Feedback control makes use of a systems output to influence an input to the same system

1
Feedback control makes use of a systems output
to influence an input to the same system
REVISION

• Components of a feedback control system
• Final control element used for adjustments
  handles e.g valves,
• motor speed
• Sensor for measument (fast and accurate)
• Control algorithm How does the controller
  determines the control action e.g on/off
  controller, proportional controller
• Setpoint Desired outcome
• Assessing a system for feedback control
• necessary components?
• Is there a cause and effect relationship between
  the manipulated variable and the control variable

Manipulated variable Feed flow rate Control
variable Tank level
2
Q1

• Components
• final control element the valve on the pump
  discharge
• sensor the level float measuring device
• control algorithm the LC shown in the process
  flow diagram
• setpoint implicit with the presence of the
  controller.

cause and effect relationship? Yes, between the
manipulated variable (exit pipe valve position)
and the controlled variable ( tank level). The
tank level changes whenever the valve position is
adjusted. gt feedback control system.
3
Disturbances and changes in setpoint

• The level control system is likely to work well
  in general.
• Problem when a significant drop in the feed to
  the tank occurs
• - specifically, if the feed to the tank drops
  below the discharge from the pump on the left
  hand side. In this instance, closing the valve on
  the right hand side pump will not be sufficient
  to prevent the level from dropping. Eventually,
  the level will go to zero, and the pump on the
  left will be damaged.

4
Q2

• Components
• final control element the valve on the pump
  discharge
• sensor on the tank
• control algorithm in the LC
• setpoint implicit in the LC

• cause and effect relationship? NO
• The tank has an internal baffle, and the flow to
  the smaller compartment on the right occurs
  because of the height of liquid above the baffle
  (or weir) in the tank on the left.
• Thus, the level in the tank on the right is
  essentially independent of the flow out of the
  compartment on the right.
• Moreover, the level in the left hand compartment
  will be essentially level, since with the baffle,
  the tank will function as a constant overflow
  device.
• gt not a feedback control system.

5
Q3

• Components
• final control element valve
• sensor thermocouple contained in a thermowell
  in the pipe
• control algorithm in the temperature
  controller TC
• setpoint implicit in the TC

• Cause and effect relationship? NO
• changing the flowrate of this stream merely makes
  fluid of the same temperature pass by the
  thermocouple faster.
• feedforward control.
• gtnot a feedback control system.

6
Q4

• Components
• final control element the valve
• sensor TIc
• control algorithm in the TiC
• setpoint implicit in the TIC

Cooling medium

• cause and effect relationship?
• YES
• adjusting the recycle will alter the inlet
  temperature of the cooling medium, thereby
  changing the degree of heat transfer and hence
  the temperature of the contents in the tank.
• gt feedback control system.