Full Field Optical Technology for Quality Control

Stamping Out Bad Parts

By John Psilopoulos, Tim Schmidt and John Tyson

Automotive, original equipment, sheet metal manufacturers and stamping houses have been going through tremendous changes due to the economic status globally. More and more of them are shifting responsibility for quality inspections to suppliers, adding extra burdens. New, complex product designs and rigorous processes, such as deep drawing and the growing use of exotic materials, are placing greater demands on both quality assurance and development departments. The current method of formability quality control is by using CGA (Circle Grid Analysis). CGA is the measuring of the elongation of electrochemically etched circles onto a stamped part. Thought this method has worked, it has also come at a great cost. Performing this task is tedious and time consuming. The measurements are conducted manually and circle by circle. The results are prone to interpretation and very difficult to repeat the results. Many of the current measurements for CGA are conducted by the steel vendors and may take anywhere from 3-5 days to receive a report in hand. The downtime and lack of information is a great cost to a manufacturer. One option for stamping quality control to help fabricators determine shape, strains, and thinning is full-field optical vision technology. Based on the principles of circle grid analysis and photogrammetry—the science of making precise measurements by recording and analyzing photographs—optical vision technology provides automated analysis and quantitative color maps for every square inch of a complex part. The quality results can be displayed in a 3-D computer model, using the actual measured dimensions of the real part, and viewed from any angle. Using this technology, stamping houses can verify finite element models with compatible data from factory tests on the real part. They also can use it for initial die verification, first-article inspection; ongoing checks for die wear, tooling alignment, and new material lots.

How It Works A circular dot pattern is electrochemically etched onto the unformed sheet, exactly as it would be with circle grid analysis. The usual dot size is 0.5 to 2 millimeters, and spacing is 1 to 4 mm. After forming, a scale bar and a series of bar-coded markers are distributed around the part (see Figure 1).

Figure 1 – On a formed part prepared to be measured, bar-coded markers allow multiple camera shots to be overlapped and combined. The user snaps a series of pictures of the stamped part with a digital camera wirelessly connected to a computer. The exact camera locations are arbitrary, as long as every area of the surface is contained in at least three pictures from different angles. An ellipse finder algorithm identifies and marks the center of every dot visible in each camera shot (see Figure 2). Each dot occupies at least eight to ten pixels on the camera, so the center can be interpolated with an accuracy of about 1/10 pixel. The system determines the 3-D coordinates of the center of each dot using the principles of photogrammetry. The software calculates both the camera locations and the distances to each dot to measure the shape of the part (see Figure 3).

Figure 2 – An ellipse finder algorithm identifies and marks the center of every dot visible in each camera shot, accurate to about 1/10 pixel.

In circle grid analysis, the stretching of individual dots is measured and compared with the known original dimensions to indicate strains. In the camera-based system, the distance between all of the dots is compared to the known originals using a larger neighborhood of points. Because the 3-D coordinates of the surface are measured, the actual deformed shape is documented, and the resultant material thinning is calculated using the constant volume assumption. As the pictures are taken they are automatically downloaded to the computer through the wireless router and calculated. Depending on the complexity and the number of pictures it can take up to 20 minutes. Once processing is complete, data can be presented in a variety of ways.

Figure 3 – After the user snaps a batch of pictures, the software determines camera locations and distances to each dot and measures the shape of the part. Strains and thinning are calculated automatically.

In one data presentation method, each of the stamping’s measurement points is shown as an individual point on both the forming limit diagram (FLD) and the 3-D color strain map. With a dynamic link between the FLD and the strain and thinning color map, when a point is clicked on either display, a second crosshair automatically highlights that same point on the other display, and a detail box presents all measured and calculated quantities (see Figure 4). Figure 4 – A cursor is dynamically linked between the 3-D graphic strain map and the forming limit diagram. When a point is clicked on either display, a second crosshair automatically highlights the same point on the other display. A detail box presents all measured and calculated quantities for that point.

This technology’s accuracy can be as high as 0.5 percent strain. At the die tryout stage of a project, and for ongoing die monitoring and material lot verification, total area coverage with vision-based technology can provide complete quality documentation for advanced product quality planning (APQP) to meet QS-9000 requirements.

