4.    NOTES ON TECHNICAL DATA

4.1.  Voltage ranges, polarity, depolarization
The piezoelectric stacks described in the following data sheet are designed for 3 basic voltage ranges:
150 V (multilayer-types)
500 V (discrete stacking)
1000V (discrete stacking)

The maximum voltage has to be applied with the right polarity, indicated for the actuators. The ceramic is thereby operated with internal electrical field strength of 1-2 kV/mm. Counter voltages (showing opposite polarity than indicated) can be applied to 15-20% of the stated max. voltage, e.g. approx. -20 V for +150 V multilayers. The limit is defined by the onset of depolarization (inactivation) of the PZT material.

4.2.  Electrical Capacitance

From the design and operating principles of piezoelectric stacks it is obvious, that the electrical behavior of such actuators resembles simple electrical capacitors (under non resonant driving conditions). The indicated data for the electrical capacitance's of the stacks are small signal values for low driving voltages/electrical fields measured at room temperature. The capacitative nature of piezoelectric actuators imply that electrical power is needed only for changing an actuator's position and no power is needed to hold a position (in contrast to magnetic actuators, where a permanent current is needed to sustain a position). To estimate the required power or currents to supply a dynamically operated actuator, an increase of the nominal capacitance by 50% has to be accepted. The reasons are due to variations of the PZT material characteristics, elevated temperature and large signal effects (application of large electrical fields).

NOTICE: the dielectric constant of high strain PZT (used for actuators) is sensitive to temperature variations and is increased up to 50% at 90oC (compared to room temperature value). The electrical resistance of piezoelectric stacks is very high and ranges from of MegaOhm (for large volume multilayers) to GigaOhm (for high voltage stacks). An expanded (charged) actuator can be disconnected from the supply and can hold its position for a long time due to the very slow self discharging of the actuators by leakage currents (time constant hours to days).

4.3   Maximum Expansion

The maximum expansion is defined as the stroke for a voltage step 0V to maximum voltage at room temperature. Exceeding the max. voltage rating will potentially show onset of saturation in actuator's expansion characteristic and reduction of lifetime. The tolerances in the expansion rates are due to variation of materials composition from batch to batch.

4.4   Maximum Load, Mechanical prestress

Fres = m ^ I / ( ^ t )2
m = mass load
^ I = max. travel
^ t = minimum reset time

4.5.  Stiffness

Like any other solid body, piezoceramics show elasticity and therefore stiffness of the piezoelectric stacks can be defined. The resonance frequency of a PZT actuator under mass load depends on stiffness and can be estimated according the simple mass/spring-model. Furthermore, the loss of travel/force generation of an activated actuator can be estimated, when in operation against a varying counterforce. The stated data for stiffness are approximate, and were measured at a prestress 10% of the maximum force load.

4.6.  Resonance Frequency

The stated resonance frequencies are defined for actuators with one side fixed/without external mass/for small electrical signals. The frequencies refer to the basic axial oscillation mode of the actuator. Note, that an actuator shows different oscillation modes and the resonance frequencies of these modes may be lower than those for the axial mode (e.g. radial mode axial mode for a short actuator with large diameter). Additional mass load applied to the actuator lowers the overall axial resonance frequency. Therefore the resonance frequency of an internally prestressed actuator with casing is lower than that of the bare stack due to the mass of the prestress mechanism. Standard stacks show low resonance gain (high damping) due to the PZT ceramic used and the compound layer) structure. Resonance has to be kept in mind during the installation of feedback control systems. The onset of phaseshifting between the exciting signal and the mechanical response of the actuator limits the operating frequency range. Note: not only the actuator shows resonance, but also any coupled mechanics. Such parasitic resonance's can be excited by the piezoelectric actuator with high efficiency (large gain factor of the mechanics), even when the actuator itself is not operating in resonance. In many applications, resonance's are unwanted and the operating frequency of these systems should be well below resonance. However, some applications exists, where resonance is used directly for high power conversion efficiency e.g. to excite large oscillation travels (ultrasonic etc.). Resonantly working actuators and transducers show a different design compared with the standard actuators.

4.7.   Thermal Effects

Multilayer -stacks: -3x10-61/oC

Discrete stacks: 7x10-61/oC

4.8.   General Material Data
(approx. values for standard actuator PZT-ceramic/room temperature)
rel. dielectric constant: approx. 4000
piezoel. Charge constant d33: approx. 500x10-12m/V
coercive field strength: approx. 600 V/mm
Young's modulus: 6x10-10 N/m2
Density: 7.5x103kg/m3
Mech.gain factor: 50-100
Thermal conductivity: approx. 1W/oK . m
Spec. heat: approx. 380 Ws/kg . oK
Curie-temperature:
Low voltage multilayers: 150 oC to 200oC
High voltage types: 250oC to 350oC
Depolarizing pressure (see sec.4.1./4.4.): approx. 4000N/cm2
Compressive strength: 6x104N/cm2
Failure strain: approx. 1%

NOTICE: The electrical and piezoelectrical properties depend on the special type of PZT ceramic. Discrete high voltage stacks can be optimized with respect to these parameters for distinct applications.

 

| 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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