FAQs - Piezo Products
Some materials exhibit what is called the piezoelectric effect, which literally means that electric charge is generated when the material is pressed (or squeezed or stretched). The reverse is also true: an applied electric field will cause a change in dimensions of the piece of material. For a positive voltage applied in the z-direction, a solid rectangular piece will expand in one direction (z) and contract in the other two (x and y); if the voltage is reversed, the piece will contract in the z-direction and expand in the x- and y-directions. This is somewhat like thermal expansion and contraction, but since electric field is used instead of temperature, a quick reaction is achieved in response to commands easily generated with electronic circuits.
A “tuned” beam means that the natural frequency of your piezo matches the frequency of the environment its harvesting energy from or the drive signal you are exciting the piezo with. A tuned piezo will drastically outperform an untuned one in both energy harvesting and actuation applications. Tuning your piezo beam is achieved by adjusting the clamp position and/or the tip mass. Adding tip mass will reduce the resonant frequency. Lengthening the beam by moving the clamp location will also reduce the resonance. The opposite is true for both as well. See Section 4.3 for more information. To test what the resonance of your piezo pack is, simply connect the piezo terminals to an oscilloscope. If the piezo is “flicked” it will vibrate at its resonant/natural frequency.
The relevant material properties are included in Section 5 for modeling and simulation of our piezos. Refer to a particular product’s specifications to determine the geometry necessary for modeling. Layer thickness is also provided. Simple simulations and calculations can be made in MATLAB or a similar analysis software. More complex 3D analysis will likely need a Finite Element Analysis software package for piezoelectrics such as ANSYS, COMSOL, or others. Midé has used SolidWorks Simulation and NASTRAN to model piezos by manipulating the thermal expansion coefficient of the material. This works well but has its limitations in regard to poling direction.
For many purposes, the electrical behavior of an actuator can be approximated by that of a simple capacitor. When the actuator is driven with a sinusoidal voltage, the required current can be found from the equation: I=(2πf)CV where I is peak current in amperes, C is capacitance in farads, V is peak drive voltage and f is frequency in hertz. The capacitance of each product is listed. Note that at resonance the current draw can be as much as 20% to 50% higher depending on the configuration. For static/DC actuation the current draw is very low. The current draw can be calculated from the following equation: I=C dV/dt where I is peak current in amperes, C is capacitance in farads, dV/dt is rate of change of the voltage (V/s). To hold the position, once actuated, the piezo draws very little current. The only current draw needed is to compensate for the very low leakage currents, even in the case of very high loads. This is true even when suddenly disconnected from the electrical source, the charged piezo will slowly discharge the electrical energy and return to the zero position slowly.
Each product must be tuned to a specific frequency for optimal energy harvesting and for the most significant displacement and force output during actuation. This resonant frequency can be adjusted by changing the clamp location and/or changing how much tip mass is on the beam. Each of the products have a wide range of resonant frequencies from as low as 20 Hz up to 500 Hz. But all of these will operate at virtually any frequency in actuation. In energy harvesting it will need at least 2 Hz of motion to produce any electrical current; but it is virtually unlimited in regard to higher frequencies.
Midé created an easy to use product selector that, if provided a vibration frequency and amplitude (refer to Section 4.3.3), it calculates the expected power output, tip mass required, open circuit voltage, and peak to peak displacement. These calculated values are meant to only act as a guide in selecting your product and determining feasibility. All of these parameters are impacted by manufacturing tolerances, clamping, temperature, even the load applied to the power output. Once an estimate has been calculated, accurate data can only be confirmed with testing in the desired environment.
Yes, when a piezo beam is excited with a shock or impact event the beam will oscillate at its own resonant frequency but quickly (depends on the resonance but within about 2 seconds) dampen out. Midé recommends directly testing the power output for your given environment and clamp conditions but you can expect as much as 0.5 mJ of energy to be harvested from a single impact event if optimal conditions are met. The following figure provides a typical output from a shock event. One will notice that the initial output is quite large; but it quickly dampens out within a second or two. This dampening coefficient will depend on the harvester, tip mass, and clamp configuration.
Yes, but over a very long period of time. These harvesters will generate at most a few mA of current on the order of 10s of volts. For easy math, we’ll assume the harvester generates 5 mA at 20 volts, or 100 mW of continuous power. Most phones have about 2,500 mAh of storage capacity, similar to a AA battery. Assuming the phone or battery is operating at 5 volts, it will take 125 hours (over 5 days) to fully charge the battery or phone. Alternatively, you will need 125 piezo energy harvesters to charge this battery in one hour. Now these numbers used were very aggressive, where most applications only have a few milliwatts of power available. In these applications it will take several thousand hours to charge the device or battery. Piezo energy harvesters are better suited for applications that require very little energy such as periodic measurements in health monitoring applications for example. They are not well suited for charging large batteries.
The half power bandwidth defines the frequency range at which the output/displacement is greater than -3 dB or greater than the maximum output/displacement divided by root 2. The Q factor is the ratio of the resonant frequency to the half power bandwidth. The higher the Q factor, the narrower and sharper the peak is.
These first generation products were discontinued and replaced by the new PPA line. The PPA line improved upon performance, cost, and ease of use over the older products. Midé can still manufacture these products but this would involve a custom order as the materials are not stocked any longer. Therefore, the minimum order size would need to be on the order of $5,000 with a 3 month lead time.
Midé is developing a piezo driver that incorporates the waveform generation and voltage amplifier in one easy-to-use package. Until that product is finalized you must use a signal generator and amplifier. Midé has used amplifiers from http://piezodrive. com/ in the past before that are easy to use and cost effective.
The piezo’s output will be a relatively high voltage, low current AC signal that needs to be conditioned for use with most other electronics. Midé has partnered with Linear Technology who offers a number of commercially available chips and demonstration boards for energy harvesting applications. The following solutions are recommended:
- DC2042A – Energy Harvesting Multi-Source Demo board
- LTC3588-2 – Nanopower Energy Harvesting Power Supply (higher voltage)
- LTC3330 –Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Life Extender
- LTC3331 – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger
Midé does not offer an educational discount. The PPA-1001 is the most cost effective product we offer and we recommend this for those on a tight budget.
Selecting the right energy harvester is critical to maximizing performance. Midé understands that this selection process can be difficult so we wrote a blog post to help you. Please check it out!