SMART SENSOR SYSTEM APPLICATION: An Integrated Compass

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1 SMART SENSOR SYSTEM APPLICATION: An Inegraed Compass Ronald J. W. T. Tangelder, Guido Diemel, Hans G. Kerkhoff MESA Research Insiue /Universiy of Twene P.O. Box AE Enschede, The Neherlands Absrac A fully inegrable elecronic compass has been designed based on he pulse posiion mehod, using micro-machined fluxgae magneic sensors. The compass has been designed o have an accuracy of one degree. The analogue and digial circuiry in he sysem fi on a single Sea-of-Gaes array of 200k ransisors. Togeher wih he sensors i will be combined on a single MCM. Keywords: 1. Inroducion mixed-signal design, micro sysems, fluxgae sensors A number of magneic sensor sysems for navigaional use have been described unil now [Pe86, Spr93, Hol94, Mal95, Go95], bu none of hem have been shown o funcion as a complee compass sysem on a single chip or muli chip module. The main problem being he requiremen of sensors and mixed analogue and digial circuiry on a single Silicon subsrae. Wih he inroducion of micro-machined fluxgae sensing elemens wih elecroplaed permalloy cores [Kaw95] his problem has been solved parly. These sensors are paricularly suiable for applicaions in he earh's magneic field range as hey combine high sensiiviy wih good inegrabiliy[go95, Kaw95], and can be produced in a CMOS compaible process or can oherwise be used in a Muli Chip Module (MCM) implemenaion. Their operaing principle ensures robus and reliable operaion of a porable compass sysem. Above menioned properies are very appropriae for a fully inegrable elecronic compass sysem. 2. Global sysem overview An overview of he oal sysem is shown in Figure 1. The elecronic compass funcions by measuring he magneic field in a horizonal plane in wo perpendicular direcions. The angle o he magneic norh is calculaed by aking he arcangen of he division of he wo measurans. The sysem comprises of a analogue fron-end which excies he sensors wih a riangular waveform and convers he resuling sensor oupu o measurable digial signals. The digial back-end performs he angle calculaion and LCD conrol. The sysem uses a muliplexing echnique by exciing one sensor a a ime. This reduces boh momenal power consumpion and chip area since only one oscillaor is needed. The supply volage is currenly 5 Vols, bu can be scaled down o 3.5V. Boh digial and analogue pars are implemened on he fishbone Sea-of-Gaes Array [Fre94] (see Figure 2). The fishbone SoG consiss of 4 quarers, each wih circa 50k pmos/nmos pairs. I is mainly inended for digial applicaions, bu can very well be used for analogue designs, oo (see [Haa95], [Don94]). Capaciors can be made by puing he second meal layer above he firs one. Very large capaciors (> 400pF) and resisors should be realised, however, on he subsrae of he MCM. wach oscillaor curren source (V/I) y_sensor x_sensor analogue swich pulse pulse coun digial swich display driver deecor conrol logic Analogue secion Digial secion Figure 1: Overview of he elecronic compass sysem ED&TC 97 on CD-ROM Permission o make digial/hard copy of par or all of his work for personal or classroom use is graned wihou fee provided ha copies are no made or disribued for fee or commercial advanage, he copyrigh noice,he ile of he publicaion, and is dae appear, and noice is given ha copying is by permission of he ACM, Inc. To copy oherwise, o republish, o pos on servers, or o redisribue o liss, requires prior specific permission and/or a fee ACM/ /97/0003/$3.50

Φ Φ b) c) H exc Vind Ho d) Figure 2: The fishbone Sea-of-Gaes Array The digial par of he inegraed compass occupies 3 quarers fully and he analogue par 1 quarer for less han 15%.

