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Low Carbon Footprint Electric Lawn Mower
Kraig Kamp, David Sharpe, Jamin WilliamsAdvised by Dr. Huggins and Mr. Gutschlag
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ProblemThe amount of pollution produced by a lawn
mower in one hour is the same as eight new cars driving 55 MPH for one hour
==
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• Project Specifications• Overall System• Motor• Battery• Solar Energy• Solar Energy• Charging the Battery• Controls• Results• Conclusions
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Project Specifications
• Performance similar to a gasoline powered mower
• Mow a 10,000 sq. ft. lawn in 1 hour• Keep mower weight under 90 lbs.• Keep mower weight under 90 lbs.• Use solar energy to recharge the battery in
1 week• Utilize intelligent controls for mowing and
charging
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Overall System
• Mower System• Charging System
Solar Panel
0 Š
21.
12 VDC
.8 VD
C
tex
t
tex
t
tex
t
tex
t
Batteries 120 VAC
Power
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Mower System
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Motor
Tecumseh 90000A
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Motor Specs
• Input Voltage = 24 VDC– Can be achieved with two 12V batteries in series
• 3200 RPM• 1.54 HP• 1.54 HP• Weight = 15 pounds
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Initial Motor Tests
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Initial Motor Tests
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Initial Motor Test Results
• Average Running Current ≈ 18 Amps• Maximum Running Current ≈ 40 Amps• Cut Grass as well as gas mower
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Choosing Batteries
• Different chemistry make-ups– Nickel Cadmium– Nickel metal hydride
• Price vs. capacity and weight• Price vs. capacity and weight
• Deep Discharge Lead Acid– Rated in amp*hours
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Choosing Batteries35 A*h battery
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Motor Modeling
• Purpose• Simulation in PSPICE
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Motor Characteristics
In order to not introduce back EMF, motor shaft cannot spin during test
sVR =
a
sa
I
VR =
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Motor Characterisitcs
At Vs=12 Volts and no load measure Vs, Ia and ωs
0=++−=++− SEaasaaas KRIVERIV ω
S
aas
TE
RIVKK
ω
−==
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Motor Characteristics
Compute the static friction coefficient( ) and the viscous friction coefficient (b).Find Ia at 8V and 12 V2 equations, 2 unknowns
..FST
Sum of the torques
0.... =−−=−− sFSaTsFSdeveloped bTIKbTT ωω
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Motor Characteristics
Perform a coast down test to find to compute moment mass of inertia (J)
( ) ( )ωω ..COsa VE =
20
25
τ*bJ =
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1-5
0
5
10
15
τωt
FS
t
eb
T
K
Vt
−
+= ..)(
τ*bJ =
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Motor Characteristics
Ω= 0825.aR
==
A
mNKK TE
*68723.0
[ ]mNT *272504.= [ ]mNT FS *272504... =
=
srad
mNb
/
*000535.
