Energy in sensor nets

1
Energy in sensor nets
2
Where does the power go

• Components
• Battery -gt DC-DC converter
• Sensors-gtADC-gtMCUMemory?Radio

3
Micro-controller unit MCU

• Intel strong arm 400mW
• Atmel AVR 16.5 mW
• Of course, strong-arm can accomplish more
  processing in a shorter amount of time
• Intel strong arm 50mW in idle and 0.16mW in
  sleep
• Battery 3000mAh
• .16mWgt781 days
• 16.5mWgt7.5 days
• 400mWgt7.5 hours

4
MCU continue

• Active
• All clocks running to all subsystems
• Idle
• Halt CPU, preserve context, able to respond to
  interrupts.
• When an interrupt occurs, processor returns to
  active
• Sleep
• Turn off power o most circuits.
• Able to monitor wake-up event
• Advanced configuration and power management
  interface (ACPI) allows the OS to interface with
  the power saving modes
• ACPI MCU has 5 states of various power,
  SystemStateS0 fully working, to SystemStateS4
• ACPI devices have similar 4 states

5
Sleep state transition

• Going to sleep and waking up is not free it
  uses power. When transitioning, power is used
  that cannot be used for any processing etc. It is
  wasted (why? Clocks are not stable. Why? PLLs
  have not stabilized.)
• Define power usages in the four power levels as
  P_i. And ?_d,k to be the time used to go from the
  active state to power level k, and ?_u,k to go
  from low power state k to active. The power usage
  decreases linearly when going to sleep
• Going to low energy is deemed useful only is more
  energy is saved during the procedure than is
  expended by going in and come out of the low
  power state.
• The energy save is
• The threshold for going to sleep power k is

state P_k Tau (ms) T
S0 1040 -
S1 400 5 8
S2 270 15 20
S3 200 20 25
S4 10 50 50
6
Active power management

• Variable voltage processing dynamic voltage
  scaling (DVS)
• The voltage and clock frequency can be decreased
  to save power.
• We can assume that the power decreases
  quadratically with voltage and linearly with
  frequency.
• Of course, decreasing clock freq. Decreases the
  MIPS so the decrease in clock does not change the
  power required for a computation. On the other
  hand, a lower voltage might be possible at lower
  clock speed, resulting in a large saving in power.

Clock only
freq volt active idle sleep
133 1.55 240 75 50microA
206 1.75 400 100 50microA
Clock and voltage
power
Clock freq
7
Active power management

• Sleep has the most power saving. Maybe getting
  there fastest is the best thing.
• E.g, 59MHz 1V, 221MHz1.75
• Reduction in speed is 59/221 0.26 (so 1/.26
  more time is needed). Reduction in power is
  (1/1.75)2 0.32.
• Total change in energy is 0.32/0.26 gt 1 gt more
  energy is used. It is better to use full power
  and go to sleep ASAP (assuming there is very
  little power used at sleep, which is true)
• On the other hand, if one is merely waiting for
  something to happen, then low power is useful.
• Also, if events occur frequently, then it is not
  useful to go to sleep and best to finish one task
  just as the next event has occurred. Running NOPs
  is a complete waste of energy.
• Clearly, the programs must be written with power
  in mind, with the processor in mind.
• A power aware OS can help

8
radio

• The radio can use a large fraction of the total
  power

MCU sensor radio power
active on Transmit36.3mW 1080.5
active On Transmit19.1mW 986.0
Active On Transmit13.8mW 942.6
Active On Transmit3.47mW 815.5
Active On Transmit2.51mW 807.5
Active On Transmit0.96mW 787.5
Active On Transmit0.30mW 773.9
Active On Transmit0.12mW 771.1
Active On RX 751.6
Active On Idle 727.5
Active On Sleep 416.3
Active On removed 383.3
Sleep On Removed 64
Active removed Removed 360

9
radio
mcu sensor radio modulation Data rate power
active on 0.7368mW OOK 2.4 24.58
0.0979mW OOK 2.4 19.24
0.7368mW OOK 19.2 25.37
0.0979mW OOK 19.2 20.05
0.7368mW ASK 2.4 26.55
0.0979mW ASK 2.4 21.26
0.7368mW ASK 19.2 27.46
0.0979mW ASK 19.2 22.06
RX Any any 22.20
idle 22.06
Off 9.72
Idle On Off 5.92
sleep off off 0.02
Not shown is that when the radio is turned on and
off, large amount of power are required
10
battery

• Batteries are specified in terms of mAh, milliamp
  hours. An AA has about 2000-3000mAh.
• The battery also has a maximum current drain to
  meet the specified lifetime.
• If the current is beyond that, then the lifetime
  is greatly reduced in that one does not receive
  the 3000mAh as promised.
• The problem is that this current is very small,
  smaller than what is required to keep the system
  running.
• Relaxation effect
• If the system is turned on and current brought to
  nearly zero, then the battery can catch-up and
  the full lifetime can be acheived