A Guide to Wall Shear Stress Measurement - #6 - Capacitive Transduction

Posted by Jared Anderson on Apr 29, 2020 10:42:50 AM

This section focuses on capacitive direct transduction methods used in IC2's DirectShear Sensors (direct methods: piezoresistivepiezoelectric, capacitive, and optical).

Capacitive Transduction

Capacitive transduction employs a variable-capacitance structure, whereby the capacitance is altered by a shear stress-induced deflection, either by changing the gap distance between plates or the overlap area. A differential capacitance scheme is often used in which two capacitors respond oppositely to an applied shear stress. Mass-produced accelerometers commonly employ this method (e.g., the ADXL line from Analog Devices) for accurate sensing in a small package, and it is now available for shear stress sensing. The earliest instances of capacitive shear stress sensors were developed by Schmidt et al. [35], employing a differential capacitance structure fabricated via surface micromachining. Measured performance agreed well with predictions, but the sensor suffered from sensitivity drift due to moisture-induced property variations of the polyimide coating. Pan et al. [36] and Hyman et al. [37] also developed a shear stress sensor using differential capacitance transduction; however, this sensor featured closed-loop force-feedback control. The design integrated sensing, actuation, and electronics onto a single chip and was able to achieve higher sensitivity via a folded tether design but required front-side wire bonds that disturb the flow.

More recently, researchers at the University of Florida [38], [39] employed interdigitated comb fingers along the floating element and the support structure to form variable gap capacitors. As the floating element mechanically deflects, the gaps of the capacitive comb fingers differentially change on either side of the floating element (Figure 1). These are integrated with through-silicon vias (TSVs) for backside electrical connections to provide a hydraulically smooth sensor surface, as shown in Figure 2.   

cap-shear-stress-blog-part-6-1Figure 1: Illustration of a capacitive shear stress sensor.

capacitive-shear-stress-sensor-infographic

Figure 2: Illustration of a capacitive shear stress sensor.with backside contacts.

In order to perform mean shear stress measurements, a modulation circuit is introduced, which shifts all shear stress content around a carrier frequency. The measured shear stress content is then extracted by shifting back to the baseband via a demodulation circuit, thus enabling the capacitive sensor to capture both mean (DC value) and fluctuating (AC data) wall shear stress measurements [39].

After licensing the core technology [40], IC2 refined the device structure and developed a patented, lower-cost approach to fabricating and packaging capacitive wall shear stress sensors [41], [42]. This sensor, combined with further maturation of the signal conditioning electronics [43], resulted in the launch of a commercial wall shear stress sensing system available in various models (DirectShear - Capacitive [44]).

Some technical challenges still exist for this transduction scheme, including inherent parasitic capacitance and finite cross-axis sensitivity.  Additionally, the open gaps and capacitive readout limit operation to inert gas flow and nonconductive liquid environments. Significant additional protective packaging is required in order to enable operation of capacitive shear stress sensors in high-temperature and conductive fluid environments.

The upcoming Part 7 of this series discusses optical transduction and its use in IC2 products for making shear stress measurements in a high temperature environment.

Table of Contents

  1. Overview
  2. Comparing Techniques - Indirect Measurements
  3. Comparing Techniques - Direct Measurements
  4. Transduction Method - Piezoresistive
  5. Transduction Method - Piezoelectric
  6. Transduction Method - Capacitive
  7. Transduction Method - Optical
  8. Transduction Method - Summary and Guidelines***
  9. Sensor Construction - Conventional
  10. Sensor Construction - MEMS
  11. Summary and References

(*** indicates latest post in A Guide to Wall Shear Stress Measurement Blog Series)

References

[35]         M. A. Schmidt, R. T. Howe, S. D. Senturia, and J. H. Haritonidis, “Design and calibration of a microfabricated floating-element shear-stress sensor,” IEEE Trans. Electron Devices, vol. 35, no. 6, pp. 750–757, Jun. 1988.

[36]      T. Pan, D. Hyman, M. Mehregany, E. Reshotko, and S. Garverick, “Microfabricated Shear Stress Sensors, Part 1: Design and Fabrication,” AIAA J., vol. 37, no. 1, pp. 66–72, Jan. 1999.

[37]      D. Hyman, T. Pan, E. Reshotko, and M. Mehregany, “Microfabricated Shear Stress Sensors, Part 2: Testing and Calibration,” AIAA J., vol. 37, no. 1, pp. 73–78, Jan. 1999.

[38]      V. Chandrasekharan, J. Sells, J. Meloy, D. P. Arnold, and M. Sheplak, “A metal-on-silicon differential capacitive shear stress sensor,” in TRANSDUCERS 2009 - 2009 International Solid-State Sensors, Actuators and Microsystems Conference, 2009, pp. 1537–1540.

[39]      C. Barnard, D. Mills, and M. Sheplak, “Development of a Hydraulically Smooth Wall Shear Stress Sensor Utilizing Through Silicon Vias,” in Solid State Sensors, Actuators, and Microsystems Workshop, 2016, pp. 234–237.

[40]      V. Chandrasekharan, J. Sells, M. Sheplak, and D. P. Arnold, "Structure and Fabrication of a Microscale Flow-Rate/Skin Friction Sensor," Sep 2014, US Patent No. 8,833,175.

[41]      D. Mills, "Microscale Sensor Structure with Backside Contacts and Packaging of the Same", Oct 2019, US Patent No. 10,461,239.

[42]      D. A. Mills, C. Barnard, and M. Sheplak, “Characterization of a Hydraulically Smooth Wall Shear Stress Sensor for Low-Speed Wind Tunnel Applications,” in 55th AIAA Aerospace Sciences Meeting, 2017.

[43]      D. A. Mills, W. C. Patterson, C. Keane, and M. Sheplak, “Characterization of a Fully-Differential Capacitive Wall Shear Stress Sensor for Low-Speed Wind Tunnels,” in 2018  AIAA Aerospace Sciences Meeting, 2018.

[44]      “DirectShear Sensing System - IC2 - Shear Stress Sensor.” [Online]. Available: https://www.thinkic2.com/products/sensors/directshear/. [Accessed: 5-Mar-2020].

IC2's DirectShear™ Sensing Systems 

Topics: Press Release, Wind Tunnel, Skin Friction, Shear Stress, DirectShear, Flight Testing, Guides

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