On Apr 6, 2007, at 12:27 PM, Kurt Peters wrote:
If you seek to estimate the power consumption, you can read
through the
data sheet for the 128L from Atmel if you're using that group of
motes. The
schematics for other sensor boards are available online, and you
find the
parts used and then can use those datasheets to determine /
estimate their
respective current draw.
In my opinion, in order to "really" calculate energy
consumption with
any credibility, you should use an external, well-regulated power
supply and
continually monitor current. Unfortunately, the sample rate for
current
monitoring might be reasonably untenable, so you could make an
integrator
(monitoring the voltage drop across a very small valued resistor
(0.01 Ohm)
and sample that periodically (and reset it each time). The
location of that
resistor might be right at the input of your DC supply. That way
you're
getting a true measure of energy consumption. It shouldn't be too
hard of a
circuit to put together. Otherwise, others have shown that doing the
power/energy estimation using just battery voltage is unreliable.
I think
measuring current and voltage continuously has it's problems in
that you may
not sample during the period your radio is on/transmitting causing
a lower
estimated average power.
One of the issues here is that the range of currents make it very
difficult to measure with a single set-up; there's four orders of
magnitude difference between the lowest and highest draws.
The approach that being seem to be settling on in the research
literature so far is to perform microbenchmarks of specific states
using a precision multimeter, correlate those values with the
observed but inaccurate oscilloscope values, use an oscilloscope
trace to determine how long the node spent in each state, and
calculate the energy that way. This mostly matters for sleep currents
-- the oscilloscope readings are go good for things like the radio
being on.
I'm sure there are better and more complex measurement setups to do
this more accurately, but this approach seems to be pretty effective.
Phil
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