A real-time FLL-based observer to enhance rotor-shaft motion sensing

Abstract

Reliable measurements of rotary machines' motion generally require the use of speed and position sensors. Conventional solutions, which are mechanically coupled to the rotor-shaft, are often considered expensive and are subject to degradation; despite their high accuracy. Low-cost alternatives provide an estimate of angular motion by sensing the magnetic field of a target fixed to the machine's shaft. Although these magnetic-based solutions have gained popularity due to their contactless feature and ease of installation under restricted dimensions, the use of linear magnetic sensors has been limited as their output signals are subject to distortion. This is relevant for alternate current machines' field-oriented control, as position error propagates towards the output torque, due to geometric transformation of coordinates over the stator phase currents. The published literature reveals that the most accurate and computationally efficient solution consists of a frequency-locked loop observer based on a fourth-order harmonic oscillator, typically used for grid synchronization, to enhance the signals of linear magnetic sensors for rotor-shaft motion sensing. However, aspects of its implementation over a real-time system have not been discussed; representing a research gap. Hence, in this thesis, the mentioned estimation method is implemented as a real-time algorithm, where it is evaluated in terms of its computational resource utilization through processor-in-the-loop tests, and both response time and accuracy through variable speed tests with a field-oriented controlled synchronous machine, using the motion estimates as feedback. As a reference, these characteristics are equally assessed by reproducing the mentioned experiments, for a quadrature encoder and the generic approach to process the output signals of linear Hall-effect sensors for motion sensing. Experimental results demonstrate that the proposed method, while requiring more computational resources, it is able to be executed over a microcontroller unit along with a field-oriented control strategy. After fine-tuning, the proposed observer is able to achieve a response time similar to the quadrature encoder, and improve the accuracy throughout the use of linear Hall-effect sensors; decreasing the angular speed estimate's stationary error under 19 % with respect to the quadrature encoder reference. Additionally, an attenuation over the dq-currents is achieved; reducing the mean dispersion in steady-state from 31.65 mA to 24.87 mA in the d-axis, and 26.52 mA to 11.4 mA in the q-axis. Thus, improving the performance of the driving machine. This statement is further supported through a harmonic analysis over the measured signals in steady state, where the proposed method demonstrated harmonic cancellation over components derived from the number of poles from the sensing array and the controlled motor.