SenSys’15—November 2, 2015
Tragedy of the Coulombs
Federating Energy Storage for Tiny, Intermittently-Powered Sensors
This Talk
1. Batteryless sensing challenges
2. Problems with centralized energy
Vision
Sophisticated sensing on
Batteryless Devices
Batteryless Devices
Batteryless Devices
Batteryless Devices
Batteryless Devices
As small as possible
• Minimal energy storage (Cap)
• Harvest energy (RF, Solar, Glucose)
Run when you can
• Frequent failures
• Erratic supply
Applications
Infrastructure Monitoring Wildlife Tracking
Building Monitoring Wearables
Pipelines
Bridges Roads
Small animal Implant once
Occupancy Energy Waste
Greenhouse Monitoring
Water waste is a problem
• Overwatering typical
• Coarse data on plants
Dense sensing is a solution
• Fine grained plant information
Greenhouse Monitoring
Computation
Harvesting
Storage
Batteryless Sensing Su p p ly V o lta g e (V) 0 1 2 3 4 5 6 MC U H u mi d it y Se n so r Leaf W e tn e ss Radio
Voltage requirements vary
Batteryless Sensing
Se
n
s
o
r
V
o
lta
g
e
0 1 2 3 4
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Batteryless Sensing
Se
n
s
o
r
V
o
lta
g
e
0 1 2 3 4
Attempt
Send
Pkt
Batteryless Sensing
Se
n
s
o
r
V
o
lta
g
e
0 1 2 3 4
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Attempt
Send
Pkt
Reset
Batteryless Sensing
Se
n
s
o
r
V
o
lta
g
e
0 1 2 3 4
Attempt
Send
Pkt
Reset
Sensor
Batteryless Sensing Se n s o r V o lta g e 0 0.7 1.4 2.1 2.8 3.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Batteryless Sensing Se n s o r V o lta g e 0 0.7 1.4 2.1 2.8 3.5 Sensor
Task coupling decreases utility
Scheduling Tasks En e rg y C o s t (m J ) 0 25 50 75 100 Task
1s Computation 10x Leaf Rds 10x Humidity Rds Send 1 Packet
Gree
nhou
se Mo
Scheduling Tasks
En
e
rg
y
Sto
re
d
(m
J
)
0 0.125 0.25 0.375 0.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
1s of Computation Sample 10x Humidity Send 1 Pkt
Scheduling Tasks
En
e
rg
y
Sto
re
d
(m
J
)
0 0.125 0.25 0.375 0.5
Scheduling Tasks
En
e
rg
y
Sto
re
d
(m
J
)
0 0.125 0.25 0.375 0.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Start Threshold
Scheduling Tasks
C
a
p
a
c
ito
r
V
o
lta
g
e
0 1 2 3 4
Time (s)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Large Small
Scheduling Tasks
C
a
p
a
c
ito
r
V
o
lta
g
e
0 1 2 3 4
Time (s)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Large Small
Scheduling Tasks C a p a c ito r V o lta g e 0 1 2 3 4 Time (s)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Large Small
Start Threshold
Batteryless Sensing
Batteryless sensing is hard
• Tasks are coupled, causing death
• Execution is not predictable
• Low energy tasks wait on high energy
tasks
Batteryless Sensing
Batteryless sensing is hard
• Tasks are coupled, causing death
• Execution is not predictable
• Low energy tasks wait on high energy
tasks
Because energy storage is centralized
Federated Energy
Benefits
• useful work starts sooner
• fewer power failures
• simpler application decisions
• relaxes voltage coupling
UFoP
Multiple capacitors
• One for microcontroller
• One for each peripheral
• Static rate and priority
Charge Controller
Core
Energy
Harvesting Sensor Radio
+
Start work sooner MCU V o lts 0 1 2 3 4 Time (s)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
MCU MCU MCU V o lts 0 1 2 3 4 Time (s)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
MCU Radio
UFoP
Ready
Decrease power failures Centralized C a p a c ito r V o lta g e 0 1 2 3 4 Time (s)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
MCU Reset
Attempt
Send
Decrease power failures Federated C a p a c ito r V o lta g e 0 1 2 3 4 Time (s)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Simplify app decisions Centralized C a p a c ito r V o lta g e 0 1 2 3 4 Time (s)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Send
Federated C a p a c ito r V o lta g e 0 1 2 3 4 Time (s)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Radio Cap MCU Cap
Simplify app decisions
Send
Pkt
Harvest more energy
Harvest more energy
Power
Harvest more energy
Power
Harvest more energy
C
u
rr
e
n
t
(I
)
0 1.25 2.5 3.75 5
Volts (V)
Harvest more energy
Federated
V
o
lts
0 1 2 3 4
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
Centralized
V
o
lts
0 1 2 3 4
Harvest more energy
Federated
V
o
lts
0 1 2 3 4
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
Centralized
V
o
lts
0 1 2 3 4
Harvest more energy
Federated
V
o
lts
0 1 2 3 4
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
Centralized
V
o
lts
0 1 2 3 4
Implementation
Energy
Harvesting
Peripheral Control Charging Control
Peripherals
DC Power
MCU (MSP430)
Radio Sensor
1st stage Capacitor
Solar Thermal RF Kinetic
UFoP Controller (Custom PCB)
or or or
Cap Cap
Results
Evaluation
• Availability
• Resiliency
• Energy harvested
Deployment
Availability
1.5
2.0
2.5
3.0
vo lts Centralized mcu radio sensor
1.5
2.0
2.5
3.0
vo
lts
Federated
mcu radio
0 1 2 3 4 5 6
time(s)
sensor
Resiliency
0 7.5 15 22.5 30
Low Voltage Events
UFoP Centralized
0% 2.5% 5% 7.5% 10%
Energy Harvested
En
e
rg
y
(m
J
)
0 3 6 9 12
Solar RF High Energy RF Low Energy
Deployment
25% 50% 75% 100%
a
v
ailab
lity
storage
Centralized MCU Centralized Radio UFoP MCU
Future Work
Dynamic UFoP
• Dynamic priority, capacity, thresholds
• Resiliency
• Energy harvested
Language
