ABSTRACT
This chapter teaches methods of tuning controllers while they are running in automatic output mode, that is, closed loop. The two main types of disturbances introduced into a controller for closed loop tuning are: a step change in controller setpoint and a step change in bad. A step change in bad can be simulated by first placing a controller in manual output; second, introducing a step change in the controller output (valve position); and third, placing the controller back in automatic. The method consists of first turning off the integral (reset) and derivative (rate) action in the controller and, second, finding the ultimate proportional gain that will just sustain continuous cycling of the process variable and measuring the ultimate peak-to-peak time period. Some controllers use proportional band, which is 100 divided by the proportional gain. Sometimes an existing process control loop is already running with sustained cycling, and tuning of the control loop can be approached as a troubleshooting activity.
