This set of Process Control Multiple Choice Questions & Answers (MCQs) focuses on “Ideal Transfer Functions”.
1. Which of the following statements is true for a servo problem?
a) Load variable remains constant and set point changes
b) Set point remains constant and load variable changes
c) Both load variable and set point changes
d) Both set point and load variable remain constant
View Answer
Explanation: For a servo problem, set point is changed continuously. However, the load variable remains constant. In regulator problem, the set point remains constant and the load variable changes.
2. Which of the following statements is correct for negative feedback control loop?
a) Summation of the measured variable and the set point of the loop serves as the set point for the controller
b) Difference of the measured variable and set point of the loop serves as the set point for the controller
c) The error is squared and serves as the set point of the controller
d) The error is multiplied and serves as the set point of the controller
View Answer
Explanation: For a negative feedback control loop the error of the loop i.e., the difference of the set point and the measured variable serves as the set point for the controller. For positive feedback, the summation of the set point and the measured variable serves as the set point for the controller.
3. Which of the following pairs correctly represent the group of measuring elements?
a) Thermocouple, displacer device, heating coil
b) Thermometer, generator, u – tube manometer
c) Thermocouple, u – tube manometer, Anemometer
d) u – tube manometer, Borescope, LASER beam.
View Answer
Explanation: Thermocouple is used to measure temperature, u – tube manometer to measure pressure and anemometer to measure velocity for gas. LASER beam doesn’t measure anything and heating coil is used to heat.
4. Which of the following defines the time constant of a first order system correctly?
a) Time constant equals 63.8% of the ultimate value
b) Time constant equals 73.8% of the ultimate value
c) Time constant equals 53.8% of the ultimate value
d) Time constant equals 83.8% of the ultimate value
View Answer
Explanation: For a first order system, y(t) = Kp(1 – e– t/₸). So, putting, t = ₸ and Kp = 1 we get,
y(t) = 1 × (1 – e– t/₸)
or, Y(t) = (1 – 0.362)
or, y(t) = 0.638.
Hence, time constant equals 63.8% of the ultimate value.
5. Which of the following acts as a final control element?
a) PI controller
b) PID controller
c) Automatic pneumatic control valve
d) Thermometer
View Answer
Explanation: Thermometer measures temperature. Both PI and PID are control elements. A final control element uses the set point received by a controller and adjusts the manipulated variable to control the controlled variable. Thus, it can be rightly said that automatic pneumatic control valve is a final control element.
6. Which of the followings is the correct transfer function of a pure capacitive process?
a) Kp / (TS + 1)
b) Kp / TS
c) Kp / S
d) Kp / S + 1
View Answer
Explanation: First order system is represented by Kp / (TS + 1). Kp / TS and Kp / (S + 1) is not associated with capacitive control system. Capacitive control system has transfer function Kp / S.
7. Which of the followings correctly represents the measured variable?
a) Heat
b) Temperature of the vessel
c) Inlet temperature of the steam
d) Coolant temperature
View Answer
Explanation: In a heater system, we want to control the temperature of the vessel. It can be done by manipulating the flow rate of the inlet steam. So, the temperature of the vessel is the measured variable.
8. Which of the followings correctly defines deviation variables?
a) Deviation variable is expressed as the difference of the measured variable and steady state value of that measured variable
b) Deviation variable is expressed as the sum of the measured variable and steady state variable
c) Deviation variable is expressed as the product of the measured variable and steady state variable
d) Deviation variable is expressed as the difference of the measured variable and manipulated variable
View Answer
Explanation: To represent the transfer function of a system we need to express a system in terms of deviation variable and the Laplace transform of the deviation variable of the input and output are determined. The ratio of the Laplace transform of the deviation variable of output and input is the transfer function.
9. Why is P – controller used in secondary loop of a cascade control system?
a) Phase lag of P – controller is short
b) Phase lag of P – controller is long
c) P – controller provides highest degree of offset reduction
d) P controller is associated with highest degree of accuracy
View Answer
Explanation: Since P – controller has the no phase lag, there is hardly a delay in the response. In cascade control system, the inner loop or the secondary loop needs to be faster so that any deviation arising in the secondary loop can be rectified in it to make the primary loop error free.
10. Which of the following controllers has Steady State output directly proportional to the error only?
a) P – controller
b) PI – controller
c) PD – controller
d) PID controller
View Answer
Explanation: For a P – controller the Steady State output is directly proportional to the error. For PI controller, Steady State output is proportional to the integral of time. For PID, the Steady State output is directly proportional to both integral of time and the derivative.
Sanfoundry Global Education & Learning Series – Process Control.
To practice all areas of Process Control, here is complete set of Multiple Choice Questions and Answers.
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