2. Module 1
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
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.
PROCESS: An operation or series of operations on fluid or solid
materials during which the materials are placed in a more useful
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.
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.
5. 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.
6. Need for Process Control
Increase in productivity (increase in quantity or number of
products): helps to increase the efficiency of both man and
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
Ensure the stability of the process.
Optimize the performance of the process.
Meet environmental regulations.
9. Batch & Continuous Processes
Batch Process: A process in which the materials or work
are stationary at one physical location
while being treated.
10. Batch Process
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.
Risk of contamination if more than one than one product
made in reaction vessel.
Expensive down time while reactor is being filled and
Larger workforce required.
Can be difficult to control highly exothermic reactions.
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
12. Continuous Process
More cost effective if large quantities of the chemical are
No expensive ‘down time’ when plant is not being used.
Automated process requires less labour.
High capital cost of setting up the plant
Costs rise if plant not operated continuously.
13. FACTOR CONTINUOUS BATCH
Cost of factory equipment High Low
Rate of production High Low
Shut-down times Rare Often
Workforce Few people Many people
Ease of automation Relatively Relatively
21. Control System Parameters
Control Parameter Range
Control Actions (Direct & Indirect)
22. CONTROLLER MODES
Discontinuous Control Modes
2 position (ON/OFF control) mode
Multi position mode
Floating control mode
Continuous Control Modes
Composite Control Modes
33. Continuous Controller Modes
Proportional Control Mode
Also known as correspondence control, droop control
and modulating control.
Control action is proportional to Error.
Kp = Proportional Gain(Proportional Sensitivity)
Proportional Band is the range of error to cover the 0%
to 100% controller output.
PB = 100/Kp
36. Proportional Control Mode
OFFSET : Whenever a change in load occurs, the proportional
control mode produces a permanent residual error.
This can be minimized
by a larger Kp
Application : used in
processes where large load
change are unlikely or with
moderate to small process
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.
p0 = Controller O/P at t=0
39. Integral Control (Reset Action) Mode
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 KI percent/ second for every
one percent error.
Transfer Function of Integral Control is:
41. Derivative Control Mode
Controller output depends on rate of change of error.
Also known as Anticipatory Control, Rate response or
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.
43. Derivative Control Mode
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:
45. Proportional – Integral Control (PI)
pt(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.
47. Proportional – Integral Control (PI)
Used in systems with frequent or large load
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.
48. Proportional – Derivative Control (PD)
o It cannot eliminate offset of proportional control
It can handle fast process load changes as long as the
load change offset error is acceptable.