Force Generation

In most applications, piezoactuators are used to produce displacement. If used in a restraint, they can be used to generate forces, e.g. for stamp- ing. Force generation is always coupled with a reduction in dis- placement. The maximumforce (blocked force) a piezoactuator can generate depends on its stiffness and maximum displacement. At maximum force generation, displacement drops to zero. Maximum force that can begenerated in an infinitely rigid restraint (infinite spring con- stant). where: Example Example What is the force generation ofa piezo actuator with nominal displacement of 30 µm and stiffness of 200 N/µm? Thepiezo actuator can produce amaximum force of 30 µm x 200 N/µm = 6000 N When force generation is maximum, dis- placement is zero and vice versa (see below for details). A piezo actuator is to be usedin a nano imprint application. At rest (zero position) the dis- tance between the piezo actua- tor tip and the material is 30microns (given by mechanicalsystem tolerances). A force of 500 N is required to emboss the material.Q: Can a 60 µm actuator with astiffness 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 actua-tor tip). In practice the force generation depends on the stiffness of the metal and the support. If the support were a soft material, with a stiffness of 10 N/µm, the piezo actuator could only generate a force of300 N onto the metal whenoperated at maximum drive voltage. If the support were stiff but the material to be embossed itself were very soft it would yield and the piezo actuator still could not gener- ate the required force. If boththe support and the metal werestiff enough, but the piezo actu- ator 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 liftinga 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 enoughforce to lift the wheels off theground. L
= max. nominal displace-ment without externalforce or restraint [m]k
T = piezo actuator stiffness[N/m] In actual applications thespring constant of the load can be larger or smaller than the piezo spring constant. Theforce generated by the piezoactuator is: (Equation 5) Effective force a piezo actuatorcan generate in a yielding restraint where:
Force generation vs. displacementof a piezo actuator (displacement 30 µm, stiffness 200 N/µm).Stiffness at various operating voltages. The points where the dashed lines (external spring curves) intersect the piezo actua- tor force/displacement curvesdetermine the force and displace-ment for a given setup with an external spring. The stiffer the external spring (flatter dashed line), the less the displacementand the greater the force gener-ated by the actuator. Maximum work can be done when the stiff- ness of the piezo actuator and external spring are identical L
= max. nominal displace-ment without externalforce or restraint [m]k
T = piezo actuator stiffness[N/m]k
S = stiffness of externalspring [N/m]
© PI 1998–2006. Subject to change without notice. This data sheet is superseded by any newrelease. The newest release is available for download at www.pi.ws. 06/10/30.0 46