A low power multi-sensor interface for injectable microprocessor-based animal monitoring system

198

Sensors and Actuators A, 41-42 (1994) 198-206

A low power multi-sensor interface for injectable microprocessorbased animal monitoring system* P Wouters, M. De Cooman, D Lapadatu and R. Puers KathokkeCJnrvemtett .Leuven, Departement Elektmtechmek, ESAT-MICAS, Knrdmaal

Mercwlaan

94, B-3001 Hewdee

(Beti)

Abstract

In the research field of several blomedlcal dtsclphnes a maJoT need exists for rehable and implantable telemetry sensor systems Apart from the demands for small SW., light weight and long operational hfetune, the sensor systems should preferably also be flexible, versatile and intelligent As a follow-up of previous work, a novel and more powerful transponder 1s developed, specifically tailored for apphcatlons m large-scale ammal husbandry It IS a read-write telemetry device based on a microprocessor configuration, contaming a dedicated sensor interface chip (SIC), SMD thermistor and subnumature capacltwe accelerometers

1. Introduction The presented sensor interface chip (SIC) IS part of a transponder wluch is the final development of the AMIES (Annnal Momtonng and Identlficatlon - the European System) project w&m the European Eclair Programme Several other devices have been developed dunng the last years [l-3] All serve as telemetry sensor inputs for a welfare and chmatlc control system m ammal husbandry apphcatlons The challenge consists m developmg a telemetry tool which meets several goals and requuements, some of which are contradictory The most stnngent design requirements arose from the demand for mJectabihty This immehately results m very lmutmg constramts extreme mmlatumatlon and ultralow power consumption For these reasons, until now, inJectable telemetry transponders have rather hnuted apphcatlons smce they are always designed to meet the desired specdicatlons m the most nmple way This normally leaves no room for addItiona features such as mtelhgence, flexlblhty and multipurpose use The need for mtelhgence and versatility 1s nevertheless mdlspensable The mcluslon of a microprocessor within the transponder partially provides these desired qualities However, most of all commercial microprocessors are not sultable because of their large dnnenslons and high power consumption One of the better candidates 1s the Philips PCD3343, but even this small size (2 52 X 3 06 mm’) microprocessor consumes too much power for this type of appllcatlon

‘Session leading paper

0924-4247/94/$07 00 Q 1994 Elsener Sequoia All rrgbts reserved SSDI 0924-4247(93)00458-G

However, the presented momtormg system had to be bmlt arcund a nucroprocessor to cope wth the requrements for on-board mtelhgence and data storage capablhty The outcome 1s to design a stand-alone cucmt lmked to such a nucroprocessor As much mtelhgence as possible, related to momtonng, calibration and signal processing, 1s shlfted towards this SIC As such, the SIC IS designed to monitor and process the physlolo@cal parameters independently of the nucroprocessor The latter 1swoken up by the SIC only when other tasks need to be performed With such an approach, mtelhgence 1s built m both hardware and software Only by creating the nght balance between the software mtelhgence of the nucroprocessor and the hardware mtelhgence of the SIC, embedded m its dedicated &gital finite state controller and cucmtry, does a very powerful telemetry transponder become feasible Such a combmatlon may be characterized as a micromstrumentation system, mergmg sensors, analog interface clrcmts and microcontrollers on a common substrate [4] It offers the capablhty of operatmg mdependently from the outslde world and allows an mtelhgent and efficzent communlcatlon means with the end user through a radiofrequency (r f ) link 2. Overview of the overall telemetry system The general set-up of the telemetry system is shown m Fig 1 The transponder, housed m a dedicated cyhndncal package havmg a length of 40 mm and an external diameter of 5 mm, 1s inJected at the ear base of the ammal It 1s activated by an external 132 kHz carrier, generated by a nearby antenna As soon as the

