This paper introduces SpaceBok, a quadrupedal robot created to investigate dynamic legged locomotion for the exploration of low-gravity celestial bodies. With a hip height of 500 mm and a mass of 20 kg, its dimensions are comparable to a medium-sized dog. The robot's leg configuration is based on an optimized parallel motion mechanism that allows the integration of parallel elastic elements to store and release energy for powerful jumping maneuvers. High-torque brushless motors in combination with customized single-stage planetary gear transmissions enable force control at the foot contact points based on motor currents. We present successful walking, trotting, and pronking experiments. Thereby, Spacebok achieved maximal jump heights in single jump experiments of up to 1.05 m (more than twice the hip height) and a walking velocity of 1 m /s. Moreover, simulation results for low gravity on the moon suggest that our robot can move with up to 1.1 m /s at an approximate cost of transport of 1 in moon gravity when using the pronking gait.
Jumping locomotion has the potential to enable legged robots to overcome obstacles and travel efficiently on lowgravity celestial bodies. We present how the 22 kg quadruped robot SpaceBok exploits lunar gravity conditions to perform energy-efficient jumps. The robot achieves repetitive, vertical jumps of more than 0.9 m and powerful single leaps of up to 1.3 m. We present the implementation of a reaction wheel, which allows for control of the robots pitch orientation during the flight phase. We also demonstrate the implementation of a parallel elasticity in the legs providing the capability of temporarily storing and reusing energy during jumping. The jumping and attitude controller are subsequently presented. Finally, we analyze the energetics of the system and show that jumping with the integrated elasticity significantly reduces energy consumption compared to non-elastic jumps.
Celestial bodies, such as the Moon and Mars are mainly covered by loose, granular soil, which is a notoriously challenging terrain to traverse with wheeled robots. Here, we present experimental work on traversing steep, granular slopes with the dynamically-walking quadrupedal robot SpaceBok. To adapt to the challenging environment, we developed passive-adaptive, planar feet and optimized studs to reduce sinkage and increase traction. Single-foot experiments revealed that a surface area of 110 cm2 per foot reduces sinkage to an acceptable level for the 22 kg robot, even on highly collapsible soil. Implementing several 12 mm studs increases traction by 22% to 66% on granular media compared to stud-less designs. Together with a terrain-adapting walking controller, we validate — for the first time — static and dynamic locomotion on Mars analog slopes of up to 25°(the maximum of the testbed). We evaluated the performance between point- and planar feet and static and dynamic gaits for safety, velocity, and energy consumption. We show that dynamic gaits are energetically more efficient than static ones, but are riskier on steep slopes. Our tests also revealed that energy consumption with planar feet increases drastically as slope inclination approaches the soil’s angle of repose. Point feet are less affected by slippage due to their excessive sinkage but, in turn, are prone to instabilities and tripping. Based on our findings, we present safe and energy-efficient, global, path-planning strategies for negotiating steep Martian topography.
Celestial bodies such as the Moon and Mars are mainly covered by loose, granular soil, a notoriously challenging terrain to traverse with (wheeled) robotic systems. Here, we present experimental work on traversing steep, granular slopes with the dynamically walking quadrupedal robot SpaceBok. To adapt to the challenging environment, we developed passive-adaptive planar feet and optimized grouser pads to reduce sinkage and increase traction on planar and inclined granular soil. Single-foot experiments revealed that a large surface area of 110 cm 2 per foot reduces sinkage to an acceptable level even on highly collapsible soil (ES-1). Implementing several 12 mm grouser blades increases traction by 22% to 66% on granular media compared to grouser-less designs. Together with a terrainadapting walking controller, we validate -for the first time -static and dynamic locomotion on Mars analog slopes of up to 25°(the maximum of the testbed). We evaluated the performance between point-and planar feet and static and dynamic gaits regarding stability (safety), velocity, and energy consumption. We show that dynamic gaits are energetically more efficient than static gaits but are riskier on steep slopes. Our tests also revealed that planar feet's energy consumption drastically increases when the slope inclination approaches the soil's angle of internal friction due to shearing. Point feet are less affected by slippage due to their excessive sinkage, but in turn, are prone to instabilities and tripping. We present and discuss safe and energy-efficient global path-planning strategies for accessing steep topography on Mars based on our findings.
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