Download celt-wireless

Wireless for CELT?
(Or: An astronomer thinks
about BSAC and BWRC)
Marshall Perrin
Ay 250
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Why consider it?
• CELT requires a tremendous number of sensors and
actuators:
• 6200 segment edge sensors
• 3000 segment position actuators
• Other sensors: temperature, vibration, wind speed, ???
• Lots of interesting technology is being developed at
Berkeley currently
• Berkeley Sensor and Actuator Center (BSAC)
• Berkeley Wireless Research Center (BWRC)
• So what, if any, potential benefits can be gained for
CELT?
Wireless Sensors for CELT
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Wireless Sensor Networks
BSAC microsensors
• Current “macro motes”:
•
•
•
temperature, humidity, barometric pressure, light
intensity, tilt and vibration, and magnetic field
sensors all in a cubic inch package, including the
bi-directional radio, the microprocessor controller,
and the battery!
20 meter communication range
one week lifetime in continuous operation, 2
years with 1% duty cycling
• Future “Smart Dust”
• Pack everything into ~ 1 mm3.
• Power: batteries, solar, vibration?
• Estimates of a few nW power, or a
few nJ per bit transmitted
Wireless Sensors for CELT
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Powering Wireless Sensors
• Batteries – low tech, easily available but limited lifetime
• Solar – probably not feasible inside
• Sunlight is .14 W/cm2, but internal light levels down a few orders
of magnitude. Solar cell efficiency ~ 15%. Result: ~ 10 mW/cm2
• Local generation from ambient vibrations
•
http://www-bsac.eecs.berkeley.edu/publications/pdf/pub1027468961.pdf
http://www2.acae.cuhk.edu.hk/~aml/iros2000/li-2.pdf
• Electrostatic or inductive power generation
• ~ 10-100 10 mW/cm3
Wireless Sensors for CELT
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Potential Benefits for CELT
• Reduced wiring complexity on the telescope
• Ease of maintenance for segment replacement?
• Reduced weight
• Reduced needs for cable wraps
• Increased capabilities from additional sensors
• Additional diagnostic capability at minimal cost and added
complexity
Wireless Sensors for CELT
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Potential Negatives
• Increased technological complexity
• These technologies still require further development,
but should be more mature by the ~2010 timescale for CELT
• Reduced Reliability
• Interference with other systems
• Wireless transmissions at 2.4 GHz
• Optical communications lasers (do we really want signal lasers
bouncing around the back of our telescope primary?)
Wireless Sensors for CELT
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The Problem: Actuator Power
• The current segment actuator candidate design requires
approximately 2 W per actuator.
• This is far in excess of what can feasibly be provided by
batteries or local generation, so at bare minimum we’ve got to
run a power cable to every single segment.
• Might as well run data cables in the same cable runs at low cost
and low complexity?
• But power distribution is much less complex than multiplexing
6000 incoming and 3000 outgoing signals, so maybe wireless
can be of benefit after all.
• Requires comparison of more detailed designs than are currently
available.
• Power needs of edge sensors not clear (~ .1-1 W?). Could this be reduced further?
Wireless Sensors for CELT
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Additional possibilities
• Distribute many temperature sensors throughout
telescope and dome to measure dome seeing
• Locate and track sources of heat inside the enclosure
• Low duty cycle so can get substantial battery lifetimes
• Vibration sensors for diagnostic purposes
• Watch bearings for frequency signatures indicating immanent
failures
• Humidity sensors for operating conditions
• Better knowledge of whether it’s safe to open the dome
Wireless Sensors for CELT
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An additional driver for low power…
• 6 kW spread over the primary mirror due to the 3000
actuators, plus whatever the 6200 edge sensors use
• For dry air at 0 degrees C on Mauna Kea, 6 kW will heat
air at a rate of 8.3 m3/s/K
• Or, spread over the 706 m2 area of CELT, that’s 1.2 cm3
heated by 1 degree per second per cm2 of mirror…
• That can’t be good for dome seeing! (Particularly given
as in reality it won’t be evenly distributed)
Wireless Sensors for CELT
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