Electromagnetic Formation Flight - NASA's Institute for ... · – MRI exposes humans to up to 2T ... Force vs. Separation Distance 1.0E-04 1.0E-03 ... Active and passive magnetic

Electromagnetic Electromagnetic Formation FlightFormation Flight

6th Annual MeetingOctober 19-20, 2004

October 29, 2004 26th Annual Meeting

EMFFTeam MembersTeam Members

• Dr. Ray Sedwick, PI• Prof. Dave Miller, Co-I

• Completed Dissertations– Dr. Edmund Kong– Dr. Laila Elias

• Current Doctoral Students– Samuel Schweighart– Umair Ahsun

• Current Masters Students– Daniel Kwon– Matthew Neave

• Undergraduate Researchers– Jr/Sr Design Class (15)– Amado DeHoyos– Laura Condon– Dawn Wheeler

• Industry Partners– Payload Systems, Inc– Benjamin Schweighart

• Sponsorship– NIAC, NASA JPL (TPF)– NRO DII, DARPA TTO– Lockheed IRAD


EMFFOutlineOutline

• Introduction– Background/Concept– Criticisms/Responses– Research Approach

• Feasibility Issues– How far apart does it work?– Thermal Design– Dipole Solutions

• EMFF Laboratory Demonstration– Basic laboratory setup– Critical enabling technologies– EMFF vehicle design description– Performance videos

• Conclusions/Wrap-up


EMFFEMFF BackgroundEMFF Background

• What?– Provide actuation of relative degrees of freedom for space-based formation flight

systems using electromagnetic forces and reaction wheels.

• Why?– “Formation keeping” for distributed satellite systems– Elimination of consumables (thrusters)

• Unlimited mission life (solar power is free)– Eliminate Thruster Plumes

• Contaminate optics and obscure vision (thermally)– Enable high ∆V formation flying Missions

• How?– Create steerable electromagnetic dipoles using three orthogonal

electromagnetic coils made of superconducting wire on each vehicle– Create coupled Forces & Torques pair-wise between vehicles in both

axial and transverse directions– De-couple torques from forces by using 3 reaction wheels per vehicle


EMFFEMFF Concept AnimationEMFF Concept Animation

• Roll video…


EMFFCriticisms of the Phase II ProposalCriticisms of the Phase II Proposal

• Effects of high B-fields on:– Humans– Avionics– Sensors– Electric motors– Communications– Plasma environment

• Large # of technologies to develop– State of the art HTS Wire

• Low Max Current density• Brittle

– Controllability– Control Stability– Limitations of 1/r4 scaling

• More complete analysis of 3D dynamics

• Development of more realistic lab demonstrations

• Difficulty with keeping the HTS wire at cryogenic temperatures in space

• Risk of delays in developing hardware using undergraduates (interesting…)

• Development of a low impedance power supply for large currents

• Effect of sudden field collapse


EMFFFirst Responses to CriticismsFirst Responses to Criticisms

• Effects of high B-fields on Humans*– Little reason to suspect quasi-static

fields to contribute to human health problems

– MRI exposes humans to up to 2T field strength for short durations

– Exposure in particle accelerators up to 300 mT for long periods

– Maximum field strength within 1 cm of wire is ~ 20 mT

• Brittleness of HTS wire – Commercially available wire formed

in a process that is ductile, and can bend into radius as small as 10 cm

• Risk of delays in developing hardware using undergraduates– Rev 2 of Testbed is nearing

completion

• Effect of sudden field collapse– Field levels are low, and coil

inductance prohibits rapid collapse

• Insufficient distance for radiation protection idea to work– Agree that preliminary analysis

shows this particular application is not viable

* http://www.mcw.edu/gcrc/cop/static-fields-cancer-FAQ/toc.html#Q1


EMFFResearch ApproachResearch Approach

• Feasibility Questions– Coil size– Coil mass– Reaction wheel size– Power requirements to:

• Maintain thermal environment• Drive the coil current• Drive reaction wheels

– Issues of control• Controllability requirements• Non-linear dynamics

– Stability issues• Coordinated control of N bodies

• Performance Questions– Practical separation distances– Comparison to propellant based

systems– Control tolerances

• Implementation Questions– Managing angular momentum

build-up in reaction wheels– Operating within the Earth’s

magnetic field– Solving for the necessary dipole

configuration to achieve a given maneuver


EMFFOutlineOutline


• Feasibility Issues– How far apart does it work?– Thermal Design– Dipole Solutions




EMFFHow Far Apart Will They Work?How Far Apart Will They Work?

