Modeling and Control

When the Fieldbus Foundation was established, I lead the team that wrote the Function Block Application Process Specifications. This was a unique experience since it offered a rare opportunity to work with and to get to know some of the best control engineers from Siemens, Yokogawa, Honeywell, Leeds and Northrup, Foxboro, ABB, Smar and others leading companies in our industry.

In the committee meetings, we were able to put aside the commercial barriers that often prevent open discussions. In many cases, we used the ground breaking work by the SP50 User layer committee as a starting point in the development of the function block specifications. However, significant changes were made to incorporate the best ideas and features from existing products. Also, the architecture and formal model defined by Part 1 of the specification were written and structured to support object oriented design and implementation of field devices – something important if you are a manufacturer of field devices.

Basic architectural components such as status and mode definitions were discussed by the function block team at length and in the end reflected the contribution of more than one company. For example, the definition of status was strongly influence by Tom Kinney, Foxboro, and Bill Hodson, Leeds and Northrup. Similarly, the basic function block set defined in Part 2 of the specification were reviewed in detail to insure the blocks addressed the requirements of each company. The advanced function block set defined in Part 3 of the specification and the specialized blocks in Part 4 and 5 were defined by members of the team that had direct experience with the functionality encapsulated by the block.

When work on parts 1-3 of the Foundation Specification for the Function Block Application Process was completed in 1994, there were very few fieldbus products on the market and even fewer control systems that supported these devices. In the function block team we discussed the need to communicate the advantages of fieldbus to inform the industry of this technology. Thus, Tom Kinney, Foxboro, and Marcos Peluso, Smar, and I worked together to put on a fieldbus tutorial at ISA1995 conference. In this tutorial, we gave an overview of the technology to a standing room only audience. To show how fieldbus would be applied, we took a waste water process example and analyzed the benefits that fieldbus had over a traditional installation. For the next seven year, Marcos – who is now with Emerson - and I continued to host fieldbus a tutorial each year at ISA. In each tutorial, we focused on a different aspect of fieldbus installation and application. Much of the material that we developed for these tutorials can be found at Fieldbus Tutorial. If you are unfamiliar with fieldbus and want to quickly get up to speed on the basics then you may find the information provided at this site to be helpful.

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.

Achieving Target Control Performance Using Fieldbus Devices

If you are planning or designing a fieldbus installation then you may find the information contained in this presentation of interest.