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Process PID Controller - Information Resource Center
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file:///E:/project%20study/IPC%20notes/Process_PID_Controller
Process PID Controller
From Information Resource Center
Objectives:
Upon completion of this lesson, the student/trainee must be able to:
Distinguish between feedback and feedforward;
Distinguish between the following control modes;
o on-off,
o proportional,
o integral and
o derivative control mode;
Define dead zone
...
It takes the difference or error signal from
the comparator and determines the amount of change
the manipulated variable needs to bring the
controlled variable back to normal
...
The
entire system with all these possible combinations is
shown in the figure
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1 Controller Classification
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Two common types in this category are electric and
pneumatic controllers
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1
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1 Electric or electronic controllers
operate on electric signals
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1
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Pneumatic controllers are used in
the chemical industries for reasons of
safety
...
However, pneumatic
controllers are difficult to interface
with digital computers, a great
disadvantage in light of the increasing
popularity of computers in process
control
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1
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Controllers are also classified according to the type of control they provide
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On-Off Control
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In On-off control, the final control element is
either on or off
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In the pasteurizing example, if
the controller were an on-off controller, the valve
would be either open or closed
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On-off control is illustrated in the figure
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It will stay
until the measured variable goes below the setpoint
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The
measured variable will oscillate around the setpoint
at an amplitude and frequency that depends on the
process capacity and
time response
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On-off control also has
domestic applications
...
If the press
metal plate of the electric flat iron is warmer than
the preset temperature, the heater is shut off
...
Thus, the control is on-off
...
1 Typical implementation of electronic On-off
Controller uses an OpAmp, configured as
comparator of the amplitude of one voltage against
another
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The process engineer under the following conditions chooses the on-off control mode:
1
...
2
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3
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On-off control is mostly found in air-conditioning and refrigeration systems
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In this application, on-off control is called a shutdown or cutback
alarm
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2 Differential-gap control is similar to on-off
control
...
When the measured variable goes
above the upper boundary of the gap, the final
control element closes
...
Some home air-conditioning systems are use this
type of control mode rather than on-off
...
In industry, differential-gap control
is often found in noncritical level-control
applications, such as keeping a tank from running
dry or from flooding
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Engineers normally choose a dead
zone of about 0
...
0% of the range of the final
control element
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On-off control is almost always the simplest
and the least expensive controller mode
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However, many industrial
processes need better, more sophisticated control
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Proportional Control (P) Action
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In proportional control, the final control
element is purposely kept in some intermediate
position between on and off
...
An example of a proportional made using a
filled-system temperature sensor is shown in the
figure
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Any change in
the temperature causes the valve position to change
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1 The actual valve position is determined by the
proportional gain
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If
the valve does not move very much for a given
temperature change, the controller gain is small
...
In this system, the setpoint is determined by the bias
(initial setting of the valve)-50%
...
More
sophisticated proportional control systems have
variable gain or setpoints
...
As the measurement
deviates from the setpoint, the final control element
setting will change from 50%
...
2 Most industrial controllers have a gain adjustment that is expressed in percent of the proportional band
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Or we can say that the
proportional band is equal to the range of values of the dynamic variable that corresponds to a full or complete change in
controller output
...
Normally, the proportional band is expressed as a
percentage:
Then:
Note that the relationship between gain (or sensitivity) and proportional band
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Each position in the proportional band produces a controller output
...
Changing the bias shifts the
proportional band so that a given input signal will cause a difference output level
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To do so, we must
know the measurement and the setpoint
...
>> When the measurement exceeds the setpoint by 10%, the output is 40%
...
The higher gain, the more the output will move for a given change in either measurement or set point
...
The following diagram shows the controlled results when a continuous external disturbance is applied to the
controlled system
...
However, hunting (the process variable "hunts" the setpoint) tends to continue for a long
period until the controlled system becomes stable
...
Although various methods for adjusting the offset exist, the practical one is to search for the proportional band
in which the optimum response can be obtained by gradually narrowing the proportional band while changing the
setpoint within the range permitted by the process
...
3 Offset
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However, it is
impossible for a proportional controller to return the measurement so that it equals the setpoint
...
