Q.1 What does the term ‘open‑loop control’ mean in a control system?
The controller uses feedback to adjust the output
The controller operates without using any feedback
The system automatically corrects errors
The controller requires a human operator at all times
Explanation - In an open‑loop system the control action is independent of the output; there is no feedback path to correct errors.
Correct answer is: The controller operates without using any feedback
Q.2 Which component is commonly used to provide a proportional‑integral‑derivative (PID) control action?
Operational amplifier
Thermocouple
Variable resistor
Inductive sensor
Explanation - An op‑amp can be configured to implement the three terms of a PID controller in analog circuits.
Correct answer is: Operational amplifier
Q.3 In a block diagram, a summing point is used to:
Multiply two signals together
Add or subtract multiple signals
Delay a signal by a fixed amount
Convert a signal from analog to digital
Explanation - A summing point (or junction) combines signals algebraically, allowing addition or subtraction.
Correct answer is: Add or subtract multiple signals
Q.4 Which of the following best describes the transfer function of a first‑order lag system?
G(s) = K / (τs + 1)
G(s) = K·s
G(s) = K·(τs + 1)
G(s) = K·e^{‑τs}
Explanation - A first‑order lag (or low‑pass) has a transfer function with a denominator (τs + 1) and a constant gain K.
Correct answer is: G(s) = K / (τs + 1)
Q.5 What is the primary purpose of a sensor in a feedback control system?
To amplify the control signal
To provide the desired set‑point
To measure the actual process variable
To store system data
Explanation - Sensors convert physical quantities (temperature, speed, etc.) into electrical signals representing the process variable for comparison with the set‑point.
Correct answer is: To measure the actual process variable
Q.6 A PLC (Programmable Logic Controller) typically runs its program in which mode?
Batch mode
Continuous scan mode
Interrupt‑only mode
Random access mode
Explanation - PLCs continuously scan inputs, execute the program logic, and update outputs in a cyclic manner.
Correct answer is: Continuous scan mode
Q.7 In a closed‑loop system, increasing the proportional gain (Kp) generally results in:
Slower response and larger steady‑state error
Faster response but possible overshoot
No change in system dynamics
Elimination of all oscillations
Explanation - Higher Kp reduces rise time but can cause overshoot and reduced stability if too large.
Correct answer is: Faster response but possible overshoot
Q.8 Which diagram is used to assess the stability of a linear time‑invariant system by mapping poles in the s‑plane?
Bode plot
Root‑locus plot
Nyquist plot
Smith chart
Explanation - Root‑locus shows how the closed‑loop poles move in the s‑plane as a system parameter (usually gain) varies.
Correct answer is: Root‑locus plot
Q.9 A motor speed controller uses a tachometer as a feedback device. The tachometer provides:
Voltage proportional to speed
Current proportional to torque
Resistance proportional to temperature
Frequency proportional to voltage
Explanation - A tachometer generates a voltage output that is directly proportional to the rotational speed of the motor.
Correct answer is: Voltage proportional to speed
Q.10 What is the effect of adding an integrator (I‑term) to a PID controller?
Eliminates steady‑state error
Improves system bandwidth
Reduces overshoot
Increases noise sensitivity
Explanation - The integral action accumulates error over time, driving the steady‑state error to zero for a constant set‑point.
Correct answer is: Eliminates steady‑state error
Q.11 In a digital control system, the sampling frequency must be at least:
Twice the highest frequency component of the signal (Nyquist rate)
Equal to the system’s time constant
Five times the bandwidth of the plant
The same as the controller update rate
Explanation - According to Nyquist, the sampling rate must be at least twice the maximum signal frequency to avoid aliasing.
Correct answer is: Twice the highest frequency component of the signal (Nyquist rate)
Q.12 Which type of controller is best suited for a process with a large dead‑time?
Proportional (P) controller
Proportional‑Integral (PI) controller
Proportional‑Integral‑Derivative (PID) controller
Smith Predictor
Explanation - A Smith Predictor compensates for known dead‑time by predicting the future output, improving stability.
Correct answer is: Smith Predictor
Q.13 A Bode magnitude plot shows a gain crossover frequency of 10 rad/s. At this frequency, the phase margin is 45°. What does this indicate about system stability?
The system is unstable
The system is marginally stable
The system is stable with moderate robustness
The system will have zero overshoot
Explanation - A phase margin of 45° generally indicates a stable system with acceptable damping and robustness to gain variations.
Correct answer is: The system is stable with moderate robustness
Q.14 In a state‑space representation, the matrix that relates the input vector to the state derivative is called:
A matrix
B matrix
C matrix
D matrix
Explanation - The state‑space equations are \u03dot{x}=Ax+Bu, where B maps inputs to state rates.
Correct answer is: B matrix
Q.15 Which of the following sensors is most appropriate for measuring fluid level in a tank?
LVDT
Thermistor
Ultrasonic sensor
Hall‑effect sensor
Explanation - Ultrasonic sensors emit sound waves and measure the time of flight to determine distance, ideal for non‑contact level measurement.
Correct answer is: Ultrasonic sensor
Q.16 The term ‘dead‑band’ in a control system refers to:
A range of input where no output change occurs
The time delay between input and output
A region where the system oscillates
The maximum allowable error
Explanation - Dead‑band is intentionally set to avoid unnecessary control actions for small variations around the set‑point.
