All questions of Time Response Analysis for Electrical Engineering (EE) Exam

The steady-state error of a feedback control system with an acceleration input becomes finite in a
  • a)
    type-3 system
  • b)
    type-2 system
  • c)
    type-1 system
  • d)
    type-0 system
Correct answer is option 'B'. Can you explain this answer?

Raj Choudhary answered
Steady state error with an acceleration input having an amplitude of A is given by

where, 
Hence, if the type of the system = 2, then Ka = some non-zero value or finite value due to which we will get some finite vaiue of Ka.

The time required for the response curve to reach and stay within the specified 2-5% of final value is referred to as :
  • a)
    Peak time
  • b)
    Rise time
  • c)
    Settling time
  • d)
    Peak overshoot time
Correct answer is option 'C'. Can you explain this answer?

Understanding Settling Time
Settling time is a critical parameter in control systems, particularly in the field of Electronics and Communication Engineering (ECE). It indicates how quickly a system can stabilize after a disturbance or a change in input.
Definition of Settling Time
- Settling time is defined as the time required for the system’s response curve to reach and remain within a specified percentage (usually 2-5%) of its final value after a step input is applied.
Importance of Settling Time
- Performance Indicator: Settling time is crucial for assessing the performance of control systems. A shorter settling time means the system responds quickly and efficiently to changes.
- Stability: It indicates how well the system can stabilize after fluctuations or disturbances, ensuring reliability in applications.
Comparison with Other Time Parameters
- Peak Time: This is the time it takes for the response to reach its first peak. It does not account for how long it takes to settle.
- Rise Time: This measures the time required for the response to rise from a specified lower percentage to a specified upper percentage of the final value.
- Peak Overshoot Time: This refers to the amount by which the response exceeds the final value before settling down.
Conclusion
- Settling time is key to evaluating a system's performance and stability. It ensures that after an input change, the system not only responds but also stays within acceptable limits of the final value, providing a clear indication of how effectively the system performs in real-world applications. Understanding this concept is essential for designing efficient control systems in ECE.

How can the steady state error can be reduced?
  • a)
    By decreasing the type of the system
  • b)
    By increasing the input 
  • c)
    By decreasing the static error constant
  • d)
    By increasing system gain
Correct answer is option 'D'. Can you explain this answer?

From the above table, it is clear that for type – 1 system, a system shows zero steady-state error for step-input, finite steady-state error for Ramp-input and ∞ steady-state error for parabolic-input.
As the type of the system increases, the steady-state error decreases.
The steady-state error is inversely proportional to the gain. Therefore, it can be reduced by increasing the system gain.

A system has position error constant Kp  = 3. The steady state error for input of 8tu(t) is
  • a)
    2.67
  • b)
    2
  • c)
  • d)
    0
Correct answer is option 'C'. Can you explain this answer?

Aryan Mathur answered
Steady state error is a measure of how well a control system is able to track a desired input signal. It represents the difference between the desired output and the actual output of the system when the input signal has reached a steady state.

In this system, the position error constant Kp is given as 3. The position error constant relates the steady state error to the input signal. It is defined as the ratio of the steady state error to the input signal. Mathematically, Kp = 1 / steady state error.

Given that the input signal is 8tu(t), where u(t) is the unit step function, we can determine the steady state error using the position error constant.

Let's calculate the steady state error step by step:

1. First, we need to determine the Laplace transform of the input signal.
The Laplace transform of tu(t) is 1/s^2.
Therefore, the Laplace transform of 8tu(t) is 8/s^2.

2. Next, we determine the Laplace transform of the steady state error.
Let the Laplace transform of the steady state error be E(s).
Then, E(s) = Kp / (s * 8/s^2) = Kp * s^2 / 8.

3. To find the steady state error, we need to take the inverse Laplace transform of E(s).
Using inverse Laplace transform tables or software, we can determine that the inverse Laplace transform of Kp * s^2 / 8 is Kp/8 * t^2.

4. Since the input signal is a unit step function, the steady state error is the value of the inverse Laplace transform at t = infinity.
Therefore, the steady state error is Kp/8 * infinity^2 = Kp/8 * ∞ = 3/8 * ∞ = 0.

Hence, the correct answer is option 'C', which states that the steady state error is 0.

When the period of the observation is large, the type of the error will be:
  • a)
    Transient error
  • b)
    Steady state error
  • c)
    Half-power error
  • d)
    Position error constant
Correct answer is option 'B'. Can you explain this answer?

Answer: b
Explanation: The error will be the steady state error if the period of observation is large as the time if large then the final value theorem can be directly applied.

The steady state error of a control system can be reduced by
  • a)
    increasing time constant of the system only.
  • b)
    increasing gain constant of the system only.
  • c)
    increasing both the time constant and gain of the system.
  • d)
    none of the above.
Correct answer is option 'C'. Can you explain this answer?

