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Aug
01

Commissioning Control Systems – Resolution and Deadband

As addressed in my July 18th and July 25th blogs, to achieve best control performance, it is important that the final control element moves to a position that matches that requested by the control system. The minimum size of change in the PID output that the control element can respond to can be quickly confirmed by placing a control loop in Manual and then adjusting the control system output in one direction in small steps, for example, changes of 0.1%, and noting when the final control element begins to move. By this means, it is possible to determine the output resolution.

Output Resolution – Smallest percent change in control system output that will be seen in the controlled parameter value.

After a series of changes have been made in one direction, it is possible to determine the deadband by making a series of changes in the opposite direction.

Output Deadband– Smallest percent change in direction of control system output that is required to change the value of the controlled parameter.

If a measurement of the position of the final control element is an input to the control system, then this input may be used to confirm that the final control element has responded to the control system output. When such a measurement is not available, movement of the final control element to moderate changes in the PID output may be visually confirmed by going
to the field and observing the final control element response to a change in the control system output. Also, by trending the control parameter on an expanded scale along with the PID output and making small step changes in the PID output, it is possible to confirm the changes in the PID output impacted the control parameter.

As addressed in Chapter 12 of Control Loop Foundation – Batch and Continuous Processes, the most common problems in commissioning a control system often can be traced to the fact that a positioner has not been provided with the valve, or the positioner provided with the valve has not been properly installed or has malfunctioned. Thus, when you are commissioning a control system, it is a good idea to establish up front which valves have positioners, and to clearly communicate the expected limitations in control performance for valves not equipped with positioners. If some of the final control elements are dampers, then it is wise to inspect the actuator linkage to determine if loose tolerances in linkage joints could be a source of deadband. An installation inspection may show that positioners are installed on all valves and that linkages associated with dampers and other final control elements are in good condition. Even so, it is always a good practice to begin loop tuning by placing the loop in Manual, making some small changes in the control loop output, and then observing to see if the process responds to these changes. When no response is observed, then an effort should be made to investigate and address any physical or electrical problems before attempting to tune the control loop.