I/O Aware Task Scheduling for

Energy Harvesting Embedded Systems

with PV and Capacitor Arrays

Kyungsoo LEE and Tohru ISHIHARAkslee@i.kyoto-u.ac.jp

Department of Communications & Computer Engineering Graduate School of Informatics

Kyoto University, Japan

Energy Harvesting Devices

2

10 thermoelectric modules generates microwatts

800 mW per shoe at a pace of two steps per second

10m/s wind speed generates 100W electrical powerwith 1m rotor diameter

1m/s water speed generates 1Wwith 0.1m rotor diameter

Energy Harvesting System

Advantages Mobile: no power wires Easier installation Lower maintenance fee Higher uptime

3

Disadvantages Dependent on availability by

harvestable energy source Strict power budget Less mature technology

Energy efficiency

Heliomote Sojourner

Energy Efficiency

Efficiency Generating efficiency

Harvesting device efficiency

Consuming efficiency Consumer device efficiency

Transferring efficiency DC-DC converter efficiency Charger efficiency• Harvested energy is not constant• Power is not available on-demand• High peak power is not available

4

Energy Generator

Transfer/Storing Consumer

Outline

Background Maximum power point tracking DC-DC converting power loss

Motivations Reduce the voltage difference I/O aware task scheduling

Design approach Experimental results Conclusion

5

Generating Efficiency

Maximum power point tracking

6

0 2 4 6 8Output voltage (V)

Out

put c

urre

nt (m

A)

IV curve

PV curve

Out

put p

ower

(mW

)400

0

100

200

300

500

80

0

20

40

60

100 MPP (5.08V, 81.8mA)

Transfer Efficiency

DC-DC converter efficiency

7

Traditional case Recent case

Voltage Difference

Reducing the power loss in DC-DC converters

8

Pow

er lo

ss (m

W)

Voltage di↵erence (Vin

� Vout

)

0

25

50

75

100

-1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 2.5Ef

ficie

ncy

(%)

100

75

50

25

0

Power loss at 10 mA outputPower loss at 100 mA outputEfficiency at 10 mA outputEfficiency at 100 mA output

TPS63030 datasheet. http://www.ti.com/lit/ds/symlink/tps63030.pdf.

Motivations

Reduce the voltage difference I/O aware task scheduling

9

Energy

source

DC-DC

Supply

voltage A

group

DC-DC

Supply

voltage B

group

DC-DC

Supply

voltage C

group

Reconfigurable Array

10

An example for a configurable array 3 configurations with 4 photovoltaic (PV) cells Each cell has 0.5V, 80mA output Should be balanced because the size of each cell is same in this example

(4,1): 0.5V, 320mA output (2,2): 1V, 160mA output (1,4): 2V, 80mA output

Reduce the difference between

the input and output voltage

in a DC-DC converter

Motivations

I/O aware task scheduling

11

(a) Conventional (b) Proposed

Time

CPU1

CPU2

CPU3

CPU4

Triggering task

I/O

Time

CPU1

CPU2

CPU3

CPU4

I/O

Vvariable ⇡ V1

Vvariable ⇡ V2

Vvariable ⇡(V1 + V2)

2

tdeadline tdeadline

Proposed System Architecture

12

Configurable array

for PV cells

Energy storage

Configurable array

for supercapacitors

DC-DC converter

DC-DC converter

A B

Energygenerator

PV(m,n)

DC-DC

DC-DC

DC-DC

PV(m,n)

DC-DC

DC-DC

DC-DC

Supercapacitor

Charger

Proposed System Architecture

System block diagram

13

Power OR’ing by Linear Technology’s ideal diode

DC-DC

DC-DC

DC-DC

DC-DC

Charger

L1

L2

Li

(mpv, npv)PV

Cap(mc, nc)

SW1

SW2

SW3

Vjunc

Three Different Mode by the Sunlight Strength

Good harvest mode Hybrid mode Bad harvest mode

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DC-DC

DC-DC

DC-DC

DC-DC

Charger

L1

L2

Li

(mpv, npv)PV

Cap(mc, nc)

