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REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes SenseApp 2017 Ulf Kulau, Daniel Brckelmann, Felix Bsching, Sebastian Schildt and Lars Wolf, 09.10.2017 Technische Universitt Braunschweig, IBR Introduction and

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REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes

SenseApp 2017

Ulf Kulau, Daniel Bräckelmann, Felix Büsching, Sebastian Schildt and Lars Wolf, 09.10.2017

Technische Universität Braunschweig, IBR

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Towards more adaptive WSNs WSNs in real environmental conditions

Various parameters (especially temperatures) affect the characteristics of WSNs

Dependability: Efficiency of transceivers, HW faults, ... Efficiency: Power dissipation, ... Energy budget: Energy Harvesting, Energy storage, ...

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 2

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Towards more adaptive WSNs Project goal: Robust but efficient WSNs by adapting operation parameters

Energy Budget Efficiency Dependability

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 3

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Towards more adaptive WSNs Project goal: Robust but efficient WSNs by adapting operation parameters

Energy Budget Efficiency Dependability Adaptive energy harvesting platform REAPer

Energy harvesting: → Varying energy budget Voltage scaling (undervolting): → Adaptive energy efficiency

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 3

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Towards more adaptive WSNs Project goal: Robust but efficient WSNs by adapting operation parameters

Energy Budget Efficiency Dependability Adaptive energy harvesting platform REAPer

Energy harvesting: → Varying energy budget Voltage scaling (undervolting): → Adaptive energy efficiency

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 3

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Undervolting in WSNs – Background Voltage Scaling increases energy efficiency significantly

Dynamic power dissipation of CMOS pdyn = CL · fcpu · V2

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 4

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Undervolting in WSNs – Background Voltage Scaling increases energy efficiency significantly

Dynamic power dissipation of CMOS pdyn = CL · fcpu · V2 DVS: Adapting fcpu to current workload and scale V(fcpu)

voltage voltage 100% 0% 100% 0%

Task 1 Task 2

T T

Task 1 Task 2

T T

DPM DVS

f cpu=8MHz f cpu=8MHz f cpu=4MHz f cpu=6MHz

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 4

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Undervolting in WSNs – Background Voltage Scaling increases energy efficiency significantly

Dynamic power dissipation of CMOS pdyn = CL · fcpu · V2 DVS: Adapting fcpu to current workload and scale V(fcpu) Undervolting: Violate specifications V(fcpu) → V(fcpu) − ∆V

voltage voltage 100% 0% 100% 0%

Task 1 Task 2

T T

Task 2

T T

Undervolting

f cpu=8MHz f cpu=8MHz f cpu=4MHz f cpu=6MHz

Task 1 Task 2 Task 1 Task 2

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 4

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Undervolting in WSNs – Background Legitimation to use undervolting

Threshold Voltage Vth of CMOS is temperature-dependent Vth(T) = Vth0 + α · (T − T0) MCUs cover a widespread temperature range with a fixed V(fcpu)

55°C
35°C
15°C

5°C 25°C 45°C 65°C 85°C 105°C 125°C Room Temperature

(19°C < T < 24°C)

→ MCUs must be able to run below V(fcpu) (under normal conditions)

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 4

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Is this a good idea?

Undervolting will lead to a higher unreliability: Operating devices outside their specification Calculation errors, losses, resets, failures may affect the application

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 5

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Is this a good idea?

Undervolting will lead to a higher unreliability: Operating devices outside their specification Calculation errors, losses, resets, failures may affect the application Our Perspective: WSNs need increased energy efficiency and offer fault tolerance (ideal) Fulfill WSN tasks even with limited energy budget!

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 5

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

IdealVolting – Adaptive undervolting scheme IdealVolting implementation on undervolting capable node INGA v1.6.1

Secondary MCU ATtiny84 Primary MCU ATmega1284p Transceiver AT86RF233 SPI I2C

1. Control loop to ascertain ideal voltage levels

→ Find most energy efficient but reliable operating point individually

2. Supervised-Learning approach

→ Collect and predict ideal operating points

Kulau et.al., IdealVolting – Reliable Undervolting on Wireless Sensor Nodes, ACM Transactions on Sensor Networks (TOSN), 2016

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 6

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Architecture of REAPer Integrate IdealVolting to energy harvesting and vice versa...

