Smart Tags Monitor Part Flow
RFID keeps an ear tuned for information that can improve quality control
By James R. Koelsch
Castings for four-cycle engine blocks pause at Station 3 for a short but important quality-control task at Briggs and Stratton's factory in Poplar Bluff, MO. A reader retrieves the pallet number from the radio-frequency identification (RFID) tag on each of the 88 pallets carrying the castings through the 54 stations in the block-machining line. A pin-stamper then imprints that number, the date, time, and crew number into the casting.
This information is crucial for tracing any problems that final inspection might uncover in the 14,000 blocks that the company makes each day for its Quantum lawnmower engines. If a block fails any of the checks, the inspectors and production staff can focus their attention on the other blocks in the batch to find the cause and determine an appropriate corrective action. The mark gives the company's engineering staff the ability to trace problems to their source, and prevent bad blocks from going to assembly.
As Briggs & Stratton and other manufacturers have learned, RFID can be an excellent way to carry crucial information through production and automate certain verification tasks. In the simplest applications, RFID tags carry identifiers such as part numbers through environments that are not conducive to bar codes and other forms of labeling. In more sophisticated applications, RFID tags function like silicon flash disks that travel with workpieces and tools to both capture information from the process and convey information to it.
In Briggs & Stratton's case, the tags are simple labels identifying each of the 88 pallets on the line. Manufacturing Engineering Technician Billy DePew had specified BISC-12805/L read-only, passive tags from Balluff Inc. (Florence, KY). The 1023-byte tags are essentially industrial EE-PROMS encapsulated in Duraplast to protect them from cutting fluids, and the extreme conditions found in factories. The Balluff BISC-60R-001-08P unit reading these tags is an eight-bit parallel (eight-byte addressable) device that resembles a 30-mm inductive proximity sensor.
Other manufacturers prefer to use RFID tags in a more sophisticated, decentralized way. For them, the tag acts as a database that travels with the engine through the manufacturing process, collecting pertinent quality-control data from each operation, and building a genealogy of the process. Among the various kinds of information typically put into these traveling databases are machine and tool identifiers, time and date stamps, and inspection results.
Manufacturers using this decentralized strategy can put the information to work in both in-process and final inspection. A reader at each operation can retrieve data that will tell the controller whether the prior operation occurred and was successful. If any data are missing, or if any failures are recorded, the workpiece is removed from the line. Having the data right there on the tag solves the time-lag problems associated with sending quality data over a network to a central location, and then attempting to retrieve it quickly enough to make decisions at the very next operation.
To use the data at the end of the process, a reader retrieves the entire genealogy from the tag so that an inspector or automated quality-control station can use it for the final check. The reader then can upload the data to a central database for statistical analysis and storage in traceability archives. "Because the information is tied to the engine's serial number, manufacturers can record the entire genealogy of the engine, from when each of its components were machined all the way through the assembly cycle," says Mark Sippel, Balluff's product manager, object identification.
Besides tracking work in process, RFID tags also can keep track of the tools used in various manufacturing processes in much the same way as they do for tracking product. In the simplest applications, a read-only tag embedded into the tool would act as an identifier to ensure that the process uses the correct tool. Fabricators like this ability because stamping dies often look alike, and costly mistakes are easy to make. "By reading the RFID tags on each component, they know right away that they have the correct die set," says Sippel.
He adds that the tags are an even bigger help for transfer presses, because managing their tools is a much more complex task. Not only do their dies come in sets, but these presses also need several sets that follow one another in a specific order.
Despite the usefulness of read-only tags, read-write tags are the tags that tend to generate the greatest returns in tool management. These returns come not only from ensuring that the right tool goes into the machine, but also from the additional ability to track tool life accurately. After a press completes a batch of stampings, the reader records the number of hits and updates the tool's remaining life. Consequently, software in the press's controller can notify the operator at the optimal time for sending the tool to maintenance for inspection and repair. This avoids both the excessive wear that comes from not pulling the tool early enough, and the expense that comes from pulling a tool too early.
A machining facility can use RFID in a similar way. Using an automatic presetter, a technician in the toolroom can write to the tag the tool's measurements, its offsets, and its expected life span. When a batch of these tools arrives at a machine, the operator can transfer the data on each chip into a machine's controller, and then load the tools into the machine's tool magazine. When the part program calls for a particular tool, a reader on the toolchanging arm retrieves the tool number and offset data while the arm loads the tool into the spindle. After the tool completes its task, the reader updates the unit's expected life span as the arm returns it to its pot in the magazine.
Shops can use this information in two ways. First, programmers can tell the CNC to warn the operator when a tool is nearing the end of its expected life, and look for a fresh one at the appropriate time. Second, toolroom technicians can load the data into analysis software that helps them to troubleshoot problems. They also can use such software periodically to analyze the historical data to identify trends, such as whether tools from one vendor are outperforming those from another, and whether tools on a particular machine are wearing abnormally fast.
Suppliers of RFID technology can easily offer 50 types of tags that broadcast in different frequencies, and that come encased in various kinds of protective covers. Beneath these covers is an inlay, which consists of a piece of foil that holds the RFID chip, and a coil that serves as the antenna. "The cover is a significant part of the cost," notes Helge Hornis, manager of intelligent systems, Pepperl + Fuchs (Twinsburg, OH). "The literature might advertise a tag at $0.30, but that's just the inlay."
