Control Notes

Bumpless Transfer

Most control practitioners have heard of bumpless transfer, a feature available in virtually all PID controllers. It prevents a sudden jump (bump) in controller output when the controller’s mode is switched from manual to auto.

Bumpless transfer is done by the controller by executing its control algorithm to calculate a pseudo controller output, comparing this to the current manual controller output, and subtracting the difference from the integral term so that the calculated output matches the manual output.  Bumpless transfer can also be achieved by using a velocity algorithm and not the more intuitive positional algorithm for calculating the controller output.

Both of these methods are pre-programmed in the controller code – an area normally available only to the controller manufacturer. Luckily, most controllers nowadays come standard with bumpless transfer.

Bumpless Tuning

Similar to bumpless transfer is the concept of bumpless tuning – a term coined by Harold Wade. In this case, without a bumpless tuning feature, the bump occurs due to changing controller gain or derivative settings during controller tuning.

Again ignoring integral and derivative control actions for now, the controller output (CO) is simply:

CO(old) = Kc(old) * E

CO(new) = Kc(new) * E

If E is not exactly zero when the change in Kc is made, the controller output will jump. Similarly, changing the controller’s derivative setting (Td) can create a jump in controller output if the rate of change of the error is not zero at the moment Td is changed.

It is because of the bump in controller output caused by changing tuning settings that it is good practice to place a controller in manual momentarily while making tuning changes. Because most controllers have bumpless transfer, it eliminates the bump when switching the controller back to auto after making the tuning changes in manual mode.  However, this becomes a problem if the tuning settings are changed programmatically, as in the case of gain scheduling.

Bumpless tuning can be achieved without the need to place the controller in manual mode by calculating how much the controller output will jump due to the new proportional and derivative settings, and subtracting an equal quantity from the integral term, so that the sum of the three terms (the controller output) remains unchanged.  Similar to bumpless transfer, bumpless tuning can also be achieved by using a velocity algorithm.

The bumpless tuning feature is pre-programmed in the controller device – and cannot be added afterward by the user. Far fewer controllers have this feature. For example, the modern C200 controllers from Honeywell do not have Bumpless Tuning, while DeltaV controllers from Emerson do.

Stay tuned!
Jacques Smuts – Author of the book Process Control for Practitioners