The main objective of the project is, taking into account the recent developments of improved Nd-Fe-B and bio-compatible materials and microelectronic and micromechanical fabrication techniques, to make research-type millimetre and submillimetre micromotors/actuators, using electromagnetic and electrostatic principles.
Miniature electric motors with dimensions measured in microns rather than in millimetres have been manufactured. Scaling analysis shows that as size is reduced electrostatic designs become advantageous over the electromagnetic versions which dominate at dimensions starting in the millimetre range.
The electrostatic micromotors which were investigated were based on the principle of capacitive induction whereby a voltage on the stator induces a potential on a conducting rotor which then moves to minimise it potential energy.
In the project much time was devoted to analyse the electrostatic field distribution for selected geometries using a three dimensional finite element software package. The forces in the electrostatic micromotor were calculated using the Maxwell stress method over all the surfaces of regions of interest of the model-design. For the torque calculation of the electromagnetic micromotors also the virtual work principle was applied.
A novel design was analyzed consisting of a double stator with a rotor between the upper and lower stator sections. The calculated axial forces on the rotor were used to predict frictional effects on the rotor by using typical friction coefficients which were analyzed in one of the work packages. The calculation of the frictional torque allows an assessment of the decrease in performance of the electrostatic micromotor.
Within the project brushless d.c. micromotors of the disc-type with etched planar windings have been built and mini linear actuators were investigated. Work was also done on synchronous micromotors with a permanent magnet rotor. As the torque of such machines depends on the Maxwell shearing stress on the rotor surface integrated over the rotor surface, due to the bigger length, the electromagnetic micromotor outstrips the flat electrostatic type with the same rotor diameter.
Work concentrated on two phase motors with tiny permanent magnets for two-pole and four-pole designs. Measurement of the induced voltage was practised to check the torque calculation. For the planar windings the thick film technology has been used successfully even for multilayer windings.
For the assembly of the micromotor subsystems the glue technology was applied for some prototypes.
Two out of three different axial flux permanent magnet motors, built at TUB, have been analysed. One with a smooth disc-type rotor and one with salient pole rotor. The following quantities have been calculated using 3D finite element technique.
- Linked flux
- Induced voltage
In most cases an agreement of around 10% has been achieved when compared with measurements.
Electrostatic analysis and optimisation
A technique has been developed to generate an equivalent circuit for an electrostatic motor having six stator poles but for the rest arbitrary design. This technique has been applied together with 2D and 3D finite elements, and the following quantities have been calculated.
All quantities for both the electromagnetic and the electrostatic motors have been calculated as a function of the rotor position.
Techniques for generating both 2D and 3D finite element models automatically has been developed. By combining the equivalent circuit technique with the automatic model builders an optimisation of both 6/8-pole and 6/4-pole configuration of the radial flux motor and the axial flux motor, design of ESIEE, has been achieved.
For various applications very small motors or actuators are required. Due to different loads the speed and torque capabilities may vary between broad limits. The size of the motor/actuator should be as small as possible, e.g. in the region of millimetres and submillimetres. Only with advanced materials (high energy permanent magnets, thin layer technology), can integrated microcomputer control and latest design considerations such micromotors and microactuators be achieved.
In order to make these microdevices of practical use in the foreseeable future, we investigate certain fundamental problems, such as micromechanical properties, mechanical handling and loading, design procedures, assembly strategies, friction and wear of microdevices.
EH14 4AS Edinburgh