Correct answer is: A range of input where no output change occurs
Q.17 Which of the following is a typical advantage of using a PLC over a hard‑wired relay system?
Higher power handling capacity
Simpler wiring and easier reprogramming
Lower cost for small applications
No need for power supply
Explanation - PLCs replace complex relay wiring with software logic, making changes quick and reducing physical wiring.
Correct answer is: Simpler wiring and easier reprogramming
Q.18 In a PID controller, the derivative term primarily helps to:
Eliminate steady‑state error
Reduce overshoot and improve stability
Increase system gain
Compensate for dead‑time
Explanation - Derivative action predicts future error based on its rate of change, damping the response and reducing overshoot.
Correct answer is: Reduce overshoot and improve stability
Q.19 A system has a transfer function G(s)=1/(s+2). What is its time constant τ?
0.5 s
2 s
1 s
5 s
Explanation - For a first‑order system G(s)=1/(τs+1); comparing gives τ=1/2=0.5 s.
Correct answer is: 0.5 s
Q.20 Which method is commonly used to design a digital PID controller from its continuous‑time counterpart?
Euler’s forward method
Tustin (bilinear) transformation
Laplace inversion
Fourier series expansion
Explanation - The Tustin method maps s‑domain to z‑domain while preserving frequency characteristics, widely used for discretizing PID controllers.
Correct answer is: Tustin (bilinear) transformation
Q.21 A step response of a second‑order underdamped system shows a peak overshoot of 20%. What is the approximate damping ratio (ζ)?
0.2
0.45
0.7
0.9
Explanation - Percent overshoot ≈ exp(‑πζ/√(1‑ζ²)). Solving for 20% gives ζ≈0.45.
Correct answer is: 0.45
Q.22 In a DC motor speed control circuit, the armature voltage is regulated using a:
Current sensor
Pulse‑width modulation (PWM) driver
Thermocouple
Resistive load
Explanation - PWM varies the average voltage applied to the armature, providing efficient speed regulation.
Correct answer is: Pulse‑width modulation (PWM) driver
Q.23 Which of the following statements about hysteresis in a control system is true?
It reduces noise by creating a dead‑band.
It increases the system bandwidth.
It eliminates steady‑state error.
It causes the system to become linear.
Explanation - Hysteresis introduces a small range where output does not change, preventing rapid toggling due to noise.
Correct answer is: It reduces noise by creating a dead‑band.
Q.24 In a ladder diagram used for PLC programming, a normally open (NO) contact will:
Conduct when its associated input is FALSE
Conduct when its associated input is TRUE
Always conduct
Never conduct
Explanation - A NO contact closes (conducts) when the input condition is true (energized).
Correct answer is: Conduct when its associated input is TRUE
Q.25 A process has a transfer function G(s)=10/(s( s+5)). Which type of controller is generally required to eliminate steady‑state error to a ramp input?
P controller
PI controller
PD controller
PID controller
Explanation - A type‑2 system (two integrators) is needed for zero steady‑state error to a ramp; adding an integral (I) term gives one integrator, and the derivative term helps shape response, thus a PID is appropriate.
Correct answer is: PID controller
Q.26 Which of the following is NOT a typical source of disturbance in an industrial control loop?
Load torque variations
Sensor noise
Controller algorithm
Ambient temperature changes
Explanation - The algorithm is part of the controller design, not an external disturbance affecting the process.
Correct answer is: Controller algorithm
Q.27 The Nyquist stability criterion evaluates closed‑loop stability by:
Counting encirclements of the point (‑1,0) in the complex plane
Measuring the gain crossover frequency
Plotting pole‑zero maps
Calculating time‑domain step response
Explanation - Nyquist plots examine how the open‑loop frequency response encircles the critical point (‑1,0) to infer stability.
Correct answer is: Counting encirclements of the point (‑1,0) in the complex plane
Q.28 In an analog PID circuit, the derivative action is usually implemented using:
A resistor‑capacitor (RC) high‑pass filter
A resistor‑inductor (RL) low‑pass filter
A pure resistor
A potentiometer
Explanation - A high‑pass RC network produces an output proportional to the rate of change of the input voltage, providing derivative action.
Correct answer is: A resistor‑capacitor (RC) high‑pass filter
Q.29 Which communication protocol is widely used for real‑time data exchange between PLCs and HMIs in industrial settings?
HTTP
Modbus TCP
SMTP
FTP
Explanation - Modbus TCP is a simple, reliable protocol for transmitting control data over Ethernet in industrial automation.
Correct answer is: Modbus TCP
Q.30 A system with transfer function G(s)=1/(s²+2ζωₙs+ωₙ²) is known as:
First‑order lag
Second‑order underdamped
Second‑order overdamped
Non‑minimum phase
Explanation - The standard second‑order denominator with ζ<1 describes an underdamped system exhibiting oscillatory response.
Correct answer is: Second‑order underdamped
Q.31 In a cascade control structure, the inner loop typically controls:
The set‑point
A fast‑acting variable
The final output to the plant
Operator interface
Explanation - Cascade control uses a fast inner loop (e.g., flow) to improve performance of a slower outer loop (e.g., temperature).
Correct answer is: A fast‑acting variable
Q.32 Which type of sensor would you use to detect the presence of a metal object without physical contact?
