3. OPERATING PERFORMANCE OF PIEZOELECTRIC STACKS

The main applications of piezoelectric actuators include:

3.1.  Positioning by piezoelectric stacks

When a piezoelectric stack is cycled within a distinct voltage range, the displacement follows the well known hysteresis curve as shown in fig. 5 nonlinearity and hysteresis are a consequence of the ferroelectric properties of PZT materials.

fig5
fig.5
Displacement/voltage diagram of a typical piezoelectric stack for different voltage levels (within Umax). U ret returnpoint of voltage for the individual cycle

The hysteresis diagrams for standard actuators are very similar for low voltage and high voltage types and are therefore not a relevant criterion for selection between these stacktypes for a distinct application. The displacement curves are valid for constant force load, meaning that an applied force does not vary during piezoelectric stack's travel. Even for very high (but static) load forces, the full extension is achieved.

3.2.   Force generation by piezoelectric actuators, Interaction with externally applied forces.
A lot of applications show varying forces acting on the piezoelectric stacks during operation e.g.

Under all these conditions, the piezoelectric actuator reacts on the varying force with elastic compression or dilatation according its stiffness and the variation of force (Hook's law). piezoelectric stacks can be used as force generators where they are operated against a proper counterforce e.g. an elastic clamping mechanism. The produced force depends on the driving voltage and the stiffness relation between actuator and clamping mechanism. The maximum force (blocking force) which can be produced by an actuator is achieved for infinite stiff clamping at maximum voltage. The blocking force can also be defined as the required force to press back a fully elongated actuator to its zero-position. Some general relations between the geometrical dimensions of a stack and its properties are stated below

fig6

fig.6

Schematic representation of force/travel relations of 3PZT actuators
Actuator 1: showing travel I1,blocking force Fb(1)
Actuator 2: double length, same crossection as actuator 1: double travel, same blocking force.
Actuator 3: same length, double crosssection as actuator 1: same travel, double blocking force

EXAMPLE: an actuator is operated versus an elastic clamping, showing the same stiffness as the actuator: the achieved travel is half the maximum travel, the generated (additional) force is half the blocking force.

3.3.  Piezoelectric actuators as Force Sensors and Electrogenerators

Besides the actuator effect, PZT stacks shows normal piezoelectricity, meaning, that a mechanical force generates a voltage signal (electrical charge). Piezoelectric actuators can therefore be used for force sensing, which is very effective due to the high currents involved. Any arrangement incorporating a piezoelectric actuator can be checked for its mechanical resonances or mechanical response by combining the actuator with a signal analyzer and exciting the mechanics by an external mechanical knock/shock. In the widest consequence, PZT actuators can also be used as generators to convert mechanical power to electrical power.

 

| Classification of piezoelectric stacks | Mounting of Piezoelectric actuators |
| Operating Performance of Piezoelectric actuators|
Notes on Technical Data | Selection Guide |
| Custom Designed Actuators | Safety Instructions |

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