Hardware
First we have to talk about the hardware aspects of the project to fully understand the software side. This section explains the concept of an electric longboard and the function of its mechanical components. A separate chapter is dedicated to the wiring and composition of the microcontroller, as it is a hardware component, but of course its construction depends on the program running on it.
When creating a small electric-powered vehicle, such as an electric longboard, there are a few things to consider before buying the components. From the shape of the board itself to the size of the batteries, these are the topics, which will be discussed in this subsection, including all the trade-offs between choosing one configuration over another.
At the end of the page in the section "External links" I listed different online calculators and guides for finding suitable components for the build.
As with the previous prototype, I decided not to build my own longboard, but rather buy a suitable one. It was mostly because building the board itself was not the goal of my project, but that does not mean, you should not care about the parts you buy.
APEX SERPENTINE MK II is a flexible half bamboo half maple flex 1 deck. It is a concave deck with raised middle (camber deck). Its shape allows for big enough clearance for the components to be mounted to the bottom.
Altera trucks 180mm came with the deck. It would be ideal to have a pair with square axle, which would allow for easier installation of the motor mount, but instead I rather designed a mount that fits around the round axle nicely.
Orangatang Kegel 83A are 80mm wheels with a plastic core. The core features holes which snugly fit a M4 screw, easing the mounting of a 3D printed pulley. The size of the wheels is also an important factor regarding riding speed. The greater the diameter of the wheel the faster it spins at a given RPM, compared to a smaller-diameter wheel.
The prototype is a single-wheel drive system. The advantages of such a configuration are the ability to fit a larger, more powerful BLDC motor under the deck and cost efficiency, but the downsides are less traction when accelerating and more stress on the drive belt. Another alternative would be a hub motor, which is already embedded in the wheel itself and consequently does not need a drivetrain, but such motors are fairly expensive and less powerful.
Turnigy Aerodrive SK3 - 6374-192KV Brushless Outrunner Motor has 4kW of power and as it is a relatively low kv motor that means it can create a lot of torque when accelerating or driving up the slope. It is suitable for 10 to 12S LiPo battery and handles up to 80A of current.
The custom made aluminium mount is designed to hold the motor on the trucks. It is bulky and heavy since it had to be made to withstand the forces the motor puts on the system when accelerating or breaking, while being mounted on a round axle, rather than a square one, which would prevent rotation of the mount.
The single-wheel drivetrain uses HTD5 pulleys and belt. It features 2.4 reduction ratio.
15t aluminium pulley was mounted to the motor shaft using a threaded pin which was coated with Loctite Threadlocker beforehand. It can fit up to 12mm wide drive belts.
I found a great 3D model of the 36t wheel pulley made for the wheels i chose on Thingiverse. I printed it from nylon as it must handle friction and the elements without loosing its shape. It can fit drive belts up to 12mm wide.
55t drive belt is 10mm wide, which ensures a good ratio between strength and friction. A wider belt would be more durable but it would introduce more friction in the system, and vice versa applies for a narrower belt.
Choosing the right electronic speed controller (ESC) might be the most important step. The first ESC, purposely build for electric skateboards was made by Benjamin Vedder and it was called the VESC. It was an open source project and Vedder's blueprints were the foundation for various iterations from other companies. The VESC has since been patented and companies are not allowed to use the same name, but the principles remain the same. I bought my "VESC" from Enertion. It is called "FOCBOX" and it allows for smooth acceleration and precise breaking, which are important, ason a skateboard there are no handlebars to lean on in case of a strong acceleration or deceleration. It features a Micro-USB connector, which allows it to connect it to the computer and program it.
It also allows the user to run their motors in a conventional BLDC mode or in FOC mode. I went with the latter. FOC is a type of motor communication which utilizes smoother phase modulation rather than fast switching phases as with BLDC. Therefore the motor generates less noise, run more efficient but reach a bit lower top speed (because there are no voltge spikes) and are more complex than BLDC.
VESC based ESCs can be configured precisely using the program called "BLDC tool". My exported configuration can be found in Miscellaneous folder.
There are a few settings that need to be adjusted before the first drive.
Motor max: sets acceleration current limit. My motor is rated up to 80 A, but the default 60 A is strong enough.
Motor min: sets deceleration current limit - in other words how "strong" the motor breaking is. I went with -40 A, as it is not too strong to throw me off, but I could go even higher.
Batt max: sets battery current draw limit. It is set to 60 A to match the Motor max, but it can be set lower for lower power. It should not exceed the maximum discharge rate of the battery pack.
Batt min: setts regenerative braking limit. I set it to -10 A to maximize the lifespan of the battery. The values should not exceed the maximum charge rate of the battery pack.
Battery cutoff start: sets at what voltage the current draw from the batteries should start to decrease to preserve battery lifespan. I set it to 37 V.
Battery cutoff end: sets at what voltage the current draw cuts off. I set it to 34 V. The 2 last values are quite conservative, because at 10 S 5000 mAh I did not want to keep the limits too thigh to improve the range at the expense of battery lifespan as I get about 13 kms as it is.
It is important to choose batteries with a relative high voltage to reduce the current flow. I also wanted to integrate the charging circuit to ease the operation of charging the battery pack.
The 10S 5000mAh battery pack was made from 2 ZIPPY Compact 5000mAh 5S 25C Lipo Packs connected in series. Their volume is small and with buying a prebuilt pack you avoid soldering the individual battery cells, which can be quite challenging without proper equipment.
To integrate a power delivery system which would allow for charging using a wall charger, a Battery Management System had to be implemented. The circuit also features a 40A overcurrent protection, which was not needed and was bypassed. The system charges using a 42V 5A wall charger and it does so in about an hour. Between the BMS and FOCBOX I put an antispark swith to turn on/off the board but still allow it to charge when it is turned off.
I designed my own enclosure in Autodesk Fusion 360. It is otherwise a very simple enclosure, but its main property is its compactness. After 3D printing it with an SLA printer, I hand made the openings for the power button, charging port, lights port and motor wires. The 3D model can be found in Miscellaneous folder.
Parts list available here.
Getting started:
- Best forum for electric skateboards
- List of commonly known parts
- Guide for building an electric skateboard
Different calculators and guides: