Consequently, we embarked on a design with four fieldbuses. At the outset, this seemed like the best choice given the diversity of I/O types. We later discovered that our first introduction to a bus I/O infrastructure would teach us to include other factors when considering a multi-bus platform. Though I believe we have a technically solid I/O subsystem with our four fieldbuses, there’s probably a more optimal design approach if one considers the troubleshooting, maintenance, spare parts, and training aspects of each bus, which are unique. Clearly, one fieldbus doesn’t get us there, but maybe someday that will be a reality. I think, at best, given the timeframe of our project, we could have reduced the bus count to three, and, today, maybe to two. Live and learn.
Perhaps more important is the advance of technology in the past two years for increased bus devices, as well as some fundamental bus topology advances with AS-i. For example, this bus can now extend 300 meters without repeaters, and instead uses a bus tuner that sits at the end of the segment to obviate earlier length limits.
We also know that fieldbus instrumentation is more expensive from an initial purchase standpoint. We didn’t track our construction costs in a way that would allow us to say with certainty that we realized installation savings. We believe that it should have been less costly due to smaller controller cabinets, fewer field cables, and fewer conduits. Our perception is that the overall cost was less, but we don’t really know by how much. I think our biggest savings are still yet to come with the predictive maintenance model inherent in Foundation fieldbus. Foundation fieldbus transmits device status along with the process variable, so it’s possible to receive device health information regarding imminent instrument failure. In the biopharm industry, this could mean the difference between a successful multi-day or month batch run and a failure, which could run into the millions of dollars in lost product. Therefore, this is the cost savings on which we’d rather focus our attention.
|FIGURE 1: A REACTOR TRAIN|
A conventional wiring system would have meant over 230 wires to each reactor. (Click image to enlarge.)
The following example shows how Genzyme’s multi-bus architecture was deployed in one of our process suites. Figure 1 shows a typical bioreactor train, which comprises a media feed tank, bioreactor, and harvest tank with their I/O requirements. If this bioreactor train had been conventionally wired, a combined total of 238 cables would have been pulled between a controller cabinet and the process equipment.
Figure 2 below depicts a cable block diagram for a fieldbus implementation of this same bioreactor train. You’ll notice that there’s a dramatic reduction in the cable quantity; only 14 home-run cables are required between the controller cabinet and the field terminal boxes (FTBs). These FTBs are strategically located in the process suite close to the process vessels to minimize instrument cabling that fans out from the FTB to each instrument.
FTB-B1, which wires to the bioreactor skid, includes two Foundation fieldbus segments, two AS-i segments, and one Profibus segment. The segments don’t extend beyond the bioreactor to other vessels, even if they had instruments nearby these segments. This was done by design to maintain segment segregation between skid equipment, such as bioreactors and field assembled fixed tanks. Bioreactors are the only process skids that use redundant instrumentation, specifically, dissolved oxygen and pH measurement. These redundant instruments share the same segment. We didn’t design for redundant segments in this case because a bioreactor is unique in that you must have all instruments and valves functional. If not, you shut down the reactor. The redundant transmitters provide assurance that we’ll likely have at least one operational by the end of a bioreactor run, even if there’s a sensor failure or probe fouling on one of the two. Sensors can’t be changed underway because they’re within the sterile boundary of the reactor.
|FIGURE 3: SPAGHETTI WIRING|
Conventional wiring takes up lot of space.
The fieldbus architecture lends itself to maintaining an orderly appearance to controller cabinets. Figure 3 below represents one of our typical conventionally wired control systems, and Figure 4 (below) is typical of fieldbus. This cabinet uses five bus I/O cards that communicate with approximately 200 I/O points.