CPU Modules CPU001 and CPU002
October 2005 GFK-1536P
6
PID Function Parameter Block
In chapter 14 of the VersaMax PLC User’s Manual, GFK-1503C, the
table showing the Parameter Block for the PID function should be
modified as described below.
For Address +12 (Config Word), the three rightmost columns are
now:
Low
Bit
Units
Range Description
Low 6
bits
used
The low 6 bits of this word are used to
modify six default PID settings. The 10 high-
order bits should be set to 0.
Set the low bit (bit 0) to 1 to modify the
standard PID Error Term from the normal
(SP – PV) to (PV – SP), reversing the sign of
the feedback term. This is for Reverse
Acting controls where the CV must go down
when the PV goes up.
Set the second bit (bit 1) to 1 to invert the
Output Polarity so that CV is the negative of
the PID output rather than the normal
positive value.
Set the third bit (bit 2) to 1 to modify the
Derivative Action from using the normal
change in the Error term to the change in the
PV feedback term.
Set the fourth bit (bit 3) to activate deadband
processing.
Set the fifth bit (bit 4) to activate anti-reset-
windup action.
Set the sixth bit (bit 5) to activate derivative
filtering.
The low 6 bits in the Config Word are
defined in detail below:
Bit 0: Error
Term +/-
When bit 0 is 0,
Error Term = SP - PV.
When bit 0 is 1,
eError tTerm = PV - SP.
Bit 1:
Output
Polarity
When bit 1 is 0, the CV output represents
the output of the PID calculation. When bit 1
is set to 1, the CV output represents the
negative of output of the PID calculation.
Bit 2:
Derivative
action on
PV
When bit 2 is 0, the derivative action is
applied to the error term. When bit 2 is 1, the
derivative action is applied to PV.
(table continued)…
Bit 3:
Deadband
action
When bit 3 is 0, no deadband action
occurs. If the error is within the deadband
limits, then the error term is set to zero.
Otherwise the error term is not affected by
the deadband limits.
If bit 3 is 1, deadband action occurs. If the
error term is within the deadband limits,
then the error is forced to zero. If,
however, the error term is outside the
deadband limits, then the error term is
reduced by the deadband limit: Error Term
= Error Term - deadband limit.
A similar adjustment occurs when the error
term is less than the lower deadband limit.
Bit 4: Anti-
reset-
windup
action
When bit 4 is 0, the anti-reset- windup
action uses a reset back-calculation. When
the output is clamped, this replaces the
accumulated Y remainder value with
whatever value is necessary to produce the
clamped output exactly.
When bit 4 is 1, the accumulated Y term is
replaced by the value of the Y term at the
start of the calculation. In this way, the pre-
clamp Y value is held as long as the output
is clamped.
Bit 5:
Derivative
Term filter
When bit 5 is 0, no derivative term filtering
occurs.
When bit 5 is 1, the rate of change of the
Derivative Term is limited. This improves
PID loop stability by reducing the effects of
random variations (noise) and step
changes in the Set Point and Process
Variable inputs.
Bit 0 – Bit
5:
Remember that the bits are set in powers
of 2.
To set bit 0, add 1 to the Config Word
parameter value.
To set bit 1, add 2.
To set bit 2, add 4.
To set bit 3, add 8.
To set bit 4, add 16.
To set bit 5, add 32.
For example, set Config Word to 0 for
default PID configuration. Add 1 to change
the Error Term from SP - PV to PV - SP,
add 2 to change the Output Polarity from
CV = PID Output to CV = – PID Output, or
add 4 to change Derivative Action from
Error rate of change to PV rate of change,
etc.
To Previous Page