Hybrid digital control of piezoelectric actuators.

Abstract

Nanopositioning, as a core aspect of nanotechnology, concerns the control of motion at nanometre scale and is a key tool that allows the manipulation of materials at the atomic and molecular scale. As such it underpins advances in diverse industries including biotechnology, semiconductors and communications. The most commonly used nanopositioner is the piezoelectric actuator. Aside from being compact in size, piezoelectric actuators are capable of nanometre resolution in displacement, have high stiffness, provide excellent operating bandwidth and high force output. Consequently they have been widely used in many applications ranging from scanning tunnelling microscopes (STM) to vibration cancellation in disk drives. However, piezoelectric actuators are nonlinear in nature and suffer from hysteresis, creep and rate-dependencies that reduce the positioning accuracy. A variety of approaches have been used to tackle the hysteresis of piezoelectric actuators including sensor-based feedback control, feedforward control using an inverse-model and charge drives. All have performance limitations arising from factors such as parameter uncertainty, bandwidth and sensor-induced noise. This thesis investigates the effectiveness of a synergistic approach to the creation of hybrid digital algorithms that tackle challenges arising in the control of non-linear devices such as piezoelectric actuators. Firstly, a novel digital charge amplifier (DCA) is presented. The DCA overcomes inherent limitations found in analog charge amplifiers developed in previous research. In order to extend the DCA operational bandwidth, a complementary filter was combined with the DCA along with a non-linear black-box model derived using system identification techniques. To maximize the model accuracy a novel method is utilized that reduces error accumulation in the model. This method is generally applicable to many dynamic models. A non-linear model is also used with a data fusion algorithm to ensure the DCA does not exhibit drift, an issue common to most of charge amplifiers. The proposed hybrid digital system is evaluated and it is shown that hysteresis is significantly decreased, while operational bandwidth is extended with no displacement drift. Experimental results are presented throughout to fully validate the proposed system.