To fieldbus or not to fieldbus

InTech, Oct 1997 by Ochsner, Marjorie, Schrier, Matthew

In today's environment, the control strategy is executed in the DCS and a transmitter's contribution to the control strategy is the process variable. If a smart transmitter is used, changing the transmitter's range is sometimes incorporated into a control strategy. This is useful in a batch application where the products being made can vary. With fieldbus, basic regulatory control can be done in a device. As new function blocks are defined, these strategies may become more complex. The advantage is in the ability to off-load or even eliminate the controller in some applications, depending on the system design.

Implementation of control in the field devices works well if the elements of control are located relatively close to each other. Proximity makes it feasible to put all the elements on the same fieldbus segment. In this way, links between the input blocks, control blocks, and output function blocks can be made without going through any obstacles. This is especially important for loops that must be executed quickly The 31.25-kilobitsper-second fieldbus allows, on average, about 30 messages per second over the bus, depending on the message size. Another beneficial application is execution of several noncritical loops in the same physical plant area. Two to three loops can be put on one fieldbus, some interlocking or cascade control strategies can be accomplished, and the loop can still be executed at 250 milliseconds. An important concern in an application is the lack of fault tolerance, which is why it is not recommended for critical loops. The nonredundant fieldbus can become a single point of failure for the loops.

Let's look at a situation where there is a need to interlock a number of control loops in a critical application. Since fieldbus is not redundant, putting more than one critical loop on one fieldbus segment is probably not a sound idea. Therefore, the host controller will do the most efficient execution of interlocking control.

If control is already implemented in the host and a user wishes to transition devices to fieldbus, it might be beneficial in some cases to move control out to the device. The most obvious reason is to take advantage of faster execution times or to off-load a controller as mentioned previously. The added functions available within fieldbus devices (e.g., trending and alarming) deliver added security.

These benefits must be weighed against the potential costs. All changes to control strategies and control strategy implementations should be documented and tested. Costs include documentation, testing, configuration, and implementation.

More field information available When a smart transmitter is digitally integrated to a DCS, more information is available from that transmitter than simply a process variable, including range values, engineering units, secondary variables, serial number or tag name, calibration information, materials of construction, and time stamping. In multivariable devices, multiple process variables as well as the algorithm parameters that relate some variables can be accessed.