Low Carbon Footprint Electric Lawn Mower

Final Report

Kraig Kamp

David Sharpe

Jamin Williams

Advisors:

Dr. Huggins

Mr. Gutschlag

May 16, 2008

2

Abstract

Environmental air pollution and carbon emissions are becoming significant

problems. A contributor to this pollution is the use of gasoline-powered lawn mowers.

Our overall project goal is to design a battery powered lawn mower and photovoltaic

charging system that will diminish emissions. The project consists of two separate

systems: a battery-powered lawn mower and a photovoltaic system to charge the battery.

The mower will use a microcontroller to control the speed of the cutting blade and

display the charge status of the battery. The charger will use the UC3909 battery

charging chip to control the charging algorithm for the battery. The system will be

designed to be competitive in function and cost versus benefits with gasoline powered

lawn mowers.

3

Table of Contents

I. Introduction 4

II. Functional Description and Block Diagrams 4-7

a. Mower Functional Description 4

b. Charger Functional Description 5

III. Functional Specifications and System Requirements 8-10

a. Mower Specifications 8

b. Charger Specifications 10

IV. Design and Analysis 11-29

a. Battery 11

b. DC Motor Modeling 12

c. Snubber Circuit 15

d. Heat Sink 16

e. PSPICE Simulation 18

f. Mower System Circuit 20

g. Solar Panel Calculations 21

h. Charging Algorithm 22

i. Buck Converter 24

j. UC3909 Equations 26

k. Measuring State of Charge 28

l. Measuring Battery Voltage 28

m. Determining Throttle setting 29

V. Implementation and Results 30

VI. Recommendations for Future Work 31

VII. Applicable Standards and Patents 33

VIII. Equipment List 36

IX. Bill of Materials 36

X. References 37

Appendix A: UC3909 Design Equations 38-44

Appendix B: Charger Circuit Components 45-47

Appendix C: Software Flowcharts 48-55

4

I. Introduction

This report describes the design, implementation and testing of the Low Carbon

Footprint Electric Lawn Mower. A top down design approach is followed with

functionality first described at the system level including detailed block diagram is

developed. Next the design equations are discussed to illustrate more specifically each

part of the system, followed by simulations, and final system testing. The report

concludes with a parts list, references and final conclusions.

II. Functional Description and Block Diagrams

The low carbon footprint electric lawn mower consists of two separate systems: a

mower block and a charger block, which are shown in figs. 1 and 2. The lawn mower is

powered using batteries, and the charger uses a photovoltaic array to recharge the battery.

A. Mower Functional Description

The main components of the mower, as seen in Fig. 1, are a dc motor, a battery, a

controller, and a user interface. The user interface consists of a power switch, a throttle

to control the speed of the mower blade, and a safety switch to start and stop the motor.

The user interface also consists of a display. The lawn mower uses a 24V DC motor that

is powered by two 12V lead-acid batteries. The motor shaft speed is controlled by a

microcontroller, which accepts inputs from the user. A PWM signal from the

microcontroller is applied to the gate driver which then drives power MOSFETS and

controls the average voltage applied to the motor. Detail specifications for the motor and

batteries are given in Section III.

Figure 2 shows the overall software flowchart for the lawn mower system.

Additional flowcharts for each software module are listed in Appendix C. The main

purpose of the software is to control motor speed and to turn the motor on or off. To

control the motor speed, the software converts a voltage measurement from a

potentiometer into a PWM signal. This signal is sent to the motor driver circuitry to

change the speed of the motor. The user must engage the safety switch in order to turn

the motor on. If the switch is disengaged, the software turns the motor off.

The software also handles other tasks. It determines the state-of-charge (SOC) of

the battery by utilizing a method known as current counting and uses the SOC to protect

the battery from being over-discharged. The software prevents the system from drawing

too much current by measuring current via a current shunt. An LCD display is controlled

by the software as well. The display provides information about the system to the user

such as SOC and terminal voltage of the battery, the throttle setting, and the current that

is flowing through the motor.

5

B. Charger Functional Description

The charger system, shown in fig. 3, consists of a solar panel which supplies

electric power to the charger circuitry which then charges the batteries. The system is

designed so that the solar charger can fully charge two completely discharged batteries in

at least 5 days. The charger circuit is controlled by the UC3909 charge controller chip.

This chip monitors battery voltage and charge current. It varies both voltage and current

according to the state of charge of the battery based on the standard charge algorithm

sequence for lead acid batteries. This allows for trickle charge, bulk charge, over charge,

and float charge as needed to maximize battery life.

Fig. 1 – Mower Block Diagram

6

Fig.2 Overall Software Flowchart

7

Solar Panel

Buck Converter

IRF640

Gate Driver

TC4424 15VPWM

Charger Controller

UC3909 5VPWM

Input Voltage Regulation

LM7815

15 VDC

SolarEnergy

Batteries

DCM0035Charged in Parallel

SolarEnergy

Voltage/CurrentFeedback

DC V/I(Higher Current)

Sunlight

Fig.3 Solar Charger Block Diagram

8

III. Functional Specifications and Requirements

Section II presented a qualitative discussion of general functionality of the mower

and solar charger systems. In this section, the various subsystems are described in more

detailed including quantitative specifications and functional requirements.

A. Mower Specifications

The lawnmower is a push-type mower with an electric motor rotating an 18 inch

blade to remove 1 ½ to 2 in. off the height of average density grass at a walking speed of

approximately 2.66 ft/s. The mower weighs no more than 90 lbs. The motor is powered

by batteries with enough capacity to mow a 10,000 sq. ft lawn in one hour. The mower

has a power button to power up the controls on the handle and a separate start button to

start the electric motor. The circuitry includes over current protection along with a safety

switch that must be held down in order to keep the motor running. The batteries that

power the mower can be removed from the mower deck but is not necessary while being

recharged.

Battery: The battery power needed for our application is provided by two 12 volt

batteries connected in a series to make 24 volts. The capacity of each battery is 35 Amp-

Hours (AH) so that the mower has enough power to mow a 10,000 sq. ft. yard in one

hour. The chemistry make-up of the battery is deep discharge sealed lead acid with a

combined weight of approximately 50 lbs.

DC Voltage Regulators: The DC voltage regulators convert the variable battery voltage

(12-24V) to either 5V or 15V. The 5V regulator is used to power the microprocessor

and display and the 15V voltage regulator is used with the gate driver.

