When planning or designing a fieldbus installation, it is important to consideration where the control will reside. The function block set supported by fieldbus devices may be used to address many common control applications. If you are dealing with a slow process then where the control is done may have little impact on the observed control performance. However, when addressing faster processes such as liquid pressure and flow, then this decision will directly impact control performance. A common misconception is that best control performance will be achieved by using the control system controller for all control and thus only use fieldbus to access IO in field devices. It turns out that in many cases that the best performance is achieved by using the control functionality of the fieldbus devices.
One of the primary reasons for this difference in performance rest with the fact that IO access and control execution can not be synchronize when a control loop is split between the controller and fieldbus devices. In a traditional control system it is possible to highly over-sample the IO and thus minimize delay introduced between an input being scanned and the value being used in control. For example, in some cases, the information from a traditional analog or discrete card is refreshed in controller memory every 50 msec. Thus, the measurement is always fresh when control is executed without the need for synchronization. However, the relatively low speed of the H1 physical layer (31.25Kbaud) does not permit the control system to over-sample the fieldbus analog input blocks. This is especially true if multiple transmitters are installed on the same segment since only one fieldbus device is allowed to communicate at any given time. Thus, fieldbus measurements used in the controller may be as old as one macro-cycle. From a control standpoint, this added delay directly impacts PID tuning and control performance.
When the Fieldbus Foundation specifications were originally developed, a considerable amount of time was spent addressing the issue of control performance. The system management capability and publisher/subscriber services of the fieldbus communication stack allow the execution of blocks and associated communications to be precisely synchronized. Time is periodically broadcast to insure that all devices on a fieldbus segment have the same sense of time. When control is done in the field using devices on the same segment, then each device that participates in the loop execution is given its portion of the schedule. Communications used in control always occur as scheduled. Thus, when control is done in the field it is possible to minimize any delay associated with communications and scheduling.
The loop execution speed that may be achieved using control in the field depends on a number of factors. For instance, the execution period will get longer as more loops are added to the same segment. Also, there is a large different between manufacturers (3X or more) in the time that is required for a function block to execute in a fieldbus device. Some of the latest generation fieldbus devices have significantly shorter block execution times. Also, if the control system supports the assignment of function blocks to the H1 interface card, then the execution of these blocks can be synchronized with other blocks on the segment since the H1interface card is just another fieldbus device. The power available to the H1 interface card allows much faster processors and thus blocks assigned to the H1 interface card execute in less time than the same blocks in field devices.
Marcos Peluso, Emerson Process Management, and I put together a presentation last year in which we addressed many of the factors that influence control performance when using fieldbus devices.
If you are planning or designing a fieldbus installation then you may find the information contained in this presentation of interest.