Project Completion Date:
- January 25, 2012
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.
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.
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
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 H-bridge 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 P-channel (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.
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
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 5-7volts 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.
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
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 P-channel 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||Parts
|Used from an old
Power Wheels HUMMER