Freely Reaped Electromagnetic Energy (FREE) 2016-2017

Only a small amount of RF energy is captured by the intended receiver. The objective of this research is to make a device that captures the remaining ambient energy and converts it back into electricity for low power applications, such as powering a sensor module.

Problem Statement
Electromagnetic radiation is used in many different devices around us. In order to transmit a wave one must first spend energy to create that wave. Many understand that energy is used in the creation of an electromagnetic wave, but fewer realize that a lot of that energy is still contained in that wave. When we receive a signal, some of the energy in the transmitted wave is collected by the receiving device, but what is not caught by the antenna just fades out into the background. The issue is that there is energy all around us that could be harvested, yet it fades away, untapped.

Solution
This team is tasked with researching and implementing a physical device that will capture electromagnetic radiation and store it.

Specifications
In order to get the best potential results out of the project, it was split into the following three phases. Each phase progresses both the complexity and capability in a relatively linear manner.

Phase 1: Wall Antenna
The goal of this phase is purely proof of concept. The design holds little limitation on antenna size, and the only device using the power is an energy storage device to collect the harvested energy.

Phase 2: WiFi Tester
This next stage constricts two of the specifications from Phase 1. First, the apparatus will have to be handheld, so there will be limitations on the size of the antenna(s). The other part will be that there will be a microcontroller and an LED to indicate the relative power that will be received.

Phase 3: Remote Sensor
This is the final phase of the project as it is both the most complex and has the highest power requirements. Continuing from Phase 2, the microcontroller will be maintained, while the LED will be replaced by a sensor. The microcontroller will periodically read the data from the sensor and store it in memory to be collected at a later date. Additional work will also be done in order to find radio frequencies to harvest from that will be accessible at the remote location.

Design Development
The design development is best split into the different focuses in this project.

Antenna Design
The goal of the antenna is that it captures the most energy while still maintaining a manageable size, form factor, and complexity. Currently the focus is set between Dipole Antennas, an array of dipole antennas and a PIFA Antennas. Dipole antennas benefits lies in their simplicity, all while maintaining a fairly uniform radiation pattern. The PIFA antenna is on the list because that is what is used in the wireless routers on campus, and when one matches the antennas used in transmit and receive, there is less power lost.

Power Rectifier Design
One of the big challenges with the rectifier design is the frequency of the signal coming into the circuitry. As the frequency increases, this means that each of the components in the rectifier must be that much faster to react in order to convert the signal. Along with this the signal has the potential of having a very small magnitude voltage. Traditional rectifiers use diodes in order to maintain a DC output, whereas with this project, research has been looking into CMOS technology in order to avoid the voltage drops that are universal to diodes.

MATLAB Simulation
Antenna design is very complicated and math intensive. MATLAB has been chosen to simulate the different types of antennas that have been considered for the project in order to decide which would be the best. One of the greatest benefits of MATLAB is it's toolboxes. These provide models from which a user can define a number of specifications and MATLAB will simulate the many aspects of that antenna's propagation and reception.



LT Spice Simulation
The rectifier that is being proposed for this design will have to operate within very specific parameters. In order to best meet these parameters and collect sufficient data on it's capabilities, it was decided that it would be best to model the circuit in LT Spice. By using LT Spice, a very realistic representation of the rectifiers output can be calculated relative to the input.

Radio Signal Testing
One issue that occurs with radio waves is that as their frequency increases, the difficulty of measuring the waves also increases. One piece of hardware that will be used to help with this is an SDR (Software Defined Radio). With this it will be possible to generate a known signal and then calculate the strength at a receiving antenna, all with one device.

Funding
With this project being a student proposed, the students were required to secure funding in order to do the project.

UISC Grant
Near the beginning of the fall semester of 2016 the team drafted and submitted a proposal asking $1700 to design and build phase one of this project. Ultimately this project was one of three groups that was awarded the grant.

Project Learning
One of the biggest goals of this project is to further the knowledge of each person taking part in it. The core concept of there being energy in waves is nothing new to the world, but beginning to think about all the radiation that is lost is something that many are oblivious to. Each of the core focuses in this project are pushed to the limits of feasibility. Even if at the end of the project, their device does not function, it is guaranteed that each of the students will have gained a greatly increased understanding of their focus. Through this project, plenty will be learned on the topics of efficiency, but also in the cutting edge of energy harvesting.

Final Design


The final result of the project did not progress through all of the stages as hoped, but the core project has shown success.

Antenna Design
The final prototype for expo was decided to utilize a 2 dipole array. This proved to be the most bang for the buck without drastically increasing price or size of the prototype.

Filtering
Additional to the concept of impedance matching, it was decided that it may be beneficial to filter the output of the antennas as to only receive WiFi frequencies to avoid any possible destructive interference. Ultimately the filter decided upon is a 3rd order band-pass that is implemented in PCB traces.

Rectification
The final design of the rectification circuit was simplified to a half wave rectifier created with a diode and capacitor. The choice to deviate from a full wave and the transistor was based on the discovery of some fast acting diodes with a low threshold voltage.

DC/DC Converter
Since the rectified voltage will be much less than any of the potential target applications, a low power and low voltage input DC/DC converter was added. This converter can take voltages as small as 200mV and effectively boost them to the needed 3.3V applications we are targeting.

Application
Upon further research it was quickly discovered the the pulsing LED suggested in phase 2 would consume much more power than the sensors and memory used in phase 3, so it was bypassed. The final design has 3 temperature sensors that will send a reading to a microcontroller through SPI and then send that reading to FRAM for long term nonvolatile storage.

Team Bios
This project is special to this team due to this being one of the few student proposed projects of this year. Since the spring of 2016, this team has been developing this project from the kernel of an idea that it started as, to the full project that it is now.