Photoelectric sensor
Inductive proximity sensor
Capacitive sensor
Ultrasonic sensor
Explanation - Inductive sensors generate a magnetic field and detect changes caused by nearby metal objects.
Correct answer is: Inductive proximity sensor
Q.33 The term ‘gain margin’ in frequency‑response analysis indicates:
How much gain can increase before the system becomes unstable
The maximum gain the plant can handle
The ratio of output to input at steady state
The phase lag at crossover frequency
Explanation - Gain margin is the factor by which gain can be multiplied before the Nyquist plot reaches the critical point (‑1,0).
Correct answer is: How much gain can increase before the system becomes unstable
Q.34 A digital controller is implemented with a sampling time of 0.01 s. According to the rule of thumb, the controller bandwidth should be less than:
1 rad/s
10 rad/s
100 rad/s
1000 rad/s
Explanation - A typical guideline is that bandwidth ≤ 1/(10·sampling period); 1/(10·0.01)=10 Hz≈ 63 rad/s, so 100 rad/s is a reasonable upper limit.
Correct answer is: 100 rad/s
Q.35 Which of the following is a primary advantage of using a fuzzy logic controller over a conventional PID?
Exact mathematical model required
Better handling of nonlinearities and uncertainties
Higher computational speed
Simpler hardware implementation
Explanation - Fuzzy controllers use linguistic rules, making them robust to model inaccuracies and nonlinear behavior.
Correct answer is: Better handling of nonlinearities and uncertainties
Q.36 In a feedback loop, the term ‘reference signal’ is synonymous with:
Disturbance input
Set‑point
Measured output
Control effort
Explanation - The reference or set‑point is the desired value the controller attempts to achieve.
Correct answer is: Set‑point
Q.37 Which component is commonly used to provide isolation between a high‑voltage power circuit and a low‑voltage control circuit?
Transformer
Relay
Opto‑isolator
Capacitor
Explanation - Opto‑isolators use light to transfer signals across an insulating barrier, preventing high voltage from reaching the control side.
Correct answer is: Opto‑isolator
Q.38 The time delay (dead‑time) of a process can be represented in the Laplace domain as:
e^{‑Ls}
1/(Ls)
Ls
e^{‑L/s}
Explanation - A pure time delay of L seconds corresponds to the exponential term e^{‑Ls} in the s‑domain.
Correct answer is: e^{‑Ls}
Q.39 Which of the following best describes a ‘pole’ of a transfer function?
A value of s that makes the numerator zero
A value of s that makes the denominator zero
A frequency where the phase is 90°
A point where gain is maximum
Explanation - Poles are the roots of the denominator polynomial; they determine system stability and dynamics.
Correct answer is: A value of s that makes the denominator zero
Q.40 In a discrete‑time system, the z‑transform of a unit‑delay element (z^{-1}) corresponds to:
Differentiation
Integration
Delay of one sampling period
Advance of one sampling period
Explanation - Multiplication by z^{-1} shifts the sequence back by one sample, i.e., introduces a one‑sample delay.
Correct answer is: Delay of one sampling period
Q.41 A PLC I/O module that can both read a sensor voltage and output a control voltage is called:
Digital I/O module
Analog I/O module
Relay output module
Communication module
Explanation - Analog modules handle continuous voltage or current signals for sensing and actuation.
Correct answer is: Analog I/O module
Q.42 If a control system has a steady‑state error of zero for a step input but non‑zero for a ramp input, the system is said to be of which type?
Type 0
Type 1
Type 2
Type 3
Explanation - A Type‑1 system has one integrator, eliminating error for step inputs but not for ramps.
Correct answer is: Type 1
Q.43 Which of the following is the most common method to implement anti‑windup in a PID controller?
Limiting the integral term when actuator saturates
Increasing the proportional gain
Adding a high‑pass filter
Using a larger derivative term
Explanation - Anti‑windup prevents the integral term from accumulating excessively when the actuator cannot follow the command.
Correct answer is: Limiting the integral term when actuator saturates
Q.44 In a motor drive, the term ‘field‑oriented control’ (FOC) refers to:
Controlling the motor speed directly with PWM
Separating torque and flux control using d‑q axis transformation
Using a Hall‑effect sensor for position feedback
Operating the motor at a fixed voltage
Explanation - FOC transforms three‑phase currents to a rotating reference frame, allowing independent control of torque (q‑axis) and flux (d‑axis).
Correct answer is: Separating torque and flux control using d‑q axis transformation
Q.45 Which of the following statements about the Ziegler‑Nichols tuning method is true?
It requires a detailed mathematical model of the plant.
It is based on closed‑loop step response data.
It uses the ultimate gain and period from a sustained oscillation test.
It always yields a critically damped response.
Explanation - Ziegler‑Nichols open‑loop method identifies ultimate gain (K_u) and period (P_u) from the point of marginal stability to calculate PID parameters.
Correct answer is: It uses the ultimate gain and period from a sustained oscillation test.
Q.46 In a feedback system, the term ‘loop gain’ is defined as:
Product of forward path gain and feedback path gain
Difference between forward gain and feedback gain
Sum of all gains in the system
Reciprocal of the system bandwidth
Explanation - Loop gain = G(s)·H(s), where G(s) is forward transfer function and H(s) is feedback transfer function.
