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FX Series Programmable Controllers Points Of Technique 10
10-27
10.17 Pre-tuning operation
10.17.1 Variable Constants
The Pre-tunin
g
operation can be used to automaticall
y
set values for the followin
g
variables:
- The direction of the process; Forward or Reverse (S
3
+1, bit 0)
- The proportional
g
ain constant; K
P
(S
3
+3)
- The inte
g
ral time constant; T
I
(S
3
+4)
- The derivative time constant; T
D
(S
3
+6)
Settin
g
bit 4 of S
3
+1 starts the pre-tunin
g
process. Before startin
g
, set all values that are not
set b
y
the pre-tunin
g
operation: the sample time, Ts (S
3
+0); the input filter
α
(S
3
+2); the
Derivative
g
ain, K
D
(S
3
+5); the Set Point, SV (S
1
); and an
y
alarm or limit values, (S
3
+20-23).
The Pre-tunin
g
operation measures how fast the s
y
stem will correct itself when in error.
Because the P, I, and D equations all react with differin
g
speed, the initial error must be lar
g
e
so that effective calculations can be made for each t
y
pe of equation. The difference in values
between SP and PV
nf
must be a minimum of 150 for the Pre-tunin
g
to operate effectivel
y
. If this
is not the case, then please chan
g
e SV to a suitable value for the purpose of pre-tunin
g
.
The s
y
stem keeps the output value (MV) at the initial value, monitorin
g
the process value until
it reaches one third of the wa
y
to the Set Point. At this point the pre-tunin
g
fla
g
(bit 4) is reset
and normal PID operation resumes. SV can be returned to the normal settin
g
without turnin
g
the PID command Off.
Durin
g
the course of normal operation, the Pre-tunin
g
will NOT automatically set new values if
the SV is chan
g
ed. The PID command must be turned Off, and the Pre-Tunin
g
function
restarted if it is necessary to use the Pre-tune function to calculate new values.
• Caution: The Pre-tunin
g
can be used as man
y
times as necessar
y
. Because the fla
g
resets,
the set bit can be turned On a
g
ain and new values will be calculated. If the s
y
stem is
runnin
g
an oven heater and the SV is reduced from 250 to 200 C, the temperature must
drop below 200 or the “Forward/Reverse” fla
g
will be set in the wron
g
direction. In addition,
the s
y
stem error value must be lar
g
e for the pre-tune variable calculations to work correctl
y
.
• Note: Set the samplin
g
time to
g
reater than 1 second (1000 ms) durin
g
the pre-tunin
g
operation. It is recommended that the samplin
g
time is
g
enerall
y
set to a value much
g
reater than the pro
g
ram scan time.
• Note: The s
y
stem should be in a stable condition before startin
g
the pre-tunin
g
operation.
An unstable s
y
stem can cause the Pre-tunin
g
operation to produce invalid results. (e.
g
.
openin
g
a refri
g
erator door, addin
g
in
g
redients to a mixture, cold start, etc.)
• Note: Even thou
g
h Pre-tunin
g
can set the above mentioned variables, additional lo
g
ic ma
y
be needed in the pro
g
ram to "scale" all operatin
g
values to those capable of bein
g
processed b
y
the special function devices bein
g
used.