Automatic Controllers & Control Modes
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Transcript of Automatic Controllers & Control Modes
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Unit 2
Automatic Controllers & Control Modes
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A Control System is an arrangement of physical components
connected/related in such a manner as to command, direct or regulate itself
or another system.
A Control System consists of subsystems and processes (or plants)
assembled to control the outputs of a process.
What Is Control System?
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1) If the aim is to maintain a physical variable at some fixed value when thereare disturbances, this is called as regulator.Example: speed-control system on the ac generators of power utilitycompanies.
2) The second class is the servomechanism. This is a control system in which aphysical variable is required to follow (track) some desired time function.Example: an automatic aircraft landing system, or a robot arm designed tofollow a required path in space.
Classification Of Control System
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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.
Open-Loop Control System
Set Point Controller Process
Block Diagram of open loop control system
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Closed-Loop Control System
Closed-loop control systems are more effective than open-loop systems.
With the addition of a feedback loop they become self-regulating.
Components of a closed-loop system include:
I. The primary element sensor
II. The controlled variable
III. The measured variable
IV. The control signal
V. The final correcting element
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Block Diagram Of Closed loop Control System
SETPOINT
ERRORDETECTOR
ERRORSIGNAL CONTROLLER CONTROL
SIGNAL
ACTUATOR & FINALCONTROL ELEMENT
SENSOR(FEEDBACKELEMENT)
FEEDBACK
PROCESSCONTROLLED
VARIABLE
ACTION
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Process
Complex assembly of phenomenon that refers to some manufacturing
sequence. It utilizes the resources to produce certain product.
Many variables may be involved in such a process, some of which have
to be controlled.
Classification of processes.
Process
Based on variables Based on operation
to be controlled
Batch(sequential) Continuous
Single variable Multi variable process process
process process
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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:1. A disturbance appears2. Load demands varies or3. Set points are adjusted.
Several process variables are controlled at once in a typical productionmachine.
Usually, only one individual feedback loop is required to control eachvariable.
Single-variable control loops consist of the following elements:o Measuring deviceo Transducer/transmittero Controllero Final Control Element
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Process Behavior Example
Flow through the pipe
is theprocess.
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.
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Sensor (Feedback Element)
It is the eye of the system.
Produces output that represents the status of the controlled variable.
The output of the sensor is called as feedback.
Examples of sensors used in process control are
Thermal sensors like RTD, Thermistor, Thermocouple etc.
Level sensors like Ultrasonic, Float, Radiation sensor etc.
Pressure sensors like Diaphragm, Bourdon tube, Bellows etc.
Flow sensors like Ventury meter, magnetic flow meter etc.
Optical sensors based on LED, LASER and Photodiode, Phototransistor.
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Pressure
Level
Flow
Temperature
pH
Humidity
Density
Speed
Thermocouples
RTDs / Thermistors
Filled Systems
Bi-metallic
Strain gauge
Piezo-electric
Capacitance
Bourdon Tube
Head meters
(orifice, venturi)
Coriolis Mass,
velocity,
Mechanical Floats
Guided Wave
Weight (load cell)
Ultrasonic
Static Pressure
Transmitters
Pressure Transmitter
Level Transmitter
Differential Pressure
Cell
Flow Transmitter
Temperature
Transmitter
Type of Signals
Pneumatic
3-15 PSI
Electrical
Current
4 20 mA
0 20 mA10 50 mA
Voltage
05 V
1 5 V
0 10 V
igitalON/OFF
Field Bus
ModBus
ProfiBus
Feedback Elements(Sensors)
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Error Detector
Compares set point(reference signal) with the feedback signal.
Produces error signal for the controller.
Error = Set point feedback signal
Examples of devices used as error detector are
OP-AMP based differential amplifier for analog signals.
Comparison soft wares for digital signals.
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Controller
The controller is the brain of the system.
Receives error signal and develops output that causes the controlled
variable to become equal to the set point value.
Examples of controller PLC, Microprocessor, OP-AMP based controller.
Different modes of controller areController modes
Discontinuous mode Continuous mode
Two position controller Proportional controller
Multiposition controller Proportional Integral controller
Floating controller Proportional Derivative
controller
Proportional Integral Derivative
controller
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Selection of a Controller
Controllers are designed to operate by using different control modes. Each ofthese modes has specific characteristics to provide different types of control
actions.
These control modes are:
1. On/Off2. Proportional3. Integral4. Derivative
The mode or combination of modes which is selected by the designer
is determined by the requirement of the process
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On-Off Controller (2 position controller)
Used for slow acting operationswhere 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 controllerto reduce cycling.
Dead band refers to the differing levels
at which a controller switches on and off
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On-Off Controller (2 position controller)The analytical equation for On-Off controller is given as
P= 0 % Ep < 0P= 100 % Ep > 0
Applications of On-Off controller: It is best adapted to large scale systems
with relatively slow process rates. Examples of such process are
1. Room heating or air conditioning system
2. Liquid bath temperature control
3. Level control in large volume tanks.
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Continuous Control
On/Off control is acceptable for process where the variable is setbetween two limits.
