Download Remote Lawn Mower

ECE 792 Senior Project Final report
Remote
Control
Lawn Mower
Date: 05-11-2011
Team members:
-Hajrush Aliu
-Neeraj Gill
Faculty Advisor:
-Professor Wayne Smith
Project Completion Date:
- January 25, 2011
Abstract:
This project was about designing a remote control lawnmower that eliminated the need of
physical power. In completing this project, there were numerous steps that were taken; finding
parts, designing and testing H-Bridges, and having a microcontroller to interface the H-bridges to
the RC receiver. Throughout this report you will learn more on how we went about completing
this project and what various parts were used that replaced the physical power needed in moving
the lawnmower.
Introduction:
The purpose of this project is to design and build a remote controlled lawn mower. This
would be beneficial because man power is not required in mowing the lawn on those hot summer
days, where you would prefer not to be out in the sun. The remote will allow the user to control
the speed and direction of the lawn mower by moving the joy sticks. For safety purposes, the
engine of the lawn mower can be turned off via remote and also turns off automatically when
there is a loss of signal.
Design specifications:
The main objective of this project is to convert a push lawn mower into a remote control
lawn mower, where pushing is eliminated by battery power. The battery will be used to power
the dc motors, which will turn the gears and cause the wheels to turn with greater torque then
what the dc motor can produce. The steering of the lawn mower will be done in a skid steer
fashion by having the wheels turn in the opposite direction causing the lawn mower to turn either
left or right, with the help of swivel wheels in the front.
Project circuitry and component explanation
H-Bridges:
An H bridge is an electronic circuit which enables the current to flow in either direction.
The H-bridge consists of four transistor arranged in H shape (see figure 1). A transistor is an
electronic switch which can be turned on or off by applying the appropriate voltage. This Hbridge consists of two P-channel transistors on the top and two N-channel transistors on the
bottom. To turn on the P-channel transistor we need 0 volts and to turn it off we need 12 volts
where for N channel we need 12 volts to turn on and 0 volts to turn off. The Arduino
microcontroller outputs are 0 volts and 5 volts which is not enough to turn the transistors fully on
or off. In order for the microcontroller to control the transistors, we designed four drivers, two
for P-channel transistors and two for N-channel transistors. The drivers which control the Pchannel (High Side) transistors takes 5 volts in and converts it to 0 volts which turns the high
side transistor on. To turn the high side transistor off, the driver takes in 0 volts and converts it to
12 volts. The other two drivers which control the N-channel (Low Side) transistors, take in 5
volts and converts it to 12 volts which turns them on and 0 volts turns them off.
The way this circuit works is fairly straight forward. To turn the motors forward, the Q1
and Q4 transistors (Fig. 1) need to be on, but Q2 and Q3 should be off. In order to turn the
motors backward, Q2 and Q3 transistors (Fig. 1) should be on, but now Q1 and Q4 should be off.
The H-bridge is also used to control the speed of the motors; this is done by varying the duty
cycle of the transistors. Duty cycle is basically the amount of time for which the transistor
remains on, of the total time under consideration. The duty cycle runs at a frequency of about
490 Hz. For example at 50% duty cycle looks like a square wave where the on and off time is the
same, this means that the transistor will remain on for half of the time and as a result, the motors
will turn at half speed.
Two H-Bridges will be designed to control the direction of the DC motors; turning the dc
motors clock-wise or anti-clock-wise. This will force the lawn mower to go forward or
backward, and in either direction, left or right. The lawn mower will be turned left or right by
turning the wheels in the opposite direction. Turning the wheels in the opposite direction gives us
skid steering maneuvering which allows one to make very sharp turns allowing the lawn mower
to get around tight corners. We have also installed LED’s in our H-bridge circuits for direction.
There is a yellow LED which represents forward and the other LED is red that represents
reverse.
Fig. 1: Designed H-bridge circuit.
Microcontroller:
The microcontroller is used as an interface between all the components; it receives the
commands from the user and sends those commands to the H-bridges. The RC remote sends a
Pulse Position Modulation (PPM) signal to the receiver. PPM is a signal that has message bits
encoded and are sent in single transmission pulse. The signal from the receiver is picked by the
microcontroller which converts it into PWM signal, see figure 2 and figure8. Depending on the
signal, it is either used by the H bridges or the safety shut off circuit as seen in Fig. 2.
Fig. 2: Flow chart.
Wireless remote and receiver:
A 2.4 GHz Spektrum DX5E remote and an AR500 receiver is used to control the
operations of the lawn mower, such as speed, direction, and safety shut off. The remote has a
range of 300 feet which allows the user to control the lawnmower from a great distance. The
remote will send pulse position modulation signals to the receiver; which then will be used by
the microcontroller to control the operations of the DC motors. The speed and direction of the
lawn mower is controlled by two joysticks on the remote. The left joystick controls the left wheel
and the right joystick controls the right wheel. Pushing the joysticks forward makes the
lawnmower go in forward direction and pushing them backward makes it go backwards. To turn
the lawnmower left, push the left joystick backward and right joystick forward, and to turn the
lawnmower right, do the opposite.
Fig. 3: Spektrum DX5E RC Remote
Fig. 4: Spektrum AR500 RC Receiver
Relay:
A relay is an electro mechanical switch which can be turned on by applying voltage. A 12
volt two pole relay (275-249) is used to turn off the gas engine. The engine can be turned off two
ways; either from switching the toggle switch which is located on the top left of the remote or
when there is a loss of signal between the remote and the receiver. This is a great safety feature
because you still have control of the gas powered engine just in case something unexpected
happens. In order to sense if there is a loss of signal, the microcontroller constantly scans one of
the channels on the receiver to see if the remote and receiver are still connected together. When
there is a loss of signal or if you turn the toggle switch on the remote that shuts down the engine,
the microcontroller send 5 volts to the driver circuit we designed which produces 12 volts and
turns the relay on. When the relay is turned on, the spark plug module gets shorted to the
lawnmower chassis, which cause the gas engine to shut off. See fig. 8.
