GATE EC 2019 Set 1 — Question 42
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Control Systems → System Modelling → Block Diagram Reduction
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Question
The block diagram of a system is illustrated in the figure shown, where is the input and is the output. The transfer function is
In the diagram, the first summing junction is , where is the output of the second summing junction (fed back from the top) and is the output of the final block (fed back from the bottom). feeds the parallel blocks and , whose outputs are added to give , and .

Correct answer
(B) H(s) = (s²+1)/(s³+2s²+s+1)
Solution
Let be the output of the first summing junction and the output of the second.
1.
2.
3.
Substituting:Then- (A) drops one of the terms in the denominator.
- (C) and (D) do not have the third-order denominator that the two integrations and feedback loops produce.
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