Buck-Converter Harvester BQ25570 GPIO Potentiometer I2C Energy Storage ADC Secondary MCU ATtiny84 Primary MCU ATmega1284p Sensor Node Energy-Harvester Settings Status Wiper-Pos. Voltage I2C 1.71 V ≤ V ≤ 2.44 V VSTOR

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 7

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Static Overhead Efficiency

Static current consumption of REAPer Quiescent current of the entire REAPer platform

Test conditions:

Energy Storage initially charged to VSTOR = 5V, no load at buck-converter

Mean (nA) Min (nA) Max (nA) Normal 567.23 ± 15.45 546.0 592.0 Normal + Buck 708.24 ± 13.93 672.0 742.0

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 8

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Static Overhead Efficiency

Static current consumption of REAPer Quiescent current of the entire REAPer platform

Test conditions:

Energy Storage initially charged to VSTOR = 5V, no load at buck-converter

Mean (nA) Min (nA) Max (nA) Normal 567.23 ± 15.45 546.0 592.0 Normal + Buck 708.24 ± 13.93 672.0 742.0

→ Reasonable overhead below 1 µA

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 8

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Static Overhead Efficiency

Charging characteristics Exemplary charging curve at Vin = 1000 mV input voltage (Energy storage: Cap 1 F)

t0→1: Cold start phase for VSTOR ≤ 1.8 V with integrated charge-pump t1→2: Boost-Converter and duty-cycled MPPT is active for VSTOR > 1.8 V

5 10 Iin[mA] Iin 100 200 300 400 500 600 700 800 1 2 3 4 5 t0 0 V t1 1.8 V t2 5.35 V t[s] VSTOR[V] VSTOR

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 9

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Static Overhead Efficiency

Efficiency of the charging Considering the energy that is stored by the capacitor (C = 1 F)

E = 1 2 · CV2 (1) Efficiency η can be derived by comparing stored Energy against input energy: η = E Eint0→t2 (2) Where Eint0→t2 is based on... the time of charge t0 → t2, the input current Iin and the input voltage Vin

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 10

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Static Overhead Efficiency

Efficiency of the charging Evaluation of the efficiency for different input Voltages 450 mV ≤ Vin ≤ 1000 mV

t0→1 t1→2 t0→2 Vin[mV] Ein[mWh] Ein[mWh] Eintotal[mWh] η (%) 450 1.42 5.62 7.04 53.69 550 0.88 5.47 6.35 59.53 700 0.79 5.20 5.99 63.11 850 0.73 5.06 5.79 65.28 1000 0.71 5.04 5.75 65.74

Higher input voltages lead to higher input power and higher efficiency

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 10

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Static Overhead Efficiency

Efficiency of the charging Evaluation of the efficiency for different input Voltages 450 mV ≤ Vin ≤ 1000 mV

t0→1 t1→2 t0→2 Vin[mV] Ein[mWh] Ein[mWh] Eintotal[mWh] η (%) 450 1.42 5.62 7.04 53.69 550 0.88 5.47 6.35 59.53 700 0.79 5.20 5.99 63.11 850 0.73 5.06 5.79 65.28 1000 0.71 5.04 5.75 65.74

Higher input voltages lead to higher input power and higher efficiency → Advice for How to connect your energy sources (serial vs. parallel)

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 10

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Voltage Meetering State of Charge and Current Drain Estimation

Software components (excluding IdealVolting) SW Implementation on both MCUs

Rudimentary functions on tiny secondary MCU More complex implementations on primary MCU

Current Drain Estimation State of Charge Estimation Voltage Metering GPIO-Control

Primary MCU

ATmega1284p

Secondary MCU

ATtiny84 I2C

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 11

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Voltage Meetering State of Charge and Current Drain Estimation

Voltage metering of the energy storage

Additional parts (OpAmps, voltage divider, ...) are inefficient

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 12

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Voltage Meetering State of Charge and Current Drain Estimation

Voltage metering of the energy storage

Additional parts (OpAmps, voltage divider, ...) are inefficient

Measurement of supply voltage Vcc = VSTOR via bandgap reference Vref

ADC = ADCmax · Vref VSTOR ⇔ VSTOR = ADCmax · Vref ADC (1)

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 12

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Voltage Meetering State of Charge and Current Drain Estimation