2 Φ Φ b) c) H exc Vind Ho d) Figure 2: The fishbone Sea-of-Gaes Array The digial par of he inegraed compass occupies 3 quarers fully and he analogue par 1 quarer for less han 15%. Since each quarer has a separae power supply, we have used wo differen power supplies for boh he digial and analogue pars. The SoG and wo micromachined sensors will be combined on a single MCM, equipped wih boundary scan es srucures [Oli96]. a) H exc Figure 3: Pulse posiion operaing principle of a fluxgae sensor, H exc I exc and V ind = dφ/d An ELDO model was derived from hese measuremens. H K has been adaped o obain a sauraion level suiable for our applicaion, which is sill an obainable goal for a new fluxgae sensor. Hence, for he ime being, a discree miniaurised fluxgae sensor has been used. The resuling volage over he pickup coil and he acual volage over he exciaion coil afer applying a riangular exciaion curren of 12 ma peak o peak wih a frequency of 8kHz have been measuremens, and are displayed in Figure Sensors For fluxgae sensors several operaing principles can be used. Mos common is he so called second harmonic measuremen [Rip92, Go95, Kaw95]. We, however, use he so called pulse posiion mehod [Rip92], which resuls in a very simple communicaion beween he analogue and digial par [Hol94], as will be explained in subsecion Operaing principle A fluxgae sensor is a form of ransformer, which is deliberaely driven ino sauraion periodically wih a symmerical exciaion field (see Figure 3, solid line). If an exernal magneic field H o is applied (he earh's magneic field in his case), he sensor remains sauraed longer in one direcion and shorer in he oher direcion (see Figure 3, dashed line). The resuling change in inducion volage in he pickup coil is a measure for he exernal magneic field. If a riangular wave exciaion field is applied, he inducion volage consiss of pulses which shif in ime due o he exernal field. An ideal sensor should reach sauraion wih an applied field wih he same magniude as he earh's magneic field. Measuremens were carried ou on an acual fluxgae sensor [Kaw95] and showed ha i reached sauraion a 15 imes he magniude of he earh's magneic field (H K =10Oe). Also he inernal resisance (77Ω) of he sensor proved o be oo high for low power applicaions. Figure 4: Real fluxgae sensor daa, wihou and wih a field applied. The firs measuremen is wihou a field. In he second measuremen a field is applied. The pulse shif is clearly visible. Noice also he change in impedance of he exciaion coil, when sauraion is reached.

3 2.1.2 Sensor srucure The micromachined fluxgae sensor is based on a wo-layer mealisaion silicon process echnology and ferromagneic films. A permalloy core is sandwiched beween boh meal layers as illusraed in Figure 5. This layou resuls in a ransformer-like srucure wih a exciaion coil and a pick-up coil. Exciaion coil and c). The frequency of he waveform generaed is 8kHz.. The layou of he riangular waveform generaor is given in Figure 7. I uses a capaciance of 10pF which is clearly visible in he upper half of he picure. An exernal resisor of 12.5MegΩ, which is realised on he subsrae of he MCM, should be conneced o i. VSST Polyim ide SiO2 Si Pickup coil 2 nd Al Ferromagneic film 1 h Al VDDT VSSH Figure 5 : Srucure of he fluxgae sensing elemen In Figure 6 a se of micromachined fluxgae sensing elemens [Kaw95] is shown, each having a differen number of windings. VDDH vss vou_osc s_gnd s_ou_no s_ou vdd resisor_in ref2p5v dis_osc resisor_ou bswich swich vssl Figure 7 : Layou of he riangular waveform generaor Figure 6: Fabricaed micro-fluxgae sensing elemen [Kaw95] 3. Analogue secion The analogue secion consiss of wo main pars: he exciaion curren source and he pulse posiion deecor. 3.1 The exciaion curren source The curren source consiss of a riangular waveform generaor or oscillaor and wo VIconverers o drive he wo sensors. I has a riangular wave wih an ampliude of 12mA peak o peak, which is sufficienly high o drive he fluxgae sensors ino sauraion (see Figure 3a, b Bes sensiiviy is obained when he applied magneic field is wice he sauraion field. The lineariy of he waveform is no very essenial bu he dc-offse is, and is herefore correced by measuring he average of he exciaion curren. The sensors have a high series resisance, which requires he use of a balanced differenial oupu. Wih he supply volage a 5 Vol, sensors wih a resisance as high as 800 Ω can be driven. The resisive characer of he sensors is used o linearise he exciaion curren sources. 3.2 The pulse posiion deecor The oupu of he sensor consiss of volage pulses which are deeced in he second par (see Figure 3d and Figure 4). Their posiion in ime wih respec o each oher is measured by deecing boh he falling edge of he posiive pulse and he rising edge of he falling pulse. The pulse posiion deecor processes a digial 1 afer he falling edge of he posiive pulse, which changes o a digial 0 afer he rising edge of he negaive pulse, and vice versa.

