digital-brain/content/zettels/fractional_order_transfer_functions.md

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title = "Fractional Order Transfer Functions"
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author = ["Dehaeze Thomas"]
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draft = false
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Tags
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: [Digital Filters]({{< relref "digital_filters.md" >}})
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## Example Using the FOMCON toolbox {#example-using-the-fomcon-toolbox}
The documentation for the toolbox is accessible [here](https://fomcon.net/fomcon-toolbox/overview/).
Here are the parameters that are used to define the wanted properties of the fractional model:
```matlab
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wb = 2*pi*0.1; % Lowest frequency bound
wh = 2*pi*1e3; % Highest frequency bound
n = 8; % Approximation order
r = 0.5; % Wanted slope, The corresponding phase will be pi*r
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```
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Then, to create an approximation of a fractional-order operator \\(s^r\\) of order \\(n\\) which is valid in the frequency range \\([\omega\_b\\, \omega\_h]\\), the `oustafod` function can be used:
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```matlab
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G = oustafod(r,n,wb,wh);
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```
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```text
G =
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79.27 s^17 + 7.93e05 s^16 + 2.918e09 s^15 + 5.143e12 s^14 + 4.782e15 s^13 + 2.453e18 s^12 + 7.103e20 s^11 + 1.175e23 s^10 + 1.119e25 s^9 + 6.138e26 s^8 + 1.942e28 s^7 + 3.534e29 s^6 + 3.675e30 s^5 + 2.157e31 s^4 + 6.984e31 s^3 + 1.193e32 s^2 + 9.764e31 s + 2.939e31
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
s^17 + 1.312e04 s^16 + 6.327e07 s^15 + 1.462e11 s^14 + 1.783e14 s^13 + 1.199e17 s^12 + 4.553e19 s^11 + 9.877e21 s^10 + 1.232e24 s^9 + 8.866e25 s^8 + 3.678e27 s^7 + 8.775e28 s^6 + 1.196e30 s^5 + 9.208e30 s^4 + 3.909e31 s^3 + 8.755e31 s^2 + 9.395e31 s + 3.707e31
Continuous-time transfer function.
```
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Few examples of different slopes are shown in Figure [1](#figure--fig:approximate-deriv-int).
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<a id="figure--fig:approximate-deriv-int"></a>
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{{< figure src="/ox-hugo/approximate_deriv_int.png" caption="<span class=\"figure-number\">Figure 1: </span>Example of fractional approximations" >}}
## Bibliography {#bibliography}
<style>.csl-entry{text-indent: -1.5em; margin-left: 1.5em;}</style><div class="csl-bib-body">
</div>