Controller: The controller is used to start and stop the motor, control the speed of the

motor, and control the display. The controller utilizes open-loop methods with a throttle

control so the user can set the speed of the motor shaft. The signal that is output to the

MOSFET’s is a PWM signal with a frequency of 4 kHz. The controller also monitors the

current draw of the motor for over-current protection. The maximum continuous current

is 40A for 5 seconds.

User Inputs: The user inputs consist of a power button to turn the controller on/off, a

start button to start the mower. This button also acts as a safety switch and must be held

down in order to keep the blade rotating. A throttle control to vary the speed of the

mower blade is also an input.

9

Display: The Optrex 24X2 LCD is the operating display, which is shown in Fig. 4. The

display has a battery symbol indicating the charge left on the battery and a speed bar

graphic to show the user the relative RPM at which the blade is rotating. In addition, the

display also shows the terminal voltage of the batteries and the current flowing through

the motor.

Fig. 4 LCD Operating Display

Power MOSFETS: The MOSFETS used to drive the motor are two HEXFET IRF044N

power MOSFETs wired together in a parallel configuration. The gate of the MOSFET

receives a PWM input signal at a frequency of 4 kHz. The MOSFETs are protected with

a snubber circuit that keeps voltages under the dsV rating of 55V.

Heat Sinks: Two Wakefield 657-15AEPN heat sinks are attached to the MOSFETs and

one Wakefield 287-1ABE heat sink is attached to the free wheeling diode in the snubber

circuit. These heat sinks maintain safe thermal operating temperatures for these

components.

Gate driver chip: The TC4424 takes a 0-5 Volt signal from the microcontroller and

outputs a 0-15 Volt PWM signal to drive the gate of the MOSFET at a switching rate of 4

kHz.

Motor: The motor rotating the blade is the Tecumseh 90000A permanent magnet

reversible motor with an input voltage of 24 VDC and generates 1.54 HP at 3200 RPM.

These specifications provide the power to rotate an 18 inch blade at a sufficient velocity

to cut 1 ½ to 2 inches off the top of average density grass.

Over current Protection: A 40 amp circuit breaker is used for over current protection on

the battery for testing and a 30 amp inline fuse is placed between the positive terminal of

the battery and the positive terminal of the Tecumseh 90000A DC motor.

Disconnect Switch: A disconnect switch is essential in the circuitry. In the case that a

MOSFET fails while mowing and acts as a short circuit, the motor still runs and cannot

be turned off by normal procedures. This manual disconnect switch is mounted by the

handle and breaks the circuit so the blade stops spinning.

10

B. Charger Specifications

The charger uses energy collected by a solar panel and directly transfers the

energy to the battery. The system charges a maximum of two 12 Volt batteries at one

time, wired in a parallel configuration. The charger needs 5 days at 530 kJ/day of solar

energy to charge the two batteries to full capacity. The charger utilizes the Texas

Instruments UC3909 lead-acid battery charger chip to control the charging process.

15V Voltage Regulator: The DC regulator for charger electronics regulates the DC

power input from the solar panel. This regulator powers the UC3909 and the buck

converter gate drive chip

Gate driver chip: The Charger system utilizes the same gate drive chip as mentioned in

the Mower specifications because it is a dual input chip. The gate driver takes the PWM

input from the UC3909 and converts it to 15V for the gate of the buck converter

MOSFET.

Solar Power: The solar power is collected with a BP350 50W panel with an open-circuit

voltage of 21.8 V and a short circuit current of 3.17 A. The panel can collect at least 530

kJ of energy per day to charge the two fully discharge 12V batteries to maximum

capacity in 5 days.

Charger Chip: The Solar Charger System is controlled by the UC3909 Switch mode

lead acid battery charger chip. This chip is used in conjunction with the buck converter

in the charger circuit. This method and the charge algorithm is further discussed in

section IV, part G.

Buck Converter: Because the charger controller chip uses a PWM signal to vary

voltage and current applied to the batteries, a buck converter is necessary to ensure that

voltage and current is constant even though the input is switching. This is further

discussed in Section IV, part H.

Batteries: The battery block consists of two 12V, 35 AH batteries, as discussed in the

lawnmower subsystem breakdown but is wired in parallel for charging.

11

IV. Design and Analysis

Section III talked about the detailed subsystems of the mower and charger. This next

section describes the design and analysis of several subsystems and how they were

implemented into the project.

A. Battery

To meet the specification of mowing a 10,000 square foot yard in 1 hour, the

batteries must be able to power the motor at full speed for the entire time. To size the

batteries appropriately, tests were carried out. This testing revealed that the average

running current of the mower system was approximately 18A. According to Fig. 5, at a

discharge rate of about 20A, a 35Ahr battery will last about an hour. After that, the

terminal voltage drops off abruptly, indicating a fully discharged battery.

It is important to note that because the mower uses a 24V DC motor, it is

necessary to use two 12V batteries in series, to obtain this voltage. Because the batteries

are in series, the current through them will be the same, thus both batteries must have the

35Ah (18Ah @ 20A rate) capacity rating to provide enough energy.

Fig. 5 – 12V, 35Ahr Lead Acid Battery Discharge Characteristics

12

B. DC Motor Modeling

In order to draw a schematic of the power electronics and the Tecumseh 90000A

in PSPICE, certain motor parameters must be determined. These parameters are the

resistance in the windings, the motor torque constant, the viscous and static friction

coefficients, and the mass moment of inertia. These mechanical parameters can be

simulated as electrical circuits in PSPICE as shown in Fig. 8.

The mass moment of inertia is modeled as an inductance in the model and the

friction coefficients are modeled as a resistance. Torque can be modeled as a current-

dependent voltage source. It is proportional to the current flowing through the motor.

The following are steps taken to measure these parameters:

1. Measure aR which represents the resistance in the windings

To do this, a voltage ( sV ) is applied that is low enough such that the shaft of the motor

does not spin, so as not to introduce the back EMF Voltage created by the motor. A

current probe is used to measure the current and then aR is determined from Ohm’s law.

Fig. 6 Schematic used to find aR

Eqn. 1

2. Find the motor constant EK = tK

The input voltage in Fig. 6 is set to ½ of the rate input voltage of the motor ( sV =12 V)

and aI is measured, then the speed of the motor shaft in radians/second is also measured.

eK = tK =s

aas RIV

ω

− Eqn. 2

a

sa

I

VR =

13

3. Find the static friction coefficient, ..FST and the viscous friction coefficient b.

sV is first set to 8 volts and then 16 volts and sω and aI are measured at each voltage.

