The second one was a leg with a parallel topology. Each leg had two DOFs, including a passive prismatic joint. presented the PAW, a four-legged vehicle with a T-shaped body and compliant legs. addressed a wheeled–legged robot called Hylos, which had 16 actively actuated DOFs, with each leg combining a two-DOF leg and the steering and rotation DOFs in the wheel. It could roll over flat smooth terrain on rotating wheels and could also use the wheels as feet to walk over irregular and steep terrain. Each leg was a 6R (rotational joint) serial mechanism with six degrees of freedom (DOFs). For example, the Jet Propulsion Laboratory (JPL) developed an articulated–wheeled lunar robot called ATHLETE. The first one is the serial leg configuration. There are three categories of leg–wheel robotic systems that differ according to leg morphology. Moreover, they have a relatively high locomotion efficiency. They can robustly deal with uncertainties or disturbances caused by the unstructured discontinuous terrain encountered during planetary exploration. Legged–wheeled robots have the merits of being both wheeled and legged robots. The hybrid leg–wheel mechanism can be used in the design of planetary rovers. Although many remarkable achievements have been made in the field of planetary exploration, the capability of wide-range autonomous exploration is still a great challenge for planetary rovers. There was a novel angle-adjusting mechanism between the two rockers to generate a wheel-step motion that could help the rover avoid wheel slip sinkage. Recently, China’s Zhurong Mars rover adopted an active rocker–bogie suspension. One wheel is fixed at one end of the rocker, while the bogie has two wheels that are mounted on the other end of the rocker. A differential mechanism is adopted to connect the two linkage mechanisms. There are two identical linkage mechanisms on each side of the rover, which consist of a rocker and a bogie. They adopt the passive rocker–bogie suspension configuration. The planetary rovers that were deployed for the exploration of the moon and Mars, such as Curiosity and Perseverance, are purely wheeled robotic systems.
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