231575 Piezo-Mechanics GB

3. Comments on datasheetThe problem in comparing data of actuators from differentsuppliers is, that specifications and data on piezostacks are often defined in different ways.Therefore it is necessary to understand to some extentthe background of the specified parameters to ensure areasonable decision for and proper use of a certainpiezo-element.Especially for highly sophisticated applications, a profoundconsulting is recommended to check for allrelevant aspects when designing a piezo active arrangement.Contact PIEZOMECHANIK, when you make firststeps towards piezo actuation.3.1. Voltage polarityPiezostacks can be operated by unipolar or semi-bipolarvoltage signals.When special PZT-material is used even a symmetricalbipolar operation is possible (see PIEZOMECHANIKcatalog: “Long life bipolar stacks”).In case of unipolar or semibipolar driving mode, attentionmust be paid for correct voltage polarity, whenconnecting a piezostack to a power supply. Otherwise,the PZT actuator can be deteriorated in its action bydepoling, when the maximum voltage is applied.A simple check for correct polarity is, when a harmlesslow voltage signal is applied and the stack actuatorexpands with increase of voltage.Bipolar actuators are insensitive to polarity reversal bydefinition, but the mechanical response invertse.g. the stacks shrinks upon application of a counterpolarvoltage.PIEZOMECHANIK’s actuators with casing and powersupplies are manufactured for positive polarity withreference to ground.Bare piezostacks wired by two pigtails are manufacturedpotential free, so any power supply polarity can be usedwhen connected correctly to the piezostack.3.2. Stroke and positioningAny PZT material can be operated to some extent with acountervoltage opposing the burned-in polarity. Comparedto unipolar activation, the semibipolar moderesults in wider total voltage swing with the expectedconsequences: increase of stroke, blocking force andenergy density.A 20% countervoltage (of the specified maximum forwardvoltage) is applicable to any material. The yield instroke (and blocking force) can be up to 30% then comparedto the unipolar situation.The energy content canbe enhanced up to 50% and more. Hard PZT materialslike PZT ceramic HS/HT can withstand still higher electricalcounterfields than the above defined 20%.The data sheet show two values for the maximum stroke.These are related to semibipolar operation (higher valueequiv. larger voltage swing) and conventional unipolaroperation (lower value) e.g. the PSt 150/7/40 VS 12 elementis specified with 55 µm/40 µm stroke meaning:40 µm stroke for unipolar 0 V/+150 V activation and55 µm for the semibipolar –30 V/+150 V swing.A piezostack’s maximum stroke is proportional tostack’s length. Strain rates of approx. 1– 2‰ are usuallyachieved.The data sheet show nominal values for “no load” conditionat room temperature.The achievable real stroke depends on the mountingconditions like preload and stiffness of attachedmechanics (see sections 1.3.3.).One of the most important features of piezoactuators isthe ultrafine positioning capability down to the subnanometerrange (provided the electronic supply shows asufficiently high quality with respect to noise and signalresolution).Please take notice, that piezo-elements show aninfinitely large relative positioning sensitivity: This meansthat an infinitely small voltage variation leads to aninfinitely small mechanical shift. Unfortunately the stepwidth for a distinct macroscopic voltage variation is notcalibrated as characterized by the hysteresis in thevoltage stroke/diagrams.To do absolute positioning a position measuring systemsmust be introduced to detect the mechanical shift andto match the voltage signal for setting to the desiredposition.An introduction to the philosophy of feedback controlledpositioning is shown in PIEZOMECHANIK’s catalog“PosiCon feed back control electronics”.Notice: The stroke characteristic becomes morelinear and non-hysteretic when stroke is set intorelation to the electrical charge content of the piezoactuator instead voltage (see chapter 7).12