1766-RM001A-EN-P MicroLogix 1400 Programmable Controllers ...

356 Process Control InstructionPID Rung StateIf the PID rung is false, the integral sum (IS) is cleared and CV remains inits last state.Feed Forward or BiasApplications involving transport lags may require that a bias be added tothe CV output in anticipation of a disturbance. This bias can beaccomplished using the processor by writing a value to the Feed ForwardBias element (word FF). (See page 343.) The value you write is added tothe output, allowing a feed forward action to take place. You may add abias by writing a value between -16383 and +16383 to word 6 with yourprogramming terminal or ladder program.Application ExamplesPID TuningPID tuning requires a knowledge of process control. If you areinexperienced, it will be helpful if you obtain training on the processcontrol theory and methods used by your company.There are a number of techniques that can be used to tune a PID loop.The following PID tuning method is general and limited in terms ofhandling load disturbances. When tuning, we recommend that changes bemade in the MANUAL mode, followed by a return to AUTO. Outputlimiting is applied in the MANUAL mode.TIP• This method requires that the PID instruction controls a non-criticalapplication in terms of personal safety and equipment damage.• The PID tuning procedure may not work for all cases. It is stronglyrecommended to use a PID Loop tuner package for the best result (i.e.RSTune, Rockwell Software catalog number 9323-1003D).Procedure1. Create your ladder program. Make certain that you have properlyscaled your analog input to the range of the process variable PV andthat you have properly scaled your control variable CV to youranalog output.Publication 1766-RM001A-EN-P - October 2008