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On Teaching the Operating Principles of Piezoresistive Sensors

Abstract

We present an approach to teaching the operating principles of piezoresistive sensors that addresses many of the limitations of the treatments encountered in most instrumentation and MEMS textbooks. Namely, we direct the presentation to an undergraduate audience rather than a research-level audience and at the same time we avoid oversimplifying the development of the principles of operation. To this end, we make a discussion of bridge analysis central to the development, use a strain-formulation for gage factor and piezoresistor placement rather than the more common stress-formulation, and keep the associated physics and mathematics at an appropriate level for sophomore engineering undergraduates. In so doing, we maintain accessibility and coherence throughout. We present several sets of learning objectives and strategies for teaching the material that can be tailored to suit the needs of a particular course.

Introduction

Piezoresistive sensors are commonplace—the dominant commercial applications are piezoresistive accelerometers for automotive airbag deployment and piezoresistive pressure sensors for both automotive and medical applications1. Because of this widespread use, particularly in micro-electro-mechanical systems (MEMS) applications, undergraduate engineering programs whose learning outcomes include instrumentation technologies generally include an introduction to the basic operating principles of piezoresistive sensors. In our opinion, however, the exposition of these principles in popular textbooks for instrumentation systems and MEMS are generally inadequate—authors tend to either oversimplify, leaving a student unaware of operational details, or write for a research-oriented audience, making the material inaccessible to undergraduates. In this paper we present an approach to teaching the operating principles of piezoresistive sensors that addresses these issues.

The distinguishing features of our approach are its accessibility and coherence. First, the technical content and mathematics are appropriate for sophomore-level engineering undergraduates. Second, the technical material is presented coherently and completely, that is, each step of the exposition is motivated by the results of the previous step. Third, since mechanical strain is the physical phenomenon relating input to output, a strain-formulation is used for gage factor and for the placement and orientation of the piezoresistors instead of the stress-formulation found in most textbooks. In this paper we share our approach with the instrumentation education community in the hope that its accessibility and coherence will help improve the teaching of the operating principles of this one important type of sensor.

Limitation

We have no student learning data to specifically support our assertion that the approach we present has greater coherence and accessibility for undergraduates than any other. However we do make the case in the following section that our work makes a contribution via a synthesis of the strengths of widely-used texts. Also, in recent years we have seen a steady increase in our

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Layton, R., & Adams, T. (2010, June), On Teaching The Operating Principles Of Piezoresistive Sensors Paper presented at 2010 Annual Conference & Exposition, Louisville, Kentucky. 10.18260/1-2--16635