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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
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MEMS Micropropulsion Components for Small Spacecraft
SMÖRGÅSBORD [ˈsmœrɡɔsˌbuːɖ]
Thrusters Flow Control Valves
MEMS Isolation Valve
Pressure Relief Valve
Filters
Pressure Sensors
Presens (N)
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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)
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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
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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
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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
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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
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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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