Cascade control involves a control loop within a control loop. It uses a secondary feedback loop to monitor a process variable that affects the primary process variable being controlled. This helps the primary controller respond to disturbances more quickly before they impact the primary process variable. Examples given include using air temperature to control room temperature more quickly, and using feed flow rate to control liquid level in a tank before pressure changes affect the level.

1. Basics of Cascade Control
EMEC125 – Lab 02

2. Basic Feedback Control
• Feedback control incorporates feedback that assists in controlling a
process.
Controller
Measurement
OutputSet Point +
-
Disturbance

3. Basic Concepts TC
1
Large Room
Steam
Condensate
Air at variable
temperature and
flow rate
TT
1Cold air return
Sun
• Only heating is needed.
• Heat provided by steam
radiator and forced air.
• Temperature xmitter is a
conventional thermostat.
• Conventional feedback.
• Disturbances:
• Inlet air temperature.
• Flow rate of steam.
• Flow rate of air in the
room.
• Variations of heat
load in the room.
• External affects.
• Temperature of the
radiator.

4. Basic Feedback Cascade Control
• Cascade control is a control loop within a control loop.
http://controlguru.com/wp-content/uploads/2015/08//cas-blockdia-big.jpg http://controlguru.com/the-cascade-control-architecture/

5. Basic Concepts TC
1
Large Room
Steam
Condensate
Air at variable
temperature and
flow rate
TT
1Cold air return
Sun
TC
2
TT
2
• Only heating is needed.
• Heat provided by steam
radiator and forced air.
• Temperature xmitter is a
conventional thermostat.
• Cascade control
secondary loop monitors
the incoming air
temperture.
• Disturbances:
• Inlet air temperature.
• Flow rate of steam.
• Flow rate of air in the
room.
• Variations of heat
load in the room.

6. Simple Level Feedback System
Column under
variable pressure
Feed surge tank
LT
1
LC
1
Varying flows
I
P
• Several streams of liquid
flowing into the surge
tank.
• Surge tank is feed into a
distillation column.
• Simple feedback system
senses tank level and
adjusts valve opening.
• Disturbances can include:
• Variations in inlet
flow.
• Changes in pressure
in the distillation
column.
• Changes in distillation
pressure cause
pressure drop across
the valve which
affects flow.

7. Cascade Level Feedback System
Column under
variable pressure
Feed surge tank
LT
1
LC
1
Varying flows
I
P
FT
2
FC
2
• Several streams of liquid
flowing into the surge
tank.
• Surge tank is feed into a
distillation column.
• Monitors feed flow rate
and can respond to
changes in pressure
without waiting for level
changes
• The flow is providing a
set point for the valve
and can modify the flow
rate before the level
change in the surge tank
is detected.