Final Design

Background

Ultraviolet (UV) Light has seen success in recent years as a method to sterilize water, the air, and even some surfaces. These environments typically have many pathogens and are high contact making an effective, efficient, and safe solution critical. Some examples of current sanitization technologies include companies like Rzero, Xtralight, and Munchkin which tackle sanitization tasks ranging from the air to baby diapers.

Jeffrey Levit, the sponsor of this design project, is CEO and CO-Founder of the startup UVc based in San Diego. The sponsor aims to tackle the challenges of sanitizing shopping carts through a fully automated sanitization system which uses UV-C to remove pathogens from the cart. A study from the CDC found that shopping carts carry more pathogens than public restrooms and door handles. As everyone experiences using a shopping cart regularly it is a goal to sanitize these high contact services to help stop and mitigate the spread of diseases, especially during pandemics like COVID-19. There are currently no existing technologies which tackle this issue, which results in drawing inspiration from other technologies: car wash, garage door, and hand-held UV-C technologies.

Levit’s idea of what the final product would look like as it is sent in one cart at a time for sanitization

The inner workings of Levit's proposed design depicted a cart during a sanitation cycle

These images are the best quality possibly from Levit's render

Our Objective

The objective for this project is to develop UV-C fixtures that can effectively sanitize shopping carts with a target threshold of log 3 (99.9%) pathogen reduction. Theoretical and experimental analysis of the light fixtures effectiveness were performed as a means to quantitatively validate the proposed design solution. Efforts in this study revealed that all shopping carts are different, making it difficult to design a general configuration. Therefore, for the scope of this project a singular Ralph’s shopping cart which has a foldable basket, was used. Future work on the validation and design for other shopping carts is left as a consideration for greater applicability to other grocery store branches.

The proposed design solution uses bulbs placed in a non-parallel fashion as a means to better illuminate each face of the cart. The bulbs are mounted onto 20 gauge sheet metal and held in place with fasteners as well as cutouts on the sheet metal that allow for a friction fit for the tombstone components. The bulbs used are 18, 24, or 36 inch T8 Germicidal light bulbs.

Fully assembled configuration of the final fixtures and respective subassemblies

Simulation Results

In order to quantitatively evaluate the proposed design a light simulation was performed on the different faces of the cart. The lighting analysis was performed using the CAD designs found in Figures 2a through d. The simulations were performed in the Lighting Analysis Software: TracePro and RayViz developed by Lambda Research. Simulations were performed with 1 million rays per bulb (a total of 93 million rays) corresponding to each face on the cart body. In order: Rear Face, Handle, Front Face, Side Face, Bottom Cart Face.

Analyzing Figures above reveals that the cart surfaces are reaching an irradiance value greater than 526 W/m2. The areas which are of some concern are the regions near the sides of the cart which are in the 500 W/m2 range, but for the most part the cart is surpassing the target of 500 W/m^2. In addition each simulation result, from each of the different cart faces, had an average irradiance ranging from 702.2 to 892 W/m2. These simulation results reveal promising results in terms of reaching the removal of COVID-19 pathogen from the cart surface.

Expiremental Results

In order to validate the simulation results, experimental trials were performed on the various cart faces using UV-c dosimeter cards. The cards are only a qualitative measure of UV dosage which has to be inferred by the naked eye. Dosimeter cards also have a range of 25, 50 and 100 mJ/cm^2 which is a fairly large range to be able to distinguish a specific UV dosage quantity. Therefore, there was a need to develop new MATLAB code to perform the necessary image analysis and obtain quantitative data values.

The numbers labeled in the image to the left of each scatter plot correspond to the test strip numbers which were evaluated. Note that some test strip numbers are missing from the plots because the test strips exceeded an exposure value greater than their maximum capacity. However, given the fact that they exceeded the maximum threshold is a good indication that those locations along the cart face were receiving the proper UV-c dosage. Analyzing both the front and rear faces, the test strip locations reveal that the average UV-c dosage is well above the standard value of 52.6 mJ/cm^2 used to evaluate industry grade UV lamps. Furthermore the target goal of 30 mJ/cm^2 was more than doubled. This means that our light fixture configuration is able to guarantee a log 5 Reduction of Small RNA viruses. There are a few spots like the corners which were anticipated to be on the low side based on the simulations, however, even these low spots were still within the acceptable threshold.

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