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MELSEC-Q/QnA
3. PID CONTROL SPECIFICATIONS
3.2.2 PID operation block diagram and operation expressions
(1) The PID operation block diagram for complete derivative is shown below.
1
1
T
I S
(P)
(I)
(D)
W
Kp
P
PV
V
SV +
-
Disturbance
Set value
+
+
-
Gain
Manipulated
value
+
MV
Control
objective
Process
value
Detected
noise
+ +
T
D S
+
(2) The operation expressions for PID operation using PID control instructions are
indicated below.
Name
Operation Expressions
Meanings of Symbols
Forward
operation
EV
n
=PV
fn
*-SV
MV=K
p
{(EV
n
-EV
n-1
)+ EV
n
+D
n
}
T
S
T
I
MV
n
= MV
T
D
T
S
D
n
= (PV
fn
-2PV
fn-1
+PV
fn-2
)
Process
value
differentiation
Complete
derivative
Reverse
operation
EV
n
=SV-PV
fn
*
MV
n
= MV
MV=K
p
{(EV
n
-EV
n-1
)+ EV
n
+D
n
}
T
S
T
I
T
D
T
S
D
n
= (-PV
fn
+2PV
fn-1
-PV
fn-2
)
EV
n
: Deviation in the present sampling cycle
EV
n-1
: Deviation in the preceding sampling cycle
SV
: Set value
PV
fn
: Process value of the present sampling cycle
(after filtering)
PV
fn-1
: Process value of the preceding sampling
cycle (after filtering)
PV
fn-2
: Process value of the sampling cycle two
cycles before (after filtering)
MV : Output change value
MV
n
: Present manipulation value
D
n
: Present derivative term
K
P
:
Proportional
constant
T
S
:
Sampling
cycle
T
I
:
Integral
constant
T
D
:
Derivative
constant
POINT
(1) *:PV
fn
is calculated using the following expression.
Therefore, it is the same as the PV (process value) of the input data as long
as the filter coefficient is not set for the input data.
Process Value after Filtering PV
fn
= PV
n
+ (PV
fn
-
1
-PV
n
)
PV
n
: Process value of the present sampling cycle
: Filter coefficient
PV
fn-1
: Process value of the preceding sampling cycle (after filtering)
(2) PV
fn
is stored in the I/O data area. (See Section 5.2)
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