The PID controller is an essential component of nearly all control loops in the process industry. We take for granted that a change in the PID controller mode or setpoint will not disturb the process. More needs to be known about bumpless transfer and how a new operating point can be achieved quickly and smoothly after the PID is switched to the automatic mode. The following question by Brian Hrankowsky, a resource in the ISA Mentor Program, helps us seek a good definition of PID bumpless transfer, how it is achieved in the PID and how options such as setpoint tracking, PID structure, setpoint dynamics and external reset feedback come into play.
Brian Hrankowsky’s Question
In our plants, I have seen two definitions for PID bumpless transfer:
- Ninety-nine percent use bumpless transfer to mean that on transition from auto to manual, the control output does not change and on a transition from manual to auto, the SP matches the PV and no control action is taken from the current controller output until the SP or PV changes (output does not jump to previous or other value; when an error occurs, output is driven by PID action from the current output value).
- One percent use bumpless transfer to mean that on transition from auto to manual, the control output does not change and on a transition from manual to auto, control action is taken from the current controller output in response to the SP not being not equal to the PV (output does not jump to previous or other value; since an error already exists, output is driven by integral action from the current output value).
Note that the only difference in the two definitions is the removal of “the SP matches the PV and“.
In reading on line, it seems that both ways of using the term are in use. The majority seems to be definition No. 1. However, I can imagine that configuring some systems to work like definition No. 2 suggests would be beneficial. In fact, I think the way some skid mount OEM equipment works is really like definition No. 2: The controller setpoint is fixed by the selected “format” or “recipe” and starting/stopping the machine just changes the controller mode. But generally in our DCS implementations, we configure the loop to behave as in definition No. 1.
What is the official (ISA?) definition?
Greg McMillan’s Answer
Good question. The transfer from manual or remote output to auto should be bumpless regardless of the current setpoint. This gives the most options. This is commonly achieved by biasing the contribution from the integral mode to counter act any contribution from the proportional mode. The ISA Dictionary 4th Edition says as much and even has a little equation that shows the bias (b) is set equal to the manual output (mo) minus the product of the gain and error (Kc e) to provide bumpless transfer. The simple equation does not show the control action sign or the effect of a beta setpoint weight factor. In real life it gets more complicated but the goal should be the same.
b = mo – Kc e
You may be able to simply check a box to enable a SP track PV option in a modern DCS. In chemical processes where we have seen benefit from the proportional step in the PID output on a setpoint change to get to the new setpoint sooner, we use a structure of PI on error and D on PV, enable the SP track PV option, and then have the batch or startup sequence automatically change the primary PID setpoint at the right time. A setpoint lead-lag or two degrees of freedom structure can minimize setpoint overshoot by optimizing the contribution from the proportional and derivative modes to the PID output.
This of course only works well if other systems are not upset by a sudden large change in the PID output. Tight pressure control of the source of the manipulated flow can reduce the upset to other users of the source (e.g., utility). However, operators may get concerned even if there is no detrimental effect. To prevent this, you can use setpoint rate limits on the analog output block or secondary PID setpoint and enable external reset feedback (dynamic reset limit) in the primary PID to slow down the change the in manipulated flow (now famously known as directional move suppression) and the consequential upset to others (including operators) without having to retune the primary PID.
In other words, “Bumpless Transfer” is a basic MUST for all loops regardless of setpoint value, and the “SP Track PV” option is chosen based on process and operator requirements. To achieve a new setpoint faster after a PID mode makes the transition from manual or remote output to auto, this track option is used and the setpoint is then changed to the desired operating point after the PID goes to auto to give a quick change in the contribution from the proportional and derivative modes as determined by the PID structure or setpoint dynamics. External reset feedback and setpoint rate limits of the manipulated flow can slow down the actual change in the PID output to reduce the upset to other loops and operators.
Any change that is to be made to a control system must be thoroughly functionally tested by realistic simulations of the process’s dynamic response. The ability of the control system improvement to deal with abnormal besides normal operating conditions must be verified. The commissioning and performance of improvements should be closely monitored to ensure they meet plant requirements.