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Jan
05

How to Succeed – Part 8

The missing piece for success is the center piece of the loop, the PID controller. There is an incredible offering of PID features and options. To help utilize the full potential of the PID, here is a check list as a guide. While the full aspects of the PID capability are book worthy, the following overview can get you started on the right path.

If you don’t get the valve action and control action right, nothing else matters. The controller output will ramp off to an output limit. The valve action (inc-open and inc-close) can be set in many different places, such as the PID block, analog output (AO) block, splitter block, signal characterizer block, current to pneumatic (I/P) transducer, or the positioner. Make sure the valve signal is not reversed in more than one location for an inc-close (fail open) valve. Once the valve action is set properly, the control action is set to be the opposite of the process action. The control action is reverse and direct if an increase in the PID output causes the PID process variable (PV) to increase or decrease, respectively. Verify with process engineer the valve action and process action and resulting control action required. Deferring considerations, here is the checklist without delay (pun intended since we are not going to consider here the use of a delay in the external reset feedback for deadtime compensation).

 Measurement scale covers entire operating range including abnormal conditions
 Valve action (inc-open for fail close and inc-close for fail open if action not reversed in another location)
 Control action (direct for reverse process and reverse for direct process if valve action set properly)
 Form (ISA standard)
 Obey setpoint limits in cascade and remote cascade mode
 Back calculate for bumpless transfer
 PV for back calculate in secondary loop
 Structure (PI action on error, D action on PV for most loops)
 Setpoint track PV in manual unless setpoint must be inherently saved in PID
 Setpoint limits to match process, equipment, and valve constraints
 Output limits to match process, equipment, and valve constraints
 Anti-reset windup (ARW) limits to match output limits
 Execution time less than 10% of minimum reset time
 Signal filter less than 10% of minimum reset time
 Tuned with auto tuner or adaptive tuner
 Rate time less than ½ deadtime (typically zero except for temperature loops)
 Dynamic reset limit enabled for cascade, AO velocity limits, and slow valve
 AO setpoint directional velocity limits set for blending, valve position control, and surge control
 Integral deadband > limit cycle PV amplitude from deadband and resolution or enhanced PID for wireless

The setting of all options and parameters must be verified as applicable. Simulations representative of the dynamic behavior of the process and the field automation system along with the actual configuration to form a virtual plant is advisable for testing and confirmation plus training and opening the door to process control improvement (see Exceptional Opportunities in Process Control – Virtual Plants).

In Parts 9 and 10 we will explore many of the considerations for the PID checklist items.