The term “bumpless transfer” is used to describe the desired action a PID controller, where the PID controller is expected to maintain its state when transferring from manual to auto mode (or vice-versa). On the other hand, if your have a “bump transfer”, will adopt the predefined setpoint when it moves between manual and auto mode (or vice versa).
Clear as mud?
Let me explanin in detail..
In auto mode, the PID controller calculates a deviation between the desired setpoint and the actual value (aka error) of the process variable, and adjusts the signal to the output to reduce the value of the deviation. However in manual mode, the operator decides what this output signal will be by pre-defining a PID setpoint (bypassing the auto mode settings), and this value could be different than what the controller has calculated.
In this case, when the operator switches from manual to auto,the controller applies its calculated output value to the process, and since there is a good chance the auto and manual outpouts do not match, there will be a “bump” in the process, where the output controlled by the PID controller will suddenly change. For example, valves will start to open/close suddenly, pumps will increase/ decrease in speed abruptly, until such that the controller adapts the output values again. This sudden motion can be bad for the process and can affect adversely the process actuators lifespan. That is, valves, pumps and motors repeatedly being applied fast changes can even damage the actuators).
In order to mitigate this adverse change in process, we introduce bumpless transfer to the PID controller will ramp slowly the auto output to the value calculated by the controller in auto when the PID controller is changed form manual to auto mode. when we do this gradual method of transition, the process shows no abrupt changes and the process actuators are not affected. The reverse is true as well when going to mauto to manual.
Ok. Ater that 2 paragraph you know what it means…. theoretically. but how do you apply it in PLC/RTU logic?
Well, this is how you would do it in RSLogix 5000 using PIDE. The first diagram below shows bump transfer.
The diagram below is for bumpless transfer.
Below is a really awesome dragram explainig the whole process of with and without bumpless transfer.
From A to B the system is under automatic control – and “on target” … at B the setpoint is changed from 20% to 40% of full flow … at C the system has settled down at the new setpoint … at D the system is switched to Manual mode – notice that the CV (orange) holds steady at its present value … in other words, no “bump” to the system when going from auto mode to manual mode … at E the CV is manually moved to a new setting … notice from D to F that the Setpoint is hidden behind the PV (blue) … during that portion of the trend, the setpoint is “following” or “tracking” the PV … at F the system is switched back to the Automatic Mode … notice that the Setpoint (yellow) holds steady at its present value … in other words, no “bump” to the system when going from manual mode to automatic mode … at G the setpoint is manually raised to a setting of 40% …
Below is an example of how to do bumpless transer PID control in Telepace.
The first PUTF block (Put Floating Point to destination) ass ign values from Modbus register 41085 (Process PV) to 40500 (PID PV).Note that this is a 2 16-bit registers, therefore it covers from register 40500 to 40501.
If you need the PID to be in Auto mode, energize Modbus register 2625 (“Auto Man”). The nuts and bolts of the PIDA block can be found in the table below, however, the important part of this example is the last 3 ladder elements. When Modbus register 2625 is energized, the output value from the PIDA block (of Modbus register 40520) is continuously assigned to Modbus register 40518 (Man Output SP). In the event, when the PID mode changes from auto to manual, the PID will start from where the output was when the PID was in auto mode, hence preventing the sudden and drastic change of setpoint. That is bumpless transfer.
And last but not least, the last PUTF block assigns the PID auto setpoint (Modbus register 40502) continuesly from Modbus register 41090 (when the PID is in auto mode). This ensures that any change in the setpoint is captured.
The table below is from Telepace Studio help file. It identifies the storage Modbus register assignment table for PIDA.