Vision system verifies high explosive material
Inspection system integrates motion, vision, and measurement hardware and software to examine high explosive material for crystal integrity.
Since the Test Ban Treaty of 1992, the US Department of Energy has relied on non-nuclear testing of bomb components to validate the effectiveness of its nuclear stockpile. One element of that testing program is the evaluation of high explosives (HEs), such as the plastic explosive PBX 9501. These high explosives surround the nuclear material and, when triggered, compress it until it reaches critical mass and explodes.
For PBX 9501 to be effective as a catalyst for a nuclear explosion, crystal granules of various HE powders—some as small as 10 µm—must be relatively smooth in shape with consistent spacing between crystals in the HE polymer binding material. Cracks in the crystal granules caused by mechanical stress of the HE affect combustion rate, shock resistance, and other performance factors.
Since 1992, evaluations of HE materials by Los Alamos National Laboratory (LANL; Los Alamos, NM) have been conducted manually. Trained operators using electron, light-diffraction, and polarized-light microscopes studied slides of polished HE material. These operators estimated the size, shape, and distribution of the HE crystals. However, this approach did not provide quantitative data and was too slow for comprehensive, on-going evaluation of the nation's nuclear stockpile. To correct these manual inspection problems, LANL turned to Honeywell Federal Manufacturing and Technologies (Albuquerque, NM) to develop an automated inspection system.
After a detailed investigation, Honeywell adapted an existing image-processing system developed to inspect rock granules in concrete. The new system uses a Pulnix America (Sunnyvale, CA) TMC-7DSP digital color camera that is C-mounted to a Navitar (Rochester, NY) 6.5X Ultrazoom lens with a 3-mm fine focus and a coaxial light illuminator to acquire images of the HE crystals. Imaging data are fed through a Coreco Imaging (Billerica, MA) Viper-RGB frame grabber across the 200-Mbyte/s Coreco Auxiliary Bus (CAB) to a Coreco Mamba-66 embedded vision processor in a standard desktop PC. Samples under test are moved across the camera's field of view for automated scanning using C+-based routines that control an Aerotech Inc. (Pittsburgh, PA) ATS100 linear ball-screw microscope stage (see Fig. 1).
FIGURE 1. Automated version of the Honeywell high-explosive (HE) material-evaluation system essentially includes a computer-controlled AST100-100 linear ball-screw stage, a Pulnix digital color camera, a Coreco Imaging Viper-RGB frame grabber and WiT image-processing algorithms, a Navitar 6.5 Ultrazoom lens, and a Schott-Fostec coaxial light source. The test sample of HE crystal is placed on a tabletop linear ball-screw stage under the camera's field of view. The camera captures 16-bit, 768 × 494-pixel interlaced images and feeds them to a 1-GHz PC. WiT image-processing software analyzes the images and describes the area of crystal granules vs. binder, the number and ratio of cracked or twinned crystals vs. whole crystals, and total crystal area, among other results.
HMX (cyclotetramethylene tetranitramine) crystals mixed with a polymeric binding are the basis for many HE materials, including PBX 9501, which is 95% HMX and 5% binder. Typically, these powders are mixed three parts coarse crystal with one part fine crystal plus the polymer binder and then hydrostatically pressed into an explosive compound. Recent experiments have shown that the pressing of the powder mix can lead to cracking or twinning (crystal defects) that change the rate of the HE combustion and its response to shock vibrations. Machining the HE material into shapes for highly controlled explosions also can put additional mechanical pressure on the material's microstructure, again inducing crystal defects.
To explore the extent of crystal damage caused by mechanical pressures, Honeywell takes PBX 9501 samples and sets them in epoxy for handling purposes. The samples are then polished to remove the epoxy at the edge of the HE material and to expose the PBX 9501 material microstructure (see Fig. 2).
FIGURE 2. Cubed samples of PBX 9501 plastic high explosive (HE) are encased in epoxy and polished to expose the crystal structure of the explosive material (top). The effectiveness of the HE material as a catalyst to trigger a nuclear explosion requires that crystal granules of various HE powders be relatively smooth in shape with consistent spacing between crystals in the polymer binding material (bottom). Cracks in the crystal granules caused by mechanical stress of the HE affect the combustion rate, shock resistance, and other performance factors. To image the cubed crystal, a polarized coaxial light source helps to differentiate the lighter colored HBX crystal granules from the polymer binder and the darker air voids and bubbles that remain after the hydrostatic pressing of the high explosive powder/polymer combination.
The material sample under test is placed on the tabletop linear ball-screw stage, which delivers 0.5-µm resolution and the ability to return to a given position to within 0.3 µm. "That's the same stage we used for concrete inspection, and we can use the same code. [The stage] comes with drivers. We signal the controller [through the serial cable], and it controls the stage," says Honeywell staff engineer Kim Dalton-Linder. The stage is connected via serial cable to an AOpen America (San Jose, CA) desktop PC, which consumes 400 W of power and needs extra cooling to handle up to seven slots and 13 drive bays. The PC includes a 1-GHz Pentium III processor with 1 Gbyte of RAM, a Zip drive, a 3.5-in. floppy drive, CD-R drives, and a 17.4-in. ViewSonic (Walnut, CA) ViewPanel VG175 flat-screen monitor (see Fig. 3).