Correct answer is: Product of forward path gain and feedback path gain
Q.47 A 4‑20 mA current loop is commonly used in industrial control because:
It provides high voltage signals
Current is less affected by resistance changes in wiring
It allows for wireless transmission
It eliminates the need for a power supply
Explanation - In a current loop, the same current flows regardless of wire resistance, ensuring signal integrity over long distances.
Correct answer is: Current is less affected by resistance changes in wiring
Q.48 Which of the following best describes a ‘dead‑time compensator’?
A filter that removes high‑frequency noise
A controller that predicts future output to offset delay
A device that amplifies low‑frequency signals
A sensor that measures time lag
Explanation - Dead‑time compensators, such as the Smith Predictor, use a model to anticipate the effect of known delays.
Correct answer is: A controller that predicts future output to offset delay
Q.49 In a motor control application, the term ‘back‑EMF’ refers to:
Voltage generated by the motor when it acts as a generator
Current required to start the motor
Resistance of the motor windings
Power supplied to the motor
Explanation - Back‑EMF is the voltage induced opposite to the applied voltage when the motor rotates, proportional to speed.
Correct answer is: Voltage generated by the motor when it acts as a generator
Q.50 When designing a controller for a system with a dominant time constant τ, a typical rule for selecting the controller bandwidth (ω_c) is:
ω_c ≈ 0.1/τ
ω_c ≈ τ
ω_c ≈ 10/τ
ω_c ≈ 1/(10τ)
Explanation - A common heuristic sets the closed‑loop bandwidth roughly one‑tenth of the plant's dominant time constant to ensure adequate response.
Correct answer is: ω_c ≈ 1/(10τ)
Q.51 Which of the following is an example of a non‑linear element often found in control systems?
Linear resistor
Ideal op‑amp
Saturation limiter
Ideal transformer
Explanation - Saturation limits output to a maximum/minimum value, introducing non‑linearity into the system.
Correct answer is: Saturation limiter
Q.52 A system whose open‑loop transfer function has a pole at the origin is said to be:
Unstable
Marginally stable
Type‑0
Type‑1
Explanation - A pole at s=0 indicates an integrator, making the system Type‑1.
Correct answer is: Type‑1
Q.53 In a cascade control system, the inner loop is usually faster than the outer loop because:
It uses a larger controller gain
It controls a variable with faster dynamics
It receives the set‑point directly
It does not require feedback
Explanation - The inner loop regulates a fast‑acting variable (e.g., flow) to improve the response of the slower outer loop (e.g., temperature).
Correct answer is: It controls a variable with faster dynamics
Q.54 Which of the following is a key benefit of using model predictive control (MPC) in industrial processes?
Requires no mathematical model
Handles multivariable interactions and constraints
Eliminates the need for sensors
Operates without any computation
Explanation - MPC optimizes control moves over a future horizon while respecting process constraints and multivariable coupling.
Correct answer is: Handles multivariable interactions and constraints
Q.55 What does the term ‘bias’ refer to in a control system context?
A constant offset added to the controller output
The gain of the plant
The phase lag at crossover frequency
The sampling period of a digital controller
Explanation - Bias (or offset) is a steady value added to the control signal, often used to compensate for steady‑state errors.
Correct answer is: A constant offset added to the controller output
Q.56 In a PWM‑based speed controller for a DC motor, the average voltage applied to the motor is:
V_supply × duty_cycle
V_supply / duty_cycle
V_supply + duty_cycle
V_supply – duty_cycle
Explanation - The average voltage is proportional to the supply voltage multiplied by the duty cycle (ratio of ON time to total period).
Correct answer is: V_supply × duty_cycle
Q.57 A control system with a transfer function that has zeros in the right‑half s‑plane is said to be:
Minimum phase
Non‑minimum phase
Stable
Critically damped
Explanation - Zeros in the right‑half plane cause inverse response characteristics, classifying the system as non‑minimum phase.
Correct answer is: Non‑minimum phase
Q.58 In a ladder diagram, a coil that is energized when its input contacts are closed represents:
An output device
A sensor input
A timer function
A counter function
Explanation - Coils in ladder logic correspond to output elements (e.g., relay, motor) that are activated when the rung conditions are true.
Correct answer is: An output device
Q.59 Which of the following is the most common method for measuring angular position in a servomotor feedback loop?
Thermocouple
LVDT
Encoder
Strain gauge
Explanation - Encoders provide high‑resolution digital pulses proportional to shaft rotation, ideal for position feedback.
Correct answer is: Encoder
Q.60 The term ‘bandwidth’ of a control system usually refers to:
The frequency range where the magnitude response is above –3 dB
The maximum gain achievable
The time delay of the system
The number of poles in the transfer function
Explanation - Bandwidth is defined as the frequency at which the closed‑loop gain falls 3 dB below its low‑frequency value.
Correct answer is: The frequency range where the magnitude response is above –3 dB
Q.61 A plant has a transfer function G(s)=5/(s+5). If a proportional controller with Kp=2 is used, the closed‑loop transfer function becomes:
10/(s+10)
10/(s+15)
5/(s+7)
5/(s+12)
Explanation - Closed‑loop TF = Kp·G / (1 + Kp·G) = (2·5/(s+5)) / (1 + 2·5/(s+5)) = 10/(s+10).
