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SpaceBok, a Locomotion Concept for Space Exploration

 

Legged robots are advancing fast here on Earth, let’s get them to space!
Walking robots show high versatility. With precise foot placements, sandy slopes or rocky terrain can be tackled. For long-range missions , more energetically efficient gaits can be employed . Such a system could take us to places where no other robot has ever been. Sites of great scientific importance such as deep extra-terrestrial craters and canyons would for the first time be within human reach. SpaceBok has been specifically built to test the feasibility of dynamic locomotion in low gravity. We hope that our findings contribute to the future of space exploration.

Technical Specifications

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20 KG Weight

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1 m Max. Jump Height

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Electric Actuation

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Battery Powered

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Parallel Elasticity

Drivetrain

SpaceBok is equipped with an in-house built drive unit. A low ratio planetary gearbox has been placed inside the stator of a powerful DC outrunner. The result is a lightweight, high torque and back drivable actuation unit capable of thrusting Spacebok to a height of 0.4m and in conjunction with the parallel elastic spring to 1.05m.

Leg Design

To minimize the leg inertia, both actuators are located in the shoulder. The kinematics are based on a parallel mechanism, so both actuators contribute to the thrust during a jumping maneuver. Furthermore, springs that act parallel to the actuators can be mounted safely into the carbon tubes. With integrated springs, more efficient repetitive jumping is possible. Additionally, the jump height is increased, which can be helpful to overcome obstacles.

Monocoque Body

The monocoque carbon body rigidly connects four pantograph legs. To minimize the leg inertia, both actuators are located in the shoulder. The kinematics are based on a parallel mechanism, so both actuators contribute to the thrust during a jumping maneuver. Furthermore, springs that act parallel to the actuators can be mounted safely into the carbon tubes. With integrated springs, more efficient repetitive jumping is possible. Additionally, the jump height is increased, which can be helpful to overcome obstacles.

Electronic Stack

The electrical hardware is mounted on a stack composed of three levels. The stack can be conveniently removed from the carbon monocoque body to ease maintenance of the system.

Locomotion

The robot is able to perform stable locomotion with all implemented gaits, namely a pronking gait, a static gait, a walking trot and a diagonal sequence walk. It can cope with disturbances such as pushes or small obstacles (8 cm).

This performance enables further research on the platform within different gravitational settings and on a simulated terrain of Mars or the moon.

Control

To reach versatile and robust locomotion, several static and dynamic gaits have been implemented using a virtual model controller.

A pronking gait is designed by simulating a virtual spring when the robot is in ground contact. A static gait, a walking trot and a diagonal sequence walk are designed through predefined gait patterns. The desired torso position is calculated as a weighted sum of the foot positions.

A Virtual Model Controller is used to control the torso pose. The calculated wrench is mapped to the foot forces using a quadratic program.

Flight Phase

Jumping in low gravity results in longer flight phases. With a reaction wheel, the pitch angle of the body can be controlled to ensure a safe landing.

Motivation

For the past few decades, wheeled robots such as lunar rovers have been extensively used for the exploration of celestial bodies. While such systems perform well in terms of stability and efficiency, they come to their limits on steep slopes or sandy terrain. A stuck or flipped rover can hardly recover from its position and would often mean the end of the mission. Difficult terrain will often be avoided in mission planning. Scientifically interesting sites on the moon or Mars, for example on crater rims or pyroclastic deposits, are therefore currently out of reach for exploration missions.

Legged systems can negotiate steeper slopes and perform better on uneven or granular ground. A legged robot that can adapt its gait to the environment has the ability to switch between robust static gaits on challenging terrain and energetically efficient dynamic gaits on flat terrain. We built SpaceBok as an experimental platform to examine dynamic and static legged locomotion. The goal of our work is to advance the technology readiness level of legged robots for space exploration.

Project

The aim of the SpaceBok project is the development of a concept and the validation for a functional jumping robot. To accomplish this task, 12 students from the ETH and the ZHAW have been working on it over a period of one year as part of their bachelor’s degree program. The project is intended to form the basis for legged robots in space expeditions.

SpaceBok is a project of the Robotic Systems Lab at the ETH Zürich.

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Supervisor
Hendrik Kolvenbach
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Head of Robotics Systems Lab
Marco Hutter
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Student Coach
Boris Stolz
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Software
Philip Arm
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Software
Patrick Barton
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Mechanical Hardware
Lars Beglinger
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Software
Alexander Dietsche
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Electronic Hardware
Luca Ferrazzini
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Software
Elias Hampp
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Mechanical Hardware
Jan Hinder
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Electronic Hardware
Camille Huber
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Electronic Hardware
David Schaufelberger
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Mechanical Hardware
Felix Schmitt
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Mechanical Hardware
Benjamin Sun
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Mechanical Hardware
Radek Zenkl
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Contact us for questions or collaborations!

SpaceBok
Robotic Systems Lab – ETH Zürich
Leonhardstrasse 21 H
8092 Zürich
Switzerland

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