mpc_python_learn/notebook/equations.ipynb

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2020-04-21 23:33:00 +08:00
{
"cells": [
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# STATE SPACE MODEL MATRICES"
]
},
{
"cell_type": "code",
"execution_count": 120,
"metadata": {},
"outputs": [
{
"data": {
"text/latex": [
"$\\displaystyle \\left[\\begin{matrix}0 & 0 & - v \\sin{\\left(\\theta \\right)} & 0 & 0\\\\0 & 0 & v \\cos{\\left(\\theta \\right)} & 0 & 0\\\\0 & 0 & 0 & 0 & 0\\\\0 & 0 & 0 & 0 & 0\\\\0 & 0 & 0 & v \\cos{\\left(\\psi \\right)} & 0\\end{matrix}\\right]$"
],
"text/plain": [
"Matrix([\n",
"[0, 0, -v*sin(theta), 0, 0],\n",
"[0, 0, v*cos(theta), 0, 0],\n",
"[0, 0, 0, 0, 0],\n",
"[0, 0, 0, 0, 0],\n",
"[0, 0, 0, v*cos(psi), 0]])"
]
},
"execution_count": 120,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"import sympy as sp\n",
"\n",
"x,y,theta,psi,cte,v,w = sp.symbols(\"x y theta psi cte v w\")\n",
"\n",
"gs = sp.Matrix([[ sp.cos(theta)*v],\n",
" [ sp.sin(theta)*v],\n",
" [w],\n",
" [-w],\n",
" [ v*sp.sin(psi)]])\n",
"\n",
"state = sp.Matrix([x,y,theta,psi,cte])\n",
"\n",
"#A\n",
"gs.jacobian(state)"
]
},
{
"cell_type": "code",
"execution_count": 121,
"metadata": {},
"outputs": [
{
"data": {
"text/latex": [
"$\\displaystyle \\left[\\begin{matrix}\\cos{\\left(\\theta \\right)} & 0\\\\\\sin{\\left(\\theta \\right)} & 0\\\\0 & 1\\\\0 & -1\\\\\\sin{\\left(\\psi \\right)} & 0\\end{matrix}\\right]$"
],
"text/plain": [
"Matrix([\n",
"[cos(theta), 0],\n",
"[sin(theta), 0],\n",
"[ 0, 1],\n",
"[ 0, -1],\n",
"[ sin(psi), 0]])"
]
},
"execution_count": 121,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"state = sp.Matrix([v,w])\n",
"\n",
"#B\n",
"gs.jacobian(state)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# PATH WAYPOINTS AS PARAMETRIZED CURVE"
]
},
{
"cell_type": "code",
"execution_count": 98,
"metadata": {},
"outputs": [],
"source": [
"import numpy as np\n",
"from scipy.interpolate import interp1d\n",
"\n",
"def compute_path_from_wp(start_xp, start_yp, step = 0.1):\n",
" final_xp=[]\n",
" final_yp=[]\n",
" delta = step #[m]\n",
"\n",
" for idx in range(len(start_xp)-1):\n",
" section_len = np.sum(np.sqrt(np.power(np.diff(start_xp[idx:idx+2]),2)+np.power(np.diff(start_yp[idx:idx+2]),2)))\n",
"\n",
" interp_range = np.linspace(0,1,np.floor(section_len/delta).astype(int))\n",
" \n",
" fx=interp1d(np.linspace(0,1,2),start_xp[idx:idx+2],kind=1)\n",
" fy=interp1d(np.linspace(0,1,2),start_yp[idx:idx+2],kind=1)\n",
" \n",
" final_xp=np.append(final_xp,fx(interp_range))\n",
" final_yp=np.append(final_yp,fy(interp_range))\n",
"\n",
" return np.vstack((final_xp,final_yp))\n",
"\n",
"def get_nn_idx(state,path):\n",
"\n",
" dx = state[0]-path[0,:]\n",
" dy = state[1]-path[1,:]\n",
" dist = np.sqrt(dx**2 + dy**2)\n",
" nn_idx = np.argmin(dist)\n",
"\n",
" try:\n",
" v = [path[0,nn_idx+1] - path[0,nn_idx],\n",
" path[1,nn_idx+1] - path[1,nn_idx]] \n",
" v /= np.linalg.norm(v)\n",
"\n",
" d = [path[0,nn_idx] - state[0],\n",
" path[1,nn_idx] - state[1]]\n",
"\n",
" if np.dot(d,v) > 0:\n",
" target_idx = nn_idx\n",
" else:\n",
" target_idx = nn_idx+1\n",
"\n",
" except IndexError as e:\n",
" target_idx = nn_idx\n",
"\n",
" return target_idx"
]
},
{
"cell_type": "code",
"execution_count": 110,
"metadata": {},
"outputs": [],
"source": [
"#define track\n",
"wp=np.array([0,5,6,10,11,15, 0,0,2,2,0,4]).reshape(2,-1)\n",
"track = compute_path_from_wp(wp[0,:],wp[1,:],step=0.5)\n",
"\n",
"#vehicle state\n",
"state=[3.5,0.5,np.radians(30)]\n",
"\n",
"#given vehicle pos find lookahead waypoints\n",
"nn_idx=get_nn_idx(state,track) #index ox closest wp\n",
"LOOKAHED=6\n",
"lk_wp=track[:,nn_idx:nn_idx+LOOKAHED]\n",
"\n",
"#trasform lookahead waypoints to vehicle ref frame\n",
"dx = lk_wp[0,:] - state[0]\n",
