Piezomechanik

Actuators are normally classified by the maximum applicable
voltage for maximum stroke, and characterised as low
voltage and high voltage types. For newcomers in piezotechnology,
this sometimes gives the impression, that the voltage
rating of an actuator is the sole criterion for selecting a
proper electronic supply. This is however not correct.
For any application of piezoactuators the electrical power/
current balance for charging and discharging the piezoactuator’s
capacitance has to be kept in mind. The variety of
electrical supplies on offer is due mainly to the different
power/current ratings of these devices.
The charge/current balance during operation is related to
the capacitive nature of actuators as shown below:
Basic capacitor equation
Q(t) = C U(t) C actuator’s capacitance
Q actual electrical charge
U applied voltage
Obviously the expansion of an actuator is also related to
the quantity Q of electrical charge stored in the actuator’s
capacitance C, when a voltage U is applied.
From this charge balance, the kinetic parameters of motion
like speed and acceleration can be derived. These relations
are the base for specifying the necessary current/power for
distinct driving conditions.
Actuator’s position l ~ charge = Q(t)
.
Speed v ~ current I = dQ/dt = Q(t)
..
Acceleration b ~ variation of current = dI/dt = Q(t)
The generation for example of a sine-wave oscillation by
a piezoactuator requires a defined supply current depending
on actuator’s capacitance and moving amplitude.
Therefore an amplifier has to be selected for both criteria:
voltage and current.
Another consequence of the above is that, during a
steady state of the actuator (constant position, constant
force) no current is flowing, therefore no power is required.
When a charged actuator is disconnected from
the supply, it holds its position. This is an important
difference to electromagnetic systems, where a constant
position requires constant electrical power due to the
sustaining current.
The speed of an actuator cannot be increased infinitely even
by very high currents, but is limited by the elastic properties
of the stack. The maximum speed of stacked elements is in
the range of a few m/sec.
Because of the very limited moving range of piezoactuators
the generation of above speeds requires high acceleration
rates up to 10 4 –10 5 g.
During operation of a piezodriven mechanical setup for highly
dynamic application, it has to be verified that the mechanics
coupled to the actuator shows a sufficiently high stiffness/
resonant frequency, otherwise the mechanics cannot follow
actuator’s motion and it is fruitless to optimize the drive for
high speed/acceleration.
2.4. Peak current, average current
Piezoactuators require electrical power/current only during
dynamic operation. Expansion and contraction are characterized
by charging/discharging currents.
The short term available maximum peak current of a supply
determines the minimum risetime/maximum speed of an
actuator. Amplifiers of the series LE provide a special booster
stage for high peak currents to get minimum risetimes.
The average current of a supply determines the longterm
cw-repetition rate of charging/discharging an actuator.
For cw sine oscillation of an actuator, the required peak and
average currents show a fixed ratio of approx. 3:1. Therefore,
the selection of a supply to obtain a distinct cw-actuator frequency
has to consider both, peak and average current data.
2.5. Power efficiency
This section will lead on the first glance to the (surprising)
result, that it is sometimes very reasonable and necessary to
combine a high voltage actuator with a low voltage supply,
where only a fraction of the actuator’s maximum amplitude
can be achieved.
The reason for this strategy are twofold:
• optimizing power efficiency of a dynamically operated
actuator system
• minimizing selfheating of a dynamically operated actuator.
The basic idea is easily demonstrated with the following
example, where the task requires the generation e.g. of a
+/–2,5 µm sine oscillation with a distinct frequency:
The first example uses an actuator type PSt 500/5/5, where
500 V has to be applied to get the full stroke of 5 µm.
A second example is to use the longer stack PSt 500/5/15
capable for a 15 µm motion at 500 V, showing an actuator’s
capacitance 3 times larger than in the 1st case.
The important fact is, that with the longer stack only 150 V
are needed to get the desired 5 µm stroke.
Comparing the actuators’ energy content 1/2 CU 2 respectively,
despite its larger capacitance the longer stack is
favoured regarding power efficiency as only 1/3 of the power
necessary to drive the shorter PSt 500/5/5 with full strain is
required. It is obvious, that a 150 V system’s total power efficiency
is further improved by using a 150 V supply showing
higher current output compared to a 500 V supply operated
at reduced voltage rating.
In the above described strategy, the problem of selfwarming
under dynamic operating conditions is minimized by the
reduced power input and by distribution of the dissipated
energy over a larger volume/surface of the longer actuator.
This is a powerful method to extend the application range of
piezoactuators to high frequency cw-operation without the
risk of overheating.
This strategy of dynamic operation of actuators with reduced
strain shows restrictions in other operating parameters: A
longer stack has a lower stiffness and resonance, and it has
to be determined, whether this is acceptable for a distinct
application.
Finally, an important contribution to the overall power
efficiency of an actuator system is the use of recharger
amplifiers (switched amplifiers).
In most applications, piezoactuators display mainly a reactive
load, where the energy content of a charged actuator flows
back to the amplifier during the discharging cycle. Switched
amplifiers RCV are able to recycle this energy with high efficiency,
so that the needed linepower for a dynamically operated
system has only to cover the (much smaller) active part
of the power balance.
This active power is drawn from the system as mechanical
power or dissipated by the selfheating of the actuators.
This technique shows the optimum of systems’s overall
power efficiency, and favours actuator applications, where
high power levels are required e.g. for active vibration cancellation
in heavy mechanical structures (vehicles, airplanes
etc.) or anywhere, where the power consumption from the
power supply is restricted i.e. battery operated systems.
Power efficiency � is defined as
� = (P r–P al) Pr = reactive power output from amplifier
� = (P r) Pal = active power consumption from line
An ideal amplifier without internal losses shows an efficiency
1.
http://www.piezomechanik.com Amplifiers, D/A Converters, Electronic HV-Switches for Piezoactuators
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