dynamic energy burst scaling for transiently powered systems · a gomez, et al. dynamic energy...

Andres Gomez1, Lukas Sigrist1, Michele Magno1,2, Luca Benini1,2, Lothar Thiele1

1D-ITET, ETH Zurich, Switzerland 2DEI, University of Bologna, Italy

Dynamic Energy Burst Scalingfor Transiently Powered Systems

A Gomez, et al. Dynamic Energy Burst Scaling

2

15.03.2016

http://www.electronicsweekly.com/new

s/research-news/device-rd/isscc-rice-

grain-solar-sensor-nodes-uses-cortex-

m3-2010-02/

http://www.seeedstudio.com/depot/Wireless-Sensor-Node-Solar-Kit-p-919.html

http://www.prnewswire.com/news-releases/cypress-

introduces-the-worlds-lowest-power-energy-harvesting-

power-management-ics-for-battery-free-wireless-sensor-

nodes-300130529.html

http://senseonics.com/product/the-sensor

Design Trends

Battery-based Batteryless


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15.03.2016

What are Transiently Powered Systems?

Energy-harvesting based systems with limited energy storage capacity

Why harvesting based?

• Low-cost

• Long-term

• Environmentally friendly

Why limit energy storage?

• Batteries (and supercaps) self-discharge, limited cycles

• Storage is expensive in terms of cost, form factor

• Storage requires energy surplus


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15.03.2016

Energy-driven embedded systems that:

• Have low cost

• Operate efficiently

• Guarantee program progress

Challenging Scenario:

• Low power harvesting conditions

Transient System Design Goals

eventTransient

Node

harvest energy

sense information

Transient Camera

actuate

Light Source


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15.03.2016

Directly Coupled Systems

Load

[Balsamo2015]

Advantage:

• Simplicity, no additional sources of leakage/losses

Disadvantage:

• Source must have the same operating power point as the load

Federating Energy:

[Hester2015]

“Checkpointing” for MCU’s:


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15.03.2016

Boost-Buck Converter Topology

Advantages:

• Source at Maximum Power Transfer Point

• Load has regulated voltage

Disadvantage:

• Introduces new sources of losses (converter inefficiencies, leakage, etc)

• No tracking of load’s optimal operating point

• Decoupled voltages:

𝑃𝑙𝑜𝑎𝑑

𝑉𝑙𝑜𝑎𝑑Load

𝑃𝑖𝑛

𝑉𝑖𝑛Source

BoostConverter

BuckConverter

[Naderiparizi2015]

StorageSource Load

Energy

Burst

WispCam:


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Our Proposed System

Energy Management Unit (EMU):

• Based on the Boost-Buck topology

• Optimized storage element

• Minimized wake-up time, cold-start energy

• Tracks load’s optimal operating point

• Feedback-based Dynamic Energy Burst Scaling


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15.03.2016

How can we design transient applications?

Basic Sense (S) and Process (P) Application

• Consists of two atomic tasks: S and P

• Known energies (ES, EP) and operating voltage (Vload)

• Two burst-based ways of executing this application: together, individually

Single Burst @ Vload

Estored

t

buffer ES + EP

Multiple Bursts @ Vload

Estored

t

buffer ES buffer EP

• Minimized buffer sizes

• Minimum start-up time

• Task-level optimization

• Minimized buffer sizes

• Minimum start-up time

• Task-level optimization

• Higher buffer sizes

• Longer timing constants

• Application-level optimization

• Higher buffer sizes

• Longer timing constants

• Application-level optimization


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Minimizing the Storage Element

Usable energy stored

between [𝑉𝑙𝑜𝑎𝑑 , 𝑉𝑚𝑎𝑥]

• For a single-capacitor system, 𝐶𝑚𝑖𝑛 ensures functionality

𝐶𝑚𝑖𝑛 =2 max(𝐸𝑡𝑎𝑠𝑘,𝑖)

𝜂𝑏𝑢𝑐𝑘 ∗ (𝑉𝑚𝑎𝑥2 − 𝑉𝑙𝑜𝑎𝑑

2 )

• 𝐶𝑚𝑖𝑛 also miminimizes cold-start energy and start-up time

𝑡𝑠𝑡𝑎𝑟𝑡−𝑢𝑝 = 𝑡 | 𝑉𝑐𝑎𝑝 𝑡 =2 0

𝑡𝐸𝑐𝑎𝑝′ 𝜏 𝑑𝜏

𝐶𝑐𝑎𝑝= 𝑉𝑙𝑜𝑎𝑑


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15.03.2016

Energy Burst Scaling

Basic Sense (S) and Process (P) Application:

• S: minimum voltage VS=3V (peripheral req.)

• P: minimum voltage VP=2V

Constant Bursts (WispCam): Dynamic Bursts (Proposed):

Estored

t

Vload=max(VS,VP)

S+P

Estored

t

Vload=VS Vload=VP

S P

To minimize load energy:

𝐸 = 𝑽𝒍𝒐𝒂𝒅 ∗ 𝐼𝑙𝑜𝑎𝑑 ∗ 𝑡 ⇒ 𝐸𝑚𝑖𝑛 = 𝑽𝒍𝒐𝒂𝒅,𝒎𝒊𝒏 ∗ 𝐼𝑙𝑜𝑎𝑑 ∗ 𝑡Dynamic Energy Burst Scaling:

+ Minimizes task energy

- Requires control mechanism!

