E L S E V I E R Sensors and Actuators A 57 (1996) 53-63

Four essential points in developing automotive sensors

H i r o s h i K o b a y a s . h i Electronics Development Department, Nissan Motor Co., 560-2. Okatsukoku, Atsugi City. Kanagawa, 243-01, Japan

Received 14 June 1996; accepted 28 June 1996

Abstract

There are four essential points in the successful development of automotive sensors. ( 1 ) Materials technology, measurement technology and electronic technology are the principal technologies required. ( 2 ) Tbe basic sensor design is derived through a process in which the object to be measured is accurately related to the capabilities of the sensing materials and devices. (3) An examination of the feaanes of the sensing method in relation to the nature of the object to be measured clarifies the optimum sensing technique. (4) An appropriate sansor ~ and constituent parts are selected to address the technological issues involved. This paper explains why these four points are esser~a~ in developing an automotive sensor, and how the most suitable sensor technology is selected. Specific examples of aatomofive sensors are presented.

Keywords: Automotive sensors; Sensor design

1. P r inc ipa l t echnologies fo r s e n s o r deve lopmen t

The technologies required for the development o f auto- motive sensors encompass a wide spectrum of technical fields, as summar ized in Fig. 1. Materials technology is essen- tial in three respects. One is to realize new physical properties

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o f sensing materials or new electrical characteristics o f sens- ing devices. Another is to design a s imple sens ing structure. The third is to improve the reliability o f sens ing parts or devices. Measurement technology is also essential in three respects. One is to measure physical or chemical phenomena accurately in automotive parts and materials, and ~o examine

• Develop ~ of wamuf~"tm-

•Fln¢ mm mo~neng or ~

Fig, I. Technologies required for developing automotive sensors.

0924-42471961515.00 Copyright © 1996 Elsevier Science S.A. All righ~ reserved Pll $0924-4247 (96 ) 01334-9

54 H. Kobayashi /Sensors and Actuators A 57 (1996) 53-63

the physical or chemical quantifies of the measured phenom- ena. Another is to find new sensing methods for detecting the object to be measured. The third is to predict the best sensing position for a sensor under development and the main tech- nological problems to be addressed in that sensing position. Electronic technology is essential in two respects. One is to design a simple electric circuit configuration and identify the required functions for signal processing. The other is to ensure the reliability of electric circuits through suitable assembly and mounting methods. Whenever we develop a sensor, it is necessary to develop technologies extending over these three categories.

An actual example will be given in this paper to illustrate the technologies developed for a new automotive sensor. This example concerns an ultrasonic vehicle-height (i .e, ground- clearance) sensor. The requirements for the sensor system were to improve riding comfort and vehicle handling and stability by varying the damping rate of the shock absorbers to match the level of vehicle vibration [ 1 ]. Information on vehicle vibration, which is indicated by the pattern of time- related changes in vehicle height, can be found by measuring the distance between the vehicle floor and the road surface.

There were two technological issues that had to be resolved in developing this sensor. One was to eliminate the effects of direct waves, which would occur because of the short meas- uring distance involved. The other was to ensure that the sensor would operate with complete reliability in the harsh environment under the vehicle floor. This paper will describe the technology developed for the vehicle-height sensor in connection with the first technological issue.

The problem posed by direct waves is that the damping edge of a direct wave signal overlaps the rising edge of a reflected wave signal, making it difficult to detect the rise timing of the latter. This problem was resolved through the use of two different approaches. One, the strength of the direct waves was reduced, and two, their arrival time was shortened. Fig. 2 illustrates how the strength of the direct waves was reduced. Direct waves can be divided into two types. Some direct waves are transmitted directly through the inside of the sensor. Other direct waves are transmitted directly through the air from the transmitter to the receiver.

To eliminate the influence of the first type of direct waves, we improved the material and structure of the rubber insulator covering both the transmitting and receiving microphones, except for the transmitting and receiving surface. Ordinary rubber generally becomes harder as the temperature decreases. Since harder, more rigid rubber is less capable of absorbing the ultrasonic wave signal, the strength of the direct waves passing through the sensor would increase. To prevent this, we made two improvements to file rubber insulator. One was to engineer the insulator with a foam composition so that the hardness of the rubber would not increase significantly at low temperatures, and also the acoustic energy would be damped by the vibration of the air in the foam. The other was to design an appropriate plasticizer made up of a polymer that did not show much increase in hardness at low temperatures.

Direct wave Intel/erence In short-dlslance measurement Transmitted • [ ] lionel 40kHz , ~ I

Re~elve~ ~ Wml 8lgnal Receiver ~ JTrsnemRtor

....... % 7 Reflected wave .l#~ll~lmu#l

Optimized sensor structure for direct wave reduction ~) Direct were level reduction .~,

[ u.,no ~...,~,,ct. ram,, .nd ] Mruoture fog the Insulator

~) Direct w a ~ v e l reduction

Using the appropriate e¢ouMl¢ horn I shape I

rn

Senior im'ut~lre Fig. 2. Ways of reducing direct wave level.

