Piezo-electrical actuator

The Bespak Piezo Electric Actuator is a novel aerosol delivery system based on a piezoelectric crystal combined with an electroformed mesh (Fig. 3). It produces droplets of adjustable size from a single metered drop or fluid reservoir. The mesh hole dimension (as small as 3 pm) determines the size of the droplets produced, whereas the size and density of the holes control the rate of fluid delivery. These can be varied according to the formulation. Although solutions are more readily nebulized, suspensions can be aerosolized if the particle size of the suspended particles is two to three times smaller than the mesh size. 

Fig. 3 Schematic diagram of the sprayhead of the Bespak Piezo Electric Actuator. (Reproduced courtesy of Bespak pic, UK.)...

ECDL External Cavity Diode Laser OI Optical Isolator L Lens AOS Acousto-Optic Switch PZT Piezo-Electric actuator Me Cavity mirror APD Avalanche Photodiode PC Pressure Controller P Pump... 

The ARES spectrograph is also equipped with an active wavelength stabilization system, which guarantees fixed shape and position of the echelle spectrum pattern relative to the detector. Altogether five micro-positioning units based on piezo-electrical actuators (four active R1- R4 and one passive thermo-mechanical R5) are implemented to control permanently the relative spectrum geometry (see Figure 3.11). 

Uchino K (1997) Piezoelectric actuators and ultrasonic motors. Kluwer series in Electronic materials Science and technology, Norwell, MA Uchino K (2000) Eerroelectric devices. Marcel Dekker, New York, NY Valasek J (1921) Piezo-electric and allied phenomena in Rochelle Salt. Phys Rev 17 475 81 Waanders JW (1991) Piezoelectric ceramics, properties and applications. Philips Components, Academic, New York... 

Claeyssen, F. A new multi-mode piezo-electric motor. Proc. Int. Conf. on Intelligent Materials, ICIM96-ECSSM96, SPIE vol. 2779, Lyon, Prance (1996), pp. 634-637 also Proc. Actuator 96, Axon, Bremen, Germany (1996), pp. 152-155... 

Here D is the vector of the dielectric displacement (size 3x1, unit C/m ), S is the strain (size 6x1, dimension 1), E is a vector of the electric field strength (size 3x1, unit V/m) and T is a vector of the mechanical tension (size 6x1, unit N/m ). As the piezoelectric constants depend on the direction in space they are described as tensors e- is the permittivity constant also called dielectric permittivity at constant mechanical tension T (size 3x3, unit F/m) and 5 , is the elastic compliance matrix (size 6x6, unit m /N). The piezoelectric charge coefficient df " (size 6x3, unit C/N) defines the dielectric displacement per mechanical tension at constant electrical field and (size 3x6, unit m/V) defines the strain per eiectric fieid at constant mechanical tension [84], The first equation describes the direct piezo effect (sensor equation) and the second the inverse piezo effect (actuator equation). 

Figure 20.2. A schematic of the MASIF apparatus. One surface is mounted at the end of a bimorph force sensor and the other is mounted at the end of a piezo-electric tube actuator. The force sensor is enclosed in a Teflon sheath mounted inside the small stainless-steel measuring chamber (volume 10 ml). The LVDT displacement sensor is used to directly measure the displacement of the upper surface during force measurements...

Fig. 3 Spotting tools for non-contact printing a Bubble ink-jet A heating coil locally heats the loaded sample, resulting in a changed viscosity and expansion of fluids. The generated droplet can be easily expelled from delivery nozzles, b Microsolenoid A microsolenoid valve, fitted with the ink-jet nozzle is actuated by an electric pulse transiently opening the channel and dispenses a defined volume of the pressurized sample, c Piezo ink-jet A piezoelectric transducer that is fitted around a flexible capillary confers the piezoelectric effect based on deformation of a ceramic crystal by an electric pulse. An electric pulse to the transducer generates a transient pressure wave inside the capillary, resulting in expulsion of a small volume of sample...

One of the techniques used is piezo-actuated microinjection, whereby a vibrating needle pierces the donor cell and oocyte plasma membrane. This technique was used to directly inject porcine fetal fibroblast donor nuclei into enucleated oocytes. The injection was followed by activation by means of an electric pulse after nuclear transfer by microinjection. 

The engine industry is pursuing the use of bipolar electric loading to enhance the mechanical actuation of piezo or PZT stacks or actuators for fuel injection. Theoretically, a reverse electric field at a limited level can introduce beneficial depolarization and increase the number of switchable domains. Hence, a greater piezoelectric response of a stack can be promoted. This is mainly based on tests of singlelayer PZT and limited work on several commercial PZT stacks However, the fatigue performance of PZT stacks under the bipolar electric loading mode, specifically semi-bipolar mode, is essentially uncharacterized. 

Certain crystals, such as quartz, feature a physical relationship between mechanical force and electric charge. When the crystal lattice ions are elastically shifted relative to one another due to an external force, an electric polarization can be detected by means of metallic electrodes on the surface. This so-called piezoelectric effect was first scientifically explained by the brothers Jacques and Pierre Curie in 1880 and forms the basis for piezo sensors (see Sect. 7.3). The effect is reversible and is then called reciprocal or inverse piezoelectric effect. If, for instance, an electric voltage is applied to a disc shaped piezo crystal, the thickness of the crystal changes due to the reciprocal piezoelectric effect. It is this property that is made use of in actuators. 

Solid-state actuators and actuators with controllable fluids show certain similarities piezo actuators and electrorheological fluids are driven with electrical fields whereas magnetostrictive actuators and magnetorheological fluids draw their actuation energy from a magnetic field. We will therefore consider the power electronics of these two superordinate groups, but still we will mention the differences between the two different actuator types of each group. 

Piezo actuators are driven with an electrical field strength of up to 2 kV/mm in large signal operation (see Sect. 6.2). The ceramic layers are 30 pm to 0.5 mm thick, leading to a driving voltage in the range of 60 V to 1000 V. 

Current Control. Since the voltage and displacement of a piezo actuator are proportional in their first approximation, the time derivatives of the voltage and the displacement have a similar relationship, that is, current and velocity correspond. This relationship is almost free of hysteresis. Therefore, when an application requires a certain velocity signal at the actuator output, it is possible to drive an actuator by its electrical current. In this case a control loop has to ensure that the operating voltage does not exceed the permissible range. 

During later operation it suffices to measure the electrical quantities in order to compute the mechanical actuator quantities on the basis of the model, thereby compensating the hysteresis and creep of the piezo actuator. Since the computing process is quite complex and has to be executed in realtime, this control method is presently only implemented for low-frequency operations. However, the applicable frequency range of this inverse control method will grow in conjunction with the technological progress in the field of microelectronics [364]. 

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