Then, the following equation is used to find both unknowns.

Eqn. 3

4. Find the mass moment of inertia, J.

This is accomplished with a coast down test by driving the motor at 24 volts. Once the

shaft is up to speed the power is shut off and the motor voltage decay is captured with the

oscilloscope:

Fig. 7 Motor Coast curve averaged in MATLAB

Voltage is on the y-axis and time is represented on the x-axis. Using this plot‘t’ is

found ,which is the time the motor took to coast down from 24 volts to 0 volts. By the

graph above t=.847 seconds. Using the next equations, the final parameter, J is

determined.

Eqn. 4a

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1-5

0

5

10

15

20

25

0.... =−−=−− sFSaTsFSdeveloped bTIKbTT ωω

τωt

FS

t

eb

T

K

Vt

−

+= ..)(

14

Setting 0)( =tω , τ can be found which will be used to find J. τ*bJ = Eqn. 4b

(Note: this can be checked at any speed, find V at any speed and set )(tω = to that

particular speed and the right side of the equation should approximately equal ).(tω )

Using the four steps above the following values were measured and used to implement in

the PSICE model shown in Fig. 8.

HL

mkgLJ

rad

mNRb

mNT

A

mNKK

RR

M

J

B

FS

et

Ma

µ153

*000912.0

sec/

*000535.0

*272504.0

*068723.0

0825.

..

=

==

==

=

==

Ω==

Here, ML is inductance of the motor terminals measured with an LRC meter.

Fig. 8 PSPICE DC Motor Model (www.ecircuitcenter.com)

15

C. Snubber Circuit

As stated earlier, a snubber circuit is required to protect the MOSFETs from being

damaged when large currents could flow into the drain. This is a problem because the

maximum voltage from the drain to source cannot exceed 55V for the MOSFETs. Diode

1 on the left side of the circuit turns on and lets the current flow through it when the

MOSFET switches off. The circuitry on the right side of the circuit essentially gives

Diode1 time to turn on all the way and accommodate the high currents. Below is a

schematic of the snubber circuit

D1

D2

To positve terminal of the motor

To Drain on the MOSFET

Cs

Rs

To negative terminal of the motor

To Source on the MOSFET

Fig 9 Snubber Circuit

In order to obtain values for sR and sC the following constraints need to be taken

into consideration. First, sR should be small enough so that the largest current that could

be flowing through the motor multiplied by sR will not exceed the dsV rating of 55 volts.

Therefore, approximately 400 amps is the greatest amount of current flowing though the

motor according to simulations so,

V=I*R

55V = 400*R Eqn. 5

R=.135 ohms.

16

In addition, Cs is determined using Eqn. 6 as follows.

off

fDo

sV

tIC

*12

*= Eqn. 6

Where DoI is the max current of 400 amps, ft is the switching frequency of the

MOSFETs (4 kHz), and

offV = onDs VV 1+ or 24 V+.4512 V=24.4512V. Eqn. 7

(Note: onDV 1 is the voltage drop across the diode)

D. Heat Sink

While making initial tests in the motor driver circuit it was noticed that the

IRFP044N power MOSFETS were becoming extremely hot. Looking into the problem

these calculations were made to find out the junction temperature of the MOSFETS.

Using the IRFP044N data sheets the following data was obtained.

W

CR

W

CR

W

CR

AI

R

VV

JA

CS

JC

D

onDS

DSS

°=

°=

°=

=

Ω=

=

40

24.0

3.1

55

020.0

55

)(

θ

ϑ

θ

Also, datasheet supplies a Normalized On-Resistance Chart at a junction

temperature of 100 C° which yields Ω= 5.1)(onDSR . This is normalized so this value needs

to be multiplied by the )(onDSR of 0.020Ω to get .03Ω .

Next, the power dissipation needs to be calculated, and the average drain current

must be known. The mower system will have a worst case scenario of 40 amps flowing

through the drain at normal operating procedures and since there are 2 MOSFETS in

parallel the current will be evenly distributed through the both of them. So the drain

current is 20 Amps, therefore:

WRIP onDSDD 123.0*20* 2

)(

2 === Eqn. 8

17

Now the junction temperature is:

CTRPT AJADJ °=+=+= 7.5177.3740*12θ Eqn. 9

Note: AT was chosen by the design team to be 37.7 C° .

517.7 C° is not an acceptable temperature for the IRFP044N power MOSFET, so a heat

sink is needed. Using a heat sink:

ATDJ TRPT += θ where SACSJCT RRRR θθθθ ++= Eqn. 10

SARθ can be found by using the natural convection characteristic chart of a specified heat

sink found on the data sheets. The heat sink chosen for this application is the Wakefield

657-15ABEP and at a power dissipation of 12 W the heat sink temperature rise above

ambient is about 58 C° which yields:

W

C

W

C

P

TR

D

SA

SA

°=

°=

∆= 83.4

12

58θ Eqn. 11

W

CR T

°=++= 37.683.424.03.1θ Eqn. 12

So now the new junction temperature can be calculated

CTJ °=+= 14.1147.3737.6*12 . Eqn. 13

114.14 C° is within the 175 C° maximum operating junction temperature of the

IRFP044N power MOSFET.

18

E. PSPICE Simulation

Having determined the electrical and mechanical properties of the motor as well

as the components of the snubber circuit, a mower simulation circuit can be developed.

The following circuit diagram is drawn in PSPICE to simulate the basic operating

conditions of the mower motor control system.

Fig. 10 Motor Model, Snubber Circuit, and MOSFETs Simulation Circuit

Source

FET2

IRFP044N

+

Motor Electrical Model

-

Motor Mechanical Model

C1

8u

R1

.14

D3

MUR405

I

V-

V+

I

I

D4

MUR405

FET1

IRFP044N

Motor

153uH

1

2

LJ

912uH

1

2

RB

.000535

+-

EMF

H1

+-

Torque

H2

Ra

.0825

PWM

TD = 0

TF = 10nPW = .0005PER = .001

V1 = 0

TR = 10n

V2 = 15

OPEN

50Meg

0

OPEN2

50Meg

V2

24Vdc Drain

19

Fig. 11 Simulation Results of Circuit in Fig. 10

The above simulation done in PSPICE shows the voltage and current curves with

respect to the motor shaft speed which is the curve in blue. As seen above the teal curve

represents the current flowing through the motor. At initial start up the inrush current is

around 400 amps and as the shaft approaches top speed the current decreases to about 3

amps. The green curve shows the current through the freewheeling diode 1 in the

snubber circuit. In this particular simulation the current is measured through one of the

MOSFETS instead of measuring it through the two of them so the simulated current is

double of the motor current. The magenta curve shows the voltage with respect to speed

and as shown it stays at a constant 24V. Note that the simulation does not take into

consideration all of the motor losses, so the simulation shows the motor pulling less

current, but the simulation shaft speed was almost the same as the experimental.