L,o]\ ] \\4-o~.,

--4O --20 0 eO Temperature ('C)

r

Ao -~o ' ~o " Temperature (C)

Fig. 3. Characteristics of new insulator robber.

The improved rubber has an independent foam composition, with the limit foaming ratio in moulding of EPDM (ethylene propylene rubber), and a good-quality plasticizer. Fig. 3 shows the change in the relative modulus temperature, Tn, of the rubber as a function of ambient temperature, along with the change in the S /N ratio. In this case, Tn is the temperature at which the hardness of the rubber is n times as hard as it is at 20°C. The S/N ratio is the ratio of the reflected wave signal strength to that of the direct wave signal. The improved rubber insulator maintains the prescribed hardness at temperatures as low as minus 30°C.

To counter the influence of the second type of direct waves, we optimized the shape of the acoustic horn. The transmitting and receiving microphones generally have fixed directivity characteristics that are dependent on the transmitting and receiving frequency and the structure and configuration of vibrating elements. The side-lobe component of the directiv- ity characteristics originates in the second type of direct waves. The side-lobe component of the transmitted and received signals was red,~:ed by developing a new simple shape for the acoustic horn, having an appropriate opening diameter and straight length. Fig. 4 shows the S /N ratio as a function of both the opening diameter and straight length of

H. Kobayashi /Sensors and Actuators A 57 (1996) 53-63 55

i z

Opening clhmleter (mini

18,Smm

!:] Straight length (mm)

Fig, 4. Direct wave characteristics with acoustic horn.

o ~ 1 ~ . . ~ Trinlmltter

~ ~ - 1 o " " ' .~

i / ,,eft ~ o ~ horn --30 , ,

_! .... !

--4~ --~0 --20 -lO o Damping qutmtllt (dB)

0' --40- --~0 --iO --Ifl 0

DImplng quinttty (dS) Fig. 5. Directivity characteristics.

the horn. The best SIN ratio is obtained with an opening diameter of 18.5 ram, which is about twice the length of the transmitting and receiving surface diameter, and with a straight length of 4.5 ram, which is about one-half the ultra- sonic wavelength. Fig. 5 shows the directivity characteristics with and without the new horn. The side-lohe component of the directivity characteristics has been substantially reduced by over 40 ° .

Next, the procedure used to shorten the arrival time of the direct waves is described. Fig. 6 illustrates the signal-proc- essing method, which is accomplished through a four-step process. First, one portion of the transmitted signal is sent

dire, cOy to the receiver by means of an electrical circumven- tion techr:que employing a resistor. The resistivity value of the resistor is set so that the strength of the electrical circum- vention signal is several times larger than that of the direct wave signal. Next, the envelope signal is superimposed on the direct and the reflected waves. The strengu~ of the enve- lope signal is adjusted so that the peak value of the supe~- impos~ direct wave appears exactly at the end of the transmitted signal. This can be acco~nplisbed because the strength of the electrical circumvention signal that generau~ the drive time of the wansmitted signal is sufficiently larger than the strength of the direct wave signal. After that, a dif- ferentiator is employed to obtain the de~vative of the received signal. As a result of this processing, the differential signal becomes zero at the point when the peak value of the direct wave appears. The differential signal is then masked. The masked direct wave signal appears within the drive time of the transmitted signal, all~wing the reflected signal to be separated from the masked direct wave signal. This procedure makes it possible to shorten the arrival time of the dil'ect wave, which means the effects of the direct wave signal can he eliminated.

Fig. 7 summarizes the various technologies ',hat were achieved in the process of developing the vehicle-height sen- sor. The new technologies indicated with a filled dot in this Figure represent measures adopted to overcome the effects of direct waves. Since they can be divided into three care- gories, it suggests that the development of a new sens~ requires the achievement of new technologies in the three basic areas of which sensors are composed.

~1., ~ o E I ~ , V . ~ . ~ . ~

Fig. 6. Signal-processing method.

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OUilltl m :Sul~i ~ ~ llSlltll ~ ll¢ltl-

,lilll~ iI lira m e MImlillilllll~ Im.'!/ll

:®lie

Fig. 7. Newly developed technologies in vehicle-height sensor.

56 H. Kobayashi /Sensors and Actuators A 57 (1996) 53--63

Therefore, in developing a sensor, it is essential to have a thorough understanding of all three disciplines. New tech- nologies are born of a process in which the best means are found for overcoming the technological issues that must be addressed.

2. Process for basic sensor design

A sensor is a structural element of an electronic control system or a measurement indication system. Sensor require- ments are derived from the performance required of a system, and they are reflected in the required specifications of the system. The required specifications represent the objectives to be met by a new sensor. Thus, sensor development is carried out as a needs-oriented process.

