Process Control Two Mark Questions and Answers

Process Control Two Mark Questions and Answers

1. List any four objectives of process control.                                                                                         Suppressing the influence of external disturbances, Optimizing the performance, Increasing the productivity, Cost effective.
2. Define process                                                                                                                         Any system comprised of dynamic variables usually involved in manufacturing and production operations
3. What is manipulated variable                                                                                       It is a variable which is altered by the automatic control equipment so as to change the variable under control and  make it conform with the desired value.
4. Define Controlled variable                                                                                          It is the quantity of control system which is directly measured and controlled

5. What do you mean by self regulation?                                                                                                The output will move from one steady state to another for the sustained change in input. This means that for change in some input variable the output variable will rise until it reaches a steady state ( inflow = outflow ).

6. Why do we need mathematical models of process?                                                                   The physical equipment of the chemical process we want to control have not been constructed .Consequently we cannot experiment to determine how the process reacts to various inputs and therefore we cannot design the appropriate control system. If the process equipment needs to be available for experimentation the procedure is costly. Therefore we need a simple description of how the process reacts to various inputs, and this is what the mathematical models can provide to the control designer.

7. Name different test inputs.                                                                                                                 Step ,Ramp, Impulse, Sinusoidal, Pulse inputs

8. Name a process giving inverse response.                                                                                             Drum boiler system, in which the flow rate of the cold feed water is increased by a step the total volume of the boiling water and consequently the liquid level will decreased for a short period  and then it will start increasing.    

9. Define interacting system and give an example. Load changes in first process affects the second process and vise versa when both are connected in series nature is called interacting system. eg. two level tanks are connected  in series.

10. A tank operating at 10ft head, 5lpm outflow through a valve and has a cross section                   
1. area of 10 sq.ft . calculate the time constant.                                                   T=R/A, R=H/Q=10/(5X5.885X10^-4)

11. What is meant by non-self regulation?                                                                                                 A system that grows without limit for a sustained change in input. (constant outflow or outflow independent of inflow condition).

12. Write any two characteristics of first order process modeling.                                     The smaller the value of time constant the steeper the initial response of the system. A first order lag process is self regulating the ultimate value of the response equal to K_p(steady state gain of the process) for a unit step change in the input

13. Distinguish between continuous process and batch process.                                                    A process in which the materials or work flows more or less continuously through a plant apparatus while being treated is termed as continuous process. The problem of continuous process is due to load changes. (e.g.) storage vessel control.                          A process in which the materials or work are stationary at one physical location while being treated is termed as batch process.  (e.g.) furnace.

14. Mention two drawbacks of derivative action.                                                                          (i) The output of controller is zero at constant error condition.                                       (ii) It will amplify the noise present in the error signal.

15. What are the steps involved to design a best controller?                                                          Define appropriate performance criterion(ISE,IAE,ITAE).Compute the value of the performance criterion using a P,PI,or PID controller with the best setting for the adjusted parameters Kp,Ti,Td. Select controller which give the best value for  the performance criterion.

16. Define proportional control mode A controller mode in which the controller output is directly proportional to the error signal  P=K_pe_p+P₀    P-controller output K_p=Propotional gain,e_p=error in percent of variable range,P₀=Bias.

17. Define proportional band.                                                                               Proportional band is defined as the change in input of proportional controller mode required to produce a full-scale change in output

18. Write the relation ship between proportional band and proportional gain.                               The reciprocal of gain expressed as a percentage is called proportional band.                             K_P=100/PB

19. Define offset.                                                                                                              It is the steady state deviation (error) resulting from a change in value of load variable.

20. Define error(deviation)?                                                                                                          It is the difference at any instant between the value of controlled variable and the set point. E=S.P-P.V

21. Sketch Pneumatic P+I controller.                                                                                 Refer Curtis Johnson, Page No.418, and Fig. 10.17.

22. Why is the electronic controller preferred to pneumatic controller?                             Electronic signals operate over great distance without time lags. Electronic signals can be made compatible with digital controllers. Electronic devices can be designed to be essentially maintenance free. Intrinsic safety techniques eliminate electrical hazards. Less expensive to install. More energy efficient. Due to the above said properties electronic controllers are preferred to pneumatic controller.