For processes where the variable needs to be kept at particular set point
level, proportional control is used.
Proportional action can be accomplished in two ways:
Time Proportioning Method
Amplitude Proportional Method
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Time Proportioning
Is a method whereby the output of the controller is continually switched
on and off. This method is also called as PWM(Pulse Width Modulation).
Here the ratio of On time to Off time called as duty cycle is varied as per
the changes in the feedback signal.
On versus off times are varied dependent upon process requirements.
Example: Speed control of DC Motor .
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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 andproportional band.
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Proportional Control Smooth relationship exists between the controller output and the error.
P= Kp Ep + PoWhere Kp- Proportional gain between error and the controller output
Po- Controller output without error
The range of error to cover 0% to 100% controller output is called asproportional band.
PB = 100/ Kp
Disadvantage of proportional controller is offset or SSE or residual error.
The offset error limits use of proportional mode to only a few casesparticularly where manual reset of the operating point is available to reset
the offset.
It is generally used in process where large load changes are unlikely or withmoderate to small process lag times.
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Characteristics of Proportional Controller
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Offset Error
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Integral Control
Because of the introduction of offset in a control process, proportional
control alone is not used. It is often used in conjunction with Integralcontrol.
Offset is the difference between set point and the measured value after
corrective action has taken place.
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Integral ControlThe offset error of the proportional mode occurs because the controller can not
adapt to changing external conditions i.e. changing loads. In other words the zero
error output is a fixed value.
The Integral mode eliminates this problem by allowing the controller to adapt to
changing external conditions by changing zero error output.
Integral action is provided by summing the error over time, multiplying that sum
by a gain and adding the result to present controller output.
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Integral mode controller action the rate of output change depends on the error.
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Characteristics of Integral Controller
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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.
P(t) = KD (dEp / dt)
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The Derivative mode must be used with great care and usually with a small
gain, because a rapid change of error can cause very large, sudden changes of
controller output which can lead to instability.
Derivative controller is not used alone because it provides no output when the
error is constant.
It is also called as rate controller or anticipatory control as it can take an action
in advance depending upon the rate of error change.
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Characteristics Of Derivative Controller
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Limitations of Derivative ControlThe derivative mode acts upon the rate of error signal change and it may cause
unnecessary upsets.
It tends to react to sudden set point changes and will amplify noise.
The control algorithm can be altered so that derivative acts on the
measurement and not on the error. This will reduce upsets.
Excessive noise and step changes in the measurements can be corrected by
filtering out any change in the measurement that occurs faster than the maximum
speed of response of the process.
DCS system provides software with adjustable filters for each variable.
The time constant of these filters is usually adjusted from 0 to 100 seconds.
In analog system Inverse Derivative control mode is often used.
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Inverse Derivative Control ModeThis control action is used on fast processes. The inverse derivative mode is
opposite of the derivative mode.
While the output of Derivative mode is directly proportional to the rate of
change in error, the output of inverse derivative mode is inversely proportional to
the rate of change in the error.
Inverse derivative is used to reduce the gain of the controller at high frequenciesand is useful in stabilizing the control loop.
The dynamic gain of the derivative function is selected to be 0.5 or less.
The gain of the inverse derivative controller decreases from the proportional gain
at low frequency to the limiting value of the proportional gain divided by this factor
at high frequency.
A proportional plus inverse derivative controller provides high gain to minimize
offset at low frequency and low gain to stabilize the loop at high frequency.
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Inverse derivative can be added to PI Controller to stabilize the loops requiring
very low proportional gains for stability.
Inverse derivative should only be added when the loop is unstable at theminimum gain setting of the PI Controller.
It is available in the separate unit can be added to the loop when stability
problem occurs.
The addition of inverse derivative when proportionally tuned has little effect on
the natural frequency of the loop.
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Proportional Integral (PI) Control Combines proportional and integral mode together and eliminates the offset
inherent in proportional controller.
However makes the action sluggish and increases the response time.
Another disadvantage of this system is that during start up of the batch process
the integral action causes a considerable overshoot of the error and the output
before settling to the operation point.
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Characteristics of PI Controller
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Proportional Derivative (PD) Control
It involves the serial (cascaded) use of the proportional and derivative modes.
The analytical expression for this mode is given as
This controller cannot eliminate the offset of proportional controllers , however it
can handle fast process load changes.
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Characteristics of PD Control
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Proportional Integral and Derivative (PID) control
One of the most powerful but complex controller mode operations combines theproportional, integral and derivative modes.
The system can be used for virtually any process condition.
The analytical expression is
This mode eliminates the offset of the proportional mode and still provides fastresponse.
