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Final Design Overview
Final Design Overview
The final design of the electric autonomous go-kart consists of three main systems: the steering, drive motor, and braking systems. The steering system is composed of a newly machined steering motor shaft with a brushless DC (BLDC) steering motor and gearbox kit with a 100:1 gear ratio. This is connected to the steering wheel shaft by a pulley and timing belt mechanism that turns the kart's wheels. The drive motor system includes a BLDC drive motor and a motor controller that helps control the speed of the kart. The braking system consists of both a mechanical brake that includes a linear actuator that extends forward to push on a lever to engage the brake calipers, and an electriconic brake that comes with the BLDC drive motor.
Steering System
Steering System
Steering System Overview
Steering System Overview
A traditional go-kart has a steering wheel attached to the top of the steering shaft for a human to turn the kart's tires. In order to turn the tires autonomously, the steering system has a pulley attached to both the motor shaft and steering shaft, which are connected by a timing belt to transfer the rotational motion of the motor and gearbox from the motor shaft to the steering shaft. The motor used is a REV Robotics NEO BLDC motor with a MAXPlanetary gearbox kit.
CAD view showing the components of the steering system
CAD view showing steering system and structure
Improvements to the Steering Motor Shaft
Improvements to the Steering Motor Shaft
Previous steering shaft design using coupler and set screws
Previous Steering Motor Shaft Design
Previous Steering Motor Shaft Design
The previous design for the steering motor shaft used a coupler to connect the 1/2" diameter hexagonal shaft to a 3/8" diameter hexagonal shaft. The set screws holding the coupler began grinding the shaft, causing it to slip and fail after just 30 left and right static turns.
New Steering Motor Shaft Design
New Steering Motor Shaft Design
The new steering shaft design uses a complete 1/2" diameter hexagonal shaft that was rounded 2/3 of the way in order to fit through the bushing and support bearing. A key insert was machined onto the round part of the shaft so that a key could fit into the bushing and prevent slipping. This design was able to withstand 250 static left and right turns of testing and achieved no failures during the race.
New steering shaft design that is round and hexagonal
Timing Belt Tension Adjuster
Timing Belt Tension Adjuster
In order to ensure the correct tension of the pulley’s timing belt, an adjustable tension mechanism was implemented on the steering wheel shaft. This was done by adding a bearing to the steering wheel shaft, which is pushed down by two high-strength screws, 4" long and 3/8" in diameter, and are attached to the steering support structure on both sides of the shaft. The two screws ensure that there is a 90° angle between the timing belt and the steering wheel shaft and the belt tension can easily be tuned by turning both screws to either loosen or tighten the belt to the optimal tension.
Overview of belt tension adjuster mechanism
Steering Shaft Encoder Mount
Steering Shaft Encoder Mount
Due to the possibility of the timing belt skipping teeth on the pulley when the kart is overturned, an encoder placed on the steering shaft would allow the autonomous program to recalibrate the center position easily. Thus, a 3D printed mount was designed to place the encoder on the very bottom of the steering wheel shaft as a last resort mechanism in case the timing belt skips.
The encoder and mount placed at the bottom of the steering wheel shaft
Welded Bushing to the Steering Wheel Shaft
Welded Bushing to the Steering Wheel Shaft
With all other mechanical components of the steering system working properly, the bushing on the steering wheel shaft became the next weak point when the steering shaft was overturned. This caused the bushing to slip and turn around the steering shaft while the shaft remained in place. To fix this problem, the bushing was welded onto the steering wheel shaft since the bushing was already tightened as much as possible. Other slip prevention methods such as inserting a shaft key were not possible with a hollow shaft.
The bushing welded to the steering wheel shaft
Mechanical Steering Stop
Mechanical Steering Stop
Although there was already a software limit switch to prevent the steering shaft from overturning, adding a mechanical stop to prevent the steering from overturning was a more secure method. Thus, an additional T-slotted support beam was added on each side of the steering structure, visualized in red on the right, to prevent the steering shaft from overturning. This causes the steering shaft to collide with the support beam if overturned.
The placement of the mechanical stop to prevent overturning
Drive Motor System
Drive Motor System
ME1719 BLDC Motor + VESC 75/300 Motor Controller
ME1719 BLDC Motor + VESC 75/300 Motor Controller
The ME1719 brushless DC motor with a VESC 75/300 motor controller was used to achieve speed control for the kart's drive system. The VESC has a built-in PID speed control algorithm that can easily achieve speed control when connecting encoders to it. It can automatically accelerate and use an electric brake to control the speed. In addition, the VESC supports CAN communication, which allows RPM values to be fed through CAN, and allows the VESC to automatically set the appropriate RPM for the motor.
The ME1719 brushless DC motor was more durable than the brushed DC motor previously used as it does not experience wear with use. In addition, the VESC is only compatible with brushless DC motors which made the drive system more reliable and easier for the software team to control the speed of the kart.
Overview of communication to the ME1719 BLDC motor
ME1719 BLDC Motor and roller chain sprocket assembly
VESC 75/300 motor controller on a heat sink
Brake System
Brake System
Mechanical Brake using Linear Actuator
Mechanical Brake using Linear Actuator
The mechanical brake system functions using a Kar-Tech linear actuator that extends forward to push the brake lever. This then supplies pressure through the brake wires to engage the brake calipers, which slows down the wheels. Testing showed that the linear actuator could withstand 250 cycles of pushing on the brake lever without overheating past its operating temperature of 85C.
Linear actuator mechanical brake
Electronic Motor Brake
Electronic Motor Brake
In addition to the linear actuator mechanical brake, the ME1719 BLDC motor used for the kart's drive motor system also allows for electronic braking, which was found to be easier to use than the mechanical brake at low speeds. Thus, the electronic brake was used as the kart's main braking system while the mechanical brake was used as a backup to provide additional braking power if needed.
The ME1719 BLDC motor with electronic braking
Improvements to Kart's Structure
Improvements to Kart's Structure
Increasing Impact and Vibration Resistance
Increasing Impact and Vibration Resistance
Previously, the shelves at the back of the kart that supported the electrical components were made of acrylic, which was not allowed for the race due to its potential for shattering in the event of a crash. It was found that delrin had a much higher impact resistance than acrylic (7.00 kJ/m^2 vs 1.60 kJ/m^2) and was also easily manufacturable by being lasercut. To provide further support to the kart's electrical components, all the acrylic shelves and mounts on the kart were replaced with delrin.
To increase the strength of the kart's steering frame, triangular gusset brackets were added in between adjacent T-slotted rails all throughout the kart. Additionally, belleville washers and loctite were added to every screw to diminish the effects of vibrations.
An example of a delrin electrical mount used on the kart.
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