Introduction and Motivation
During my gap year, I aimed to improve my engineering skills through a project which would challenge both my theoretical understanding and practical capabilities. I wanted to have a better grasp of control systems due to my curiosity and their intensive use in the aerospace industry.
I had an old Segway-style scooter called Smart Balance Wheel, also known as a hoverboard, and I decided to build another one from scratch. The platform has two wheels in total located on both sides of the rider, making the platform inherently unstable. Without the use of active stabilization algorithms the platform can not stand up right. This was a totally new concept for me; I have experimented with simple feedback control loops but never with an unstable system. This fact gave the project the complexity I was searching. In addition, I wanted to experiment with the control input, changing it to a center of gravity driven system. The board would react based on where the riders center of gravity was, instead of light sensors used in my Smart Balance Wheel.
Mechanical Design
Load Cell Configuration
For the load cells I decided to use the Keli AMIB C3 with a capacity of 20 kg because they were already present at the lab. I decided to use eight load cells in total, four under each foot, giving me a total measuring capacity of 160 kg.
I arrived at this load cell configuration to minimize the probability of errors during center of gravity calculations. Under ideal conditions the pressure plate would be larger than the rider’s feet, and the load cell sensors would be placed outside of the pressure points of the rider’s feet. This configuration allows larger loads to be calculated and tames the shifts in the calculations since the lever arm of the sensors are longer than the rider's pressure points. If the opposite was true, meaning the feet’s support points were further er from the center of the pressure plate than the sensors, even small shifts in pressure would be magnified due to torque and could possibly saturate the load cells. A larger pressure plate also ensures that the feet always land inside of the sensors whenever someone mounts the device.
Pressure Plates
Given the facts above, I chose a plate with dimensions of 190 mm x 99 mm x 8 mm to keep the design compact and lightweight. I chose 7075 series aluminium as the pressure plate material due to its low modulus of elasticity and light weight I decided on the pressure plate dimensions based on a foot that measures 260 mm long, 100 mm wide in the toe box and 60 mm wide in the heel, which approximately corresponds to a size-41 shoe per the European system. Since the actual support points of a foot are shorter and narrower (approximately 150mm for the given sample) the 190mm length and the 99 mm width is sufficient for my chosen feet model. These plates also sit three millimeters higher than the upper plate, ensuring that even riders with larger feet would transfer their whole weight to the pressure plates. Thus, load cell measurement would stay unaffected. In terms of load cell placement, I located the load cells facing backwards inside of the ones facing forwards since the heel is narrower than the toe box; as such they could be located closer to each other without hindering the quality of the center of gravity calculations.
Load Cell Spacers
I added load cell spacers to create 10 mm of additional height in the electronics compartment. I needed more height in the electronics compartment because of the battery I planned on using. Since the load cells were going to be mounted on this piece to be rigid for accurate measurements, thus I decided on 7075 aluminium as the material of choice. This extra space between the load cells and the bottom plate also benefited the motors. The power and Hall sensor cables needed to pass through the load cell compartment before reaching the electronics compartment. This space introduced a route where those cables could pass through without interfering with the load cells.
Bottom Plate
The bottom plate has the dimensions of 580 mm x 228 mm x 5 mm and houses all the other parts. I had previously decided on the material as 6061 series aluminium to better resist the bending forces. However, the supplier did not have 6061 series aluminium for the given thickness thus I ended up using 5754 aluminium.
Load Cell Compartment
I added walls around the load cells, creating a compartment to minimise bending and torsion under load, allowing more accurate data to be received from the load cells. I punched holes in these pieces to save weight without sacrificing too much strength. These holes also allowed cables to be fed through. I decided on the long sides of this compartment to be 7075 series aluminium with dimensions of 204 mm x 50 mm x 8 mm. The short sides of the compartment are identical to the long sides in terms of function and material, with the only difference being size: 129mm x 50mm x 8mm.
Motor Mounts
The purpose of the motor mounts is to create sturdy mounting points for the motors which are also the wheels. The piece has dimensions of 68 mm x 50 mm x 10 mm and is machined out of 7075 series aluminium. I deliberately designed the motor shaft to be eccentric in order to increase the ground clearance. This allows for steeper tilt angles thus allowing the platform to climb steeper slopes. The current tilt angle with this configuration is 21 degrees according to my calculations. However, realistically the tilt angle will be lower to prevent the board’s edge from scraping on the ground. The tilt angle will also be limited with software for the balance loop to function as desired. For the platform to apply brakes, it needs to move ahead of the center of gravity and that can only be achieved by accelerating if the board is already cruising forwards at a given speed. For that reason, the top speed and tilt angle will be capped at a certain value.
Upper Plate
The upper plate is added as a cover on top of the board, it conceals the electronics compartment and helps with distributing the bending and torsion forces. I used 5754 series aluminium with dimensions of 580 mm x 229 mm x 5 mm due to the same reason I used 5754 aluminium for the bottom plate.
Electronics Compartment Supports
I added the electronics compartment supports to prevent the upper plate from caving in on the electronics compartment. They are located on the middle point of the 280 mm-long span between the two load cell compartments. I used 7075 series aluminium with dimensions of 50 mm x 30 mm x 10 mm for these pieces.