Plate Quality Inspection System
| By: Paul Falkenstein, VP - Automated and Inspection Systems
Providing a conveyor-based automated inspection system that can visually inspect both the top and bottom surfaces of more than 50 types of dinnerware plates and bowls for a variety of hard-to-identify defects, and that can acquire a 3D model of the ware shape in real time to measure geometric distortion.
Manufacturing dinner plates can introduce several types of defects. Warp is defined as the variability of the plate height around its circumference. Trim defects include both "trim gouge,” where there is an indentation at one or more angular positions, and “trim bulge,” where the circumference bows out from a perfect circle. The other defect, glass adhesion, occurs when small molten glass balls adhere to the top or bottom surfaces in the region adjacent to the plate rim.
The system requirements called for detecting each of the four defect types when the defect size is >250 micron (0.01 in.) for plates up to 12 in. in diameter. Figure 1 shows a highly magnified view of a plate with both trim bulge and glass adhesion defects that were detected and highlighted by PQIS. The system throughput is up to 60 plates per minute and inspection should run 24 hours per day, seven days per week, with minimal down time.
Figure 2 shows the PQIS layout. The system has three sections of 12 in.-wide conveyors positioned end to end with approximately 0.25 in. gaps between conveyors. All conveyors are driven by a single variable-speed drive capable of 65 ft/min.
Positioned below the first gap is a 4,096 pixel line-scan camera and lens that acquires images of the tops of the inverted ware shapes. Images are acquired via NI Vision Acquisition Software. Directly above the second gap is another 4,096 pixel line-scan camera and lens that acquires images of the plate bottoms. Both cameras are connected via Camera Link protocol to an NI PCIe-1433 dual Camera Link acquisition device. Each camera has an independent electronic photo sensor to trigger collection through a Camera Link I/O extension device.
The plate warp is measured at another station on one of the conveyors. A set of red laser lines is used to create a laser line incident on the bottom surface of the inverted plate or bowl. Vertical displacements of this line are viewed in respect to a reference reading and are measured using an area scan camera. Triangulation is used to generate a point cloud model of the sample as it passes under the camera. The software corrects for lens perspective and laser alignment to produce models with 100 µm height tolerances and cross-section resolution of less than 150 µm. The two lasers can generate more than 250,000 points per second. Point clouds are analyzed to determine sample warp. Figure 3 shows a typical warp model generated by PQIS.
Rapidly Developing Advanced Image Analysis and Control Software
We used LabVIEW and the NI Vision Development Module to develop the PQIS software. These tools helped us quickly develop the complex vision analysis routines required, as well as provide the users with an easy-to-use, informative user interface. Figure 4 shows a screen shot of the PQIS main application window.
It presents images of the top and the bottom of each plate, an interactive 3D plot of the plate rim warp measurement, and a summary table of cumulative run statistics including the number of failures of each type. As shown in Figure 1, the PQIS highlights the plate edge and adhesion defects found for each plate view.
Michael Coleman, President
Paul Falkenstein, VP - Automated and Inspection Systems