FX Series Programmable Controlers Applied Instructions 5
5-102
5.9.8 PID (FNC 88)
Operation:
This instruction takes a current value (S
2
) and
compares it to a predefined set value (S
1
). The
difference or error between the two values is then
processed throu
h a PID loop to produce a
correction factor which also takes into account previous iterations and trends of the calculated
error. The PID process calculates a correction factor which is applied to the current output
value and stored as a corrected output value in destination device (D). The setup parameters
for the PID control loop are stored in 25 consecutive data re
isters S
3
+0 throu
h S
3
+24.
Points to note:
a) Ever
PID application is different. There will be a certain amount of “trial and error”
necessar
to set the variables at optimal levels.
b) The PID instruction is
only
available on FX and FX
2C
Main Processin
Units fitted with
CPU
versions 3.11 or greater
.
c) On FX
2N
MPUs a Pre-tunin
feature is available that can quickl
provide initial values for the
PID process. Refer to pa
e 10-28 for more details.
d) As 25 data re
ister are required for the setup parameters for the PID loop, the head address
of this data stack cannot be
reater than D975. The contents of this data stack are
explained later in this section. Multiple PID instructions can be pro
rammed, however
each PID loop must not have conflictin
data re
isters.
e) There are control limits in the PLC intended to help the PID controlled machines operate in
a safe manner. If it becomes necessar
to reset the Set Point Value (S
1
) durin
operation,
it is recommended to turn the PID command Off and restore the command after enterin
the
new Set Point Value. This will prevent the safet
control limits from stoppin
the operation
of the PID instruction prematurel
.
f) The PID instruction has a special set of error codes associated with it. Errors are identified
in the normal manner. The error codes associated with the PID loop will be fla
ed b
M8067 with the appropriate error code bein
stored in D8067. These error devices are not
exclusive to the PID instruction so care should be taken to investi
ate errors properl
.
Please see chapter 6, ‘Dia
nostic Devices’ for more information.
) A full PID iteration does not have to be performed. B
manipulation of the setup parameters
P (proportional), I (Inte
ral) or D (derivative) loops ma
be accessed individuall
or in a user
defined/selected
roup. This is detailed later in this section.
Mnemonic Function
Operands
Program steps
S
1
S
2
S
3
D
PID
FNC 88
(PID
control loop)
re
ister each
Receives a data
input and
calculates a
corrective action
to a specified
level based on
PID control
D
Note: S
1
and S
2
use a sin
le
data re
ister
D
Note: S
3
uses 25
consecutive
data re
isters
D
Note: D
uses a sin
le
data re
ister
PID:
9 steps
X10
D 19D18PID
[ S1 ] [ S2 ] [ D ]
D 20 D 46
[ S3 ]
