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| Figure 1 : Controller Principle block diagram
SP = Set point
MV = Measured value
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Controller is a
device which receives input from two points :
(i)
a value which is sent by transmitter
(ii)
a value which is set by set point
There
are two types of controller :
1. ON OFF Controller
-like switch
2. Analog Controller
-Proportional
controller
-Integral
controller
-Derivative
controller
TWO POSITION MODE
A two position
controller is a
device that has
two operating conditions:
completely on or completely off. Figure 1 shows the input to
output, characteristic waveform for a two position controller that switches from
its "OFF" state to its "ON" state when the measured
variable increases above these tpoint.
Conversely, it switches
from its "ON" state
to its "OFF" state
when the measured variable decreases below the
set point. This device provides an output
determined by whether the error signal is above or below the set point. The magnitude of the error signal is above or
below the setpoint. The magnitude of the
error signal past that point is of no concern to the controller.
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| Figure 1 : Two position controller |
Example of Two Position Control
A
system using a two position controller is shown in Figure 2.The
controlled process is the volume of water in the tank. The controlled
variable is the level in
the tank. It
is measured by
a level detector
that sends information
to the controller.
The output of the
controller is sent
to the final
control element, which
is a solenoid
valve, that controls the flow of
water into the tank.As the water level decreases initially, a point is
reached
where the measured variable drops below the set point. This creates a
positive error signal. The controller opens the final control
elementfully. Water is subsequently
injected into the tank, and the water level rises. As soon as
the water level
rises above the
setpoint, a negative
error signal is
developed. The negative
error signal causes the controller to shut the final control element.
This opening and closing of the final control element results in a
cycling characteristic
of the measured variable.
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| Figure 2 Two Position Control System TYPES OF CONTROLLER
There are a few types of controller used to control a process
either in a form of Proportional output to the error, Proportional and Integral
to the error or Proportional and Derivative output to the first
error.Controller can be used in the form of single mode of Proportional,
Integral, or Derivative, two mode of Proportional and Integral (P+I) and
Proportional and Derivative(P+D), and three mode of Proportional, Integral and
Derivative (P+I+D).
Proportional Controller (P-Controller)
One of the
most used controllers is the Proportional Controller (P-Controller) who
produce an output action that is proportional to the deviation
between the set point and the measured process value.
The gain or action factor - kP
The gain kP can be expressed as
The proportional band P, express the value necessary for 100% controller output. If P = 0, the gain or action factor kP would be infinity - the control action would be ON/OFF. A
proportional controller will have the effect of reducing the rise time
and will reduce, but never eliminate, the steady-state error.
Integral Controller (I-Controller)
With
integral action, the controller output is proportional to the amount of
time the error is present. Integral action eliminates offset.
The
integral controller produce an output proportional with the summarized
deviation between the set point and measured value and integrating gain or action factor.
Integral controllers tend to respond slowly at first, but over a long period of time they tend to eliminate errors.
The integral controller eliminates the steady-state error, but may make the transient response worse. The controller may be unstable.
The integral regulator may also cause problems during shutdowns and start up as a result of the integral saturation or wind up
effect. An integrating regulator with over time deviation (typical
during plant shut downs) will summarize the output to +/- 100%.
During start up the output is set to 100%m which may be catastrophic.
Derivative Controller (D-Controller)
With
derivative action, the controller output is proportional to the rate of
change of the measurement or error. The controller output is
calculated by the rate of change of the deviation or error with time.
The
derivative or differential controller is never used alone. With sudden
changes in the system the derivative controller will compensate the
output fast. The long term effects the controller allow huge steady
state errors.
A derivative controller will in general have
the effect of increasing the stability of the system, reducing the
overshoot, and improving the transient response.
Proportional, Integral, Derivative Controller (PID-Controller)
The
functions of the individual proportional, integral and derivative
controllers complements each other. If they are combined its possible to
make a system that responds quickly to changes (derivative),
tracks required positions (proportional), and reduces steady state
errors (integral).
Note that these correlations may
not be exactly accurate, because P, I and D are dependent of each other.
Changing one of these variables can change the effect of the other
two.
Control Mode Proportional and integral (P + I)
The process uses a large space Proportional to reduce cycle are usually fixed error (offset). Another great space Proportional or more large load changes, improved error is greater still. With the integration mode is required. Use proportional integral response but reduce the return on set point is more important. When the integral is used it will continue to change as long as there is an error output until the error becomes zero.
Control Mode Proportional and integral (P + I) are: -
P (t) = + Kp KpEp
available from Ep area under the graph (%) versus time (t
Mod Kawalan Berkadaran + Terbitan (P+D)
Pengawal
jenis ini tidak dapat menghindarkan ralat tetap. Tetapi berguna untuk prosees yang mengalami
perubahan beban dengan pantas selagi
ralat tetap yang ditimbulkan boleh diterima.
Mod
kawalan nya ialah:-
QUESTION
1. Design the
schematic circuit for controller action types below.
(i) Proportional
controller (P)
(ii) Integral controller (I)
(iii) Derivative controller (D)
2. Give
the advantages and disadvantages of Proportional, Integral, Derivative,
Proportional + Derivative and Proportional + Integral.
3. Explain the
operation of three mode controller system (P+I+D)
4. Sebuah pengawal numat jenis
berkadaran digunakan untuk mengawal suhu dalam proses melebur. Suhu titik
set ialah 750°C dan julat alat suhu ialah 0 - 1000°C. Ruang berkadaran ditentukan pada 15%. Julat
keluaran tekanan dari pengawal ialah 20 –100 kN/m2 dan nilai
keluaran tekanan meningkat apabila suhu meningkat. Jika nilai keluaran
keluaran tekanan diset pada 60kN/m2 untuk titik set suhu, cari:
a) Nilai suhu untuk keluaran
tekanan 20 kN/m2
b) Nilai suhu untuk keluaran
tekanan 100 kN/m2
c) Nilai tekaaaanan bila suhu
735°C
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