Legged robots are very promising for use in real-world applications, but their operation in narrow spaces is still challenging. One solution for enhancing their environmental adaptability is to design a small-sized biomimetic robot capable of performing multiple motions and carrying payloads. In this case, rats that live in caves have attracted a great amount of attention owing to their unrivaled agility and adaptability. Lots of efforts have been made trying to mimic the morphology or motion characteristics of rats.
Recently, a team led by Prof. Qing Shi from Beijing Institute of Technology, China, has developed a novel robotic rat name SQuRo (small-size quadruped robotic rat).
Prof. Shi’s team has developed robots that are able to reproduce their movement or behavior. Through bio-inspired design, they have developed a wheeled robotic rat capable of multiple rat-like behaviors. Recently, they replaced the wheels with legs to further enhance the movement agility. In this study, published in the journal IEEE Transactions on Robotics, experimental results revealed that the legged robot SQuRo is capable of mimicking the motion of actual rats inside narrow spaces.
Rats can adapt to narrow spaces owing to their elongated slim body and unrivaled agility. To copy the movement agility, the team made full use of the morphology and motion characteristics of rats moving in caves. The team first extracted the key movement joints (KMJs) of rats and completed the (degree of freedom) DOF configuration. Specifically, they designed 2 DOFs in each limb to reproduce the limb movement, 2 DOFs in waist and 2DOFs in head to replicate the flexible spine movement. Benefiting from a long and flexible spine, SQuRo can bend its body and quickly turn around.
Besides copying the morphology of rats, a robotic rat also features the locomotion characteristics of rats. To this end, the team proposed a hierarchical open-loop controller to achieve multimodal motion similar to that of rats. The control framework mainly consists of three layers: 1) a multi-motion planner with four basic motion modes and established a direct relationship between the control variable and ground reaction forces (GRFs); 2) parameter optimization with the consideration of the stability and actuation limits; 3) trajectory generation of each joint. The control framework makes agile movements and fast transitions possible.
Due to the biomimetic flexible structure and multimodal motion control, SQuRo can perform various motions, such as crouching-to-standing, walking, crawling, and turning, and can recover after falling by controlling its limbs and cervical parts to appropriately adjust its center of mass (CoM). Moreover, through field tests, SQuRo successfully passed through an irregular narrow passage (inner width of 90 mm), crossed an obstacle with a height of 30 mm, and achieved stable locomotion on a slope with an inclination of 15°, which demonstrates its potential application to inspection tasks inside narrow spaces.
Compared with state-of-the-art quadruped robots of a similar scale, the newly developed SQuRo has a relatively more elongated, slimmer body and smaller weight. The minimum turning radius of 0.48 body length is much smaller compared with that of other robots. In addition, SQuRo can achieve steady locomotion even after carrying a load equal to 91 percent of its own weight, which demonstrates its superior payload-carrying compared with small-sized quadruped robots. These capabilities allow SQuRo to agilely pass through narrow spaces and rugged terrains and perform tasks, such as detection or transportation in relevant scenarios.