• PID operations are conducted as follows.
Item
Deviation (DVn)
Output variation (MV)
Bn
Kp: Gain
Ti: Integral time
Td: Derivative time
Md: Derivative gain
CT: Control cycle
DVn: Deviation
DVn-1: Last deviation value
PVn: Process variable
PVn-1: Last process variable
PVn-2: Process variable before the last value
SVn: Engineering value conversion processing result
The integral term and derivative term are as follows under the following conditions.
Item
Condition
Derivative
When Td = 0
term
When the control mode is MAN
When the control mode is CMV
Integral
When Ti = 0
term
When either of MH or ML error has occurred, MVP > MH and the
following expression is satisfied
CT
×DVn>0
Ti
When either of MH or ML error has occurred, MVP < ML and the
following expression is satisfied
CT
×DVn<0
Ti
Ti: Integral time
CT: Control cycle
DVn: Deviation
MH: Output high limit value
ML: Output low limit value
MVP: MV Internal operation value
Set an integral multiple of the execution cycle (T) as a control cycle (CT).
Set 0.0 or a value equal to or larger than the control cycle (CT) as an integral constant.
PID operations of this tag access FB are performed every control cycle (CT) (MV output).
In other execution cycles (T), the last value is held (MV = 0).
■Engineering value conversion
This function block converts the setting value (%) from the primary loop in the CAS or CSV mode into an engineering value.
RH-RL
SV=
×
Setting value (%) from the primary loop + RL
100
RH: Engineering value high limit, RL: Engineering value low limit, SV: Setting value
Direct action
DVn = PVn - SVn
CT
ΔMV = Kp × { (DV
- DV
) +
n
n-1
Ti
Proportional
Gain
The following shows a proportional term, integral term, and derivative term of MV.
■Proportional term
MV = Kp (DVn - DVn-1)
■Integral term
CT
ΔMV=Kp×
×DVn
Ti
■Derivative term
MV = Kp Bn
Md×Td
B
=B
+
×
n
n-1
Md×CT+Td
CT×B
n-1
{(PV
-2PV
+PV
)-
n
n-1
n-2
Td
10.4 Velocity Type PID Control (Disable Tracking for primary loop) (M+P_PID)
Reverse action
DVn = SVn - PVn
× DV
+ B
}
n
n
Derivative
Integral
(imperfect derivative)
B
=B
+
n
n-1
Md×CT+Td
{-(PV
}
n
Processing
Bn = 0
CT
×DV
=0
n
Ti
10 LOOP CONTROL OPERATION
Md×Td
×
CT×B
n-1
-2PV
+PV
)-
}
n-1
n-2
Td
10
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