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Instrumentation and Control Engineering Materials-Free Download

26. Write the relation ship between proportional band and proportional gain

The reciprocal of gain expressed as a percentage is called proportional band. Kp=100/PB

27. Define offset.

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

28. 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

29. Sketch Pneumatic P+I controller

Refer Curtis Johnson, Page No.418, and Fig. 10.17.

30. 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. 

31. Explain the function of controller.

The element in process control loop that evaluates error of the controlled variable and initiates corrective action by a signal to the controlling variable. 

32. 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. 

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

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

34. 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.

35. 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. 

36. What are the two modes of controller.

Discontinuous and continuous mode are the two modes of controller.

37. Define Discontinuous mode of controller

If for only two values of error, control action is taken, it is Discontinuous mode of controller.

38. Define Continuous mode of controller

If for every value of error, control action is taken, it is Discontinuous mode of controller.

39. Give an example for Discontinuous and Continuous mode of controller.

Discontinuous-ON-OFF controller.

40. Define cycling.

Oscillations of error about zero is called cycling.

42. Define controller turning.

Deciding what values to be used for the adjusted parameters of the controller is called controller turning.

43. What is reaction curve.

In process controller, 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. 

44. What performance criterion should be used for the selection and turning of controller?

Keep the maximum error as small as possible. Minimize the integral of the errors until the process has settled set Point.

45. Define ultimate gain

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

46. 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. 

47. What are the parameters required to design a best controller?

Process Parameters (K,ô), Controller parameters (Kp,Ti, Td),performance creation (ISE, IAE, IATE)

48. Write any tow 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 turning. 

Typically, a control designer synthesizes a feedback system with gain margin larger than 1800.

49. 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. 

50. What is turning 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.

51. Name the time integral performance criteria measures.

Integral Square Error (ISE), Integral of absolute value of error (IAE), Integral of time weighted absolute error.

52. Define Integral Square Errors (ISE)

If we want to suppress large errors, ISE is better than IAE Because errors are squared and contribute more to the value of integral.

53. Define Integral Absolute Errors (IAE)

If we want to suppers small errors, IAE is better than ISE Because when we square small numbers, they even become smaller.

54. Define Integral of Time weighted Absolute Error (ITAE)

To suppress errors that persist for long times, ITAE criterion will tune the controllers better because the presence of large t amplifies the effect of even small errors in value of integral. 

55. Define One-quarter decay ratio

It is reaso nable trade off between fast rise time and reasonable setting time.

56. Give the satisfactory control for gas liquid level process.

Proportional Control is the satisfactory control for liquid level process.

57. Give the satisfactory control for gas pressure process.

Proportional Control is the satisfactory control for liquid level process.

58. Give the satisfactory control for vapour pressure process.

PI Control is the satisfactory control for vapour pressure process having fast response.

59. Give the satisfactory control for temperature process.

PID Control is the satisfactory control for temperature process.

60. Give the satisfactory control for composition process.

PID Control is the satisfactory control for composition process.

61. Define ratio control

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

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