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Mechanical Considerations for Dynamic Operation of PZTs
Dynamic ForcesEvery time the Piezo drive voltage changes, the piezo element changes its Dimensionss (if not blocked). Due to the inertia of the Piezo mass (plus any additional mass), a rapid change will generate a force (pushing or pulling) acting on the piezo. The maximum force is equal to the blocked force described by:
Fmax » ± kT*DL0 (4-8)
Maximum force available to accelerate the piezo mass plus any additional mass.
where
DL0 = max. nominal displacement without external force or restraint [m]
kT = Piezo actuator stiffness [N/m]
Tensile forces must be compensated for by a mechanical preload (inside the actuator or external) in order to prevent damage to the ceramics. Preload should be around 20 % of the compressive load limit, with soft preload springs soft compared to the Piezo stiffness (1/10 or less).
In sinusoidal operation with frequency f and the amplitude DL/2, peak forces can be expressed as
Fdyn = ±4p2*meff*(DL/2)*f2 (4-9)
Dynamic forces on a Piezo in sinusoidal operation with frequency f.
where
Fdyn = dynamic force [N]
meff .= effective mass [kg]
DL = peak to peak displacement [m]
f = frequency [Hz]
The maximum permissible forces must be considered when choosing an operating frequency.
Example:
Dynamic forces at 1000 Hz, 2 µm peak-peak, 1 kg load are approximately ± 40 N.
Note:
A guiding system (e.g. diaphragm type) is recommended when heavy loads or large mechanical parts (compared to the piezo actuator diameter) are moved dynamically. Without a guiding system there is a potential for tilt oscillations and other non-axial forces that may damage the Piezo ceramics.
Recommended guilding for large masses.
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