# botamochi6277/trochoid-py

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# trochoid.py

Python script to draw a trochoid A trochoid is the curve described by a fixed point on a circle as it rolls along a fixed path. A trochoid family is categorized with a relationship between the radius of a drawing circle ($r_d$) and the radius of a rolling (moving) circle ($r_m$), location of the rolling circle (inside/outside) and the shape of a fixed path.

Cycloid $r_d = r_m$ - Straight line
Hypotrochoid $r_d \neq r_m$ In Circle
Epitrochoid $r_d \neq r_m$ Out Circle
Hypocycloid $r_d = r_m$ In Circle
Epicycloid $r_d = r_m$ Out Circle

## How to use

python3 trochoid.py


## Calculation for Drawing Point

Name Definition
$s$ Length of path of rolling
$r_d$ Radius of drawing circle
$r_m$ Radius of rolling circle
$p_m$ Position of rolling circle
$p_d$ Position of drawing point
$\theta$ orientation of rolling circle
$p(s)$ Fixed path

The length of path of rolling ($s$) is summation of $\Delta s$ :

\begin{align} \Delta s &= \sqrt{dx[i]^2 + dy[i]^2} \
s &= \sum_0^n \Delta s \end{align}

The moving circle rolls along the fixed path without slippage.

\begin{align} r_m\theta &= s \end{align}

This equation is transformed for the orientation of a rolling circle.

\begin{align} \theta & = s/r_m \
\Delta \theta &= \Delta s/r_m , \end{align}

A tangential vector ($t[i]$) and a normal vector ($n[i]$) at $p[i]$ are

\begin{align} t[i] &= \frac{p[i+1]-p[i-1]}{2} \
n[i] &= \frac{t[i+1]-t[i-1]}{2} \
&= \frac{p[i+2]-p[i-2]}{4} \end{align}

The position of a rolling circle ($p_m[i]$) is described with a unit normal vector ($n_u[i]$): \begin{align} p_m[i] &= p[i] + r_m n_u[i] \end{align}

This $n_u$ depends on concavity and convexity of the fixed line. $n_u$ is redefined with a following equation,

\begin{align} n_u[i] &= R（\frac{\pi}{2}） t_u[i] \end{align}

where $R$ is a rotation matrix.

Finally, $p_d[i]$ is described a following equation.

\begin{align} p_d[i] &= p_m[i] + r_d R(\theta[i]) p_d \end{align}

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