Nowadays, automotive paint-shop processing is a far cry from the messy, manual operation that, less than a century ago, was a commonplace. Today, spray-painting and powder coating are fully automated in every major manufacturing plant, and parts are generally scheduled and transported between operations without any human intervention.
Enhanced RFID transponder and Read/Write Modules (RWM) capabilities have played a big part in enabling this change. Although in-process event monitoring has long been possible, designing RFID systems that are able to withstand the environment is still a challenge for the industry.
Harsh Environment
Motorcycles have relatively few colour-coordinated parts. However, synchronising delivery and ensuring correct sequencing is far from being a trivial task. Two significantly harsh, yet unavoidable, environmental conditions place heavy demands on the RFID system.
Firstly, the transponders must withstand process temperatures that may reach 250 deg C. Secondly, relatively long-distance sensing is essential to eliminate the risk of mechanical damage to RWMs that arises whenever the motion of moving parts is not fully constrained.
Plant-wide process control requires seamless data transfer from one end of the production line to the other — including associated feeder lines. It can typically be up to 31 RWMs, often many meters apart and connected as a simple daisy chain network, communicate via the a fieldbus protocol.
Step-By-Step Processing
Body-in-white parts, in this instance the motorcycle’s fuel-tank cover and rear fairing, are loaded onto purpose-designed transport frames suspended from an overhead conveyor; each frame is fitted with an RFID tag-carrier and a high-frequency, high-temperature tag.
As a frame approaches the first process station (pre-treatment), the remote scheduling system allocates the parts to a specific vehicle in the assembly master schedule and instructs a local, static RWM to write corresponding process identification data, including body colour, to the tag.
Simultaneously, the scheduling system programs the pre-treatment or degreasing station to execute the wash cycle specified for the vehicle in question. On completion of the cycle, the parts exit the process station.
From this point on, the frame, complete with parts, is destined to marry with a known vehicle on the final-assembly line. A second RWM positioned immediately outside the station may record the transit. Alternatively, the arrival at the subsequent process area serves to confirm the completion of the prior operation.
Since the transport frame is not constrained from moving about its vertical axis, the exact path of the tag carrier may vary from frame to frame — often by several millimeters. Certain RWMs allow for this movement and produce reliable results that are independent of the exact tag position. Some high-frequency RWMs also feature an anti-collision algorithm that identifies and addresses a specific tag, disregarding other transponders in the immediate vicinity.
Turning Up The Heat
A similar sequence takes place at the automatic paint booth. Pre-treated parts arrive; the scheduling system interrogates the transponder and activates the correct spray nozzle. Depending on the coating process employed at the plant, overspray may be deposited on the tag carrier — a common cause of read/write performance degradation for some RFID systems.
However, certain RFID transponders and RWMs are known to operate reliably when embedded in or coated with most common materials, including plastic and metal, resulting in few degradation or loss of data.
In addition, at the paint-curing station where temperature is high, the entire transport frame, complete not only with parts but also with transponder, passes through the curing oven. During the curing cycle — again programmed by the central scheduling system — temperatures may rise to 250 deg C; temperatures this high are beyond the capabilities of many RFID devices. High-temperature transponders, which are able to operate in this range, eliminate the complex mechanical arrangements otherwise needed to shield devices from elevated-temperature areas.
Staying In Control
As the painted parts arrive at the final-assembly area, additional RWMs confirm the presence of the correct transport frame for the motorcycle waiting on the production line. After the parts are unloaded from the carrier and fitted to the vehicle, the scheduling system carries out a final interrogation to confirm completion of the process, following which the transport frame is returned to the loading station.
The flexibility of RFID systems allows plant managers to stay in control of a changing situation. Should the assembly sequence be altered as a result of variations in customer demand, in-process parts that have not yet been painted can be reallocated at the next read/write station by overwriting the transponder data. The RFID system accommodates day-to-day variations with little or no disruption to operations.
What Is RFID?
It is a device that uses wireless non-contact medium like radio-frequency electromagnetic fields to transfer data, for the purposes of automatically identifying and tracking tags attached to objects.
Like a bar code, the RFID device provides a unique identifier for a particular object. However, it does not require to be within the line of sight of the reader like a bar code and it may be embedded in the tracked object.
