Energy in sensor nets

Where does the power go Components: –Battery -> DC-DC converter –CPU + Memory + Flash –Sensors + ADC –DAC + Audio speakers –Display –Radio

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

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

Dynamic voltage scaling Power  frequency * V 2 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 3.5/7.875=44%

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 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 state. The power usage decreases linearly when going to sleep state PkPk  d,k (ms)  u,k (ms) S S S S S41050

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 P Active interrupt P1P1 P0P0 Option 1 – after event is processed, go to deepest sleep Option 2 – after event is processed, go to light sleep There is a significant amount of time that the deep sleep uses more power than the light sleep

Optimal Sleep Depth state PkPk  d,k (ms)  u,k (ms) S S S S S41050 In matlab plot([( ) ( ) ( ) ( )], [ ])

Optimal Sleep Depth

Multiple power save modes are not that useful. The deepest sleep is most likely the best.

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 freq power Clock only Clock and voltage freqvoltactiveidlesleep mi croA mi croA

Active power management Sleep has the most power saving. Maybe getting there fastest is the best thing. E.g, 59MHz = 1V, 221MHz=1.75 Reduction in speed is 59/221 = 0.26 (so 1/.26 more time is needed). Reduction in power is (1/1.75)^2 = Total change in energy is 0.32/0.26 > 1 => 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

Battery capacity Batteries are specified in terms of mAh, milliamp hours. An AA has about mAh. Capacity is often measured in J/cm^3 (recall a 1 J = 1 watt * sec) –So an AA battery = 2.5Ah*1.5V*3600 = J

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

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

Energy scavenging Photovoltaics –10 microW/cm^2 indoors and 15 mW/cm^2 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 / cm^2 at 1V from 5C temperature difference. Vibrations –Depending on the amplitude and frequency, it is possible to generate between 0.1 microW/cm^3 to 10mW/cm^3 –Practical device of 1 cm^3 can generate 200 microW/cm^3 from 2.25 m/s^2 at 120 Hz. –How much is this? Displacement = A*sin(2*pi*120*t) -> acceleration=(2*pi*120)^2*A = 2.25 => A=4e-6m….? Pressure vibration –Sneakers with lights –330 microW/cm^2 –This could be used for sensors in roads Air/liquid flow

Energy scavenging vs energy capacity