Since 1975, the Food and Drug Administration (FDA) has
required that all prescription tablets or capsules have a unique identification
in terms of size, shape, color and imprint. Drug makers use the imprints to
identify the chemical substance and dosage as well as for promotional purposes.
Acceptable identification has traditionally been achieved by debossing - pushing the image below the level of the product, embossing - pushing the image above the level of the paper, or printing. The problem with these two approaches is that coating systems have a tendency to fill in debossed or embossed areas, sometimes rendering them unidentifiable, especially by those with less than perfect eyesight.
Tablet and capsule printing is normally accomplished by using a hard roll engraved with the image to be printed. The hard roll contacts the ink, then a wiper removes all of the ink except for that in the engraved area. The ink is then transferred to a soft roll that contacts the tablet or capsule.
Inspection is critical because, if the wrong ink formulation is used, the ink might not adhere to the tablet, or the coating might flake off and contaminate the ink to the point that the printing becomes indistinct. Manual inspection has been used before, but with hundreds of thousands of tablets or capsules being printed per hour, it's impossible to inspect more than a small sample of product. This makes it impossible for the manufacturer to meet the FDA product identification regulations.
Ackley Machine originally incorporated a vision system consisting of a camera, an image processing card and image processing software running on a PC into its systems. The company was about to ship such a system to a customer when a computer failed and had to be replaced. The computer ran on an older version of Windows that was no longer available so it had to be replaced with a computer that ran a newer version. As a result, the new software, hardware and operating system each needed to be validated to meet FDA requirements.
To avoid encountering this problem again with next Windows upgrade, Mark Ford, engineering manager for Ackley Machine, sought out a self-contained vision system that could easily be validated based on functionality alone.
Validation and TrainingFord ended up selecting the Cognex In-Sight vision system because it provides a camera and image processing hardware and software within a protective enclosure. This small package approach makes it possible to write a short validation procedure, which also means that upgrading to a new model of vision system is a relatively simple procedure. An added benefit is that higher performance is achievable when multiple vision systems are used, because each vision system processes its own images.
After printing the tablets, Ackley Machine's latest tablet printing machines drop them into the pockets of a carrier bar that holds up to 24 tablets. The carrier bar moves under an array of four In-Sight vision systems where each camera inspects six tablets. The vision system program first uses a histogram tool to check for the presence of the light-colored tablet against the dark background of the carrier bar. If a tablet is present, then the vision system performs a print quality check.
The vision system's pattern recognition algorithm looks for the printed image regardless of its location in the image acquired by the camera. This eliminates the need for precision positioning of the camera or tablet. The system identifies and isolates the key individual features within an object image and measures characteristics such as shape, dimensions, angle, arcs and shading. It then correlates the spatial relationships between the key features of the trained image to the run-time image, encompassing both distance and relative angle. By analyzing the geometric information from both the features and spatial relationships, the object's position can be determined without regard to the object's angle, size or appearance.
After being initially trained with an image of a good quality printed table, the system then compares the good image to the most recently acquired image and rates the quality of the recently acquired image based on the match. The location of the center of the printing on the product that was inspected is also determined to ensure the logo is centered on the tablet.
Damage InspectionThe next step for the system is to see if the tablet is broken. A broken tablet shows up as several light-colored sections against a dark background. It is identified by using the vision system's "blob" tool to look for light-colored blobs smaller than the tablet. The final step is to look for a coating defect which can be identified as a smaller white spot, because the core of the tablet is white. The blob tool is used to search for white spots.
The camera acquires the image and performs each of these inspections and builds a binary word that contains the inspection results for each of the six tablets in its field of view. The vision system sends the results to the PLC that runs the machine. The vision system also sends each inspection image to the corporate network where it can be called up by the machine operators or engineers.
The PLC then operates a vacuum system that picks up the tablets from the carrier bar and moves them to a discharge chute. Based on the signal from the PLC, specific positions on the vacuum shoe are operated individually to either pick up or leave behind individual tablets. The tablets that have passed the inspection are picked up and placed in the discharge chute, while those that have failed remain in the carrier bar. In the next step, the tablets remaining in the carrier bar are dumped into a reject bin. This approach makes it possible to eliminate in a positive manner individual tablets that fail inspection and ensure that only good tablets are passed along for packaging.
The vision systems enable Ackley's machines to print and inspect more than 400,000 tablets per hour, which is "substantially faster than any other tablet printing and inspection machine," Ford says. "The machine also offers a higher level of quality because it is able to reject individual tablets while other machines on the market can only reject batches of tablets which is wasteful when there is only one bad tablet in a batch."
Philip A. Gulotta is sales engineer for Ackley Machine Corp.