Pid Instruction Timing - Allen-Bradley Logix5000 Reference Manual

Special Instructions (FBC, DDT, DTR, PID) Chapter 13
The PID instruction also 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 K = 0). In this case
i
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 will maintain its last value. You can disable back-calculation of the
.BIAS term by setting the .NOBC bit in the PID data structure. Be aware that
if you set .NOBC true, the PID instruction no longer provides a bumpless
transfer from manual to auto when integral control is not used.

PID Instruction Timing

The PID instruction and the sampling of the process variable need to be
updated at a periodic rate. This update time is related to the physical process
you are controlling. For very slow loops, such as temperature loops, an update
time of once per second or even longer is usually sufficient to obtain good
control. Somewhat faster loops, such as pressure or flow loops, may require an
update time such as once every 250 ms. Only rare cases, such as tension control
on an unwinder spool, require loop updates as fast as every 10 ms
or faster.
Because the PID instruction uses a time base in its calculation, you need to
synchronize execution of this instruction with sampling of the
process variable (PV).
The easiest way to execute the PID instruction is to put the PID instruction in
a periodic task. Set the loop update time (.UPD) equal to the periodic task rate
and make sure that the PID instruction is executed every scan of the
periodic task.
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011 521