[ ]2*000912. mkgJ =
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Motor Driver
IRFP044N• Vds Max = 55V• Rds (on) = 0.02Ω• Id max = 53 A
TC4424• Gate Driver Chip• Takes 0-5V input from
Microcontroller• Outputs 0-15V PWM to the
Gate of the MOSFET
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Snubber Circuit
• Purpose: To keep voltage across Vds on the MOSFET below 55 volts
To negatiive terminal of motor
1DSR
To positve terminal of the motor
To Drain on the MOSFET
To Source on the MOSFET
1D
2D
SR
SC
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Issues with Motor Driver
• FETs overheating• Snubber capacitor failure
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PSPICE circuit schematic
+
Motor Electrical Model
-
Motor Mechanical Model
R1
.14I
D4
MUR405
Motor153uH
1
2
+-
EMF
H1
Ra.0825
Source
FET2
IRFP044N C18u
D3
MUR405
I
V-
V+
I
I
FET1
IRFP044N
LJ912uH
1
2
RB.000535
+-
Torque
H2
PWM
TD = 0
TF = 10nPW = .0005PER = .001
V1 = 0
TR = 10n
V2 = 15
OPEN50Meg
0
OPEN250Meg
V224Vdc Drain
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PSPICE Simulation
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Solar Charger System
• Solar Panel– BP 350
• Charger Controller– UC3909 Switchmode Lead-Acid Battery – UC3909 Switchmode Lead-Acid Battery
Charger
• Charger Circuit– Buck Converter– Protective Circuitry
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Solar Charger System
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Solar Panel Sizing
Things to consider:• Voltage and Current Output
– Need to be Higher than 12V– Current needs to break down sulfation– Current needs to break down sulfation
• Power Rating– Need enough energy to charge the batteries
• Efficiency– Used to calculate energy collected from
available solar radiation
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Solar Panel Sizing
BP 350 Specifications at Peak Power:
• 50 W• 17.5V• 17.5V• 2.9A• 10% efficiency
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Solar Panel Sizing
Source: NREL.gov
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Solar Panel Sizing
Month KW-Hrs/day Solar
Energy Emmitted(NREL)
KJ / day of Solar Energy Collected
actual days to charge 2 - 35AH batts
January 2.0 353 8.6 February 3.0 530 5.7 March 4.0 706 4.3 April 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
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UC3909 Features
• Controls Charge States– Trickle– Bulk– Overcharge– Overcharge– Float
• Charge state output to microcontroller• Battery Temperature Input for optimal
charging
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Charging Lead Acid Batteries
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Charger Circuit
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Buck Converter
Purpose:• To Provide Constant Current/voltage to a
load (batteries)• To adjust the constant current/voltage • To adjust the constant current/voltage
based on a PWM input.– High duty cycle means higher V/I– Low duty cycle means lower V/I
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Buck Converter
Source: Wikipedia.org
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Buck Converter
Source: Wikipedia.org
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Issues with Charger Subsystem
• Voltage is not up to the proper level for charging
• Need to further debug buck converter• Need to charge batteries using the solar • Need to charge batteries using the solar
panel
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Controlling the Mower
• User Interface• Speed Control• Safety• Display• Display• State of Charge measurement
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Microcontroller
ATMEGA168
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User Interface
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Speed Control
• Voltage Divider using 10k potentiometer• A/D Conversion• PWM output
– Lowest setting outputs 50% duty cycle– Lowest setting outputs 50% duty cycle– Highest setting outputs 100% duty cycle
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Safety
• Safety Switch• PWM output is set to zero when
disengaged• Over-current protection• Over-current protection• Fuses• Battery Protection
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DisplayOptrex 2x20 character LCD
with HD44780 Controller
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Display
Mowing
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Display
REMAINING SOLAR
TIME 3 2d
Charging
TIME 3.2d
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Display4-bit control vs. 8-bit control
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State of Charge
• Importance• Methods
– Terminal Voltage– Specific Gravity of Electrolyte– Specific Gravity of Electrolyte– Current Counting
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Current Counting
Current into battery
Current out of battery≈
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Current Counting
• SOC = SOC0 ± 1/capacity*∫Idt• Measure current every 10ms• Integrate for 1 second• Recalibrate SOC after a rest of four hours • Recalibrate SOC0 after a rest of four hours
using terminal voltage method
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Issues with Controls
• Inaccurate Current Measurement• Battery Voltage Measurement Circuitry• Too much current drawn
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Final System Test
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Final System Test
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Final System Test
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Final System Test
• Grass Conditions– Extremely dense in spots– Wet– 3” tall– 3” tall
• Mower settings– Blade spinning at full speed– 1.5” cut off the top of grass
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Final System Test
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Results
• Mowed 13,000 sq. ft.• Elapsed Time = 1.5 hrs• Initial Voltage = 26.6V• Final Voltage=23.77V• Final Voltage=23.77V
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Results
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Improvements
• Brushless DC Motor• Charging from AC power• Self Propelled• Better Mower Deck• Better Mower Deck
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Questions