Design and Analysis of a Tri-Axis Gyroscope Micromachined by Surface Fabrication

Outline

Motivation

Introduction

Background

Applications

Design

Principle of Operation

Fabrication of the Gyroscope

Experimental Setup

Results

Conclusion

Motivation

Recent Progress in microfabrication techniques.

Tri-axis Gyroscope

Miniature size and low cost.

Reliable batch fabrication techniques and reduction of production cost gradually become the focus of evolution of this device.

Various applications.

Introduction

What is Gyroscope?

A gyroscope  is  a  device  for  measuring  or  maintaining orientation,  based  on  the  principle  of  preserving  angular momentum.  Mechanical  gyroscopes  typically  comprise  a spinning wheel or disc  in which the axle  is free to assume any orientation. 

Background

In 1991, Greiff et al. designed conventional macroscale gyroscope with detective resolution of 4deg/sec and 1Hz bandwidth.

In 2006, Vinay et al. proposed a micro tri-axis gyroscope in which a single proof mass is embedded.

However, accuracy, bandwidth and decoupling efficacy of measurements are insufficient.

Applications

Image stabilization

Inertial Navigation Systems(INS).

Rollover detection

Active suspension for advanced vehicles

Satellite posture control.

Design

The microgyroscope mainly consists of four key components

Seismic inertial mass modules

Flexure pivots

Sensing differential capacitor sets

Rotational comb-drive electrodes

Design

The proposed tri-axis microgyroscope is schematically shown in below.

Design

The seismic inertial mass modules includes: Inner disk, outer-ring, distributed translational proof mass(DTPM)

In fact, the seismic inertial mass modules are nothing but the angular rate detection elements whose responding motions are induced by Coriolis effect in three axes.

1- Outer-ring2- DTPM3- Inner-disk

Design

Flexure pivots include four spring sets.

Spring#1(marked as S1),which is used to suspend the outer-ring(used for detection of angular rate about Y-direction).

The inner-disk(used for detection of angular rate about x-direction) is suspended by a set of torsion spring, named as Spring#2(marked as S2).

Spring #3(marked as S3) and Spring #4 are, respectively, used to resist the radial and tangential acceleration of DTPM due to external angular excitations about z-direction.

11- S112- S213- S314- S4

Design

The sensing capacitor sets include two pairs of differential capacitor and four pairs of comb electrodes.

The comb electrode pairs by the DTPM are used to measure the induced radial displacement of DTPM if any regular excitation about Z axis is present.

4,6,7 – Sensing electrodes5,8,9 – Comb electrodes

Operation

Operation

Change in o/p voltage ~ Measure of angular rates

Operation

Suspension flexure(S1)

Fabrication

Cleaning of waferInsulation layer 4μm and Structure layer 40μmAll mechanical components are micro machined on bottom layer.

Fabrication

Al material deposited (mask#1) 

Fabrication

Spin Coat Photo Resist (mask#2) The glass photomask(mask#2) is used to pattern the desirable PR by photolithography.

Fabrication

PR patterned and layer of Al partially removedIt is noted that the hard etching mask is concurrently patterned with the glass photo mask, by photolithography process.

Fabrication

PR removed and structure layer then etched by DRIEThe mechanical structure is formed by DRIE

Fabrication

Al removed and insulation then etched by the BOEThe purpose of deposited Al is to resist DRIE so that Geometry of structure layer by SOI wafer can be defined. 

Fabrication

Wafer cleanedPyrex has high optical transmittance over wide range wave length.

Fabrication

Spin coat PR

Fabrication

PR patterned

Fabrication

HF (Hydrogen Fluoride) etchingBy standard liftoff process after the corresponding trenches to house 

Fabrication

PR removed and Au deposited as electrode

Fabrication

Bonding of Dual layers

Fabrication

A couple of scanning electron microscope(SEM) Images

Experimental Setup

Driving circuit Sensing Circuit

Results

As the External angular velocity about x axis or y axis is exerted, the inner-disk or outer ring responds to tilt owing to Coriolis effect.

Hence the air gap between the corresponding mass module and differential capacitors is altered so that the voltage across capacitors is accordingly changed, as shown in results.

Results

The relation between capacitance variation and the corresponding induced voltage is found to be fairly linear.

In other words, it can be conveniently used for measure of detected angular rate,

Results

Conclusion

Capability of detecting tri-axis angular motions.

Designed in geometry such that the stress of proposed microgyroscope is considerably reduced but the resolution for angular displacements is increased.

The mass production cost has been considered at the design stage but the resolution, bandwidth and decoupling capability of tri axis detection can be expected to be much upgraded .

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