High-frequency PWM amplifier for high precision motion

Abstract

High quality motion with nanometer resolution is indispensable in many systems such as atomic force microscopes and high-precision 3D- printers. In these systems, Lorentz actuators or piezoelectric actuators are typically utilized to satisfy the requirements on the positioning resolution and bandwidth, and they are usually driven by linear power amplifiers. They have low noise for high-precision motion; however, their energy efficiency is low, resulting in energy dissipation and a cooling system requirement. Although the energy efficiency can be improved by using switching power amplifiers, they generate current ripples that impair motion precision. To drive high-precision actuators in an energy efficient way without degrading the motion resolution, this thesis investigates the feasibility to utilize switching power amplifiers based on Gallium Nitride (GaN) transistors, which enable a higher switching frequency in comparison with conventional silicon-based transistors. For the experimental investigation, a switching power amplifier is developed, consisting of a PWM generator, logic and dead-time adjustment circuit, an output stage and an LC-filter. Evaluation of the developed amplifier is conducted with inductive load and piezoelectric actuator. Experimental results with the inductive load show that the increase of the switching frequency from 20 kHz to 1MHz decreases the current ripple of the coil current by a factor of 13 while power dissipation is constantly low. However, the energy dissipation analysis reveals high energy losses when the developed power amplifier is used for driving the piezoelectric actuator. Analysis shows that cause for these losses is the hard switching effect occurring when amplifier is used for driving a capacitive load. Experimental results also show that the high-frequency switching is beneficial for a high positioning bandwidth and reduction of a standard deviation of voltage on the piezoelectric actuator up to switching frequency of 450 kHz.