Photoelectric inclination sensor

Sensors and Actuators, A21-A23

289

(1990) 289-292

Photoelectric Inclination Sensor HISAO KATO, MASAHIKO KOJIMA and YOHZOH OKUMURA Nagoya Mwuctpal Industnal Research Imtrtute, 3-4-41 Rokuban, Atsuta-ku, Nagoya-shr (Japan)

ATSUSHI OZAKI Atchz Instttute of Technology, Yachrgusa, Yaguea-rho, Toyota-sht (Japan)

Abstract A method for obtammg the tilt angle and direction m the two-dimensional plane by using a photoelectnc inclination sensor, consisting of LED, a hermsphencal splnt level and a photodiode array 1s explained Light from the LED projects a bubble m the splnt level and throws a shadow onto the surface of the array composed of four equivalent p-n Junction diodes, isolated from each other by a cross on a wafer When the sensor 1s kept honzontal, the area of the shadow m each diode is exactly equivalent But when it IS mchned, the area 1s not eqmvalent Assummg that the shadow on the array m a low tilt angle 1s circular and the output current per umt area 1s constant over the surface of the diodes, the tilt angle and dnectlon m a two-dnnenslonal plane is obtained by combmmg the diode outputs The prmclple of the sensor, its fabncation process, theoretical analysis of the sensor output agamst tilt angle and &rectlon, and the system to display them m CRT are explained Finally, the expenmental values are discussed 1. Introduction The gyroscope 1s a well-known device to measure the tilt angle Its accuracy IS excellent and measurement m three dimensions can be done A simple sensor conslstmg of a single pendulum and phototransistor or magnetic sensor 1s commercially available but measurements by this device are very much affected by external noise, such as mechanical vibration Recently, a new sensor usmg the capacitance or conductance or inductance of a hqmd m the splnt level appeared [l] These sensors have their own excellent charactenstics, for example, accuracy, wade dyuarmc range and measurement stability against vibration, but their resolution IS not enough We have proposed a new photoelectnc mchnatlon sensor The resolution of thus device is excel0924-4247/90/%350

lent and measurement m two dlmenslons can be done The prmclple and output charactenstlcs of the sensor to detect the honzontal were reported previously [2,3] In thy paper, a method for obtalmng the tilt angle and dmzctlon using this sensor 1s explained and the system to compute the angle and dn-ectron 1s also fabncated

2. Prmeiple This sensor [2,3] 1s shown m Fig 1 and consists of a hght enuttmg diode and a henusphencal splnt level mounted on a p-n Junction photodiode array A shadow of the bubble m the hqmd 1s thrown onto the surface of the photo&ode array by the LED When the sensor 1s kept honzontal, the shadow on the array surface 1scircular, as shown m Fig 2(a), and the area of the shadow on each diode of the array IS eqmvalent However, when It IS mchned, the shadow becomes shghtly elliptic as shown m Fig 2(b), lmplymg that the output current IS no longer eqwalent Assummg that the crosslmes m the array correspond to the x and y axes of the coordmates and output currents of the &odes m the first, second, thrd and fourth quadrant are, respectwely, Z, , Z,, ZSand Z,,

(4 Rg 1 Components of the photoelectnc mchnatlon sensor (a) arrangement of hght source (LED), spmt level and p-n Junction photodmdc array m a black box, (b) spmt level, (c) surface geometry of the photodmde array

0 Elsevler Sequola/Pnnted m The Netherlands

290

3. calculPtioo of the Sensor output

d

( Fig 2 PosItIon of the shadow of a bubble protected onto the surface of the photodmde array The area of PD,(I = 1-4) IS S, the area of the shadow on the photodmde IS Sh, (I = l-4), c( IS the tdt angle and d shows th.e mncbneddmctmn (a) The level IS honzontal, (b) the level IS mchned

the tilt angle and dlrectlon can be obtained by calculatmg z,=z*+z,-(z,+z,) z,=z,+z,-(Z,+Z*) These output current values, however, are too small, and they are converted into voltages and amphfied by the clrcmt shown m Fig 3 Thus the outputs of the arcmt, expressed as V, (I = 1, 2, 3, 4, x and y), are proportional to the Z, (I = 1, 2, 3, 4, x and y) Then V, and V, are expressed V, = Vz+ V3- (V, + V,) = AZ,( 1 - y)[Sh, + S/z, - (Sh, + Lsh,)] V”= V, + V, - (V, + V,)

(2)

= AZ,( 1 - y)[Sh, + Sh, - (Sh, + Sh,)]

