When a control loop is placed in manual, the output is fixed. It doesn't change any more, it stays where it was or where the operator sets it. As a result, the process can change significantly, the measurement or PV (Process Value) can go much higher or lower than where the setpoint was last time the loop was in automatic control.
This can cause a problem if the loop is put back into automatic control, and there is still a big gap between the PV and the setpoint. The loop can take aggressive action and move the output by a large amount to try and get the process back where it wants it. These big moves can upset the process and cause more problems with downstream equipment.
To avoid this, the operator would need to move the setpoint close to the current PV before changing it back to automatic. And the need to do this every time. As you can imagine, it is just a matter of time before this gets missed and a large jump is made.
A solution to this is to use PV Tracking. Whenever the loop is placed in manual, the setpoint tracks the PV. It the PV increases, the setpoint also increases. They are stuck together. When the loop is placed back in automatic, there is no large movements, the system tries to keep everything where it is. The operator can then decide what setpoint they actually want and can move it there, perhaps in multiple steps to make sure that the moves are kept small.
There are occasions when you don't want to use PV Tracking, but in most cases you probably do.Go Top