199

-

antenna

A

1 decoder

U

LJ

Fig 1 OvervIew of the overall telemetry system

dew&s receiver stage picks up this frequency, It til transmit its stored data usmg a 66 kHz AM-modulated tamer, vvlth a maxmmm range of 50 cm These data contam mformation on physlologlcal monitored parameters, as well as all relevant mdlvidual mformatlon on its SubJect history of the ammal, ldentticahon number, etc Data may also be wrltten towards the transponder whilst It is within the actwatlon range of the antenna The wntten data are related to the functlonahty of the transponder and the mformatlon on its sub@, which 1sretained m memory The external antenna ISconnected to a dedicated decoder unit, developed by the Eureka Company (Poole, UK) This decoder interrogates and reprograms the mjected transponders. It is nucroprocessor-based and controlled via a senal communlcatlons link by a personal computer, whrch processes and stores the mcommg data

3. The qjectable transponder The general and sunphfied block diagram of the mJectable transponder is shown m Fig 2 Four major bmldmg blocks are dlstmgulshed The first part IS the Philips PCD3343A microprocessor, responsible for telemetry control, commumcatlon with the SIC and the

outside world, reading momtonng data from and wntmg programmmg settmgs to the SIC The second part 1s the sensor interface chip, which combines several functlons It contams two sensor mterface channels a temperature channel using a commercial SMD thernustor and a physical actlvlty detection channel, relymg on a submmlature capacltlve accelerometer, developed at our laboratory [5, 61 Apart from these purely analog comhtionmg circuits, the SIC contains dedicated d@al circuitry formed by its A/D convertor, local memory, fimte state controller, movement processmg algorithm and several tuning functions and clock control circuits Additional analog circmtry IS incorporated onto the SIC three different types of oscillators, r f modulation and demodulation cucultry and a fully mtegrated battery check clrcult The three different types of oscdlators control all tmnng functions of the transponder The low frequency (W) oscillator, runmng at 64 Hz, is an extremely low power consummg cmxut for general and basic tunmg of the momtonng actlons Since it 1s running at all times, its current consumption 1s kept as low as 150 nA at 3 V The nud-frequency (MF) oscillator controls the actual momtonng functions when the SIC 1s active and runs at 8 5 kHz The thud clock oscdlator 1s the actual microprocessor clock or high-frequency (HF) clock with a frequency of 4 5 MHz It 1s only actwe at mlcroprocessor wake up The remauung two building blocks are formed by the compopents necessary for the receiver and transnutter stages The mcommg earner IS ASK (amplitude shift keymg) modulated at 132 kHz, while the transnusslon output tamer 1s ASK or PSK (phase shift keymg) modulated at 66 kHz, denved from the mcommg frequency 4. The sensor interface chip

I

Fig 2 Stmphfied block &gram

I

of the qectable

transponder

4 1 General descnptwn Prunanly because of the small size and low power consumption demands, a dedicated low voltage (3 V)

CMOS sensor interface chrp IS developed as part of the transponder A nucrophotograph of the SIC 1s shown m Fig 3(a), whereas Fig 3(b) situates the different modules on the chip A 2 pm n-well CMOS process 1s used for the integrated cucmt, which 1s fabricated by Mletec NV (Oudenaarde, Belgium) The dlmenslons of the SIC are 9 97 X 2 68 mm2 The long, small shape 1s dictated by the cylmdncal package of the transponder A large part of the slhccm area 1s required to nnplement additional features not directly related to the momtormg of the physiologIca parameters, such as additional tlmmg circuits, control logx for commumcatlon, etc Moreover, extreme care IStaken to incorporate as many components as possible on chip Instead of usmg SMD components The motivation for both these choices IS based on the need for extreme mnuatunzatlon Although the chip area increases about ZO%, m the end of a lot of space 1s gamed on the thick film substrate carrymg the SIC, microprocessor and external components The complete configuration of all electromc components, placed on a 200 pm double-sided, through-hole-prmted thick-film alumina substrate, ISpresented m Fig 4 Apart from the external sensors (one SMD thenmstor and two capacitive accelerometers), the external components are part of the r f mput and output cmxutry, both of which consist of tuned LC tanks