23.  Explain the function of controller.                                                                              The element in a process control loop that evaluates error of the controlled variable and initiates corrective action by a signal to the controlling variable.
24. Write any two limitations of single speed floating control.                                                      The present output depends on the time history of errors and such history is not known ,the actual value of controller output floats at an undetermined value If the deviation persists controller saturates at either 100% or 0% and remain there until an error drives it towards opposite extreme

25. Sketch the input – output characteristic of a single – speed floating controller.

      Refer Curtis Johnson, Page No.368, and Fig. 9.7.

26. Why derivative mode of control is not recommended for a noisy process?                             The series capacitor in the derivative controller will amplify the noise in the error signal.
27. Define integral(reset) windup? The over charging in the presence of a continuous error of the integral capacitor which must discharge through a long time constant discharge path and which prevents a quick return to the desired control point.

28. Write Ziegler-Nicolas tuning formulae.                                                                                    For proportional controller: K_p = 0.5 K_u.                                                            For proportional-integral controller:  K_p = 0.45 K_u , T_i = P_u/1.2 .                            For Proportional-integral-derivative controller: K_p = 0.6 K_u, Ti = P_u/2, T_d = P_u/8.

29. Define controller tuning.                                                                                                          Deciding what values to be used for the adjusted parameters of the controller is called controller tuning.

30. What is reaction curve?                                                                                                           In process control ,the reaction curve is obtained by applying a step change(either in load or in set point)and plotting the response of the controlled variable with respect to time.

31. What performance criterion should be used for the selection and tuning of controller?          Keep the maximum error as small as possible.                                                        Achieve short settling time.                                                                  Minimize the integral of the errors until the process has settled to its desired set point.

32. Define ultimate gain.                                                                                                   The maximum gain of the proportional controller at which the sustained oscillations occur is called ultimate gain (K_u).

33. What is ITAE and when to go for it?                                                                                       ITAE means Integral Time Absolute Error. To suppress the errors that persist for long time, the ITAE criterion will tune the controllers better because the presence of large t amplifies the effect of even small errors in the value if integral.

34. What are the parameters required to design a best controller?                                      Process parameters (K,τ), Controller parameters(Kp,Ti,Td),Performance criterion(ISE,IAE,ITAE)

35. Write any two practical significance of the gain margin.                                                          It constitutes a measure of how far the system is the brink of instability.                                Higher the gain margin (above the value of one), the higher the safety factor we use controller tuning.                                                                                                                Typically, a control designer synthesizes a feedback system with gain margin larger than 180˚.

36. Why is it necessary to choose controller settings that satisfy both gain margin and phase margin?                                                                                                                  The gain margin and phase margin are the safety factors which is used for the design of a feedback system. Beyond the phase margin and gain margin the system goes to unstable position.           
37. What is tuning a controller based on quarter – decay ratio?                                         It is the procedure in which adjusting the proportional gain of controller upto ¼ th decay ratio waveform is obtained

38. Define ratio control.                                                                                                                Ratio control is a special type of feed forward control where two disturbances are measured and held in a constant ratio to each other.

39. Define cascade control.                                                                                                           Cascade control is defined as a control system composed of two loops where the set point of one loop (the inner loop) is the output of the controller of the other loop(the outer loop)

40. When cascade control will give improved performance than conventional feedback control?                                                                                                                                    In some process the secondary variables in it introduce disturbance throughout the system is measured and controlled by a separate loop.
41. Explain the purpose of cascade control for heat exchangers?                                      In heat exchangers, the control objective is to keep the exit temperature of stream. But the flow rate of the stream creates the low disturbance throughout itsa function. The secondary loop is used to compensate the flow rate of the stream.

42. What is meant by auctioneering control?                                                                                             Such control configurations select among several measurements the one with the highest value and feed it to the controller. Thus it is a selective controller which possesses several measured outputs and only one manipulated input.

43. Give any two types of selective control system.                                                                                  Override control for the protection of process equipment, auctioneering control.

44. What is limit switch?                                                                                                                In some cases it is necessary to change from the normal control action and attempt to prevent a process variable from exceeding an allowable upper or lower limit. This can be achieved by the use of special type switches called limit switches.

45. Mention the  types of limit switches.                                                                           High Select Switch(HSS), Low Select Switch(LSS).

46. What is HSS? High Select Switch(HSS) is a limit switch which is used whenever a variable should not exceed an upper limit.

47. What is LSS?                                                                                                               Low Select Switch(LSS) is a limit switch which is used whenever a variable should not exceed a lower limit.

