bird PIEZO SYSTEMS, INC.
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FREQUENTLY ASKED QUESTIONS
Physics
Questions

Handling & Preparation
Questions

Application
Questions

Piezo Technology
Questions


PHYSICS ANSWERS
Why is there piezoelectricity? Because some atomic lattice structures have as an essential unit (or "cell") a cubic or rhomboid cage made of atoms, and this cage holds a single semi-mobile ion which has several stable quantum position states inside the cell. The ion's post ion state can be caused to shift by either deforming the cage (applied strain) or by applying and electric field. The coupling between the central ion and the cage provides the basis for transformation of mechanical strain to internal electric field shifts and vice versa.

What is electric field? An electric field is always associated with the presence of electric charges. It fills the space around the charge and is the mechanism of interaction between charges. A test particle with small known charge (Q) placed near a charge concentration will experience an accelerating force (F) due to the field. The value of the electric field (E) at that location is the ratio F/Q (a vector).

What is strain? When a solid object like a rod of length (L) is stretched to a new length (L + delta L), the strain in the rod is defined as the ratio (Δ L)/(L). This is a dimensionless measure of stretching or compression often stated as "inches per inch", "millimeters per meter", or "microns per meter (microstrain)" for convenience of visualization.

What is elastic modulus
(or Young's modulus)?
A material property of all elastic solids, Young's modulus (Y) is used to describe "stiffness" of materials. When rod or plate of cross section (A) and length (L) is pulled with force (F) resulting in an elongation (Δ L), the Young's modulus can be computed as follows:

Y = (L/A)*(F/ΔL)

In piezo applications Y is frequently used to estimate the equivalent spring constant of a rod or a plate of material that is in contact with a piezo actuator (F/ΔL).

What is tensile strength? Tensile strength is the stress (measured in Newtons/m2 or psi) at which a sample of solid material will break from tension.

What is poling/depoling in piezoceramic materials? The piezoelectric property of ceramics does not arise simply from its chemical composition. In addition to having the proper formulation the piezoceramics must be subjected to a high electric field for a short period of time to force the randomly oriented micro-dipoles into alignment. This alignment by application of high voltage is called "poling". At a later time, if an electric field is applied in the opposite direction it exerts a "dislodging stress" on the micro-dipoles. Low level applied fields result in no permanent change in the polarization (it bounces back upon removal). Medium fields result in partial degradation of the polarization (with partial loss of properties). High applied fields result in repolarization in the opposite direction.

What is damping? 'Damping' is the term used for the general tendency of vibrating materials or structures to lose some elastic energy to internal heating or external friction.

Can piezoceramic actuators be used at cryogenic temperatures? Yes. All piezo actuators continue to function right on down to zero degrees Kelvin. This may seem counter-intuitive at first; however, you must remember that the basis for the piezoelectric effect is inter-atomic electric fields, and electric fields are not affected by temperature at all. Quantitatively, the piezo coupling of most common piezoceramics does decrease as temperature drops. At liquid helium temperatures, the motion of most materials drops to about one-seventh of that measured at room temperature.

What is the pyroelectric effect? The tendency of some materials to exhibit a change in internal electrical polarization state in response to a change in temperature. If the materials are equipped with electrodes on two surfaces, a voltage will arise between the electrodes in response to temperature shifts.

HANDLING & PREPARATION ANSWERS
How do I cut up a sheet of piezoceramic into the size I want? Ceramic is best cut using a special diamond saw. Small prototype parts can be cut from piezoceramic sheet stock by using a razor blade and a straight edge to score the piezo surface and then making a controlled break. Even with practice this method does not yield straight-sided parts or repeatable cuts. Use at your own risk.

How do I bond/attach piezoceramic sheet to a structure like an aluminum beam? Cyanoacrylate and two part epoxies have proven useful in many applications. We suggest that you contact a manufacturer of these materials and explain your specific bonding requirements, including:
  1. The metal surfaces to be joined
  2. Temperature of operation
  3. Any unusual shear stress requirements
Note: University of Missouri at Rolla has a very helpful section on this.

