Description
The second generation of our home fertility tracking device at Inito was planned with home replaceable coin cells. This project was targeted at creating a user replaceable design for a battery cover system in a pre-established design space and Industrial Design boundary.
Inito Monitor - Battery End
Contributions
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Problem definition and first requirements
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Ideation and Concept generation
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Prototyping and concept selection
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Concept reviews, iteration and final design
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Creation of User Manual
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Field Survey with actual users
Ideation
Considering that the device architecture including the optical circuit as well as the electronic circuit board had been fixed, the problem consisted of creating an enclosure to comply with the industrial design of the device.
It was supposed to house two CR2032 lithium cells to power the device. The structure of the PCB commanded an anti-parallel stacked arrangement of the two cells.
PCB Design - Battery End
Based on the available requirements, three primary principles were identified for the architecture:
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Direct insertion of the cells onto the contacts and a simple 'window' enclosure : This would prove difficult for recovery and replacement of cells at the user end and risked damage to the device through wrongful insertion or abuse
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A cell crate to house and load the cells, covered by a simple lid
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An integrated lid and cell rack
Prototyping and Concepts Review
The latter 2 ideas of integrated and separate crates were explored through detailed CAD, prototypes were created using 3D printing.
These were tested in house with different users with and without context of the application.
Independent Cell Rack
Side Entry of Cells
Front Entry of Cells
Integrated Cell Rack and Cover
While the independent rack would be the simplest to manufacture, it caused a critical increase in the number of steps needed to assemble the system. The entry of the independent rack into the device cavity was also unguided, hence risking misalignment and possible failure.
The integrated rack design was found to be more useful. It was possible to include a guide rib to ensure poka-yoke while reassembling the cover.
Keeping in mind design for injection molding, the front entry variant of the integrated rack model was chosen as it would allow for a simpler tool without compromise on functionality.
Final Design
The front entry design variant of the integrated rack and cover was chosen. The cells would be assembled into the rack first as a push-to-snap fit.
The rack would then be guided into the device using the guide rib and locked into place through snaps of its own. Snap fits were chosen as they are the simplest compliant mechanism which allow for reliable and robust mating, specially in plastic parts with low fatigue life requirements.
Base
(Device End, Option 1)
Battery Cover (Option 1)
Base
(Device End, Option 2)
Battery Cover (Option 2)
The concept was further finetuned for the ideal mating and alignment arrangement. While option 1 shown above had a sleek locating rib and generous locking snaps for the cover and base, option 2 utilized a larger area for the locating rib created by inverting the locking snaps.
This would also allow the cover - base locking snaps to be much smaller and concealed within the top ledge of the battery cover, out of sight and interruption to the user.
Option 2 was chosen as the final design for the system. It was finetuned for injection molding and tooled.
Final Design
(Device End)
Final Design
(Battery Cover)
Verification and User Study
The device batteries are supposed to last a regular use of 12-14 months based on regular use calculations. Considering the indicated change-over frequency and the use of coin CR2032 cells, it was considered appropriate to design for cell replacement at watch shops.
Considering this, a brief user manual was created to elucidate the process of removal and replacement of cells into the device, also explaining the critical cell orientations.
This manual was then tested on the field with 25+ individual watch shops and was found to succeed unanimously. While some technicians read the manual in advance, others followed through the process with each step.
We also extended the study to untrained users to evaluate the feasibility of at-home replacement and received encouraging results.
The design was tooled for manufacturing and rolled out in the new generation device.
