Week 4

     During the class meeting, we showed Dr. Gordon the current components we have for our project. The prototype was not in its final stage yet, as the brass had not been ordered yet and the 3D printed leg braces needed to be redone. Dr. Gordon had two main things to talk about: he wanted us to talk with Dr. Hekman (the vibrations professor) to see if enough force will actually be transmitted axially along the leg/bone as we are hoping. He also had a possible redesign that he wanted us to consider. The image below shows the proposed redesign.

The circle on the left side represents the motor, which would be connected by links to a "track" on the leg (shown on the right side of the picture.)

     The redesign would allow linear motion along the leg, ensuring that (at least most of) the force would be transmitted in the desired directions, rather than in all directions due to the rotational manner. Three concerns that were immediately brought up were that this system would likely be bigger/bulkier than the current design, that excess heat would likely be generated between the "track"and "slider," and that there would be a higher likelihood of material/structural failure (given our budget and machining capabilities). However, Dr. Gordon encouraged us to spend some time this week thinking over the two designs and coming up with a concrete decision by Week 5. 

     An Excel mathematical model was created for the redesign, with some of the reference equations being drawn from Wikipedia. These were used to calculate the acceleration with respect to the crank angle. This can be seen in the images below:

Equations used (credit: Wikipedia). 

The results from the mathematical model. 

     The CAD files for the leg braces were also updated and sent to Josh for 3D printing. The leg braces remain largely the same to the ones created last week, except they are scaled slightly larger for better fit and also have the motor mount on the opposite side now (so that the motor is mounted to the outer part of the ankle, rather than the instep). 

     Isaac and Jake created a MasterCAM file for milling the brass once it arrived. (It arrived late Thursday evening.) We set up a time with Dr. Hekman to help us mill the brass weight on Friday morning, as we are not permitted to use the CNC machine on our own. 

Setting up the CNC machine to mill out our piece from stock brass. 

Dr. Hekman helping us use the CNC machine.

The completed brass part that will be used in the motor system. This design is far less likely to shear off, as the screw and mass set did. 

     We could not run the full tests as the leg braces had not finished printing yet, but we tested out the new brass weight with the motor and casing. We ran the motor at slow speeds, but found that the rotations per minute were very close to the values we got from previous testing with the screw and offset mass (both tests were measured with a tachometer). 

The brass part mounted to the motor via screws. The brass part can spin freely without hitting the case.

The lid now fits onto the motor casing, as we removed the bulky bracket that was used to hold the screw and mass arrangement. 

The other side of the case. The metal washer is where the casing will be attached to the leg braces once they are printed.

The motor spinning with the brass piece on it (shown without the cover for observation purposes). 

    We also looked into what types of hand stitching are the strongest, as we are planning on sewing loops into the elastic bands and attaching them to the harness. Through a general internet search, it was found that back stitching is the strongest hand stitching method. None of us are very experienced at sewing, so we practiced sewing a back stitch of extra pieces of the elastic band. Based on our preliminary attempts, we agreed that the stitch seemed to be strong enough for what we are intending.  

Example of a back stitch (credit: http://sewing.wikia.com/wiki/Back_Stitch). 

     Our 3D printed parts finished printing over the weekend, so we were able to fully assemble our prototype for the first time. We also did some preliminary testing of the motor system while Isaac was wearing it on his leg and standing on Dr. Kim's force plate. The force plate is able to record force in the x,y, and z directions.  The data could not be exported, however, so further testing will have to be done once Dr. Kim is available to help us. Despite not being able to export the data and get a concrete value, it appeared as though enough force was being generated in the z-direction to continue on with this design . These tests were performed at low speeds, so we are confident that with further testing and refining, we can reach our desired goal of force transmitted axially along the leg. Because of this, we have decided to stick with our current design rather than pursuing the redesign.

Isaac wearing the system on his leg while standing on the force plate.

Close-up view of the motor system. The final version will likely use Velcro straps or a ratchet system rather than duct tape to hold the system to the leg. Additionally, future versions will secure the wires so that they are not dangling. 

This is a screenshot of the program screen during testing with the force plate. The force in the z-direction is currently shown. Although it is difficult to tell, each division is 150 N. The peak-to-peak measurement appears to be about half of this, which would equate to ~75 N.