The most important stage in sensor development is the initial one, because the work done at this stage sets the direc- tion for all subsequent development activity. Fig. 8 illustrates the development process in the initial stage. The most impor- tant step in this process is to determine the optimum sensing method, which is done on the basis of two investigations. One involves an accurate prediction of the best sensing posi- tion, and the other involves the determination of the optimum sensing device and material for the sensing position.

The best sensing position can be found through a study of the sensor specifications in connection with the measurement materials. In this case, the measurement materials refer to the relation between the sensing position and the phenomenon to be detected. It boils down to a question of what kinds of phenomena should be detected in what sensing position of automotive parts or materials. As one example, the relations between the sensing position and the various phenomena detected by sensors in chassis-control systems are summa- rized in Table 1.

The optimum sensing device or material is found through a study of the behaviour of the object to be measured and environmental conditions in the sensing position in connec- tion with the abilities of the sensing devices and materials. This includes determining which features of the sensing devices or materials are best suited for meeting the require-

I ~"~ I~ P°~II°"I l l~'m~ "~ ~"I

I"

Fig. 8. Flowchart of initial sensor development pro~ess.

Table 1 Sensors used in chassis control systems

Conlrol ~ M ~ po~lon Pl~momen~ d ~ d

TrllCllofl i ld ~'llle --'--'I-- 01~ l ~ e - - - ' r Tor~l~,r,~Im~r- l~l~e cm~Iml r r I ' - Gllll~er ~ - -

~-- Idl~er cylinder PrNmm) I - Ilmto ;ednl ---f-- m~.~ m d - - Dl~mmm.~ L ~ Pedal I[',~lel efk~l I - l~meol ----r-- Tlnl ~ ~l~Immlol

'. L o ~ , . , ~ ~ - , o m , ~ / g o ~ n torsi

r.~oer, e~on i $WeM~n co~o; r~em

• oe~g ......

~_ Qnamd clutm~ Qroued

~ Acaelm.lllonl goun~lo Iml~lor Lold ~ ' ~ , --c ~ , ~

T ~ IIn~ I . o l d $ulpnlon ~ L

llCCelOnlllon St~:dllza. gOrl~ NONI

L Growl ekmnme

I ~- ~ ~F4r,~

I i - StN~g gee;' - - ~ Tom~ torte Power.ml~t fOrml [ L . . . . . . . . . .

| ~ r ~

! L-- Wheel Tim - - HodzoMal

o~ Iome

ments of the intended automotive use. Determinations must be made as to what kind of device or material is appropriate for the kind of object to be measured. As an example, Table 2 summarizes the capabilities of sensing devices and materials for automotive use. These two studies thus ensure that the predicted sensing position and the chosen sensing device and material are suitable for the specifications required of the sensor to be developed.

An actual example will be given to illustrate the two studies noted above. This example concerns two kinds of capaci- tance-type position sensors, designed to measure directly the linear motion of vehicle parts. One sensor measures large travels ranging from 50 to 500 mm and the other sensormeas- ures small travels in a range of 5 to 50 mm.

Linearity is the most important specification for a position sensor measuring linear travels. Excellent linearity can be achieved with the following two design approaches. (1) The sensing structure is designed to convert positions directly into capacitance so that hysteresis can be mitigated. To do this, the new sensors have been constructed of sensing electrodes that convert the relative displacement of automotive parts directly into the relative movement of sensing electrodes. (2) The electrode spacing is designed to provide a uniform gap between the electrodes so that nondinearity can be dimin- ished. To do this, the new sensors have been constructed of concentric cylindrical electrodes that readily assure the accu- racy of the elecU'ode gap. Table 3 summarizes the structure

H. Kobayashi I Sensors and Actuators A 57 (1996) 53-.63 5 7

Table 2

Capabilities o f sensing devices and materials

I ~ ~ ~ I I ~ m ] -- ~ q N ~ l ~ ' t l | - I f ~ 'F l i ~ : T t l ~ F l | ¢ o t ~ t l l

rtmu~t, t,oeogtm~. ~ . . . . P t ~ megt~t Nd-F*-8 meg~et : M I ~ t t te~ I ~ i I 80~k,~d.

-- M~q~l~ ~nleondu~ d , ~ m

~ -H~I e t~ t devk:e 8#-Ae Hd

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: Tomp~l~O ~oeMd~t of H~q

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to I,lO~nm, ~ PhM0¢Ot~lUCtIvO it:~t~t NOtt~ : Dttfk era'lint art 1~0C la I00 pA.

effect dev~cu d~v(~

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-- ~ndn t~mtN~t d e ~ e t

F Metal i t~y ty!~ NI-Cr I~y : Tor~em~Jn ~ dl~hmt M ~ugo

fwte¢ ml mt~mm im I~ne~ ovw wt~ tm~mtlm mmgt.

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t --Amom~s ~ Fe-Sl-8 qmm : ~ ~m~tmt t~ I'1~d11~1 ,30 X 10 "e.

F~Go*St'8 : I ~ t ~ it trodem I X 1 0 ~.

