Certain control function blocks can run in the fieldbus devices or in the DeltaV controller. This topic discusses the pros and cons of each method.
Consider a simple regulatory control loop with an AI, PID, and AO function block. The AI block must run in the transmitter and the AO block must run in the valve. The PID block can run in either of the field devices or in the DeltaV Controller. The configuration of the control module's function block diagram is similar regardless of where the PID algorithm runs. The difference is in how you assign the PID block. In DeltaV Control Studio, you can assign the PID block to fieldbus and run it in either the transmitter or valve, or you can drag the PID block off the palette and run it in the controller.
Running the PID block in the device:
Improves controllability due to synchronous execution
Conserves controller CPU resources
Reduces the number of VCRs on the segment
Running the PID block in the controller:
Provides additional control options, such as form and structure
Can facilitate conditioning of the input and output, if required