Tactile Feet and Online Learning for Robust Control of a Quadruped Robot in Challenging Terrain

触觉传感在物体操作和检测领域已经得到了广泛的探索，特别是利用TacTip触觉传感器，但在步行机器人的触觉脚领域还做得很少。脚的问题与手的问题类似：通过与环境交互，机器人遮挡了需要量化的表面(无论是在纹理、表面方向、边缘检测等方面)。因此，应用触觉传感器直接测量被接触表面，似乎比计算机视觉(CV)、激光雷达或声纳等无法感知被接触表面的常用非接触方法更合适。增加触觉脚可以使行走机器人更可靠地估计脚和地板之间的滑动、地板距离和方向以及识别光滑/凹凸不平的表面(用于选择稳定的脚点)，所有这些都提高了四足动物在不平坦的自然地形上行走和/或跑步的成功率。大多数行走机器人都很难做到这一点。触觉脚对触觉手中的人构成了额外的挑战。硬件必须能够支撑体重，能够承受行走/奔跑的冲击，并能抵抗在自然地形中遇到的意外物体(如尖锐物体)。软件必须处理由被支撑机器人的移动引起的传感器的更大失真和噪声，这本质上比手中的噪声更复杂和动态，并且反馈回路必须更快，因为行走或跑步需要比抓取和操纵对象更快的运动。此外，考虑到行走机器人的大小和移动特性，任何软件都必须在车载微型计算机上运行，这限制了计算能力，或者必须与车载计算机进行高速连接，速度足够快，以实现对机器人的实时控制。此外，正如在第一年的项目中探索的那样，使用在线学习方法来学习触觉数据和有用的控制输入之间的映射可能是训练机器人的一种数据高效和健壮的方式。离线方法往往是数据密集型的，需要跨越所有可能的变化维度的数据，并且不能学习适应测试期间遇到的新输入。此外，对于可能随时损坏并需要更换的特定传感器，学习模型，虽然替换传感器将是相似的，但有足够的差异，不能使用相同的模型，必须重复所有训练。在线学习的使用解决了这些问题，反而带来了创建全面的在线数据收集策略的挑战，以便自主学习正确的映射。第一年的项目探索了边缘跟踪任务中数据高效的在线传感器培训和控制，这是一个巧妙的扩展，开发了一种梁平衡机器人。给四足动物装上触觉脚将使它能够沿着仅略宽于腿部跨度的随机弯曲的横梁行走，而不会摔倒。有了触觉感应，就有可能识别边缘以及边缘沿着传感器有多远(就像第一年项目中用单个传感器探索的那样)。这与没有触觉感知的机器人形成了鲜明的对比，后者无法判断脚是否完全在横梁上-使用机载摄像头监视脚将被脚或地形(如草地)直接遮挡在脚下，因此可能不可靠。以这种方式使用触觉传感器将使机器人能够将双脚移回横梁上，并防止摔倒(在使用地图的更复杂的系统中，可以注意到边缘的位置，以帮助未来规划稳定的路线)。

Tactile sensing has been extensively explored in the domain of object manipulation and detection, especially with the TacTip tactile sensor, but very little has been done in the domain of tactile feet for walking robots. With feet there is a similar issue as in hands: by interacting with the environment the robot occludes the surface that needs quantifying (whether this be in texture, surface orientation, edge detection etc). Therefore the application of a tactile sensor, which directly measures the contacted surface, seems more appropriate than commonly used non-contact methods such as computer vision (CV), lidar or sonar which cannot sense the surface under contact. The addition of tactile feet may give a walking robot more reliable estimation of slip between the feet and floor, floor distance and orientation and identification of smooth/bumpy surfaces (for selection of stable footholds), all of which improve the success of a quadruped walking and/or running over uneven natural terrain. Most walking robots struggle to do this. Tactile feet pose additional challenges to those faced in tactile hands. The hardware must be capable of supporting the body weight and withstand the impact from walking/running and be resistant to unexpected objects encountered in natural terrain (e.g. sharp objects). The software must handle the greater distortion and noise of the sensor caused by the movements of the supported robot, which is inherently more complex and dynamic than the noise experienced in hands, and the feedback loop must be faster as walking or running require faster movements than with object grasping and manipulation. Additionally given the size and mobile nature of a walking robot, any software must either run on an onboard microcomputer, which restricts computational power, or there must be a high speed connection to an offboard computer which is fast enough to enable real time control of the robot. Additionally, as explored in the first year project, the use of online learning methods to learn the mapping between tactile data and useful control inputs could be a data efficient and robust way of training the robot. Offline methods tend to be extremely data intensive, needing data across all possible dimensions of variation, and cannot learn to adapt to novel inputs encountered during testing. Additionally the models are learnt for specific sensors which may break and need replacing at any time, and while replacement sensors will be similar there is enough difference that the same models cannot be used and all training must be repeated. The use of online learning solves these problems, introducing instead the challenge of creating comprehensive online data collection policies in order for the correct mappings to be learnt autonomously. A neat extension from the first year project, which explored data-efficient online sensor training and control in edge following tasks, is the development of a beam-balancing robot. Fitting a quadruped with tactile feet would enable it to walk along a randomly curving beam only slightly wider than its leg span, without falling. With tactile sensing it is possible to identify an edge and how far along the sensor the edge is (as explored with a single sensor in the first year project). This is in contrast to a robot without tactile sensing which would be unable to tell if the foot is fully on the beam or not - the use of onboard cameras watching the feet would be obscured from seeing directly under the feet by the feet or the terrain (e.g. grass) so may be unreliable. Using tactile sensors in this way would allow the robot to move the feet back onto the beam and prevent falling (in a more complex system using maps the locations of the edge could be noted to aid future planning of stable courses).