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design approach used in developing these position sensors. The basic structure of the long-travel measuring sensor resembles that of a vehicle shock absorber, while the short- travel measuring sensor is designed to be built into a power cylinder.

Capacitance-type sensors consist of a variable capacitance section and a fixed capacitance section. In order to improve capacitance measurement accuracy, the ratio of the variable capacitance section to the fixed capacitance section must be increased. The following discussion will focus on this ratio in the two kinds of capacitance-type position sensors.

In the large-travel measuring sensor, two approaches were employed to increase the ratio. One was to increase the ~) io of hie case inside diameter, d4, to the cylinder outside diam-

eter, d3. The other was to reduce the capacitance formed in the resin insulation layer by using a resin having the smallest dielectric constant possible. Fig. 9 shows the temperature dependence of the dielectric constant in some typical plastic resins. A polypropylene resin has the smallest dielectric con- stant, as well as the most stable dielectric-constant character- istics. Thus, it was decided to insulate the electrodes with polypropylene resin.

On the other hand, the ratio in the small-travel measuring sensor is determined only by the dielectric-constant ratio of the capacitance-forming dielectric substance to the air, because, it is independent of structural factors. To increase the ratio, the dielectric substance should have as large a die- lectric constant as possible. In addition, this sensor is con-

58 H. Kobayashi /Sensors and Actuators A 57 (I 996) 53-63

Table 3 Structural design approach used for position sensors

Largo travel meauudng sensor Small travel meauudng sensor

OebN:~ng range 50turn to 500ram for linear mo~on of automotive 5 mm to 50ram for linear motion of automotive ~ t s ~u~

6aZic sth/CtUm 1. ConceMdc cytiodflcal electrode construction 1. Double concent~c cyllnddonl electrode supports accurate e ~ spacing, conslRICttun raises vadable capacitance.

2. Seflsor Interior is filled with oit to keep out 2. Dlaleutde substance with high dleleutdc foreign mutter, constant improves vadable capacitance.

3. Simile' in sln~um to e shock absod)ar since 3. Sensor Is Incorporated in power cylinder to I m ~ was placed on mudng applicability provide greeter protection. to veMcJe components.

Factors ~e~ng muanrement oucuracy

= I L ,J

: d~ : rod diameter d=: cylinder Inside diameter s,,..~t,,,, of..~,,t ,,,,,. ~.*.0. ,,,,.o, d3 : cylinder outside diameter d4: case Inside diameter x : percentage of rod Inserted Irl cylinder L : cyimdc, r lan35h

i C:,,2'," t .L.x/In (d2/dl) I

{vadsble capm'.Uanca section)

I +2"rr. -L/In (dJd..)-l-moln layer (r, se4 cede~tance secllon)

"Olelecthc -r-Change with time in constant of |enc lonod case oil: e I"-Frequency dependence

i L._Temperature dependence r-Eiectmde --r-Individuel vadatlon ]gap : L--Change with time / (d2-dl) /2,

ea aye T emperature dependence Lof dlelectflo constant Frequency dependence of dielectric constant

dl : first groundelectrode outside diameter d2: annde-electrode inside diameter d3: anode.elecVode outside diameter d4: cylinder Inside diameter

C=2w( ac - , oX.,x(l/In (de/d,)+1/tn(d,/d,,.J] ] (variable capacitance section)

+2w z eL[l/In (d2/dl)'l" 1/in (d,,/d~J] (fixed cap~itmce zecUon)

- DlelecUlc -1-Chanpo with time In constant of [enclosed case composite ~Frequency dependence matudal: E c Temperature dependence

- Electrode --.q-individual vadal|on gap : L -Change with time (dz-diV2 (d,-d~)/2'

structed of double concentric cylindrical electrodes in order to increase the variable capacitance for small-travel measure- ment. This means the dielectric substance also has a complex double concentric cylindrical structure. The dielectric sub- stance must therefore meet two requirements. One is to have a dielectric constant as large as possible and stable dielectric- constant characteristics. The other is to be producible by a processing method that can fabricate a complex configuration repeatedly at a low cost. In order to fulfil these two require-

8 66 Nylon resin

~ ] 6 N y l . . . . sln~/bt Phenol resin

~. Polypropylene resin

() 50 100 150 Temperature ("C)

Fig. 9. Temperature dependence of relative dielectric constant for various resins.

ments, we have developed a new composite dielectric mate- rial for use as the dielectric substance.

This composite material comprises a ceramic dielectric and a plastic resin. The ceramic material should have stable di- electric characteristics, as well as a large dielectric constant. The plastic resin must be stable in shape, and also provide stable dielectric-constant characteristics.