An approximate calculation of the sensor output against the tilt angle 1s carned out assummg the shape of the shadow on the photodiode 1s circular The cross section of the splnt level and the shadow of the bubble on the photodmde array are shown m Fig 4 Here 0 is the center of the bottom of the splnt level, AE 1s the cross section of the level, the area surrounded by BCD and BD shows the bubble, C 1s the center of BD, 0 1s the tdt angle from the honzontal level, r 1s the dameter of the hemlsphencal glass, B”, C’ and D” are, respectively, the proJections of B, C and D on the photodlode array, 0’ 1s the proJectIon of 0 corresponding to the cross pomt of the aolation hne on the photodlode array, r’ IS the diameter of the shadow on the array, + 1s the deviated angle of C’O’ from the separation hne NE/SW, and Sh, (z = 1,2,3,4) 1s the area of the shadow on the photodmde PD, (I = 1,2,3,4) The dtameter of the LED 1s 10 mm, the &stance between the LED and the splnt level IS 50 mm, and dmrneters of the henusphencal glass and bubble are 17 mm and 9 mm respectively, wth the mcldent light beam on the splnt level perpemhcular to the diode array When the light beam stnkes the glass surface at II’, It penetrates mto the level, but outside the area total reflection occurs Thus, It 1s dark outslde the circle with center 0’ on the array, It 1s bnght between the circles with centers 0’ and C’, and it 1s also dark wlthm the circle wth center C’, since the shadow of the bubble 1s proJected onto the array Thus, the position of the circle C’ moves wlthm the fixed circle 0’ depending on the tilt angle In this case, the distance between the LED

where Sh, (I = 1, 2, 3 and 4) 1s the area of the shadow on the photodlode PD, (I= 1,2,3 and 4), A is the gam of the amplifiers m Fig 3, I,, 1s the diode output current per umt area of the &umnated region and y is the ratlo of diode output current per umt area of the shadow to the dlumlnated region In eqn (2) zero voltages (V, and V, = 0) indicate that the transducer 1s honzontal m the two-dlmenslonal plane The tilt angle and inclination direction of an ObJect can be deduced by measurmg the values of V, and V,,

Rg 3 Electnc arcmt connected to the sensor

Rg 4 Relation between the splnt level and shadow of the bubble on the photodmde array

291

Rg 6 Arrangementof the photodmde array on an alumma SttXU

c Rg 5 Relation between the tdt angle and dmsbon, and a plane When the plane IS mchned clockwtse, B rpvesthe tdt angle, 8, and @,are the x and y componeots of B, and JIshows the dlrect10n

and the level IS adjusted so that the shadow frmge IS not emphastzed by the Influence of surface tension at B and C m the level Thus observation 1searned out by posttiomng the prmtmg paper, not the diode array, and projectmg the unage onto the paper Usmg eqn (2), V, and V, can be calculated v XY

=

-2fi2AZ0( 1 - y)(r sm a cos &,)* x [an-‘(tan ff, y ~tu)+~n~~~~ta x {(l-tan@,,

cot &)$I

(3) where fl ISthe ratio of the projected bubble dunenslon to the real one, and ISsmaller than umty smce the level 1s a convex lens and the proJected image IS reduced by thts ratio A IS the gam of the amphfiers and SX, imphes the tdt angle m the x and y direction, respectively R@ and 6 m Figs 4 and 5 are expressed as $I = tan- ‘(tan eYftan 0,)

(4)

19= tan- I( tan2 0, + tan2 0,) Ii2

(5)

4. Preparationof the S~IWXW

The p-n Junction &odes fabncated by thermal dtffuslon were used as the photodmde array of the Sensor The surface of the nurror-pohshed n-type slhcon wafer, wrth a reslstmty of about 5 Q cm and onentation (NO), was oxldrzed at 1100 “C under an atmosphere of Hz and O2 (wet 0,) for 1 h The thickness of the slhcon dioxide (SIOz) layer was 700 nm The oxide on the surface was selectively etched away, then a spm coating Glm contimng boron (Tokyo Oka, PBF-3M-31) was coated by the spinner Predeposition of the nnpunty was camed out at 900 “C for 0 5 h and its dmmg was done at 1100 “C for 1 h Thus four eqmvalent square-shaped diodes were fabncated on a SI wafer The oxide on the wafer was taken away completely and a 700 mn field oxide film was

grown The wmdows of the diodes were fabncated by etchmg and anti-reflection film was prepared by oxtdrzmg the wafer at 1000 “C for 3 h The contact holes were opened by etchmg and metal electrodes were formed on the &odes by evaporation of ahnnmum and photoetchmg The omde on another surface was removed by sandblastmg Ohnnc contact was formed on tbs surface by burnt eva~ra~on and heat treatment m pure Hz at 500 “C for 40 mm Thrs photodmde array was mounted on a stem fabtlcated by screen prmtmg the gold electrodes, electrodes m the diode array were connected to them Htlth 50 pm diameter gold wlies that were molded by epoxy resm A~angement of the array on the stem IS shown m Fig 6 The h~~he~~ spmt level was mounted onto the array Except for the Qode area, onto whch the spent level was rno~t~ dzrectly, the whole part of the d&e was pamted with black wax to shelter the l~~na~on The sensor thus fabncated was mounted onto the iron plate and mstalled m a plastic black box to shelter it from the external hght noise The LED was attached to the center of the box cedmg 5. Tilt Angle aad Dkeetion Me -at

