Download proportional band

Process Control Methods
1
Open-Loop Control
• Process control operations are performed automatically by
either open-loop or closed-loop systems
• Processes controlled only by set-point commands without
feedback are open-loop
• Open-loop systems are used in applications where simple
processes are performed
• Open-loop systems are relatively inexpensive
2
Closed-Loop Control
• Closed-loop control systems
are more effective than openloop systems
• With the addition of a
feedback loop they become
self-regulating
• Components of a closedloop system include:
– The primary element sensor
– The controlled variable
– The measured variable
– The control signal
– The final correcting element
3
Process Behavior
• The objective of process control is to cause a
controlled variable to remain at a constant value at
or near some desired set-point
• The controlled variable changes because of:
– A disturbance appears
– Load demands vary
– Set points are adjusted
4
Process Behavior Example
• Flow through the pipe is the
process
• Fluid flow rate is the controlled
variable
• Valve position is the set point
• Demand for the fluid
downstream is the load
• Variance in upstream pressure
is the disturbance
5
Single-Variable Control Loop
• Several process variables are controlled at once in a typical
production machine
• Usually, only one individual feedback loop is required to control
each variable
• Single-variable control loops consist of the following elements:
–
–
–
–
Measuring device
Transducer/transmitter
Controller
Final Control Element
6
Response Time of the Instrument
• All instruments have a
time lag - the time from
when a variable is
measured till the
corrective action is
taken
• Factors affecting lag:
– Response time of sensor
– Time lag of transducer
– Distance the signal must
travel
– Time required for the
controller to process
– Distance control signal must
travel
– Time lag of the final
correcting element
7
Time Lag
• The controlled variable itself may contribute to lag
problems because of inertia in the variable
• Lag due to inertia of the variable is referred to as
pure lag
• Another factor that affects time lag is dead time this is the elapsed time from when the deviation
occurs and corrective action takes place
8
Illustration of Pure Lag
9
Selecting a Controller
• The controller mode selection is based upon the
requirements of the process and one of the following
control modes will be used:
–
–
–
–
On/Off
Proportional
Integral
Derivative
10
On-Off Control
•
•
•
•
Used for slow acting operations
where lag is unavoidable
Final correcting element is either
fully-on or fully-off
The primary drawback of on-off
control is the rapid switching of the
final control element
On-off differential or hysteresis is
programmed into the controller
• Deadband refers to the
differing levels at which a
controller switches on and off
11
Continuous Control
• On/Off control is acceptable for process where the variable is
set between two limits
• For processes where the variable needs to be kept at
particular setpoint level, proportional control is used
• Proportional action can be accomplished in two ways:
– Time Proportioning Method
– Amplitude Proportional Method
12
Time Proportioning
• Is a method whereby the
output of the controller is
continually switched on
and off
• On versus off times are
varied dependent upon
process requirements
13
Amplitude Proportional
• Most common technique to produce a
proportional signal
• The control signal is proportional in amplitude to
the error signal
• The signal may be amplified and the
amplification may be referred to as proportional
gain and proportional band
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Proportional Gain Comparison
Level control at a gain of 1
15
Proportional
Gain Comparison
(Gain set at 2)
16
Proportional Band
• Proportional band is defined as the percentage
change in the controlled variable that causes
the final correcting element to go through 100
percent of its range
Controlled Variable % Change
PB =
Final Correcting Element % Change
17
Integral Control
• Because of the introduction of
offset in a control process,
proportional control alone is
often used in conjunction with
Integral control
• Offset is the difference between
set point and the measured
value after corrective action has
taken place
18
Reset Action
• Integral control is also referred
to as reset control as the set
point is continuously reset as
long as an error is present
• Integral adjustments that affect
the output are labeled 3 ways:
– Gain - expressed as a whole
number
– Reset - Expressed in repeats
per minute
– Integral Time - Expressed in
minutes per reset
19
Derivative Mode
•
•
•
•
For rapid load changes, the
derivative mode is typically used to
prevent oscillation in a process
system
The derivative mode responds to
the rate of change of the error
signal rather than its amplitude
Derivative mode is never used by
itself, but in combination with
other modes
Derivative action cannot remove
offset
20
Control Mode Summary
21
Tuning the Controller
• Fine-tuning is the process to optimize the controller operation
by adjusting the following settings:
– Gain setting (proportional mode)
– Reset rate (integral mode)
– Rate (derivative mode)
• Three steps are taken when tuning a systems
– Study the control loop
– Obtain clearance for tuning procedures
– Confirm the correction operation of the system components
22
Trial-and-Error Tuning
• Does not use mathematical methods, instead a chart
recorder is used and several bump tests are made in
the proportional and integral modes
• Trial-and-error tuning is very time consuming and
requires considerable experience on the part of the
technician or operator
23
Ziegler-Nichols Tuning Methods
• Two formal procedures for tuning
control loops:
– Continuous cycling method
– Reaction curve method
24
Continuous Cycling Method
• The continuous cycling method analyzes the process by
forcing the controlled variable to oscillate in even, continuous
cycles
• The time duration of one cycle is called an ultimate period.
The proportional setting that causes the cycling is called the
ultimate proportional value
• These two values are then used in mathematical formulas to
calculate the controller settings
25
Continuous Cycle Calculations
• Proportional only controller
• Proportional Gain
– Kc = Gu x 0.5
• KC = proportional gain,
• Gu= ultimate gain
• Proportional Band
– PB = Pbu x 2
• PB = proportional band
• Pbu = ultimate proportional band
26
Continuous Cycling Formulas
27
Ziegler-Nichols Reaction Curve Tuning Method
• This method avoids the forced oscillations that are found in
the continuous cycle tuning method
• Cycling should be avoided if the process is hazardous or
critical
• This method uses step changes and the rate at which the
process reacts is recorded
• The graph produces three different values used in
mathematical calculations to determine the proper controller
settings
28
Reaction Curve Tuning Formulas
29
Direct Synthesis Method
•
•
•
•
The direct synthesis method uses the
same procedures to analyze the process
times as the Ziegler-Nichols reaction
curve method
One advantage of this method is that it
directly matches (synthesizes)
synthesizes the process
The first step in this method is perform a
bump test and recording the dynamic
settling time
Depending upon the complexity of the
process, one of several dynamic
response signals may develop as shown
on the right
30
Direct Synthesis Tuning Calculations
31
Advanced Control Techniques
• Some complex manufacturing operations require
more precise control than available with PID
controllers
• Four Techniques frequently used are:
–
–
–
–
Cascade control
Feed forward control
Ratio control
Adaptive control
32
Cascade Control
• Cascade control systems
use a second feedback
loop with a separate
sensor and controller
• Cascade control is
effective in overcoming
lag in some systems
33
Feedforward Control
• Feedforward control
measures a variable that
enters a process and takes
corrective action if it is
affected by a disturbance,
reducing or eliminating
deviation from the set
point
34
Ratio Control
• Used in mixing systems
where an uncontrolled
flow of material (wild
flow) is monitored and
used to control the
second material which is
controlled according to
the desired ratio between
the two components
35
Adaptive Control
• To accommodate a nonlinear process, a
microelectronic controller uses software that
has adaptive control capabilities to
compensate for nonlinear transducers and
sensors
36
END
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