The SIC-mxroprocessor interface consists of eight lines, as described m Table 1 They combme clock signals, control and communication lmes All communication between the SIC and the nucroprocessor IS issued using three lines Data are shifted serially m and out of the SIC usmg the bldlrectlonal ‘data L/o’ lme by the positwe edge of the clock signal on the ‘data clock’ hne All commumcatlon IS initiated by the SIC and controlled by the microprocessor, which configures the ‘data l/o’ hne for read or write mode using the ‘read-not wnte’ lme To preserve battery power, the microprocessor and Its 4 5 MHz clock are always m sleep mode They are only woken up by the SIC when really necessary Two events may tngger a wake-up signal either the detection of an external 132 kHz tamer to initiate telemetry commumcatlon or the signal of avallablhty of SIC momtormg data, ready to be read m by the nucroprocessor When the nucroprocessor IS woken up, it ~111detect the ongm and decide which action has to be taken When the appropnate action IS fimshed, it goes back to sleep mode

4 2 operahon modes

The SIC has three modes of operation, as vlsuallzed m Fig 5

cl dIgital

:: 0 I? tl

Rg

uproc

cwcultry

communication

memory

controller

movement

r-f clrcults

processqj

circuitry

00 53

I

3 (a) Mwxophotograph of the sensor Interface

0

chtp (9 97x 2 68 mm’)

(b) LocatIon

of the major modules on the SIC

201

FE

4 SchematIc

TABLE Interface

1 Descnptlon lme

Data I/O Data clock Rea¬

dqram

write

Data awlable Wake up Transnusslon data 45 MHz clock 8250 Hz clock

of the transponder’s of the SICmvxoprocessor

electromcs mterface

lines

Function bldlrectlonal data line data strobe used by the nucroprocessor to write data to or read data from the SIC, when wntmg, the data ht 1s placed on the ‘data I/O’ lme pnor to a knv-blgh tr?sltton on the ‘data clock’ line selects the mode of the ‘data I/o’ he, when high thbs hne IS m microprocessor-read mode, when low It IS m rmcropmcessor-write mode this lure goes high as soon as momtormg data come avadable from the SIC, these data (17 bit) are to be read by the rmcmpmcessor goes high to wake up the mxropmcessor when an r f signal IS pIcked up or data are avadable from the SIC used by the muzroproccssor to send the data bit tram to the rf cwcmtry on the SIC for transrmssron rmcropmcessor clock started by ‘wake up’ signal clock signal derwed from the mcommg 132 kHz r f signal to synchronue outgomg transmlsslon

The first mode 1s its normal momtormg mode, controlled by its own finite state machme Tlus operation cycle starts off Hnth a general reset signal, resetting all internal dtgtal arcmtry, except the memory retaining the settings as defined by the microprocessor After reset the ultralow power (150 nA at 3 V) LF clock oscdlator 1sstarted The extreme low power consumptton is imperative because this clock 1s almost runnmg at all tunes The function d this oscillator 1s to tune the intervals between consecutive momtormg samples The latter are defined by the nucroprocessor, on user request, and range from 1 to 128 mm Durmg 011s penod all clrrrults of the SIC are m standby mode As soon as such an interval has expired, a new sample tngger IS generated At this point the SIC’s mtemal clrcmtry is woken up and the measurements are started Accordmg to the user’s specficafions three types of measurements may be performed the measurement of temperature,

detection of physical act&y and a check on the battery voltage level The circuits perfonnmg these measurements are only actwated when needed m order to reduce the consumed power The temperature measurement consists of dlgltlzmg the analog voltage denved from a thenrustor into an eight-bit code, after amphficahon by a low power instrumentation amphfier The movement detection, however, 1s more complex On the one hand, it makes no sense to sample the movement signal at a smgle pomt m tune, on the other hand, It takes too much battery power to momtor physical actrvlty contmuously A specific algonthm has to be nnplemented As a compronuse, physical actlvlty 1smomtored for a set penod of time, defined by the user between 4 to 512 s The movement detection algonthm (see below) filters out the relevant mfonnatlon and encodes it mto a mere eight-bit d@al code Finally, the last data bit IS produced by the battery voltage check arcmt,

