Embedded Systems and Software. Some Power Considerations

Transcription

1 Embedded Systems and Software Some Power Considerations Slide-1

2 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) Slide-2

3 An Embedded System Slide-3

4 An Embedded System Radio-Serial Interface Serial Interface Ultrasonic Sensor Slide-4

5 An Embedded System Creek or Small River Slide-5

6 An Embedded System Attach Distance Sensor Slide-6

7 An Embedded System Wake up, Measure Distance to Surface Slide-7

8 An Embedded System Wake up, Measure Distance to Surface Slide-8

9 An Embedded System To UI Via Cell Modem Relay Data Back to Servers on Internet Slide-9

10 Simplified System Diagram Slide-10

11 Ultrasonic Sensor Electronics Cell Modem (Note Green Dot) Serial-Interface Connector Cell Antenna LED Serial-Radio Interface Antenna Electronics Battery Solar Charge Controller GPS Antenna Slide-11

12 Electronics LP3500 Development PCB Cell Modem Glue Board SD Card External Memory GPS Slide-12

13 Where Does The Power Go? Typically contains it own MCU + firmware May be part of microcontroller Slide-13

14 Principles The following is a simple model for the power dissipated by a CMOS-based system Power dissipated (Use simpler uc) Capacitance (Lower voltage) Voltage P CV 2 f Number of active gates (turn off unused systems) Switching frequency (lower clock) Slide-14

15 Digital Switching Levels Time Reducing power consumption is the main reason for push towards lower voltage levels Slide-15

16 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 Slide-16

17 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 Slide-17

18 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 Slide-18

19 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 o C. Battery Chemistries: Alkaline, Lead-Acid, Li-ion, NiMH, etc. Each chemistry has its own characteristics Slide-19

20 Constant Current Discharge Curve 1.5 Constant Current Discharge (100 m A) Voltage (V) Duracell AA Energizer NH Eveready Gold Kodak AA Panasonic HHR-160AAB RadioShack Enercell Rayovac Maximum Plus Discharge Tim e (min) Slide-20

21 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 Slide-21

22 Current (ma) Battery Capacity Rayovac Maximum Plus ( Alkaline, AA) Voltage (V) % Capacity is Area Discharge Time (min) Slide-22

23 Capacity Dependence on Load Peukert s Law is often used as a model: C I n t 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 C is the capacity removed from the battery. Empirical formula Dimensionally inconsistent, and often interpreted and used incorrectly. Slide-23

24 Capacity Dependence on Load Better formulation of Peukert s law: Here C 0 is the capacity measured at I 0, and C I is the capacity at a load I. The state of charge (SOC) of a battery is defined as: I t SOC 1 n1 Where C I is the capacity at a load I C C I 0 I I 0 C I Slide-24

25 Example An NiMH cell has Peukert number n = 1.1 and has a capacity of 2,500 mah measured at 50 ma and 20 o C. (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? C C I 0 C I I I 0 n , , mah SOC 1 I t C I , % Slide-25

26 Battery Capacity Capacity vs Constant Load Current 3000 Linear fit (no Peukert) Capacity (mah) 2500 EA EB ED EE EF 2000 EG EH EI EJ EX Load Current (ma) Slide-26

27 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 Slide-27

28 Service (%) Capacity (%) Service (%) Capacity (%) Effects of Temperature Temperature ( o C) Storage Time (days) Temperature ( o C) Storage Time (years) Slide-28

29 Modeling Temperature Effects Simple linear model for %Service S T) 100 1( T T ( 0 Here S(T) is % Service, T 0 is where Service = 100%, and 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 C( T, d) C0 1 Td Where C(T,d) is the capacity of the battery after d days of storage at temperature T, and has units day -1 o C -1 Slide-29

30 Estimating Battery Capacity Constant Current Discharge (100 m A) May be possible to use curve to gauge battery state. Must be under load conditions. 1 Voltage (V) Duracell AA Energizer NH Eveready Gold Kodak AA Panasonic HHR-160AAB RadioShack Enercell Rayovac Maximum Plus Discharge Tim e (min) May not be possible to use curve to gauge battery state. Slide-30

31 Power Supplies In normal operation, the 5 V linear regulator provides clean 5 V. D 1 drops V D(on). This is greater than (3 V + V D(on) ), so D 2 is reverse-biased and open. Without main power, D 2 is forward biased turn on, and powers the controller. 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. Slide-31

32 Coin & Button Cells Capacities: mah Designed for A or few ma pulsed load Enough to power CMOS Used for RAM backup WSN: Power RTC Li chemistry typical => single cell => very long service Slide-32

33 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. Slide-33

34 Power Supplies MAX604 Linear Regulator Quiescent current for MAX604 a few micro-amps, thus much more suitable for battery-operated equipment. Slide-34

35 DC/DC Converter Simple Buck Converter Feedback provides regulation Slide-35

36 Power Supplies MAX724 DC/DC switching Regulator Very wide input range, efficient conversion to output voltage Note that switching regulators can step up voltages Slide-36

37 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 Slide-37

38 Physical Battery Models Consists of differential equations that describe electrochemical processes in detail: 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 Most accurate, but some can require up to 50 parameters, thus hard to configure model Not feasible to embed in an embedded system Slide-38

39 Empirical Battery Models These models attempt to capture/describe behaviors of interest using simple equations Examples include Peukert s law, self-discharge, etc: C n1 I I 0 C( T, d) C 1 Td 0 C 0 I 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 Slide-39

40 Li-Ion battery Coulomb Counter/Battery Fuel Gauge ICs Fuel gauge; Q = I t Coulombs = > Coulomb counter Example DS bit (1.6 V / 78A) measurement Unique ID Slide-40

41 Smart Batteries Transient Voltage Suppression Slide-41

42 Smart Battery Smart battery monitor Slide-42

43 Sample Exam Questions Question 1. (10 points) Consider a battery-operated consumer electronics device that uses an embedded microcontroller that will accept a 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 AVRbased embedded system. Slide-43

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