20

F. Mower System Circuit

After simulations and software programming the mower system circuit was

implemented. The system schematic is shown in Fig. 12 below.

Vcont3

R/W5

7D0

8D1

RS

E

D7

D6

4

6

14

13

VCC

2

D29

D512

D310

D411

GND

1

LCD

PC61

PD02

3PD1

4PD2

PC5

PC4

PC3

PC2

28

27

26

25

PD35

PD46

7VCC

8GND

PC1

PC0

GND

AREF

24

23

22

21

PB69

AVCC20

PD511

PD612

13PD7

14PB0

PB4

PB3

PB2

PB1

18

17

16

15

PB710

PB519

Atmega168

SafetySwitch

M

.005Ω 5W

10kΩ

1kΩ

217Ω

1kΩ

NC1

In A2

3GND

4In B

NC

Out A

VCC

Out B

5

6

7

8

7815

I G O

220u/50V

.33u

.33u

7805

I G O

IRFP044NIRFP044N

STPS20120D

STPS20120D

.1Ω

.68u

.68u

From UC3909Stat 0

From UC3909Stat 1

12V 12V

24VBatteries

Fig. 12 Mower System Circuit Diagram

The mower circuit above contains all of the circuitry used to power and control

the mower system. The controller (Atmega 168) software flowcharts are further

discussed in Appendix C. The circuit also references the UC3909, which is discussed in

part I of this section.

21

G. Solar Panel Calculations

The charger system is powered by a photovoltaic panel. This component is the

most expensive part of the entire project, so the smallest size that can charge the batteries

in 5 days must be used. The solar panel must be able to supply enough voltage and

current to charge the two 12V batteries, as well as collect enough energy in 5 days to do

this. A 50W solar panel, the BP350, was the most appropriate solar panel to meet the

requirements. It is important to note that in the solar charger system, the batteries are

charged in parallel, so that they can be charged at 12V, because to charge at 24V, it

would require a much larger solar panel. The solar panel is selected based on the

minimum solar energy the U.S receives in places that can still grow grass. According to

Fig. 13 from the National Renewable Energy Laboratory website, the upper parts of the

country receive the least amount of solar energy, and therefore this number should be

used when calculating the amount of solar energy the panel can collect.

Fig. 13 Minimum Daily Solar Radiation Per Month (NREL.gov)

The efficiency of the BP350 is 10%, which means 10% of the solar radiation

energy incident on the panel collected is converted to electric energy. Taking into

account this efficiency combined with the panel area and minimum radiation of 3.5

KWh/m2/day yields the energy collected per day by the panel.

22

( ) dayMJhr

s

W

sJ

KW

W

daym

KWh

ft

mft /4682.0%10

1

3600/1

1

1000

/

5.3

1

092903.4

22

22 =

Eqn. 14

Another important note is that the solar energy the US receives varies monthly

based on the seasons. So given all the above information, a charge time per month chart

was developed.

Table 1 Monthly Battery Charge Times

Month KW-Hrs/ m2

/day Solar Energy Emitted

KJ / day of Solar Energy Collected

Actual days to charge 2 - 35AH

batteries

January 2.0 353 8.6

February 3.0 530 5.7

March 4.0 706 4.3

April 4.0 706 4.3

May 5.0 883 3.4

June 5.0 883 3.4

July 5.0 883 3.4

August 5.0 883 3.4

September 4.0 706 4.3

October 3.0 530 5.7

November 2.0 353 8.6

December 1.0 177 17.1

From the data presented in table 1, it is evident that the 50W solar panel works for

this project. During the grass cutting months of March through September, the batteries

can be charged in under 5 days. The winter months have a much higher charge time

because less the US receives less solar energy. This is acceptable because the charger

system only needs to keep a float charge on the batteries while they are not used during

the winter.

H. Charging Algorithm

Now that the solar panel has been selected, it is interfaced with the charger circuit.

But first, some of the basics of charging a lead acid battery must be discussed. Lead acid

batteries are charged in different stages because of the nature of the battery chemistry.

The charging stages can be seen on the following figure taken from the technical paper

by Laszlo Balogh.

23

| Trickle | Bulk | Over | Float |

Fig. 14 Charge Algorithm Voltage and Current Characteristics

1. Trickle Charge

This is the first stage of charging, where the battery is completely discharged.

Here, battery current kept low and constant in order to bring the terminal voltage high

enough to start the next stage of charging.

2. Bulk Charge

This is the high current stage where most of the battery’s charge is returned. Here

battery current is kept constant while terminal voltage gradually increases further until it

reaches the over charge cut off voltage.

3. Over-Charge

This stage is constant voltage stage where the remaining charge is returned to the

battery. The voltage is held a couple volts higher than the rated operating voltage, but

24

only for a short period of time. Again, the charge algorithm is based on the internal

chemistry characteristics of lead acid batteries.

4. Float Charge

This is the final stage of the battery charge algorithm. Once the charger reaches

this stage, the battery is fully charged. This is a constant voltage stage just slightly over

the operating terminal voltage. This stage just maintains the battery so it is ready for use

by charging at the same rate the battery naturally self-discharges.

I. Buck Converter

The charger circuit is controlled by the UC3909 Switchmode lead acid battery

charger chip. The circuit consists of the controller, voltage dividers for voltage and

current, input voltage regulation, and a buck converter to supply constant current and

voltage to the batteries. The controller controls the current and voltage via PWM signal

sent to the buck converter’s MOSFET. The basic concept of a buck convert can be seen

in the following circuit (wikipedia.org):

Fig. 15 Basic Buck Converter Circuit

The red parts of the circuit represent current flow. The switching is done by a

MOSFET that is connected directly to the solar panel. When the switch is on, the source

both supplies power to the load as well as the inductor and capacitor. When the switch is

25

off, the diode conducts to complete the circuit, and the energy stored in the inductor and

capacitor is then supplied to the source. This effectively provides constant current and

voltage even though the source is switching as seen in the next figure (wikipedia.org).

