Override Control

12 Oct 2019 at 20:56 // under Simple Saturdays Process Control

I do not think that we covered override control loops while at university, the first time I came across them was in the sugar factory. Yet they are used a lot and are incredibly useful. I don't understand why they aren't covered in introductory control classes.

Override control can come in two forms, min select or max select. In each case the outputs of two or more control loops are fed into the loop. This could be a flow controller that wants to set the valve position to 80%, and a level controller that wants the valve set to 75%. In the case of min select, the controller chooses the minimum output, in this case 75%. Max select would choose the maximum value, 80% in this case. The loop that is chosen remains in control. The main select would choose the smallest value and so the level would remain in control. The flow rate would be lower than the setpoint wants. The override controller looks at two measurements, though only one can be in control at any time.

Why would you use the this?

This type of control set up allows you to keep two different measurements under some form of control. While only one is in control at a time, the other will be further away from any limit that you are concerned about. It is useful to help maintain a unit at maximum rate when the restriction that limits the units throughput changes.

Take a case of a feed drum and a flow controller. The unit has a maximum flow rate that it can process, so to maximize the utilisation, you want to measure the flow and adjust the control valve to keep the flow as close to that limit as possible. But there are times when there is not enough feed getting to the feed drum. We need to ensure that we don't empty the feed drum completely.

We could add a level control loop that adjusts the valve position to try and maintain the level at a low value, like 30%. If not enough feed is available, the valve gets closed, reducing the flow rate. Once more feed is available, the valve will open up to maintain the low level.

If the override controller is set to min select, then the control loop that wants the lower flow rate will be in control. If the drum is at low level, the level control loop is in control and there will be a lower flow. If the amount of feed available increases, eventually the flow will increase up to the maximum that the unit can handle. In that case, the flow controller will ask for a lower valve position and will take over control. The level in the drum will no longer remain at 30%, but will increase away from the low level.

If override controlled are set up well, the unit can be ran in such a way that the control system always keeps the the unit at its highest rate, no matter what the limit at that time is.

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