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Key Feature: Redesigned Horns
The original design of the heat staking component consisted of a heating element and thermocouple attached to a plate. Brass horns were then attached below this plate. The thermocouple obtained the temperature of the horns by checking the temperature of the plate and relaying this temperature to the PID controller which was responsible for adjusting the temperature of the heating element. Our sponsor relayed to us that the temperature tolerance was roughly ± 20˚C which is fairly high for our application. We theorized that the disparity in temperature was attributed to two main factors: 1) loss of heat due to the compatibility between the two different metals and 2) the amount of area available for heat transfer to horns.
The original plate was made of aluminum while the horns were made of brass. Since these are two different materials, the heat traveling from the heating element must travel through the aluminum plate before it reaches the horns thus making the temperature read by the thermocouple inaccurate. In reality, the thermocouple was only obtaining the temperature of the aluminum plate and not the intended component - the horns. Additionally, the conductivity of brass played a major role in the high-temperature tolerance that was observed. Implementing another material would greatly improve the temperature readings obtained by the thermocouple. Both original plate and horn designs are shown below.
Our final design consists of two key features that we have redesigned. The first feature is a new horn component that will generate more consistent temperatures and the second is a linear actuator system that will automate the nest movement and pressing processes. Multiple safety features have also been implemented in our final design such as a safety shield, two-button start, and emergency stop. A CAD model of the final design is presented below.
Original plate design Original horns
To improve the temperature issues with the original design, the horn and plate components were combined into one component and the material of the component was changed. By combining the two components together, the thermocouple can obtain an accurate reading of the horns through the plate since they are now the same component. The new design is shown here:
The material of the component was changed to copper which has a higher thermal conductivity than brass, ~390 W/m*K compared to ~120 W/m*K. Thermal conductivity affects the rate of heat transfer along with the material's ability to conduct heat. The temperature of the original brass horn component was gathered at 2-second intervals for roughly 50 minutes and the resulting data is shown in the figure below. The temperature recorded was 170.2 ± 3.8˚C which is a significantly lower tolerance than we expected. The low average temperature (compared to 180˚C) may have been caused by the small surface contact in which the temperature probe had with the horns.
Running the same test on the newly redesigned horns yielded the results below. As can be seen, the temperature fluctuations are now much smaller in magnitude, being only about ± 3˚C. Just as before, the average temperature of 171.1˚C was slightly lower than 180˚C due to the small contact area between the temperature probe and the horns.
Key Feature: Linear Actuator System
The original heat staking process required a torque lever to push the horns onto the polypropylene disks which rested on the nest and consistently apply 90 pounds of force for 10 seconds as shown in the video above. This process was time-consuming and required the operator to manually push the lever down every time a unit was to be produced resulting in a greater task time.
To improve this, we implemented a linear actuator. The linear actuator is connected to an Arduino Mega 2560 and is used to control the position of the horns. The Arduino controls the speed of the linear actuator through Pulse Width Modulation (PWM) and feedback from load cell sensors located below the nest. This allows for complete and precise control over the amount of time the heat staking occurs and force being applied to the polypropylene disk and adhesive patch. The accuracy obtained from the Arduino driven system improves the quality and repeatability of the heat staking process. A video of the process is shown below.
Key Feature: Load Cell/Hx711 Driver
A Hx711 driver was used to help measure the signal outputted by the load cell. Without the load cell, we would apply 5V to the load cell and measure the amount of excitation voltage that came in through the signal wires which would allow us to determine the amount of resistance. During testing, however, a multimeter was used, but the precision was not high enough as the voltage kept fluctuating within .1mV. To solve this, a Hx711 driver along with its library was used to transform the voltage signal into a readable “lb” output. During testing, a calibration factor needs to be inputted for each individual load cell. By iterating through various factors, a calibration factor of 91 was found and allows for force readings of within 0.1 lb tolerance.
Project Performance Summary
Currently, the system works and operates at planned efficiency. However, there are currently precision issues in regard to the alignment of the horns with the energy directors on the polypropylene disks. Two sources of error have been found for this.
The first source of error happens when the actuator is retracting and is in between the extending arm of the linear actuator, and the main body of the linear actuator. The cause was found to be an improperly sized aluminum rod, which was colliding with the drive screw and causing unwanted rotation. This was fixed.
The second source of error occurs when the arm is fully extended and is in the same place as the above error source, as well as in between the copper horns and the extending arm of the actuator. Currently, we are looking into solutions to prevent the unwanted rotation, with two of our top contending fixes being either a guide rail or a cone locator pin.
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