SW1

SW2

SW3

Vjunc

Good Harvest Mode

Enough sunlight to operate the loads

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DC-DC

DC-DC

DC-DC

DC-DC

Charger

L1

L2

Li

(mpv, npv)PV

Cap(mc, nc)

SW1

SW2

SW3

Vjunc = VMPP

Control the charging current

to keep MPP

Good Harvest Mode

Enough sunlight to operate the loads

16

DC-DC

DC-DC

DC-DC

DC-DC

Charger

L1

L2

Li

(mpv, npv)PV

Cap(mc, nc)

SW1

SW2

SW3

Vjunc = VMPP

Control the charging current

to keep MPP

Control the configuration

to minimize the total loss

Control the configuration

to minimize the total loss

Hybrid Mode

Not enough sunlight to operate the loads

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DC-DC

DC-DC

DC-DC

DC-DC

Charger

L1

L2

Li

(mpv, npv)PV

Cap(mc, nc)

SW1

SW2

SW3

Vjunc = VMPP

Control the output voltage

to keep MPP

Control the configuration

to minimize the total loss

Control the configuration

to minimize the total loss

Bad Harvest Mode

The power loss in the Conv_cap is more than the amount of generated power from the PV array

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DC-DC

DC-DC

DC-DC

DC-DC

Charger

L1

L2

Li

(mpv, npv)PV

Cap(mc, nc)

SW1

SW2

SW3

Vjunc = VCap

Control the configuration

to minimize the total loss

Control the configuration

to minimize the total loss

Control the charging current

to keep MPP

Two Phase Control

Very simple two phase control

19

Task scheduling

Task set

Configuration

Component set

Status of system

Estimate the power consumption

in components

1st phase

2nd phase

Pow

er lo

ss (m

W)

(1,6

,1,1

2)

0

30

60

90

Configuration (mc, nc,mpv, npv)

(2,3

,1,1

2)(3

,2,1

,12)

(6,1

,1,1

2)(1

,6,2

,6)

(2,3

,2,6

)(3

,2,2

,6)

(6,1

,2,6

)(1

,6,3

,4)

(2,3

,3,4

)(3

,2,3

,4)

(6,1

,3,4

)(1

,6,6

,2)

(2,3

,6,2

)(3

,2,6

,2)

(6,1

,6,2

)

120

One Case Result for the Array Configuration

Power loss in each converter by the configuration

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The lowest power loss

configuration

Experimental Results

21

0

30

60

90

120

single single single Untitled 1 Untitled 5 Untitled 9cam. mod.proc. loss_camloss_mod loss_ploss_char loss_conv

Ener

gy c

onsu

mpt

ion(

mJ)

sing

lem

ultip

lepr

opos

ed

Sun 100 60 20SoC 80 20 80 20 80 20

0%

20%

40%

60%

80%

single proposed multi single proposed multi

Total reduction Converter loss reduction

Experimental Results

Normalized reduction of the energy consumption Compared with the single core system

22

mul

tiple

prop

osed

Conclusion

We suggest Array configuration and I/O aware task scheduling

To reduce the voltage difference between input and output of the DC-DC converters Considering the multiple supply voltage loads

Three power path control method By the sunlight strength

Our proposed technique will reduce The system cost by increasing the energy efficiency of the system Without increasing a PV array or an energy storage

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Thank you

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Condition System Processor Etc. Loss Total (mJ) Saving

Case 1Single-core

Case 1 Multi-coreCase 1Proposed

Case 2Single-core

Case 2 Multi-coreCase 2Proposed

13.94 61.56 24.78 100.28 -7.91 61.56 24.32 93.79 6.47%7.91 61.56 18.48 87.95 12.30%

13.94 61.56 25.90 101.40 -7.91 61.56 20.22 89.69 11.55%7.91 61.56 14.00 83.47 17.68%

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