48. What is override control?                                                                                             During the operation of the plant, it is possible that some of the process variables exceed the limit. In such cases it is necessary to change from the normal control action and attempt to prevent a process variable from exceeding an allowable upper or lower limit. This can be achieved by the use of special type switches called limit switches (HSS and LSS). This type of protective control is called override control.

49. What is split-range control?                                                                                                     To control A single process output can be controlled by co-coordinating the actions of several manipulated variables all of which have same effect on controlled output. Such systems are called split-range control systems.

50. Differentiate split-range control and selective control.                                                             Split-range control system  involves one measurement and more than one manipulated variables but selective control system involves one manipulated variables and several controlled outputs. 

51. Why are fuel and air sent at a specified ratio into a combustion chamber?                              To obtain the most efficient combustion.

52. What are decouplers?                                                                                                              The special element introduced in a system with two strongly interacting loops to cancel the interaction effect between the two loops and thus render two non-interacting control loops is called decoupler.

53. When is inferential control used?                                                                                It is used in some cases where the output of the process and the influence of the disturbance cannot be measured.

54. What is flashing in control valve?                                                                                            When a liquids enters a valve and the static pressure at the vena contracta less than the fluid vapour pressure and the valve outlet pressure is also less than the fluid vapour pressure the condition called flashing exists.

55. When do you use a valve positioner?                                                                          If the diaphragm actuator does not supply sufficient force to position the valve accurately and overcome any opposition that flowing conditions create a positioner may be required.

56. Give two examples for electric actuators.                                                                                Motor, Solenoids.

57. What is the need of I/P converter in a control system?                                                            In some process loop the controller is electronic and the final control element is electronic one. To interconnect these two we need a device that should linearly converts electric current in to gas pressure(4-20mA-3-15 psi). Such device is called I/P converter.

58. Why installed characteristics of a control valve is different from inherent characteristics?     Inherent characteristics is which the valve exhibits in the laboratory condition where the pressure drop is held constant. Installed or resultant characteristics is the relationship between flow and stroke when the valve is subjected to pressure conditions of the process.

59. Explain the function of pneumatic transmission lines.                                                             Used  to transmit the input signals into standard instrumentation pneumatic output signals (3 to 15 psi or 20 to 100 KPa).

60. What is the purpose of final control element?                                                             Components of a control system (such as valve) is used to  directly regulates the flow of energy or materials to the process. It directly determines the value of manipulated variable.

61. What is meant by cavitations in a control valve?                                                                     When a liquid enters a valve and the static pressure at the vena contracta drops to less than the fluid vapor pressure and then recovering to above fluid vapour pressure, this pressure recovery causes an implosion or  collapse of the vapour bubbles formed at the vena contracta This condition is called cavitation.

62. What is “equal percentage” in the equal percentage valve?                                                     For equal increment of stem travel at constant pressure drop an equal percentage change in existing flow occurs.

63. What are the characteristics of control valve?                                                  Inherent characteristics, Installed characteristics.

64. Differentiate inherent characteristics and installed characteristics.                               Inherent characteristics is which the valve exhibits in the laboratory condition where the pressure drop is held constant. Installed or resultant characteristics is the relationship between flow and stroke when the valve is subjected to pressure conditions of the process.

65. What are the advantages of digital controllers.

Ø  The digital controllers can perform large and complex computation with any desired degree of accuracy at very high speed.

Ø  The digital controllers are easily programmable and so they are more versatile

Ø  Digital controllers have better resolution.

66.. Explain  the terms sampling and sampler.

Sampling is a process in which the continuous-time signal (or analog signal) is converted into a discrete-time signal by taking samples of the continuous time signal at discrete time instants. Sampler is a device which performs the process of sampling

.

67What is periodic sampling?

The periodic sampling is a sampling process in which the discrete time signal or sequence is obtained by taking samples of continuous time signal periodically or uniformly at intervals of T seconds. Here T is called sampling period and 1/T = F s is called sampling frequency.

68. State Shanon's  sampling theorem.

Sampling theorem states that a band limited continuous-time signal with highest frequency fm, hertz can be uniquely recovered from its samples provided that the sampling rate Fs is greater than or equal to 2fm samples per second.

69. What is meant by quantization?

The process of converting a discrete-time continuous valued signal into a discrete­ time discrete valued signal is called quantization. In quantization the value of each signal sample is represented by a value selected from a finite set of possible values called quantization levels.

72. What is coding?

The coding is the process of representing each discrete value by a n-bit binary sequence (or code or number).