Anuj Kapoor answered
Time constant of the system, 

When  is increased means damping is reduced, thus steady state error reduces.
Also, when gain of the system increases, then damping reduces and therefore, steady state error reduces.

With negative feedback in a closed loop control system, the system sensitivity to parameter variation:
  • a)
    Increases
  • b)
    Decreases
  • c)
    Becomes zero
  • d)
    Becomes infinite
Correct answer is option 'B'. Can you explain this answer?

Mahi Bose answered
Answer: b
Explanation: Sensitivity is defined as the change in the output with respect to the change in the input and due to negative feedback reduces by a factor of 1/ (1+GH).

A third order system is approximated to an equivalent second order system. The rise time of this approximated lower order system will be:
  • a)
    Same as the original system for any input
  • b)
    Smaller than the original system for any input
  • c)
    Larger than the original system for any input
  • d)
    Larger or smaller depending on the input
Correct answer is option 'B'. Can you explain this answer?

Answer: b
Explanation: As order of the system increases the system approaches more towards the ideal characteristics and if the third order system is approximated to an equivalent second order system then the rise time of this will be smaller than the original system for any input.

The identical first order system have been cascaded non-interactively. The unit step response of the systems will be:
  • a)
    Overdamped
  • b)
    Underdamped
  • c)
    Undamped
  • d)
    Critically damped
Correct answer is option 'D'. Can you explain this answer?

Kalyan Patel answered
Answer: d
Explanation: Since both the systems that is the first order systems are cascaded non-interactively, the overall unit step response will be critically damped.

A system has a single pole at origin. Its impulse response will be:
  • a)
    Constant
  • b)
    Ramp
  • c)
    Decaying exponential
  • d)
    Oscillatory
Correct answer is option 'A'. Can you explain this answer?

Avik Saha answered
Answer: a
Explanation: For a single pole at origin the system is of type 1 and impulse response of the system with single pole at the origin will be constant.

Assertion (A): With the increase in bandwidth of the system the response of the system becomes fast.
Reason (R): Damping ratio of the system decreases with the increase in bandwidth.
  • a)
    Both A and R are true and R is a correct explanation of A.
  • b)
    Both A and R are true but R is not a correct explanation of A.
  • c)
    A is true but R is false.
  • d)
    A is false but R is true.
Correct answer is option 'B'. Can you explain this answer?

Explanation:


The statement "With the increase in bandwidth of the system the response of the system becomes fast" is true because bandwidth is a measure of the frequency range that a system can handle. A system with a higher bandwidth can handle higher frequency signals, and thus respond faster to changes in the input signal.

However, the statement "Damping ratio of the system decreases with the increase in bandwidth" is not always true. The damping ratio is a measure of how quickly the system oscillations decay in response to an input. It is related to the natural frequency of the system and the amount of damping in the system.

Counterexample:


Consider a system with a low damping ratio and a high natural frequency. This system will have a high bandwidth because it can respond to high-frequency signals. However, the damping ratio is low, meaning that the system will oscillate for a long time before settling to a steady-state response. Therefore, in this case, the response of the system is not necessarily fast even though the bandwidth is high.

Conclusion:


Therefore, both statements are true, but the second statement does not always provide a correct explanation for the first statement. The relationship between bandwidth and response time is more complex and depends on other factors such as the damping ratio and natural frequency of the system.

The system with the open loop transfer function 1/s(1+s) is:
  • a)
    Type 2 and order 1
  • b)
    Type 1 and order 1
  • c)
    Type 0 and order 0
  • d)
    Type 1 and order 2
Correct answer is option 'D'. Can you explain this answer?

Kajal Yadav answered
Answer: d
Explanation: Type is defined as the number of poles at origin and order is defined as the total number of poles and this is calculated with the help of the transfer function from the above transfer function the type is 1 and order is 2.

If a type 0 system is subjected to step input, what is its effect on steady state error?
  • a)
     It increases continuously
  • b)
     It remains constant
  • c)
     It decreases monotonically
  • d)
     It gets subjected to another input
Correct answer is option 'B'. Can you explain this answer?

Introduction:
A system's steady-state error is the difference between the desired output and the actual output of the system when it reaches a stable condition. In this case, we are considering a type 0 system, which means that the steady-state error is determined by the input to the system.

Effect of a Step Input:
When a step input is applied to a type 0 system, the output of the system gradually approaches the desired value over time. The response of a type 0 system to a step input can be represented by a ramp function.

Explanation:
The steady-state error of a type 0 system is defined as the difference between the desired value and the output value when the system reaches a stable condition. In the case of a step input, the desired value is constant over time.