Correct answer is: 10/(s+10)
Q.62 Which of the following is an example of a ‘soft sensor’ in industrial control?
A temperature probe
A model‑based estimator that calculates flow from pressure and valve position
A Hall‑effect sensor
A proximity switch
Explanation - Soft sensors use mathematical models and available measurements to infer variables that are not directly measured.
Correct answer is: A model‑based estimator that calculates flow from pressure and valve position
Q.63 In a digital PID implementation, the integral term can be approximated using which numerical integration method?
Euler forward (rectangular) method
Runge‑Kutta fourth‑order
Trapezoidal (Tustin) method
Simpson’s rule
Explanation - The trapezoidal (Tustin) method offers better frequency response preservation compared to simple rectangular integration.
Correct answer is: Trapezoidal (Tustin) method
Q.64 A control system exhibits a 180° phase shift at the gain crossover frequency. What does this imply about its stability margin?
The system is stable with large margin
The system is marginally stable
The system has zero phase margin and is on the verge of instability
The system is unstable
Explanation - Phase margin = 180° – phase at gain crossover; 180° shift gives 0° margin, indicating critical stability.
Correct answer is: The system has zero phase margin and is on the verge of instability
Q.65 Which of the following statements about the ‘Z‑transform’ is true?
It is the Laplace transform evaluated at s = jω
It maps discrete‑time sequences into the complex z‑plane
It only applies to continuous‑time systems
It cannot be used for stability analysis
Explanation - The Z‑transform is the discrete‑time counterpart of the Laplace transform, used for analysis of digital control systems.
Correct answer is: It maps discrete‑time sequences into the complex z‑plane
Q.66 In a high‑speed CNC machine, which control strategy is typically used for axis positioning?
Open‑loop voltage control
PID position control with feed‑forward
Bang‑bang control
Dead‑band only control
Explanation - Combining PID feedback with feed‑forward compensates for known dynamics, achieving high precision and speed.
Correct answer is: PID position control with feed‑forward
Q.67 The term ‘hysteresis’ in a relay contact means:
The contact opens and closes at the same voltage
The contact opens at a higher voltage than it closes
The contact never opens
The contact operates only in one direction
Explanation - Hysteresis provides a voltage difference between turn‑on and turn‑off points, preventing chatter.
Correct answer is: The contact opens at a higher voltage than it closes
Q.68 A system with transfer function G(s)=1/(s^2+2ζωₙs+ωₙ²) is critically damped when:
ζ = 0
ζ = 0.5
ζ = 1
ζ > 1
Explanation - Critical damping occurs at ζ = 1, giving the fastest response without overshoot.
Correct answer is: ζ = 1
Q.69 Which sensor type is most suitable for measuring very low pressures (near vacuum) in a control system?
Piezoelectric sensor
Capacitive pressure sensor
Strain‑gauge pressure sensor
Thermocouple
Explanation - Capacitive sensors can detect minute changes in pressure with high sensitivity, making them suitable for low‑pressure applications.
Correct answer is: Capacitive pressure sensor
Q.70 In a control loop, the ‘actuator’ is responsible for:
Measuring the process variable
Comparing set‑point with measured value
Applying corrective action to the process
Storing historical data
Explanation - Actuators (motors, valves, etc.) execute the controller’s command to influence the process.
Correct answer is: Applying corrective action to the process
Q.71 A proportional controller with gain Kp is applied to a first‑order plant G(s)=1/(τs+1). The closed‑loop time constant τ_cl is:
τ / (1+Kp)
τ·(1+Kp)
τ + Kp
τ – Kp
Explanation - Closed‑loop TF = Kp/(τs+1+Kp) → τ_cl = τ/(1+Kp).
Correct answer is: τ / (1+Kp)
Q.72 Which of the following is the main advantage of using a digital signal processor (DSP) for implementing control algorithms?
Unlimited processing speed
Ability to handle high‑frequency real‑time calculations efficiently
No need for power supply
Inherent analog filtering
Explanation - DSPs are optimized for fast arithmetic operations, making them ideal for real‑time control computations.
Correct answer is: Ability to handle high‑frequency real‑time calculations efficiently
Q.73 In a state‑space model, the matrix C relates the state vector to:
The input vector
The output vector
The derivative of the state vector
The disturbance vector
Explanation - The output equation is y = Cx + Du; C maps states to measured outputs.
Correct answer is: The output vector
Q.74 What is the primary purpose of a ‘feed‑forward’ control path?
To correct errors after they occur
To predict and compensate for measurable disturbances
To increase the loop gain
To provide hysteresis compensation
Explanation - Feed‑forward adds a control action based on known disturbances, improving response before the feedback loop detects error.
Correct answer is: To predict and compensate for measurable disturbances
Q.75 Which of the following is NOT a typical step in the design of a PID controller?
Model identification
Tuning of Kp, Ki, Kd
Selection of sensor type
Verification of stability margins
Explanation - While sensor choice is important, it is not part of the PID design methodology itself.
Correct answer is: Selection of sensor type
Q.76 A control system with a transfer function having a pole at s = –10 and a zero at s = –5 is said to be:
Unstable
Minimum phase
Non‑minimum phase
Critically damped
Explanation - Both pole and zero are in the left‑half plane, so the system is minimum phase.
Correct answer is: Minimum phase
Q.77 In a PLC, what does the term ‘scan time’ refer to?
