Capacitive Micro Flow-Sensor

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Working principle of device

Calculations and Design Iterations

Acknowledgements and some references

The idea used is that a gas flow exerts a pressure (force) on any object which lies in its path. This mechanical force is utilized to cause change in position of a moveable part of the device. This change in position causes an electrical change to take place in the system. This electrical change is easier to measure than the gas flow per se.

Problem Statement

To measure a significant capacitance change

The aims of the capacitive micro flow-sensor are:

1. To detect the presence of gas flow

2. To measure the rate of a small gas flow if it exists

Components of the device:

1. Rotating beam

2. Sail hinged to polysilicon beam at one end

3. Angular comb capacitor electrodes attached to other end of beam

4. Springs (2)

5. Damping coil

6. Connection pads

Working principle of device:

• Sail moves with force of flow thus moving moveable capacitor in and out of fixed capacitor

• There is a change in capacitance for a degree change in area of overlap between electrode fingers

• Springs return the beam to its original position

• Damping coil damps out the oscillation of the beam caused by the springs

Design Details

• Rotating poly1 beam of maximum length of 200 microns. This is to avoid bending or breaking of the beam.

• Sail of Poly1 attached to beam by poly2 hinges
  • sail raised out of plane after release

  • kept in vertical plane by means of a hinge

  • for greater efficiency, sail is perforated

• The hinge has poly2 staples that are anchored to the beam.

  When the sail is raised after release, the central poly1 and 2 connection rotates into the vertical plane to be stapled down. The sail does not release down onto the substrate. The hinge stands on three legs for stability

• Poly2 springs are attached to beam on both sides. The stiffness of the spring can be controlled by its design structure.

  Two springs were used, on either side of the beam, to help stabilize the structure.

• The damping coil has to be placed under the beam so that it can produce eddy currents in the beam which can damp its motion

• The moveable poly1 bi-directional rotary comb electrodes were anchored to the beam.

  Dimensions to be considered included height, air gap and length of electrodes.

  Efficiency dependence was placed on more electrodes, more overlap and thus more sensitivity in capacitance readings.

  The fixed plate were anchored to the silicon substrate.

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Calculations

Some of the constants to be taken into consideration included

1. density
2. pressure
3. spring constant

Some of the variable were:

1. area of sail
2. weight of sail
3. weight of beam
4. weight of electrodes
5. area of overlap of electrodes

Design Iterations

• With different sail dimensions

• With different beam lengths

Notes:

• For accuracy, reading of two capacitance signals may be preferable

• Overall design
  • cover sensor with plastic tubing to direct flow and protect circuitry

  • use device for uni-directional flow sensing

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Final design

On one of the designs (see fig.), the hub was moved closer to the electrodes for greater radius of rotation of the beam. This was due to the fact that the final comb electrodes design was much smaller than the beam which meant that it had a small radius of curvature.

Due to MUMPs design rules, design iterations could not be included.

Conclusion

Although the final design was completed many of the anticipated calculations and design aspects could not be incorporated and achieved on time.

Design review

• Completion of calculations necessary to capacitance measurements

• Design consideration for hinge stability in the vertical plane thus considering greater gas pressure tolerance of sensor

  At present, the hinge may not be stable in the vertical plane and a small pressure may blow the sail down

• A greater radius of curvature for the comb electrodes so as to have the.hub centered on the beam.

  This would lead to greater sensitivity and larger capacitance readings

• More flexibility in the springs so as to have greater motion of the beam

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Acknowledgments

Special thanks to Prof. Albert Henning and Dr. Christopher Levey of Thayer School of Engineering, Dartmouth College, for their continuous help and support through the progress of this project, in appreciation of their efforts to teach the technology of micromachines.

Some References

P. N. Gadgil, M. Parameswaran, J. McEwen, A micromachined pressure-time recorder for medical applications, Sensors and Actuators A 45 (1994) 17-21

P. Gravesen, J. Branebjerg and O. S Jensen, Microfluidics- a review, Danfoss A/S, DK-6430 Nordborg, Denmark, J. Micromech. Microeng. 3 (1993) 168-182