231575 Piezo-Mechanics GB

1.3. Actuator characterization by strokeand forceLike any other “electrical motor”, a piezo actuator convertselectrical energy into motion and/or force what iscoupled to an external mechanism. This can be in thesimplest case a component, what shall be shifted fromone position to another one. More complex applicationsare dynamic applications like valve control or the setupof adaptive/smart structures where piezo-mechanicalelements are incorporated.In all these cases the interaction between the actuatorand the driven mechanism must be analyzed. Thismechanical interaction is ruled by the stiffnesses(= inverse compliances = spring constants) of the twosystem parts: “actuator” and “actuated mechanics” (seeSection 3.6.).Fig. 4: Schematic of a piezo actuated system:A piezo actuator interacts with a coupled mechanism,where the piezomechanical performance is defined bythe two interacting stiffnesses S piezo and S mech .To characterize the actuator performance two basicexperiments are carried out to determine actuator’sstroke and actuator’s force generation as functions of1.3.1. Voltage – stroke characteristicCondition:The coupled mechanics show stiffness S mech = 0For S mech = 0 a piezo stack shows maximum strokeNo force variation is generated (constant preload)1.3.2. Voltage – blocking force characteristicCondition:S mech = ∞ , actuator cannot expandFor S mech = ∞ , the piezo stack generates maximumchange of force: the blocking force.No stroke is generatedFig. 5:Schematic voltage/stroke diagram of a stack actuatorFig. 6:Schematic voltage/blocking force diagram for a piezostack.The force response shows remarkably lower hysteresisthan the stroke diagram (depends on preload conditions).6