Friday, August 14, 2015

Final Posting

There were two main objectives this summer:
1 - to design a pressure sensing circuit/housing
2 - to reduce the form, weight and profile of a power transfer mechanism for the PPAFO.

The circuit design and testing went very quickly early on in the project.  The most challenging part was designing the housing to work around the limitations (poor tolerances/shrinkage/expansion/rough surface finish) of a the Cura Lulzbot Mini 3D printer we were using.  After a few iterations adjusting the initial dimensions the two halves of the housing fit into each other and around the circuit board.

A second challenge was getting the push to connect fitting to seal around the barbed fitting of a component we were using.  Eventually, wrapping silicon tape around the barbed fitting ended up solving this problem.

The second objective took longer to complete.  First, gear size was optimized.  THe optimization was a challenge in itself.  Because of an inverser relationship between the expected weight and expected stress in the gear system, gear geometry themselves had to be designed to minimize stresses.  In the end we settled on 16 diametral pitch gears with a 35o pressure angle for their high tooth strength.

Material selection was another challenge detailed in the last post.  AR500 steel was the best solution given all the project requirements (cost, preformance, ease of manufacture) but not the best solution for performance alone.

Some parts were outsourced to the UIUC machine shop, others machined ourselves, and others produced via SLA additive manufacturing.  Below is the final system assembled (minus linear actuators & orthotic footbed)
Two nuts were on backorder from McMaster for several weeks and were never delivered.  This prevented attachment of the actuators. Once the nuts arrive, they need to be milled to .25" thickness and mounted to the gear mechanism with the actuators.  Once the acutators are attached, testing can commence.  Strength of the SLA gearbox can be evalueated, and, if need be, the gearbox can be milled from an aluminum block.

In the future, work with wire EDM tolerances and post machining surface treatment (polishing, electropolishing, etc.) might be necessary for gear system with better longevity -- the better the tolerances for gear meshing, the more even the load sharing between gear teeth, thus the longer the gear life.

Additional future work might involve designing a printed circuit board for use with surface mount components on the pressure sensing circuit to further reduce size and weight of the whole board and housing assembly.


I am so grateful to the lab for teaching me throughout the summer and allowing me to work on the orthosis.  After a summer learning about machine design and implementing a design myself, I am amazed at the tolerances to which so many components are mass produced on a daily basis.  It is baffling to think about the amount of work that goes into any engineered object -- not only on the individual components but on the effort it took over generations to accumulate the immense body of engineering knowledge which allows the components to be built in the first place.  This summer I was allowed to reiterate a design that has been in the works for several years.  I hope I have the chance to do it again.

Thanks to Ziming Wang and Elizabeth Hsiao-Wecksler for helping me through this project.


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