Energy in sensor nets

1
Energy in sensor nets
2
Where does the power go

• Components
• Battery -gt DC-DC converter
• CPU Memory Flash
• Sensors ADC
• DAC Audio speakers
• Display
• Radio

3
CPU Energy

• 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 to 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

4
CPUs

• Intel strong arm
• Full power 400mW
• Idle mode CPU clocks are stopped, but peripheral
  clocks are active (so peripheral interrupts can
  occur) 100 mW
• Sleep mode only a real time clock. Only timed
  wake up can occur. 50 micro W (some cell phones
  have alarms that can ring even when turned off)
• Texas instruments MSP 430
• Wide range of modes
• One fully operational mode 1.2 mW
• 4 sleep modes
• Deepest sleep only external interrupts can cause
  wake up 0.3 micro W
• Next deepest sleep the clock can cause wake ups
  50 micro W
• Atmel Atmega
• Active modes range from 6mW to 15mW.
• Sleep mode uses 75 micro watts

5
Dynamic voltage scaling

• Power ?frequency V2
• If the frequency is reduced, or the voltage is
  reduced, power can be saved.
• As all us overclockers know, there is a
  relationship between voltage and frequency (if
  the voltage is decreased, the frequency must also
  be decreased)
• Transmeta Crusoe
• 700 MHz at 1.65 V
• 200 MHz at 1.1V
• Power is reduced by a factor of 7.8, but speed is
  only reduced by a factor of 3.5
• Hence energy per instruction is reduced by
  3.5/7.87544

6
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.
• Waking
• Wait for clocks to become stable and PLLs to
  stabilized
• Waking from deep sleep might require moving data
  from static ram (or rom) to dynamic ram)
• Sleep
• Move data
• Discharge of currnent
• The deeper the sleep, the more time it takes to
  wake up (compare waking up in the morning to
  waking up from dozing off as I speak)
• Let the power usages in the four power levels be
  Pi. 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 state. The power
  usage decreases linearly when going to sleep

state Pk ?d,k (ms) ?u,k (ms)
S0 1040 -
S1 400 5 8
S2 270 15 20
S3 200 20 25
S4 10 50 50
7
Deep Sleep vs. Light Sleep

• If delay is important, then deep sleep might not
  be better than deep sleep.
• But to determine the trade-off between delay and
  energy requires a user model
• Without user models, deep sleep might use more
  energy

Option 1 after event is processed, go to
deepest sleep Option 2 after event is
processed, go to light sleep
PActive
P1
P0
interrupt
interrupt
There is a significant amount of time that the
deep sleep uses more power than the light sleep
8
Optimal Sleep Depth
state Pk ?d,k (ms) ?u,k (ms)
S0 1040 -
S1 400 5 8
S2 270 15 20
S3 200 20 25
S4 10 50 50
In matlab plot((1040-10) (1040-200) (1040-270)
(1040-400), 50 22.5 17.5 6.5)
9
Optimal Sleep Depth
10
Optimal Sleep Depth
Multiple power save modes are not that useful.
The deepest sleep is most likely the best.
11
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
12
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

13
Battery capacity

• Batteries are specified in terms of mAh, milliamp
  hours. An AA has about 2000-3000mAh.
• Capacity is often measured in J/cm3 (recall a 1
  J 1 watt sec)
• So an AA battery 2.5Ah1.5V3600 13500 J

14
Battery issues

• Capacity under load
• If too much energy is drawn from the battery, the
  battery will not be able to supply the specified
  amount of energy it may even break.
• Typically, sensors will draw more power than the
  battery can supply for optimal lifetime.
• Self discharge
• Batteries will lose energy over time even if no
  energy is drawn from them.
• E.g. zinc air batteries have a lifetime of a few
  weeks
• Efficient recharging
• Some techniques, e.g. solar, can only generate
  very low current, but over a very long time.
• However, batteries require fairly high current to
  charge
• Relaxation
• Batteries are based on a chemical process
• Once a battery is drained, if left alone, it
  may regain some energy.
• If the relaxation is understood, then the sensor
  could take advantage of it and extract more power
  from the battery

15
DC-DC Converter

• The battery voltage might be larger or smaller
  than the sensors and processors require.
• DC-DC converter converts from one voltage to
  another
• DC-DC converters are not 100 efficient

16
Energy scavenging

• Photovoltaics
• 10 microW/cm2 indoors and 15 mW/cm2 outdoors
• A single cell creates 0.6V, which is not high
  enough the charge a battery. So many cells are
  put in series.
• Solar cells is an active area of research
• Temperature gradient
• A difference of 5C can, theoretically, produce
  considerable power.
• But it is difficult to achieve the theoretical
  limit
• Seebeck effect-based thermoelectric generators
  might achieve 80 microW / cm2 at 1V from 5C
  temperature difference.
• Vibrations
• Depending on the amplitude and frequency, it is
  possible to generate between 0.1 microW/cm3 to
  10mW/cm3
• Practical device of 1 cm3 can generate 200
  microW/cm3 from 2.25 m/s2 at 120 Hz.
• How much is this? Displacement
  Asin(2pi120t) -gt acceleration(2pi120)2A
  2.25 gt A4e-6m.?
• Pressure vibration
• Sneakers with lights
• 330 microW/cm2
• This could be used for sensors in roads
• Air/liquid flow

17
Energy scavenging vs energy capacity