Energy Management System for Prosthetic Limb Work Day
Date: 10.15.18
Time: 1:00 – 3:30 PM
Location: Logan HS
Goal: Research piezo and electromechanical solutions and begin to look at ordering components to experiment with.
Students: Alex, Seth, Austin and Thomas
We talked to a tech from Piezo.com. It was a very enlightening conversation. Here is a summary of the conversation:
1. He recommended buying the cheap piezo speaker elements (we have about 12 of them now) and experiment bending them back and forth to see what we can get out of them.
2. Use JB Weld or an epoxy to attach them to a 1/16" or 1/8" metal plate that can bend or use springs in the arch of the foot to get it to flex. (Not sure how that would be mounted to the artificial limb. We would have to drill small holes to attach.)
3. Do not wire them in parallel (even though that would seem sensible). Wire them in series with each one having a separate rectifier or diode. Using a full bridge rectifier would be the best option here. One rectifier is enough as everything can tie into that.
4. The output voltage is proportional to the number of times you bend the element.
5. If you go to the link below you will find a recommendation he made for a harvester that is somewhat more developed and easier to mount. The kids would have to be careful with this as it does not bend like a playing card and is designed to be mounted to a surface. He felt the wear and tear under the foot would be ok though.
http://piezo.com/prodproto4EHkit.html
6. He also said, “Don’t go off of the output specs noted on the web page.” These can vary.
7. The kit costs $660 which is within our budget. If we needed just the bender and not the circuit (because we can design our own circuit) that price is cut in half.
Alex-Worked on a plate for connecting the piezo components. Learned that the flex of the foot can be used as a power generation point for the piezo. The Piezo element creates a high but short jump in voltage (of 5-10 volts), as discovered by using the oscilloscope. The element jumps between positive and negative voltage when flexed. It was discovered that the voltage reached the highest point when the lever on the element was higher (having it’s own wires supporting it). The element also jumps from negative voltage to positive voltage first depending on what direction it is flexed. Piezo elements should also be wired in series and NOT parallel to generate power if using two on the same circuit.
Seth-Worked on electromechanical component that would fit in the shoe. He also found a magnet and went to Menards with Thomas and Austin to look for materials (PVC pipe .4” inside diameter and unsheathed wire)
Thomas and Austin worked on software drawing for lower leg tube.
This was a research meeting to continue learning about the hardware options for our project. Alex Magnuson continued to work on looking into piezo hardware.
The electromechanical team including Thomas Kujak and Seth Peterson worked on design options. Thomas was working on creating a casing to hold the electronic components and Seth tried to find the magnets that would be inside of the casing. They then went to Menards and continued to search for parts.
Austin Research Notes:
I did some research on the Raspberry Pi Zero W and found out that it would require a digital to analog converter in order to measure voltage as the Zero W does not have any analog pins in it. It would take around 3.3 to 5v in order to power the chip(MCP3208) alone in order to do the conversion. We would need to do some testing to see if it would draw too much power or try to find an alternative. An idea would be to cut out a circuit board to put the chip into and use the board to supply power and receive the inputs via the Zero W. This would also add more size to the system and mounting on/in the leg would be more difficult. Some testing and more research would be needed to find out the best solution for the voltage monitor.
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| Alex checking piezo outputs with an o-scope. |
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| Students working on the problem. |
