November 2005 Trend Graphic.Greg: Well-placed tools on control valves is a time-tested practice to jar a shaft, flapper, or linkage free, but hitting a digital valve positioner is not advisable. Also, in this case the hesitation in the approach to the set point was caused by a controller reset time that was too large. The contribution from the proportional mode was greater than the contribution from the reset mode. Reset action will keep increasing the controller output as long as the process variable (PV) is less than the set point (SP). However, gain action will decrease the controller output as the PV increases before the PV crosses the SP for this reverse acting controller. The result is a hesitation that is a common problem particularly when the process time delay (dead time) is larger than the time lag (time constant) and the reset time was set as a ratio of the loop period typical for closed loop oscillation methods.For dead time dominant processes, the ratio of reset time to loop period decreases dramatically. Lambda tuning, where you set the reset time basically equal to the process time constant, eliminates this problem. However, for processes where the process dead time is small compared to a very large process time constant, setting the reset time equal to the process time constant can also cause a faltering and reversal of the controller output from gain overwhelming reset action. Here, a batch controller developed in the 1970s for analog controllers but lost in the 1980s and forgotten in many a distributed control system (DCS) can reduce the time to reach set point. Of course, the DCS allows you to come up with your own strategy to hold an optimum valve position until the PV is within one dead time of reaching the set point. Just remember to release the controller in time to do its job and that while your customization is job security, you may have to maintain someone else’s creation someday.Stay tuned for a future CONTROL article on how to generically optimize the set point response for batch, startup, and transition sequences. Stan: To encourage readers to reply even if the puzzler or trend is missing, we are offering to send an autographed free copy of “Control Talk – The Early Years” if the reply includes a mailing address. For January, we are looking for the readers to send in their own problem and solution. Trends are nice if you remove the process engineering units and stream names.