Application Notes
Piezoelectrics are transducers that convert mechanical energy to electrical and vice versa. This makes the applications piezos serve quite vast! This article will answer some common questions and link to other user guides with more information.
Common Questions
| How much energy can I get with a piezoelectric energy harvester or generator? |
This depends heavily on your vibration environment (which is why we developed the Slam Stick originally). But in general a piezo energy harvester can generate on the order of a few to tens of milliwatts in ideal conditions (clean sine wave at the matching frequency of the resonance of the harvester). In practice though because most environments have broadband vibration and the resonance is constantly shifting, one can expect power to be measured in microwatts. |
| What is the frequency limit of piezoceramic sheet? |
There is no inherent frequency limit for a piezoceramic sheet. In practice the frequency limits of applications are usually determined by resonances associated with the shape and/or size of the transducer design. A typical 2.85" square, .0075" thick sheet of PSI-5A material has a thickness mode vibration in the neighborhood of 13 MHz and a planar dilatation mode at around 14 KHz. At ultrasonic frequencies large surface area parts draw considerable current and resistive heating of the electrodes becomes the limiting factor. |
| What is the highest voltage that I can drive a piezoceramic sheet to? |
For low frequency operation (0 to 5 KHz) a conservative recommendation for applied bi-polar voltage for a .0075" thick single sheet of PSI-5A ceramic is ±90 volts. Voltage applied in the poling direction only can be raised up to ~300 volts. Use caution! |
| How much mechanical power can I get out of one sheet? |
In theory, one standard PSI-5A sheet (1.5" x 2.5" x .0075") used as an "extender" can do .00035 joules of work on the outside world in a quasi-static cycle (i.e. a slowly executed sinusoidal cycle). When operated just under its first longitudinal resonance of 15 KHz, the theoretically available output power from the sheet would be around 5 watts. In practice it is difficult to collect more than 10% of this work. Resonant designs can be considerably more efficient. |
| How much electrical power can I get out of one piezo sheet in principle? |
Assuming that we stretch a PSI-5A (1.5" x 2.5" x .0075") sheet to ±500 microstrains quasistatically at a frequency just below its fundamental longitudinal resonance of 15 KHz, and that we collect 100% of the stored electrical energy at its height twice per cycle we would get approximately 9 watts of electrical power from the sheet. The mechanical energy input under these assumptions would be in excess of 100 watts. Resonant designs can be considerably more efficient.
However, the mechanical apparatus for achieving the above mentioned 15 KHz high strain excitation is not available, and there is no known electronic method for extracting 100% of the available energy. |
| How much electrical power can be extracted from a typical piezo bender element in practice? |
A "Double Quick Mount" bending element bolted to a rigid surface provides a convenient demonstration of a cantilever mount generator. Applying 80 gram force to its tip at a frequency of 60 Hz produces an open circuit voltage of 15V peak between its two electrical leads. When the leads are connected to a 8 Kohm resistive load, the output to the load is 5.3 Vrms, representing a power output of 3.6 mW. |
| Can piezo transducers be used as static and dynamic force sensors? |
Piezo transducers are not suitable for static force measurements because of charge leakage. They can be used effectively for transient force measurements lasting less than 0.1 second. |
| What is the expected fatigue life of piezoelectric material? |
The "fatigue life" is pretty difficult to estimate; although we've had a piezo fan running constantly here since 1982, no conclusive tests have been done. It would depend on mounting, voltages, etc. |
| How reliable is the packaging/lamination process? |
Our packaging process was specifically developed to improve reliability in normally brittle and fragile piezoelectric wafers. We have undergone very stringent reliability testing for our piezo fan development, here is a brief presentation on some of the tests completed. |
| Is a "spice model" available for piezo sensors? |
We do not have any spice models. As you probably have guessed, for each new thing the piezo is glued to, a new "AC source" characteristic arises. With so many various applications for piezo, we do not have the resources to comment on application-specific questions. |
| Can piezoceramic sheet be used to pick up vibrations in machinery? |
Yes. Almost any size or shape of piezoceramic element will give off a measurable signal when fastened somewhere on machinery. See 'strain gages'. |
| Can piezoceramic sheet be used as a strain gage? |
Yes. Piezoceramic is one of the most sensitive strain gage technologies existent, and it is the only one which is self-powered. |
| How repeatable are the voltage outputs from a piezo strain gage? |
Outputs from piezoceramics which are 'following' surface vibrations are generally very repeatable and stable. If the sensor is initially calibrated it can be trusted for years of accurate service. |
| How repeatable is the motion of a piezo actuator? |
A piezoceramic actuator which is cyclically driven at a constant cycle time between the same two points will perfectly repeat its path every time. However, if the cycle time or either endpoint is changed, hysteresis and creep effects cause non-repeatable motions. |
| What are the effects of temperature on piezoceramic transducers? |
Temperature changes cause a voltage to appear across the electrodes of any piezo transducer. This is due to the pyroelectric properties of piezoceramic. Temperature also affects every property of piezoceramics (elastic, dielectric and piezoelectric coupling). There is no general trend. Each dependence must be looked up or better yet measured in the context of your experiment. |
| What is the resonant frequency of a piezoceramic sheet? |
There is no one 'resonance'. There are many resonances. The number of them and their location in the frequency spectrum depend on the shape and thickness of the part. For a flat sheet as shipped, three obvious resonances are the ones associated with the length, width, and thickness of the sheet. |
| Can I drive a piezo transducer with a 'square wave'? |
The answer is application dependent. If the square wave voltage is low (i.e., less than 30 V), then the answer is usually yes. If the square wave voltage is higher, there is a good chance for shockwave, damage, cracking, reduced life, or other failures. Careful control of the square wave rise time/fall time is the solution. |
| How do I tune my piezo? |
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. |
| What is the frequency range the piezo can operate in? |
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. |
| Can my piezo energy harvester harvest energy from shock/impact events? |
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. We recommend 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. 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. |
| Can my piezo energy harvester charge a phone or battery? |
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. |
