The P.E.T is an open source multi-use personal urban transporter, designed to be used for commuting on sidewalks and for connecting the last mile between the MRT Train Station to home. It has a small size which makes it suitable being brought into trains. The benefit of it is that: It is cheap, easy to build, highly adaptable to any spare parts the end user has, small footprint, and relatively acceptable travelling speed.
What its made of
It is made of a pair of repurposed car wiper motors recycled from a previous polytechnic final-year project, bolted to a piece of 40 x 45 cm plywood. An Arduino Uno receiving commands from an app on an Android Smartphone through the HC-06 Bluetooth module is the brain of the whole transporter. It also gives commands to the Sabertooth Motor Controller through Serial UART. The app is readily available on the Google Play Store here. The P.E.T is powered by a rechargeable 24V LiFePO4 battery also recycled from a previous project.
The user can choose the panel size, motor, battery and motor driver.
For example, the battery I'm using is very expensive as it is designed for very high current draw devices like electric bikes.The user can switch the batteries for one that is cheaper like an array 12V/24V NiMh/LiPo/SLA battery. I had it salvaged from a previous polytechnic project.
For the motor driver, I used the sabertooth 2X25A V2 motor driver which costs about $125. This was also salvaged from a previous school project, so I got it for free. It can drive up to 24A at 30V which is overkill. One motor only draws about max 2 amps at the highest speed, so 2 motors equate to about 4 amps. Another alternative for this is to use a power MOSFET such as a IRF520/540 with a heatsink and regulate the speed using PWM.
For the motor, an alternative would be a bicycle motor. Or you could just go to a car scrapyard and get some windshield wiper motors
Why build it?
1. The P.E.T is multi-purpose. It can be used as a personal tranporter, a remote controlled 'Trolley' to carry heavy items, or a remote controlled robot.
Step 1: Parts List
1 x Plywood (Cut according to desired size) I cut mine to 40cm X 45cm
2 x Car Wiper Motor with mount
1 x Arduino Uno
1 x Motor Driver (Use a motor driver that can handle up to 6A) or a Power MOSFET IRF540 or a Sabertooth 2X5 regenerative motor driver
1 x HC06/HC05 Bluetooth Module (Preferably HC06)
Batteries - I used a 24V LiFePO4 battery pack. I recommend using a SLA (Sealed Lead Acid) battery as they are quite cheap. You can connect 2 in series to get 24V if you want higher speed.
Battery charger for the specific battery that you're using
Wires - Try to use 24AWG wires and above for the motor.
Crimp Connectors (Fork,Ring,Push On) for connecting motor to motor driver
Step 2: Testing Everything Out
The first thing to do is to test out everything first. The arduino is connected to a breadboard which has the HC-06 Bluetooth module and to the sabertooth motor driver with 24V powering the motor driver. Solder/Crimp fork and push on connectors for easy disconnecting/connecting of the motors. Remember to use sufficiently thick wires. The program for the arduino can be downloaded here. What I did was to just connect the motors directly to the power supply. When everything works, its time to try to interface the motor to the arduino through the motor driver. If you're using the sabertooth motor driver, I have written a simple arduino code for it. Use Final3.ino If you use a power MOSFET, you would have to modify the code to output PWM analogWrite to the MOSFET. If everything is ok, move on to the next step.
Motor Driver Vin -> + 24V/+12V
Motor Driver Ground -> Arduino Ground and Power Supply/Battery Ground
Motor Driver 5V output (If present, if not connect arduino to USB) -> Arduino Vin
Arduino Pin 1 -> Motor Driver S1
Arduino Pin 2 and 3 -> Bluetooth Tx and Rx to Arduino
Arduino 5V and Ground -> Bluetooth Vcc and Ground
Note: If you're using the same Motor driver as me, I have set the jumpers as:
1 - Off
2 - On
3 - On (For lithium battery protection, only if you're using Lithium batteries, otherwise, set this to off)
4 - On
5 - Off
6 - On
Follow the instructions of the respective motor driver you're using for more info
Edit: the above is incorrect. Use the config below.
