Here is another installment in our continuing series of screencasts showing the intersection of Foundation fieldbus (FF) digital communications with automation systems. The intent of these screencasts is to demonstrate visually how the information in these smart field devices interacts with the automation system to help improve the process.
Emerson’s Rune Reppenhagen shows a control loop with a Coriolis flowmeter, a digital valve controller, and a connection to a redundant pair of Foundation fieldbus H1 cards. Rune describes the control strategy where the analog input runs in the Micro Motion transmitter, the PID control block and analog output block run in the Fieldvue DVC6000 digital valve controller and fieldbus segments connects to the pair of DeltaV H1 cards.
In this 3:21 screencast, Rune shows how control around the loop is maintained by running even in the event of loss of both H1 cards. As you might expect, the information is no longer transmitter to the operator, but the loop will continue to operate. Operators can monitor the loop locally at the devices with their local indicators until the communications are reestablished.
Different applications and operating philosophies may prompt where you might want to locate your control strategies–in the automation system controller or in Foundation fieldbus devices. John Rezabek, a contributing editor for Control magazine, implemented this approach seven years ago as he describes in his article, Not jazzed about fieldbus? Try it. He describes the additional benefit of mode-shedding to manual when the process variable (PV) of the PID is bad or uncertain in the devices. John writes:
While it’s a diligent piece of work, this user-coded mode shedding is utterly unnecessary in fieldbus–it’s already hard-coded into the blockware and happens automatically. In the same way, the “actual” mode of a PID block sheds to manual when its PV status is bad or uncertain, holding the last output computed before the input’s signal status changed.
For bad or uncertain transmitter information, John writes:
Bad or uncertain PV status will cause appropriate mode-shedding in the same scan (macro cycle) in which the condition is detected, so no “new” valve output is passed to the AO block; it dutifully holds last value.
All this happens by interconnecting the FF signals via your programming interface. No additional code or external interlocks are necessary. It’s built-in, out-of-the-box and standard in certified FF devices that have implemented PID.
The bottom line from the screencast and what John writes is that Foundation fieldbus provides a lot of robustness in control in addition to the digital diagnostics it delivers.