Dynamic Energy Burst Scaling:

+ Minimizes task energy

- Requires control mechanism!


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15.03.2016

Dynamic Energy Burst Scaling

Feedback Loop:

• Minimizes task energy required to execute application

• Load configures EMU to provide Eburst @ Vload

• Task execution is triggered by EMU interrupt


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15.03.2016

Energy Burst-Based Flow

𝑃𝑙𝑜𝑎𝑑

𝑡

𝑉𝑐𝑎𝑝

𝑡

deep sleepactiv

e

activ

e

VSVP

deep sleep

EMU

interrupt

EMU

interrupt

Pload=60 nW Pload= ~4 mW

Estored (t) = ES Estored (t) = EP

Pload=60 nW Pload= ~3 mW


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15.03.2016

Modeling Transient Applications

task set

capacitance

EMU

Modelpower consumption

Pin(t)Vcap(t)

Eload / Eharvested

Pload(t)

Discrete-Time Simulation:

• Calculate voltage changes in capacitor (with non-ideal converters, leakage, etc)

• Verify understanding of EMU behavior

Inputs:

• Source’s power trace

• Etask, Vtask, Ptask

Output:

• Energy Efficiency

• Total Executions


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15.03.2016

Experimental Set-Up

Energy Management Unit

Control Circuit

HarvesterBQ25505

BuckTPS62740

Optimal Cap80 μF

ControlInterface

𝑷𝒊𝒏 𝑷𝒍𝒐𝒂𝒅

𝑽𝒍𝒐𝒂𝒅Load

MP3-25Solar Panel 𝑽𝒊𝒏

Load:

• Sensor: Centeye Stonyman (3V)

• Processor: MSP430 (2V)

Metrics:

• Energy harvested

• Energy consumed by the load

• Application execution rate

Control Signals

Energy Flow

PEMU=~10 uW


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15.03.2016

Constant Bursts - Characterization

Stage Voltage [V] Time [ms] Energy [μJ]

Boot 3.0 3.20 6.4

Sense 3.0 47.0 174.0

Process 3.0 50.6 220.8

Total 100.8 401.2

Estored

t

Vload=3V

S+P

ES+P=401.2 μJ

Pavg=3.98 mW


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15.03.2016

Dynamic Bursts - Characterization

1st Burst = Sense

Phase Voltage [V] Energy [μJ]

Boot 3.0 6.1

Sense 3.0 183.9

Total 190.0

2nd Burst = Process

Phase Voltage [V] Energy [μJ]

Boot 2.0 3.1

Process 2.0 143.7

Total 146.8

Constant Bursts: 401.2 μJ required per execution

Dynamic Bursts: 336.8 μJ required per execution

Constant Bursts: 401.2 μJ required per execution

Dynamic Bursts: 336.8 μJ required per execution

Estored

t

Vload=3V Vload=2V

S P

ES+P=(190 + 146.8) μJ = 336.8 μJ

Pavg,S+P= ~3.34 mW


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15.03.2016

Experimental Evaluation

EMU Testing Methodology:

• Constant Bursts Experiment

• Dynamic Bursts Experiment

• Run simulations with recorded input power traces

• Compare measured/simulated outputs

Harvesting Scenarios:

• Constant/variable input power

• Low power ranges: 0 – 400 μW


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15.03.2016

EMU System Efficiency (Eload/Eharvested)

Efficiency is bounded by converter efficiencies (~78 %)Efficiency is bounded by converter efficiencies (~78 %)

Dynamic Bursts Constant Bursts


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15.03.2016

Application Execution Rate

Dynamic Bursts: ~50 exec/min @ 400 uW

Constant Bursts: ~40 exec/min @ 400 uW

Dynamic Bursts: ~50 exec/min @ 400 uW

Constant Bursts: ~40 exec/min @ 400 uW

Dynamic Bursts Constant Bursts


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15.03.2016

Variable Input Power Experiment

Constant Bursts

Avg. Pin Metric Simulation Experiment

111.9 μWExec. Rate 9.87 min-1 9.93 min-1

ηsystem 67.76 % 68.01 %

Dynamic BurstsAvg. Pin Metric Simulation Experiment

92.3 μWExec. Rate 9.93 min-1 10.33 min-1

ηsystem 66.11 % 68.82 %

Experiment 1:

Experiment 2:

Camera worn around the lab for 15 mins. in various lighting conditions


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15.03.2016

Summary

Proposed Energy Management Unit (EMU):

• Large operating ranges (source/load decoupling)

• Minimized storage element, wake-up time, cold-start losses

• Optimized Power Point Tracking for source and load

• Feedback-based technique (Dynamic Energy Burst Scaling)

dynamic energy burst scaling for transiently powered systems · a gomez, et al. dynamic energy...

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