Module 1. Need for process control – mathematical model of first – order level, pressure and...
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- Slide 1
- Module 1
- Slide 2
- Need for process control mathematical model of first order
level, pressure and thermal processes higher order process
interacting and non- interacting systems continuous and batch
process self-regulation servo and regulator operation. Basic
control actions characteristics of on-off, proportional,
single-speed floating, integral and derivative control modes P+I,
P+D and P+I+D control modes pneumatic and electronic controllers to
realize various control actions.
- Slide 3
- Introduction PROCESS : An operation or series of operations on
fluid or solid materials during which the materials are placed in a
more useful state. The objective of a process is to convert certain
raw materials into desired products using available sources of
energy in the most economical way. CONTROL: means methods to force
parameters in the environment to have specific values.
- Slide 4
- PROCESS may be controlled by measuring a variable representing
the desired state of the product and automatically adjusting one of
the other variables of the process. A desired quantity is kept at
set point irrespective of external influences. AUTOMATIC PROCESS
CONTROL is the maintenance of a desired value of a quantity or
conditions by measuring the existing value, comparing it to the
desired value and employing the difference to initiate action for
reducing this difference. This requires a feedback control system
which does not require human aid.
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- Areas - Process Control 1.Processing industries such as
petroleum, chemical, steel, power and food for the control of
assembly operations, work flow, heat treating and similar
variables. 2.Goods manufacturers such as automobile parts,
refrigerators and electronic equipments like television sets, radio
etc. for the control of assembly operations, work flow, heat
treating and similar variables. 3.Transportation systems such as
railways, airplanes, free missiles and ships. 4.Power machines such
as machine tools, compressors and pumps, prime movers, and electric
power supply units for the control of position, speed and
power.
- Slide 6
- Need for Process Control Increase in productivity (increase in
quantity or number of products): helps to increase the efficiency
of both man and machine. Improvement in quality of products by
meeting the product specifications overcoming operational
constraints. Improvement in the consistency of product dimensions,
performance and length of service. Economical improvement by way of
savings in processing raw materials, savings in energy, effective
utilization of capital and human labour etc. Minimize/ suppress the
influence of external disturbances on the process. Ensure the
stability of the process. Optimize the performance of the process.
Meet environmental regulations.
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- Batch & Continuous Processes Batch Process: A process in
which the materials or work are stationary at one physical location
while being treated. Eg:
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- Batch Process Advantages A variety of different products can be
made using the plant. Slow reactions can be carried out. Can use
reactants in any state including solids. Disadvantages Risk of
contamination if more than one than one product made in reaction
vessel. Expensive down time while reactor is being filled and
emptied. Larger workforce required. Can be difficult to control
highly exothermic reactions.
- Slide 11
- Batch & Continuous Processes Continuous Process: A process
in which the materials or work flows more or less continuously
through a plant apparatus while being manufactured or treated.
Eg:Almost all chemical plant processes
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- Continuous Process Advantages More cost effective if large
quantities of the chemical are being made. No expensive down time
when plant is not being used. Automated process requires less
labour. Disadvantages High capital cost of setting up the plant
Costs rise if plant not operated continuously.
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- FACTORCONTINUOUSBATCH Cost of factory equipmentHighLow Rate of
productionHighLow Shut-down timesRareOften WorkforceFew people Many
people needed Ease of automationRelativelyRelatively easy
difficult
- Slide 14
- Self Regulating Systems Some systems have the capability that
is designed to produce continuous balance.
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- Refer: Krishnaswamy/ Stephanopoulos CSTR Self Regulation
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- General Closed Loop System
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- Equation for Feedback
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- Servo & Regulator Operation SERVO OPERATION Set point only
changes ; disturbance does not change d(s) = 0 REGULATOR OPERATION
Disturbance only changes ; set point does not change y SP (s) =
0
- Slide 20
- Process Characteristics Process Equation Process Load Transient
Process Lag Self Regulation
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- Control System Parameters Error Variable Range Control
Parameter Range Control Lag Dead Time Cycling Controller Modes
Control Actions (Direct & Indirect)
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- CONTROLLER MODES Discontinuous Control Modes 2 position (ON/OFF
control) mode Multi position mode Floating control mode Continuous
Control Modes Proportional Control Integral Control Derivative
Control Composite Control Modes PI PD PID
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- DISCONTINUOUS CONTROLLER MODES 1. Two-Position Mode (ON-OFF
controller)
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- Electrical Two Position Controller
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- Pneumatic Two Position Controller
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- Two Position Controllers APPLICATIONS Adapted to LARGE SCALE
SYSTEMS with relatively SLOW PROCESS rates. Eg: AC in a Hall
Disadvantage: Oscillation
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- 2. Multi position Mode
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- Requires more complicated Final Control element
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- 3. Floating Control Mode (Single speed) dp/dt = K F
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- 4. Multiple Speed Floating Control
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- Applications Well suited to self regulation processes with a
very small lag or dead time.
