NATIONAL TECHNICAL UNIVERSITY OF ATHENS DEPARTMENT OF MECHANICAL ENGINEERING CONTROL SYSTEMS LAB...

NATIONAL TECHNICAL UNIVERSITY OF ATHENS

DEPARTMENT OF MECHANICAL ENGINEERING

CONTROL SYSTEMS LAB

A Comparison of the Use of a Single Large vs a Number of Small Robots

in On-Orbit Servicing

Georgios RekleitisGeorgios Rekleitis

Evangelos PapadopoulosEvangelos Papadopoulos

May 17, May 17, 20120133

National Technical University of AthensDepartment of Mechanical Engineering

A Comparison of the Use of a Single Large vs. a Number of Small Robots in On-Orbit Servicing

G. Rekleitis & E. Papadopoulos

MotivationMotivation On-οrbit Servicing / OOS

(construction, maintenance, docking, inspection, orbital debris handling/disposal)

Currently done by astronaut EVA

Robotic OOS can relieve astronauts fromrisky activities

Successful OOS experiments:ETS VII, Orbital Express.

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OOS object handling can be done by a number of servicers with on-off thrusters and manipulators

This results in improved performance in terms of accuracy and fuel consumption, compared to pure on-off control

Is the performance better when the passive object is handled by a number of small servicers or by a single, large one?

Problem StatementProblem Statement

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Manipulation by free-flying servicers

Space Robotic System Dynamics and Control

One Large vs. Several Small Servicers

Simulation Results

Conclusions

RoadMapRoadMap

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Single manipulator servicers.

Point contact / firm grasp between end-effectors and object.

Deactivated thrusters towards the object.

Manipulation by Free-Flying ServicersManipulation by Free-Flying Servicers

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Space Robotic System Dynamics Space Robotic System Dynamics and Control- 1and Control- 1

, , , , , , T TT T T

i i i i i i i i ix y z q r

3 3 3 33 3

3 3 3 3

* *

diag( , , )

x xx i i ii T T

x xi i i i i

i i

m m m

0 0IH

0 0E R I R E

E H

i i iE

(1) , Ti i i i i i i H q C q q J Q

*1 3 1 3, ,

T TT TT T T Ti x i i i i i i i i i i i i i x i i

C = 0 E R I R E E R I R E 0 E C




Passive object Model Based Control

Constraints:

(1) Friction constraints: For the point contact case

(2) Manipulator repulsive force.


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0 0 0 0 0 0 0 0des P D Q C H q K e K e (2)




Constraints notwithstanding, the redundancy of the system leads to the absence of analytical solution.

Constrained nonlinear optimization, as a force distribution method

B Design parameters: the components of the end-effector generalized forces


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i th – Servicer Controller:• apply the required generalized force• maintaining base desired position and attitude • take into account workspace and collision avoidance

requirements.

Model Based Control

Switching strategy for the on-off thrusters

No need for asymptotic stability


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For the case of a number of small servicers:

Thrusters facing the passive object are turned off.

Additional manipulator force to push the servicer away from the passive object.

The other servicer manipulator forces compensate, keeping total applied force on the object as in Eq. (2).

One Large vs. Several Small One Large vs. Several Small Servicers - 1Servicers - 1

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For the case of a single, large servicer:

• No other servicers to compensate for additional manipulator force.

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Appropriate servicer base configuration.

Simpler controller

Force components opposing each other.




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New switching strategy for the single, large servicer case:

• Calculate the required continuous control force fi at the base frame.

• Examine if the force components exceed a preset threshold.

Far fewer components opposing each other




Other differences between the two cases:

1. Contact type between passive object and manipulator:

• Number of small servicers: Both point contact and firm grasp are feasible.

• Single, large servicer: Firm grasping is required.

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2. Servicer size limitations

• The single, large servicer case can be affected, whenever very large objects have to be handled.

• In the several small servicers case, we only need to add more servicers.

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3. Robustness in servicer performance failure

• In the several small servicers case, failure in the performance of one servicer may not be catastrophic

B The remaining servicers may be able to compensate

• In the single, large servicer case, there are no other robots to compensate for any performance failures.

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Simulation ResultsSimulation ResultsSeveral small servicers casem0 = 180 Kg, mi = 70 Kg, i = 1,2,3

fm = 20 N, ft = 10 N

Single large servicer casem0 = 180 Kg, m1 = 210 Kg,

fm = 60 N, ft = 25 N




Simulation ResultsSimulation Results

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Thruster and reaction wheel forces/torques (both cases)




Three small servicers: More frequent but less powerful thrusting.

• More flexible in terms of gain tuning, servicer base position tuning and thrusting application.

Single large servicer: More scarce but more powerful thrusting.


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Manipulator reach for both cases





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ConclusionsConclusions Post-docking manipulation of a passive object in zero-g,

utilizing manipulator equipped free-flyers, was studied.

The case of passive object handling by a number of small servicers was compared to handling by a single large servicer.

Several small servicers case was found superior, since:

1. It is more robust in terms of contact type requirements, size limitations and servicer performance failures.

2. It is more flexible in terms of gain tuning, servicer base position tuning and thrusting application

3. Superior performance in terms of fuel consumption, for the same object tracking errors.




THANK YOU!

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NATIONAL TECHNICAL UNIVERSITY OF ATHENS DEPARTMENT OF MECHANICAL ENGINEERING CONTROL SYSTEMS LAB...

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