Voltage metering of the energy storage

Additional parts (OpAmps, voltage divider, ...) are inefficient

Measurement of supply voltage Vcc = VSTOR via bandgap reference Vref

ADC = ADCmax · Vref VSTOR ⇔ VSTOR = ADCmax · Vref ADC (1)

160 165 170 175 3.25 3.3 3.35 3.4 Multimeter ATtiny84 t[s] VSTOR[V]

Result: → Measurement error below 1.5 %

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 12

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Voltage Meetering State of Charge and Current Drain Estimation

Further utilization of voltage metering

Assumption: → Energy storage is a capacitor (normal case) → Exploit the linear dis-/charge behavior

20 40 60 80 2 3 4 5 t1 = 20 s 4.083 V t2 = 80 s 2.543 V t[s] VSTOR[V]

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 13

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Voltage Meetering State of Charge and Current Drain Estimation

Further utilization of voltage metering

Assumption: → Energy storage is a capacitor (normal case) → Exploit the linear dis-/charge behavior

State of charge

Relative state of charge is trivial SoC(t)[%] = VSTOR(t) − Vmin Vmax − Vmin (2)

20 40 60 80 2 3 4 5 t1 = 20 s 4.083 V t2 = 80 s 2.543 V t[s] VSTOR[V]

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 13

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Voltage Meetering State of Charge and Current Drain Estimation

Further utilization of voltage metering

Assumption: → Energy storage is a capacitor (normal case) → Exploit the linear dis-/charge behavior

Current drain estimation

Estimation of the average current consumption I State of charge Q (t) at two points in time I = ∆Q ∆t = Q (t2) − Q (t1) t2 − t1 (2)

20 40 60 80 2 3 4 5 t1 = 20 s 4.083 V t2 = 80 s 2.543 V t[s] VSTOR[V]

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 13

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Voltage Meetering State of Charge and Current Drain Estimation

Evaluation – Current drain estimation Result: Accuracy is suitable for a rough current drain estimation

−40 −20 20 40 Error [%] Error [%] 100 200 300 400 500 600 700 800 2 4 6 8 t[s] I[mA] Multimeter Estimation

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 14

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary Voltage Meetering State of Charge and Current Drain Estimation

Evaluation – Current drain estimation Result: Accuracy is suitable for a rough current drain estimation

−40 −20 20 40 Error [%] Error [%] 100 200 300 400 500 600 700 800 2 4 6 8 t[s] I[mA] Multimeter Estimation

→ Limitation: Harvesting must be deactivated during measurement

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 14

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

First tests and future perspective First test with different energy sources

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 15

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

First tests and future perspective First test with different energy sources

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 15

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

First tests and future perspective First test with different energy sources

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 15

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Summary

More adaptive WSNs Robust but energy efficient WSNs by adapting operational parameters (REAP)

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 16

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Summary

More adaptive WSNs Robust but energy efficient WSNs by adapting operational parameters (REAP) REAPer Integration of voltage scaling (undervolting) to energy harvesting Evaluation and characteristics of REAPer

Overhead, charging characteristics, Efficiency

Software implementation

Voltage metering → State of charge, Current drain estimation

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 16

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Summary

More adaptive WSNs Robust but energy efficient WSNs by adapting operational parameters (REAP) REAPer Integration of voltage scaling (undervolting) to energy harvesting Evaluation and characteristics of REAPer

Overhead, charging characteristics, Efficiency

Software implementation

Voltage metering → State of charge, Current drain estimation

Future perspective Focusing on smart farming applications (e.g. utilizing soil temperature)

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 16

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Introduction and Motivation Undervolting in WSNs REAPer HW Design REAPer Evaluation Software Implementation Field Test Summary

Summary

More adaptive WSNs Robust but energy efficient WSNs by adapting operational parameters (REAP) REAPer Integration of voltage scaling (undervolting) to energy harvesting Evaluation and characteristics of REAPer

Overhead, charging characteristics, Efficiency

Software implementation

Voltage metering → State of charge, Current drain estimation

Future perspective Focusing on smart farming applications (e.g. utilizing soil temperature) Thank you for your attention! Questions? Ulf Kulau kulau@ibr.cs.tu-bs.de

09.10.2017 Ulf Kulau REAPer Adaptive Micro-Source Energy-Harvester for Wireless Sensor Nodes Page 16