"dy = lk_wp[1,:] - state[1]\n",
"\n",
"wp_vehicle_frame = np.vstack(( dx * np.cos(-state[2]) - dy * np.sin(-state[2]),\n",
" dy * np.cos(-state[2]) + dx * np.sin(-state[2]) ))\n",
"\n",
"#fit poly\n",
"coeff=np.polyfit(wp_vehicle_frame[0,:], wp_vehicle_frame[1,:], 3, rcond=None, full=False, w=None, cov=False)\n",
"\n",
"def f(x,coeff):\n",
" return coeff[0]*x**3+coeff[1]*x**2+coeff[2]*x**1+coeff[3]*x**0"
]
},
{
"cell_type": "code",
"execution_count": 111,
"metadata": {},
"outputs": [
{
"data": {
"text/plain": [
"array([-0.12829547, 0.96144073, -1.47313104, -0.47433648])"
]
},
"execution_count": 111,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"coeff"
]
},
{
"cell_type": "code",
"execution_count": 113,
"metadata": {},
"outputs": [
{
"data": {
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"text/plain": [
"<Figure size 432x288 with 2 Axes>"
]
},
"metadata": {},
"output_type": "display_data"
}
],
"source": [
"import matplotlib.pyplot as plt\n",
"plt.style.use(\"ggplot\")\n",
"\n",
"x=np.arange(0,3,0.001) #interp range of curve \n",
"\n",
"# VEHICLE REF FRAME\n",
"plt.subplot(2,1,1)\n",
"plt.scatter(0,0)\n",
"plt.scatter(wp_vehicle_frame[0,:],wp_vehicle_frame[1,:])\n",
"plt.plot(x,[f(xs,coeff) for xs in x])\n",
"plt.axis('equal')\n",
"\n",
"# MAP REF FRAME\n",
"plt.subplot(2,1,2)\n",
"plt.scatter(state[0],state[1])\n",
"plt.scatter(track[0,:],track[1,:])\n",
"plt.scatter(track[0,nn_idx:nn_idx+LOOKAHED],track[1,nn_idx:nn_idx+LOOKAHED])\n",
"plt.axis('equal');"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# COMPUTE ERRORS"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"* **crosstrack error** cte -> desired y-position - y-position of vehicle: this is the bias term of the fitted polynomial (road curve)\n",
" \n",
"$\n",
"f = K_0 * x^3 + K_1 * x^2 + K_2 * x + K_3\n",
"$\n",
"\n",
"Then for the origin cte = K_3\n",
" \n",
"* **heading error** epsi -> desired heading - heading of vehicle : is the inclination of tangent to the fitted polynomial (road curve)\n",
"\n",
"The derivative of the fitted poly has the form\n",
"\n",
"$\n",
"f' = 3.0 * K_0 * x^2 + 2.0 * K_1 * x + K_2\n",
"$\n",
"\n",
"Then for the origin the equation of the tangent in the origin is $y=k2$ \n",
"\n",
"epsi = -atan(K_2)\n",
"\n",
"in general:\n",
"\n",
"$\n",
"y_{desired} = f(px) \\\\\n",
"heading_{desired} = -atan(f`(px))\n",
"$"
]
},
{
"cell_type": "code",
"execution_count": 119,
"metadata": {},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"-0.47433647872956336\n",
"55.83031014720696\n"
]
}
],
"source": [
"#for 0\n",
"\n",
"cte=coeff[3]\n",
"epsi=-np.arctan(coeff[2])\n",
"print(cte)\n",
"print(np.degrees(epsi))"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# ADD DELAY (for real time implementation)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"It is necessary to take *actuation latency* into account: so instead of using the actual state as estimated, the delay factored in using the kinematic model\n",
"\n",
"* $x_{delay} = 0.0 + v * dt$\n",
"* $y_{delay} = 0.0$\n",
"* $psi_{delay} = 0.0 + w * dt$\n",
"* $cte_{delay} = cte + v * sin(epsi) * dt$\n",
"* $epsi_{delay} = epsi - w * dt$\n",
"\n",
"Note that the current position and heading is always 0; this is becouse the path is parametrized to vehicle reference frame"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": []
}
],
"metadata": {
"kernelspec": {
"display_name": "Python 3",
"language": "python",
"name": "python3"
},
"language_info": {
"codemirror_mode": {
"name": "ipython",
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},
"file_extension": ".py",
"mimetype": "text/x-python",
"name": "python",
"nbconvert_exporter": "python",
"pygments_lexer": "ipython3",
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"nbformat_minor": 4
}