Compositions based on BaTiO3 are often used in the prep- aration of PTC resistors and high-dielectric-constant capaci- tors. On the other hand, TiO2-rich compositions may be used for the preparation of dielectrics with a low temperature coef- ficient of capacitance over a wide temperature range. Ceramic dielectrics fabricated of the selected compositions in the BaO-TiO2-Nd203 system [ 2 ] exhibit room-temperaturedie- lectric constants of 80 to 100 and low temperature coefficients of capacitance ( - 1 0 0 p p m K - I ) . In addition, the BaO- PbO-Nd203-TiO2 systems [3] afford high dielectric con- stants and good temperature stability. Table 4 shows the compositions and their dielectric characteristics. As a result of this materials study, we chose the sample no. 4 in Table 4 as the ceramic material for the composite dielectric.

H. Koboyashi ~Sensors and Actuators A 57 (1996) 53-63 59

Table 4 Microwave characteristics of BaO-PbO-Nd203-TiO2 dielectric resonators

No. Composition (tool%) g' Q (at Tt 3 GHz) b (ppm °C) ¢

B~) PbO NdO3/2 TiO2

I 7 0 38 55 60 ! 100 very slight 2 9 2 31 58 65 1700 very slight 3 9 5 29 57 85 1800 very slight 4 8 7 27 58 88 2000 very slight

"e=dielectric constant. ~Qffidielectricloss. cetffitemperaturecoeflicientofresonancefrequcncy.

Measured at 10KHz

- o . . . .

i . , . - - - . - - -*--* = _ = (BoO 8mol%-PbO 7mol%*NbaOa 21mol%-TIO= 58mo1~)

13 --~ Composite matedal

J IOeraml¢ material 4OVol%-I-Poly(~atal rosin 60Vo1%)

5 Polyacetsl rosin - _- - .

3 r " " ° "~ ' e

i i i l i i --60 --20 20 60 100 140

Temperature (C) Fig. 10. Temperature dependence of dielectric constant.

A polyacetal resin has stable dielectric-constant character- istics as well as a relatively large dielectric constanL as shown in Fig. 9. It is stable in shape after injection moulding. For these reasons, we chose the polyacetal resin for the composite dielectric.

Fig. 10 shows the temperature dependence of the dielectric constant of the new composite material for a typical speci- men, which is a mixture of 40 vol.% ceramic dielectric and 60 vol.% polyacetal resin and is produced by injection mould- ing. It has a large dielectric constant, about 3.2 times as large as that of the polyacetal resin, and the dielectric constant shows little temperature dependence. The dielectric constant also shows good frequency characteristics from 1 kHz to 10 MHz. The capacitance variable ratio to travel when the composite material is used as a capacitance-formingdielectric substance is about two times as great as that for the polyacetal resin. The newly developed ct~mposite material thus shows good dielectric characteristics for use in a position sensor. In addition, the dimensional variability of the double concentric cylindrical configuration is so small, as little as 0.03 mm, that the linearity difference between position sensors is a slight 0.01 mm.

In developing the composite dielectric material, two inves- tigations were very useful. One examined what sensing struc- ture was best suited for the capacitance-type position sensor m e a s a r ' ~ ;:.near travels. The other focused on the kinds of

characteristics required for the capacitance- forming dieleca'ic substance of position sensors.

Reviewing the initial development stage of the two kinds of capacitance-type position sensors, we see that there are two aspects which must be studied in executing a basic sensor design. One is that the sensor specifications must match the measurement materials. The other is that the capabilities of the sensing materials and devices must be suitable for the phenomena in the sensing position. These two studies thus ensure that the basic sensor design fits the required sensor specifications.

3. Suitable techniques for sensor development

The automobile is one of the most convenient and widely used means of transportation, and it must provide diverse forms of product value. One way of achieving diverseproduct value is to use electronic control systems. This requires sen- sots for a variety of different purposes.

Different sensing methods or devices can require different application techniques even when they are being applied to the same type of sensor. Conversely, the same type of sensing method or device can require a different application approach when it is used in different types of sensors. Therefore, the particular nature of the sensing methods or devices must be suited for the phenomena and environmental conditions in the se,sing area.

An actual example is given here to illustrate that the same type of sensing method can require a different application approach when it is used in different types of sensing systems. This example concerns two capacitance-type sensors. One is the capacitance-type position sensor mentioned earlier for measuring iarge travels, and the other is a capacitance-type fuel gauge [4].

Electrodes of capacitance sensors are gener, qy constructed in one of two ways. A concentric cylindrical construction is suitable for measuring one-dimensional physical phenomena, while a parallel flat electrode plate construction is effective in measuring two-dimensional physical phenomena. Linear motion is a one-dimensional physical phenomenon, while fuel quantity is a three-dimensional physical phenomenon. Because of these differences, the position sensor has a con- centric cylindrical electrode construction, and the fuel gauge ha.~, a parallel tote electrode plate construction. The new gauge has multiple pairs of electrode plates located across the fuel tank, making measurement of three-dimensional physi- cal phenomena possible.

