A customer had a poor performing loop in the brewery. The problem would really become an issue once in awhile depending on the operating conditions in the plant. The problem would be passed from person to person, shift to shift with no real answer.
In discussing the problem with a controls engineer, the engineer asked if an “open loop” bump test had been performed. The answer was no. Question: What is a bump test? A bump test was performed and initial tuning parameters were calculated and installed. The loop began to settle down.
An “open loop” bump test is performed by placing the loop in manual, making a step change in the manual output and observing or trending the result of the process variable. With the resultant process curve displayed, the process gain, dead time and process time constant can be determined. With these three pieces of information the technician can determine the initial tuning parameters by selecting among hundreds of tuning strategies. Strategies encompass the type of process, process characteristics and tuning objectives. Common or classical strategies include: Ziegler-Nichols, Cohen-Coon and Lambda.
The initial problem solving method is discussed in ISA, Troubleshooting: A Technicians Guide, 2nd Edition.
From page 65:
“Consultation, also known as the “third head” technique, means that you and the equipment operator use a third person, perhaps someone from engineering or an outside consultant, with advanced knowledge about the system or the principles for troubleshooting the problem. This person may not solve the problem but may ask questions that make the cause apparent or spark fresh ideas for you. This process allows you to stand back during the discussions, which sometimes can help you distinguish the trees from forest.”