202

telemetry mode

monttormg

Fig 5 Ovetvlew of the operatmn

mode

progmmmtng mode

modes of the SIC

which sets a flag d the battery voltage drops below a user predeiined level When all measurements have been performed, the mxroprocessqr IS woken up and the custom-demgned, low power 4 5 MHz clock is started When data are available, the SIC remams m Idle mode until the microprocessor has read m all 17 data bits which are stored m Its memory As soon as the monltormg data has been read, the SIC automatically resumes its normal operation mode, startmg agam with a general reset signal and the microprocessor goes back to sleeping mode The second mode IS the telemetry mode The SIC contams all mod&&on and demodulation arcutts for the r f communlcat~on between the transponder and the outside world As soon as it detects the external 132 Hz signal, it wakes up the nucroprocessor and Its clock In the meantnne, its momtormg operations continue Independently from all r f actions When the rmcroprocessor IS woken up, it has to investigate the cause of its wake-up signal telemetry, avadabdlty of momtormg data, or both, and take the appropnate action In the case of r f commumcation, the microprocessor reads m the demodulated data commg from the SIC and acts on them accordingly If the mlcroprocessor needs to estabhsh a bldxectlonal commumcatlon, it will send the data bit tram to be modulated and transnutted to the SIC Thts consists of the monltormg data as well as the mformation on the subject When all telemetry actions are performed, the microprocessor resumes its sleepmg mode

The third and last mode of operation 1s the SIC programming mode In this mode the nucroprocessor reprogrammes the SIC on user request ‘I%LY 1sachieved through the bldlrectional telemetry link Up to 16 different commands can be used to program the SIC, as shown m Table 2 The commands can be dlvlded into three categories The fxst category consists of operational commands the stop, run and control commands They control and define the operation modes of the SIC, e g , they allow the selection of momtormg cmxuts and the configuration of the telemetry unit Each prograrnmm g sequence starts Hrlth the ‘stop and reset’ command to attract the SIC’s attention and ends with the ‘run’ command, after which the SIC resumes its normal momtormg mode usmg the newly defined settings The second senes of commands 1s used to calibrate and adJust the momtonng channels and oscillators This d@al cahbratlon 1snecessaxy to cope mth the devlattons caused by the integrated cxcuit processing and to allow the cahbratlon of the momtonng channels without the necessity of trnmnmg additional external components Even sensor dnft and the dependence of circuits’ performances on the battery voltage may be partially or completely cancelled A last category of commands 1s related to the overall operation of the SIC In fact, it 1s these commands that render the SIC flexible and versatile They program the sensltw@ of the temperature and movement channel, and several voltage references and current sources used for analog cucmts, thereby makmg the SIC suitable for dtierent types of apphcatlons and even dtierent types of sensors Two nnportant commands are the ones that program the time between consecutive monltormg samples and the tune during which physical TABLE 2 Overview of the SIC commands OP code

Command

0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

stop and general reset telemetry control system control J_F tuning MF tunmg HF hmmg ampbficatlon temperature channel voltage reference temperature channel cahbratlon movement channel sensmmty movement channel samplmg penod momtonng penod voltage reference battery check cahbratlon temperature measurement actwatlon nucroprocessor oscdlator run with current settrngs

No of data bits

4 4 5 4 9 4 8 5 3 3 3 5 1 1

m3

actmty IS detected and processed All programmmg data exchange between the nucroprocessor and the SIC 1s estabhshed accordmg to a spe&ic custom communication protocol

