Understanding Piezoelectric Accelerometer Resonance Frequency
Listed on your piezoelectric accelerometer data sheets for both IEPE and charge output types is a specification listed as "resonance frequency." What does this term mean?
Resonance frequency is the point in frequency within an accelerometer's frequency response where maximum sensitivity is outputted. It is specified in units of hertz (Hz). Typical piezoelectric accelerometers exhibit a resonance frequency above 20 kHz, though some show as high as 90 kHz. As the name implies, resonance frequency is the result of the natural resonance of the mechanical structure of the accelerometer itself. For completeness, this term should be phrased "mounted resonance frequency." Certainly, if the accelerometer resonance was measured in terms of "free space", it would be different than if mounted to a structure. However, this is an impractical application for a piezoelectric accelerometer, thus the designation "mounted" is added. It is not a design goal of manufacturers to produce an accelerometer that has a resonance frequency within a certain tolerance. Instead, resonance frequency is specified as a minimum, ensuring that this resonance point will not occur below minimum. As such, resonance frequency is a rough "figure-of-merit" that sets the upper limit of accelerometer frequency bandwidth.
For piezoelectric accelerometers whose mechanical structure is almost completely undamped, the amplitude of the resonant peak can be quite high, resulting in a sensitivity many times higher than that of the specified reference sensitivity (i.e. at 100 Hz). As such, any vibration at or near the frequency of the resonant peak will be highly amplified, resulting in distorted measurements and corrupted data. A design goal of manufacturers, then, is to push the resonance frequency point as high as possible in an accelerometer's structure, with intent that the point shall be well beyond any vibration frequencies of the user's measurement application. The user also must ensure that no vibration frequency components are at or near the resonance frequency point.
It is important to note that resonance frequency is specified assuming ideal accelerometer mounting conditions. Just as a manufacturer can influence the resonance frequency point with the accelerometer's mechanical structure itself, so too can external structural factors (which the user controls). As mechanical resonance characteristics are generally dependent upon material stiffness and damping, it is critical that the accelerometer be mounted as correctly and as stiffly as possible. Improper mounting techniques can decrease stiffness and increase damping, causing the resonant peak frequency to decrease and the width of the resonant rise to increase (i.e. mechanical Q is lowered). If allowed to degrade enough, the ultimate result shall negatively impact accelerometer frequency response. Proper accelerometer mounting techniques are beyond the scope of this article, though much literature can be found on this subject.