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RFID Tightens Up Caterpillar's Assembly Process
The company's Belgian hydraulic-valve plant deployed a real-time locating system to ensure that workers use the correct wrench and torque to install hoses.
In November 2008, De Jaeger installed RFID interrogators in Caterpillar's production area where the main high-pressure hydraulic valves are assembled. The RFID system works with the Protrac software. At the same time, the company switched its manual torque wrenches to six electronic models fitted with active RFID tags. Each wrench is designated for a specific assembly location, and is preset to tighten a specified amount. When an operator applies the maximum amount of force preset for that valve, the wrench begins to click, indicating the hose is properly attached, and the measured torque is transmitted to the corresponding PC. By placing an RFID tag on each wrench, the RTLS system can ensure that the appropriate wrench is used in the proper location.
The amount of torque is crucial, Degraux says, because if those hydraulic hoses are not properly tightened, failure could result in leaking valves when the complete machine is assembled and tested prior to shipping for retail sale. "The main target of implementing traceability on this zone was to ensure that the correct number of tightenings are applied to the valve," he states. The company needed to know not only whether the proper tightness was employed, but also that the right tool (preset to tighten a specific amount) was used in the correct location.
Today, compact Ubisense tags are mounted on the torque wrenches. Each battery-powered 7 GHz ultra-wideband (UWB) tag transmits a unique ID number only when in motion, and goes to sleep when immobile, in order to conserve battery life. Eight Ubisense interrogators are mounted on the ceiling, each tuned to read tags in an area measuring 6 meters by 30 meters (19.7 feet by 98.4 feet). The readers cover six production positions, capturing the unique tag ID of the wrench each worker uses, then sending that ID number to Caterpillar's back-end system, as well as to PCs at the six stations at which the valves are assembled. The Protrac software determines whether the correct wrench is being operated in the proper location, based on that tool's calculated location.
Before beginning to assemble a valve, a worker inputs his or her ID number, which is linked to employee-related data, including which assembly that individual is authorized to work on. The worker then uses a bar-code scanner attached to the PC in his or her work area to scan the ID number on the machine part. The system links that bar-code number with the location and identity of the torque wrench the employee begins using, to ensure the correct wrench is in the appropriate location for the work about to be completed. The screen then displays instructions regarding the procedures that should be undertaken, as well as an alert if the wrong tool is in the work station. The color green indicates the correct wrench is being operated in the proper zone, while yellow means the tool is not at the correct assembly station and red signifies the worker's PC has lost its network connection with the back-end server.
The results, thus far, have been positive, Degraux says. The majority of hydraulic valves and hoses are now being found to be properly assembled when tested—a higher accuracy rate than the company had experienced in the past—and that has led to less rework on leaking valves discovered during the testing process.
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