Table 5 summarizes the basic structural design points of these capacitance-type sensors. The electrodes of the position sensor are of concentric cylindrical construction, which facil- itates precise electrode spacing and easy processing. On the other hand, the electrodes of the fuel gauge are of parallel flat electrode plate construction, which allows the shape and arrangement of the electrode plates to be readily adapted to

60

Table 5 Design approach for capacitance-type sensors

H. Kobayashi /Sensors and Actuators A 57 (1996) 53-63

Position sensor Fuel gauge

Basic structure and aims I. The basic structure resembles that of vehicle shock absorbers. o Low cost and high reliability can be achieved using automotive parts. 2. The electrodes have a concentric cylindrical construction. o Precision of electrode spacing is readily achieved, and processing is easy.

Structural design

I. Stray capacitance is minimized

2. Dielectric substances show stable dielectric- constant characteristics

3. Accurate prediction is made regarding factors disturbing capacitance

o The detector circuit is located in close proximity to the sensor. ° By estimating the stray capacitance at 3 pF, the variance in capacitance and the electrode spacing are determined.

o The shock-absorber oil shows stable dielectric- constant characteristics for long periods ~n an enclosed environment. o Polypropylene resin used for insulation shows stable dialectric-constant characteristics for tong periods in the oil. o Materials getting into the shock absorber are examined as well as changes with time at the dielectric constant of the oil and polypropylene resin.

I. The electrode pairs consist of parallel electrode plates. o The shape of the electrode plates can be adapted to non-linear liquid level characteristics resulting from an irregular tank space. 2. The .~ansing electrode is distlibuted among several pairs of electrodes o The shape and arrangement of electrodes can be adapto] to ~ymmetrical inclination charecteri~tics of the liquid level. o Same as at left.

o By estimating the variable capacitance in the electrode edge at 5 pF, the variance in capacitance and the surface area of electrodes are determined. o The dielectric constant of gasoline is stable for long periods of time. o Polyacetal resin used for insulation shows stable dielectric-constant characteristics and shape for long periods in gasoline and alcohol.

o Dielectric-constant characteristics of additives in a fuel tank are examined. o The motion of residual water in a fuel tank is clarified.

non-linear liquid-level characteristics and asymmetrical fuel- level inclination characteristics.

In designing the structure o f capacitance-type sensors, con- sideration mus t be given to three points in particular.

(1) The structure mus t be as free as possible from the influence o f stray capacitance.

(2) The capaci tance-forming components and dielectric substance mus t have stable dielectric -constant characteristics.

(3) Accurate prediction must be made of factors disturbing capacitance, and o f measures effective in countering them.

The followirtg discussion will focus on the third point. The position sensor is tightly enclosed in a case to protect it against foreign matter because the sensor is installed under the vehi- cle floor. For this sensor, it was found that there were two main factors disturbing capaci tance One was the dielectric- constant characteristics o f the oil used as the capacitance- forming dielectric substance and the other was those o f the insulation resin. The dielectric constant of the capacitance-

forming dielectrics mus t be stable for long periods o f use o f up to approximately ten years in an enclosed environmental condition. It was clarified through experimental tests that the paraffin oil used for automotive shock absorbers was one of the best dielectrics for the vehicle-position sensor. Table 6 shows the long-term stability o f the dielectric constant o f the

shock-absorber oil. The dielectric constant is stable for more than ten years. On the other hand, the dielectric constant o f the insulation resin mus t be stable for a i ong period in the oil. W e found that polypropylene resin provided the best dielec-

Table 6 Long-term stability of dielectric constant of shock-absorber oil

New oil Used oil Used oil

Production date October 85 Match 77 October 73 Relative dielectric constant 2.22 2.2 i 2.23

tric constant for use as the insulator. These two factors affected the long-term stability o f the dielectric constant o f the dielectric materials in the enclosed condition, and deter- mined what dielectric material was best suited for the vehicle-

position sensor.

The fuel gauge is built into the fuel tank, which makes it subject to the influence o f the environmental conditions in the tank. It was found that there were two main factors dis- turbing capacitance. One was moisture inside the tank result- ing from condensation, wbich occurs because the fuel tank

takes in outside air. The other was the presence o f additives

in the tank, which include a detergent for the fuel route and a solvent for moisture. Both o f them easily dissolve in gaso- line. The former is a factor affecting the electrode gap, which mus t be designed so that water does not collect in tt. The latter is a factor affecting the electrode surface area, since the

total capacitance formed in the sensing electrodes mus t be

high enough to eliminate the effects o f the additives. These two factors were related to changes in the dielectric constant

o f gasoline caused by foreign matter and determined what

H. Kobayashi / Sen.~ors and Actuators A 57 ¢1996) 53-63 61

structure of the sensing electrode was best suited for the vehicle fuel gauge.

Reviewing the development apprt~.~,'~: for the two kinds of capacitance-typo sensors, we see that different sensing tech- niques are used depending on the application involved. The reason for this is that the nature of the detected objects varies according to the behaviour of the object to be measured and environmental conditions in the sensing position. Therefore, the best-suited sensing technique is selected to match the object even though the detection method is the same.