Initially, when the step input is applied, the output of the system starts to change. As time progresses, the output approaches the desired value, and the steady-state error decreases. Eventually, when the system reaches a stable condition, the output value becomes equal to the desired value, resulting in zero steady-state error.

Since the desired value remains constant throughout the response of a type 0 system to a step input, the steady-state error also remains constant. This is because once the system reaches a stable condition, the output value doesn't change further, and the steady-state error is determined solely by the difference between the desired value and the output value at that point.

Conclusion:
In summary, when a type 0 system is subjected to a step input, the steady-state error remains constant. This is because the output of the system eventually reaches a stable condition where it matches the desired value, resulting in zero steady-state error.

Given a unity feedback system with G (s) =K/ s (s+4). What is the value of K for a damping ratio of 0.5?
  • a)
    1
  • b)
    16
  • c)
    4
  • d)
    2
Correct answer is option 'B'. Can you explain this answer?

Answer: b
Explanation: Comparing the equation with the standard characteristic equation gives the value of damping factor, natural frequency and value of gain K.

Consider a causal second-order system with the transfer function with a unit-step  as an input. Let c(s) be the corresponding output. The time taken by the system output c(t) to reach 94% of its steady-state value  rounded off to two decimal places, is 
  • a)
    5.25
  • b)
    2.81
  • c)
    4.50
  • d)
    3.89
Correct answer is option 'C'. Can you explain this answer?

Imtiaz Ahmad answered
Concept:

A = 1
B = -1
C = -1

Applying inverse Laplace transform:
C(t) = 1 – e-t – te-t 
at steady state i.e. at t → ∞

C(t) = 1
94% of steady state = 94/100 x 1
= 0.94.
0.94 = 1 – e-t – te-t 
Substitute all options
Let us substitute options
option (a) t= 5.25
1 – e-5.25 – 5.25 e-5.25 

= 0.967
option (b)
t = 4.50

1 – 0.938
≈ 0.94

Match List - I (Transfer function of systems) with List - II (Nature of damping) and select the correct answer using the codes given below the lists:

Codes:
  • a)
    a
  • b)
    b
  • c)
    c
  • d)
    d
Correct answer is option 'A'. Can you explain this answer?

Ameya Gupta answered
Characteristic equations are:
• s2 + 8s + 1 2 = 0
Here, ωn = √12 and 

• s2 + 8s + 16 = 0
Here, ωn = 4 and 2 ξωn = 8
or, ξ = 1 (critically damped)
• s2 + 8s + 20 = 0
Here, ω = √20
and 2ξω= 8
or, ξ = 0.894 (ξ < 1 ∴ underdamped)
• s2 + 4 = 0
or, ωn = 2
and ξ = 0 (∴ undamped)

The transient response of a system is mainly due to
  • a)
    internal forces    
  • b)
    stored energy
  • c)
    friction    
  • d)
    external forces
Correct answer is option 'B'. Can you explain this answer?

Transient response of a system in Electrical Engineering

Transient response is the response of a system when it is subjected to a sudden change in its input or operating conditions. It is a temporary response that eventually settles down to a steady-state response. In electrical engineering, transient response is an important concept that is used to analyze the behavior of electrical systems. The transient response of a system is mainly due to stored energy.

Stored Energy

In electrical systems, energy can be stored in various forms such as electrical, magnetic, or mechanical. When there is a sudden change in the input or operating conditions of a system, this stored energy is released and causes a transient response. For example, when a capacitor is suddenly charged, it stores electrical energy. When the input voltage is removed, this stored energy is discharged and causes a transient response.

Effect of stored energy on transient response

The effect of stored energy on transient response can be understood by considering the example of a simple RC circuit. When a step voltage is applied to the circuit, the capacitor starts charging and stores electrical energy. As the capacitor charges, the voltage across it increases and the current through the resistor decreases. This causes a transient response in the circuit. Eventually, the capacitor reaches its fully charged state and the steady-state response is achieved.

Conclusion

In conclusion, the transient response of a system in electrical engineering is mainly due to stored energy. When there is a sudden change in the input or operating conditions of a system, the stored energy is released and causes a transient response. Understanding the effect of stored energy on transient response is important for analyzing the behavior of electrical systems.

Consider a unity feedback control system as shown in figure below:
The error signal e(t) for a unit ramp input for T > 0 is
  • a)
    T
  • b)
    T - e-t/T
  • c)
    T(1 - e-t/T)
  • d)
     t + T . e-t/T
Correct answer is option 'C'. Can you explain this answer?

Abhay Khanna answered

Given, 
∴ 
Here, A = -T, 6 = 1, C = T2
So, 

Taking inverse Laplace transform on both sides, we get:
C(t) = (-T + t + Te-t/T)
∴ Error signal,
e(t) = c(t) - r(t)
= c(t) - t = - T + Te-t/T
= T(1 - e-t/T)

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