FIGURE 3. In the Honeywell high-explosive (HE) material-evaluation system, the Coreco Auxiliary Bus passes 200-Mbyte/s image data directly from the RGB frame grabber to the embedded image-processing board, which houses its own Pentium II processor. The HE sample under test moves across the camera's field of view for automated scanning thanks to C+-based routines that control an Aerotech ATS100 linear ball-screw stage.
The digital camera captures 16-bit, 768 × 494-pixel interlaced images and feeds them to the PC via a Coreco OC-VIPC-TMC7D cable. This custom cable connects to the camera's 12-pin connector and the frame-grabber's DB9 connector. Light from the coaxial illuminator passes through a polarizing filter. The polarized light reflects off the HE sample's surface and provides the greatest contrast between the crystal granules and the polymer binder, adds Dalton-Linder. For more light, Dalton-Linder uses a Schott-Fostec (Auburn, NY) A08575 dual-gooseneck light source with a polarizing cap lens.
The frame grabber—which also controls asynchronous shuttering, color, and white balance of the digital camera across a separate BNC connector—gathers the image data while synchronizing the camera's electronic shutter to the ball-screw stage movements as this linear stage moves the sample across the camera's field of view. Image data are transferred across the hardwired CAB at speeds to 200 Mbytes/s to the image processor board, which contains an embedded Pentium II processor.
"To extract all of the information we need to properly analyze the material, we need to cover the entire surface area of the sample," says Dalton-Linder. "We expect that the embedded vision processor and other system improvements will increase our system speed to handle 400 highly complex, 768 × 494-pixel RGB images in less than two hours. Right now, that same scan takes much longer."
To maximize the overall system image-processing speed, Dalton-Linder intends to change—at the suggestion of systems-integrator Image Labs International (previously Vision1, Bozeman, MT)—to a Sony Electronics (Park Ridge, NJ) XC-999 NTSC color digital camera and Coreco Viper-CamLink combination. According to Dalton-Linder, the systems integrator claims that the Sony camera with a Camera Link interface tends to work better with the Viper-CamLink frame grabber than other NTSC digital color cameras. The Sony/CamLink combination uses a Camera Link cable based on the National Semiconductor (Santa Clara, CA) Channel Link chip set and is capable of transferring data at rates exceeding 2 Gbits/s into the PC.
The vision processor uses Coreco's WiT image-processing programming environment to perform a number of analyses on the HE sample images, including thresholding and equalization of individual images, during a single HE sample scan. Explains Dalton-Linder, "We use WiT software to develop the sequence of imaging-processing techniques for new applications. We can pick blob analysis or thresholding, for example, and put it at the bottom [of the graphical user interface], connect the lines, and go. It's easy to change the threshold or delete it." WiT uses an object/icon-based programming language called igraphs that allows programmers to process multiple, even parallel, algorithms on single or multiple processors for machine-vision applications.
Importing custom routines was another benefit of WiT, according to Dalton-Linder. "We had some image-processing techniques that we developed for the concrete analysis, such as the total area represented by the granules, which we developed in C and imported into WiT for explosives evaluations. WiT also lets us use Visual Basic for the GUI, which is our preference," she adds.
WiT outputs two files for eventual delivery to LANL. The first is a text file that describes the overall area of granules versus binder, the number and ratio of cracked or twinned crystals versus whole crystals, and total crystal area, among other data. A second file contains intensity histograms for each image and profile histograms created by sampling intensity values along a mouse-drawn line that dissects the image file. Actual images of each sample are not used as part of the on-going HE evaluation program and so are discarded.
Dalton-Linder says that while the PC contains an Ethernet card that connects it to the remainder of the Honeywell's local-area network, data of the HE samples are typically stored on a floppy or zip disk and sent to LANL.
Each system module has been tested and proven separately. The HE samples have been analyzed using the Pulnix/Viper-RGB combination. The WiT algorithms as well as the custom C+-based image-processing procedures have all been tested. All the results are certified through manual inspection of the samples. According to Dalton-Linder, the next step is to upgrade the camera and frame grabber to the Sony/Viper-CamLink combination and integrate all the modules into a single system for delivery to LANL.
"That's when the Mamba board and the CAB data bus will come in handy. When we use the system in full automation mode, all the operator will have to do is put the sample on the stage and start the system," says Dalton-Linder.
R. Winn Hardin, Contributing Editor
AOpen America Inc.
San Jose, CA
Honeywell Federal Manufacturing & Technologies
Image Labs International
Los Alamos National Laboratory
Los Alamos, NM
Santa Clara, CA
Pulnix America Inc.
Park Ridge, NJ