Capacitive Micro Flow-Sensor

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Introduction
Working principle of device

Calculations and Design Iterations

Acknowledgements and some references

Definition

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
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Working principle of device:

Design Details

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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

Notes:

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Final design

This included some dimples to prevent stiction on the beam.

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

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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


Jennifer F. Kaira <jfka@dartmouth.edu>
Leave a message at: jfkaira@mhc.mtholyoke.edu.