Project Scope

 

•  Problem at hand

Our project will portray the design, fabrication and performance of an axial flux electromagnetic micromotor based on MEMS technology.

•  Background Information

The stator of the micromotor consists of multiple layers of copper windings, as shown in Fig.1. The rotor is a disc-shaped permanent magnetic alloy and it is magnetized with pairs of magnetic poles. The rotor and stator are assembled together to form the micromotor (see Fig. 2). These thick rotors are magnetized by a meander-shaped conductor equivalent to single-turn coils, supplied by a current pulse. Magnetizing the pairs of poles would be magnetized one way in a strong field and likewise with an intermediate opposite field.

•  Project Outcome

Our project outcome is building a prototype to show how the micromotor works. This process needs to obtain a high-quality result via a scale up model or a possible MEMS version. Also a report on the function and design of the micromotor is to be written.

Figure 1 , Configuration of the micromotor stator

 

Figure 2 , Schematics of electromagnetic micromotor

 

Executive Summary

This project involves the understanding and reverse engineering of an axial flux permanent magnet micromotor found in a novel safety device. The motor is based on MEMS technology and is of millimeters in size, 0.66 cm diameter to be exact. To understand the functionality of the micromotor, extensive research was conducted with the help of Dr. Philippe Masson and Dr. Chi-Fu Wu. From the research conducted, it was determined that the motor operates in such a matter that includes the rotation of an eight pole permanent magnet rotor sandwiched between two six pole electromagnetic stators. Two coils of each stator are induced with electrical current in a sequence that creates a change in the magnetic field at each step. This change results in a physical repulsion of the same poles as well as attraction of the opposite poles between the rotor and stator, which subsequently produces torque. With the aid of the Maxwell software package, force and torque estimates were obtained. From Maxwell 2D, it was estimated that the micromotor would have a force and torque performance of 103mN and 206µNm respectively. Maxwell 3D estimated the torque to be 650 µNm for the micromotor motor. Since both of the results are estimates there is a discrepancy in the value. It can be seen, though, that the values are in the same order of magnitude.

With the understanding of the actuation of the motor and simulation of the model, focus was set on understanding the commutation of the device. Upon research, it was determined that the stepping sequence of the stators would be controlled through the implementation of a circuit with a controller and a driver mechanism. This circuit, with the controller chip (MINI-MAX 51-C2) and the driver chip (L6234, driving mechanism), allows for the input of an algorithm or logic code that will permit automatic commutation.

Fabrication of the original MEMS micromotor could not be reproduced due to technological restrictions. However, focus was made on fabricating a scale-up model with similar physical properties e.g. motor configuration and material selection. The scaled version is fifteen times greater relative to the MEMS micromotor device (9.9 cm in diameter) and has an expected performance torque output of 0.055 Nm. All dimensions of the micromotor were scaled up by that factor except the air gap, core thickness, and wire dimensions. Modifications have been made to the scale-up model due to fabrication cost, which is explained in Chapter IV, Section VI.