By definition, the output must equal the bias setting (normally 50%) when the measurement equals the setpoint
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Proportional control may reduce the effect of the load change, but it can never eliminate it
...
From this equation, we see that as the proportional band goes toward zero (gain approaches infinity), offset will
approach zero
...
We know that we cannot have an offset in an on-off controller
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Temperature Sensor
Proportional Plus Reset (PI) Control Action
A deviation caused by an abrupt external
disturbance can be immediately eliminated by the
proportional control action
...
The offset
caused by proportional control is often times not
acceptable
...
1 Integral control (also called reset action) in a
process control system will perform corrective
action for as long as deviation exists by giving the
controlled system a manipulated variable
proportional to the integrated value of deviation so
that the offset is eliminated
...
4
...
An integrator circuit utilizes
negative-feedback
...
A square wave input
will produce a triangular voltage waveform while a
cosine will be produced at the output for a sinusoidal
input voltage
...
3 The response of pure integral controller (against
time) is shown in the figure
...
This figure also assumes that an open-loop
condition exists where the controller' output is not
connected to the process
...
Proportional
plus integral control (PI) combines the characteristics of both types of control
...
In the figure, reset time is the amount of time
required to repeat the amount of output change y
...
In this example, a 50% proportional band is centered
around the setpoint
...
Proportional
response will be seen immediately in the output, followed by integral action
...
This shift causes a new controller output
for a given difference between measurement and setpoint
...
Integral control action does exactly what an operator would do manually adjusting the bias in the proportional
controller
...
It is shifted in a direction opposite to that of the
measurement change
...
Because
integral control acts only on long-term or steady-state errors, it is seldom used alone
...
These small changes may occur for a long time before finally they go beyond the
allowable error limits
...
Consequently, PI control is a very popular mode of control in industry today
...
This situation is especially true
in processes that have a small capacitance
...
5
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Differentiator circuit responds to the input rate of
change, or slope
...
The voltage across resistor
R is equal to the voltage at the output terminal
...
The
output will stay at zero until the input will change to
a negative going signal, at which time, another pulse
is generated at the output
...
By adding derivative control to
proportional control, we get a controller that
responds to the measurement's rate of change as well
as to its size
...
2 Proportional plus derivative control (PD) is
illustrated in the figure
...
Derivative
control is never used alone because it can only react
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to measurements when they are changing
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The deviation control action provides a manipulated variable proportional to the differential value of the deviation
to prevent the deviation from increasing by performing a large scale corrective action when the deviation is about to
occur
...
The manipulated variable is also increased at a fixed speed
with the proportional action used alone as shown in the figure
...
As a consequence of this
combination, the controller output follows the composite output curve as shown by the figure
...
The
best results of the control can be obtained since this
control action is performed in relation to the
magnitude of a deviation and the manner of time
transition
...
Since a
large-scale correction is performed in the initial
stage when the deviation has occurred, the control
action is affected quickly
...
1 Step response of PID control action output
...
2 Rampwise response of PID control action
output
...
3 Controlled Output results of PID action
...
Two types of controllers are common, for common
types of actuators: step controllers for electric actuators and continuous controllers for pneumatic and hydraulic
actuators
...
1 We distinguish two types of controller: three-position step controller switches the electric motor of the actuator to
clockwise rotation by means of relays or semiconductor switches, and can affect the positioning speed of the final control
element by means of different on/off ratios
...
2 Two position controllers are used to activate
relays, contactors, or thyristor switches for switching
electric heaters or coolers
...
A continuous oscillation
occurs whose frequency depends on the delay time
of the controlled system and the switching hysteresis
of the controller
...
3 Continuous controllers are mainly used with
pneumatic actuators
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7
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7
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1 Analog controller processes, computes and
provides an output in a continuously variable signal
level
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4
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The figure illustrates
how a digital controller performs a corrective action
( even if it is called "linear") - incremental
...
However, the control output is determined at each sampling time in the digital type B controller
and therefore is changed progressively by a fixed increment C, For example the typical resolution of the Iinear output of
common PID controller is divided into 205 increments Accordingly, the control output of the Iinear sensor controller is
changed toward a maximum of 205 increments on a step-by-step basis
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168
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