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FX Series Programmable Controllers Points Of Technique 10
10-25
Method 2
Durin
g
manual operations the PID instruction is kept runnin
g
but the calculated MV is i
g
nored;
instead the operator controls MV. In order to prevent the PID instruction from runnin
g
out of
control the MV value set b
y
the operator should be fed in to the MV
n-1
re
g
isters of the PID data
block in the same wa
y
as for MV ran
g
e control earlier (i.e. Set S
3
+18, S
3
+19 to MV x 100).
When switchin
g
back to PID control the internal values of the PID instruction are alread
y
set
and full PID operation starts immediatel
y
.
10.15.3 Using the PID alarm signals
Included as part of the data block there are four alarm values. These set the maximum positive
and ne
g
ative chan
g
e that should occur to MV and PV. The PID alarm si
g
nals are used to warn
of the s
y
stem
g
oin
g
out of control.
When the s
y
stem is startin
g
from cold it is usuall
y
not
g
ood to include the Derivative numbers
of the in the calculation; the chan
g
es to PV are lar
g
e and the Derivative introduces too much
correction. Also, if the s
y
stem starts to move rapidl
y
awa
y
from the SV then sometimes the use
of D can over correct and cause chasin
g
.
B
y
havin
g
an 'alarm' fla
g
for the chan
g
e in PV and MV it is possible to monitor the state of the
s
y
stem and ad
j
ust the PID parameters to appropriate settin
g
s.
When the s
y
stem is close to the SP the chan
g
es in PV (and MV) should be minimal.
In this situation usin
g
full PID is ver
y
useful in keepin
g
the s
y
stem close to the SP. (Full PID is
appropriate).
However, if the conditions chan
g
e (e.
g
. openin
g
a refri
g
erator door, addin
g
in
g
redients to a
mixture, cold start, etc.) the s
y
stem reacts. In some cases (especiall
y
cold start) the reaction is
too much for the D to be useful (PI or sometimes
j
ust P onl
y
is better). In these cases the alarm
fla
g
s can be used to chan
g
e to PI control until the s
y
stem returns to a more stable condition,
when full PID can then be used.
Basicall
y
, rather than use actual values of the PV to determine the chan
g
e over point from PI to
PID (or PID to PI), use the size of the chan
g
e in PV (or MV). This means chan
g
es to the Set
Point do not require different ran
g
es for the PI - PID chan
g
e over point (at least, in theor
y
).
10.15.4 Other tips for PID programming
• It is recommended that an input value for PV is read before the PID is activated. Otherwise,
the PID will see a bi
g
chan
g
e from 0 to the first value and calculate as if a bi
g
error is
occurrin
g
.
• The PID instruction is not interrupt processed. It is scan dependent and as such the
samplin
g
can not occur faster the FX scan time. It is recommended that T
S
is set to a
multiple of the pro
g
ram scan time.
• To keep timin
g
errors to a minimum it is recommended that constant scan is used.
• To improve samplin
g
rates it is possible to put the PID instruction inside a timer interrupt
routine.
• It is better to have the PID onl
y
perform P until the input value (PV) reaches the workin
g
ran
g
e.
• When settin
g
up it is a
g
ood idea to monitor the input and output of the PID instruction and
check that the
y
are about the expected values.
• If the PID s
y
stem is not operatin
g
properl
y
check the error fla
g
s for PID errors (D8067).