Capacitive displacement sensor: Difference between revisions

Revision as of 08:23, 30 March 2012

Revision as of 02:57, 12 May 2012

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'''Capacitive displacement sensors''' “are non-contact devices capable of high-resolution measurement of the position and/or change of position of any conductive target”.<ref name="LionCapOverview">[http://www.lionprecision.com/capacitive-sensors/index.html#apps Lion Precision Capacitive Sensor Overview], An overview of [[capacitive sensing]] [[technology]] from Lion Precision.</ref> They are also able to measure the thickness or density of [[Non-conductor|non-conductive]] materials.<ref name="SensorTechHandbook">{{cite book|isbn=07506772950-7506-7729-5|title=Sensor Technology Handbook|page=94|url=http://books.google.com/books?id=fdeToUK8edMC&pg=PT94|author=Jon S. Wilson|publisher=Newnes|year=2005}}</ref> Capacitive displacement sensors are used in a wide variety of applications including [[semiconductor]] processing, assembly of precision equipment such as [[disk drive]]s, precision thickness measurements, [[machine tool]] [[metrology]] and [[assembly line]] testing. These types of sensors can be found in [[machining]] and [[manufacturing]] facilities around the world.

==Basic capacitive theory==

[[Capacitance]] is an electrical property which is created by applying an [[electrical charge]] to two conductive objects with a gap between them. This property is most commonly illustrated using the example of two parallel conductive plates with a gap between them and a charge applied to them. In this situation, the Capacitance can be expressed by the [[equation]]:

:<math> C = \dfrac{\varepsilon_0 K A}{d} </math> <ref name="Physics">{{cite book|isbn=08790113510-87901-135-1|title=Physics Second Edition|author=Paul Allen Tipler|pages=653–660|publisher=Worth Publishers|year=1982}}</ref>

Where ''C'' is the capacitance, ε₀ is the [[permittivity of free space]] constant, ''K'' is the [[dielectric constant]] of the material in the gap, ''A'' is the area of the plates, and ''d'' is the distance between the plates.

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===Machine tool metrology===

Capacitive displacement sensors are often used in metrology applications. In many cases, sensors are used “to measure shape errors in the part being produced. But they also can measure the errors arising in the equipment used to manufacture the part, a practice known as machine tool metrology”.<ref name="LLNL">[https://www.llnl.gov/str/Blaedel.html Lawrence Livermore National Laboratory: Engineering Precision into Laboratory Projects], Examples of advances made by LLNL in the field of precision measurement.</ref> In many cases, the sensors are used to analyze and optimize the rotation of spindles in various machine tools, examples include [[surface grinder]]s, [[lathe]]s, [[milling machine]]s, and [[air bearing]] spindles.<ref name="PrecisionSpindleMetrology">{{cite book|isbn=16059500331-60595-003-3|title=Precision Spindle Metrology|author=Eric R. Marsh|publisher=Destech Pubns Inc|year=2009}}</ref> By measuring errors in the machines themselves, rather than simply measuring errors in the final products, problems can be dealt with and fixed earlier in the manufacturing process.

===Assembly line testing===