1 - up
2 - down
3 - up
4 - down
5 - up
6 - up
This works for me so you might want to try this too if the above doesn't work.
Step 3: Layout
The second thing to do is to think of the layout, how everything is mounted to the plywood. Then measure the total size. I chose the wheels to be flush/’hidden’ within the plywood so as not to hit other surrounding things easily. Use a pencil and draw guide lines and circles for the holes that needs to be drilled.
Step 4: Cutting and Drilling the Plywood
The next thing to do is to cut the plywood to size. I had it cut to 40 X 45 cm. Using an automatic hand jigsaw, it was cut from a larger piece of plywood. Then, draw guide lines for drilling the mounting holes. Before drilling, make sure that the mounting holes align with the markings. Double check! Using a hand drill, with a piece of scrap wood underneath, drill the holes needed. I used a 8mm drill bit as the bolts used are M8, giving it a tight fit. Screw a nut on the other side, tight but not so tight that it will crack the plywood. After that, mark and drill the holes for the arduino, HC06/05 and the motor driver.
Step 5: Screwing Everything In
Using an appropriate length screw size, screw the motor bracket to the plywood. As mine is a hexagonal shaped head, I used a spanner to tighten in. Insert a nut on the other end, if available.
Next, mark out the holes to mount the Arduino Uno, motor driver and battery. Drill the holes for the Arduino and motor driver, but using a smaller bit (1 mm down) than the required hole. As for the battery, I used a bit size that fits the cable tie, which is what I am going to use to mount the battery because it did not have any mounting hole.
Use self tapping screws for all the electronics.
As for the battery, I simply used cable ties to secure it as the battery does not have any mounting hole
Step 6: Wire Connections
The battery I'm using has an Anderson Powerpole connector, so I used a fork adapter to connect the battery to the motor driver. For the motors, I used a push on crimp connector and a fork connector on the motor driver side. As for the arduino connections, I simply used individual solid core wires into the arduino, although soldering headers to the wires would be better as the wires kept coming out.
For convenience, you might also want to connect a switch between the battery and the motor driver's input.
Step 7: Android App
As for the app, I used Andi.co's App 'Arduino Bluetooth RC Car'. The app can be downloaded from the link below. The app is originally used for controlling RC-sized cars, but it'll work well for our purposes. You can download it here.
Once installed, enable bluetooth on your android device, and then connect to your HC06. It should show as HC06. The default password is 1234. Consult the manual/datasheet if you're using a different bluetooth module for the default password and pinouts. Open up the app, then select the gear icon. Scroll down, make sure that 'Continuous Stream (every 50ms)' is ticked. Below that is all the commands that the app will send for your reference in case you're writing your own Arduino code.
Press forward and both motor should spin in the same direction. Do the same for the left and right. 1 motor should turn while the other stopped. 1 motor should also spin slower/faster than the other when you press forward/backwards and left/right at the same time. Adjust the polarity of the motors accordingly. Remember to bring up the speed slider if your motors do not spin. You also might want to adjust the turning radius by adding/subtracting the numbers that I added/subtracted behind 'velocity' in the arduino sketch. I find 30 to be the best for mine.
The App can be downloaded here
I may consider developing my own android app in the future using MIT's AppInventor 2 as I have just started learning it.
Step 8: Others/Improvements
Right now, the whole P.E.T is running on 2 wheels and it is sort of using the frame as the 3rd wheel. In the future, I plan to put a caster wheel in front so that it will balance correctly.
An improvement I would like to do is to reduce the overall weight of it. It has quite a hefty weight due to mainly the car wiper motors being mounted on a solid aluminium block. In the future, I would like to change those out to something like wood or a smaller aluminium frame which is much lighter.
Another improvement that I would like to make is to design and 3D print a bracket to mount the battery in. The cable tie method is unsightly and not very secure.
Visit my website at hassanulmakers.wordpress.com for more projects!
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