3 fundamental type of controller
TRANSCRIPT
Regulatory and servo control
• For process that the controlled variable deviates from set
point because of disturbance – regulatory control
• For process that the most important disturbance is the set
point itself – servo control
Terminology
Controlled
variable
Sym: c, PV
“The parameters
that indicate
product quality or
the operating
condition of the
process.” (L.M.
Gordon)
Manipulated
variable
Sym: m, CO, MV
A process being
adjusted (controlled)
by a final control
element (such as
control valve,
damper, motor
speed).
Load variable
Sym:
“All other
affecting
variable, other
than the one
being
manipulated.”
(L.M. Gordon)
Set point
Sym: r, SP, SV
The desired
operating
condition.
TC
TT
controlled variable
(temperature of
sterilised saline)Load variable
Manipulated variable
(saturated steam)
(Flow, temperature
of saline solution)Uncontrolled variable
(Saturated steam)
Heat exchanger
30°C
100 L/min
95°C
80°C
I/P
100°C
Terminology
© Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)
• Load variable = set of all other variables that affect the CV to deviate from SP. Any one of the load variables may be made into MV.
= DV (Seborg page 263)
• Process Load = the set of all load variables that cause a change or deviations of CV from SP. It excludes CV
• Do you still remember the sentence :
“Change of process loading of load variable”?
- you are actually changing MV
Control strategies
TC
TT
controlled variable
(temperature of
sterilised saline)
What is feedback control?A method of control that the action taken is
based on past result.
Merit?Simple design. Easy to tune (for linear, fast &
short deadtime processes: flow, level &
pressure).
Demerits?Slow recovery for slow process such as
temperature control upon load disturbance or
change in setpoint. A process with long deadtime
may exhibit oscillatory process response with
long settling time.
Load variable
Manipulated variable
(saturated steam)
(Flow of saline solution)
Manipulated variable
(Saturated steam)
Heat exchanger
30°C
100 L/min
95°C
80°C
The ProcessSaline solution at 30°C is sterilized by
heating the solution in a heat-exchanger
system to 80°C. The temperature of the
sterilized saline solution is maintained by
regulating the control valve of steam input
to the heat-exchanger.
Feedback control I/P
100°C
© Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)
Explanation:
1. With feedback control, a process deviation occurring near
the beginning of the process will not be recognized until the
process output.
2.The feedback control will then have to adjust the process
inputs in order to correct this deviation.
3.This results in the possibility of substantial deviation
throughout the entire process.
Demerits?Slow recovery for slow process such as temperature control
upon load disturbance or change in setpoint. A process with
long dead time may exhibit oscillatory process response with
long settling time.
4.The system could possibly miss process output disturbance
and the error could continue without adjustment.
5.Generally, feedback controllers only take input from one
sensor.
6.This may be inefficient if there is a more direct way to control
a system using multiple sensors.
7.Operator intervention is generally required when a feedback
controller proves unable to maintain stable closed-loop control.
8.Because the control responds to the perturbation after its
occurrence, perfect control of the system is theoretically
impossible.
MERITS
• Corrective action occurs as soon as the controlled
variable deviates from the set point, regardless of
the source and type of disturbance.
• reduces the sensitivity of the controlled variable to
unmeasured disturbances and process changes.
• Simple design
- do not require detailed knowledge of the system and, in
particular,
- do not require a mathematical model of the process,
- can be easily duplicated from one system to another.
DEMERITS
• no corrective action is taken until after the disturbance has
upset the process, that is, until after the controlled variable
deviates from the set point.
DEMERITS
• It does not provide predictive control action to
compensate for the effects of known or measurable
disturbances.
• It may not be satisfactory for processes with large
time constants and/or long time delays. If large
and frequent disturbances occur, the process may
operate continuously in a transient state and
never attain the desired steady state.
• In some situations, the controlled variable cannot be
measured on-line, so feedback control is not feasible
• Feedback control
Feedback controller
Process
RC
D
BLOCK DIAGRAM
M
R = set point D = disturbanceM= manipulated variable C = controlled variable
Control strategies
Feedforward control
30°C
100 L/min
95°C
80°C
I/P FFC
What is feedforward control?A method of control based on process model:
mass and energy balances.
Merit?Faster recovery than cascade or SISO.
Compensate for disturbances before they effect
the controlled variable.
Demerits? Cannot work if any of the sensors fail esp. load
variable.
Requires process model in order to design
feedforward system ---that’s why PV@CV is not
measured
Characteristic of feedforward
control:• Measurements of load variables online
(disturbance variable is measured, but the
controlled variable is not.)
• Utilization of computational blocks.
