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The state-space representation of a separately excited DC servo motor dynamics is given by \[ \begin{bmatrix} \dfrac{d\omega}{dt} \\ \dfrac{di_a}{dt} \end{bmatrix} = \begin{bmatrix} -1 & 1 \\ -1 & -10 \end{bmatrix} \begin{bmatrix} \omega \\ i_a \end{bmatrix} + \begin{bmatrix} 0 \\ 10 \end{bmatrix}u \] where \(\omega\) is the speed of the motor, \(i_a\) is the armature current, and \(u\) is the armature voltage. The transfer function \(\omega(s)/U(s)\) of the motor is
- 10/(s²+11s+11)
- 1/(s²+11s+11)
- (10s+10)/(s²+11s+11)
- 1/(s²+s+1)
Correct answer: 10/(s²+11s+11)
Solution
For the given state model, take \(x=[\omega\ i_a]^T\), \(A=\begin{bmatrix}-1&1\\-1&-10\end{bmatrix}\), \(B=\begin{bmatrix}0\\10\end{bmatrix}\), and \(C=[1\ 0]\). Using \(G(s)=C(sI-A)^{-1}B\), the transfer function simplifies to \(\omega(s)/U(s)=10/(s^2+11s+11)\).
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