4 3 lYte sensor mterface cucwts 4 3 1 The tenperatum channel The sensor for the temperature channel (which 1s presented m Fig 6) 1s an NTC surface mountable themustor, namely, the Fenwall 196-204QAG-001, with a resistance of 200 f 20 kil at 25 “C A current source dnves a current of 1 +4 through a senes conneCtlon of the themustor and a bias res=tor Rb. The latter 1s required to generate a voltage ll, well withm the mput range of the ampler This voltage serves as the negatrve voltage input for the dtierential amphlier The positwe input voltage U, 1s produced by an eight-bit bmary weighed current source, which 1s dwtally controlled by one of the programmmg aunmand’s controlhng switches SRl SR8, m combmation Hrlth the on-&p reference resistor Rrcf The U, voltage 1s a reference defining the maximum temperature of the measurement window The voltage difference between the two mput nodes 1s amphfied by a de&cated two-opamp-based instrumentation amphfier, which 1s optmuzed towards low power consumptlons (<30 fi) Agam, the ampbficatlon factor of the mstrumentatlon amphfier IS dlgrtally controlled usmg switches SAl SA4 These define directly the range of the temperature wmdow and thereby the resolution of the temperature measurement since the analog output voltage of the mstrumentatlon amphfier IS dlpbzed into an eight-bit

code by the A/D convertor A last feature of the temperature channel IS the posslblhty to exchange the external thermistor for an on-chip cahbratlon resstor Rd Tim feature allows recahbration of the temperature channel automatically dunng nnplantatmn Only one of all the components shown 111Fig 6 IS external to the chp: the SMD thermistor 4 3 2 lRe capacuwe movement chmnel 4 3 2 I The capacttweaccelemmeterand itsconduwntng cmmt At ESAT-MICAS a subnumature sd~~n capacltlve accelerometer has been developed (FG 7) The main obJectives are low power consumption, h@ sens~tmty and very small size (II 1 mm’) Low power consumption 1s obtamed by adoptmg the capacltwe sensmg prmcrple In order to acbeve the extreme mmlatunzatlon and also to unprove the senslt~ty, new suspension techmques are investigated: torsional, meander- and sprmg-shaped beams. A very high sensitrvlty (15 Lwn/g) can be obtamed usmg the latter The rest capacttance 1s typ&ly 4 pF, with a AC of maxunum 200% The general design concept, electllcal conslderatmns and the most unportant fabncation steps are presented m ref 6 The qualities of such a highly muuatunxed capacltlve sensor can only be fully utdued when the appropnate condltionmg cucmtry based on switched capacitor techniques 1s incorporated (Fig 8) It performs conversion of the small sensmg capacitance mto a noise msensltlve output signal This cu-cmt 1s designed to suppress all negative effects of paraslfic capacitors, leakage resstors and electrostatic forces and it IS manly optmuzed

SAl

2ook

sA2

BOk

-+

-.-&j/L_ Sk3

4(3(

R3

.

eo=(Ur-Ut)[41+2(4OOWR3)] Fig 6 Cwcmt diagram of the temperature channel

SMD component

Fig 7 SEM photograph of the accelerometer

L

I

Rl

l

1p

Fig 8 Diagram of the swttched capacitor cwcud

towards low power consumption (35 pW) and a lugh PSRR The analog actwlty qnal 1s gwen by V,,, = VOPx (C, + C&)/C, To filter out the d c component of the capaatlve sensor, which IS related to the position of

the sensor (and the animal) at rest, an identical, but mechanically overdamped, capaatwe sensor C,,, IS used to generate a threshold Vmef,, to compare the activity signal V,,,, with The analog sensltlvlty of the cacmt can be defined digitally by controlling the switches

