Sensors and Achutors A, 41-82 (1994) 439-441
439
A thick-film pressure transducer for cars propelled by natural gas
L Fragi, D. Lupl and L Malatto
Research Centre for Electronrcs and Infonnaacs Technology, CC 157, 1650 San Martln, Buenos Aves (Argentma)
Abstract
The development of a thick-film pressure sensor on a stamless steel substrate for apphcation m cars propelled by compressed methane gas ISpresented The development ISbased on the classic model of a cxcular diaphragm on which four thxk-film resistors are screen-prmted They are connected m a Wheatstone bndge configurabon The sensor response between 0 and 400 bar shows a senslhwty of 1131 mV/V with a 0 1% Fs hneanty, 0 2% FS hysteresis and 2 CcvrvpC zero thermal drift The constant-temperature zem shift due to the creep effect IS nunmxzed by annealmg the samples
Introduction
Transducer design
One of the main properties of thick-film resistors IS their reversible plezoreslstwe effect, which has been studied smce the early 197Os, although Its utlhzatron for the development of pressure sensors started around 1980 Thick-film strain gauges, their long-term stability and the mfluence of temperature on then properties have been studled Since then Different types of thlckfilm pressure transducers on alumma substrates have been developed for different apphcatlons, particularly for the automobile Industry [l]
Automotive mdustnes reqmre low-cost and rugged sensors, which means inexpensive and non-bnttle matenals, capable of operating m extreme conditions of temperature, vlbratlon and humidity Although tradltlonally alumma has been used almost exclusrvely as the transducer substrate, new technologcal reqmrements demand a suitably insulated metallic substrate for certain apphcahons [2] Machmablhty, which allows the sensor body and diaphragm to be obtained m one piece, 1s the most appreciated advantage of a metalhc substrate This makes it a safe and easily manufactured component In particular, the use of stainless steel substrates renders higher sensltlvlty and better mechanical properties m terms of bendmg stress and maxunum allowed strain
The development of a thick-film pressure sensor on a stamless steel substrate 1s presented m this work The sensor was developed for apphcatlon m compressed methane gas-propelled cars, which represent a cleaner alternative to gasoline-propelled ones 111terms of envlronmental protection
The classic model of a circular diaphragm with an embedded border, where four thick-f&n resistors are connected m a Wheatstone bridge cotiguratlon, was used for the pressure-sensor development In order for the m-urn bndge unbalance to be obtained, two resistors were placed around the centre and the other two close to the embedded border, with their lengthwlse axis parallel to the diaphragm radius
In the mechamcal design of the body and diaphragm a O-200 bar range was considered, as well as comphance with standards of usage that require hydrauhc pressure testing at 0400 bar workmg ranges Space restraints also reqmred the device to be small
Figure 1 shows an outhne of the transducer mountmg, the 3 6 mm thick diaphragm has an effectlve radius of 9 mm These parameters were carefully selected m order to remam wltlun the diaphragm elastic hmlt and
,,Cover & connector
Electronics board
Sensor diaphragm
Rg 1 Pressure transducer assembly
0924-4247/94/$07 00 0 1994 Elsevler Sequoia All rlgbts reserved SSDI 0924-4247(93)00536-D
440
avoid breakage A stress level of 1015 kg/cm’ was obtamed for this diaphragm design
The onepece transducer was machmed m AISI 316 stainless steel In order to obtam the four thick-film stram-sensitive resistors, the followmg steps were performed with a standard screen-prmtmg, drymg and firmg cycle
(I) two layers of thick-film dlelectnc paste (Heraeus IP 211) fired at 950 “C for 10 mm,
(u) one layer of thick-film dielectric paste (Heraeus IP 9117) fired at 920 “C for 10 mm,
(111o)ne conductive layer of thick-film paste (Heraeus H 1214B) fired at 850 “C for 10 mm,
(iv) one reslstlve layer of thxk-fihn paste (Heraeus R 8041) fired at 850 “C for 10 mm
The electromc design was based on an instrumentation amplfier circuit wth controlled supply voltage and over-pressure detection, which was mounted on the sensor body As the sensor has to be located close to the combustion gases, the associated electronics were deslgned followmg mtrmslc safety rules [3]
In order to study the sensor stablhty m unloaded condltlons, the output of the bndge was measured at constant temperature for four weeks Results are shown in Fig 3 The repstered zero dnft was 0 6%, which 1s rather high for mdustnal apphcatlons Such a dnft at constant temperature 1s a typical phenomenon m metallic substrates, which accumulate residual stresses during heatmg It 1swell known that these stresses can be relieved by an appropnate heat treatment, which m this case was an annealmg at 230 “C for 24 h The constant-temperature zero dnfi after heat treatment 1s also shown m Fig 3
L&e ceramic pressure sensors, metalhc ones show a zero dnft with temperature In order to determme this dnft, the sensor was subjected to a thermal sweep from 0 to 100 “C and the bndge output was measured unloaded Results can be seen m Fig 4, whch shows a zero thermal dnft of 2 WV/WC This value LSsmular to that obtained m commercial ceranuc-substrate pres sure sensors
Results and discussion
SensitMy, hnearlty and hysteresis tests were carned out on a Degranges et Hout 5300 pressure balance, an HP 6282A power supply and a Kelthley 197 d@al voltmeter of 5 l/2 digits
The sensor temperature performance was tested m a Heraeus HC4020 chmatlc chamber
Figure 2 shows the sensor response between 0 and 400 bar The maxmmm measured senslhvlty was 1131 mV/V mth a 0 1% full-scale (FS) lmeanty and 0 2% FS hysteress The calculated gauge factor was 16 The temperature coefficient of resistance (TCR) was also measured for each resistor and the value of 320 ppm/ “C m the range 25-100 “C was obtamed
AVo% 07
at25°C 06
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01’
I
I/
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I
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700
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Rg 3 Zero dnft response (A) before beat treatment, (B) after heat treatment
Vo (mV)
Vo LmV)
0 093
I
Pressure (bar)
Fig 2 Pressure sensor response wtb unbalanced v SUPPlY
bridge at 10
0
10 20
30
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’
w) 100
Temp (“C)
Fig 4 Zero thermal drift of pressure sensor
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Conclusions
gas-propelled cars, where mtrmsrcally safe devrces are needed
The sensor described showed the followmg charactenstxs sensrtrvrty, 1131 mV/v FS, Imeanty, 0 1% FS, hysteresis, 0 2%FS, zero thermal dnft, 2 pVjV/ “C
The sensor proved to be robust, rehable and safe and it can be produced at compeutrve prices A pilot productron of these sensors rs currently under field testmg The results encourage Its apphcatton m methane
References
A Cattaneo, R Dell’Acqua, F Forlam and L Pxozw, Low
cost ttuck film pressure sensors, ME SP 454 Detw, USA,
Feb 25-29, 1980, pp 49-54
2
NM Wlute, An assessment of thck f&n Insulated steel substrates, Hybnd Cmum,
ptezoreslstors on 20 (1989) 17-21
3 Intemahonnl Standard IEC 79-11, Electrical apparatus for
exploswe gas atmosphere Part II Safely ‘I’, 1990
Ver+/-
