1. Process Control Instrumentation II
Module 1

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 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.

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.

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
Processing industries such as petroleum, chemical, steel, power and food for the control of assembly operations, work flow, heat treating and similar variables.
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.
Transportation systems such as railways, airplanes, free missiles and ships.
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 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.

9. Batch & Continuous Processes
Batch Process: A process in which the materials or work
are stationary at one physical location
while being treated.
Eg:

10. Batch Process Advantages Disadvantages
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.

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 Advantages Disadvantages
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.

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
needed needed
Ease of automation Relatively Relatively
easy difficult

14. Self Regulating Systems
Some systems have the capability that is designed to produce continuous balance.

16. Refer: Krishnaswamy/ Stephanopoulos
CSTR – Self Regulation

17. General Closed Loop System

18. Equation for Feedback

19. 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

20. Process Characteristics
Process Equation
Process Load
Transient
Process Lag
Self Regulation

21. Control System Parameters
Error
Variable Range
Control Parameter Range
Control Lag
Dead Time
Cycling
Controller Modes
Control Actions (Direct & Indirect)

22. CONTROLLER MODES Discontinuous Control Modes Continuous 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

23. DISCONTINUOUS CONTROLLER MODES
1. Two-Position Mode (ON-OFF controller)

24. 1. Two-Position Mode (ON-OFF controller)

25. Electrical Two Position Controller

26. Pneumatic Two Position Controller

27. Two Position Controllers APPLICATIONS
Adapted to LARGE SCALE SYSTEMS with relatively SLOW PROCESS rates.
Eg: AC in a Hall
Disadvantage: Oscillation

28. 2. Multi position Mode

29. 2. Multi position Mode
Requires more complicated Final Control element

30. 3. Floating Control Mode
(Single speed)
dp/dt = ±KF

31. 4. Multiple Speed Floating Control

32. Applications
Well suited to self regulation processes with a very small lag or dead time.

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

34. p = Kpep +p0 Proportional Control Mode p = Controller Out put (%)
ep = Error (%)
p0 = Controller output with no error (%)

35. Proportional Control Mode

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 lag times.

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,
p0 = Controller O/P at t=0

38. Integral Control (Reset Action) Mode

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:
Integral Windup

40. Integral Control (Reset Action) Mode
** Offset eliminated

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.

42. Derivative Control Mode

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:

44. COMPOSITE CONTROL MODE
PI Control
PD Control
PID Control

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.

46. Proportional – Integral Control (PI)

47. Proportional – Integral Control (PI)
Used in systems with frequent or large load changes.
Disadvantage
Because of the integration time, however, the process must have relatively slow changes in load to prevent oscillations induced by the integral overshoot.
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)
Disadvantage:
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.

49. Proportional – Derivative Control (PD)

50. Proportional – Integral - Derivative Control (PID) (three mode controller)

51. Proportional – Integral - Derivative Control (PID)

52. ELECTRONIC CONTROLLERS
TWO POSITION CONTROLLER

53. ELECTRONIC CONTROLLERS
FLOATING TYPE CONTROLLER

54. ELECTRONIC CONTROLLERS
PROPORTIONAL MODE CONTROLLER

55. ELECTRONIC CONTROLLERS INTEGRAL MODE CONTROLLER

56. ELECTRONIC CONTROLLERS DERIVATIVE MODE CONTROLLER

57. ELECTRONIC CONTROLLERS
PI MODE CONTROLLER

58. ELECTRONIC CONTROLLERS
PD MODE CONTROLLER

59. ELECTRONIC CONTROLLERS
PID MODE CONTROLLER

60. PNEUMATIC CONTROLLERS PROPORTIONAL MODE CONTROLLER

61. PNEUMATIC CONTROLLERS
PI MODE CONTROLLER

62. PNEUMATIC CONTROLLERS
PD MODE CONTROLLER

63. PNEUMATIC CONTROLLERS
PID MODE CONTROLLER