Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 236)

EAF
Process Control Instructions
Chapter 16
16-25
The following ladder logic solution to the de-scaling equation assumes that
Smax
1
and Smin
1
are between 0-999. For values less than 0 or greater
than 999, additional math instructions would have to be programmed.
Rung 1 Subtraction of Smin1 from M1. In this case, the input is at
maximum, 550 o C.
Rung 2 This is the EAF math multiplication instruction necessary to
handle the possible 4 digit BCD answer, in this case, 4095.
Rung 2A Display branch. Displays the product. The four locations
must be stored consecutively starting with Store 5.
Example: Store 5 through Store 8 = 400 through 403.
Rung 2B Display of multiplier. You must allocate 4 consecutive words
even though only one is used. The unused words must
store zero.
Rung 2C Display of floating point multiplicand. Note implied decimal
point between store 10 and 11. You must include this branch.
The first two branches are for display only and can be
omitted. Note that 12 words must be allocated to use
this instruction.
Rung 3 BCD to Binary conversion. The 2 word value to be
converted is in store 5 and 6, the answer is in store 13.
Store 13 will be the PV for your PID instruction.
For systems requiring more than one de-scaled input, store this program in
a subroutine and call it as many times as it is needed.
Anti-Reset Windup
This is a feature that prevents the integral output term from becoming
excessive when the output reaches a limit. When the sum of the PID and
bias terms in the output reaches the limit, the instruction stops calculating
until the error changes sign causing the output to come back in range.
Bumpless Transfer (Word 02, Bit 11)
The ability to switch from manual to auto mode without causing a sudden
shift or “bump” in the Control Output. When this bit is set, the Bias Term
is adjusted to provide a bumpless transfer when the Integral Gain is zero
(i.e., when no integral control is used).
PID Glossary