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Nov
20

PID Design

The PID is by far the most common feedback control technique used in the process industry. Thus, during the development of DeltaV we placed a special emphasis on the features that should be included in the PID block. There were differences in the PID implementation of the two control system manufactured by Emerson at that time, Provox and RS3. For example, Provox uses the series form of the PID and the standard form of the PID (also know as the ISA form) is used in the RS3 control system. Also, there were some differences in the features supported by the PID and in its implementation. As we looked at the PID in other control systems of competing manufacturers, we noticed similar differences in the PID form, features, and implementation. In some cases, the manufacturers included multiple PID blocks within their system to support multiple forms of the PID and to provide different levels of capability. As part of this background investigation, we reviewed a draft copy of EnTech’s Automatic Controller Dynamic Specification that Bill Bialkowski had sent me earlier that year for comment. This specification contained ideas that influenced some of the features that we included in the PID.

In the end we included in the PID design what we considered to be the best PID features found in industrial control systems. We set a goal of incorporating this functionality in one function block. The core parameters of the block were based on the PID definition in the Fieldbus Foundation Function Block Specification. By taking this approach, the names and data types of the basic PID parameters were consistent with the PID parameters included in Foundation fieldbus devices. Also, the units of the PID parameters were selected to be consistent with those defined by the fieldbus Foundation function block specification. The fieldbus Foundation function block specification does not specify the form of the PID. Thus, we were free to choose the form of the PID. However, rather than selecting one form of the PID algorithm, the block was structured to allow the user to select Series or Standard form using the FORM parameter.

A key decision in the PID design was the method used to realize the reset component. In our investigation, we noted that were significant differences in the approach that major manufactures have taken in their reset implementation. Thus, we examine the dynamic behavior of the most common designs (including that of Provox and RS3) in cascade strategies and under the conditions of override and downstream limit conditions. Based on this analysis, the external-reset feedback technique was selected for the reset implementation. This approach fits especially well into a system that is designed around the Foundation fieldbus block design. For example, in a cascade control strategy, the standard block options allow the PV of the downstream block to be automatically provided through the connection to the BKCAL_IN parameter. Thus, this value is available to the PID for use in the reset calculation independent of whether the downstream block is in the same controller, another controller or a fieldbus device.

One of the key points of the Entech specification was that the user should be able to independently select whether proportional and derivation action are based on PV or error. Thus, based on this input, we included the STRUCTURE parameter in the PID to allow the user to independently select whether proportional and derivative act on PV or error. In addition, as one of the STRUCTURE selections, we allow the user to specify the fraction of PV or error that is used in proportional and derivative action. Through this added selection it possible to achieve the same response provided by a two-degree of freedom controller and thus eliminating the need to choose whether to tune a loop for best response for setpoint or load disturbance.

If you are interested in learning more about the features that we selected to include in the PID block, then more information can be found in the white paper Key Features of the DeltaV PID Function Block .