The time required to power up the PLC
The duration of one complete read‑execute‑write cycle
The delay between two consecutive inputs
The period of the internal clock
Explanation - Scan time is the total time the PLC takes to read inputs, execute the program, and update outputs.
Correct answer is: The duration of one complete read‑execute‑write cycle
Q.78 Which of the following best characterizes a ‘dead‑time’ (transport lag) in a process?
A delay that can be modeled as a pure gain
A delay that appears as a phase lag increasing with frequency
A delay that reduces system bandwidth without affecting phase
A delay that only affects steady‑state error
Explanation - Dead‑time introduces a frequency‑dependent phase lag, making high‑frequency response more difficult.
Correct answer is: A delay that appears as a phase lag increasing with frequency
Q.79 In the context of industrial control, the term ‘HMI’ stands for:
Human‑Machine Interface
High‑Magnitude Input
Hybrid Motor Inverter
Hardware Monitoring Interface
Explanation - HMI provides a graphical interface for operators to monitor and control processes.
Correct answer is: Human‑Machine Interface
Q.80 A system is said to be ‘asymptotically stable’ if:
Its response grows without bound
All trajectories converge to zero as time approaches infinity
It oscillates forever with constant amplitude
It has a pole on the imaginary axis
Explanation - Asymptotic stability implies that any initial condition decays to zero over time.
Correct answer is: All trajectories converge to zero as time approaches infinity
Q.81 In a DC motor, the torque constant K_t relates:
Torque to armature current
Speed to voltage
Back‑EMF to speed
Resistance to temperature
Explanation - Torque = K_t × I_armature; the constant links current to generated torque.
Correct answer is: Torque to armature current
Q.82 Which of the following is a common method for reducing measurement noise in sensor signals before they are used by a controller?
Increasing the controller gain
Using a low‑pass filter
Adding a dead‑band
Applying a high‑pass filter
Explanation - Low‑pass filtering attenuates high‑frequency noise, providing a smoother measurement for control.
Correct answer is: Using a low‑pass filter
Q.83 The term ‘cascade control’ typically involves:
Two independent controllers acting on the same plant
A primary controller feeding a set‑point to a secondary controller
A controller with multiple outputs
A controller that operates without feedback
Explanation - In cascade control, the outer loop’s output becomes the set‑point for the inner loop, improving disturbance rejection.
Correct answer is: A primary controller feeding a set‑point to a secondary controller
Q.84 When using a PID controller, the term ‘integral wind‑up’ occurs when:
The derivative term becomes too large
The controller output saturates while the integral term continues to increase
The proportional gain is set to zero
The sampling time is too short
Explanation - Wind‑up happens because the integrator accumulates error even though the actuator cannot follow, leading to overshoot when it recovers.
Correct answer is: The controller output saturates while the integral term continues to increase
Q.85 A system with a transfer function G(s)= (s+2)/(s^2+4s+5) has how many poles?
One
Two
Three
Four
Explanation - The denominator is a second‑order polynomial, giving two poles.
Correct answer is: Two
Q.86 Which of the following control strategies is best suited for a highly nonlinear process with large variations in operating point?
Linear PID control
Gain‑scheduled PID control
Bang‑bang control
Open‑loop control
Explanation - Gain scheduling adjusts PID parameters based on operating conditions, handling nonlinearity more effectively.
Correct answer is: Gain‑scheduled PID control
Q.87 In a digital control system, quantization error is introduced by:
The sampling period
The finite resolution of the analog‑to‑digital converter
The controller algorithm
The power supply ripple
Explanation - Quantization error arises because the ADC maps a continuous voltage into discrete levels.
Correct answer is: The finite resolution of the analog‑to‑digital converter
Q.88 Which of the following is true about a ‘Bode plot’?
It shows the time‑domain step response of a system.
It displays magnitude and phase versus frequency on logarithmic scales.
It is used only for discrete‑time systems.
It requires the system to be non‑linear.
Explanation - Bode plots consist of two graphs—magnitude (dB) and phase (degrees) versus log frequency—used for frequency‑domain analysis.
Correct answer is: It displays magnitude and phase versus frequency on logarithmic scales.
Q.89 A control system with a transfer function having a pole at s = +2 is:
Stable
Unstable
Marginally stable
Critically damped
Explanation - A pole in the right‑half s‑plane (positive real part) leads to exponential growth, causing instability.
Correct answer is: Unstable
Q.90 In a PLC ladder diagram, a ‘timer on‑delay’ (TON) instruction:
Activates its output immediately when its input becomes true
Delays the activation of its output by a preset time after the input becomes true
Turns off the output after a fixed time regardless of input
Counts the number of input pulses
Explanation - TON waits for the preset time while the input is true before energizing its output coil.
Correct answer is: Delays the activation of its output by a preset time after the input becomes true
Q.91 Which of the following is a characteristic of a second‑order underdamped system?
No overshoot
Oscillatory response with decaying amplitude
Pure exponential decay
Infinite settling time
Explanation - Underdamped systems (ζ<1) exhibit oscillations that gradually diminish over time.
Correct answer is: Oscillatory response with decaying amplitude
Q.92 In the context of industrial networks, the fieldbus protocol ‘PROFIBUS’ is primarily used for:
High‑speed video streaming
Connecting PLCs to sensors and actuators
Internet browsing
Wireless communication
Explanation - PROFIBUS is a fieldbus standard for real‑time data exchange between automation devices.
