Piezo-electric drives

A more recently developed force measurement technique, coined the liquid siu-face force apparatus (LSFA), brings a drop made from a micropipette close to a flat liquid/liquid interface [29-32]. A piezo electric drive is used to change the position of the micropipette while the deflection of the pipette and the radius of the drop are recorded with piezo motion. The drop radius and thus the film thickness between the two liquid/liquid interfaces are recorded using interferometry. The method requires a calibration of the interferometer, where the drop must come into contact with the other liquid interface. The distance resolution of the film is about 1 nm at a 50-nm separation and 5 nm at a separation of 10 nm. This is a very robust technique where the authors have proposed attaching a particle to the end of the pipette instead of a drop [29]. In comparing this method to AFM, the only drawback of the LSFA is the weaker distance resolution. It is important point out that both methods required a contact point for distance calibration. 

Figure 6.26 STM set up Scanning of the surface takes place in the near field regime in which piezoelectric drives Px and Py raster the STM tip across the surface of a sample. Piezo electric drive Pz ensures that the tip remains a short distance dj (a few A) from surface encouraging a tunnelling current, Ij, to be established under the influence of a tip to surface potential Uj.

The heart of control in an STM apparatus is the exquisite sensitivity and fine control of distances exercised by the Px, Py and Pz piezo-electric drives that control tip motion. These drives are in the form of bars, tubes or bimorphs all comprised of piezo-electric ceramics in contact with Macor. A piezo-electricbar changes in length, A /p, as a function of the potential difference. Up, applied between two opposing bar electrodes according to... 

Figure 3.37. A schematic diagram of a scanning force microscope. The tip-cantilever assembly can be driven in the x, y ot z direction by piezo-electric drives. The deflection of the tip can be detected both in the vertical and in the lateral direction by measuring the deflection of an incident laser beam with a quadrant photodiode.

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. 

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