Transcript Energy in sensor nets

Energy in sensor nets
Where does the power go
• Components:
– Battery -> DC-DC converter
– Sensors->ADC->MCU+MemoryRadio
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
– .16mW=>781 days
– 16.5mW=>7.5 days
– 400mW=>7.5 hours
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
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.
Pk
Pk
The energy save is E save,k :
P 0 P k 
t k  P o
d,k  P o 
u,k
2
2
•
The threshold for going to sleep power k is T th,k  12 d,k 
state
P_k
Tau
(ms)
S0
1040
-
S1
400
5
8
S2
270
15
20
S3
200
20
25
S4
10
50
50
T
P 0
Pk
P 0 P k
u,k
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
Clock and voltage
power
Clock freq
freq
volt
active
idle
sleep
133
1.55
240
75
50mi
croA
206
1.75
400
100
50mi
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 = 0.32.
• 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
radio
• The radio can use a large fraction of the total power
MCU
sensor
radio
power
active
on
Transmit=36.3mW
1080.5
active
On
Transmit=19.1mW
986.0
Active
On
Transmit=13.8mW
942.6
Active
On
Transmit=3.47mW
815.5
Active
On
Transmit=2.51mW
807.5
Active
On
Transmit=0.96mW
787.5
Active
On
Transmit=0.30mW
773.9
Active
On
Transmit=0.12mW
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
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
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