Correct answer is: Connecting PLCs to sensors and actuators
Q.93 A control system that uses a ‘fuzzy logic controller’ instead of a conventional PID is most beneficial when:
The plant model is perfectly known
The system is linear and time‑invariant
The process exhibits high non‑linearity and uncertainty
The controller must operate at extremely high frequencies
Explanation - Fuzzy logic uses linguistic rules, making it robust to model inaccuracies and nonlinear behavior.
Correct answer is: The process exhibits high non‑linearity and uncertainty
Q.94 The term ‘settling time’ for a step response is defined as:
The time required for the response to reach the final value for the first time
The time required for the response to stay within a specified error band around the final value
The time required for the controller to compute the output
The time required for the system to become unstable
Explanation - Settling time is measured when the response remains within a predefined tolerance (e.g., ±2 %) of the final value.
Correct answer is: The time required for the response to stay within a specified error band around the final value
Q.95 In a discrete‑time PID algorithm, the derivative term is often approximated by:
Forward difference of the error
Integral of the error
Multiplication by the error
A constant gain
Explanation - The derivative can be approximated as (e(k)‑e(k‑1))/T_s, where T_s is the sampling period.
Correct answer is: Forward difference of the error
Q.96 Which of the following is an advantage of using a ‘vector control’ (field‑oriented control) technique for AC induction motors?
Eliminates the need for a power supply
Provides independent control of torque and flux
Reduces motor weight
Allows operation without sensors
Explanation - Vector control transforms three‑phase currents to a d‑q frame, separating torque (q‑axis) and flux (d‑axis) control.
Correct answer is: Provides independent control of torque and flux
Q.97 A control system with a phase margin of 5° is considered:
Highly robust
Marginally stable
Very lightly damped and close to instability
Over‑damped
Explanation - Small phase margin (≈5°) indicates low robustness and a high likelihood of oscillations.
Correct answer is: Very lightly damped and close to instability
Q.98 Which type of filter is commonly used to smooth the output of a derivative term in an analog PID circuit?
Low‑pass RC filter
High‑pass RC filter
Band‑stop filter
All‑pass filter
Explanation - Derivative action amplifies high‑frequency noise; a low‑pass filter attenuates this noise.
Correct answer is: Low‑pass RC filter
Q.99 In a control system, the term ‘gain scheduling’ refers to:
Changing controller gains based on operating conditions
Keeping gains fixed for all conditions
Using only proportional control
Scheduling tasks in a PLC program
Explanation - Gain scheduling adapts controller parameters to different regions of operation, improving performance for nonlinear plants.
Correct answer is: Changing controller gains based on operating conditions
Q.100 Which of the following is a typical symptom of a controller with excessive derivative gain?
Slow response time
Increased steady‑state error
High sensitivity to measurement noise
Large overshoot
Explanation - Derivative action amplifies high‑frequency noise, leading to noisy control signals.
Correct answer is: High sensitivity to measurement noise
Q.101 A digital controller uses a sampling period of 5 ms. According to the Nyquist theorem, what is the maximum frequency that can be accurately represented?
100 Hz
200 Hz
50 Hz
400 Hz
Explanation - Nyquist frequency = 1/(2·T_s) = 1/(2·0.005) = 100 Hz.
Correct answer is: 100 Hz
Q.102 In a PLC, which module would you select to convert a 4‑20 mA current signal into a usable voltage for the controller?
Digital input module
Analog output module
Analog input module
Relay output module
Explanation - Analog input modules condition current or voltage signals for processing by the PLC.
Correct answer is: Analog input module
Q.103 When a system’s open‑loop transfer function has a gain margin of 6 dB, this means:
The system can tolerate a gain increase of 6 dB before becoming unstable
The system can tolerate a gain decrease of 6 dB before becoming unstable
The gain is exactly 6 dB at crossover
The phase margin is 6 degrees
Explanation - Gain margin indicates how much gain can be increased before the loop reaches the critical point (‑1,0).
Correct answer is: The system can tolerate a gain increase of 6 dB before becoming unstable
Q.104 Which of the following is an example of a ‘hard‑real‑time’ requirement in an industrial control system?
Updating a display once per second
Ensuring a motor shuts off within 5 ms of a fault detection
Logging temperature data every minute
Sending a status email after a batch completes
Explanation - Hard real‑time constraints demand strict timing guarantees; a 5 ms shutdown is a safety‑critical deadline.
Correct answer is: Ensuring a motor shuts off within 5 ms of a fault detection
Q.105 In a feedback control system, the term ‘reference tracking’ refers to:
Minimizing the effect of disturbances
Ensuring the output follows the desired set‑point over time
Reducing measurement noise
Increasing the controller bandwidth
Explanation - Reference tracking evaluates how well the system follows the commanded trajectory.
Correct answer is: Ensuring the output follows the desired set‑point over time
Q.106 Which of the following best describes a ‘sliding mode controller’?
A linear controller using only proportional action
A nonlinear control strategy that forces system trajectories onto a predefined surface
A controller that slides between different gain values based on temperature
A PID controller with variable sampling time
Explanation - Sliding mode control switches control actions to drive the system state onto a sliding surface, offering robustness to disturbances.
