F u n d a m entals of Piezoelectric Act u ators

Force Generation Example Example 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 alwayscoupled with a reduction in dis-placement. The maximum force (blocked force) a piezo actuator can generate depends on its stiffness and maximum displacement (see also p. 4-23). At maximum force generation, displacement drops to zero.(Equation 3)Maximum force that can begenerated in an infinitely rigidrestraint (infinite spring con- stant). What is the force generation ofa piezo actuator with nominaldisplacement of 30 µm andstiffness of 200 N/µm? The piezo actuator can produce a maximum force of 30 µm x 200 N/µm = 6000 N When force generation is maximum, dis- placement is zero and vice versa (see Fig. 19 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 30 microns (given by mechanical system 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 motiondue 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 asoft material, with a stiffness of10 N/µm, the piezo actuator could only generate a force of 300 N onto the metal when operated at maximum drive voltage. If the support were stiff but the material to be embossed itself were very softit would yield and the piezoactuator still could not gener- ate the required force. If both the support and the metal were stiff enough, but the piezo actu- ator mount was too soft, the force generated by the piezowould push the actuator awayfrom 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 enough force to lift the wheels off the ground. Where: 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 4)Effective force a piezo actuatorcan generate in a yielding restraint Where:
Fig. 19. Force generation vs. dis-placement of a piezo actuator (dis- placement 30 µm, stiffness 200 N/µm). Stiffness at various operat-ing voltages. The points where thedashed lines (external spring curves) intersect the piezo actua- tor force/displacement curves determine the force and displace- ment for a given setup with anexternal spring. The stiffer theexternal spring (flatter dashed line), the less the displacement and the greater the force gener-ated by the actuator. Maximumwork 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 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 4-22