Fig. 16 Buck Converter Characteristics

The figure above illustrates the constant current and voltage. The UC3909 can

control the current and voltage by varying the duty cycle of the switching waveform. A

higher duty cycle means the MOSFET is on longer, thus the average current and voltage

are higher, and the converse is true for a lower duty cycle.

26

J. UC3909 Circuit Equations

Now that the basics of the buck converter operation have been discussed, it is

necessary show how the UC3909 charger circuit works. The controller measures the

battery terminal voltage as well as the current through a current sense resistor. The

controller also must know what voltage and current cutoff points are in order to change to

the next stage of the charging algorithm previously mentioned. It does this by various

resistor and capacitor networks. The formulas for calculating these values and the rest of

the components in the charger circuit are provided in a technical paper published by the

chip’s manufacturer (Balogh). See the UC3909 Battery Charger Appendix A for the

formulas required to design the charger circuit.

All the charger circuit equations are based on the parameters inherent to the

battery. Again, it is important to note that because the batteries are charged in parallel,

the voltage is the same at 12V, but the capacity must be doubled to obtain the correct

charging currents. Using the characteristics of the batteries selected for this project, the

DCM0035 by Interstate, the component values were calculated using the extensive design

equations given in Appendix A. The components are connected as shown to the UC3909

as shown in Fig. 17, which is the complete charger circuit.

27

Fig. 17 UC3909 Charger Circuit

28

K. Measuring State of Charge (SOC)

As discussed in the Functional Description section, the microcontroller on the

mower displays the state of charge of the battery. This requires that the State of Charge

(SOC) be measured. Of all the methods researched, current counting (Zhu), as given in

Eqn. 15, is implemented on mower controller to determine the SOC.

∗±= ∫ IdtCapacity

SOCSOC1

0 Eqn. 15

SOC is the calculated state-of-charge of the battery, SOC0 is the initial state-of-charge of

the battery, Capacity is given by the battery manufacturer, and current (I) is measured.

Current is measured using a current sense resistor. These are typically resistors

that are very small and accurate. A current sense resistor is placed in the ground path of

the circuit and the voltage drop was measured across it. Using a .005Ω resistor made it

possible to omit a conversion step to determine the actual current because the A/D

register was very close to the actual current.

sensesense RIV ∗= Eqn. 16

Imeasured

AREF

sense

V

V 1024∗= Eqn. 17

Ex.

I=30A

Rsense=.005Ω

VAREF=5Vdc

Vsense=30*.005=.15 Eqn. 18

Imeasured=30.72

L. Measuring Battery Voltage

The microcontroller on the mower also displays the terminal voltage of the

batteries which requires measurement of the voltage. The maximum voltage of each

battery at full charge is approximately 14Vdc. Since the batteries are in a series

configuration, the maximum total battery voltage is 28Vdc. Since the A/D channels on

the microcontroller can only measure 0-5Vdc, a voltage divider circuit must be used. The

maximum output voltage of the divider circuit needs to be 5Vdc. This corresponds to

28Vdc as well as the maximum value that the A/D register can store (210

-1=1023 = 5V

when VAREF=5V).

29

21

2

28

5

RR

R

Vdc

Vdc

+= Eqn. 19

A 1kΩ was selected for R1 to limit current at the microcontroller. This yields a value of

217Ω for R2.

V batt(Register Value)

AREF

batt

V

V 102

2171000

217∗

+

∗= Eqn. 20

If Vbatt = 28Vdc and VAREF = 5Vdc, then V batt(Register Value) is1022. V batt(Register Value) is

divided by the actual voltage to determine how they are related.

5.3628

1022= Eqn. 21

Eqn. 20 is used to convert V batt(Register Value) to the actual voltage on the

microcontroller. The microcontroller can only divide by whole numbers. To fix this, it is

multiplied by 10 then divided by 10*36.5=365.

=∗

365

10 V Value)ter batt(RegisV batt(actual). Eqn. 22

M. Determining the Throttle Setting

A voltage divider with a 10kΩ potentiometer is used as the user input for the

throttle. The input voltage to the divider is 5Vdc. The output voltage will be between 0

and 5Vdc. 0Vdc will correspond to a 50% speed setting, and 5Vdc will correspond to a

100% speed setting. Timer1 on the microcontroller controls the PWM signal. A counter

is incremented until it equals 2046 and then resets to 0. Every time the counter is

incremented, the count is compared to a compare register. If they are equal, a pin is

toggled. This creates the PWM signal. For example, if the compare register had 1023

stored, this would result in a 50% duty cycle. In order to have 0Vdc correspond to a 50%

duty cycle, 1023 should be added to the A/D register value and then stored in the

compare register.

Compare Register 10231024

+∗

=AREF

out

V

V Eqn. 23

30

V. Implementation and Results

Mower System Results

The various subsystems of the mower system and charger system, discussed in

Section IV, were implemented and tested. In case of the motor control subsystem, the

microcontroller PWM signal with throttle control input, voltage regulators, gate driver

and power MOSFETs were successfully implemented. The motor control subsystem was

first tested in the lab using a PWM signal supplied by a function generator and system

operation was verified. Next the microcontroller was interfaced to the gate driver and

speed control of the motor using the throttle input to the microcontroller was verified.

With these successful tests, the complete motor control subsystem was finally mounted

on the mower platform with the Tecumseh motor with blade and tested by mowing grass.

The test was carried out by mowing dense wet grass and cutting 1 ½ inches off the top of

the grass. The pace at which the mower was pushed was at a walking speed of

approximately 2.66 ft/s. The mower ran for almost an hour and a half and mowed

approximately 13,000 sq. ft, before the batteries were completely discharged. This

mowing test exceeded the requirement to mow a 10,000 sq. ft yard in one hour. As far

as the weight expectation, the final mower system weighed 91 pounds as a prototype.

The 90 pound specification certainly could be met by improved mower deck design,

optimizing battery size and weight and using a brushless DC motor. Furthermore, the

speed control worked correctly. When the throttle knob was adjusted, the motor RPM

would decrease or increase as the microcontroller varied the PWM signal accordingly.

This change in speed was also displayed correctly. Unfortunately the current and state of

charge were not measured or displayed correctly due to problems with the current sense

resistor. Though the SOC algorithm was correct, the current measurement was wrong

precluding a correct SOC calculation. The problem with the current measurement was

noise due to the small current sense resistor. The problem with the voltage display is due

to the battery voltage divider not working correctly.