Can I just use 'superglue'? Good quality temporary bonds may be made with cyanoacrylate (e.g. "super glue"). An added benefit of cyanoacrylate bonds is that the bond easily achieves electrical contact. The length of time the bond will last will be application dependent, from seconds to years. For a short time the performance of the part is very close to that achieved using the best bonds, which makes it useful for exploratory work.

Are piezoceramic sheets fragile and easy to break? Yes, they are very fragile! Single sheet piezoceramic should always be handled with great care. Dropping them almost always results in a shattered part. When two piezo sheets are bonded together with a metal shim between them, as most standard bending elements are, they become rugged enough to be dropped without being damaged.

How do I make electrical contact to the side of the piezoceramic that is bonded down? The most common method is to make a conductive bond between a metal substrate and the piezo part. Then one electrical lead is attached to the substrate, and one to the outward face of the piezoceramic sheet. In cases where a conductive bond is not possible (i.e. when the substrate is glass or plastic), a wire must be soldered to the "down" side of the ceramic at some location and a corresponding 'dish', 'cutout', or 'overhang' must be used to allow room for the wire when bonding the piezo sheet to the substrate.

How do I attach wire leads to the piezoceramic? All of the PSI piezoceramic parts come with a thin (~3000 Angstrom units) metallic electrode already on the ceramic. Wire leads can be soldered (use ordinary 60/40 resin core solder) anywhere on the electrode to suit the application/experiment. Most PSI ceramics have thin nickel electrodes and require the use of an additional liquid flux for uniform results. Our Solder/Flux Kit was designed to make this task much easier.

How can I access the center shim? Support the bender underneath; using a milling machine, take .001 - .002" passes to remove ceramic and expose center shim. Water can be used to lubricate the cutting.

How do I hook up a bender element so it works? This depends on how the two piezoceramic plates are polarized. If 5A-type, .0075" thick plates are poled for series operation (i.e. poling arrows pointing in opposite directions) then a wire is attached to each of the outer electrodes of the bender. ±180 volts is then applied between the wires. If the plates are poled for parallel operation (i.e. poling arrows pointing in the same direction) then the two outer electrodes are shorted together forming one lead, and a wire is attached to the center metal shim forming the second lead. ±90 volts may be applied between these leads. (See Introduction to Piezo Transducers) Poling and Wiring.

How can I remove the electrode? You can chemically etch, sandblast, sandpaper, or laser ablate the electrode.

How far can I stretch a sheet before it breaks? A sheet can be stretched to a strain of approximately 500 microstrain (micrometers per meter) in regular use. Higher surface strains can be achieved, but the statistics of survival get worse. Proceed with caution.

How far can I bend a bimorph before it breaks? If a T220-type bender (.020" thick) is cantilevered to a distance of one inch, the tip can be pushed approximately 0.055 inch before the bender is heard to "snap".

If a sheet loses some properties, can it be repoled? Yes. For 5H material, an electric field of 40 - 60 volts/mil will restore nearly all lost polarization. For 5A, use 50 - 100 volts/mil.

APPLICATION ANSWERS
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.

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.

PIEZO TECHNOLOGY ANSWERS
Where in everyday life do I find piezo devices? All 'watch beepers' are piezoceramic audio transducers, most battery operated smoke detector alarms, fish finders, some cigarette lighters, many gas grill igniters.

How are piezoceramics used in vibration cancellation? Two piezoceramic sheets can be bonded directly to the surface of a structure (such as a strut, or beam) close to one another at a site where unwanted bending occurs. One is used to sense surface strain. The output from the strain sensor is fed into a "smart box" (which can be anything from a simple op-amp to an elaborate Digital Signal Processing computer) which in turn controls a power amplifier that drives the other piezoceramic sheet. Ideally the resulting mechanical contractions of the second piezo sheet inject a vibration into the structure which is equal and opposite of the initially detected one so that the net vibration is canceled.

Will piezo technology replace magnetic technology? No. Fundamentally, magnetic technology is based on a force which arises 'at a distance', without physical contact. Piezo technology is based on physical contact and elastic coupling. On an application by application basis one is usually better than the other. Take solenoid actuators as an example. Piezo actuators can be designed to replace almost any solenoid but they always come out bulkier and often heavier so it is unlikely that full scale replacement will ever occur. On the other hand, they always take much less power to operate; so in any application where power consumption is an issue, piezo actuators are preferred.


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