A Novel Piezoelectric Strain Sensor for Simultaneous Damping and Tracking Control of a High-Speed Nanopositioner

This paper presents a novel piezoelectric strain sensor for damping and accurate tracking control of a high-speed nanopositioning stage. 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. Once the low-frequency filter is accounted for, any standard control system can be applied. In this paper, an analog integral resonant controller together with an integral tracking controller are implemented on a flexure-guided nanopositioner. The resulting closed-loop bandwidth is experimentally demonstrated to be 1.86 kHz. The nanopositioner is installed in an Atomic Force Microscope to obtain open- and closed-loop images at line rates of 40 and 78 Hz. Images recorded in closed loop show a significant improvement due to the elimination of nonlinearity.