Limit Sensor EVT
Vital Biosciences | 2022
Objective: Create a fixture that autonomously and accurately verifies a limit sensor’s lifespan to be greater than that of Vital Bioscience’s blood diagnostic device; 5 years. It is estimated that the sensor will be triggered approximately one million times over this 5-year duration within the device.
Initial Design
Following numerous meetings with teammates and various design changes, a CAD model for the initial design was finalized, and fabricated. The first model consisted of 4 custom parts, modelled in SolidWorks and subsequentially 3D printed, 15 distinct pre-manufactured parts, and 5 distinct electrical components.
Issues & Solutions
Issues
Poor efficiency; the EVT was completing one cycle every 4 seconds, equating to 50 days of non-stop testing to reach the one million cycles.​
Non-isolated test subject; there was no method of deciphering if an inconsistency in cycle duration is a fault of the sensor or the stepper motor.
Instability; there was an alarming amount of wobble in the flag (sensor triggering object).
Solutions
Although the “Z” like shape of the flag was used to mimic the flag in the blood diagnostic device, efficiency was more important, so the flag was altered to a rectangular prism. This altercation decreased travel distance by 35%, consequently reducing each cycle duration to 2.6 seconds and total testing time to 32.5 days.
In order to isolate the optical limit sensor, a mechanical limit sensor was implemented into the design to act as a homing position for the flag. This addition eradicates the possibility of the motor interfering with the test, as any inaccuracies induced by the motor will be abolished by the homing sequence at the end of each cycle.
The presence of only a single linear motion shaft is what allowed the entire moving plate to wobble. Although the plate was seemingly fixed in two places, the point that was connected to the motor applied a torque about the point connected to the linear motion shaft, which induced the observed wobble. As such, the design was modified to accommodate the addition of a second linear motion shaft.
Final Design
All of these changes were implemented into the final design, among some other minor adjustments, which contributed to a successful EVT. The final design is composed of 4 custom parts—three of which were 3D printed, one was laser cut—16 distinct pre-manufactured parts, and 5 distinct electrical components. Although the quantity of distinct parts remained relatively the same, many of the quantities for repeated parts have been altered, and custom parts have been modified for this final rendition. It should also be noted that the script for this project was written in Python and transmitted through a Raspberry Pi.
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Over the course of the next month, the one million sensor triggers were carried out and the sensor successfully passed the EVT, deeming it acceptable to be used in Vital Bioscience’s blood diagnostic device.
