Time Response Analysis

Question 1
Consider the following closed loop control system

where G(s)=\frac{1}{s(s+1)} and C(s)=K\frac{s+1}{s+3}. If the steady state error for a unit ramp input is 0.1, then the value of K is _______.
A
10
B
20
C
30
D
40
GATE EC 2020   Control Systems
Question 1 Explanation: 
Open loop transfer function for the system =C(s)\times G(s)=\frac{K(s+1)}{ (s+3)}\times \frac{1}{s(s+1)}
Since the system is type-1, so far a given unit ramp input steady state
e_{ss}=\frac{1}{K_{V}}
where, K_{V}=\lim_{s\rightarrow 0}S\times \frac{K}{S(S+3)}=\frac{K}{3}
so, \; e_{ss}=\frac{1}{K/3}=\frac{3}{K}
Given that, \; e_{ss}=0.1
So, 0.1=\frac{3}{K}\Rightarrow K=30
Question 2
A system with transfer function G(s)=\frac{1}{(s+1)(s+a)}, a \gt 0 is subjected an input 5\cos 3t. The steady state output of the system is \frac{1}{\sqrt{10}} \cos (3t-1.892). The value of a is ____.
A
3
B
4
C
5
D
6
GATE EC 2020   Control Systems
Question 2 Explanation: 
Given that,
\begin{aligned}G(j\omega )&=\frac{1}{(1+j\omega )(\alpha +j\omega )} \\ \left |G(j\omega ) \right |&=\frac{1}{\sqrt{(\omega ^{2}+1)(\omega ^{2}+\alpha ^{2})}} \\ \text{According to }& \text{question,}\\ \left | G(j\omega ) \right |_{\omega =3}&=\frac{1}{5\sqrt{10}}\\ \Rightarrow \frac{1}{\sqrt{(\omega ^{2}+1)(\omega ^{2}+\alpha ^{2})}}&=\frac{1}{5\sqrt{10}} \\ \Rightarrow \frac{1}{\sqrt{10(a^{2}+9)}}&=\frac{1}{5\sqrt{10}} \alpha ^{2}+9=25 \\ \alpha ^{2}&=16 \\ \alpha &=4\end{aligned}
Question 3
The loop transfer function of a negative feedback system is

G(s)H(s)=\frac{K(s+11)}{s(s+2)(s+8)}

The value of K, for which the system is marginally stable, is ________.
A
160
B
120
C
180
D
200
GATE EC 2020   Control Systems
Question 3 Explanation: 
Characteristic equation q(s) for the given open loop system will be
q(s)=s^{3}+10s^{2}+16s+Ks+11K=0
Using R-H criteria,


For System to be Marginally Stable
\begin{aligned}\frac{10(16+K)-11K}{10}&=0 \\ 160+10K-11K&=0 \\ K&=160\end{aligned}
Question 4
Consider a causal second-order system with the transfer function

G(s)=\frac{1}{1+2s+s^2}

with a unit-step R(s)=\frac{1}{s} 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 \lim_{t \to \infty }c(t), rounded off to two decimal places, is
A
5.25
B
4.5
C
3.89
D
2.81
GATE EC 2019   Control Systems
Question 4 Explanation: 
\begin{aligned} G(s)&=\frac{1}{s^{2}+2 s+1}=\frac{1}{(s+1)^{2}} \\ R(s)&=\frac{1}{s} \\ \alpha (s)&=G(s) R(s)=\frac{1}{s(s+1)^{2}} \end{aligned}
Using partial fraction expansion, we get,
\begin{aligned} C(s) & =\frac{A}{s}+\frac{B}{(s+1)}+\frac{C}{(s+1)^{2}} \\ A\left(s^{2}+2 s+1\right)&+B\left(s^{2}+s\right)+C s=1 \\ A&=1 \\ A+B & =0 \Rightarrow B=-1 \\ 2 A+B+C & =0 \Rightarrow C=-1 \\ \therefore \quad C(s) & =\frac{1}{s}-\frac{1}{(s+1)}-\frac{1}{(s+1)^{2}} \\ \text{and}\quad c(t) & =\left(1-e^{-t}-t e^{-t}\right) u(t) \\ \lim _{t \rightarrow \infty} c(t) & =1 \end{aligned}
In order to reach 94 % of its steady-state value,
\left(1-e^{-t}-t e^{-t}\right)=0.94
By trial and error, we get,
t \approx 4.50 \mathrm{sec}
Question 5
The open loop transfer function G(s)=\frac{(s+1)}{s^{P}(s+2)(s+3)}
Where p is an integer, is connected in unity feedback configuration as shown in figure.

Given that the steady state error is zero for unit step input and is 6 for unit ramp input, the value of the parameter p is _________.
A
1
B
2
C
3
D
4
GATE EC 2017-SET-1   Control Systems
Question 5 Explanation: 
Steady state error of type 1 system, for step input is zero, for ramp input is \frac{1}{K_{v}} and for parabolic input is infinity.
So, the given system must be of type- 1 and p=1
Question 6
For the unity feedback control system shown in the figure, the open-loop transfer function G(s) is given as

G(s)=\frac{2}{s(s+1)}

. The steady state error e_{ss} due to a unit step input is
A
0
B
0.5
C
1
D
\infty
GATE EC 2016-SET-3   Control Systems
Question 6 Explanation: 
\begin{aligned} E(s) &=\frac{R(s)}{1+G(s) H(s)} \\ R(s) &=\frac{1}{s} ; G(s)=\frac{2}{s(s+1)}; \\ H(s) &=1 \\ E(s) &=\frac{\frac{1}{s}}{1+\frac{s}{s(s+1)}}=\frac{s+1}{s^{2}+s+2} \\ e_{s s} &=\lim _{s \rightarrow 0} s E(s) \\ &=\lim _{s \rightarrow 0} \frac{s(s+1)}{s^{2}+s+2} \\ e_{s s} &=0 \end{aligned}
Question 7
In the feedback system shown below G(s)=\frac{1}{(s^{2}+2s)}. The step response of the closed-loop system should have minimum settling time and have no overshoot.

