55:036 Embedded Systems and Systems Softwares-iihr64.iihr.uiowa.edu/MyWeb/Teaching/ece_55036_2013/... · 2013-05-07 · Some Power Considerations Embedded Systems and Software, 55:036

Some Power Considerations 1 Embedded Systems and Software, 55:036. The University of Iowa, 2013

Embedded Systems and Software

Some Power Considerations


Energy/Power Considerations

• Terms – Cell, Battery – Energy (Joule) – Power (J/s or Watt) – Ampere-hour (Ah) – Deep-cycle – MCU – Sleep Modes – ADC – Data rate (BPS)


An Embedded System


An Embedded System

Radio-Serial Interface

Serial Interface Ultrasonic Sensor


An Embedded System

Creek or Small River


An Embedded System

Attach Distance Sensor


An Embedded System

Wake up, Measure Distance to Surface


An Embedded System

Wake up, Measure Distance to Surface


An Embedded System

Relay Data Back to Servers on Internet

To UI Via Cell Modem


Simplified System Diagram


Electronics

Ultrasonic Sensor

Cell Modem (Note Green Dot)

“Electronics” Solar Charge Controller

Battery

Cell Antenna

GPS Antenna Serial-Radio

Interface Antenna

LED

Serial-Interface Connector


Electronics

LP3500 Development PCB

GPS SD Card External Memory

Glue Board

Cell Modem


Where Does The Power Go?

Typically contains it own MCU + firmware

May be part of microcontroller


Principles

fVCP 2α=

Power dissipated Capacitance Voltage

Switching frequency

Number of active gates

The following is a simple model for the power dissipated by a CMOS-based system

(turn off unused systems)

(lower clock)

(Lower voltage) (Use simpler uC)


Digital Switching Levels

Time

Reducing power consumption is the main reason for push towards lower voltage levels


Microcontroller Unit (MCU)

• Intel’s StrongARM, Atmel AVR, PIC, … • Several low power modes

– Active, idle, nap, shutdown, sleep modes Various MCU subsystems are turned off Different clock speeds

– For some MCUs, in deep sleep modes, the power consumption can almost be negligible

– Takes longer to wake from a deep sleep than just a nap – Wakeup time also takes power – Wakeup impacts processing


Radio

• Radio typically contains an embedded controller that provides many functions – Uses RSSI to adjust transmit power – Error detection and correction in hardware/firmware

• Several modes – Receive only, transmit + receive, idle, etc.

• In general, transmit requires most power • Carefully consider radio spec and modes • Mode change can consume a lot of power

– May be better to shutdown completely rather than go into idle mode


Sensors

Passive & low power (~mW and smaller) – Soil moisture, temperature, light, humidity

• Active & high power – Anemometers, disdrometers, cameras

• Many sensors are inherently analog, but some sensors have digital interfaces (provided by embedded controllers)

• Conditioning/wakeup times need to be considered • Analog-Digital Converters (ADC)

– Can be a major power consumer – More bits and high conversion rate requires more power – Don’t over-specify


Batteries

• Uses chemical reaction to provide electrical energy • Battery vs Cell • Batteries are often the most bulky part of a device • Capacity measured in Ampere-hours (Ah) or mAh • Note that the capacity does not consider voltage… • The capacity is the nominal number of hours it can

supply a given current. Normally specified at given load conditions and temperature. For example, 100 mA constant current discharge, 70% of open circuit voltage and 20 oC.

• Battery Chemistries: Alkaline, Lead-Acid, Li-ion, NiMH, etc. Each chemistry has its own characteristics


Constant Current Discharge Curve

0 200 400 600 800 1000 1200 1400

0.25

0.5

0.75

1

1.25

1.5Constant Current Discharge (100 mA)

Volta

ge (V

)

Discharge Time (min)

Duracell AA Energizer NH15-2500 Eveready Gold Kodak AA Panasonic HHR-160AAB RadioShack Enercell Rayovac Maximum Plus


Battery Capacity

• AA Size batteries – Few hundred mAh (Alkaline) to ~ 3,000 mAh (Li-ion)

• Factors – Previous number of discharges. Capacity decreases as the cycle

number increases. – Temperature. Overall, capacity decreases with decreasing

temperature, but over some temperature ranges, capacity my increases

– Load. Higher loads reduce apparent capacity


Battery Capacity

0 200 400 600 800 1000 1200

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 Rayovac Maximum Plus ( Alkaline, AA)

Volta

ge (V

)

0 200 400 600 800 1000 1200

50

100

150

200

250

300

350

400

Current (m

A)


70%

Capacity is Area


Capacity Dependence on Load

• Peukert’s Law is often used as a model:

• t is discharge time, I is discharge current, and n is Peukert’s number that depends on battery chemistry and battery history. Values for n range between 1.1 – 1.2. C is the capacity removed from the battery.

• Empirical formula • Dimensionally inconsistent, and often interpreted

and used incorrectly.

tIC n=


Capacity Dependence on Load

• Better formulation of Peukert’s law:

• Here C0 is the capacity measured at I0, and CI is the capacity at a load I.

• The state of charge (SOC) of a battery is defined as:

Where CI is the capacity at a load I

10

0

−

=

nI

II

CC

ICtI ×

−=1SOC


Example

• An NiMH cell has Peukert’ number n = 1.1 and has a capacity of 2,500 mAh measured at 50 mA and 20 oC. (a) What is the cell’s capacity at a load current of 300 mA? (b) What is the SOC after 2 hours at a load of 300 mA?

mAh 090,2836.0500,2

836.030050 11.11

0

0

=×==>

=

=

=

−−

I

nI

CII

CC

%7171.0090,2

230011SOC ==×

−=×

−=ICtI


Battery Capacity

50 100 150 200 250 300 350 400 4501500

2000

2500

3000

Load Current (mA)

Cap

acity

(mAh

)

Capacity vs Constant Load Current

EAEBEDEEEFEGEHEIEJEX

Linear fit (no Peukert)


Effects of Temperature

• Temperature affects rate of chemical reactions and influences battery capacity

• Lower temperatures => lower (apparent capacity) • Broadly speaking, temperature effects are

reversible – Some manufacturers specify “% Service vs.

