinate system—all measured from the same machine. Merging 6DoF laser-tracking and portable-scanning technologies can now make it straightforward for an operator to precisely scan very large parts.
This article was first published in the April 2007 edition of Manufacturing Engineering magazine.