Process Control BEE4343

1
Process Control (BEE4343)

• Chapter 6Application of Feedback Control

2
From last class

• Chapter 6 Application of Feedback Control
• Equipment Specification
• Accuracy
• Reproducibility
• Cost
• Input Processing
• Validity check
• Conversion for nonlinearity
• Engineering units
• Filtering
• Set point limits

3
Todays discussion

• Application of Feedback Control
• Equipment Specification
• Input Processing
• Feedback Control Algorithm
• Output Processing

This week
4
Simplified control loop drawing, showing
application topics
5
Recap on Equipment Specification

• Why sensors are selected for specification
• To provide and indication of true controlled
  variable
• How sensors are selected
• Based on
• Accuracy
• Reproducibility
• Cost

6
Recap on Accuracy Reproducibility

• Poor reproducibility is OK for accurate sensor
• Solution for both accuracy reproducibility is
  in Cascade Feed-forward control

7
Recap on Input Processing

• Subtopics under input processing
• Validity check
• Current signals 4-20mA
• Conversion for Nonlinearity
• Convert signal to a better measure of the actual
  process variable
• Engineering Units
• Simplify the analysis of data by operations
  personnel
• CV Z R(S3-S30)
• Filtering
• Set-point limits
• Prevent an incorrect value being introduced
• Inadvertently by the operator
• By poor control of a primary in a cascade control
  strategy

8
Recap on Filtering
9
Recap on Filtering
10
Recap on Filtering
11
Recap on Filtering
12
Block diagram of the control system with filter
filter
Closed loop transfer function with filter
13
First Order Transfer Lag
with
CVf(s) value after the filter CVm(s)
measured value before the filter tf
filter time constant
14
Guidelines for reducing the effects of noise
15
Simplified control loop drawing, showing
application topics
From last class
This week
16
6.3 Feedback Control Algorithm
17
(No Transcript)
18
Proportional Mode
Sense switch
19
Simple example of direct/reverse acting
A pneumatic direct acting valve will travel to
the open position as the air signal to the valve
increases.
A reverse acting valve will travel to the close
position as the air signal increases.
A direct acting valve(air to open) will fail
close.
A reverse acting valve(air to close) will fail
open
20
Proportional Mode
21
Proportional Mode
22
Proportional Mode
With (Kc)s dimensionless (scaled) controller
gain Kc(CVr/MVr) MVr range of manipulated
variable 100 for a control valve CVr range
of the sensor measuring the controlled variable
in engineering units
23
Proportional Mode
24
Integral Mode
25
Integral Mode
Effect of reset windup ? poor control performance
26
Reset Wind Up
Proportional plus reset controllers act to
eliminate the offset error found in proportional
control by continuing to change the output after
the proportional action is completed and by
returning the controlled variable to the
setpoint.
An  inherent  disadvantage  to  proportional
 plus  reset  controllers  is  the  possible
 adverse  effects caused  by  large  error
 signals. The  large  error  can  be  caused  by
 a  large  demand  deviation  or when initially
starting up the system.   This is a problem
because a large sustained error signal will
eventually cause the controller to drive to its
limit, and the result is called "reset windup."
Because of reset windup, this control mode is not
well-suited for processes that are frequently
shut down and started up.
27
Integral Mode
28
Integral Mode
Anti Reset Wind Up External Feedback
Block diagram of a PI control algorithm with
external feedback
When limitation is not active
When limitation active
constant
29
Integral Mode
In digital system
30
Derivative Mode

• Effects
• reduce amplification of noise
• retain some of the good control performance
  possible with derivative mode

31
6.4 Output Processing
32
(No Transcript)
33
Closure

• Feedback Control Algorithm
• P mode
• I mode
• D mode
• Output processing
• Bumpless transfer
• Fail closed vs Fail open

34
Coming Up next

• Chapter 7
• Cascade control