205 SW0 25 to SW4 and, hence, the amplticatron of the stage

5. Conclusions

factor

43 2 2 Processwg of the act&y data Unhke the temperature signal, the frequency content of the movement srgnal is quite hrgb (50 Hz) Where temperature is only sampled Just once, this method 1s plainly msufficient for detecting physrcal actrvrty According to the Nyqurst cntenon, the samphng of the movement results m a massive amount of data to be stored and transmuted Therefore, the movement data are processed by the SIC cucmts The processing algonthm, as explamed below, 1s embedded m the logrc crrcmts on the chip Frgure 9 vrsuahzes the used algorithm For the grven application the degree of acttvrty, rather than the exact value of the amplitude of the signal, versus time 1s relevant In fact one 1s merely interested m a relative expression for the actrvrty of the mdrvrdual animal dunng the chosen momtormg interval Therefore, the analog movement signal, denved from the sensing capaatrve accelerometer, IS compared to an adjustable d c reference value The output of this comparator 1s fed mto the drgrtal processmg circuit, which measures the trme dunng whrch rt remams high, and relates this to the overall sampling penod (from 1 to 512 s) Thn eventually results m only one byte of mformatron representing the percentage of the time durmg which the ammal had a certam predefined degree of actrvrty The major advantage of thus processing technique 1s the drastrc data reduction (100 samples/sx 8 bus x s/ momtormg penod into one byte) wrthout losing any vrtal mformatton It also improves the efficrency of the telemetry lurk and reduces power consumptton and required memory substantially 5

4 -

reference level

-5

z g

This work 1s supported by the Comnnssron of the European Cknnmunity m the Eelan Programme, No 0106, AGRE 004

0

s z

Acknowledgement

-+V.

= k

A dedicated CMOS sensor interface chip 1sdeveloped as part of an unplantable microprocessor-based mlectable transponder Prevrous research showed that detection of physical actrvrty and the measurement of body temperature are relevant mdtcators for the ammal’s welfare Apart from the analog condrtromng crrcmts for a thermrstor and capacitive accelerometer, the SIC also contams addrtronal crrcurtry necessary for Its mteractron with a mmlature Philips mrcroprocessor, which controls the implant Nevertheless, a lot of mtelhgence IS mcorporated wrthm the SIC, allowmg cahbratron of several cu-cults and the processing of sensor data rn srnr The major design ObJectives for the SIC are extreme mrmatunzatron and ultralow power consumptron to guarantee a long rmplantatron hfetrme of the transponder The mean power consumption dunng monrtonng IS as low as 25 PA, and during transmission 4 mA, both at 3 V Powered by a small 20 mA h hthmm cell, the estnnated hfetlme IS more than srx months Addrtronal and unique features are rts flexrbrhty m operatron and rts multrpurpose apphcatrons It makes extensive use of the capabrhtres of on-board mtelhgence by allowmg adlustments of the momtonng crrcmts wrth respect to, e g , sensor dnft

-10

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10

15

20

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30

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l-kc 14th Int Conf IEEE EngmeenngUIh4twkxneand WorogY Socztj, Pans, Fmnce, 1995 pp 2665-2666

References

P Wouters, M De Cooman, K. Van Schuylenbergh, B Puers and W Sansen, InJectable blotelemetry CMOS chip for Ident&xmon and temperature measurement system, Proc

ESSCIRC ‘91, Nan,

Italy, 1991, pi 45-48

B Puers, P Wouters, M De Cooman and S Vergote, A low power m&l-channel sensor mterfaw for use m dl@tal telemetry, Sensors and Actuators A, 37-38 (1993) 260-267 P Wouters, R Puers, R Geers and V Goedscels, Implantable blotelemetry devwes for anunal momtormg and tdenttficatlon,

4 K.D Wise and N Najafi, The commg opportumtles m mlcroscnsor systems, Tech Lk@, 6th Int Conf on Sohd-State Sensors and Actuators (Tbamdwers XV), San Fmncuco, C4, USA, June 1991, pp 2-l 5 S Vergote, M De Cooman, P Wouters and B Puers, A compostte membrane movement detector wth dedicated mterface electmmcs for ammal actlvlty trackmg, Senws and

Actuators A, 37-38 (1993) 86-92 6 B Puers and D Lapatadu, Extremely mmlatunzed movement sensors usmg new suspenaon Actuators A, 4142 (1994) 129-135

capantlve systems, Sem0r.s and