• Writing the force in terms of the coil radius (R), separation distance (d) and total loop current (IT), the force scales as

• We see that for a given coil current, the system scales ‘photographically’, meaning that two systems with the same loop current that are simply scaled versions of one another will have the same force

• For design, it is of interest to re-write in terms of coil mass and radius, and physical constants:

• The current state-of-the-art HTS wire has a value of

And the product of coil mass and radius becomes the design parameter.

42

03~2 T

RF Id

π µ ⎛ ⎞⎜ ⎟⎝ ⎠

22 47 2

0 4

3 3 1~ (10 ) ( )2 2 2

C C CC C

M I IRF M RR d d

π µπ ρ

− ⎛ ⎞⎛ ⎞ ⎛ ⎞ = ⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠ ⎝ ⎠

16, 250 /CI A m kgρ

⎛ ⎞= −⎜ ⎟

⎝ ⎠


EMFFForce vs. Separation Distance

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1 10 100

Separation (m)

Fo

rce (

N)

MR = 0.1MR = 0.3MR = 1MR = 3MR = 10MR = 30MR = 100MR = 300

kg-m

EMFORCE Testbed

EMFF Effectiveness GraphsEMFF Effectiveness Graphs

With further simplification:2

4

1~ 31.2 ( )C CF M Rd

The graph to the right shows a family of curves for various products of MC and RC

2 3-7 C

2

I3 m(10 ) = 39.6 2 kg-sρ

⎛ ⎞⎜ ⎟⎝ ⎠

2 3-7 C

2

I3 m(10 ) = 396 2 kg-sρ

⎛ ⎞⎜ ⎟⎝ ⎠

Example:• 300 kg satellite, 2 m across, needs

10 mN of thrust, want MC < 30 kg• EMFF effective up to 43 meters

Force vs. Separation Distance

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1 10 100

Separation (m)

Fo

rce

(N

)

MR = 0.1MR = 0.3MR = 1MR = 3MR = 10MR = 30MR = 100MR = 300

kg-m


EMFFEMFF Thermal ControlEMFF Thermal Control

• High Temp Superconducting (HTS) Wire – Wire needs to be kept below its critical temp (< 115 K) – The lower the temperature, the higher the critical current– On ground the problem solution is Liquid Nitrogen (77 K)– In space the problem must not use consumables

• General Objectives:– Quantify heat extraction to maintain superconducting

temperatures– Determine a design that will minimize the power needed

by the thermal subsystem


EMFFThermal Analysis SetupThermal Analysis Setup

• Material Selection and Geometry– Selected two designs: insulation and vacuum-gap– For maximum heating, coil is perpendicular to solar radiation

• Assumptions: – Steady-state analysis used to find the heat into the superconducting coil

Spacecraft in Earth trailing orbit, Qsun = 1367 W/m2 at 1 AU– No heat conduction by standoffs in vacuum-gap case– Temperature at rc is known (Superconducting temperature)– R and rc is given by mission specifications

R rc

rh

R

rc= HTS wire radius

rh= container radius

A

A


EMFFEarth Trailing ResultsEarth Trailing Results

• Observations:• Vacuum gap that has a tight

packaging works best• For a larger packaging coil

radius, Aerogel is favorable (rh > 2.08 cm)

• Heat can be rejected with known cryocoolers

7.54Cooling Power (W)

7777Temperature (K)

2.73Mass (kg)

150100Input Power (W)

M87M77Model

SunPowerSunPowerCompany• Results used the following:– Spacecraft at 1 AU (Gs = 1367 W/m2)– Design: rc = 1 cm, R = 1 m– Optical Solar Reflector (Quartz over

Silver): α/ε = 0.079/0.79 = 0.1


EMFFLEO ResultsLEO Results

• Results for a satellite at ISS altitude (~ 400 km)– Worst-case: albedo = 0.35– qIR= 258 W/m2

– Amean= 4 π rh R = 0.2356 m2

75. WQTotal

42.5 WQIR

7.9 WQalbedo

25.4 WQsun

• Results Comparison:– Greater heat rejection needed in LEO– Breakpoint favors smaller rh as heat rejection increases


EMFFThe Dipole Solution ProblemThe Dipole Solution Problem

• Given an arbitrary trajectory for the satellites in formation, what are the required magnetic dipoles strengths?

• Can any force profile can be created using EMFF?– Yes, As long as the formation center of mass does not change,

and the maximum dipole strength is not reached.• Given a certain forcing profile, what is the correct amount of current

that must be run through each coil?– There are multiple solution possibilities for any given force

profile• Other Considerations

– Angular momentum distribution– Maximum dipole strength– Selection of the free dipole.


