ResearchFields

12-Electrode Piezoelectric Tube Scanner

A 12-electrode piezoelectric tube scanner was used for fast atomic force microscopy (AFM). The novel electrode pattern allows simultaneous sensing and actuation of the piezoelectric tube scanner in an efficient way. The external electrode is segmented into 12 equal sections of 30deg each. It has a small continuous electrode at the top of the tube for z-axis actuation. The inside of the tube is covered by a continuous electrode which is grounded at all times. Fig. 1 illustrates the wiring of the tube for actuation and sensing in the x-axis alone. The two outer electrodes on opposite sides are used for actuation. When voltages with equal magnitude but opposite polarity are applied to the opposite electrodes, one side of the tube extends and the opposite side retracts, resulting in bending. The strain experienced on each side of the tube is translated into a voltage at the respective central electrode due to the piezoelectric effect. Due to the symmetry, the voltages induced at the opposite electrodes are equal in magnitude but 180deg out of phase. The voltage induced in one electrode is inverted and added to that obtained from the opposite electrode. The resulting signal is then used as an indication of the tube's deflection. Actuation and sensing in the y-direction can be obtained in a similar manner. For z-axis actuation, a voltage is applied to the continuous electrode (z-electrode) near the free end of the tube.

The electrodes are arranged such that the tube is driven in an anti-symmetrical manner, resulting in a collocated system suitable for positive position feedback (PPF). A PPF controller is designed to damp the scanner's resonance. Piezoelectric strain-induced voltage is used as measurement.

The device is then installed into an AFM to obtain open- and closed-loop images of a grating at 15.6Hz scan rate. 10um x 10um images (with 256x256 scan lines) of the grating were recorded (see Fig. 2). The closed-loop images are noticeably superior to the open-loop images, illustrating the effectiveness of the proposed scanner when used simultaneously as a sensor and an actuator.

Fig. 1
Left: Side and top views of the 12-electrode piezoelectric tube scanner
Right: An atomic force microscope

Fig. 2
Left: Oscillations are clearly visible in the open-loop scan
Right: Vibrations are eliminated in the closed-loop image