Planning and Control for Autonomous Drives of the Mars Sample Recovery Helicopter
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The planned Mars Sample Return (MSR) campaign is humanity’s first attempt at returning samples from the Martian surface to Earth. This campaign is composed of multiple Martian surface assets, including the Perseverance rover and the Sample Retrieval Lander; which collaborate to collect, retrieve, and ultimately return a diverse set of Martian rock and regolith samples. The Sample Recovery Helicopter (SRH) is a backup system of two hybrid aerial-ground robots that transport sample tubes from the Three Forks sample depot to the Sample Retrieval Lander (SRL) in the case that the Perservance rover is not able to transport its own cache of samples to SRL.An SRH robot is a hybrid aerial-ground vehicle, capable of flying between Three Forks and SRL, performing accurate drives from landing sites to tube pickup and dropoff locations, and handling sample tubes with a manipulator arm. With the weight constraint induced from flying in the Martian atmosphere, the ground mobility and sample manipulation subsystems are each subject to significant mass constraints (< 150g each). The mobility system has a four wheel skid-steer configuration, with 10 cm diameter wheels, and a maximum rock height traversal capability of 3.5 cm. To enable the manipulator arm to pickup the tube, the robot must drive to within a pose error of ±2 cm in x, ±4cm in y, and ±5◦ in yaw. With this tight accuracy requirement, the low control authority of lightweight skid-steer vehicles, and low rock-height traversal, the mobility system requires a highly capable autonomous driving capability.This paper presents the design of the planning and control strategies for SRH autonomous driving, along with a quantitative evaluation of the platform’s ability to drive to a desired pose on Martian analogue terrain. The ABIT* anytime optimizing sampling-based planner is used to plan feasible and distance-efficient paths assuming an occupancy grid representation of the local environment. A path-following controller is then used to follow the generated path by using spot turn and straight drive motion primitives. A separate controller is used for the final approach to a target pose using drives along closed-loop shallow arcs. Performance evaluation results show that such a strategy reliably drives the skid-steer platform to within the required pose error.The decision to implement Mars Sample Return will not be finalized until NASA’s completion of the National Environmental Policy Act (NEPA) process. This document is being made available for information purposes only.
规划中的火星样本返回（Mars Sample Return, MSR）任务是人类首次尝试将火星表面样本带回地球的行动。该任务由多套火星表面载荷组成，包括毅力号火星车与样本取回着陆器（Sample Retrieval Lander），二者协同完成多样的火星岩石与表岩屑样本的采集、取回并最终送返地球。样本回收直升机（Sample Recovery Helicopter, SRH）是一套由两套混合式空陆机器人组成的备份系统，当毅力号火星车无法将自身缓存的样本运送至样本取回着陆器（Sample Retrieval Lander, SRL）时，该系统负责将样本管从三岔口样本存放点运送至SRL。 SRH机器人属于混合式空陆平台，可在三岔口与SRL之间飞行，也能从着陆点位精准行驶至样本管的取放位置，并通过机械臂完成样本管的操作。由于在火星大气中飞行带来的重量限制，其地面移动与样本操作两个子系统均受到严格的质量约束（单系统质量均不超过150克）。该移动系统采用四轮滑移转向构型，配备直径10厘米的车轮，最大可翻越高度为3.5厘米的岩石障碍。为了让机械臂能够抓取样本管，机器人需要行驶到位姿误差满足：x方向±2厘米、y方向±4厘米、偏航角±5°的范围内。鉴于这一严苛的精度要求，再加上轻型滑移转向车辆的控制权限有限、翻越岩石障碍能力不足，该移动系统需要具备高性能的自主驾驶能力。 本文介绍了SRH自主驾驶系统的规划与控制策略设计，并针对该平台在火星模拟地形中行驶至目标位姿的能力开展了定量评估。本文采用ABIT* anytime优化采样规划器，基于局部环境的占用栅格表示，规划出可行且距离最优的路径。随后通过路径跟踪控制器，利用原地转向与直行运动基元跟踪生成的路径。针对最终接近目标位姿的环节，则采用另一套控制器，通过沿闭环浅圆弧的行驶完成该阶段操作。性能评估结果表明，该策略能够可靠地将滑移转向平台行驶至满足要求的位姿误差范围内。 火星样本返回任务的实施决策需待美国国家航空航天局（National Aeronautics and Space Administration, NASA）完成《国家环境政策法案》（National Environmental Policy Act, NEPA）流程后方可最终敲定。本文件仅用于信息分享。