EMFFSolving for the DipolesSolving for the Dipoles

• The EOMs are a system of 1st and 2nd degree polynomial equations.

• Polynomial equations can be solved by– Numeric solvers (Newton’s Method)

• Must be randomly seeded• Only provides one solution• Fast

– Reduction (Groebner’s Basis)• Does not work for large systems• Very poorly conditioned numerically

– Continuation Method– Produces all solutions (Real and Imaginary)

• Time Intensive

1 1 2 2 2 3 3

1 2 2 2 3 3

1 2 2 2 3 3

( ...)

( ...)

( ...)

x x x y z x y

y x y z x y

z x y z x y

F C C C C C

C C C C C

C C C C C

µ µ µ µ µ µ

µ µ µ µ µ µ

µ µ µ µ µ µ

= + + + + + +

+ + + + + +

+ + + + +

r


EMFFNumber of SolutionsNumber of Solutions

• The degree of the system determines the number of solutions.– For EMFF, each equation has a degree 2 or 1.

• From Bezout’s Theorem, the number of solutions is

• If we define the free dipole, then there are

• These solutions can lie– In the real or complex plane– In finite space or at infinity

• If we don’t define the free dipole, there are an infinite number of solutions.

1 2 3 3 3( ... )NNumof Sol d d d d −=

3 62 NNumof Sol −=


EMFFThe Earth’s Magnetic FieldThe Earth’s Magnetic Field

• Many EMFF missions will operate within the Earth’s magnetic field. – The Earth’s magnetic field will have an effect on the vehicles that must

be accounted for.• The Earth’s magnetic field can be approximated by a large dipole with

– A typical EMFF vehicle will have

• Even though the Earth’s dipole is 16 orders of magnitude larger, the force applied by the Earth’s magnetic field is negligible.– The force depends on the field gradient– The gradient of the B field at that

distance is negligible.

22 28 10Earth Amµ = g6 210EMFF Amµ ≈

2( )F Bµ= ∇r rr g


EMFFOutlineOutline


• Feasibility Issues– How far apart does it work?– Thermal Design– Dipole Selection




EMFFEMFF Testbed VehiclesEMFF Testbed Vehicles

EMFF Testbed Parameters

3 m x 1.5 mTable dimensions

~ 20 minutesOperating duration

0.90 mMaximum Diameter

1.18 mHeight

36 kgMass (single vehicle)

Metrology

Gas supply tank

Magnet and cryogenic

containment Electronics boards

Power Boards & Batteries

Base & Gas carriage

Reaction wheel

LN2 Tank


EMFFEMFF Critical TechnologiesEMFF Critical Technologies

25Distributed control

2

2

2

3

4

2

4

TRL (EMFF)

Demonstrated on EMFF Testbed

6Active and passive magnetic shielding

6Non-linear dynamics and control

2Efficient near field electromagnetic dipole interaction algorithms

6High angular momentum storage reaction wheels

6High current, low voltage power systems

9Cryogenic cooling and thermal control

6High temperature superconducting (HTS) wire

Current & Future Testbed Developments

TRL (Current)

Critical Technology


EMFFTestbed OverviewTestbed Overview

• Functional Requirements:– System contains 2 vehicles with EM control replacing thrusters– Each vehicle is:

• Self-contained (no umbilicals)• Identical/interchangeable

• Vehicle Characteristics– Each is 36 kg mass, 2 electromagnets, 1 reaction wheel

• Communication and processing– 2 internal microprocessors (metrology, avionics/control), Motorolla Tattletale– Inter-vehicle communication via RF channel– External “ground station” computer (operations, records)

• Metrology per vehicle– 1 rate gyro to supply angular rate about vertical axis– 3 ultrasonic (US) receivers synchronized using infrared (IR) pulses

DR2000 Comm. Board


EMFFElectromagnet DesignElectromagnet Design

• Coil wrapped with alternating layers of wire and Kapton insulation– 100 wraps– Radii of 0.375m and 0.345m

• Toroid-shaped copper casing– Operable temperature at 77 K– Filled with liquid nitrogen

• American Superconductor Bi-2223 Reinforced High Temperature Superconductor Wire– Dimensions

• 4.1 mm wide• 0.3 mm thick• 85 m length pieces

– Critical Current• 115 amps, 9.2 kA/cm2

– Below 110 K


EMFFCryogenic Containment SystemCryogenic Containment System

• Requirements:– Keep the wire immersed in liquid nitrogen.– Stiff enough to support liquid N2 container,

superconducting wire, and its own weight.• Original design: Foam with fiberglass and epoxy

