24
450.2
500.0
Master
Set Point
(MSPt)
Master
Process
Variable
(MPV)
70
OP1
1997
2000
SLV
70
OP1
MST
Slave
Set Point
(SSPt)
Slave
Process
Variable
(SPV)
•1
•1
…2
OPERATOR LEVEL
2.8
Cascade with Feedforward (Template 13)
For cascade control, two internally-linked PID controllers are used, with the first (master) PID
controller providing the set point for the second (slave) controller. The feedforward disturbance
variable signal is added to the master output (slave set point). The disturbance signal is weighted by
the feedforward gain (FFGn) and the feedforward bias (FFbs) values.
Master Process Variable (MPV)
Master Control Set Point (MSPt)
['SPLO' to 'SPHI' – see Section 4.5]
Adjustable in Local control only
Slave Control Output
[0 to 100% (–10 to 110% for analog outputs)]
Adjustable in Manual mode only.
Slave Process Variable (SPV)
Slave Set Point (SSPt)
['SPLO' to 'SPHI' – see Section 4.5]
Adjustable in Manual mode only
Note. The
L
L
R
key can be used in this frame to
change between cascade and local slave set points.
Continued…
•1
With the Ramping Set Point function enabled (see Section 3.3, Set Points/ Ramp Rate), the
bargraph shows the actual (ramping) set point value and the digital display shows the target set point
value.
MSPt
I/P1
I/P2
OP1
Local Set Point
Master PV Input
Slave PV Input
Local Slave
Set Point
Slave
PID
Control Loop
MPV
LSPt
SSPt
I/P3
Feedforward Disturbance Variable
M.OP x
CrtO
+
CbiA
Master
PID
Control Loop
∑
DV x
FFGn
+
FFbS
Manual
Output
SSPt
L
L
R
FFGn
=
0
SPV
Fig. 2.7 Cascade Controller with Feedforward