Correct answer is: A nonlinear control strategy that forces system trajectories onto a predefined surface
Q.107 The term ‘actuator saturation’ in a control loop means:
The actuator is operating at its maximum or minimum limit
The actuator is damaged
The actuator is providing feedback
The actuator is offline
Explanation - Saturation occurs when the control signal exceeds the physical limits of the actuator, causing clipping.
Correct answer is: The actuator is operating at its maximum or minimum limit
Q.108 In a process control system, a ‘set‑point’ is:
The measured variable
The desired value the controller should achieve
The disturbance input
The feedback gain
Explanation - The set‑point (reference) defines the target for the control loop.
Correct answer is: The desired value the controller should achieve
Q.109 Which of the following devices converts a digital control signal into an analog voltage for a valve actuator?
Digital to Analog Converter (DAC)
Analog to Digital Converter (ADC)
Relay
Transistor
Explanation - DACs generate analog voltages from digital words, allowing precise control of analog actuators.
Correct answer is: Digital to Analog Converter (DAC)
Q.110 A process with a dead‑time of 2 s and a dominant time constant of 5 s is best approximated by which model?
First‑order plus dead‑time (FOPDT)
Second‑order underdamped
Integrating process
Pure delay only
Explanation - FOPDT models combine a first‑order lag with a pure time delay, matching the given parameters.
Correct answer is: First‑order plus dead‑time (FOPDT)
Q.111 In a control loop, the term ‘gain scheduling’ is most often used with which controller type?
PI controller
PID controller
On‑off controller
Bang‑bang controller
Explanation - Gain scheduling adapts PID parameters based on operating point, making it suitable for nonlinear processes.
Correct answer is: PID controller
Q.112 When designing a digital controller, why is it important to include anti‑aliasing filters before the ADC?
To increase the sampling frequency
To remove high‑frequency components that could fold into the baseband
To amplify low‑frequency signals
To convert voltage to current
Explanation - Anti‑aliasing filters limit the signal bandwidth to below half the sampling rate, preventing aliasing.
Correct answer is: To remove high‑frequency components that could fold into the baseband
Q.113 A control system with a pole at s = –0.5 and a zero at s = –5 is:
Unstable
Minimum phase with faster zero dynamics
Non‑minimum phase
Critically damped
Explanation - Both pole and zero lie in the left‑half plane; the zero at –5 is farther left, giving faster dynamics but still minimum phase.
Correct answer is: Minimum phase with faster zero dynamics
Q.114 Which of the following is a primary cause of ‘limit cycle’ oscillations in a digital PID controller?
Too high proportional gain
Quantization and sampling effects combined with non‑linearities like saturation
Large integral time constant
Low derivative gain
Explanation - Limit cycles arise from discrete implementation errors and non‑linear elements causing sustained oscillations.
Correct answer is: Quantization and sampling effects combined with non‑linearities like saturation
Q.115 In a cascade control arrangement, which loop typically reacts faster?
Outer loop
Inner loop
Both loops have equal speed
Neither loop reacts
Explanation - The inner loop controls a fast‑acting variable, providing rapid correction to the outer loop.
Correct answer is: Inner loop
Q.116 Which type of sensor uses changes in capacitance to detect position?
Inductive proximity sensor
Capacitive sensor
LVDT
Thermocouple
Explanation - Capacitive sensors detect variations in capacitance caused by the presence or movement of an object.
Correct answer is: Capacitive sensor
Q.117 For a second‑order system, the relationship between damping ratio (ζ) and percent overshoot (PO) is given by PO ≈ 100·e^{‑πζ/√(1‑ζ²)}. If PO = 5 %, ζ is approximately:
0.2
0.5
0.7
0.9
Explanation - Solving the equation for PO = 5 % yields ζ≈0.7.
Correct answer is: 0.7
Q.118 In a PLC, what does the term ‘rung’ refer to?
A physical wire connecting modules
A horizontal line in a ladder diagram representing a logical expression
A type of sensor
A memory address
Explanation - Each rung contains contacts and coils that define a piece of the control logic.
Correct answer is: A horizontal line in a ladder diagram representing a logical expression
Q.119 Which of the following is a common method for reducing the effect of measurement noise on the derivative term of a PID controller?
Increasing proportional gain
Adding a low‑pass filter to the derivative term
Removing the integral term
Increasing the sampling rate
Explanation - A low‑pass filter attenuates high‑frequency noise that the derivative term would otherwise amplify.
Correct answer is: Adding a low‑pass filter to the derivative term
Q.120 In a control system, the ‘phase crossover frequency’ is defined as:
The frequency where the magnitude plot crosses 0 dB
The frequency where the phase plot crosses –180°
The frequency where gain equals 1
The frequency where the system becomes unstable
Explanation - Phase crossover frequency is where the phase lag reaches –180°, used together with gain margin calculations.
Correct answer is: The frequency where the phase plot crosses –180°
Q.121 Which control strategy is most appropriate for a system that must follow a rapidly changing trajectory while respecting actuator constraints?
PID control
Model Predictive Control (MPC)
Bang‑bang control
Open‑loop control
Explanation - MPC optimizes future control actions while handling constraints, making it suitable for fast trajectory tracking.
Correct answer is: Model Predictive Control (MPC)