DECEMBER 12--Because of the current increased emphasis on quality control, manufacturers must be able to trace the origin of products coming off the production line. For example, the manufacturer needs to know from which batch a faulty product came, when the product was made, and what are the test results. This traceability can be obtained by giving each product unique identification marks and then using industrial machine-vision systems to read the product marks through all stages of production. If a product subsequently fails in use, the manufacturer, the date of production, and the batch number can be quickly determined. Identification marks can also be used to determine whether the product is counterfeit.
A recent seminar at Warwick University (Coventry, UK) showed how marking and machine-vision systems can work together to provide defined traceability. It was one of several seminars jointly organised by Cognex UK (Epsom, Surrey, UK; www.cognex.co.uk) and Technifor (Atherstone-on-Stour, Stratford-upon-Avon, UK; www.technifor.com). Technifor initially specialised in mechanical indent marking systems, but it now also offers laser-based marking systems. Cognex specialises in machine-vision systems and applications for product identification and verification. The two companies work in complementary fields that fit well together.
If a permanent mark is required to last through the life of a product, then temporary pen and ink markings are obviously unsuitable. Hand-stamping can be used but it is labour-intensive and too slow for use in a production line. Chemical etching is also slow to implement. An indent marking at up to 0.4 mm in depth proves satisfactory. In addition, it can be applied rapidly, but only if the product can be securely clamped during marking. Indent marking is widely used in the automotive industry, laser marking is popular for pharmaceuticals and medical products, and ink-jet marking is dominant in the food and beverage industry. Traceability is especially important in the pharmaceutical industry, where it is vital to track products when medical problems emerge.
In laser marking a thin layer of the product's surface is oxidised; this allows the formation of a highly accurate pattern over a small area. It also is suitable for two-dimensional codes, and can also be used for barcodes and alphanumeric coding. However, laser marks can be worn away by heavy abrasion and might degrade due to corrosion. These marks can be too small to be seen by the human eye, but they are readily detected by a machine-vision system. Technifor hopes to achieve marks as small as 300 μm across.
Technifor also demonstrated a pneumatic indent-marking system using a tungsten carbide-tipped point for marking. Mark Worlidge, Technifor director, said that the carriage in such a system can move quickly and, with the marking point reciprocating at 300 Hz, work faster than an electromagnetic-solenoid indent-marking system. However, electromagnetic systems offer more accurate markings than do pneumatic systems.
Automatic machine-vision-inspection systems can check products as they are being manufactured, identify thousands of parts per minute, and automatically reject those with defects at an early stage before further value has been added. Leigh Simpson, applications engineer at Cognex UK, said that machine-vision systems for reading component identification marks act as "computers with eyes." They can check product appearance, detect surface defects, verify assembly operations, gauge critical components, guide robot systems, read identification codes, and align objects prior to insertion, among others. Some machine-vision systems can deliver processed information over a factory-wide Ethernet network.
Identification marks are generally one-dimensional barcodes or two-dimensional data-matrix codes. Matrix codes provide a higher date storage density than barcodes or alphanumeric text readable by humans. Simpson said, "We can encode about one hundred times as much information within a data matrix as that within a barcode of the same area." It is often undesirable and impractical to print text readable by technicians by the side of two-dimensional coding marks, as is often done with barcodes. An automated reader is therefore required to verify that the two-dimensional codes have been correctly written.
Two-dimensional codes are essential in the aerospace industry where manufacturers must meet the ATA Spec 2000 requirements because many individual aircraft components must marked and verified. Two-dimensional coding is also employed in the fibreoptics and pharmaceutical industries. It is also used to mark high-value beverages and liquids in an attempt to resist counterfeiting. The coded information may include a part number, a serial number, a supply reference, date and time of production, a batch reference, a machine operator number, and test results. Product information on identity, location, and dimensions can also be fed to a robotic system for a pick-and-place operation. The machine-vision system provides the information and the robot moves accordingly.
During the past two years enormous advances in reading codes have been made, such as reading codes when image contrast is poor. Codes can be read and processed even if they are misaligned, have low contrast, are distorted, or somewhat damaged. Codes can be monitored so that if there is a continuing deterioration in marking quality, adjustments can be made to the marking process before the marks become unreadable.
