Brian Mulcahey
Ronald L. Spangler
Active Control eXperts
Cambridge, Mass.
Vibration causes headaches for equipment designers. It creates problems that range from noisy industrial machines and chattering skis to fatigue failures on fighter aircraft. And it’s often tough to pinpoint sources of vibration.
The traditional solution is to mount machines on rubber pads or attach foams and mechanical dampers to problem areas. Many times this provides a quick, economical solution. But when it doesn’t, someone faces the unpleasant task of redesigning the system from the ground up or simply living with the problem.
A new class of vibration-control technology — based on piezoceramics — now provides another option. Piezo-based dampers are efficient and easily tuned to the exact frequency that needs damping. As a result, they often outperform conventional dampers in a more compact package.
PIEZOCERAMICS
Piezoceramics such as lead zirconium titanate eliminate vibration by converting physical motion into electrical charge. The basic phenomena was discovered over 100 years ago by French scientists Jacques and Pierre Curie but, until recently, piezo dampers remained a laboratory curiosity with few practical applications.
A major roadblock to widespread use was the inability to economically manufacture reliable devices. Raw piezoceramics are brittle and require special handling during assembly. Attaching wires to create a damping circuit proved to be problematic as well because solder adheres poorly to the thin metal electrode that covers the ceramic. Results were high manufacturing costs, low yields, and quality problems that simply made the devices impractical.
However, stepped up R&D efforts in recent years have paid off in an automated process that mechanically bonds electrical connections to ceramic wafers and then encapsulates the package in a plastic protective coating. The structurally rigid package stands up to normal handling, moderate impact loads, and exposure to harsh environments and chemicals. It carries a moderate price tag, too.
The biggest hurdle to widespread use now appears to be a lack of understanding of piezo damping among the design-engineering community. Most engineers are well versed in the workings of rubber dampers as well as electromechanical devices such as solenoids and motors. Few, however, know much about piezos. As they are successfully installed in products ranging from recreation equipment to industrial valves, that perception is expected to change.
PIEZO DAMPERS
Most piezo dampers are passive devices. The simplest is a piezo wafer electrically wired to a resistor. Called an RC damper (because piezoceramic acts as a capacitor) the piezo converts mechanical vibrations to electricity which the resistor dissipates as heat. RC dampers provide fairly broadband damping over about one decade of frequency — for instance from 10 to 100 Hz or 100 to 1,000 Hz. Decreasing resistance increases the center damping frequency.
For instance, two different models of ski contain the same standard piezo module, but each with a different resistor. One is tuned for the low frequencies of a recreational skier, the other for high-frequency vibrations generated by a racer.
The passive-resistive dampers are most effective when the circuit’s RC frequency (resistance × capacitance) matches the structure’s natural frequency. Tuned for maximum performance, passive-resistive dampers convert about 10% of mechanical-vibration energy to heat, an efficiency on par with other passive dampers.
A second passive-damping option, an RL damper, adds an inductor in series with the resistor. Tuning the frequency of the inductor- capacitor circuit to the structure’s resonant frequency directs more electrical energy to the resistor. This increases damping efficiency by roughly an order of magnitude over an RC damper. The drawback is a much narrower frequency band than with RC dampers, and they cost more.
Active damping provides a third option. Unlike passive devices, active systems use piezoceramics as an actuator instead of a damper. Applying a voltage stretches or compresses the material.
These are closed-loop systems in which a transducer senses mechanical vibrations and relays the data to control electronics. The controller, in turn, signals piezo actuators to generate motion of the same frequency and amplitude as the external vibration, but shifted in phase. In essence it generates antiforces that counteract and eliminate the vibration.
Active systems are highly efficient, offering up to 10 times more damping than RL systems, and they can target this capability at conceivably any number of resonant frequencies. The trade-off is a larger, morecomplicated package and higher price tag than passive systems. Currently, an external power source drives the piezo actuator, but work is progressing on self-contained circuits powered by piezo dampers.
COMPARING SYSTEMS
Application engineers generally recommend piezo dampers only after exhausting more-conventional options such as constrained- layer viscoelastics and vibrationabsorbing foams. For example, simple rubber isolators are inexpensive, widely available, require little design effort, and are quick and easy to install.
Piezo damping systems generally take longer to design and cost more. Therefore, even though they will likely improve performance, they are not cost-effective for relatively simple vibration-control applications. The exception is when conventional approaches cannot alleviate uncontrollable vibrations or incremental vibration control is absolutely critical.
For instance, viscoelastics do not work well across wide temperature ranges. Piezo dampers offer excellent performance from several hundred degrees Fahrenheit to well below zero.
Traditional damping materials cannot perform at low frequencies, while piezo dampers address broadband vibrations, including low frequencies. In fact, there are few limits in the type of vibrations piezos can handle, with frequency bandwidth from dc to the ultrasonic range above 20 kHz. Piezo dampers can also be tuned to target specific vibration modes or frequencies, something viscoelastics cannot do. This focuses damping capabilities where the problem is worst, usually at the first or second mode of vibration. Active piezo-damping systems handle multiple frequencies with high efficiency.
Reliability is another issue. Mechanical dampers fatigue over time with an accompanying loss in damping performance. Temperature extremes or chemical exposure can accelerate the decline. Because piezoceramic devices have no moving parts, they are unaffected by resonance that plagues standard mechanical dampers, and damping ability generally remains constant over the life of the device. Units routinely run for hundreds of millions of cycles without failure. However, because piezoceramic is brittle, even though packaged to increase fracture strength, recommended practice is to maintain strain levels below about 1,000 microstrain, or 0.1% strain.
Finally, unlike traditional materials, piezo dampers are structural components. Therefore, using the vibration damper as a built-in structural element — such as in the composite construction of a ski — helps reduce size and weight. Traditional dampers, on the other hand, are often mounted on the outside of a structure where they are vulnerable to damage and take up space.