Many discrete manufacturers ensure complete quality control over their production processes by directly marking parts with machine-readable codes and tracking those parts through their life cycle. This process is known as Direct Part Mark Identification (DPM ID). As part tracking becomes essential from production through the supply chain, automotive, aerospace, electronic, and medical device manufacturers require fast ID readers that provide accurate and reliable results.
Fast, accurate, and reliable DPM ID is a challenge because codes can be difficult to read due to low contrast, variations in part surfaces, and partial damage due to process and environmental conditions. If a code is unreadable, the part is not processed, and production may stop, and an automated machine may go down. Consequently, ID readers must provide accurate, reliable performance and high-read rates to prevent machine interruptions, while simultaneously reading at speeds fast enough to keep pace with high-throughput equipment.
Manufacturers that implement DPM ID may integrate ID readers on existing production lines and equipment, and specify the same ID readers on any new automated equipment supplied by machine builders. Nonetheless, when choosing an ID reader, the needs of manufacturers, systems integrators, and machine builders are generally the same -- fast, reliable, flexible, and easy-to-use ID readers.
Performance is of the utmost importance in order to minimize equipment downtime and keep the ongoing cost of ownership low. This means that a DPM ID reader must provide accurate, reliable, and fast reading regardless of changes in mark appearance due to variations in mark quality, marking process, part material or position to camera, coatings, damage, and other image variations that occur under normal production conditions.
The DPM ID reader should also provide flexibility in working distance (distance from the camera to the part), which is desirable to accommodate space constraints within various types of automation equipment such as automatic storage/retrieval systems; handling, loading, and feeding equipment; packaging, assembly, marking, or other machines used in production environments where DPM ID is a required part of the manufacturing process.
In addition, look for a rugged DPM ID reader that meets IEC specifications for shock and vibration, and achieves an IP67 rating for dust and washdown protection -- without the need for a separate industrial enclosure. Also, DPM ID readers that incorporate lighting, optics, and processing within the unit make mechanical and electrical integration easy.
Finally, connectivity options, such as serial and Ethernet communications, offer important benefits. Serial I/O provides an interface between the DPM ID reader and a controller or PLC within a work cell or piece of equipment for communicating read or verification results. Network connectivity, such as Ethernet, enables the reader to communicate decoded results data to PCs and databases at the enterprise level, and provides a centralized way of managing multiple readers in a factory.
Due to advances in the power of digital signal processors, imaging sensors, and decoding algorithms, ID readers are currently available that are both cost effective and deliver the reading results to achieve the objectives of discrete parts manufacturers. Following is an in-depth look at the most important factors to consider when implementing DPM ID.
Code Selection
The automotive, aerospace, electronics, and other industries have adopted 2D code standards and formats to meet their process application needs, with industry standards groups frequently defining the appropriate code for a given application. The DataMatrix code is the most widely supported for DPM ID applications involving metal, glass, ceramic, or plastic materials.
Because this code is in the public domain, marking and reading equipment suppliers have invested significant R & D resources to improve the performance of supporting equipment. As a result, there are more product choices, a situation that generally results in higher performance and lower cost equipment as compared to proprietary codes that lock you into a single supplier, or other code symbologies with fewer product offerings.
Data Encoding
The Data Matrix code offers a number of advantages for DPM ID applications, including small size, high data encoding capacity, and error correction that allows a code to be successfully read even though as much as 60% of the code may be damaged. Data encoding refers to the amount of information that is "stored" within the matrix when the Data Matrix code is generated. There are 24 square formats and 6 rectangular formats available in ECC200 to provide users the flexibility for encoding between 6 and 3,116 digits in a single code.
Code size can affect readability and is generally determined by the amount of data to be encoded, module (cell) size, and surface roughness of the area on the part where the code will be applied. When trying to comply with an industry specification, an application specification will define the size that is needed in order to be in compliance. Deciding on what information to encode is typically driven by the end user's requirements of the traceability project.
The available space on the part must also be considered. Limited space may require the Data Matrix code be used as a "license plate" to simply identify the part, reducing the amount of data encoded and size of the code. In this case, a centralized database containing manufacturing and historical data referring to the part is updated as the part is identified during manufacturing and supply chain processes. When space isn't an issue, users may take advantage of the large data capacity of the code and encode much more information about the part, creating a "portable database."
