new planar air-bearing microgravity simulator for

New Planar Air-bearing Microgravity Simulator for Verification of Space Robotics

Numerical Simulations and Control Algorithms

Tomasz Rybus1, Janusz Nicolau-Kukliński1, Karol Seweryn1, Tomasz Barciński2,

Monika Ciesielska1, Kamil Grassmann1, Jerzy Grygorczuk1, Michał Karczewski1,

Marek Kowalski1, Marcin Krzewski1, Tomasz Kuciński1, Jakub Lisowski1,

Rafał Przybyła1, Konrad Skup1, Tomasz Szewczyk1, Roman Wawrzaszek1

(1) Space Research Centre of the Polish Academy of Sciences

(2) Department of Control and Measurements, West Pomeranian

University of Technology

1 2 t h S y m p o s i u m o n A d v a n c e d S p a c e T e c h n o l o g i e s i n

R o b o t i c s a n d A u t o m a t i o n A S T R A 2 0 1 3

1 6 . 0 5 . 2 0 1 3

• Nearly one thousand of active satellites

orbits around Earth (April 2013).

• Malfunctions of satellites:

- failure of deployment mechanisms,

- failures of attitude control systems,

- premature fuel depletion.

• It is considered to use manipulator-

equipped autonomous satellites for on-

orbit servicing.

• Space debris (spent rocket stages, defunct satellites, small

fragments of satellites) pose real thread for active satellites and

human presence in orbit.

• Manipulator-equipped satellites could also be used for capturing

and removing from orbit specific large debris.

Applications of satell ite-

manipulator systems

Commercial satellite (conceptual illustration)

Applications of satell ite-

manipulator systems

Conceptual illustration of the satellite capture manoeuvre

Equation of satellite-manipulator dynamics:

Satell ite-manipulator

dynamics

Mass matrix

Coriolis matrix

Potential forces

Torques

Space manipulators

(free-floating)

� � � � � ��

��

� � 0

� � ��, �, … , ��

� � �� , �� ( 1 )

Ordinary manipulators

(fixed-base, working on Earth)

� � � �

� � � 0

In space:

- Dedicated demonstration missions,

- Tests of certain technologies on the ISS,

- Tests on-board the ISS.

On Earth:

- Suspension systems,

- Tests in the under-water environment,

- Tests on parabolic flights,

- Planar air-bearing microgravity-simulators.

Solutions for testing

space robots

Air-bearing microgravity

simulators• Free two-dimensional motion of the satellite-manipulator system.

• Satellite mock-up with attached robotic arm is mounted on air-

bearings.

• Air-bearings allow almost frictionless motion on the table surface,

thus simulating in two dimensions microgravity conditions.

• Possible applications:

- demonstration of control and trajectory planning algorithms,

- verification of numerical simulations of the satellite-manipulator

systems,

- tests of specific components of docking mechanisms ,

- tests preceding on-orbit demonstration missions.


simulator in the SRC PAS



Parameter Value1 Base mass 12.9 kg

2Base moment

of inertia0.208 kg·m2

3 Link 1 mass 4.5 kg

4Link 1 moment


5 Link 1 length 0.62 m6 Link 2 mass 1.5 kg

7Link 2 moment


8 Link 2 length 0.6 m

9 (m1 + m2)/ m0 0.465

Geometrical and mass properties of the planar satellite-manipulator system

Planar air-bearings

• Generate a thin (5 ÷ 15 μm) film of pressurized air and slide on it.

• Based on a porous media technology.

• Pressurized air is supplied through a hole on a side of the bearing.

• Airflow is controlled across the entire bearing surface through

millions of holes in the porous carbon.

• Air pressure remains almost uniform across the whole surface.

• Immune to scratches and hard to clog.

Planar air-bearing

Mechanical design of

manipulator joint

• Each joint consists of a DC motor,

harmonic drive, two resilient

suspension plates and absolute optical

encoder.

• Five air-bearings are used - all support

points must be ideally coplanar.

• Each air bearing is mounted on a ball

stud.

