Download Itenberg_Piezoelectrics_USB_handheld_HW5

Section 1 Group Contributions
For this assignment Bryan Wilson showed interest to be part of the group to work on Piezoelectric.
However, due to miscommunication the group could not complete the assignment in a timely matter. To
regroup, at the conclusion of Spring Break the group met at the library to get the project back in order.
Research was done together and each member wrote a few pages of the above report.
Section 2 Report
1. Introduction to Piezolectrics
Many materials exhibit a property where the introduction of stress results in an electric potential
difference. This is called the piezoelectric effect. Certain ceramics exhibit the piezoelectric effect. A
necessary characteristic of piezoelectric ceramics is that they are formed as a single crystal. A random
orientation of molecules cancels out the effect. The amount of energy harnessed from a piezoelectric
material depends on several factors, such as the molecular structure and geometry of the material, the
piezoelectric properties, and the geometry of the forces applied.
Piezoelectric materials are different from other ceramics because their dielectric constant decreases
with an applied mechanical load and Young’s Modulus, the stiffness of a material, changes with an
applied voltage. The change in the dielectric constant is of interest. The equation for the dielectric
constant is below
r FREE
(1 – k²) =
r CLAMPED
The dielectric constant is reduced when a load is applied. “FREE” is the state when the material has not
changed dimension and “CLAMPED” is the state when the material is under a load.
The plates of a normal capacitor become charged with the application of voltage, however with a
piezoelectric capacitor the plates become charged when stress is applied. This phenomenon is known as
the Direct Piezoelectric Effect. The guiding equation for this effect is below.
𝐷 = 𝑑𝜎
D = Electric Displacement
d = Piezoelectric Modulus
σ = Strain applied.
The piezoelectric modulus is the ratio of charge density to applied mechanical strain.
One necessary property of piezoelectric is that the material must be formed as a single crystal. This
poses a problem as ceramics have a multi-crystalline structure. The impurities of the grains cancel out
the piezoelectric effect. In order to align the grains a process called poling takes place. During poling the
material is introduced to a DC voltage which aligns the grains of the materials. Not all grains align; the
amount of grains that align depends on aspects such as voltage, temperature and time. During poling
the material changes shape due to the applied voltage, it increases in size between the electrodes and
decreases in parallel. The poles during poling determine the poles of the piezoelectric once a force is
applied. The geometry of the poling and the stress is worth researching for this project in order to
maximize the energy harnessed from the footstep. Below is the illustration of this.
It can be seen that based on the poling direction and the direction of stress the piezoelectric crystal
produces and electric potential with different poles.
It is important to note that when piezoelectric materials are conducting electricity they are also
changing shape. This is what makes it possible to create Piezoelectric Benders. To maximize the power
produced from the piezoelectric materials; two piezoelectric materials are stacked on top of one
another, with the negative poll touching. This means that when a force is applied to a piezoelectric and
electricity is produced, one layer will be getting longer and thinner while the other layer is getting
shorter and thicker. This creates a bending moment. Illustration below.
http://www.eetimes.com/electronics-news/4197064/Piezoelectric-Technology-A-Primer
2. Generation of Piezoelectric Energy from Bi-Pedal Locomotion
The generation of power through the deformation of a piezoelectric material presents many challenges,
and these challenges become increasingly difficult as the frequency of generation decreases, and this
severly impacts our expected project performance, as the frequency of steps may be less than one hertz
(per foot). To address this concern, extensive research was performed to try and identify new avenues
of success. A study performed by MIT has shown that the heel of the shoe offers the best source of
piezoelectric generation, as the quick impact of the heel during the beginning of a new step offers more
energy than the slow curving of the sole. The study also outlines graphically the type of power
developed by an average weight person walking at a slightly brisk pace (1 step per second).
From the results of the MIT experiment it was shown that the “heel strike” offers greater than 5
times the power produced by the sole of the shoe. While the power generated by was small, other
experiments have sought ways of reconfiguring the design to allow for increased usable production from
the same stepping force. Experiments performed at Virginia Tech showed that by using smaller
piezoelectric crystals arranged in parallel the time required to charge equivalently sized batteries
decreased dramatically. This experiment did not use a real shoe, as with the MIT counterpart, but
instead developed a consistent, easily repeatable, mechanical model. While the results of this model
may not be seen quite as dramatically in the shoe itself it is reasonable to assume that by using
paralleled piezoelectric crystals more usable energy can be derived from a step, and that the addition of
further paralleled crystals would yield further decreased battery charging times. As this is the purpose of
the project in question (to provide USB charging power), it stands to reason that a grid design using
many paralleled piezoelectric materials would be the most efficient route of deriving useful energy from
the human step.
