San José State UniversityCollege of Engineering/Electrical Engineering

EE174: Analog Peripheral for Embedded Systems

Section 01, Spring 2018

LAB4 Energy Harvesting (EH)

Goals:The goal of the experiment is to generate power from piezoelectric sensor and store in a supercapacitor.

Required Equipment:

Breadboard and Components: Piezoelectric sensor, supercapacitor 2.7V, 1MΩ resistor, FF

References:https://www.allaboutcircuits.com/technical-articles/how-why-of-energy-harvesting-for-low-power-applications/

https:// www.mouser.com /applications/ rf_energy_harvesting /

http:// www.instructables.com /id/Free-Energy-From-Thin-Air/

The Building Block of Energy HarvestingEnergy harvesting is the capture and conversion of small amounts of readily available energy in the environment into usable electrical energy. The electrical energy is conditioned for either direct use or accumulated and stored for later use.

Transducer/harvester: This is the energy harvester that collects and converts the energy from the source into electrical energy. Typical transducers include photovoltaic for light, thermoelectric for heat, inductive for magnetic, RF for radio frequency, and piezoelectric for vibrations/kinetic energy.

Energy storage: Such as a battery or super capacitor. Power management: This conditions the electrical energy into a suitable form for the

application. Typical conditioners include regulators and complex control circuits that can manage the power, based on power needs and the available power.

Energy Harvesting Technologies:Harvesting Solar EnergySmall solar cells are used in industrial and consumer applications such as satellites, portable power supplies, street lights, toys, calculators, and more. These utilize a small photovoltaic cell which converts light to electrical energy. For indoor applications, light is usually not very strong and typical intensity is about 10 µW/cm².

Harvesting Thermal EnergyThermoelectric energy harvesters rely on the Seebeck effect in which voltage is produced by the temperature difference at the junction of two dissimilar conductors or semiconductors. The

energy harvesting system consists of a thermoelectric generator (TEG) made up of an array of thermocouples that are connected in series to a common source of heat. Typical sources include water heaters, an engine, the back of a solar panel, the space between a power component such as a transistor and its heat sink, etc. The amount of energy depends on the temperature difference, as well as the physical size of the TEG.

Harvesting RF EnergyAn RF power receiving antenna collects the RF energy signal and convert RF energy into DC

power. Harvesting Kinetic EnergyPiezoelectric transducers produce electricity when subjected to kinetic energy from vibrations, movements, and sounds such as those from heat waves or motor bearing noise from aircraft wings and other sources. The transducer converts the kinetic energy from vibrations into an AC output voltage which is then rectified, regulated, and stored in a thin film battery or a super capacitor.

AC Voltage SourcePiezo sensors are unique because they produce an alternating current (AC) voltage when stressed, converting mechanical energy to electrical. If you hooked an oscilloscope up to a piezo sensor, you might see waveforms below when the sensor shakes.The Minisense 100 from Measurement Specialties is a low-cost cantilever-type vibration sensor loaded by a mass to offer high sensitivity at low frequencies. Useful for detecting vibration and ‘tap’ inputs from a user. A small AC and large voltage (up to +/-90V) is created when the film moves back and forth. A simple resistor should get the voltage down to ADC levels. Can also be used for impact sensing or a flexible switch.https://www.sparkfun.com/datasheets/Sensors/Flex/MiniSense_100.pdf

The signals above were generated by simply inserting a large, weighted piezo into a breadboard and flicking it a few times. Note the voltage spikes are reaching almost +90V and -40V. Signals at that level have the potential to permanently damage a microcontroller’s analog-to-digital converter (ADC) pins.To dampen those voltage spikes, we have a few simple tricks up our sleeve. The easiest fix is to load the piezo with a large resistor. By placing a 1MΩ resistor in parallel with the sensor, for example, we can drop the voltage spikes down to safer levels.

More complex piezo damping circuits might include zener diodes to clamp the voltage or op amps to buffer a signal, but this simple resistor-loading circuit is a good place to start.

Example CircuitUsing the 1MΩ load resistor dampening method described above, here’s a simple example circuit demonstrating how to hook up the vibration sensor:

Now, you can create oscilloscope measurements of your own! The graph helps demonstrate the piezo sensor’s voltage spikes and ringing. Try flicking, shaking, or stomping the table to see how those movements affect the measurements of the piezo sensor.

Conclusions