Bumpless Transfer is present in PID algorithms to prevent a sudden change in the control output on transition from Manual to automatic control. Since we have an additional parameter that limits the maximum ramp rate of the output (slew rate), isnt bumpless transfer superfluous. Especially since if you are operating without I (integral) control, Bumpless transfer does not work for you.

I am asking this because we are trying to code a simple PID algorithm.

Thanks in advance for your response.

 

ANS:
Yes. It is meant only one controller mode is changed from Manual To Auto or vice Versa. The output should not be changed at the instance of mode change.

<Since we have an additional parameter that limits the maximum ramp rate of the output (slew rate),>

ANS:

Max Ramp rate plays role when controller mode is Auto. For Eg., Limit is 60% per minute.
Assume as per PID calculation, calculated output is 90% then controller output will reach 90 in one and half minute. If there is no Max Ramp rate mentioned, controller output will reach 90% immediately.

<isnt bumpless transfer superfluous. Especially since if you are operating without I (integral) control, Bumpless transfer does not work for you.>

ANS:

No Bumpless transfer is not superfluos, it may not be required for propotional only configuration. But it is must for other algorithms.

 

For example, in manual control your output was 0% and your calculated output in automated control is 90%. The bumpless transfer will prevent you from transitioning to 90% in one shot by putting in an initial integral term value. Since upon on transitioning to auto you are in automatic contorl, the Max Ramp rate would have done exactly the same. The function of both these parameters is to prevent a sudden change in the controller output, the bumpless transfer working only when transitioning from manual to auto, and Max rate of control working under all conditions of automated control. The Max ramp rate gives the user explicit control, why would you need bumpless transfer in addition?

 

For example, in manual control your output was 0% and your calculated output in automated control is 90%. The bumpless transfer will prevent you from transitioning to 90% in one shot by putting in an initial integral term value.

ANS:
Assume you have a controller with PI configuration.

The instance you change to AUTO from MAN,
in your case bumpless transfer will not work. You cannot expect also. Definetly you will see a bump in output. Integral action does not have any role in bumpless transfer.

Normally whatever controllers having bumpless transfer feature, will be configured to copy the PV value to SP (PV tracking), so at the instance of changing the mode, the Error will be zero and also the output stays in the value which was in Manual (Assume it is 40%). [This is bumpless, i.e. output should not be driven to zero, since error is zero.]

Now if you are increasing the setpoint 10% more, (Assume gain as 1 and Integral as 1 min), you can see 10% sudden increase in the output [i.e. Output will go to 50% and not 10%] and every second integral part also will be added [0.16% will be added for above config if there is no change in Error (i.e. 10%)].

 

What Anonymous describes is true for most analog PI controllers. However, for many years starting with computer control and now with almost all commercial DCSs, there is a second option for bumpless transfer that may be configured for any PI or PID controller: If you set the option for bumpless transfer, the setpoint remains at its previous position when the controller was in manual, and the controller sees a large error, but no reset remainder (windup), and no derivative term. This causes the control output to drive the process to the desired setpoint using only the proportional and integral terms.

This configuration is generally used for the inner loop of a cascade control where the operator sometimes changes the mode of that term to Manual in order to take over the control valve temporarily. The setpoint is calculated by the outer loop controller, and is the correct setpoint.

Dick Caro
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