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PLC User's Manual - Analog Control Edition
PID Instruction (FNC 88)
1.2 Basic Operation Expressions in PID Instruction (Reference)
1.2 Basic Operation Expressions in PID Instruction (Reference)
The PID instruction executes using the speed type or measured value differential type operation expression.
According to the contents of +1, bit 0 (operation setting (ACT)) specified by in the PID control,
either for forward operation or backward operation is executed.
Each value required in the operation is specified by a corresponding parameter or later.
1. Basic operation expression for PID control
1) Symbols
EVn : Deviation in sampling at this time Dn : Differential term at this time
EVn
-1 : Deviation in previous cycle Dn-1 : Differential term in previous cycle
SV : Target value K
P : Proportional gain
PVnf : Measured value in sampling at this time (after filter) T
S : Sampling cycle
PVnf
-1 : Measured value in previous cycle (after filter) TI : Integral constant
PVnf
-2 : Measured value in two cycles before (after filter) TD : Differential constant
ΔMV : Output variation K
D
: Differential gain
MVn : Operation quantity at this time
2) Expression for calculating the measured value (after the filter) in sampling at this time (PVnf)
The value "PVnf" is obtained from the following expression based on the read measured value.
Measured value after filter: PVnf = PVn+L(PVnf
-1-PVn)
PVn : Measured value in sampling at this time
L : Filter coefficient
PVnf
-1 : Measured value in previous cycle (after filter)
Operation
direction (ACT)
+1, b0
PID operation expression
Forward operation
(OFF)
Backward
operation (ON)
S
3
S
3
S
3
S
3
S
3
Δ
MV = K
P
{(EVn
−
EVn
-1
) + EVn + Dn}
EVn = PVn
f
-SV
Dn = (
−
2PVn
f
−
1
+ PVn
f
+ PVn
f
−
2
) +
Dn-
1
MVn =
ΣΔ
MV
T
S
T
I
T
D
T
S
+ K
D
T
D
K
D
T
D
T
S
+ K
D
T
D
Δ
MV = K
P
{(EVn
−
EVn
−
1
) + EVn + Dn}
EVn = SV
−
PVn
f
Dn = (2PVn
f
−
1
−
PVn
f
−
PVn
f
−
2
) +
Dn-
1
MVn =
ΣΔ
MV
T
S
T
I
T
D
T
S
+ K
D
T
D
K
D
T
D
T
S
+ K
D
T
D