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PID controller characteristics
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Control Mode Summary
Control System Design Process
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Electronic Controllers1. On Off Controller with dead band
Here VH = Vsp and VL= Vsp- (R1/R2) Vo
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2. Proportional Controller
We know that for proportional mode
p = KPEP+ PO
For implementation of electronic controller we have
Vout = GpVe+ VO
Where GP = R2/R1 = Gain of the controller
A li ti f ti l t l f F t t t l
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Application of proportional control for Furnace temperature control
Application of proportional control for Robot Arm control
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Application of proportional control for Robot Arm control
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3. Proportional Integral Controller
We know that the control mode equation for this mode is given as
For electronic implementation we have
Where Proportional gain = R2/R1
Integral gain = 1/ R2.C
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Proportional Integral Controller
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Application of Proportional Integral Controller to Robot arm control
4 P i l D i i C ll
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4. Proportional Derivative Controller
We know that for the PD Control mode the equation is given as
For electronic implementation we have
Where Proportional gain = R2/(R1+R3)
Derivative gain = R3.C
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Proportional Derivative Controller
5 P ti l I t l D i ti C t ll
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5. Proportional Integral Derivative Controller
We know that for PID the analytical expression is given as
For electronic implementation we have
Where Proportional gain= R2/R1
Integral gain = 1/RI. CI
Derivative gain = RD . CD
P ti l I t l D i ti C t ll
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Proportional Integral Derivative Controller
Application of Proportional Integral Derivative Controller for Robot Arm Control
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Application of Proportional Integral Derivative Controller for Robot Arm Control
P i R l
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Pneumatic Relay Also called as pneumatic amplifier or booster. It raises the pressure and /or air
flow volume by some linearly proportional amount from the input signal.
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Nozzle/Flapper systemIt is used to convert the pressure to mechanical motion and vice versa.
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Current to pressure converters The I/P Converter is an important element in process control and used to signal
condition the output of controller to equivalent pressure signal.
P ti C t ll
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Pneumatic Controllers
The pneumatic controller is based on the nozzle/flapper system.
Here also we can implement different control modes.
1. Proportional Controller
Pout= (x1/x2)* (A1/A2)*(Pin-Psp) + Po
Where Kp= (x1/x2)* (A1/A2)
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Pneumatic Proportional mode
2 Proportional Integral controller
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2. Proportional Integral controller
3 Proportional Derivative Controller
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3. Proportional Derivative Controller
4. Proportional Integral Derivative Controller
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p g
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Final Control Operation
Control signalSignal
conditioning
Actuator
(motor)
Final control
element(valve)
Process
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Control Systems in Robotics Perspective
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The Future of Control Systems
Tuning the Controller
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Tuning the Controller
Fine-tuning is the process to optimize the controller operation byadjusting the following settings:Gain setting (proportional mode)Reset rate (integral mode)Rate (derivative mode)
Three steps are taken when tuning a systems1. Study the control loop2. Obtain clearance for tuning procedures3. Confirm the correction operation of the system components
T i l d E T i
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Trial-and-Error Tuning
Does not use mathematical methods, instead a chart recorder is used and
several bump testsare 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.
Ziegler-Nichols Tuning Methods
Two formal procedures for tuning control loops:
1. Continuous cycling method
2. Reaction curve method
Continuous Cycling Method
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Continuous Cycling MethodThe continuous cycling method analyzes the process by forcing the controlled variableto oscillate in even, continuous cycles.
The time duration of one cycle is called an ultimate period. The proportional settingthat causes the cycling is called the ultimate proportional value.
These two values are then used in mathematical formulas to calculate the controllersettings.
Ziegler-Nichols Reaction Curve Tuning Method
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Ziegler Nichols Reaction Curve Tuning Method
This method avoids the forced oscillations that are found in the continuous cycletuning method.
Cycling should be avoided if the process is hazardous or critical.
This method uses step changesand the rate at which the process reacts isrecorded.
The graph produces three different values used in mathematical calculations todetermine the proper controller settings.
This method is applicable only to processes with self regulation characteristics.
From the response curve the following parameters are calculated
L: Lag time in minutesCp : Controlled variable change in %T : Process reaction time in minutesN = Cp / T = Process reaction rate in % / min.
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The calculations for Ziegler Nichols Process reaction method are
Mode Kp Ti = 1/ Ki Td = 1/ KdProportional P/ NL
Proportional
Integral (PI)
0.9 P/ NL 3.33 L
ProportionalIntegral
Derivative (PID)
1.2 P/ NL 2L 0.5 L
Frequency Response Method
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Frequency Response MethodThis method involves use of Bode Plot for the process and control loops.
The method is based on the application of the Bode plot stability criterion and
the effects that the proportional gain, integral time and derivative time have on the
Bode plot.
Bode plot stability criterion
1. If the phase is less than 140 degrees at the unity gain frequency the system is
stable. This then is 40 degrees phase margin from the limiting value of 180degrees .
2. If the gain is 5 dB below unity when the phase lag is 180 degrees the system is
stable. This is then 5 dB gain margin.
Tuning : The operations of tuning using frequency response method involveadjustments of the controller parameters until the stability is proved by the
appropriate phase and gain margins.
Proportional Action : Multiplies gain curve by a constant and no effect on phase.
Integral Action : Integral gain= Ki/ and Integral phase = - 90 degrees (lag)
Derivative Action : Derivative gain = Kd*
and Derivative phase=90 degrees (lead)
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