Solar Charger System Results

The solar charger system was implemented as shown in figure 17. The solar

panel input was simulated by the Agilent power supply, and the battery was simulated as

a load resister. First, the UC3909 chip operation was verified by supplying an input

voltage similar to the solar panel output at peak power, about 19V. Then the PWM signal

was measured with the oscilloscope along with the timing capacitor for the switching

frequency. Once they were verified, the output of the UC3909 was interfaced to the buck

converter which was loaded by the resistor. Then, voltage and current measurements

were made on the load. The input voltage was slowly increased until peak power to

simulate how the sun moves from dawn to the peak power point. The Chip turned on

around 8V, and the load voltage increased to about 10.5V, and current was about 150 mA

when the input voltage was at 19V. Once the load voltage reached this level, the charger

system regulated it there for further increases of the input voltage up to the power supply

limit of 26V. This indicates the charger system was functioning, but not within the

specifications necessary to charge two 12V batteries. This is most likely due to the

31

tolerance of the components calculated in appendices A and B. These components are

used to set the voltage and current levels for the charge algorithm. Another issue may be

noise. Because the UC3909 is analog, it is more susceptible to noise issues, and most of

the measurements made were fairly noisy. Similar to the current sense issue mentioned

for the mower system, the charger system also uses a low value resistor for current

sensing.

VI. Recommendations for Future Work

In conclusion, the mower exceeded most of the initial specifications and the

charger system remained out of the initial specifications. This project is interesting and

offers the opportunity for future work as follows.

Mower System Recommendations

• Design the mower with a brushless DC motor. This would dramatically increase

efficiency and therefore battery sizing and other parts of the power electronics

may have to be redesigned

• Add a self propelling functionality, because of the increased weight of the

batteries. This change would also need further design considerations for the

battery size and power usage.

• Utilize a Hall Effect sensor for measuring motor current. This would increase the

accuracy of sensing current for the SOC algorithm.

• Implement the AC power back up. This was included in the original

specifications to allow for quick charging and cases of prolonged cloudy weather,

but it was omitted to reduce the scope of the project

• Obtain a sturdier mower deck. This project used a plastic deck that was not

designed to work with the motor and batteries in the system, so it was awkward to

push around.

32

Charger System Recommendations

• Redesign and simulate the charger circuit. Because the circuit displayed correct

functionality in the laboratory, it appears the problem is with the choice of

components. It is recommend that new component values computed and

simulated. However this will require a model for a lead acid battery.

• Interface the redesigned charger circuit to the solar panel.

• Develop a digital charging system. A digital system would be much less

susceptible to noise, and it could be implemented on the same microcontroller

used in the mower system. This may turn out to be a project in itself because it

will most likely need a closed loop feedback controls design to integrate the

charge algorithm.

33

VII. Applicable Standards and Patents

Standards

Document # Title Developer

ASAE S440.3

MAR2005

Safety for Powered Lawn and Garden

Equipment ASABE

UL 1447 (Ed. 4) Standard for Electric Lawn Mowers UL

UL 1447-2006

The Standard for Safety for Electric Lawn

Mowers UL

UL 82 (Ed. 6) Standard for Electric Gardening Appliances UL

IEC 62093 Ed. 1.0

b:2005

Balance-of-system components for photovoltaic

systems - Design qualification natural

environments IEC

IEC 60086-1 Ed. 10.0

b:2007 Primary batteries - Part 1: General IEC

IEC 60086-2 Ed. 11.0

b:2007

Primary batteries - Part 2: Physical and electrical

specifications IEC

Electricity. Magnetism.

General Aspects (IEC)

Electricity. Magnetism. General Aspects

Collection IEC

Other Standards Related

to Electricity and

Magnetism (IEC)

Other Standards Related to Electricity and

Magnetism Collection IEC

IEC 60287-1-1 Ed. 2.0

b:2006

Electric cables - Calculation of the current rating

- Part 1-1: Current rating equations (100 % load

factor) and calculation of losses - General IEC

CISPR 14-2 Ed. 1.1

b:2001

Electromagnetic compatibility - Requirements

for household appliances, electric tools and

similar apparatus - Part 2: Immunity - Product

family standard IEC

IEC 60730-2-10 Ed. 2.0

b:2006

Automatic electrical controls for household and

similar use - Part 2-10: Particular IEC

C 4512 Small Switches for Single-Phase Motors (E) KSA

UL 60730-2-10A (Ed. 1)

Automatic Electrical Controls for Household

and Similar Use; Part 2: Particular Requirements

for Motor Starting Relays UL

GB/T 6656-1986

DC ferrite permanent magnet motors (TEXT OF

DOCUMENT IS IN CHINESE) SPC

UL 1004 (Ed. 5) Standard for Electric Motors UL

34

UL 1012 (Ed. 7) Standard for Power Units Other Than Class 2 UL

IEC 60335-2-29 Ed. 4.1

b:2004

Household and similar electrical appliances -

Safety - Part 2-29: Particular requirements for

battery chargers IEC

IEC/TR 61955 Ed. 1.0

en:1998

Primary batteries - Summary of research and

actions limiting risks to reversed installation of

primary batteries IEC

A-20 Battery Charging Devices ABYC

TA-27 Batteries and Battery Chargers ABYC

A-A-1741

BATTERY CHARGER (FOR 6/12/18/24

VOLTS) US DoD

IEC 62093 Ed. 1.0

b:2005

Balance-of-system components for photovoltaic

systems - Design qualification natural

environments IEC

A-A-744 NOT 1

MOWER, LAWN, ROTARY, WALK BEHIND

(HAND PROPELLED WITH BLADE STOP)

(NO S/S DOCUMENT) US DoD

IEC 60931-3 Ed. 1.0

b:1996

Circuit-breakers for over current protection for

household and similar installations - Part 2:

Circuit-breakers for a.c. and d.c. operation IEC

35

Patents

Patent Number Description

US Pat. 4987729 Lawn Mower w/ solar panel attached

US Pat. 4942723 Lawn Mower w/ solar panel attached

US Pat. 5906088 Lawn Mower w/ solar panel attached

US Pat. 5084664 Solar Powered Lead-acid battery charger

US Pat. 4871959 Solar Powered Lead-acid trickle charger

US Pat. 6236175

Process and device for detecting the speed of rotation of a DC electric motor

US Pat. 5321627 Battery monitor and method for providing operating parameters

US Pat. 5656920 Method and apparatus for charging a lead-acid battery

36

VIII. Equipment List

Test Equipment:

