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As their name suggests, strain gauges are sensors that measure strain, or the displacement of a body compared to its original length, when subjected to an applied force. It is common when discussing strain to include the idea of stress, which is the force per unit area. Combining these two concepts in a simple fashion leads to the idea of stress being a result of strain! Young’s modulus is defined as the ratio between stress and strain and is a commonly quoted property of metals. This property is often used to discuss the elasticity of metals, that is to say, a stress applied to a metal will cause it to elongate, but the metal will return to its original length when the stress is removed. This is a mechanical property of the materials and one of great interest to optical engineers who are interested in high-precision alignments. Environmental changes ranging from temperature and pressure to irradiation by light to movements in a gravity field can all make small but measurable changes in an optical system. Other types of strain gauges are available, such as fiber optical gauges, which can be very useful; however, here we will restrict ourselves to the classical strain gauges. In this chapter we will be concentrating on the electronics and electronics systems used to process the signal from strain gauges and strain gauge bridges. We will also take this opportunity to outline how to move information into a microcontroller or a computer. Strain gauges are remarkably sensitive devices and are commonly used in almost every science and engineering discipline.
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