1 Wireless for CELT? (Or: An astronomer thinks about BSAC and BWRC) Marshall Perrin Ay 250

2 Wireless Sensors for CELT 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?

3 Wireless Sensors for CELT 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 mm 3. Power: batteries, solar, vibration? Estimates of a few nW power, or a few nJ per bit transmitted

4 Wireless Sensors for CELT Powering Wireless Sensors Batteries – low tech, easily available but limited lifetime Solar – probably not feasible inside Sunlight is.14 W/cm 2, but internal light levels down a few orders of magnitude. Solar cell efficiency ~ 15%. Result: ~ 10  W/cm 2 Local generation from ambient vibrations Electrostatic or inductive power generation ~  W/cm 3

5 Wireless Sensors for CELT 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

6 Wireless Sensors for CELT 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?)

7 Wireless Sensors for CELT 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?

8 Wireless Sensors for CELT 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

9 Wireless Sensors for CELT 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 m 3 /s/K Or, spread over the 706 m 2 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)