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TitleA robotic testbed for low-gravity simulation
LanguageEnglish
File Size28.0 MB
Total Pages50
Table of Contents
                            Abstract
Abstrakt
Acknowledgments
Contents
Acronyms
1 Introduction
	1.1 On-orbit servicing
	1.2 Active debris removal
	1.3 Landing on low-gravity bodies
	1.4 Microgravity simulation
	1.5 Thesis overview
2 Ground-based testing in microgravity
	2.1 Drop towers and parabolic flights
	2.2 Neutral buoyancy pools
	2.3 Robotic-arm facilities
	2.4 Air-bearing facilities
	2.5 Case study of ERA
3 A robotic testbed
	3.1 Limitations of air-bearing facilities
	3.2 From a flat surface to a mobile robot
	3.3 Artificial gravity
4 Prototype development
	4.1 Mobile robot
	4.2 Suspension
	4.3 Payload
	4.4 Pneumatics
	4.5 Control system
		4.5.1 Payload position measurement
		4.5.2 Relative position control
		4.5.3 Safety
	4.6 Software
	4.7 Hardware integration
5 Verification experiments
	5.1 Experimental setup
	5.2 1D Contact model
	5.3 Methods and data
	5.4 Discussion
		5.4.1 Feedback control
		5.4.2 Errors
		5.4.3 Limitations
6 Future work and conclusions
	6.1 Future work
		6.1.1 Automatic levelling
		6.1.2 Omnidirectional wheels
	6.2 Conclusions
a Diploma thesis assignment
Bibliography
                        
Document Text Contents
Page 1

A R O B O T I C T E S T B E D F O R L O W- G R AV I T Y
S I M U L AT I O N

jakub tomášek

DIPLOMA THESIS

Czech Technical University in Prague

Page 2

DIPLOMA THESIS
Jakub Tomášek: A robotic testbed for low-gravity simulation

supervisor :
Jiří Zemánek

Czech Technical University in Prague
Faculty of Electrical Engineering
Department of Control Engineering

© May 2016

Page 25

4
P R O T O T Y P E D E V E L O P M E N T

I built a prototype of the testbed. There were five function require-
ments for the platform:

• unconstrained planar movement,

• decouple the robotic platform from the payload,

• provide safe operation,

• allow moving over common floors,

• and recreate artificial gravity scenarios.

In this first iteration the focus was on testing the first three functions—
free-floating payload on top of a mobile robot—but during the design
it was necessary to take into account all the points for easy upgrades
in the future.

The prototype integrates five core subsystems: a mobile robot, sus-
pension, payload, pneumatic system, and control system. The con-
trol system includes the computer and software. Figure 10 gives the
system overview and illustrates how the hardware subsystems are
integrated. All the subsystems are discussed below in detail.

Position sensor

Mobile robot

Payload

Suspension

Ball transfer units

Air bearings

Interface plate

Figure 10: System overview.

4.1 mobile robot

The prototype is based on Youbot from Kuka. Youbot is a commercially-
available holonomic mobile robot widely used by academic researchers

15

Page 26

16 prototype development

[4]. Figure 11 shows the robot. This solution allowed to benefit from
the community during the development, to shorten the development
time, and to lower the total cost.

The core requirement was the unconstrained planar locomotion.
Youbot is equipped with mecanum wheels so the robot can move
in any direction from any configuration. Each wheel is propelled by
an independent motor controlled by Trinamic TMCM controller. The
motors are connected to an onboard computer with Intel Atom Dual-
Core central processing unit. Besides that, it features power system
with a battery providing up to 1 h of operation. It was not necessary
to use the battery as the robot can be powered all the time.

Figure 11: Youbot from Kuka. It provides unconstrained planar movement
with the use of mecanum wheels.

Mecanum wheels feature series of rollers mounted at 45° angle
around its circumference. By rotating each wheel with different com-
puted velocity one can move in any direction. The maximum velocity
in the forward direction of the robot is 0.8 m s−1.

4.2 suspension

The drawback of the mecanum wheels is the discontinuous point of
contact with the ground. There is a gap between each roller and
as the wheel rotates the rollers collide with the ground. This motion
introduces significant vertical vibrations to the robot and may disturb
the motion of the payload. Figure 12 shows the vibrations measured
on top of the robot and its frequency spectrum. The vibrations peak
at frequency 30 Hz.

The vibrations are intolerable for the simulation of microgravity.
In order to damp the vibrations, I built a frame around the Youbot
which carries the air bearings and the payload. The frame is made
of aluminium profiles. The weight of the frame and payload is sup-
ported by ball transfer units which allow holonomic movement with
small rolling resistance.

Page 49

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