Capacitive position sensor. A position sensor is made as follows: Two tubes with

Question

Capacitive position sensor. A position sensor is made as follows: Two tubes with very thin walls are placed one inside the other so that they are concentric, separated by a Teflon layer, and can move in or out between two preset limits x1 10 mm and x2- X2 50 mm, as shown in Figure 5.99. The tubes are L 60 mm long a. Calculate the minimum and maximum capacitance between the two tubes, assuming that an electric field may only exist in that area in which the tubes overlap. b. Show how this device can be used as a sensor, that is, calculate its sensitivity for a4 mm, b 4.5 mm, 10 mm, x2- 50 mm, and the relative per- mittivity of Teflon Ep = 2.0. FIGURE 5.99 Capacitive position sensor. Te flon -- Moving tube 2a 2b

Explanation / Answer

The key features of capacitive sensors are:

(a)minimum capacitance=eA/d

=8.85*10^(-12)*(area)/10

=8.85*10^(-12)*(60*10)/10=5.31*10^(-10) farad

maximum capacitance=8.85*10(-12)*(60*50)/50=5.31*10(-10)farad

(b) It can be used as a sensor because it measures the changes in an electrical property called capacitance. Capacitance describes how two conductive objects with a space between them respond to a voltage difference applied to them .

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