12 Volt, 12Ah Lead Acid Battery:
The 12 volts battery (see fig. 5) is used to power the drive system, safety shut-off, and
cooling system. The battery can be fully charged within four hours. The DC motors we are using
have a running current of 1 amp which means when the battery is fully charged it would last for
about six hours. We are also using a four D cell battery pack which gives us about 6 volts to
power the microcontroller and RC receiver. The microcontroller and the RC receiver are low
power devices which mean the four D cell batteries are less often replaced. The reason we are
using a different battery pack for the microcontroller and the receiver is because they need 57volts to operate, which made the 12 volt battery not suitable for this purpose. First we decided
to use a 12 to 5 volts voltage regulator, but after testing, we found out that if both motors stalled,
it caused the battery voltage to drop significantly causing our voltage regulator to not function
properly. In order to keep our microcontroller and receiver safe, we then decided to have a
separate battery pack for the receiver and the microcontroller.
Fig. 5: Fisher Price 12 volt 12Ah battery.
DC Motors:
12 volt DC Johnson motors. They have a stall current of 43 amps and continuous running current
of 1 amp.
-
Dimension = Ø 35.8 x 57, shaft Ø 3.175 mm
Torque = @ constant 6.68 mNm/A
Number of poles = 3
Weight = 242 grams
Fig. 6: 12 volt Johnson DC motors
Cooling system:
A 12 volt 18 mW fan and heat sinks are used to keep the power transistor cool and also keep the
air circulating inside the box where all of our electrical components are located, as seen in figure
7.
Fig. 7: Drive and control system.
Fig. 8: Electrical components wiring diagram.
Lawnmower Gas Engine:
The blade of the lawnmower is powered by the 3.5 horse power gas engine. This is the
most economical and effective way in powering the blade. The lawnmower has a 20 inch
diameter blade.
Testing and Implementation:
The H-Bridges were built and tested under different loads. We also tested the speed
control by varying the PWM signal (0.1% to 99.9%) of the signal generator. The speed control
test showed us that the motor does not run at below 8% duty cycle. After the project was
completed, a series of tests were performed to see how the lawn mower performed at different
landscapes (hills and flat areas). The safety shut off switch was tested by using the remote.
Additionally, the lawn mower engine was shut off when the remote signal was lost by shutting
the remote control off.
In mid October, we decided to build H-Bridges using P channel Mosfets on the high side
and N channel Mosfets on the low side. It was really difficult to find P channel Mosfets that can
take high current because the major carriers of P channel Mosfets are holes that have half the
mobility of electron hence twice the on resistance. The ones we found were surface mount
transistors (30A, MTB30P06vT4G) which we thought we could work with but found it very
difficult to solder wires to the pins. New P-channel (IRF9540) 23 Amp Mosfets were ordered
and we decided to put two in parallel with heat sinks to meet our stall current requirement of 43
Amps.
The H-Bridge was built using the new transistors and when we tested it, we found out the
motor was not spinning fast enough. The reason was the two P-channel mosfets on the High side
were not turning on and off completely. After testing each pin, it was found out that the driver
circuit was not working as expected. Based on the specifications from the data sheet, the Pchannel mosfet needs 0v to turn on and about 12v to turn off due to the Vgs voltage as
mentioned above.
The H-bridge and driver circuit once again were redesigned and rebuilt. The results were
far better than the previous H-Bridges we built. The Mosfets were no longer heating and the
speed of the motor improved which means there was no power loss in the circuit. Now the next
step was to interface the receiver with the microcontroller because the receiver outputs a PPM
signal which we need to convert it into PWM signal in order to control the speed of the motor.
We decided to order an Arduino Mega 2560 because it uses C language and can be easily
programmed. After we received the microcontroller, it was interfaced to the microcontroller with
the receiver and was programmed to control the H-bridges. Everything went as planned, and the
project was completed on January 25.
Fig. 9: Testing H-Bridge for direction control.
Fig.10: Testing redesigned H-bridge
Budget (spent)
Category
Lawn mower
Drive system
Wireless Controls
Circuitry
Others
TOTAL
Parts
-Motors
-Gears
-Wheels
-Swivel wheels
-Remote
-Receiver
-Wires
-Circuit board
-Microcontroller
-Circuit comp.
Miscellaneous
Price
$ 40.00
Used from an old
Power Wheels
HUMMER
$ 25.00
$ 60.00
$ 50.00
$ 25.00
$ 10.00
$ 70.00
$ 52.00
$ 20
$ 352.00
Parts used in designing the H-Bridges
Name and part number
Used in final design
(Yes/No)
NO
Replaced with
Comments of why it
was replaced
IRF9540
N channel FDU8780
NO
FQP50N06
BJT 2N3904
YES
BJT 2N3906
YES
The Mosfet was a surface
mount which was hard to
solder and put a heat sink
on.
The Mosfet was replaces
because of its low drain
current and was hard to
put a heat sink on.
Used to drive
PMOS/NMOS
Used to drive NMOS
P-channel
MTB30P06VT4G
Conclusion:
The project can be seen as a success. We met all the requirements and completed our
goals for this project. This project eliminated the physical power required in pushing the lawn
mower without sacrificing safety. The lawnmower can now be controlled through a RC remote
just by moving joysticks which would be preferred on those hot summer days. The project is also
equipped with safety features, such as an engine shut-off when there is a loss of signal and also
the engine can be turned off via remote.