Update Content - 2024-12-13

content/zettels/relative_gain_array.md

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+title = "Relative Gain Array"
+author = ["Dehaeze Thomas"]
+draft = false
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+
+Tags
+:
+
+Imagine a 2x2 plant:
+\\[ G(s) = \begin{bmatrix} g\_{11}(s) & g\_{12}(s)\\\ g\_{21}(s) & g\_{22}(s) \end{bmatrix} \\]
+
+Now suppose a controller \\(k\_{1}(s)\\) is closed on the first loop.
+The new transfer function from \\(u\_2\\) to \\(y\_2\\) (while the first loop is closed) is :
+
+\begin{equation}
+y\_2 = g\_{22} \left[ 1 - \underbrace{\frac{g\_{21}g\_{12}}{g\_{11}g\_{22}}}\_{\phi} \underbrace{\frac{g\_{11} k\_{11}}{1  + g\_{11}k\_{11}}}\_{T\_{11}} \right]
+\end{equation}
+
+\\(\phi\\) is called the **interaction index**.
+\\(T\_{11}\\) is the complementary sensitivity of the first loop (equal to 1 in the bandwidth of the first controller).
+Therefore, we want \\(\phi \approx 0\\) to have no interaction.
+
+Similarly, the **relative gain** is defined as:
+\\[ \Lambda = \frac{1}{1 - \phi} \\]
+And we want \\(\Lambda \approx 1\\) to have no interaction.
+
+Note that the scaling of inputs or outputs of the MIMO plant has no effect on \\(\phi\\) or \\(\Lambda\\).
+
+The **relative gain array** is defined as:
+\\[ \Lambda(G) = G \star (G^{-1})^{T} \\]
+where \\(\star\\) means element wise multiplication.
+
+```matlab
+RGA = G.*pinv(G.')
+```
+
+
+## Bibliography {#bibliography}
+
+<./biblio/references.bib>