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Discrete state equations and pulse TF (T = 1)

Short answer

For T=1 s, the discrete state matrices are G=[1 0.4323; 0 0.1353], H=[0.2838; 0.4323], C=[1 0]. The pulse transfer function is Y(z)/U(z)=(0.2838z+0.1485)/(z^2−1.1353z+0.1353).

Obtain the discrete-time state and output equations and pulse transfer function (when the sampling period T = 1) of the following continuous-time system: G(s)=Y(s)/U(s)=1/s(s+2), which may be represented in state space by the equations [ẋ₁; ẋ₂]=[0 1; 0 -2][x₁; x₂]+[0; 1]u, y=[1 0][x₁; x₂].

With a zero-order hold, the discrete model is x(k+1)=Gx(k)+Hu(k)\mathbf{x}(k+1) = \mathbf{G}\mathbf{x}(k) + \mathbf{H}u(k), y(k)=Cx(k)y(k) = \mathbf{C}\mathbf{x}(k), where G=eAT\mathbf{G} = e^{\mathbf{A}T} and H=(∫0TeAλdλ)B\mathbf{H} = \left(\int_0^T e^{\mathbf{A}\lambda}d\lambda\right)\mathbf{B}.

Step 1: State transition matrix of the continuous system

(sI−A)−1=[s−10s+2]−1=[1s1s(s+2)01s+2](s\mathbf{I}-\mathbf{A})^{-1} = \begin{bmatrix} s & -1 \\ 0 & s+2 \end{bmatrix}^{-1} = \begin{bmatrix} \dfrac{1}{s} & \dfrac{1}{s(s+2)} \\[2mm] 0 & \dfrac{1}{s+2} \end{bmatrix} eAt=[112(1−e−2t)0e−2t]e^{\mathbf{A}t} = \begin{bmatrix} 1 & \tfrac{1}{2}(1-e^{-2t}) \\ 0 & e^{-2t} \end{bmatrix}

Step 2: G and H for T = 1 s

e−2=0.1353e^{-2} = 0.1353:

G=eA=[112(1−e−2)0e−2]=[10.432300.1353]\mathbf{G} = e^{\mathbf{A}} = \begin{bmatrix} 1 & \tfrac{1}{2}(1-e^{-2}) \\ 0 & e^{-2} \end{bmatrix} = \begin{bmatrix} 1 & 0.4323 \\ 0 & 0.1353 \end{bmatrix} H=∫01[12(1−e−2λ)e−2λ]dλ=[12[1−12(1−e−2)]12(1−e−2)]=[0.28380.4323]\begin{aligned} \mathbf{H} &= \int_0^1\begin{bmatrix} \tfrac{1}{2}(1-e^{-2\lambda}) \\ e^{-2\lambda} \end{bmatrix}d\lambda = \begin{bmatrix} \tfrac{1}{2}\left[1 - \tfrac{1}{2}(1-e^{-2})\right] \\ \tfrac{1}{2}(1-e^{-2}) \end{bmatrix} = \begin{bmatrix} 0.2838 \\ 0.4323 \end{bmatrix} \end{aligned}

Discrete state and output equations

[x1(k+1)x2(k+1)]=[10.432300.1353][x1(k)x2(k)]+[0.28380.4323]u(k)\begin{bmatrix} x_1(k+1) \\ x_2(k+1) \end{bmatrix} = \begin{bmatrix} 1 & 0.4323 \\ 0 & 0.1353 \end{bmatrix}\begin{bmatrix} x_1(k) \\ x_2(k) \end{bmatrix} + \begin{bmatrix} 0.2838 \\ 0.4323 \end{bmatrix}u(k) y(k)=[10][x1(k)x2(k)]y(k) = \begin{bmatrix} 1 & 0 \end{bmatrix}\begin{bmatrix} x_1(k) \\ x_2(k) \end{bmatrix}

Step 3: Pulse transfer function

(zI−G)−1=1(z−1)(z−0.1353)[z−0.13530.43230z−1](z\mathbf{I}-\mathbf{G})^{-1} = \frac{1}{(z-1)(z-0.1353)}\begin{bmatrix} z-0.1353 & 0.4323 \\ 0 & z-1 \end{bmatrix} F(z)=C(zI−G)−1H=0.2838(z−0.1353)+0.4323×0.4323(z−1)(z−0.1353)=0.2838z+0.1485(z−1)(z−0.1353)=0.2838z+0.1485z2−1.1353z+0.1353\begin{aligned} F(z) &= \mathbf{C}(z\mathbf{I}-\mathbf{G})^{-1}\mathbf{H} = \frac{0.2838(z-0.1353) + 0.4323\times0.4323}{(z-1)(z-0.1353)}\\ &= \frac{0.2838z + 0.1485}{(z-1)(z-0.1353)} = \frac{0.2838z + 0.1485}{z^2 - 1.1353z + 0.1353} \end{aligned}

Check by direct transformation

(1−z−1) Z[1s2(s+2)]=(2T−1+e−2T)z+(1−e−2T−2Te−2T)4(z−1)(z−e−2T)=1.1353z+0.59404(z−1)(z−0.1353)(1-z^{-1})\,\mathcal{Z}\left[\frac{1}{s^2(s+2)}\right] = \frac{(2T-1+e^{-2T})z + (1-e^{-2T}-2Te^{-2T})}{4(z-1)(z-e^{-2T})} = \frac{1.1353z + 0.5940}{4(z-1)(z-0.1353)}

This is the same result.

Answer: F(z)=0.2838z+0.1485(z−1)(z−0.1353)=0.2838z−1+0.1485z−21−1.1353z−1+0.1353z−2F(z) = \dfrac{0.2838z + 0.1485}{(z-1)(z-0.1353)} = \dfrac{0.2838z^{-1} + 0.1485z^{-2}}{1 - 1.1353z^{-1} + 0.1353z^{-2}}

✓ Answer checked by Claude Opus 5.5 (high effort)

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