Even after the optimum sensing method has been deter- mined, technological issues peculiar to the sensor under development must be addressed. These issues concern the relation between the nature of the measured object and the features of the sensing method. Two aspects are of particular importance. One is to examine the best position for making the required measurement and also the distinctive behavionral and environmental conditions present in that position. The other is to examine the best sensing method for measuring the object, and also the development issues related to the particular features of the sensing method.

4. Overcoming technological issues

Each sensing method has its own particular features. The technological issues involved in developing a sensor are posed by the peculiar features of the sensing method, and the particular nature oftbe behavionr of the object to be measured and the environment in the sensing position. The following four steps are especially important in overcoming these tech- nological issues.

( 1 ) The technologies to be developed for *he sensor must be tailored to meet the specific requirements of automotive USe.

(2) Factors obstructing the attainment of the required sen- sor specifications should be analysed and classified into those originating in vehicle operating conditions, and those in the behavionral and environmental conditions in the sensing position. This will help clarify the best approaches to take in resolving them.

Table 7 Structural design approach for fuel gauge

(3) The various elements making up the factors should be related to the peculiar nature of the sensing method, and compared against the proposed sensor construction and con- stituent parts. This will ensure that the development isstw.s are fully reflected in the design and construction of the sensor. Furthermore, it will help guarantee that the sensor is designed and built with the capabilities needed for overcoming the technological issues involved.

(4) The interrelationship between the required specifica- tions and the sensor construction should be made clear, as well as clarifying how the material properties of the constit- uent parts affect sensor performance. As a result, it will be possible to design the sensor and determine the constituent parts so that the sensor delivers the performance needed to satisfy the required specifications.

An actual example is given here to illustrate this develop- ment process. This example concerns the capacitance-type fuel gauge mentioned earlier. Table 7 summarizes the design approach for the fuel gauge, in which the sensor construction is related to the special requirements of automotive use. An automotive fuel gauge has four special requirements origi- nating in the operating conditions of an automobile and in the structural peculiarities of the fuel tank, as listed in Table 7.

First, an automotive fuel tank generally has an irregular shape due to space limitations, which results in non-liuear liquid level characteristics. The use of parallel flat electrode plates allows a high degree of freedom for electrode design. As a result, the gauge can be easily adapted to non-linear level characteristics and asymmetrical fuel-level inclination characteristic.

Next, an automotive fuel gauge must provide accurate fuel measurements even when a vehicle is stopped on an incline. In the case ot'a symmetrical fuel tank, there is ~. locus of fixed points where the fuel level does not change even if the fuel surface is inclined. These points lie on a line, called an equiv- alent line, which runs through the centre of the tank. By positioning the electrodes symmetrically along this equiva- lent line, the fuel quantity can be measured accurately without being influenced by inclines. However, the equivalent line for an irregularly shaped tank takes the form of a non-linear curve. Fig. I 1 shows the loci of fixed points where the fuel

Sensor construction Aims Special requirements of automot~',e use

The electrode pairs consist of parallel electrode The shape of the electrode plates can be adapted The fuel tank has an in'egular shape, due to space plates, to non-linear liquid level characteristics resulting requirements.

from an in'egular tank shape. The sensor is distributed among several pairs of The shape and arrangement of electrodes can be It is nece~ary to obtain accurate measurements electrodes, adapted to asynunetrical inclination ~f th~ amount of fuel even when the vehicle is

characteristics oftha liquid level, stopped on an incline. A signal-processing method appropriate to liquid- Methods used for time averaging and elimination It is necessary to obtain accurate measurements level ~haviour is selected to support sensing of abnormal values from sensing electrode signals of the amount of fuel even when the vehicle is electrode signal, can be adapted to localized fuel-level fluctuations, movi,g. The electrode spacing is 3 mm. Water is not apt to collect in the electrode gap. Dew condensation tends to cause water to collect

in the fuel tank.

62 H. Kobayashi / Sensors and Actuators A 57 (1996) 53-63

Inclined 15 degrees to left and rlgi.t

~ ~orlzon~l liquid lnvnl Fig. I I. Loci of fixed points.

Baffle plate /

+ - - - - / - . . . . . Electrode plate ..p, t+" ~ ' , ,

Fig. 12. Structure of distributed sensor.

level intersects the horizontal level when inlined 15 ° to the left and right for various quantities of fuel. Based on the inclined fuel-level characteristics, a geometric method [4] was devised for configuring the electrode pairs and their lay- out. The newly developed fuel gauge consists of three sets of electrodes, as shown in Fig. 12, so as to counter the influence of fuel-level inclination.

Thirdly, an automotive fuel gauge must deliver accurate fuel measurements even when the vehicle is moving. The motion of the vehicle can result in localized fuel-level fluc- tuations or cause the fuel to stick to the tank wall. As a result, the values measured by the sensing electrodes may show large and momentary fluctuations. This was countered by devel- oping a signal-processing method [4] appropriate to liquid- level behav~our.