The required value of gain k to achieve this is ________
A
0.5
B
1
C
2
D
3
GATE EC 2016-SET-2   Control Systems
Question 7 Explanation: 
\begin{aligned} G(s)&=\frac{1}{s^{2}+2 s} \\ \frac{Y(s)}{R(s)}&=\frac{\frac{K}{s^{2}+2 s}}{1+\frac{K}{s^{2}+2 s}}=\frac{K}{s^{2}+2 s+K} \end{aligned}
Minimum settling time and no overshoot implies critical damping i.e.
\begin{aligned} \xi &=1 \\ \omega_{n} &=\sqrt{K} \\ \therefore \quad 2 \times \xi \cdot \omega_{n} &=2 \\ \omega_{n} &=1 \\ \Rightarrow \quad \sqrt{K} &=1 \text { or } K=1 \end{aligned}
Question 8
The response of the system G(s)=\frac{s-2}{(s+1)(s+3)} to the unit step input u(t) is y(t). The value of \frac{dy}{dt} at t=0^{+} is _________
A
0
B
1
C
2
D
3
GATE EC 2016-SET-2   Control Systems
Question 8 Explanation: 
\begin{aligned} L\left(\frac{d y}{d t}\right) &=(s Y(s)-y(0)) \\ Y(s) &=G(s) \times \frac{1}{s}=\frac{s-2}{s(s+1)(s+3)} \\ y(0)&=\lim _{s \rightarrow \infty} s Y(s) \\ \text{(Applying } & \text{initial value theorem) } \\ &=\lim _{s \rightarrow \infty} \frac{s-2}{(s+1)(s+3)} \\ &=\frac{\left(1-\frac{2}{s}\right)}{s\left(1+\frac{1}{s}\right)\left(1+\frac{3}{s}\right)} \\ y(0)&=0 \\ L\left(\frac{d y}{d t}\right) &=s Y(s)=\frac{s \times(s-2)}{s(s+1)(s+3)} \\ &=\frac{s-2}{(s+1)(s+3)} \\ \left.\frac{d y}{d t}\right|_{t=0} &=\lim _{s \rightarrow \infty} s L\left(\frac{d y}{d t}\right) \\ &=\lim _{s \rightarrow \infty} \frac{s \times(s-2)}{(s+1)(s+3)}\\ &=\frac{\left(1-\frac{2}{s}\right)}{\left(1+\frac{1}{s}\right)\left(1+\frac{3}{s}\right)}=1 \end{aligned}
Question 9
The open-loop transfer function of a unity-feedback control system is given by

G(s)=\frac{K}{s(s+2)}

For the peak overshoot of the closed-loop system to a unit step input to be 10%, the value of K is____________
A
1.6
B
2.2
C
2.8
D
4.2
GATE EC 2016-SET-1   Control Systems
Question 9 Explanation: 
G(s)=\frac{K}{s(s+2)} ; H(s)=1
Characteristic equation =1+G(s) H(s)=0
\begin{aligned} 1+\frac{K}{s(s+2)} &=0 \\ s^{2}+2 s+K &=0 \Rightarrow \omega_{n}=\sqrt{K} \\ 2 \xi \omega_{n} &=2 ; \xi=\frac{1}{\sqrt{K}} \\ M_{p}&=e^{-\pi\xi/\sqrt{1-\xi^{2}}}-0.1\\ -\frac{\pi \xi}{\sqrt{1-\xi^{2}}} &=\ln (0.1) \\ \Rightarrow \quad\frac{\pi \xi}{\sqrt{1-\xi^{2}}} &=2.3 \\ \pi^{2} \xi^{2} &=(2.3)^{2}\left(1-\xi^{2}\right) \\ 15.16 \xi^{2} &=(2.3)^{2} \\ \Rightarrow \qquad \xi&=0.59 \\ \Rightarrow \quad \text{Also}, \quad K&=\frac{1}{\xi^{2}}=2.8 \end{aligned}
Question 10
The output of a standard second-order system for a unit step input is given as y(t)=1-\frac{2}{\sqrt{3}}e^{-t}cos(\sqrt{3}t-\frac{\pi }{6}). The transfer function of the system is
A
\frac{2}{(s+2)(s+\sqrt{3})}
B
\frac{1}{s^2+2s+1}
C
\frac{3}{s^2+2s+3}
D
\frac{4}{s^2+2s+4}
GATE EC 2015-SET-2   Control Systems
Question 10 Explanation: 
The output of a second order system for unit step
input
y(t)=1-\frac{e^{-\xi \omega_{n} t}}{\sqrt{1-\xi^{2}}} \sin \left(\omega_{d} t-\theta\right) \qquad\ldots(i)
From given data
\begin{aligned} y(t)&=1-\frac{2}{\sqrt{3}} e^{-t} \cos \left(\sqrt{3} t-\frac{\pi}{6}\right)\\ \therefore \quad \alpha&=\xi \omega_{n}=1 \\ \text{and }\quad \omega_{d}&=\sqrt{3} \\ \omega_{n}&=\sqrt{\alpha^{2}+\omega_{d}^{2}}=2 \\ \xi&=\frac{1}{2} \end{aligned}
Therefore, the transfer function
T(s)=\frac{\omega_{n}^{2}}{s^{2}+2 \xi \omega_{n} s+\omega_{n}^{2}}=\frac{4}{s^{2}+2 s+4}
There are 10 questions to complete.
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