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