Warning

# Lab05 - Randomized Sampling-based Algorithms

Motivations and Goals
Become familiar with sampling-based motion planning
Understand the probabilistic roadmap approach and rapidly growing random tree approach
Implement the RRT and PRM approaches for robot path planning in the maze
Lab resources

### Lab code for task05 PRM algorithm

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import sys
import math
import numpy as np

import collections
import heapq

import matplotlib.pyplot as plt

import Environment as env

class PRMPlanner:

def __init__(self, limits):
"""
Parameters
----------
limits: list((float, float))
translation limits in individual axes
"""
self.limits = limits
x_lower = limits
x_upper = limits
y_lower = limits
y_upper = limits
z_lower = limits
z_upper = limits
#calculate the maximum step length
self.max_translation = 1/250.0 * np.max([ x_upper-x_lower, y_upper-y_lower, z_upper-z_lower ])
self.max_rotation = math.pi/6

def plan(self, environment, start, goal):
"""
Method to plan the path

Parameters
----------
environment: Environment
Map of the environment that provides collision checking
start: numpy array (4x4)
start pose of the robot given in SE(3)
goal: numpy array (4x4)
goal pose of the robot given in SE(3)

Returns
-------
list(numpy array (4x4))
the path between the start and the goal Pose in SE(3) coordinates
"""
path = []

#random sample x points in the configuration space
#for RRT iterate for the specific number of iterations
n_points = 300

#sample the points in the full configuration space - we are sampling in the full space
samples = np.random.rand(6,n_points)

#apply the limits in individual axes
i = 0
for limit in self.limits:
scale = limit - limit
samples[i,:] = samples[i,:]*scale + limit
i += 1

#add start and goal sonfigurations to the samples
samples = np.append(samples, start.reshape(6,1), axis=1)
samples = np.append(samples, goal.reshape(6,1), axis=1)

#debug plot samples
#environment.plot_environment(P_start)
#plt.plot(samples[0,:],samples[1,:],samples[2,:],'r.')

#check the transition between individual configurations and filter the ones that are not feasible
#it is recommended to construct the navigation graph in this step, for that, you will need a container to save the information about edges and their cost (preferably an oriented graph that will come handy in the 6th task)

tx = samples.shape
#in this implementation make the full transition graph
for i in range(0,tx-1):
for j in range(i+1,tx):
p1 = samples[:,i]
p2 = samples[:,j]

#check, that there is a feasible path between the two configurations
dist, edge = self.check_path_for_collision(environment, p1, p2)

#if there is the path, add it to the list of feasible edges
if dist > 0:
#add it to the oriented graph

#debug plot the path
#xx = [x[0,3] for x in edge]
#yy = [x[1,3] for x in edge]
#zz = [x[2,3] for x in edge]
#plt.plot(xx,yy,zz,'b', linewidth=0.5)

#plan the path on the resulting graph -> result is a sparse path

#reconstruct the dense path to adhere to the distance limit between the samples, i.e. recalculate the proper path pose matrices

return(path)

def construct_pose(self, state):

R = self.rotation_matrix(state,state,state)
T = state[0:3]

#construct the SE(3) matrix
P = np.hstack((R,T.reshape((3,1))))
P = np.vstack((P,[0,0,0,1]))

return P

def check_path_for_collision(self, environment, sample1, sample2):
dist = 0
edge = []

#Note, decouple the translation and rotation parts ...

#get the translation direction towards the target
T_start = sample1[0:3]
T_goal = sample2[0:3]
T_dir = (T_goal-T_start)

#get how many samples there is necessary to sample along the line to adhere to the maximum translation limit
dist = np.linalg.norm(T_dir)
samples_translation = int(dist/self.max_translation)

#get the difference in the rotation angles
dphi_x = sample1 - sample2
dphi_y = sample1 - sample2
dphi_z = sample1 - sample2

#get how many samples there is necessary to sample the rotation to adhere to the maximum rotation limit
samples_rotation = int(np.max([np.abs(dphi_x/(self.max_rotation)), np.abs(dphi_y/(self.max_rotation)), np.abs(dphi_z/(self.max_rotation))]))

#select the maximum of sampling
n_points = np.max([samples_translation, samples_rotation])

# iterate and check for collision
for i in range(0,n_points):

#construct the translation part of the pose
T = T_start + T_dir*i/n_points

#construct the rotation part of the pose
yaw = ...
pitch = ...
roll = ...

#construct the SE(3) pose matrix
P = ...

#check for collision with the environment
ret = environment.check_robot_collision(P)

#if in collision return...
if ret:
return 0, []
else:
edge.append(P)

#don't forget to calculate the distance

#return
return dist,edge

def rotation_matrix(self, yaw, pitch, roll):
"""
Constructs rotation matrix given the euler angles
yaw = rotation around z axis
pitch = rotation around y axis
roll = rotation around x axis

Parameters
----------
yaw: float
pitch: float
roll: float
respective euler angles
"""
R_x = np.array([[1, 0, 0],
[0, math.cos(roll), math.sin(roll)],
[0, -math.sin(roll), math.cos(roll)]])
R_y = np.array([[math.cos(pitch), 0, -math.sin(pitch)],
[0, 1, 0],
[math.sin(pitch), 0, math.cos(pitch)]])
R_z = np.array([[math.cos(yaw), math.sin(yaw), 0],
[-math.sin(yaw), math.cos(yaw), 0],
[0, 0, 1]])

R = R_x.dot(R_y).dot(R_z)
return R 