– Constant leaking, and time consuming repair process

• Redesign – copper container tested without leaks

Redesign

HTS Wire Insertion


EMFFPower SubsystemPower Subsystem

• Coil & Reaction Wheel Power: – Rechargeable NiMH D-cell batteries (Capacity, C = 9 A-hr)– MOSFET controller – uses H-bridge circuit to control current through gates– T = C/In = 20 minute power duration, Peukert’s Formula

- 3.6 V + 35 Amps

35 A35 A

> 100 Amps

Electromagnet1

4

2

3


EMFFMore efficient High Current Control SystemMore efficient High Current Control System

• Rev2 uses a similar H-bridge design but exploits chip logic– Closes switch 3 to keep current

through electromagnet contained –free wheeling (coasting)

– Periodically pumping current• cooler (no fans needed)

Electromagnet1

4

2

3


EMFFAir Carriage and Reaction Wheel Air Carriage and Reaction Wheel

• Reaction Wheel– Store angular momentum– Provide counter-torques to electromagnets– Provide angular control authority– 0.23 Nm Torque at 10 Amps

• Flywheel Requirements: – non-metallic Urethane Fly Wheel– Maximum wheel velocity at 7000 RPM

• Motor tested in EM field with no variation in performance

• 2-D Friction-less environment provided by gas carriage– allows demonstration of shear forces, in concert with

reaction wheel– Porous Membrane, Flat air bearings provide pressurized

cushion of gas – CO2 gas supply: rechargeable compressed gas tank, 20

minute duration


EMFFTestbed Open Loop ManeuversTestbed Open Loop Maneuvers


EMFFTestbedTestbed Closed Loop HoldClosed Loop Hold


EMFFOutlineOutline


• Feasibility Issues– How far apart does it work?– Thermal Design– Dipole Selection




EMFFConclusionsConclusions

• Feasibility Questions Answers– Using current HTS wire, coil size and

mass are reasonable compared to what would be allocated for actuation

• This will improve over time– Reaction wheels sufficient for many

anticipated maneuvers are commercially available

– Power requirements to drive the coil current can be made quite low

– The limiting power draw is with thermal control, but is still found to be within the 100’s of Watts range

– The system is fully controllable, and linear control appears to be able to stabilize the system over large perturbations

– Techniques of extending this range using gain scheduling and other techniques are being investigated

• Performance Questions Answers– Separation distances are on the order of

several 10’s of meters at 10 mN Thrust– Systems using EMFF compare favorable

with those using propellant based actuation

• Especially as mission life (∆V) is increased

• Implementation Questions Answers– Methods for redistributing angular

momentum have been identified and are being further developed

– The Earth’s field can be used to an advantage, as one method of transporting angular momentum

– Methods of solving for the required dipole configuration have been found and methods of finding the ‘best’ configuration are being investigated


EMFFYear 2 PlansYear 2 Plans

• Feasibility questions have been mostly addressed

• Most of the remaining issues are ones of implementation

• B-field interactions with certain electronic components is still an issue that should be addressed– What components are affected– How difficult is it to shield them

• More detailed Design of thermal management system for use in space to keep HTS wire at cryogenic temperatures

• Automating the process of dipole selection– Trajectory planning phase– Real-time implementation phase

• Robust determination of dipole solutions for trajectory planning

• Extending stability regime by gain scheduling or other means

• Investigating other applications of EMFF and identifying other potential sources of development funding


EMFFOffice of Exploration Systems BAA Office of Exploration Systems BAA

• The BAA divides into 2 programs– Advance Space Technology (ASTP)– Technology Maturation (TMP)

• Under TMP is the Advanced Space Operations (ASO) Technology area– In-space Assembly/Maintenance– EVA Systems– Intelligent/Affordable On-Board Ops– Reliable/Responsive Ground Ops– Novel Space Ops Demonstrations

• Example of In Space Assembly given– Large scale (cranes)– mid-scale (robots)– Small-scale (micromanipulators)

• Focus is on systems providing– Reliable in-space deployment– Reliable in-space assembly– Intelligent/robust docking

mechanisms– Robust, autonomous rendezvous and

docking technologies and testbeds

• EMFF offers a potentially robust and reusable platform for assembly of large space structures– Can be integrated into first mission

and re-used on subsequent ones– Not as limited by size of structure as

arms or as massive as ‘cranes’– Can actuate against Earth’s field for

positioning large structures


EMFFApplications to Space AssemblyApplications to Space Assembly

• Roll Video…

Documents

Electromagnetic Formation Flight - NASA's Institute for ... · – MRI exposes humans to up to 2T ... Force vs. Separation Distance 1.0E-04 1.0E-03 ... Active and passive magnetic