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Project Supervisor: Teo Shin Jen (http://shin-ajaran.blogspot.sg)
The P.E.T is an open source 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 MRT 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.
It is made of a pair of repurposed car wiper motors from a previous 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/NiMH battery also recycled from a previous project.
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.
Motor Driver Vin -> + 24V
Motor Driver Ground -> Arduino Ground and Power Supply/Battery Ground
Arduino Pin 1 -> Motor Driver S1
Arduino Pin 2 and 3 -> Bluetooth Tx and Rx to Arduino
5V and Ground -> Bluetooth Vcc and Ground
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.
The third thing to do is to cut the plywood to size. I had it cut to 40 X 45 cm. Using an automatic 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 6mm drill bit as the bolts used are M6, giving it a tight fit. Next, using the appropriate screw size, screw in the motor to the plywood. As the screw head has a hexagonal shape, use a spanner to tighten it. Insert a nut to the other end, if available. The fourth step is to drill holes for the battery, Arduino and the Sabertooth motor driver. This should be fairly simple. Use self tapping screws for this as these screws do not go fully go through the wood. The last step is to fasten the battery to the plywood, connect the respective motors to the respective ports on the motor driver. Remember, one of the motors has to have its polarity reversed. Test the forward direction first to see which motor has to have its polarity reversed. If you don’t have the correct connector for the battery, use crocodile clips. Add a switch between the battery to the motor driver input. As the motor driver has a 5V out, use that to power the arduino through the Vin header. Refer to the earlier wiring diagram on top of this post. Solder wires to the Bluetooth module and a connector to the other end. Make sure the wires are all not hanging freely. Use wire fasteners to secure them.
And you’re done! Wasn’t that hard, was it? Now go out there and show the world what you have made!
In the previous post, I talked about the Tan Kah Kee Young Inventor’s Award (TKKYIA). In this post, I will be talking about what are my detailed plans for the personal transporter.
The motors are from a car windshield wiper which was also used previously on another personal transporter. It has a gearbox to give it some torque while sacrificing speed, which is not really what is needed. Its fairly heavy, so this will ensure stability.
A motor driver will be used to drive those motors using PWM. The PWM speed is regulated by the motor shield which receives serial commands via a Rx line from the arduino. This particular model is the Sabertooth 2X25. It can drive up to 25A per channel, but we only need about 800mA – 1.5A, so at normal operation, the currents are way lower than its limit.
Right now I’m just testing different android apps that send char characters to the bluetooth module and using the serial monitor to see the output. Originally I wanted it to be accelerometer controlled, but the app I found doesn’t really work on newer versions of android. There was another app of which I can customise every slider and button on the phone side. However, it is a paid app so in free mode I could only use it for 10 secs. The link to app can be accessed here.
The other apps only send a ‘f’ for forward, ‘b’ for backward and so on. There are predefined in the app so in the arduino sketch I have to match those commands. They also support moving forward while turning slightly. (8 directions of movement). Most likely I will use this method.
The wood panel used is plywood and I had it cut to 40cm X 45cm. I gave some extra tolerance so that I don’t have to recut in case everything doesn’t fit. I can cut the wood to the smallest possible size later on.
The battery is rather large, so I have to see how it fits into the transporter. I have not gotten it yet, so I did not drill holes for the motor mounts. Currently I have 2 possible layouts as shown below, if the battery is really large, I would have to place it in between the motors.
The motors have a M8 standard holes, so what I’ll do is screw them in from the top of the wood panel.