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3. Practical aspects of dynamically operated Piezoelectric actuators

3.1. Preloading, reset mechanisms

Piezoceramic is sensitive to tensile stress, it shows a damage strain of only 1%o.

Note, that this tensile stress can be created externally and also internally by dynamic operation. This fact is easily seen in Fig 4/sec. 2.8., where the application of an electric pulse leads to overshooting of the actuator relative to the steady state position. This overshooting can cause tensile stress and thereby damage to the actuator when the relevant forces are not compensated by other means.

To prevent damage by tensile forces the following strategies are commonly applied:

  • passive preloading/reset at actuators

This technique is mostly applied to stack actuators:

An elastic spring compresses the Piezoelectric stack with a defined force shown in fig. 5a, b. A preloaded stack is less sensitive to externally applied tensile stress for several reasons, i.e. a reduction in stacklength is achieved by the preload force.

fig5a
Fig. 5a: Mirror tilter with passive prestress/reset

fig5b
Fig. 5b:Linear stackactuator with passive prestress/reset

A real tensile stress is acting on the ceramic only, when the external force lengthens the stack beyond the original (loadfree) state.
Furthermore, the elastic counterforce slows down the moving mass in the overshoot phase during dynamic operation. So, the applied preload force can be chosen according the

fig6a.JPG (16896 bytes)
Fig. 6a:Mirror filter with active (push-pull) reset

fig6b
Fig. 6b: Linear push-pull arrangement of Piezoelectric stacks for active reset

simple mass acceleration law to accommodate the accelerated masses within the desired short rise/fall-times. The standard preloading VS of US EuroTek, Inc. actuators cover a wide range of applications. It is possible to apply higher preload forces, which can be supplied on request or can be applied externally (see "Piezoelectric actuators").

active reset (push-pull mode, antagonistic configuration)

A more sophisticated reset mechanism for compensating dynamic forces is the arrangement with two complementary working actuators shown in fig. 6a, b. The advantages include a symmetric force balance for both directions of motion, and higher resonance frequencies compared with passive preloading.

3.2. Selfheating
Another aspect of dynamically operating Piezoelectric actuators is their selfheating. Due to the ferroelectric nature of PZT ceramics, the electrical operating power transferred to the actuator is partially dissipated as heat. For example an actuator PSt 150/5/15 with full amplitude operation heats up to the operating temperature limit at about 600 Hz. Higher temperatures will shorten an actuators lifetime. A further increase of frequency therefore requires cooling or an equivalent reduction of amplitude (see sec. 2.5).
Simple surface cooling results in limited success for large volume actuators, because PZT ceramics have poor thermal conductivity. Furthermore measuring the actuator's temperature on its surface does not reflect the internal conditions. A good parameter for checking the volume temperature is the temperature dependence of the electrical capacitance of the actuator leading to a shift of the current balance.

fig7
Fig. 7: Temperature dependence of the electrical capacitance of a typical Piezoelectric actuator (relative to capacitance at roomtemperature)

3.3. Vibration control, acoustical noise
Every dynamic excitation of a Piezoelectric actuator attached to a mechanical structure acts back on this structure. Pulsed or oscillating actuators generate vibrations in the mechanical structure. In case of a resonance a large amplitude response can emerge even for small excitation levels, which can interfere with the regular function of the structure. Therefore, dynamically operated structure have to be designed for sufficiently large resonant frequencies, and include sufficient damping to avoid these unwanted side effects at the driving frequency.
Vibration suppression can be done in passive or active ways. An example for active pulse compensation is shown in fig. 8, where a counteracting Piezoelectric stack compensates for the repulse of the original stack, e.g. shifting a mirror. Generally, Piezoelectric elements are powerful tools for vibration control, both for generating vibrations (shakers) and for cancellation (active vibration isolation and damping). Active cornpensation can be done in feedback controlled systems, where a transducer detects an incoming vibration, and excites an antivibration with proper amplitude and phase relation via an actuator.
From ergonomic aspects, it must be kept in mind, that actuator vibrations can produce acoustical noise which may be very uncomfortable for the operator.

fig8.jpg (18967 bytes)
Fig. 8: Mechanical impulse compensation

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