• Resilient suspension plates are used

for compensation of possible vertical

misalignments between components of

the system.

Manipulator joint with resilientsuspension plates

Electronics and control

system

Visual pose estimation

• Visual pose estimation system is used

to track the satellite-manipulator

system.

• Each marker is a black concave

pentagon containing a square area in

the middle which holds a pattern that

makes the markers distinguishable.

• The pose is initially estimated from

the homography between the marker

and the camera plane.

• Pose is further optimized in an

iterative process by minimizing the

error between the reprojected marker

position and its detected position.

Frame captured by visual pose estimation system camera

Visual marker used in the test (squares inside the pentagon

define marker index)

We follow the General Jacobian Matrix (GJM) approach introduced by

Umetani and Yoshida (1989).

End-effector trajectory is given in the velocity space. Two equations

are solved simultaneously:

Inverse dynamics –

torque computation

��

��

� �

� ��

� � � � �� , ��

Matrices H2 and H3 dependent not only on configuration of the

manipulator but also on the state of the servicing satellite.

For obtained trajectory of manipulator’s joints control torques Q are

computed:

( 2 )

( 3 )

( 4 )

Experimental results:

straight line trajectory

• Straight-line path of the end-effector (in the inertial reference

frame).

• Total reference end-effector translation: 0.6 m.

• Positions of manipulator joints are computed taking into account

free-floating nature of the satellite-manipulator system.

• Joint controllers are responsible for trajectory following in the

manipulator configuration space.

• We test the accuracy of the test-bed: no feedback from the end-

effector position is used.

• Experimental results are compared with reference trajectory and

results of numerical simulation.



0 5 10 15 20-150

-100

-50

0

50

100

150

Join

t positio

n [

deg]

Time [s]

Joint 1

Joint 2

0 5 10 15 20-30

-25

-20

-15

-10

-5

0

5

10

Join

t velo

citie

s [

deg/s

]

Time [s]

Joint 1

Joint 2

Positions of manipulator joints for straight-line trajectory (data used by the control system in the experiment)

Velocities of manipulator joints for straight-line trajectory

Error of position of manipulator joints during the experiment (difference between the given trajectory

and data obtained from encoders)

0 5 10 15 20-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

Time [s]

Join

t positio

n e

rror

[deg]





Comparison between the given end-effector straight-line trajectory and end-effector position

measured during the experiment

0.7 0.8 0.9 1 1.1 1.2 1.3 1.41.7

1.8

1.9

2

2.1

2.2

2.3

2.4

x [m]

y [

m]

Experimental results

Reference trajectory

0 5 10 15 20-40

-20

0

20

40

60

80

100

Time [s]

Sate

llite

orienta

tion [

deg]

Experimental results

Numerical simulations

Comparison between the satellite orientation obtained from numerical simulations and orientation of manipulator base measured

during the experiment

What is new here?

• New air-bearing microgravity simulator has two distinctive features:

- large area for the experiment (2x3 meters),

- separate air-bearings supporting each link of the manipulator.

• New possibilites:

- to perform complex manoeuvres of the satellite-manipulator

system,

- to add third link to the current 2DoF manipulator,

- to test systems with high ratio of the manipulator mass to the

mass of the satellite,

- to test manipulator with long links (flexibility).

• This new test-bed is important for Space Reserach Centre PAS for

tests of control algorithms and for validation of numerical

simulations.

Conclusions

• Tests of the satellite-manipulator systems are difficult on Earth.

• Air-bearing table is one of possible solutions, as it is simulating

microgravity conditions (in two dimensions) and is taking into

account free-floating nature of the satellite-manipulator system.

• New planar air-bearing microgravity simulator constructed recently

in the Space Research Centre PAS was presented and exemplary

experimental results were shown.

• In performed test no feedback from the end-effector position was

used (joint controllers were only responsible for trajectory following

in the manipulator configuration space).

• End-effector trajectory obtained from the experiment is very close

to the planned reference trajectory.

Thank you for

your attention!

new planar air-bearing microgravity simulator for

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