One way of classifying RFID technology is to divide the tags into two basic types: active and passive. "Active tags have a battery that runs the microchip's circuitry and broadcasts a signal to the RFID reader, which can read these tags from up to 1000' [305 m] away," says Gordon Fraser, RFID application engineer, Tyco Electronics (Middletown, PA). "This is particularly useful when access is restricted, movement is being monitored, and position data or multiple reads are required."
Rather than using batteries as their power source, passive tags rely instead on induction. "They draw their power from the reader, which induces current in the RFID tags' antennas," explains Fraser. "Because they rely on RF electromagnetic energy for both power and communication, this can restrict their read and write ranges." But they tend to be inexpensive, and last a long time.
A third type of tag, called a semipassive-semiactive, offers a bit of a compromise. Although a battery powers its chip, the reader provides the power for transmitting data. "This allows the tag to respond to the reader from a slightly longer distance," says Fraser.
As attractive as the active tags' greater range might seem at first glance, passive tags are often better suited to manufacturing operations. The reason is that most production lines access the tags many times a day—as frequently as hundreds of times/day on some high-volume lines. So they tend to drain their batteries much more quickly than do logistical, retail, and other applications of RFID.
"If the power source dies, the tag is useless until you either replace the tag or power source," says Sippel at Balluff. Not only would operators have to stop production to replace the batteries, but power losses also sometimes carry the risk of losing data. And the need to change the battery every so often means that the manufacturer cannot encase the tag completely in epoxy to harden it to survive in the factory environment.
Another reason that active tags might lose their luster is that their chief advantage, their range, is unnecessary for many production lines. The shorter range of passive tags is often adequate, as long as you consider the need to read them up front, during the design of the process. You need to remember to mount the tag in a place that gives the reader ready access, and minimizes interference from metal and other fields.
In most applications that track work, experts recommend mounting the tag to the pallet or other carrier transporting the workpiece between operations. "The problem with putting it on the part is that you don't typically leave it there," explains Sippel. "The tags are too expensive." And, also, you face the extra cost of designing and building stations to add and remove them.
Although most manufacturers prefer to avoid this expense, a few find it necessary, especially if the workpiece does not return to the same pallet or carrier when an operation is completed. Sippel reports that some automobile manufacturers attach RFID tags to engine blocks using what Balluff calls a data bolt. The tag's electronics are embedded in a bolt that automatic drivers can insert at the beginning of the line and remove at the end.
The transmission frequency is another fundamental way of classifying RFID technology. Four standard frequencies exist today, in addition to a number of older proprietary frequencies supported by some vendors. The lowest of the standard frequencies is 125 kHz. Technology using it is well established and supported by a number of chip and RFID-system manufacturers, according to Hornis at Pepperl + Fuchs.
"At this frequency, you get a relatively high immunity to mounting in or around metal," he adds. "Another advantage is that a number of tags are available with large memory." These tags, moreover, can use FRAM (ferroelectric random-access memory) chips, which allow an unlimited number of reads and writes.
A higher frequency that has become well established in manufacturing is 13.56 MHz. Like the 125-kHz chips, several big players offer a wide variety of 13.56-MHz chips. An advantage of this frequency is that vendors can start printing and etching the coils that serve as antennas, which reduces the cost of producing long-range antennas. Reading distances at this frequency are as great as 3' (0.9 m), but they shrink considerably when they are used around metal and fluids.
"Typically metal and fluids don't really work well with RFID, because metal reflects the signal and liquid absorbs it," explains Alex Stuebler, business manager, factory automation sensors, Siemens Energy & Automation (Alpharetta, GA). "So embedding the tag into metal limits your reading distance to a fraction of an inch—maybe up to a half inch [12.7 mm]."
Another advantage of this frequency, as well as higher ones, is that it can carry more information per unit time. So not only does the quality of the signal tend to be better, but the tags also can pass by the readers faster. "Although it appears that they are read simultaneously, that's not really the case," says Hornis at Pepperl + Fuchs. The hardware reads each tag one at a time, but does so very quickly.
The next highest frequency is in the ultrahigh frequency (UHF) range. These 928-MHz chips broadcast as far as 20' (6.1 m). "Their specifications allow for low-cost tags and high baud rates, in addition to the longer distances," says Stuebler. For this reason, major retailers are pushing this frequency for tracking their deliveries and inventories.
The UHF range has not been quite as popular in manufacturing, however. "The drawback is that these tags are very susceptible to interference from the background material that you put them on," says Hornis. Not only is getting good results near metal difficult, but water also can cause problems, because it is not transparent to radio waves at this frequency.
Siemens, nevertheless, believes that many production lines can benefit from UHF, and has developed what it calls metal-mountable tags. "You can mount them on metal bins, drums, and other containers," says Stuebler. These tags are better suited for industrial applications than paper-backed labels.
The highest frequency used for RFID is 2.4-GHz, which is in the microwave portion of the spectrum. Hardware can read these tags from long distances, which is why toll roads and gas stations have adopted the underlying technology for their speed passes. So far, this technology finds most of its use in manufacturing in tracking the location of mobile assets like vehicles moving about in large plants.
As important as tags are, Hornis urges users not to focus on the tags to the exclusion of the reader. He points out that noise patterns and other sources of interference have a habit of changing over time as factories evolve. Because high-speed lines cannot afford the delays that any unanticipated noise can cause in reading tags, he urges users to specify quality hardware for their high-speed production lines. "It doesn't matter how good your tag is if you have a poor reader," he explains. So be smart and think any implementation of smart tags all the way through.
This article was first published in the August 2007 edition of Manufacturing Engineering magazine.