Question Details

The Laplace transform of the step response of a system is given by Y(s) = 100 /s(s+100) . The rise time is defined as the time taken for the response to go from 0.1 to 0.9 of its final value. The settling time is defined as the time taken for the response to reach 0.98 of its final value. For this system, the rise time (Tr), settling time (Ts), and time constant (Tc), all expressed in seconds, are

Options

A

Tr = 0.022, Ts = 0.04, Tc = 0.01

B

Tr = 0.22, Ts = 0.404, Tc = 0.01

C

Tr = 2.2, Ts = 4.04, Tc = 1.01

D

Tr = 22, Ts = 40.4, Tc = 10.1

Show Answer

Correct Answer :

Option A

Tr = 0.022, Ts = 0.04, Tc = 0.01

Solution :

The correct option is Tr = 0.022, Ts = 0.04, Tc = 0.01.

Here is the step-by-step educational explanation of how to arrive at this solution:

Step 1: Finding the time-domain response y(t)
The Laplace transform of the output response is given as:
Y ( s ) = 100 s ( s + 100 )
We can expand this expression using partial fractions:
100 s ( s + 100 ) = 1 s - 1 s + 100
Taking the inverse Laplace transform, we get the time-domain response:
y ( t ) = 1 - e - 100 t
for t ≥ 0.

Step 2: Identifying the Time Constant (Tc)
A standard first-order system response has the form:
y ( t ) = A ( 1 - e - t / T c )
Comparing this with our system's response y(t)=1-e-100t, we have:
1 T c = 100 T c = 1 100 = 0.01 seconds
Also, note that the final steady-state value of the response as t approaches infinity is:
y final = lim t ( 1 - e - 100 t ) = 1

Step 3: Calculating the Rise Time (Tr)
The rise time is the time taken to go from 0.1 to 0.9 of its final value (which is 1). Let t1 and t2 be the times when the response reaches 0.1 and 0.9 respectively.
For t1 (0.1 of final value):
1 - e - 100 t 1 = 0.1 e - 100 t 1 = 0.9 t 1 = - ln ( 0.9 ) 100 0.00105 seconds
For t2 (0.9 of final value):
1 - e - 100 t 2 = 0.9 e - 100 t 2 = 0.1 t 2 = - ln ( 0.1 ) 100 0.02303 seconds
Therefore, the rise time is:
T r = t 2 - t 1 = ln ( 0.9 ) - ln ( 0.1 ) 100 = ln ( 9 ) 100 2.197 100 0.022 seconds

Step 4: Calculating the Settling Time (Ts)
The settling time is the time taken to reach 0.98 of its final value:
1 - e - 100 T s = 0.98 e - 100 T s = 0.02
Taking the natural logarithm of both sides:
- 100 T s = ln ( 0.02 ) - 3.912
T s = 3.912 100 0.03912 0.04 seconds

Conclusion:
The calculated parameters are Tr = 0.022 seconds, Ts = 0.04 seconds, and Tc = 0.01 seconds, which matches the option Tr = 0.022, Ts = 0.04, Tc = 0.01.

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