The first few sections of this guide discussed indirect vs. direct measurement of wall shear stress. The remainder of this guide will dive deeper into direct, floating element shear stress sensors, focusing on transduction methods and how to select the best method for a particular application. There are four main transduction methods commonly employed with floating element shear stress sensors: piezoresistive, piezoelectric, capacitive, and optical. They all work in very different ways, but ultimately convert the shear stress into a measurable electrical signal. Each offers various strengths and weaknesses, making some methods better suited than others for particular applications.

Piezoresistive Transduction

The piezoresistive effect [25] describes a change in electrical resistance that occurs when certain materials (e.g., some semiconductors and metals) undergo mechanical strain. Sensors using piezoresistive transduction typically employ doped silicon regions arranged in a Wheatstone-bridge configuration. A Wheatstone bridge is a simple circuit used to accurately convert the change in resistance into a measurable voltage. Through appropriate design and doping, the bridge can be made partially or fully active, depending on whether one or more legs of the bridge are active piezoresistors. A fully-active bridge employs four piezoresistors, while a half bridge and quarter bridge employ two or one piezoresistors, respectively, with the remaining bridge resistors replaced by passive elements.The advantage of a fully-active bridge is increased sensitivity along with common-mode rejection to reduce pressure sensitivity. Ion implantation is commonly used to fabricate the piezoresistors on the sensor tethers, targeting regions that undergo high strain from an applied shear stress.

Figure 1: Piezoresistive floating element sensor example.

Piezoresistive transduction has a long history of use for pressure sensors and has been recently developed into macroscale shear stress sensors by Ahmic Aerospace [26]. For microscale sensors, piezoresistive technology has been developed at the research stage [27], [28], but no commercial microscale devices are currently available. Piezoresistive transduction enables a low sensor output impedance that eases performance requirements on signal conditioning electronics; however, it requires significant power to operate since it draws a steady supply of current through the piezoresistors and the amplifier. Furthermore, piezoresistive sensors are sensitive to temperature which results in thermal drift and requires temperature compensation.

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

References

[25]      L. E. Hollander, G. L. Vick, and T. J. Diesel, “The Piezoresistive Effect and its Applications,” Rev. Sci. Instrum., vol. 31, no. 3, pp. 323–327, Mar. 1960.

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

[27]      Y. Li, T. Nishida, D. P. Arnold, and M. Sheplak, “Microfabrication of a wall shear stress sensor using side-implanted piezoresistive tethers,” 2007, p. 65290D.

[28]      A. A. Barlian, S.-J. Park, V. Mukundan, and B. L. Pruitt, “Design, Fabrication, and Characterization of Piezoresistive MEMS Shear Stress Sensors,” in Microelectromechanical Systems, 2005, vol. 2005, pp. 531–536.

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