27 real Process Control Basics questions from the Chemical Core bank, as asked in Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd — free to start.
1. What is Feedback control?
Junior
A.Laplace-domain ratio of output to input deviation variables for a linear system with zero initial conditions
B.raise proportional gain until sustained oscillation, then set PID constants from the ultimate gain and ultimate period
C.controller acts on the error between set point and measured variable only after a disturbance has already moved the output
D.choice of air-to-open or air-to-close so the valve moves to the safe position (fuel valve shuts, cooling valve opens) on loss of instrument air
A.Feedback control — span of measurement over which controller output moves from 0 to 100%, equal to 100 divided by the proportional gain
B.Feedback control — closed loop is unstable when the open-loop amplitude ratio exceeds 1 at the frequency where phase lag reaches 180 degrees
C.Feedback control — integral term keeps accumulating while the valve is saturated, causing large overshoot when control is regained; cured by anti-reset-windup logic
D.Feedback control — controller acts on the error between set point and measured variable only after a disturbance has already moved the output
A.Offset — span of measurement over which controller output moves from 0 to 100%, equal to 100 divided by the proportional gain
B.Offset — steady-state error left by a proportional-only controller after a load change, removed by adding integral action
C.Offset — closed loop is unstable when the open-loop amplitude ratio exceeds 1 at the frequency where phase lag reaches 180 degrees
D.Offset — first-order-plus-dead-time approximation using gain, time constant and delay, fitted from a step test and used for Cohen-Coon or open-loop tuning
A.Transfer function — closed loop is unstable when the open-loop amplitude ratio exceeds 1 at the frequency where phase lag reaches 180 degrees
B.Transfer function — raise proportional gain until sustained oscillation, then set PID constants from the ultimate gain and ultimate period
C.Transfer function — Laplace-domain ratio of output to input deviation variables for a linear system with zero initial conditions
D.Transfer function — integral term keeps accumulating while the valve is saturated, causing large overshoot when control is regained; cured by anti-reset-windup logic
14. Which term means: "first-order-plus-dead-time approximation using gain, time constant and delay, fitted from a step test and used for Cohen-Coon or open-loop tuning"?
A.FOPDT model — integral term keeps accumulating while the valve is saturated, causing large overshoot when control is regained; cured by anti-reset-windup logic
B.FOPDT model — choice of air-to-open or air-to-close so the valve moves to the safe position (fuel valve shuts, cooling valve opens) on loss of instrument air
C.FOPDT model — span of measurement over which controller output moves from 0 to 100%, equal to 100 divided by the proportional gain
D.FOPDT model — first-order-plus-dead-time approximation using gain, time constant and delay, fitted from a step test and used for Cohen-Coon or open-loop tuning
17. Which term means: "integral term keeps accumulating while the valve is saturated, causing large overshoot when control is regained; cured by anti-reset-windup logic"?
A.Integral windup — span of measurement over which controller output moves from 0 to 100%, equal to 100 divided by the proportional gain
B.Integral windup — Laplace-domain ratio of output to input deviation variables for a linear system with zero initial conditions
C.Integral windup — integral term keeps accumulating while the valve is saturated, causing large overshoot when control is regained; cured by anti-reset-windup logic
D.Integral windup — choice of air-to-open or air-to-close so the valve moves to the safe position (fuel valve shuts, cooling valve opens) on loss of instrument air
A.Ziegler-Nichols closed-loop tuning — choice of air-to-open or air-to-close so the valve moves to the safe position (fuel valve shuts, cooling valve opens) on loss of instrument air
B.Ziegler-Nichols closed-loop tuning — span of measurement over which controller output moves from 0 to 100%, equal to 100 divided by the proportional gain
C.Ziegler-Nichols closed-loop tuning — first-order-plus-dead-time approximation using gain, time constant and delay, fitted from a step test and used for Cohen-Coon or open-loop tuning
D.Ziegler-Nichols closed-loop tuning — raise proportional gain until sustained oscillation, then set PID constants from the ultimate gain and ultimate period
A.Bode stability criterion — choice of air-to-open or air-to-close so the valve moves to the safe position (fuel valve shuts, cooling valve opens) on loss of instrument air
B.Bode stability criterion — steady-state error left by a proportional-only controller after a load change, removed by adding integral action
C.Bode stability criterion — first-order-plus-dead-time approximation using gain, time constant and delay, fitted from a step test and used for Cohen-Coon or open-loop tuning
D.Bode stability criterion — closed loop is unstable when the open-loop amplitude ratio exceeds 1 at the frequency where phase lag reaches 180 degrees
26. Which term means: "choice of air-to-open or air-to-close so the valve moves to the safe position (fuel valve shuts, cooling valve opens) on loss of instrument air"?
A.Fail-safe valve action — first-order-plus-dead-time approximation using gain, time constant and delay, fitted from a step test and used for Cohen-Coon or open-loop tuning
B.Fail-safe valve action — controller acts on the error between set point and measured variable only after a disturbance has already moved the output
C.Fail-safe valve action — span of measurement over which controller output moves from 0 to 100%, equal to 100 divided by the proportional gain
D.Fail-safe valve action — choice of air-to-open or air-to-close so the valve moves to the safe position (fuel valve shuts, cooling valve opens) on loss of instrument air
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