Marking Processes
The choice of marking process is typically incorporated into the component design; deviations from this design may require engineering change approval. The primary methods used to produce machine-readable symbols for DPM ID include dot peening, laser marking, electrochemical etching, and inkjet printing. Important factors influencing the marking process decision include part life expectancy, material composition, environmental wear and tear, and production volume. Other considerations include surface texture, the amount of data to be encoded on each part, as well as the available space and location of the mark on the part.
Mark Placement
The location of the code on the part can directly impact the readability of the code. The location should be clearly visible throughout the manufacturing process and it is best to mark on a flat site on the part. Also, choose a location where the mark is in a prominent position on the part that is easily viewed by the reader. Avoid locations where there may be a surrounding surface relief that could potentially affect the illumination of the code by the reader's illumination source.
Wherever possible, it is best to provide a "clear zone" around the mark where no features, part edges, noise, or other interference comes into contact with the code itself. In those cases where the mark must be placed on a cylindrical part (see illustration), care must be taken in selecting the size of the code. Surface curvature can create distortions to the code and make proper illumination of the code very difficult. In order to mitigate this problem, a code size that is no larger than 16% of the diameter or 5% of the circumference of the part is required.
Readability
Readability defines how easy or difficult it is for a reader to successfully read a code. DPM ID is rapidly becoming a required part of the manufacturing process, so if a code is not readable, the part is not processed, or the production equipment stops. Until recently, manufacturers implementing DPM ID have lived with varying levels of read rates, in some cases approaching the upper 90% level; however, this level of performance is no longer acceptable.
DPM ID solution providers must demonstrate that they can achieve six sigma read rates, a level of quality that equates with only 3.4 defects per million opportunities. To achieve this read rate it is important to plan, understand, and implement a marking solution that builds in quality to ensure these results. A user looking to implement DPM ID must understand all of the factors that affect the readability of the code. A good baseline for this is to understand the design of the Data Matrix symbology and associated issues that might impact readability.
The features that comprise the Data Matrix symbol are the quiet zone, the finder pattern, the clocking pattern, and the data region. Each individual element is referred to as a module (cell). The actual appearance of the code depends on the type of mark placed. For example, a Data Matrix code formed with a laser marking machine or printer would appear with a "continuous L pattern" and square modules; whereas, dot peen and inkjet markers produce codes that have a "noncontinuous L pattern," with a data pattern made up of round modules.
A code pattern with individual modules that are consistent in shape and size, and modules that are distinctively different in shape and size from other features on the surface of the part, provide the basis for robust and reliable code location and reading. However, in DMPI applications this can be challenging due to variations in the surface texture, variations in part presentation during the marking process, and inherent variability of the marking machine. For example, the bumps on the surface of a cast part will show up in an image. If these bumps are similar in size and/or shape to the dot peen marks of the code, readability suffers because the code blends in with the bumps in the surrounding image.
An objective verification of the code at the marking machine will help assure that the Data Matrix mark produced meets the specifications of the standards, and greatly contributes to readability success downstream.
Verification
In order to assure that the marking equipment is applying a mark that will meet the requirements for achieving the highest read rates, it is highly recommended that a code verification system that includes lighting, optics, camera, and software be implemented at the marking station. This is not only a critical factor for downstream reading performance, but it reduces nonconformance and the associated costs of rejected parts due to unreadable codes.
A verification system will immediately detect a problem with the marking process which could be due to poor fixturing of the part, damage to the machine such as a broken tip on a dot peen machine, or incorrect settings during part changeover. Additionally, a code verification system can also provide process feedback on the marking process that can be used in preventive maintenance. For example, the verifier can monitor the wear of the tip on a dot peen machine by monitoring dot size and flag the operator on the floor when a pin should be changed.
Types of DPM ID Reading Solutions
Three types of reader (decoder) products for DPM ID are in general use today: fixed-mount readers, presentation readers, and handheld readers.
Fixed-mount readers are used to identify parts that are handled and moved
automatically by conveyor, indexer, or robot. Typically, fully automated manufacturing equipment, as in electronics and automotive manufacturing, uses fixed-mount readers.
In operation, this type of reader is mounted in a fixed position where the mark can repeatably be placed in front of the reader in either continuous or indexed motion. A "trigger" signals the reader that the part is ready for reading. This trigger event is performed by an external sensor that detects the presence of the part, or by an encoder that knows the position of the part at all times and can signal the reader to decode.
