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Advanced MEMS components in closed-loop micro

propulsion applications

Pelle Rangsten, Johan Bejhed, Håkan Johansson, Maria Bendixen, Kerstin Jonsson, and Tor-Arne

Grönland

NanoSpace Uppsala Science Park e-mail: [email protected]

SE-751 83 Uppsala http: www.sscspace.com/nanospace SWEDEN


MEMS Micropropulsion Components for Small Spacecraft

SMÖRGÅSBORD [ˈsmœrɡɔsˌbuːɖ]

Thrusters Flow Control Valves

MEMS Isolation Valve

Pressure Relief Valve

Filters

Pressure Sensors

Presens (N)

http://en.wikipedia.org/wiki/Wikipedia:IPA_for_Swedish_and_Norwegian




MEMS Micropropulsion Components

•First generation MEMS micropropulsion:

– Miniaturised, accurate and open-loop

• Next generation MEMS micropropulsion:

– Closed-loop control

Xenon flow control module CubeSat propulsion module


Motivation - Advanced Nano- and Cube Sats

• Propulsion to enable new missions

– Drag free flights, Orbit change, FF & RV , docking, de-orbit…

-> New scientific results

-> Commercial applications

-> Space debris mitigation


Challenging requirements

Flow Control Requirements for next generation mini Ion engines

Flow rate range 5 – 50 µg/s

Flow rate control accuracy +/- 5% across the flow range

+/- 5% above 25 µg/s and

+/- 10% below 25 µg/s

Flow rate control resolution +/- 0.5 µg/s

Mission thrust requirements given by CNES for MICROSCOPE

Thrust range 1 – 300 µN

Thrust resolution 0.2 µN

Response time 250 ms

RIT-µX by Astrium

MICROSCOPE by CNES


Closed–Loop Flow Control

ON/OFF

Valve

MEMS Proportional

Flow Control Valve

Mass flow

sensor

Filter

Feed back loop

to control FCV

Including front

end electronics

Integrated mass flow sensor provides

control signal to the proportional flow

control valve

Schematic view of a complete closed -loop control thruster.

Closed-loop flow control

Thruster chip and front-end electronics


Key capabilities – Like any other

ON/OFF cycles, full thrust range

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70 80 90 100 110 120

Time [s]

Th

rus

t [µ

N]

Test result of MEMS thruster operating in ON/OFF mode

(open loop, using solenoid valve only) to show thrust range. Full thrust can be set in the range 50 micro-Newton to 5 milli-Newton


Key capabilities – Unlike any other

Test result of a MEMS valve operating in closed-loop control mode showing the thrust

response to commanded steps of 5 μN.

Low thrust regime step response: 5µN steps

0

5

10

15

20

25

45 55 65 75 85 95 105 115 125 135

Time [s]

Th

rus

t [µ

N]

Commanded Thrust

Delivered Thrust


Unique performance

Test result of a MEMS valve operating in closed loop control mode responding to the

commanded steps of 0.1 μN.

Low thrust regime response: 0.1µN steps

4,3

4,4

4,5

4,6

4,7

4,8

4,9

5

5,1

5,2

5,3

40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Time [s]

Th

rust

[µN

]

Commanded Thrust

Delivered Thrus


Low flow rates in combination with the wish for fast

response

Physics problem

MEMS Proportional

Flow Control Valve

Thrust chamber

volume

Mass flow

sensor and

feed back loop

to control FCVFeed line

volumes

Thruster case:

Requirement: 250 ms in response time

Flow rate: 5 µg/s

Response time increases linearly with the internal dead volume

Simplified estimate: V ~ 10 mm3

Tubing Length Volume

1/8” 5 mm (0.2”) 9 mm3

1/4” 0.62 mm (0.025”) 10 mm3


The Solution - MEMS

In our view, using MEMS technology and integrating the flow control valve,

mass flow sensor and chamber/nozzle on a single chip is the best –if not the

only- way to realise a closed-loop control thruster that can meet the

challenging requirements with low flow rates in combination with fast

response.


Results – Xenon Flow Control

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Mas

s Fl

ow

[µ

g/s]

Time [s]

Mass Flow vs Time

Capable to operate in full flow regime


Results – Xenon Flow Control

5.5

5.7

5.9

6.1

6.3

6.5

6.7

6.9

7.1

7.3

7.5

0 20 40 60 80 100 120 140

Mas

s Fl

ow

[µ

g/s]

Time [s]

Mass Flow vs Time

Capable to performe minute flow changes


Record shattering resolution

Capable to resolve extremely small changes: 0.2 µg/s (200 ng/s)

7.4

7.45

7.5

7.55

7.6

7.65

7.7

0 10 20 30 40 50 60 70 80

Mas

s Fl

ow

[µ

g/s]

Time [s]

Mass Flow vs Time


Summary – XeFCM H/W

• Designed, manufactured, and

tested a Xenon closed-loop flow

control module!

• Mass: 63 grams

• Excellent dynamic range

• Step regulation < 200ng/s

• Fast response time

Next step:

• Testing together with mini Ion engine (Astrium´s µN-RIT engine)


Summary – Micro Thruster H/W

• Four 1mN thrusters with closed-loop thrust control

• Thrust resolution: <10µN

• Propellant: Butane

• Total impulse: 40Ns

• Size: 10*10*3cm

• Mass: 250g

• Operating pressure: 2-5 bar

• Power consumption: 2 W

(average, operating)

• Mechanical interface: CubeSat payload I/F (Pumpkin)

• Electrical interface: 52 pins analog (0-12V) and digital (SPI)

Next step:

• Finalise the assembly, integration, and testing

• Closed-loop thrust control demonstrated with unique

performance (in terms of thrust and response time in the low thrust regime)

• Developing a CubeSat propulsion module


Outlook

• Return to SmallSat in Logan next year for a live demonstration!!!

• Fly our closed-loop products in space

Thank you for your attention!

Swedish coins for size reference… SSC booth 48-49


NanoSpace

starts operation

2005

Final delivery

to Prisma

phase B

contract 1st complete

thruster module

First test of

MEMS thruster

1 st delivery

of flight H/W

Launch!

2006 2007 2008 2010 2009

End-to-end

testing on S/C

First generation developed for Prisma


Thruster Pod Assembly – Plenty of MEMS inside

Six-wafer-stack MEMS Thruster Chip

= 44 mm (1.73”)

Four thrusters per pod

10 µN – 1 mN

Mass: 115 g


Understanding the physics, an example:

Assume that the MEMS valve is closed. This implies (in vacuum) zero

pressure and zero mass flow through the nozzle. Now assume that the

valve immediately opens to allow a flow rate of 5μg/s (which corresponds to

2 μN). Also assume that the nozzle throat is sized such that this flow rate

corresponds to 0.1 bar pressure in the chamber (which corresponds to

thruster dimensioned for ~100 μN at full thrust).

Now, to reach this new steady state condition, the total volume between the

valve and the nozzle throat must be “filled up” with gas from zero to 0.1 bar.

Assume that the total volume of feed lines and thrust chamber is 10 mm3.

A first order estimate of the response time of such a system is 230 ms.

Response time increases linearly with volume.

This is an optimistic estimate neglecting a number of effects such as valve

opening response, reduced flow rate as the pressure increases, delays in

the control loop, etc which in reality will slow down the response time

significantly.

However, this example illustrates how crucial it is to minimise the internal

volumes in a regulated system with low flow rates.

Physics Lessons

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