Provides Additional Control Options
The FFPID extended block may not support all the parameters supported by the PID block in the fieldbus device. You can configure the full set of parameters when the PID block runs in the controller. Or, you can use the FFPID_RMT extended block with compatible devices to use the full set of parameters.
May Facilitate Input Output Conditioning (if required)
When a control loop goes beyond the simple AI-PID-AO combination, there may be a rationale for having the PID block reside in the controller. In most cases however, there are calculation blocks available in fieldbus devices for this purpose. Examples of conditioning include, but are not limited to the following:
Selecting the input from multiple sensors
Compensating a flow for temperature and pressure
Characterizing the control output
Rate limiting the control output during a startup or shutdown situation
If you find it necessary to perform this type of conditioning in the controller, you do not lose anything by also running the PID block in the controller because anything done in the controller (such as conditioning) eliminates synchronous control. However, it is usually possible for the entire loop to reside in field devices on the segment, thus maintaining synchronous execution. For example:
Use the Input Selector function block to select the input from multiple sensors.
Use the Arithmetic block for pressure-temperature compensation.
Use the Signal Characterizer block to characterize both the forward and backward paths of the control output.
Enable rate limiting by placing the AO block in Auto mode and manipulating the setpoint during startup or shutdown (Setpoint rate limits apply in Auto mode but not in Cas mode).