73. What are hold circuits?

Hold circuits are devices used to convert discrete time signals to continuous time signals.

74.What is a signal flow graph?

A signal flow graph is a diagram that represents a set of simultaneous algebraic equations .By taking L.T the time domain differential equations governing a control system can be transferred to a set of algebraic equations in s-domain.

75. What is transmittance?

  The transmittance is the gain acquired bythe signal when it travels from one node to another node in signal flow graph.

76.What is sink and source?

     Source is the input node in the signal fow graph and it has only outgoing branches. Sink is a output node in the signal flow graph and it has only incoming branches.

77.Define nontouching loop.

 The loops are said to be non touching if they do not have common nodes.

78.Write Masons Gain formula.

Masons Gain formula states that the overall gain of the systm is

    T = 1/  ΔΣk Pk  Δk

k-o.of forward paqths in the signal flow graph.

Pk- Forward path gain of kth forward path

  Δ = 1-[sum of individual loop gains ] +[sum of gain products of allpossible combinations of two non touching loops]-[sum of gain products of allpossible combinations of three non touching loops]+…

  Δk -  Δ for that part of the graph which is not touching kth forward path.

79.Write the analogous electrical elements in force voltage analogy for the elements of mechanical translational system.

Force-voltage e

Velocity v-current i

Displacement x-charge q

Frictional coeff B-Resistance R

Mass M- Inductance L

Stiffness K-Inverse of capacitance 1/C

80.Write the analogous electrical elements in force current analogy for the elements of mechanical translational system.

Force-current i

Velocity v-voltage v

Displacement x-fluxφ

Frictional coeff B-conductance 1/R

Mass M- capacitance C

Stiffness K-Inverse of inductance 1/L

81.Write the force balance equation ofm ideal mass element .

    F = M d2x /dt2

82. Write the force balance equation of ideal dashpot element .

    F = B dx /dt

83. Write the force balance equation of ideal spring element .

    F = Kx

84.Distinguish between open loop and closed loop system

Open loop	Closed loop
1.Innaccurate 2.Simple and economical 3.The changes in output due to external disturbance are not corrected 4.They are generally stable	Accurate Complex and costlier The changes in output due to external disturbances are corrected automatically Great efforts are needed to design a stable system

85.What is servomechanism?

    The servomechanism is a feedback control system in which th output is mechanical position (or time derivatives of position velocity  and acceleration,)

86.Why is negative feedback invariably preferred in closed loop system?

    The negative feedback results in better stability in steady state and rejects  any disturbance signals.

87.What is transient response

The transient response is the response of the system when the system changes from one state to another.

88.What is steady state response.

The steady state response is the response of the system when it approaches infinity.

89.What is an order of a system.

The order of asystem is the order of the differential equation governing the system.The order of the system can be obtained from the transfer function of the given system.

90.Define Damping ratio.

Damping ratio is defined as the ratio of actual damping to critical damping.

91.List the time domain specifications.

The time domain specifications are

i.Delay time

ii.Rise time

iii.Peak time

iv.Peak overshoot

92.Define Delay time.

The time taken for response to reach  50% of final value for the very first time is delay time.

93.Define Rise time.

The time taken for response to raise from 0% to 100% for the very first time is rise time.

94.Define peak time.

The time taken for the response  to reach the peak value for the first time is peak time.

95.Define peak overshoot.

Peak overshoot is defined as the ratio of maximum peak value measured from the Maxmium value  to final value

96.Define Settling time.

Settling time is defined as the time taken by the response to reach and stay within specified  error

97.What is the need for a controller?

   The controller is provided to modify the error signal for better control action

98.What are the different types of controllers.

    Proportional controller

    PI controller

    PD controller

    PID controller

98.What is proportional controller?

    It is  device that produces a control signal which is proportional to the input error signal.

100.What is PI controller?

          It is  device that produces a control signal  consisting of two terms –one proportional to error signal and the other proportional to the integral of error signal.

101. What  is PD controller

   PD controller is a proportional   plus derivative  controller which produces  

   an output signal consisting of two time -one proportional to error singal

   and other proportional to the derivative of the signal.

102What is the significance of integral controller and derivative 

     controller in aPID controller.

The proportional controller stabilizes the gain but produces a steady state error. The integral control reduces or eliminates the steady state error.

103.Why derivative controller is not used in control systems.

The derivative controller produces a control action based on the rate of change of error signal and it does not produce corrective measures for any constant error.