Fixed-mount readers are configured with either an integrated light source or with an external light source as required by the application. Advantages of a fixed-mount reader without an integrated light source are that it can be mounted in varying working distances from the part and supplemental lighting can be selected to meet the application needs. It is advantageous to use fixed-mount readers that can be easily set up, viewed, and maintained over an Ethernet network or with the production equipment controller for efficiency
Similar to a fixed-mount reader, a presentation reader is mounted in a fixed position; however, it operates in a continuous reading cycle, automatically performing the decoding task once the operator places the part in front of the reader. Presentation readers can provide a very fast way of reading part codes in areas where parts are handled manually. A presentation reader can be implemented with either a fixed-mount reader or handheld reader. Using a handheld reader in presentation mode provides the opportunity for multiuse, as one can also remove the reader from its stand and bring it to the part.
Handheld readers are preferred in those environments where part handling is not automated or parts vary greatly in size. Handhelds are used in job shop manufacturing operations, QC test stations, and in logistics areas. Handheld readers come in either tethered (with a cord), or cordless configurations; tethered handheld readers have the advantage of not being displaced from the application location, while cordless operation is required in cases where part size or position are a practical limitation to cord length.
Selecting a Reading Solution
The process of evaluating and selecting a reading solution is critical to the success of the overall DPM ID application. Many factors go into the selection process of any new equipment, but in the case of a DPM ID reader, none is more important than the read rate. The capability of the reader to consistently read codes throughout the process is critical.
In DPM ID applications, distortions to the code are quite common due to part material composition, variations in part presentation, or variability caused by the manufacturing process. It is important to select a reading solution that can tolerate a wide range of distortions to the appearance of the code, no matter what the cause.
A set of sample parts that are representative of the range of mark quality a reader will need to handle should serve as the basis for a preliminary test of a reader's read rate. However, a more extensive pilot test is recommended so that more read rate statistics can be gathered and analyzed.
The "model image" represents how a code might appear under ideal conditions. In order to consistently read codes and achieve the required read rates, the reading solution should tolerate changes in contrast, focus, and degradation to the code without a need to change underlying parameter settings. With a fixed mount reader this is easily tested by starting with an optimal set-up as represented with the "model image" and adjusting the aperture to simulate change in contrast, focus to simulate change in depth of field, and position of the light to simulate background problems. A reading solution that can consistently read under these conditions will minimize startup issues, lower support costs, and ultimately is critical to the overall implementation success of a DPM ID application.
In addition to read rate, it is important that the reader return a result quickly. In operations requiring handheld or presentation DPM ID readers, accurate and fast decoding is important. Although the main driver for the use of an automatic reader is to eliminate data accuracy errors, the implementation of the readers cannot slow down the process or automated equipment.
Connectivity/IO
Connectivity refers to the method of interfacing the reader or verifier to a controller such as a PLC or PC so that the result of the decoding is communicated. The connectivity method depends upon the application and the type of reader involved.
In fixed-mount DPM ID applications read results are usually sent to process equipment or to a database over the factory network, so the DPM ID reader should offer both serial and network communications. Serial communications are used typically in applications where the read or verification results stay "local" to the work cell or factory automation equipment. Network connectivity enables the reader to communicate decoded results data to PCs and databases at the enterprise level. For establishing a communications link between a reader and a PC at the enterprise level, make sure the reader supports a broad range of standard network protocols.
Finally, as more and more ID readers are used throughout the manufacturing process, it becomes important to have a centralized way of managing them. Make sure the ID Reader will allow you to manage and control vision activity over the network from remote locations in the plant and beyond.
In the case of handheld readers, connectivity methods depend on whether the reader is tethered (uses a cord) or cordless. Tethered readers often communicate the read result through what is called a "keyboard wedge" interface, which emulates the keystrokes of a keyboard, making integration to a PC very simple. Alternatively, communications can be made over an RS232-C interface. A cordless handheld reader uses wireless technology, such as Bluetooth®, in order to communicate to the base PC station or other controller.
Vendor Selection
DPM ID is a very challenging application requiring technology and know-how in solving difficult image analysis problems. Typically, companies experienced in industrial machine vision have the right know-how and technology for providing the highest DPM ID read rates.
The vendor should provide the support necessary to thoroughly qualify your DPM ID application, guide you through the considerations in assuring success, and insure that the installation is a success. Also important, look for a vendor with a global network of offices offering both pre- and post-sales support. This way, you can get the same consistent high level of product support anywhere in the world -- especially important if the system is commissioned in one location and shipped to another. And finally, the selected supplier should have a successful track record and financial stability to maintain its role as your DPM ID reading solutions provider for the long term.
