|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.
Fig. 5a: Mirror tilter with passive prestress/reset
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
Fig. 6a:Mirror filter with active (push-pull) reset
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.
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.
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.
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.
Fig. 8: Mechanical impulse compensation