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Mechanical Considerations
It must be understood that a piezo actuator can only generate considerable force if it is directly coupled (no slack!) to an element which is stiff when compared to the Piezo.
Force Generation
In most applications, piezo actuators are used to produce displacement. If used in a restraint, they can generate forces. Force generation is always coupled with a reduction in displacement. The maximum force (blocked force) a piezo actuator can generate depends on its stiffness and maximum displacement. click here for "Displacement with External Forces"
Fmax » kT*DLO (4-3)
Maximum force that can be generated in an infinitely rigid restraint (infinite spring constant). At maximum force generation, displacement is zero.
where
DL0 = max. nominal displacement without external force or restraint [m]
kT = Piezo actuator stiffness [N/m]
In actual applications the load spring constant can be larger or smaller than the Piezo spring constant. The force Fmax eff generated by the Piezo is:
Fmax eff » kT*DL0 (1-kT/(kT+kS)) (4-4)
Effective force a piezo actuator can generate in a yielding restraint
where
DL0= displacement (without external force or restraint) [m]
kT = Piezo actuator stiffness [N/m]
ks = spring stiffness [N/m].
Example:
Force generation of P-845.20 (click here for "Piezo Actuators" section). The Piezo can produce a maximum force of 30 mm * 200 N/mm = 6000 N. When force generation is maximum, displacement is zero. At full displacement no force can be generated (see Fig. 4.23/1 for details).
Force Generation vs. Displacement of a P-845.20 LVPZT actuator at
various operating voltages. The points where the dashed lines (external spring curves)
intersect the Piezo force/displacement curves determine the force and displacement
for a given setup with an external spring. Maximum work can be produced when the stiffness
of the Piezo actuator and external spring are idenical.
Example:
A piezo actuator is to be used in a metal sheet embossing application. At rest (zero position) the distance between the Piezo tip and the sheet is 30 microns (given by mechanical system tolerances). A force of 500 N is required to emboss the metal.
Q: Can a 60 µm actuator with a stiffness of 100 N/µm be used?
A: Under ideal conditions this actuator can generate a force of 30 x 100 N = 3000 N (30 microns are lost motion due to the distance between the sheet and the Piezo tip). In reality the force generation depends on the stiffness of the metal and the support. If the support was a soft material, with a stiffness of 10 N/µm the Piezo could only generate a force of 300 N onto the metal when operated at maximum drive voltage. If the support was stiff but the metal itself was very soft (gold, aluminum, etc.) it would yield and the piezo actuator still could not generate the required force. If both the support and the material were stiff enough, but the Piezo mount was too soft, the force generated by the Piezo would push the actuator away from the material to be embossed. The situation is similar to lifting a car with a jack. If the ground (or the car's body) is too soft, the jack will run out of travel before it generates enough force to lift the wheels off the ground.
Example: High Force Piezo Ceramic Actuators: Click Here
New Piezo Tutorial: Click Here