A microcomputer performs a time-averaging operation on sensor signals to eliminate abnormal values. This signal-proc- essing method involves two principal operations. The first is refueling detection, illustrated in Fig. 13. This is done by comparing the average of the preceding 256 values with the average of the most recent eight values. If the difference between the two averages exceeds 5 !, the system judges that the vehicle is being refueled. The averaging interval is short- ened only during refueling so that data are displayed without any delay.

The second operation is abnormal data rejection, also illus- trated in Fig. 13. Abnormal data are rejected when the meas- ured value exceeds a specified threshold when compared with the preceding average value. Fig. 14 shows a flow chart of the abnormal data rejection procedure. When abnormal data occur, the microcomputer performs two operations. The first is to retain and display the preceding average value. The second operation is to increase the threshold value by 0.1 I in order to increase the data input needed for averaging.

tell tank

Fig. 13. Signal-processing Olmmtions.

"1 / + + / o~lllMor

l Gompule diNeml~m between lUmlpldd I data I

I oo°.+ I I .... ' + 1 ...... I~erl~e vml,n da~ for display

+ +

I . . . . . . ..... l I .. . . . . . . . . . . l dimshold threshold

l' I / ....... /

diIplay

I Fig. 14. Flowchart for signal processing.

When the system judges that the measured data are normal, two computer operations are also performed. The first is to instruct the system to input the measured data as sampling data for averaging. The second is to reduce the threshold value by O. 1 I in order to prevent discrepancies between the average value and the actual fuel quantity. To ensure that the average value does not deviate from the actual fuel quantity, the system constantly checks for the occurrence of abnormal data and also adjusts the threshold value.

Fourth, water condenses inside the fuel tank because the tank structure allows the intake of outside air. Since the amount of water that dissolves in the gasoline is minute, it has a negligible effect on the dielectric constant of gasoline.

H. Kobayashi l Sensors and Actuators A 57 (1996) 53-.-63 63

However, the surface tension of water is approximately 2.5 times that of gasoline. For this reason, it was necessary to increase the electrode gap to 3 mm to prevent water from accumulating in the gap, and thus reduce the effects of resid- ual water on sensor performance. Temporary shorting of the electrodes may still he caused by water clinging to them momentarily, but the signal-processing method used to counter fluctuations in the fuel level is also effective in elim- inating abnormal values arising from electrode shorting.

Reviewing the development process of the fuel gauge once more, the following three aspects are evident.

First, all four development issues originated from the vehi- cle operating conditions or the structure of fuel tank, and were peculiar to the nature of the automobile. In addition, the vehicle operating conditions were known to affect the behav- iour and environmental conditions of the fuel which was the object to be measured.

Secondly, the devei:opment issues were clarified through an examination in which the hehaviour and environmental conditions of the fuel were analysed in connection with the features of the capacitan~e sensing method. Although the capacitance sensing method is one of the best techniques for use in an automotive fuel gauge, there were some peculiar issues that had to be addressed in developing the automotive fuel gauge described here,. This )mderscored the importance of accurately understanding the basic behavionr of the object to be measured and its environmental conditions.

Thirdly, the technological means for overcoming the development issues were reflected in the sensor construction and constituent parts, thereby giving the sensing system a design and construction unique to automotive use.

5, Conclusions

The most crucial factor in developing an autonu3five sensor is to clarify the major development issues and their ~ . To do this, we must first obtain accura~ measureme~ of the physical or chemical p roperfies of automotive parts and mate- rials. The distinctive t~ature of these properties must be made clear along with the distinctive features oftbe environmental conditions in the measuring position. Secondly, wemnstclar- ify the critical environmental conditions and environmem~ characteristics of each sensing device and martial, and exam- ine the distinctive features of each sensing method. Based on that information, we are able to execute the basic sensor design, and can comprehend the main development issues and their causes. Finally, we can understand the direction in which the sensor development work should proceed.

References

(I ] F. Sugasawa and H. Kobay~hi, Electronically ¢.ommlled shock ab~rber system used as a mad senso~ which utilizes supersoaic waves,

Passenger Car Meeting. Dearborn, USA. Sept. 1985. Paper No. 851652.

[2] D. Ko~ar. 7,. S~llee, S. Gabersc~ and D. Suverov. Cesan~ and dielecuic ptopegti~ of ude~ed comp~itioas in the BaO-TtO-z--Nd203 sys~m, Ber. Deatach. Kerom. Ges., .ST (19"/8) 346-348.

[3] K. Wakino, K. Minai and H. Tanmra, Microwave c ~ of (Zr,Sn)TiO, and BaO--PbO--TiO~ dielectric resoalors, J. Am. Cera~ See., 67 (1984) 278-281.

[ 4 ] H. Kobayashi, Conqm~er-aided capacitive-sensing fuel level memu~ng systems for automofive use, Proc. lECOlq "84, Tokyo.Japan. Oct. 1984.