Final Design

New Brake System: Hydraulic Brakes

Past project team members installed a hydraulic brake system that contains the following components: the brake pedal, a master cylinder piston, calipers, brake pads, and brake fluid that travels through the brake line. A hydraulic brake system works as follows:

Brake Pedal: When the driver presses the brake pedal, it pushes the master cylinder arm.
Master Cylinder Arm: The movement of the master cylinder arm compresses the brake fluid in the master cylinder.
Brake Fluid: The compressed brake fluid is forced through the brake lines towards the brake calipers.
Brake Calipers: The brake fluid pressure causes the brake calipers to move.
Brake Pads: The calipers press the brake pads against the rotor.
Rotor: The rotor, installed on the axle, is squeezed by the brake pads.
Friction: The friction between the brake pads and the rotor slows down the rotation of the axle, thereby braking the vehicle.

Prior to the new master cylinder arm and linear actuator mount, the braking system consisted of a purchased master cylinder arm and linear actuator, both acquired from Top Kart. In this previous design, the linear actuator arm struck the master cylinder arm at an angle, causing inconsistent contact from zero to maximum actuator displacement and resulting in irregular pressure outcomes. Through redesign of the brake system, a gradual relationship between actuator displacement and brake pressure was achieved.

The team was able to utilize the hydraulic brake system using a brake caliper and rotor setup, which applied braking torque on the rear axle and allowed the car to slow down or come to a complete stop without skidding. The team was able to quantify the maximum braking torque that must be applied to avoid skidding under heavy braking or the wheels locking.

Linear Actuator Mounting Plate

The actuator generates 90 lbs of force, and remounting had created a larger moment arm in order to increase the braking force applied. A ¼-inch thick steel plate was machined to elevate the linear actuator by 2 inches from its original position while keeping the master cylinder in its initial location.

Master Cylinder and Clevis Arm

A clevis arm, threaded to fit the end of the linear actuator, was added, and ensured reliable connection and consistent contact as the brake pedal angle changes with the actuator’s movement. A new brake master cylinder was designed and machined from 0.2 inch thick steel to provide adequate and constant surface area contact throughout the actuator’s range of motion.

Voltage Divider Circuit Diagram

Since the RTC (Real Time Controller) of the go kart has a limited voltage input of 3.3V, a voltage divider was created using a combination of resistors in series. Resistor 1 has a resistance of 1.5KOhm while resistor 2 has a resistance of 3.3KOhm. This outputs a voltage of 3.3V which provides good resolution and is safe from exceeding the maximum 3.3V. The output of the sensor will then be utilized by the Triton AI team to create a loop that will control the brake actuation through setting a targeted voltage at different vehicle speeds.

Speed Encoder System

When a hall effect sensor detects a magnet, it generates a square wave in the digital signal (some hall effect sensors produce analog waves, however, the sensors selected for this project were intended to be digital). This occurs because the sensor switches between a high and low state as the magnetic field passes by, producing a series of pulses that represent the presence or absence of the magnetic field. These pulses are then interpreted as a digital signal, where the width of each pulse can be used to measure the speed or position of the rotating ABS ring. The parts selected were an ABS ring from Jeep Wrangler 2007-12 models and a Littelfuse Inc. Digital Hall Effect Sensor.

Diagram of ABS reluctor ring and hall effect sensor. Image credit: Apec Automotive

Example of what the signal could look like when all teeth are detected versus when some teeth are skipped.

To ensure accurate readings between the hall effect sensor and ABS ring, where every tooth on the ring is being detected by the ring at any given speed, both components must be mounted efficiently so that the necessary gap (distance between the sensor and ring) is maintained.

ABS Ring Mount

Because the diameter of the ABS ring was significantly larger than the go kart axle, a mount was created to securely install the ABS ring onto the axle without independently spinning or experiencing excessive vibration. This mount was machined from aluminum 6061 and press-fit into the bore of the ABS ring, with three (1/4th in.) set screws threaded into it to securely grip the axle.

Hall Effect Sensor Mount

The final design for the hall effect sensor mount prioritized simplicity in design and fabrication. The individual beam components and the slot sliders allow for adjustability of the sensor’s position. The sensor is able to move in/out from the axle face, as well as up and down in order to ensure orthogonality to the teeth on the ABS ring.

Level Converter Wiring Diagram

To ensure compatibility with the go kart’s Real Time Controller (RTC), which requires a 3.3V signal, a level converter was used to step down the 5V output signal from the sensor. A perfboard was then soldered according to a custom wiring diagram created by the team, as shown in Figure 2.13. This setup allowed the converted 3.3V signal to interface with the RTC.

Final Performance

Gradual Brake Pressure Increase

Pressure testing was conducted on the previous brake mount system using a 1000 psi pressure sensor attached to the brake system and an oscilloscope. Voltage data was collected from the oscilloscope for each 0.1 inch of actuator displacement and then converted to pressure values in psi based on the pressure sensor’s reference sheet. The max pressure obtained when testing this system was 137 psi and brake pressure was only displayed in the last 12.5% of actuator displacement as shown in the figures below.

Diagram and graph of brake pressure vs. linear actuator displacement prior to redesign.

After designing, mounting, and machining all the required hardware for reliable brake pedal actuation, the redesigned brake system underwent the same pressure testing. The resulting graph, shown in the right, displayed a gradual relationship between actuator displacement and brake pressure, with a maximum pressure of 927 psi, which is approximately seven times the maximum pressure obtained with the initial Top Kart purchased system. The results demonstrated that the redesigned system produced a mechanical advantage with consistent contact between the master cylinder arm and actuator, leading to improved brake performance.

Diagram and graph of brake pressure vs. linear actuator displacement after redesign.

Reduced Wheel Skidding

WhatsApp Video 2024-05-07 at 12.06.12_7ec1b46e.mp4

Braking Pre 156A Team

Skidding occurs due to regenerative braking and car reverses back

WhatsApp Video 2024-05-07 at 12.04.55_e44515b0.mp4

Braking Post 156A Team

No skidding and car comes to a complete stop once brakes are applied

Precise Measurement of ABS Ring

After finalizing and machining both the three set screw shaft collar and the 2-DOF sensor mount, they were securely mounted onto the go kart, ensuring the correct distance between the sensor and ABS ring. Testing was then conducted to confirm that all teeth were being accurately read by the sensor. With the finalized mounts and correct distance between the ring and sensor, all teeth were successfully detected at high and low speeds without the presence of missing waves in the signal.

finalmount.MOV

oscilloscope_halleffect.mp4

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