Temperature” rather than “Capacity vs. Temperature” • At very high and very low temperatures damage

to battery occur • Temperatures affect self-discharge & shelf life


Effects of Temperature S

ervi

ce (%

)

Cap

acity

(%)

Temperature (oC) Storage Time (days)

Ser

vice

(%)

Cap

acity

(%)

Temperature (oC) Storage Time (years)


Modeling Temperature Effects

• Simple linear model for “%Service”

• Here S(T) is “% Service”, T0 is where Service = 100%, and α is the temperature coefficient of the “%Service”. For batteries that exhibit a plateau, this equation would apply to the relevant part of the plot.

• Simple linear model for self discharge

( ))(1100)( 0TTTS −+= α

( )TdCdTC β−= 1),( 0

Where C(T,d) is the capacity of the battery after d days of storage at temperature T, and β has units day-1 oC-1


0 200 400 600 800 1000 1200 1400

0.25

0.5

0.75

1

1.25

1.5Constant Current Discharge (100 mA)

Volta

ge (V

)


Duracell AA Energizer NH15-2500 Eveready Gold Kodak AA Panasonic HHR-160AAB RadioShack Enercell Rayovac Maximum Plus

Estimating Battery Capacity

May be possible to use curve to gauge battery state. Must be under load conditions.

May not be possible to use curve to gauge battery state.


Question: what type of diodes should D1, D2, be, and why?

Answer: Schottky diodes, because they have lower turn-on voltages than Si diodes. Thus, these diodes dissipate less power.

In normal operation, the 5 V linear regulator provides clean 5 V. D1 drops VD(on) . This is greater than (3 V + VD(on) ), so D2 is reverse-biased and open. Without main power, D2 is forward biased turn on, and powers the controller.

Power Supplies


Coin & Button Cells

• Capacities: 50–300 mAh • Designed for 3–200 µA or few mA pulsed load • Enough to power CMOS • Used for RAM backup • WSN: Power RTC • Li chemistry typical

– => single cell – => very long service


Power Supplies

Linear Three-Terminal Regulator

Quiescent current for LM78xx regulators are large, and can waste lots of energy not suitable for battery-operated equipment.


Power Supplies

MAX604 Linear Regulator

Quiescent current for MAX604 a few micro-amps, thus much more suitable for battery-operated equipment.


DC/DC Converter

Simple Buck Converter

Feedback provides regulation


Power Supplies

MAX724 DC/DC switching Regulator

Very wide input range, efficient conversion to output voltage

Note that switching regulators can step up voltages


Battery Models

• Motivation – Understand battery behavior – Simulate battery behaviors – Predict battery lifetime. Think about the battery-icon

on your cell phone • Physical models • Abstract models

– Electrical circuit models – Discrete time models – Stochastic models – Mixed models

• Empirical Models


Physical Battery Models

• Consists of differential equations that describe electrochemical processes in detail:

• Most accurate, but some can require up to 50 parameters, thus hard to configure model

• Not feasible to embed in an embedded system…

Write sets of differential equations that describe reaction and diffusion processes (at both electrodes). Incorporate effects of temperature on electrolyte, ion mobility, passive film forming at electrodes etc. Solve equations


Empirical Battery Models

• These models attempt to capture/describe behaviors of interest using simple equations

• Examples include Peukert’s law, self-discharge, etc:

• Simplest to configure and use • Generally not very accurate (Peuket’s law) • Model parameters are chemistry- and battery specific. For

example, Peukert’s number for an alkaline AA may be different for a same-brand AAA battery.

• However, a well designed empirical model based on good experimental data for a specific battery model can be very accurate, and simple to embed

10

0

−

=

nI

II

CC ( )TdCdTC β−= 1),( 0


Coulomb Counter/Battery Fuel Gauge ICs Fuel gauge; Q = I×t Coulombs = > Coulomb counter

Example 15 bit (1.6 µV / 78µA) measurement Unique ID DS2740

Li-Io

n ba

ttery


Smart Batteries

Transient Voltage Suppression


Smart Battery

Smartbattery monitor


Question 1. (10 points) Consider a battery-operated consumer electronics device that uses an embedded microcontroller that will accept a 2.7–5.5 V power. The device can also be powered from a power supply that plugs into a mains outlet. Consumers’ expectation is that the switchover between battery- and mains power is transparent. That is, the instant a user inserts the power supply connector, the device switches to the power supply, and the instant the user unplugs the device, it switched to battery power.

Draw a block diagram/schematic that shows how to implement this functionality using diode(s), linear regulator(s), battery, etc. Explain how the circuit works. Provide as much details as you can. For example, specify the type of diodes, indicate voltages on the diagram, include critical capacitors, and so on. You can assume that unregulated 9 V dc power is available.

Question 2. (10 points) List (1 point) and the briefly explain (1 point) five design considerations that one can employ to reduce the power consumption in an AVR-based embedded system.

Sample Exam Questions


Documents

55:036 Embedded Systems and Systems Softwares-iihr64.iihr.uiowa.edu/MyWeb/Teaching/ece_55036_2013/... · 2013-05-07 · Some Power Considerations Embedded Systems and Software, 55:036