3. Power Conversion to USB Standard
Assuming that more energy can be generated than the initial experiments the next obstacle is to
rectify, filter, and possibly store the energy being developed so that a USB device can utilize it. As per
the USB 2.0 standard found at USB.org one USB port must be capable of supplying 500 milli-Amps at
precisely 5 Volts. From the MIT experiments it was seen that voltages up to 200 volts were produced
with a 500 kilo-Ohm load. Additionally the bipolar production of the piezoelectric system must be
considered as the current produced is not direct. As the heel first strikes occurs deformation of the
crystalline material produces a voltage across the material which will always be in the same direction
relative to the circuit (assuming the type of step is consistent). However, as the “heel strike” completes,
the material returns to its original shape and dimensions, and generates a voltage opposite, and greatly
less than the initial. This phenomenon is found in the original MIT experiment and can be seen on the
next page.
The voltage spike at
approximately 0.9 seconds
represents the heel first
striking the surface below the
shoe. The next spike is the
rolling of the heel, and the
return of the material to its
original shape. The MIT
researchers chose to view this
information as the absolute
value of the voltage, but from
the perspective of power generation it is also useful to view the bipolar voltage production. It should
also be noted that the while the initial spike is of greater amplitude, the area under underneath the
initial and secondary generations is approximately equal.
4. Efficiency of Energy Conversion for a Piezoelectric Power Harvesting System
This paper studies the energy conversion efficiency for a rectified piezoelectric power harvester. An
analytical model is proposed, and an expression of efficiency is derived under steady-state operation.
In addition, the relationship among the conversion efficiency, electrically induced damping and ac–dc
power output is established explicitly.
The researchers show that the conversion efficiency is dependent on the frequency ratio, the
normalized resistance and, in particular, the ratio of the electromechanical coupling coefficient to the
mechanical damping ratio. In general, the conversion efficiency can be improved with a larger coupling
coefficient and smaller damping.
Their results can be applied to the investigation of charging a battery from a vibrating piezoelectric
harvester. It is shown that direct charging will result in a low efficiency of energy conversion, since the
equivalent impedance of the battery may not match that of the optimal load in most situations.
Another researcher has developed an adaptive electric circuit to optimize the energy transfer from the
piezoelectric element to the stored device. It is based on the principle of load impedance adaptation by
tuning the load impedance to obtain a higher power flow.
Consider an energy conversion device which includes a vibrating piezoelectric structure together with an
energy storage system. If the modal density of such a device is widely separated and the structure is
vibrating at around its resonance frequency, we may model the power generator as a mass + spring +
damper + piezo structure, as schematically shown in figure 1.
The power generator considered here is connected to a storage circuit system, as illustrated in figure 1.
Since the electrochemical battery needs a stabilized dc voltage while a vibrating piezoelectric element
generates an ac voltage, this requires a suitable circuit to ensure the electric compatibility. Typically an
ac–dc rectifier followed by a filtering capacitance Ce is added to smooth the dc voltage, as shown in
figure 1. A controller placed between the rectifier output and the battery is included to regulate the
output voltage. Figure 2 is a simplified energy harvesting circuit commonly adopted for design analysis.
It can be used to estimate an upper bound of the real power that the piezoelectric generator is able to
deliver at a given excitation. Note that the regulation circuit and battery are replaced with an equivalent
resistor r and Vc is the rectified voltage across it.
The rectifying bridge shown in figure 2 is assumed to be perfect here. Thus, it is open circuited if the
piezo voltage |Vp| is smaller than the rectified voltage Vc. As a result, the current flowing into the
circuit vanishes, and this implies Vp(t) varies proportionally with respect to u(t). On the other hand,
when |Vp| reaches Vc, the bridge conducts and the piezo voltage is kept equal to the rectified voltage,
i.e., |Vp|= Vc. Finally, the conduction in the rectifier diodes is blocked again when the absolute value of
the piezo voltage |Vp(t)| starts decreasing.
The efficiency of mechanical to electrical energy conversion is a fundamental parameter in order to
compare energy harvesters of various sizes and with different vibration inputs. If the generator is
excited by a harmonic forcing function, the energy conversion efficiency is commonly defined as the
time-averaged power ratio.
It is clear from the equations that the conversion efficiency depends on the normalized resistance r, the
applied frequency ratio Ω and, in particular, the relative magnitudes of the electromechanical coupling
coefficient k2e and the mechanical damping ratio ζm.
These researchers have established the relation among the energy conversion efficiency, electrically
induced damping and power transfer for a rectified piezoelectric power harvester. An analytical model
is proposed, and an exact formula for the conversion efficiency is derived under the steady-state
operation. It is shown that the efficiency depends on the normalized resistance r, the frequency ratio Ω
and, in particular, the relative magnitudes of the electromechanical coupling coefficient k2e and the
mechanical damping ratio ζm. In general, high energy conversion efficiency can be achieved with large
k2e/ζm.
