Capacitive pressure transducer and accelerometer

Capacitive pressure transducer and accelerometer - SEN2MET...

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SEN2MET Capacitive pressure transducers and accelerometers SEN2MET, Chapter 8
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MEMS technology/product map
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Rough comparison between capacitive and piezoresistive sensors: Piezoresistive: Ease of signal conditioning Good linearity High signal level Immunity to electro-magnetic interference Capacitive: Low power consumption Principle also useful for non-silicon tech (e.g. LIGA) High sensitivity (due to progress in signal processing tech) Good long term stability Less temperature dependent (than piezoresistive) Capacitive designs have become a great success
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Sensor types Capacitive, Chapter 8 Chapter 9
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Part 1: Capacitive Pressure Transducers
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Capacitive pressure transducers Flat circular diaphragms 2 element combination in earlier designs: Circular diaphragms Radius of electrodes are different on variable and fixed capacitors Capacitors sandwiched between glass-plate and silicon substrate Reference capacitor Variable capacitor
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Capacitive pressure transducers Flat circular diaphragms Movement characteristics of circular diaphragms: Chapter §2.6.3 in textbook w r ( ) w 0 1 r 2 a 2 2 w 0 w 0 ( ) p a 4 64 D 0 D 0 E h 3 12 1 ν 2 ( ) C 0 π γ 0 2 a 2 ε ε 0 d 0 C x 0 γ a r ε ε 0 2 π r d 0 w r ( ) d π ε ε 0 a 2 2 w 0 d 0 ln 1 γ 2 w 0 d 0 + γ 2 w 0 d 0 + w 0 d 0 1 γ 2 w 0 d 0 γ 2 w 0 d 0 + w 0 d 0 ref [1] in book Flexural rigidity Displacement function At centre, Δp pressure difference Fixed capacitor
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Capacitive pressure transducers Flat circular diaphragms, calculation of C x 22 2 4 00 2 2 0 0 2 0 0 0 ( 1) 2 (1 ) Substitute: 1 , 2, 2 11 ) ( 1 )( 1 ) 1 2 rr x x r aa xx C dx dx dd ww x s x ds xdx xdx ds C ds ds d s ss w a w d w γγ πε = = −+ − −− = = = = = −+ +−   = +  ∫∫ 2 0 0 2 r a ds ds dw d d = 
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Capacitive pressure transducers Flat circular diaphragms, calculation of C x 22 2 4 00 2 2 0 0 2 0 0 0 ( 1) 2 (1 ) Substitute: 1 , 2, 2 11 ) ( 1 )( 1 ) 1 2 rr x x r aa xx C dx dx dd ww x s x ds xdx xdx ds C ds ds d s ss w a w d w γγ πε = = −+ − −− = = = = = −+ +−   = +  ∫∫ 2 2 0 0 2 0 0 0 2 0 0 0 0 2 1 ln ln Q.E.D 1 r s s a ds ds dw d d d s w d d d s w γ = = =  + = =
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Differential capacitance read-out circuitry Flat circular diaphragms Diode-quad structure: Page 179 in textbook For a specific γ, γ 0 may be selected for zero output for a specific pressure 2 0 2 0 Sensor sensitivity: 4( ) ( 2) Typical value for 0.6 is maximizing the sensitivity out out x x xP x PF xp dV dV dV S dp d V dp C C C dC VV C C C dp γ = = ∆∆ = ++ A problem with flat diaphragm structures are the resulting large non-linearity of the sensor, particularly when the sensitivity S is large. The displacement of a flat diaphragm Is not linear and is difficult to linearize. 0
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This note was uploaded on 05/30/2011 for the course SEN 2MET taught by Professor Ulrikhanke during the Spring '11 term at VCU.

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Capacitive pressure transducer and accelerometer - SEN2MET...

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