• Agilent DC Power Supply E3634A

• Agilent DC Power Supply 3630A

• GE DC Ammeter

• Pioneer Digital Photo Tach DT-36M

• Power Patrol SLA1079 Sealed Lead Acid Batteries

• GME PM89 Ammeter and Current Shunt

• Fluke 87 Digital Multimeter

• Fluke 45 Digital Multimeter

• Fluke 337 Clamp Ammeter

• Tektronix TDS3012B Digital Oscilloscope

IX. Bill of Materials

Part Part Number Bulk Unit Cost Quantity Line Cost

DC Motor Tecumseh 9000A 57 1 $57.00

24V Battery Interstate DCM0035 26 2 $52.00

50W Solar Panel BP350 269 1 $269.00

Mower Frame (estimate) 20 1 $20.00

Motor MOSFET IRFP044N 1.3398 2 $2.68

Buck MOSFET IRF640 1.799 1 $1.80

MOSFET Heat Sink Wakefield 657-15ABPN 0.98 3 $2.94

Diode Heat Sink Wakefield 287-1ABE 0.659 1 $0.66

30A Fuse Little Fuse 0297030.WXNV 0.2973 1 $0.30

Fuse Holder Little Fuse 01530009Z 0.6627 1 $0.66

Std. 0.25W Resistors (various) 0.00855 20 $0.17

Std. Ceramic Capacitors (various) 0.027 13 $0.35

Speed Potentiometer 3852A-282-103AL 4.27 1 $4.27

Safety Switch 8125SHZBE 2.5558 1 $2.56

Buck Inductor 2216-V-RC 1.7085 1 $1.71

Input Capacitor UHE1H681MHD 0.2241 1 $0.22

Output Capacitor UHE1E471MPD 0.1232 1 $0.12

5V Regulator LM7805 0.22828 1 $0.23

15V Regulator LM7815 0.252 1 $0.25

Gate Driver TC4424 1.33 1 $1.33

Buck Diode MUR405 0.14204 1 $0.14

Protection Diode STPS20120 0.493 5 $2.47

Microcontroller ATMEGA 168 2.39 1 $2.39

Charger Controller UC3909 3.721 1 $3.72

LCD MDLS-24269-HT-HV-S 5 1 $5.00

Total Parts Cost $431.97

37

X. References

http://www.ecircuitcenter.com

http://www.mindfully.org

http://www.batteryuniversity.com

http://www.wikipedia.org

http://NREL.gov

Balogh, Laszlo. “Implementing Multi-State Charge Algorithm with the UC3909

Swichmode Lead Acid Battery Charger Controller.” (Unitrode) Texas

Instruments. 1999.

Zhu, C.B.; Coleman, M.; Hurley, W.G. “State of Charge Determination in a Lead-acid

battery: combined EMF estimation and Ah-balance approach” Power Electronics

Specialists Conference, 2004, PESC 04. 2004 IEEE 35th

Annual vol.3,20-25 June

2004 pp.1908 – 1914

38

Appendix A

Battery Data Equations (Unitrode)

39

Appendix A

Buck Converter Operating Parameters (Unitrode)

40

Appendix A

Power Stage Design Equations (Unitrode)

41

Appendix A

42

Appendix A

Controller Design Equations (Unitrode)

43

Appendix A

Controller Design Equations Cont. (Unitrode)

44

Appendix A

Controller Design Equations Cont. (Unitrode)

45

Appendix B

Charger Circuit Components List

Parameter Description Value/Part# Unit

Battery Data for DCM0035 Lead-Acid Battery

V Nominal Battery Voltage 12 Vdc

NC Number of Cells 6

Crate Battery Capacity 3.6 Ah

Vc Cell Float Voltage 2.25 Vdc

Vc,max Maximum Cell Voltage 2.483 Vdc

Vc,min Minimum Cell voltage 1.75 Vdc Itrickle Trickle Charge Current Limit 0.036 A

Ibulk Bulk Charge Current Limit 1.8 A

IOCT Over-charge Terminate Current Threshold 0.45 A

TC Cell Voltage Temperature Coefficient -0.0035 V/C

Tmin Minimum Operating Battery Temperature -23 C

Tmax Maximum Operating Battery Temperature 60 C

Vbat Battery Float Voltage 13.5 Vdc

Vbat,min Minimum Battery Voltage 9.765 Vdc

Vbat,max Maximum Battery Voltage 15.906 Vdc

Pch,max Maximum Output Power 28.6308 W

Buck Converter Operating Parameters

Vin,min Minimum Input Voltage 12 Vdc

Vin,max Maximum Input Voltage 22 Vdc

fs Switching Frequency 50000 Hz

Vd1f D1 Forward Voltage Drop (estimate) 0.59 Vdc

Vd2f D2 Forward Voltage Drop (estimate) 0.73 Vdc

Dmax Maximum Duty Ratio 1.353181461

Dmin Minimum Duty Ratio 0.487681478

Buck Converter Power Stage Components Design Sheet VRMM (D1) Diode Breakdown Voltage 23.859 V IO,MIN (D1) Diode Current Rating 3.6 A D1 Discharge Protection Diode MUR405

PD1 Diode Power Dissipation 1.062 W VRMM (D2) Diode Breakdown Voltage 33 V IO,MIN (D2) Diode Current Rating 3.6 A D2 Buck Freewheeling Diode MUR405

tRR Diode Reverse Recovery Time 3.50E-008 s

IRRM Diode Peak Reverse Recovery Current 0.5 A

PD2 Diode Power Dissipation 0.677999038 W

46

VDSS (Q1) Switch Breakdown Voltage 33 V ID,MIN (Q1) Transistor Current Rating 7.2 A Q1 Buck Main Switch IRLZ14PBF RDSON (Q1) Switch ON Resistance 0.28 Ω

COSS (Q1) Drain Source Capacitance 1.70E-010 F

IGATE Gate Charge/Discharge 0.8 A

QGS (Q1) Gate-To-Source Charge 3.50E-009 C

QGD (Q1) Gate-To-Drain Charge 6.00E-009 C

tOFF; tON Approximate Switching Times 1.19E-008 s

PQ1 Switch Power Dissipation 1.902 W PHS Heat sink Power Dissipation 3.64162787 W

