Question

For the system 2/s+1, the approximate time taken for a step response to reach 98% of its final value is:

(a) 1s

(b) 2s

(c) 4s

(d) 8s

I got this question during a job interview.

Query is from Time Response of Second Order Systems in section Time Response Analysis, Design Specifications and Performance Indices of Control Systems

Answer 1

Correct choice is (c) 4s

For explanation: C(s)/R(s) = 2/s+1

R(s) = 1/s (step input)

C(s) = 2/s(s+1)

c(t) = 2[1-e^-t]

1.96 = 2[1-e^-T]

T= 4sec.