Progress
This week I did literature review for the energy harvesting part of the project. I made a number of interesting notes from these papers, with the most important ones being:
Piezoelectric energy harvester impedance matching using a piezoelectric transformer
Though sourcing a piezoelectric transformer is infeasible, this paper did give some insights on some of the challenges to getting optimal power transfer from piezoelectric elements. The major concern was the fact that at low frequencies (the typical frequencies of human motion), the impedance of a piezoelectric element is largely capacitive. The inductance required to match this capacitance is typically infeasibly large to achieve on a wearable device, so instead mostly resistive matching is done.
A Coaxial Wrist-Worn Energy Harvester for Self-Powered Internet of Things Sensors
This paper proposed the use of a motor and weight for energy harvesting. Surprisingly, it achieved power generation of ~3mW average at a normal walking pace, which far exceeds the capability of the state of the art in piezoelectric generators (as far as I could find). Originally we discarded the idea of using a motor for energy harvesting due to mechanical complexity, but after some discussion we realized that we may be able to use a standard gimbal motor and 3D print a planetary gearbox for the top. We will be experimenting with both methods to see which one is more effective.
Low-Power Design of a Self-powered Piezoelectric Energy Harvesting System With Maximum Power Point Tracking
This paper presented the most thorough design for the voltage regulation and conversion that I’ve found so far, and goes into great detail on the design considerations for implementing a circuit for low-power use. I expect that my design will be primarily based on the techniques discussed here.
Pacing
I am roughly on pace right now.
Planning
Next week, I will start the more detailed design of the energy harvesting circuit, and do some hands-on testing of piezoelectric elements I have laying around. My goals are:
- Create preliminary design for Boost/Buck Impedance Matching
- Test Boost/Buck Impedance Matching on Breadboard