Heat exchangerTT
12 TTWH
CW p
s
p
s
© Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)
controlled variable
(temperature of
sterilised saline)
Load variable
Manipulated variable
(saturated steam)
(Flow of saline solution)
Manipulated variable
(Saturated steam)
FT
Demerits? Cannot work if any of the sensors fail esp. load variable.
Explanation:
In the house example, a feed-forward system may measure
the fact that the door is opened and automatically turn on the
heater before the house can get too cold.
For instance, if a window was opened that was not being
measured, the feed-forward-controlled thermostat might still
let the house cool down.
MERITS
• corrective action is taken before the controlled variable
deviates from the set point.
- Ideally, the corrective action will cancel the effects of the
disturbance so that the is not affected by the disturbance.
DEMERITS
• the disturbance variable must be measured (or accurately
estimated). In many application, this is not feasible.
• no corrective action is taken for unmeasured disturbances
• Ideal feedforward controllers that are theoretically capable
of achieving perfect control may not be physically
realizable. Fortunately, practical approximations of these
ideal controllers can provide very effective control
• Feedforward control
Feedforward controller
ProcessR C
D
M
R = set point D = disturbanceM= manipulated variable C = controlled variable
Control strategies
Cascade control
30°C
100 L/min
95°C
80°C
I/P FC
100°C
What is cascade control?A method of control of which the inner loop
controller obtains its setpoint from an outer loop
controller.
Merit?Faster recovery time as compared to SISO
feedback control.
Demerits?Controllability will be worst than SISO if both
controllers were not properly tuned.
Characteristic of cacade control:Two transmitters, two controllers and one
control valve (2-level cascade).
Application: Transform from SISO to
cascade strategy if faster recovery time is
desired.
Heat exchangerFT
TC
TT
© Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)
controlled variable
(temperature of
sterilised saline)
Load variable
Manipulated variable
(saturated steam)
(Flow of saline solution)
Manipulated variable
(Saturated steam)
TSP
FSP
The cascade control loop structure has two distinguishing features:
1. The output signal of the master controller serves
as the set point for the slave controller.
2. The two feedback control loops are nested, with
the secondary control loop (for the slave controller)
located inside the primary control loop (for the
master controller)Master controller
Slave
controller
Primary @Outer@
@Master Control Loop
Secondary @
Inner@Slave
Control Loop
Control strategies
Feedforward/Feedback control
30°C
100 L/min
95°C
80°C
I/P
Heat exchangerTT
12 TTWH
CW p
s
p
s
TC
TT
© Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)
controlled variable
(temperature of sterilised saline)
Load variable
Manipulated variable
(saturated steam)
(Flow of saline solution)
Manipulated variable
(Saturated steam)
FT
What is feedforward/feedback control?Feedback compensation added to feedforwardcontrol
Merit?Feedforward control compensates for majordisturbances, while feedback control compensatesfor all other disturbances.
FFC+
+
Control strategies
Ratio control
Wild flow 30°C
100 L/min
95°C
80°C
I/P FC
What is ratio control?Keeps a flowrate in proportion to another
flowrate.
Merit?Keep two flow in proportion.
Demerits?Inherits PID characteristics
Heat exchanger
FT
FT
X
© Abdul Aziz Ishak, Universiti Teknologi MARA Malaysia (2009)
controlled variable
(temperature of
sterilised saline)
Manipulated variable
(saturated steam)
(Flow of saline solution)
Manipulated variable
(Saturated steam)
Ratio Control
• Type of feedforward control
• The objective is to maintain the ratio of two
variables at a specified value
• One variable is manipulated to keep it as ratio of
another.
• Implemented in two basic schemes.
multiplier
divider
•Measuring the flow in steam A
•Multiplying it with the desired ratio
to obtain the required flow rate in
stream B (set point to the flow
controller of stream B)
•FBset = R x FA
•If the flow of stream A varies, the
set point of the controller of stream
B will vary.
Multiplier : set the ratio
The flow of stream B must vary, as the flow rate of
stream A is varies
(Wild flow)
•Measuring both stream
•Divide them in FY16 to obtain the
actual ratio flowing through the
system
•R=FB/FA
•The calculated ratio is the sent to
the controller which manipulated
the flow B to maintain set point.
•The set point to this controller is
the required ratio.
Divider
• Feedback control
Feedback
controllerProcess
Y(s)Y
D
•Feedforward control
Feedforward
controllerProcess
Y(s) Y
D
•Cascade control
Primary
controllerProcess II
Y(s)Y
Process ISecondary
controller
BLOCK DIAGRAM
• Self – exercise
- How to ensure that the level in the tank remains at or near the set
point? Propose the following control strategy:
- Feedback
- Feedforward
- Feedforward/Feedback
Self - exercise
• The objective of the following figure is to control the
temperature in the tank. Draw the block diagram for the
following feedfoward control.