The induced damping added to the system due to the removal of mechanical energy from the vibrating
structure is obtained based on the derived formula of conversion efficiency. It is shown that the
maximum conversion efficiency corresponds to the maximum induced electric damping as well as the
optimal power transfer in the case of weak electromechanical coupling. This result is consistent with
that observed by Lesieutre.
However, unlike Lesieutre, a new finding shows that the optimal electric load maximizing the conversion
efficiency and induced electric damping is very different from maximizing the harvested power in
strongly coupled electromechanical systems. This gives completely distinct optimization schemes, since
the harvested power has two identical peaks evaluated at two different electric loads at the respective
operating points, provided that k2e/ζm >> 1, while it has only one peak otherwise.
http://homepage.ntu.edu.tw/~yichung/power_harvesting-jmm-2006.pdf
5. Multilayer Piezoelectric Energy Generation
Piezoelectric material has usually been considered as a high voltage and low current source of
electricity. When charging a thin film battery or capacitor for an electrical device, a larger current is
needed to shorten the charging time. In order to increase the current in the piezoelectric energy
harvesting, multilayer ceramics were fabricated. N-layer multilayer ceramics decreased the voltage but
increased the current N times. The impedance of the multilayer ceramics are matched to 1 kΩ which is
similar to the impedance of general electrical devices, so the multilayer piezoelectric generator could be
directly employed for electrical devices without the additional electrical circuit in order to improve
efficiency.
While the standard electronic circuitry requires a high current at a low voltage, piezoelectric materials
generally produce from several tens to hundreds of volts at micro scale current. In addition, energy
storage devices for electronics operation also need the larger current to shorten the storage charging
time. Since normal battery voltage capacity is about 3V, further higher voltage is unnecessary to
recharge. So it is necessary to reduce the voltage and increase the current of the piezoelectric
generator.
In this study, to increase the generating current, they made multilayer ceramics and characterized the
energy harvesting properties.
These researchers were able to show how multilayered ceramic piezoelectrics could generate a more
significant amount of current with the voltage level being much lower. Compared with the bulk
ceramics, thin multilayer ceramics generated a large current by lowering the voltage and the electric
current was increased with proportion to the number of layers. The impedance of the piezoelectric
generator was decreased by increasing the number of layers. As a result, the impedance of the
multilayer piezoelectric generator comes to be similar to that of general electronic devices. So the
multilayer energy harvester could be directly used for electrical devices without the additional electrical
circuit. It is promising to use multilayer ceramics to charge the battery and to power an electronic
device.
http://www.springerlink.com/content/9v23732061q6847t/
Section 3.
1. There are various difficulties in developing a piezoelectric generator for hand-held devices. The
main constraint is that the power output is not enough to charge most USB devices. Also
sustainability of the product would also be in question, due to the fact that it is always under
mechanical stress. Eventually the material will break down and produce even less
power.However, manufacturing the product will not be difficult as there will only be 1 circuit
and a few other parts. The product will also be environmentally friendly as it is using the
person’s energy output instead of burning coal.
2. The professional and ethical responsibilities of the group are to make this product produce the
energy marketed and make it long lasting. The product will undergo extensive testing. Testing
the power output and how it can deal with mechanical stress over time, will be two of the main
properties tested since they insure a product that is promised.
Sites
http://www.eetimes.com/electronics-news/4197064/Piezoelectric-Technology-A-Primer
http://www.springerlink.com/content/9v23732061q6847t
http://homepage.ntu.edu.tw/~yichung/power_harvesting-jmm-2006.pdf
Articles –
“Generated power characterization of piezoelectrics with electromechanical coupling coefficient and
quality factor” J. H. Cho, R. F. Richards, D. F. Bahr, and C. D. Richard
“Energy Scavenging with Shoe-Mounted Piezoelectrics” Shenck, N.S. Paradiso, J.A.
Organizations –
The main organization we have involved in this research is IEEE. Most of our articles come from there as
they publish research from the most prestige technology schools in the country.
Self Analysis
Power harnessing through piezoelectric devices can be very useful for various applications other than
charging a handheld device (since the amount of power required may not be feasible). However, by
maximizing power generated, other devices could be sufficiently charged. Such devices could be given
to people out in remote locations with no easy access to electricity. This could include adventurers,
military and rescue personnel. Their location as well as the state of their health could be easily
monitored and sent back to a computer. Even though it was disappointing to find that not enough
power could be generated for a hand-held device, the group is optimistic about various applications of
piezoelectric.