4 This digial compaible signal which is sampled by a high speed digial up-down couner. The fracion of ime in a period a which he oupu of he pulse deecor is high is a direc indicaion of he field componen measured. Since he analogue oupu consiss only of one digial compaible signal, a complicaed AD-converer is no necessary, which would have been he case for mehods based on second harmonic measuremens. 4. Digial secion The digial conrol logic has wo main funcions. I enables he analogue secion and he digial high speed up-down couner only when hey are needed, in order o diminish he power consumpion furher, and i conrols he muliplexing of he wo sensors. The pulse coun par conains a high-frequency ( MHz) up-down couner, which ransforms he oupu of he pulse deecor ino wo ineger values x and y, each indicaing he field componen of he x- and y-sensor. The arcangen par ges an x- and an y-value from he up-down couner and compues arcan(x/y), using a cordic-like algorihm (see Figure 8) [Spa76]. I used only 8 cycles o calculae he direcion wih an accuracy of one degree. The calculaion mehod is insensiive o local variaions of he magniude of he earh's magneic field, which is necessary since he magniude varies beween 25µT in souh America and 65µT near he souh pole. y_reg := y * 128; x_reg := x * 128; res := 0.0; oal_delay := 0 ns; ready <= '0'; coun := 0; shif := 1; while coun /= 8 loop oal_delay := oal_delay + comp_delay; if y_reg >= (x_reg / shif) hen y_reg_prev := y_reg; x_reg_prev := x_reg; y_reg := y_reg_prev - x_reg_prev / shif; x_reg := x_reg_prev + y_reg_prev / shif; res := res + aanrom(shif); end if; dir <= ranspor res afer oal_delay; coun := coun + 1; shif := shif * 2; end loop; ready <= ranspor '1' afer oal_delay; Figure 8: The main par of VHDL descripion of he arcan par. The digial par conains also common wach opions as added feaures. The display driver selecs eiher he direcion or he ime o display. The pulse coun par and he arcan par can be modified easily o compue he direcion wih an arbirary precision. However, here will always be a bole neck in he previous pars as he sensiiviy of he fluxgae sensor and he analogue secion are limied. 5. Simulaions and design flow Simulaions were carried ou wih Compass Design Auomaion Tools and Anacad ELDO for he analogue and mixed-signal simulaions. A model of he sensors based on realisable specificaions was used. The layou has been realised wih he Ocean Design Sysem [Gro93]. 6. Conclusions A miniaurised elecronic compass sysem has been designed. The analogue and digial pars fi on a single Sea of Gaes array. I combines sensing devices and analogue and digial circuiry in a single sysem by means of a Silicon based MCM. Simulaions indicae ha an accuracy wihin one degree is possible. The ideal sensor is no available ye bu he sysem is designed o broad specificaions so i can operae wih fluxgae sensors which will be realised in near fuure. References [Pe86] T. J. Peers, Auomoive Navigaion Using a Magneic Flux-Gae Compass, IEEE Trans. Veh. Tech., VOL VT-35, No. 2, May 1986, pp [Fre94] A. Frehe, The division of he Sea of Gaes Chip ino 4 quarers, Inernal repor, Technical Universiy Delf, he Neherlands [Haa95] P.E. de Haan, Analogue Circui Design on Digial Sea-of-Gaes Array, Ph.D. Thesis, Universiy of Twene, Sepember 1995, he Neherlands [Don94] R. van Dongen and V.Rikking, Advanced Analogue Circui Design on a Digial Sea-of- Gaes Array, proc. ED&TC 1994, pp , Paris, France [Oli96] J. Oliver, H.G. Kerkhoff, Tes Srucures on MCM Acive Subsrae: Is i Worhwhile, proc. ED&TC 1996, pp , Paris, France [Spr93] A. Sproe, e al, CMOS Magneic-Field Sensor Sysem, proc. ESSCIRC 1993, 1993, pp [Hol94] J. S. Hollema, e al, Compass Wach, Repor on he design of an Inegraed Compass Wach, D1 projec repor, ICE, 1994, Twene Universiy, The Neherlands [Mal95] P. Malcovai, e al, A Magneic Sensor wih Curren-conrolled Sensiiviy and

5 Resoluion, Sensors and Acuaors, A46-47, 1995, pp [Go95] R. Gofried-Gofried, e al, A Miniaurized Magneic Field Sensor Sysem Consising of a Planar Fluxgae Sensor and a CMOS Readou Circuiry, Transducers '95, Sensors and Acuaors, A12, 1995, pp [Gro93] P. Groeneveld and P. Sravers, Ocean: he Sea-of-Gaes Design Sysem Manual, 1994, Delf Universiy of Technology, The Neherlands [Kaw95]S. Kawahio, e al, Micro-Fluxgae Magneic Sensing Elemens using Closely Coupled Exciaion and Pickup coils', Transducers 1995, Sensors and Acuaors, A12, 1995, pp [Rip92]P. Ripka, Review of fluxgae sensors, Sensors and Acuaors A, 33, 1992, pp [Spa76] O. Spaniol, Arihmeik in Rechenanlagen, Teubner Sudienbücher Informaik, 1976

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