system

Tilt angle and drectioon were measured by the system shown m Fig 7 The outputs of the cmzut shown m Fig 3, expressed as V,(z = 1,2,3 and 4), were converted mto d@al signals by an A/D converter The values of V, and V, were obtained from eqn (2) By preparmg look-up tables of these values agamst the tdt angle on the computer using eqn (3), 0, and 0, (x and y components of the tilt angle) can be obtained from the values of V, and V, Then the tdt angle and dlrechon m the two-dimensIona plane, expressed as 0 and $, can be obtained from eanq (41 and (5), and the

Fig 7 Tdt angle and dnrx%onmeasunng system

292

tabulation of V, (I = 1, 2, 3 and 4), V,, V,, O,, O,, 0 and $ was displayed on a CRT

NWY-9ul NW’W-675”)

t-

6. Output of the Sensor Values for 13and $ were measured by changmg the tilt angle and direction of the transducer The offset of the operational amplifiers was adjusted so that V, and V,, were set at 0 V, when the sensor

was positioned on a plate surface Then the sensor was mclmed step by step using an angle gauge, and the tilt angle 8 and direction $ were measured by using the system shown m Fig 7 The dlrectlons m this expenment are shown m Fig 8, where NW, SE and NE, SW axes correspond to the separation lmes on the photodlode array The hght from the light source was kept constant throughout this expenment Figure 9 (a-e) shows the relation between the tilt angle 0 displayed m the CRT and the sensor tilt angle where the sensor was mchned counterclockwise along NE, NNE, N, NNW and NW respectively The gams of the operational amphfiers were adjusted so that V, and V,, are 1 5 V when the tilt angle of the sensor, mclmed in the x and y dlrectlons, respectively, was 9” The values of 1(1m this expenment are 0, 22 5, 45, 67 5 and 90” In Fig 9, for angles less than 10” the plots show linear relatlonshlps with slopes of 45”, N NW,

NNE

Fig ICI The dnechons of the sensor &splayed m CRT and real ones concentnc nrcles show the trlt angles, 0 shows the data m CRT, and sohd hues md~cate duect~ons

meaning the system shown m Fig 7 can measure the tilt angle For angles above 10” the measured data deviate from the real ones This 1s due to the fact that for high tilt angles the shadow on the photodlode array 1s no longer arcular, but elhptlc, and thus the circular shadow model shown m Fig 4 cannot be adopted An alternative way to solve this problem ISto use an elhptic shadow model, but m this case the tilt angle and direction cannot be obtained from the values of V, and V,,m eqn (2), since V, vanes mth 0, at constant 8, A rather shorter hnear relationshp range than that m a previously published paper [2] 1s due to the dlfferent sizes of the splnt level and the bubble Figure 10 shows the dlrectlons of the sensor displayed m CRT and real one as a function of tilt angle This also shows that direction can he measured by this system Slight dlstrrbuhon of the data m Fig 9(a) IS due to the fact that the outputs of the sensor (1, and 1,) are very small and the offset dnft of the amphfiers greatly m!luences the result To solve this problem, the use of a bnghter LED IS suggested

S

Ftg 8 Dlrechons and the photodlode array

2d

1

tut

Pit

Acknowledgements The authors are indebted to T Shnmzu and T Ilda at Nagoya Mumclpal Industrral Research Institute for their cooperation m carrymg out the expenments The authors wsh to express their sincere thanks to K Mizuno, K Hasegawa, S Inagalu and A Imal at the Nagoya Mumclpal Industnal Research Institute References

Fig 9 Plots of the outputs &splayed m CRT against the real tdt angle 0 shows the expenmental values, and sohd hues show the hnes of slope 45” through the ongm Rrectlons are (a) NE, (b) NNE, (c) N, (d)NNW, and (e) NW

K Nlsluhara, S Matsumoto and K Hashoto, Inelme sensor using electrolyte, J Jpn SW Precls fig, 53(1987) 85-87 (m Jaoanese) H Kat;, M ko&, M Hayash and S Sasalu, Photoelectnc mchnatlon sensor, Reu Scr IFISWCUII , 57( 1986) 1207-1208 H Kato, M KoJlma and S Sasak~, Photoelectnc mchnahon sensor, Proc 6th Sensor Symp , Tsukuba, Japan, 1986, pp 229-303