Anti-reset Windup and Bumpless Transfer From Manual To Auto (PID)
The PID instruction automatically avoids reset windup by preventing the integral term from accumulating whenever the CV output reaches its maximum or minimum values, as set by .MAXO and .MINO. The accumulated integral term remains frozen until the CV output drops below its maximum limit or rises above its minimum limit. Then normal integral accumulation automatically resumes.
The PID instruction supports two manual modes of control.
Manual Mode of Control | Description |
|---|---|
Software manual (.SWM) | This mode is also known as set output mode and allows the user to set the output % from the software. The set output (.SO) value is used as the output of the loop. The set output value typically comes from an operator input from an operator interface device. |
Manual (.MO) | This mode takes the tieback value, as an input, and adjusts its internal variables to generate the same value at the output The tieback input to the PID instruction is scaled to 0-100% according to the values of .MINTIE and .MAXTIE and is used as the output of the loop. The tieback input typically comes from the output of a hardware hand/auto station that is bypassing the output from the controller. Important: Manual mode overrides software manual mode if both mode bits are set on. |
The PID instruction automatically provides bumpless transfers from software manual mode to auto mode or from manual to auto mode. The PID instruction back-calculates the value of the integral accumulation term required to make the CV output track either the set output (.SO) value in software manual mode or the tieback input in manual mode. In this manner, when the loop switches to auto mode, the CV output starts off from the set output or tieback value and no ‘bump’ in output value occurs.
The PID instruction can also automatically provide a bumpless transfer from manual to auto even if integral control is not used (that is Ki = 0). In this case, the instruction modifies the .BIAS term to make the CV output track either the set output or tieback values. When automatic control is resumed, the .BIAS term maintains its last value. Disable back-calculation of the .BIAS term by setting the .NOBC bit in the PID data structure. If you set .NOBC true, the PID instruction no longer provides a bumpless transfer from manual to auto when integral control is not used.
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