A Guide to Wall Shear Stress Measurement - #8 - Transduction Summary

Posted by Jared Anderson on May 26, 2020 8:15:00 AM

The previous four sections of this guide discussed the four primary direct transduction methods commonly employed with floating element shear stress sensors: piezoresistive, piezoelectric, capacitive, and optical. This section discusses key performance criteria for floating-element shear stress sensor measurements and provides a comparison of these criteria for the four transduction methods to aid in sensor selection.

Selection Guidelines

Selecting the best transduction method for a given measurement application is a complex decision that depends upon multiple factors. As discussed above, there are four main transduction types; each with their own set of advantages and technical challenges.  An ideal sensor for all shear-stress measurement applications is non-intrusive, provides a known and quantifiable gain and phase relationship and demonstrates immunity to unwanted inputs (e.g. temperature, EMI, etc). Further, key measurement-specific performance criteria must be met including: linearity over the expected dynamic range, flat bandwidth over the operating frequencies, sufficiently high spatial resolution to minimize spatial averaging, and must be robust enough to survive and function in the testing environment. Any or all of these criteria may sway the decision for one transduction method over another. Performance varies across manufacturers, however some basic comparisons can be made to simplify the selection process, as shown in Table 1. From these basic guidelines, initial down-selection of sensors can be made based on transduction method.

Transduction Method

Mean or Dynamic Shear

Pressure Rejection

High Temp Operation

EMI Immunity

Commercially Available

Power Requirements

Piezoresistive

Both

High

No

Moderate

Yes [26]

High

Piezoelectric

Dynamic

Low

No

Poor

No

Low

Capacitive

Both

High

No

Moderate

Yes [44]

Moderate

Optical

Both

High

Yes

High

Yes [44], [45]

Moderate

Table 1: Transduction method strengths and weaknesses

Sensor construction will be presented in the next two sections to provide an introduction to MEMS technology and its comparison to conventional techniques.

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

[26]      “Sensors — Ahmic Aerospace.” [Online]. Available: http://www.ahmicaerospace.com/products. [Accessed: 10-Aug-2018].

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

[45]      “RealShearTM Wall Shear Stress Sensor - Lenterra.” [Online]. Available: http://lenterra.com/realshear-wall-shear-stress-sensor-tech/. [Accessed: 10-Aug-2018].

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