DIL,MAX Inductor Ripple Current 0.72 A

L1 Buck Inductance 1.53E-004 H

IL1,PEAK Inductor Peak Current 2.16 A L1 Buck Filter Inductor 2.20E-04 H

VC3 Input Capacitor Voltage Rating 33 V

IC3,RMS Input Capacitor RMS current 0.9 A C3 Input Capacitor (electrolytic) 680µF/63V C18 High Frequency Bypass For Switches 1µF/63V

VC5 Output Capacitor Voltage Rating 23.859 V

IC5,RMS Output Capacitor RMS Current 0.208 A

C5 C5 Output Capacitor (electrolytic) 2.20E-04 F

RC5,ESR Output Capacitor’s ESR 0.084 Ω

PSN,MAX Snubber Power Dissipation 0.429462 W

VC4 Snubber Capacitor Voltage Rating 33 V

C4 Snubber Capacitor(polypropylene or metalized film) 3.55E-008 F

C4 Snubber Capacitor(polypropylene or metalized film) 10nF/63V

R3 Snubber Resistor (non-inductive) 11.21039121 Ω

R3 Snubber Resistor (non-inductive) 43 Ω

PR4,MAX Current Sense Resistor Power Dissipation 0.429462 W

R4 Current Sense Resistor 0.1 Ω

R4 Current Sense Resistor 0.1 Ω

F1 Output Fuse Rating 2.25 A

Controller Part Values

C6 Bypass Capacitors 100 nF

C7 Bypass Capacitors 100 nF

C13 Bypass Capacitors 100 nF C14 Bypass Capacitors 100 nF

C15 Bypass Capacitors 100 nF

C16 Bypass Capacitors 100 nF

C17 Bypass Capacitors 100 nF

Auxiliary Power Supply

(Voltage Regulator) 15 V

47

Gate Drive

(Dual Channel Gate Driver from the motor driver circuit)

Differential Voltage Sense

(optional) Charger Control - IC Setup - Housekeeping and Temperature Sensing

U1 TI UC3909

C8 Timing Capacitor 1.5 nF

fs Switching Frequency 50000 Hz

R8 RSET Oscillator 11000 Ω

R7 reference resistor - thermistor 10000 Ω

RP1 thermistor emulation Potentiometer 50000 Ω

Charger Control - IC Setup - Current Levels

R9 OVCTAP set resistor 100000 Ω

R10 OVCTAP set resistor 8333.1 Ω

R11 Trickle Current Limit Set 1721.74068 Ω

R12 Bulk Current Limit Set 5165.22204 Ω

Charger Control - IC Setup - Voltage Levels

R15 Battery Voltage Divider 1% recommended 77564.19048 Ω

R16 Battery Voltage Divider 1% recommended 6422.47619 Ω

R17 Battery Voltage Divider 1% recommended 17247.2619 Ω

R18 Battery Voltage Divider 1% recommended 138175.4888 Ω

Charger Control - IC Setup - Current Error Amplifier

R14 Current Error Amplifier Compensation Resistor 3078.46354 Ω

C11 Current Error Amplifier Compensation Capacitor 1.03399E-08 F

C12 Current Error Amplifier Compensation Capacitor 1.03399E-09 F

Charger Control - IC Setup - Voltage Error Amplifier

fo Voltage Loop Cross Over Frequency 1000 Hz

R13 Voltage Error Amplifier Compensation Resistor 3.22E+05 Ω

C9 Voltage Error Amplifier Compensation Capacitor 5.73055E-11 F

C10 Voltage Error Amplifier Compensation Capacitor 4.34048E-08 F Charge State Controller

(N/A - will be used by the micro controller)

48

Appendix C: Software Flowcharts

Software Initialization

Start

Setup Stack Pointer

Clear Variables

Setup Timer1 for Fast PWM mode using

16 bit resolution

Setup Timer2 for 1ms interrupt

Enable Interrupts

Initialize LCD

Set pin PB2 as an

output

Clear TCNT1

Clear OCR1B

Load OCR1A with $7FE

Setup TCCR1 for Fast PWM

mode and 16 bit resolution

Start Timer1

Divide internal

clock by 32

Clear TCNT2

Load OCR2A with $A5

Setup TCCR2 for Counting up and

resetting on match with

OCR2A

Start Timer2

Call LCD InitReturn from

LCD Init

Jump to State 01

The SOC calculations are made during an interrupt that occurs every 1ms.

49

Appendix C

Interrupt Service Routine

50

Appendix C

State 1

51

Appendix C

State 2

Jumped to from State

01

Measure Throttle

Potentiometer Voltage

Setup A/D control register to enable

A/D, start conversion, and enable interrupt

Choose A/D channel

AD5

Disable remaining

pins on Port C

Is A/D conversion complete

Store A/D register as

Speed variable

Yes

No

Measure current

Setup A/D control register to enable

A/D, start conversion, and enable interrupt

Choose A/D channel

AD4

Disable remaining

pins on Port C

Stop Timer2

Is A/D conversion complete

Store A/D register as

Current variable

Start Timer2Enable pins on

Port C

Set PB2 to ‘1’

Set PB2 to ‘0’

Jump to State 03

Yes

No

52

Appendix C

State 3

53

Appendix C

State 4

Jumped to from State

01

Jumped to from State

03

Copy Speed Variable to PWM

Register

Jump to State 05

State 5

54

Appendix C

State 6

Jumped to from State

05

Update Battery Icon

on LCD

Convert Current to

ASCI value

Update Speed Icon

on LCD

IsSOC greater

than80%?

IsSOC between

60% and79%?

IsSOC between

40% and59%?

IsSOC between

20% and39%?

IsSOC between

10% and19%?

Display 5 Battery Bars

Display 4 Battery Bars

Display 3 Battery Bars

Display 2 Battery Bars

Display 1 Battery Bar

Display “Please

Recharge Battery”

Has1 second elapsed?

Display Empty

Battery Icon

Convert Voltage to ASCI value

Display Voltage Value

Display Current Value

IsSpeed Variable

Icon =5?

Display 5 Speed Bars

Display 4 Speed Bars

Display 3 Speed Bars

Display 2 Speed Bars

Display 1 Speed Bar

Don’t Display

Speed Icon

IsSpeed Variable

Icon =4?

IsSpeed Variable

Icon =3?

IsSpeed Variable

Icon =2?

IsSpeed Variable

Icon =1?

Jump to State 01

55

Appendix C

LCD Initialization