Mechanical_measurement - Measurement of Mechanical...

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Measurement of Mechanical Quantities Author: John M. Cimbala, Penn State University Latest revision: 15 April 2008 Introduction Some examples of mechanical quantities that need to be measured include position (or displacement ), angular velocity (rotation rate of a shaft) , force , torque , and shaft power . Instruments of various kinds have been invented to measure each of these quantities. In this learning module, several of these instruments are discussed; the list here is by no means exhaustive. The purpose here is to give you a feel for how mechanical instruments work, and to make you aware of the variety of ways to measure things. Position and Displacement Measurement The standard unit of length in the SI system is the meter (m). The standard meter is 1,650,763.73 wavelengths of krypton 86 in a vacuum . (Krypton 86 is an isotope of the element krypton, and is an orange color.) Various kinds of instruments have been invented to measure distance, length, or displacement. Some of these (especially the mechanical devices) are simple and straightforward, and are discussed first. Some more sophisticated measurement devices also exist for measuring displacement, and these are discussed later. Mechanical devices Principle of operation : o A simple comparison is made between a displacement (or an object’s length) and that of a pre-measured displacement or length. o The length or displacement is inferred to be the same as that of the (known) pre-measured length. Examples include: o Lengths of various magnitudes are measured with a standard ruler or tape measure . o Small lengths are measured with a more precise device called a micrometer . o Cylindrical or spherical diameters are measured with vernier calipers . o Various gaps and the inner diameters of small tubes are measured with gage blocks (a set of precision ground hardened steel objects of known size). Interferometer Principle of operation : o Interacting light waves produce interference patterns. Surface being analyzed Optical flat Defect o Displacement is inferred from the interference patterns produced by light reflecting off a surface. An interferometer uses single wavelength light (like that from a laser) for optimum quality. A schematic diagram is shown to the right, and the fundamentals of interferometry are analyzed here. o The light is passed through a precision ground quartz disk called an optical flat . The optical flat is tilted at some small angle, as shown. o Incoming light passes through the optical flat, and is reflected as sketched. Interference fringes Optical flat Defect o Interference patterns are visible when viewing the reflected rays of light, since the optical path of individual rays differs. Each dark and light band represents a distance equal to half of the wavelength of the light .
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This note was uploaded on 07/23/2008 for the course ME 345 taught by Professor Staff during the Spring '08 term at Penn State.

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Mechanical_measurement - Measurement of Mechanical...

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