• Piezoelectric Strain Sensor
    Piezoelectric sensors have the benefit of simple interface circuitry, low cost, high sensitivity, and high bandwidth. Although piezoelectric sensors have been successfully used as vibration sensors in smart structures, complications arise when they are used in a feedback loop for tracking. As piezoelectric strain sensors exhibit a capacitive source impedance, a high-pass filter is created, typically with a cut-off frequency of 1 to 10 Hz. This filter can cause significant errors and destabilize a tracking control system. Here, we overcome this problem by using a low-frequency bypass technique to replace the low-frequency component of the strain measurement with an estimate based on the open-loop system.

  • High-speed Non-raster AFM Scanning
    A traditional AFM utilizes a scanner to scan over an area of a sample in a zig-zag raster pattern. The fast axis of the AFM scanner is forced to track the non-smooth triangular waveform that contains frequencies beyond the scanner's mechanical bandwidth. The high-order dynamics of the triangular waveform tends to trigger the resonance frequencies of the scanner. This leads to image distortions. Therefore, smooth scan patterns are proposed to achieve much higher speed scans than a raster pattern.
    [Left image: Distorted raster scanned image; Right image: Non-raster scanned image]
  • Flexure-based Nanopositioners
    The design of the flexure-based nanopositioning stage is based on the concept of flexible mechanisms (flexures) where motions are generated through the elastic deformation of the structures. There are no moving and sliding joints; therefore, the problems of wear, backlash, friction and the need for lubrication are eliminated. This provides repeatable and smooth motions to fulfil the requirement of accurate nanoscale positioning. Piezoelectric stack actuators are commonly used to drive flexure-based stages due to their capability of achieving repeatable nanometer resolution over a very high bandwidth. They can also generate large forces and high accelerations which are desirable for the design of a high bandwidth nanopositioner.

    This project is aimed to improve the dynamic performance of a nanopositioning scanner for high-speed scanning applications of an AFM (Atomic Force Microscope). The project objectives are: (a) to design a nanopositioning scanner which has relatively high resonance frequency, large scan range & low cross-coupling between the X-Y axis, and (b) to implement a well-performing control scheme that provides substantial damping & accurate high-speed scanning performances.
  • Piezoelectric Tube Scanner
    Piezoelectric tubes with quartered external electrodes have been widely used as scanners in modern scanning probe microscopes. Various feedback control techniques have been developed to improve bandwidth and accuracy of these scanners. Non-contact displacement sensors, e.g. capacitive and inductive sensors, have been used for positioning feedback. This project has several goals: (a) To investigate different electrode sizes & patterns for sensing and actuation; (b) To design a piezoelectric tube with improved dynamic performances; and (c) To implement a control scheme that provides substantial damping & accurate high-speed scanning performances.