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Sep
26

Commissioning Control Systems – Setting Controller Action

The process gain may be negative or positive depending on the process design. To provide the correct response a feedback control block must take into account the process gain direction—whether to increase or decrease the manipulated input to correct for a control error. For example, the inlet flow to a tank may be manipulated to maintain the tank level at setpoint.
When the level increases above setpoint, then the inlet flow must decrease for the level to return to setpoint. In this case, the control response is described as reverse acting. Conversely, if tank level is maintained by adjusting the flow out of the tank, then for an increasing level, the outlet flow must increase. The control response in this case is described as direct acting. Control blocks include a parameter to select direct/reverse response as a means to accommodate this difference in control response. This selection is made internally in the block by changing the sign of the error term used in the control calculation. As part of the initial control block setup, it is important to verify that the default setting of the parameter that specifies direct/reverse action matches the application requirement.

Typically the parameters used to specify direct/reverse action have a default setting of reverse since this setting is the most common. As address in Control Loop Foundation – Batch and Continuous Processes, the guideline shown below should be followed in configuring the direct/reverse parameter selection.

If the direct/reverse action setting is set incorrectly, then in automatic control the manipulated process input will be driven to full open or full closed. Based on the initial controller error at the time the control system was put in Automatic, the manipulated parameter will be driven in the direction opposite to that required to reduce the error. Such a mistake can be made when initially commissioning a control system, and in some cases can cause a serious upset or shutdown of a process. So, when you are initially working with a control system, it is important to always verify the configuration of the control action setting before placing control in Automatic.