How it’s Used When a camera is wirelessly connected to the computer, no image import steps are necessary. The system automatically downloads the images and processes them as they are taken. On the camera the operator has the capability to control lighting factors by adjusting the F-stop and the shutter speeds. It also has an onboard flash that assists in the lack of lighting. These controls help to overcome potential problems with highly reflective surfaces such as aluminum. Data analysis is automatic, and there is no need for calibration before each measuring session. Once the camera is focused and a standoff distance is set, it must be maintained for the duration of the session. Data to create an FLD for any known material can be imported. Alternatively, a two- camera version of the technology, called 3-D image correlation photogrammetry, can be used to determine the FLD experimentally for any new material or to study the strength of the materials under load, as a full-field optical strain gauge. Limited areas can be measured in minutes, and entire parts can be documented fully and archived for quality control purposes.

Optical systems can be used with many material types and forming processes, as long as the applied pattern has high contrast. Based on the material type, the proper fluid for the etching of the grids must be select to get the needed contrast. Beyond conventional stamping operations, vision systems are applicable to other processes, such as inverted toggle drawing and hydroforming.

An optical system is suitable for large, complex parts; when material is being taken near its forming limit; and for investigating new materials and processes. For example, the system can inspect the entire surface of a 1.2-meter floor member (see Figure 5), display the distance to the forming limit curve for each point, and present the graphic from any angle. It also can indicate hot spots, or areas that exceed the forming limit diagram (see Figure 6a, 6b). Identifying such hot spots allows engineers to optimize the die to bring strain and thinning within acceptable limits.

Figure 5 -- The entire surface of this 1.2-m-long floor panel was inspected with an optical system. The quantity displayed is the distance from the forming limit curve, which shows the maximum strain areas are approximately 5 percent under the limit.

Figure 6a, 6b—(a) The first-article strain map indicates excessive material strain. (b) After die optimization, all strains and thinning are within allowable limits.

Geometry Verification

One of the most important processes in manufacturing is checking the part shape and spring back of the actual part to CAD data. The current CMM method measures the actual part and provides a point cloud. The point cloud is then imported and compared to the CAD data. This process can be very time consuming going from machine to machine. This process is based on touch probe deflection to the part. Full field optical measuring now offers the ability to compare cad data to the entire part within the same system. There is no longer a need to go to a different computer or machine to check the results. The reporting can also be done on the same machine. The CAD data can be imported and is then used as reference geometry. The measurement file is then transformed onto the CAD data using a best fit feature. The algorithm does not only use a point to point alignment. It uses an actual shape to shape alignment which is more accurate. From the alignment the system can now compare the CAD, which is the reference data to the actual part or measurement produced. The system will give you a color gradient indicating high and low spots in the deviation (see Figure 7a, 7b).

Figure 7a, 7b—(a) this diagram shows the CAD and the Measurement before the alignment. (b) After the alignment a color gradient shows the high and low spots between the CAD and the measurement.

FEA Comparison

During the beginning stages of manufacturing a part, a simulation is run to understand the pressures needed to form the part correctly and without splits. Once the tool is built and parts are run, CGA (Circle Grid Analysis) is performed. As mentioned before CGA is a point to point analysis. Full field optical analysis allows you to take the measurement of the entire part and overlay it to the FEA simulation within the system. Just as in geometry verification, FEA comparison is conducted the same way. Once the alignment is done the systems provides a color gradient. This allows the operator to see conclusively how the actual part differs from the simulation. The system allows the operator to pick individual points, cut sections and report in a number of different ways. This allows for customization in the reporting to fit into any quality system (see Figure 8).

Figure 8— Shows the FEA simulation and Optical measurement result. It also shows the overlay color gradient between the two.

John Psilopoulos is Applications Engineer, Tim Schmidt is Vice President and John Tyson is President of Trilion Quality Systems, 500 Davis Drive, Suite 200, Plymouth Meeting, PA 19462, 215-710-3000, fax 215-710-3001, [email protected], www.trilion.com.