- Slide 33
- Continuous Controller Modes Proportional Control Mode Also
known as correspondence control, droop control and modulating
control. Control action is proportional to Error. K p =
Proportional Gain(Proportional Sensitivity) Proportional Band is
the range of error to cover the 0% to 100% controller output. PB =
100/K p
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- Proportional Control Mode p = K p e p +p 0 p = Controller Out
put (%) e p = Error (%) p 0 = Controller output with no error
(%)
- Slide 35
- Proportional Control Mode
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- OFFSET : Whenever a change in load occurs, the proportional
control mode produces a permanent residual error. This can be
minimized by a larger K p Application : used in processes where
large load change are unlikely or with moderate to small process
lag times.
- Slide 37
- Integral Control (Reset Action) Mode the value of manipulated
variable is changed at a rate proportional to the deviation. If
deviation is doubled over a previous value, the final control
element is moved twice as fast; when CV at SP the FCE remains
stationary. Controller output, p 0 = Controller O/P at t=0
- Slide 38
- Integral Control (Reset Action) Mode
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- If the error is zero, the output stays fixed at a value to what
it was when the error went to zero. If the error is not zero, the
output will begin to increase or decrease at a rate of K I percent/
second for every one percent error. Transfer Function of Integral
Control is: Integral Windup
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- Integral Control (Reset Action) Mode ** Offset eliminated
- Slide 41
- Derivative Control Mode Controller output depends on rate of
change of error. Also known as Anticipatory Control, Rate response
or lead component. The controller anticipates what the error will
be in the immediate future and applies action which is proportional
to current rate of change of error.
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- Derivative Control Mode
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- Drawback: for a noisy response with almost zero error it will
compute large derivatives and thus large control action, which is
not needed. Not used alone. For zero or constant error, no control
action. Transfer Function of Derivative mode:
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- COMPOSITE CONTROL MODE PI Control PD Control PID Control
- Slide 45
- Proportional Integral Control (PI) p t(0) = Integral term value
at t=0 (initial value) One-to-one correspondence of the
proportional mode is available and the integral mode eliminates the
inherent offset.
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- Proportional Integral Control (PI)
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- Used in systems with frequent or large load changes.
Disadvantage o Because of the integration time, however, the
process must have relatively slow changes in load to prevent
oscillations induced by the integral overshoot. o During start up
of a batch process, the integral action causes a considerable
overshoot of the error and output before settling to the operating
point.
- Slide 48
- Proportional Derivative Control (PD) Disadvantage: o It cannot
eliminate offset of proportional control Advantage: It can handle
fast process load changes as long as the load change offset error
is acceptable.
- Slide 49
- Proportional Derivative Control (PD)
- Slide 50
- Proportional Integral - Derivative Control (PID) (three mode
controller)
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- Proportional Integral - Derivative Control (PID)
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- ELECTRONIC CONTROLLERS TWO POSITION CONTROLLER
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- ELECTRONIC CONTROLLERS FLOATING TYPE CONTROLLER
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- ELECTRONIC CONTROLLERS PROPORTIONAL MODE CONTROLLER
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- ELECTRONIC CONTROLLERS INTEGRAL MODE CONTROLLER
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- ELECTRONIC CONTROLLERS DERIVATIVE MODE CONTROLLER
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- ELECTRONIC CONTROLLERS PI MODE CONTROLLER
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- ELECTRONIC CONTROLLERS PD MODE CONTROLLER
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- ELECTRONIC CONTROLLERS PID MODE CONTROLLER
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- PNEUMATIC CONTROLLERS PROPORTIONAL MODE CONTROLLER
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- PNEUMATIC CONTROLLERS PI MODE CONTROLLER
- Slide 62
- PNEUMATIC CONTROLLERS PD MODE CONTROLLER
- Slide 63
- PNEUMATIC CONTROLLERS PID MODE CONTROLLER