EBand Local Planner基本思想讲解

这里介绍一个local planner，论文是这篇Elastic Bands: Connecting Path Planning and Control
目前我们的一些规划的方法主要思想就是先规划出Path，接着根据Path生成Trajectory，让机器人跟着(tracking)这个Trajectory走就不会有事。
主要介绍了Elastic Bands这种Connecting Path Planning and Control的方法
首先简介了一些传统方法的解决问题的办法
1.The conventional solution is first to convert the path to a trajectory by time parameterization, then to track the trajectory. Path planners are often designed to find any feasible path, with little attention to its suitability for execution. Even if the time optimal parametrization is used,
the path may have abrupt changes in direction or maintain little clearance from obstacles, requiring the robot to move slowly
一些path planner也就是global_planner很少会去考虑机器人的运动，所以在有一些场景下面只能让机器人走的很慢去解决这个问题

2.In addition, if the controller is to implement some sort of real-time obstacle avoidance scheme then it must be able to deviate from the path. Once the robot is off the path, however, the controller has no global information on how to reach the goal
如果遇到一些动态的障碍物，即使global planner可以重新规划新的路径，但是也有可能出现当前位置偏离规划出的路径。(这里其实说的应该是路径平滑的问题)

提出了这个新方法Elastic Band，主要分三层
1 Path planning: A world model is used to generate global solutions to specified tasks.
2 Elastic bands: The path from the planner is deformed
in real time to handle local changes in the environment
detected by sensors and to smooth the path.
3 Control: A conventional control law is used to move
the robot along the elasticband
比较特别的一点就是第二条，用了一个Elastic Band去优化前面规划出来的路径，也就是(path smooth)

a是global planner可能规划出来的路径，b是经过Elastic band优化之后的，主要是根据障碍物(obstacles)来施加一个力(contraction force)让原本看起来松弛的路径变的紧绷(removes any slack in the path)

接着提到了Bubble

Bubble就是一个代表路径上的点与障碍物不冲突的圆形范围(not to collide with an obstacle)，如果距离障碍物近，bubble会小，距离障碍物远，bubble会大
An elastic band is represented by a finite series of bubbles

To insure that a collision free path can be generated between the via points, we impose the condition that the bubbles at consecutive via points overlap
为了确保path 是 collision free的，我们会让生成bubbles是overlap的，这一点图中也可以看得出来

最后就是操纵生成的Bubble(Manipulating the Bubbles)
比如对bubble前移或者后移，在障碍物近的地方生成bubble更加密集