Project Advisor : Teo Shin Jen (http://shin-ajaran.blogspot.sg/) and funded by Singapore Polytechnic
If you have not read the article on the P.E.T V1, read here.
My first version of the personal transporter was really bulky, heavy and quite slow. I can literally workout with that thing. It was made of recycled parts from previous projects, so it wasn’t really a good one and I have very little control over the size and customisability of the parts. The 24Ah LiFePO4 battery is too overkill and too heavy. The Sabertooth is too overkill as it is rated at 25A and the motor mount is too heavy as it is made of a solid aluminium block. So I am going make a new one from scratch using brand new parts, so that I can customise everything. It is going be smaller, lighter and has faster speed.
For the overall size, it will be about half as much as the current P.E.T. As for the motors, I am going to use a geared motor which is much lighter and smaller than the wiper motor. For the wheels, I am going to use 100mm scooter wheels that are going to be driven by the motors. It is going to be mounted on a bearing. As the wheels have a 22mm diameter mounting hole, I am going to use pololu’s 6mm scooter wheel adapter. Both of the motors will be driven by a RoboClaw 2x15A motor driver, which is cheaper than the sabertooth. All of these will be powered by a 5000mAh 4S 14.8V LiPo battery. Lastly, there will be a handle added to the PT.
Where I got my parts:
Motors – Ebay
Wheels – Ebay
Batteries and charger accessories – Hobbyking.com
Motor Driver and scooter wheel adapters – Robot R Us (Singapore – based online robot shop)
Coupling, bearing holders, shaft – Misumi
Plywood, PVC pipes and pipe joints – Local hardware shop
The motors are the most important thing in this project. I used a 160 rpm worm gear motor that has 10kgcm of torque which should be enough for this application. At no load, they take about 1A of current. They cost about SGD55/USD41 each. You can buy it here. If you’re choosing other motors, aim for one that has high torque (~10kgcm or higher).
As for the handle, I thought of using aluminium extrusions but they are quite expensive. I then decided to use PVC pipes with a T-joint at the top to make it like a handle. I used a 32mm diameter one, but I’m not sure why its labeled as 25mm. Its cheap though, so I’m not complaining. I paid SGD 4.20 for about 2 meters of the 32mm pipe.
I used 100mm scooter wheels as I thought that they would give me a balance between moderate speed and high torque. Scooter wheels also have high traction against the ground. Most of them have bearings installed in them, so try to get one that doesn’t have the bearings installed as you would have to knock them out, possibly damaging them in the process. I bought this particular one. They have a 22mm bore hole after the bearings are removed, so I used pololu’s scooter wheel adapter. I used the one with 6mm bore. You can get them here or if you’re in Singapore, here.
Batteries and Accessories
I bought the LiPo batteries from hobbyking.com , as they are priced reasonable and has fast delivery service. I bought a hard-cased Turnigy 5000mAh 4S battery. It has a rating of 20C (which means it can discharge at a rate of 20 times its capacity i.e. 20C x 5000mA = 100A). They are quite cheap, at USD25.85/SGD35, a price that other shops definitely can’t beat. They are in fact genuine.
I also bought a Hobbyking ECO6 50W Balance charger for USD19/SGD25. It is a DC charger, so I also bought a hobbyking 60W 15V Switchmode power supply for it. Along with that, I also bought some adapters for the 4mm HXT bullet connector for the battery, 2 battery voltage alarm/checker and velcro straps for the battery.
Previously I used the Sabertooth 2X25 on the P.E.T. However, they are too overkill and quite expensive, although I got it for free. I decided to use Orion Robotics’s roboclaw 2x15A motor driver. It has regenerative braking, which means the kinetic energy from the spinning motors will be converted to electrical energy to charge the battery when it is braking/slowing, similar to how electric trains brake. This saves power as the battery does not have to be very large, allowing for extended range. It has a simple serial mode which I plan to use. In the future when I have extra time I might use the packet serial mode which can feedback to the arduino about motor voltages, battery voltages as well as have better control over the motors. You can buy it here or if you’re in Singapore, here. If you’re going for a cheaper one, you might want to consider getting Pololu’s Simple Motor Controller series or even use a simple MOSFET to control the motors using PWM.
The wheels are direct-driven and have to be mounted on a mount. I originally wanted to use a pillow block but I could not find one with 6mm bore. I then thought of using a pillow block with plastic bearing but having limited experience of using plastic plain bearings (I only used it once), and I’m not sure whether it works for high load applications. I then decided to use a customisable bearing holder that has a 626 bearing(6mm bore) installed from misumi. Misumi does not sell to individual customers, they only sell to companies so I ordered them through my school.
As for mounting of the motors, I am planning to use steel 90° shelf mounting brackets. I will drill holes based on the spacings of the holes on the motor’s gearbox.
Fabrication of The Personal Transporter
(As for the fabrication part, my friend Max also helped me)
We started with cutting the plywood to a suitable size. Originally we thought of cutting it to 40cm x 30cm but it proves uncomfortable to stand with my feet so close together. I decided to cut it to 40cm x 40cm square, 5 cm smaller than the previous personal transporter.
As for the PVC pipes, my friend had a really awesome idea of storing the handle under the transporter when not in use. They would be secured to the plywood board using velcro straps. So we decided to cut the 1m pvc pipe into 30cm sections so they could fit below. Using straight joints, they could be combined into a 90cm handle. At first we thought of using a 3D printed mount to mount the handle to the wood, but 3D prints are not very strong so we decided to use a screwed coupler for the handle. We had to ‘mill’ a large hole in the wood to get in the screw coupler. Tedious, but works better than the 3D print mount.
For the mounting of the motor, we used the 90° angle bracket used for shelves. We counterbored holes for the M4 screw heads so they would not protrude out as that would look unsightly.
We spray painted both the plywood and handle so that it will look nicer and cleaner. For the handle, we used a matte silver colour to give it a metallic look and as for the wood we used dark grey so that it would not get look dirty easily. A few coats had to be given in order to hide the underlayer.
We used misumi’s bearing holder with a 626 bearing preinstalled. It is made of solid aluminium so it should be more than enough for a human’s weight. Similarly, we also counterbored holes on the plywood side so that the screw would not look unsightly.
The magic smoke of the motor driver
I was testing the battery with the motor driver at one time. The 5V BEC pin of the motor driver was connected to the Vin pin of the arduino as the 5V pin was used for the HC06 bluetooth. I unplugged the battery from the motor driver, and then inserted it in to the HXT 4mm to 2.1mm connector and then connected the 16V battery to the arduino, not realising the Vin pin is still connected to the 5V pin of the motor driver. The magic smoke was released, and that was the end for the motor driver. Luckily, upon further scrutiny, the MOSFETs look ok, but the 5V switching regulator has burned. I might be able to salvage it, but that will be in a future blog post. I had to use an old sabertooth 2×5 from another student’s project for now, although now I also have to add in a 7805 5V regulator as the sabertooth 2×5 can only supply 10mA for battery voltages above 12.6V.
Attaching Couplers to Motor Shafts
Completed P.E.T V2
Read Personal Transporter V2.5 here