Pid Operation Block Diagram And Operation Expressions - Mitsubishi QCPU Programming Manual

3. PID CONTROL SPECIFICATIONS

3.1.2 PID operation block diagram and operation expressions

Set value
Name
EV
Forward
D
operation
MV
Process
value
differentiation
EV
Incomplete
derivative Reverse
D
operation
MV
(1) The PID operation block diagram for incomplete derivative is shown below.
SV +
1
1
+
T s
- I
(P)
(I)
T S
D
1
+(T /K ) s
D D
(D)
(2) The operation expressions for PID control using PID control instructions are
indicated below.
Operation Expressions
=PV *-SV
n fn
T
S
MV=K {(EV -EV )+ EV
p n n-1 n
T
I
T
D
= (PV -2PV +PV
n fn fn-1
T
D
T +
S
K
D
= MV
n
=SV-PV *
n fn
T
S
MV=K {(EV -EV )+ EV
p n n-1 n
T
I
T
D
= (-PV +2PV -PV
n fn fn-1
T
D
T +
S
K
D
= MV
n
POINT
(1) *:PV is calculated using the following expression.
fn
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
PV
n
PV
fn-1
(2) PV is stored in the I/O data area. (See Section 5.2)
fn
+
-
+D }
n T
D
K
D
)+
D
fn-2
n-1
T
D
T +
S
K
D
+D }
T
n
D
K
D
)+
D
fn-2 n-1
T
D
T +
S
K
D
= PV
fn
: Process value of the present sampling cycle
: Filter coefficient
: Process value of the preceding sampling cycle (after filtering)
3 - 2
MELSEC-Q/QnA
Disturbance W
MV +
Kp
Manipulated
Gain Control
value
objective
Meanings of Symbols
EV : Deviation in the present sampling cycle
n
EV : Deviation in the preceding sampling
n-1
cycle
SV : Set value
PV : Process value of the present sampling
fn
cycle (after filtering)
PV : Process value of the preceding
fn-1
sampling cycle (after filtering)
PV : Process value of the sampling cycle
fn-2
two cycles before (after filtering)
MV : Output change value
MV : Present manipulation value
n
D : Present derivative term
n
D : Derivative term of the preceding
n-1
sampling cycle
K : Proportional constant
P
T : Sampling cycle
S
T : Integral constant
I
T : Derivative constant
D
K : Derivative gain
D
+ (PV - -PV )
n fn 1 n
PV
P
Process
value
+ +
V
Detected
noise