Micrograph of a 400 micron diameter disk during levitation
There is a large market for turn rate sensors (gyroscopes) for applications such as inertial guidance systems and active suspension systems in cars. The current "state of the art" technologies include fibre optic gyros, and micromachined Corriolis force gyros. Fibre optic gyros offer high performance but are large and expensive; micromachined Corriolis force gyros promise small cheap sensors, but with moderate performance. There is clearly room for a new type of gyroscopic sensor that falls between these two categories in providing a small cheap sensor with good performance.
To fulfil these criteria, we have designed a rotating gyroscope that consists of a magnetically levitated bearingless rotor spun at a very high rate. Due to its simple planar construction, the proposed device can easily be mass produced using standard microelectronic fabrication techniques.
Micrograph of a 400 micron diameter disk during levitation
At very high rotation speeds the tiny rotor has sufficient angular momentum to act as a gyroscope. When the sensor is rotated at a constant rate about one axis, the levitation forces cause the rotor to precess (tilt) about the orthogonal axis at a constant angle which is proportional to the rate of rotation of the gyroscope. Thus, the gyro is a turn-rate sensor.
The performance of the gyro can be improved by using a feedback loop to force the rotor back until it is once-again parallel to the stator, and the amount of force needed to achieve this is proportional to the rate of rotation of the gyro. This improves the linearity of the device and gives it a large dynamic range.
Advantages of the micromachined rotating-gyro are:
The 500 micron diameter rotor is levitated using electromagnetic induction which makes it frictionless, unlike other rotating micromotors described in the literature. In these designs, the torque was barely sufficient to overcome friction, and the lifetime was limited to only few hours. In the literature, several other levitation schemes have been investigated using phenomena such as the electroquasistatic induction principle and the Meissner effect of superconductors, but these are expensive to implement either because of the complicated fabrication requirements or the need for a cooling system.
Finite-element analysis using the commercially-available packages MEGA and ANSYS was used to design the novel coil topology to achieve stable levitation, and to optimise the performance of the motor.
The design has been coupled with careful material selection and layout which is essential for optimised performance. The planar coils and aluminium disks were fabricated by standard microelectronic patterning techniques. The stator was fabricated on a soft magnetic backing plane electrically isolated by a layer of silicon nitride, with the 1.3µm thick gold coils defined by metallisation and lift-off. The 500µm diameter, 10µm thick rotors were fabricated separately by wet-etching thermally evaporated aluminium deposited onto a polyimide sacrificial layer
A British Patent filed in April 1995 has just been secured embracing this levitated and rotated micro-disk technology (European Patent No.: 9519461.9).
Micrograph of 3 phase micro-motor.
Micrograph of
micromotor with rotating aluminium disk!
