SAW Temperature Sensor

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SAW TEMPERA TURE SENSOR AND REMOTE READING SYSTEM

X. Q ao, W. Burkhard, V. V. Varadan and V. K. Varadan

Research Center for the E ngineering

of

Electronic and Acoustic Materials

Department of Engineering Science and Mechanics

The Pennsylvania State University

University Park, PA 16802

A B S T R A C T

A system fo r remotely reading a SAW temperature sensor has been adapted

from an existing, commercially available personnelherchandise detector

system. The SAW sensor is a Lithium Niobate wafer with an inter -digital

transducer IDT)which is directly connected to a small transducer which is

directly connected to a small microwave antenna and two reflectors. The

reading system is a special

Fhl

radar. The FM electromagnetic signal is

transmitted by the system and picked up by the small antenna that is

connected to the IDT and is subsequently, converted into a surface acoustic

wave in the Lf ib 0 3 wafer, reflected by the reflectors, converted back to an

electromagnetic wave and returned to the Mradar. The acoustic velocity

varies as a function of the ambient temperature and results in varying time

delay of the echoes, which is detected by the system. The resolution an d

accuracy of such a system

arr

investigated theoretically and experimentally.

The operating principle is also suitable

for

other remote reading SAW

sensors.

I. I N T R O D U a I O N

Various applications of SAW sensors have been reported recently. for

example, sensors for temperature, pressure, force, electric voltage, humidity

and gases

[ l

- 51. Most of them are based on detecting the change in phase

velocity of the surface acoustic wave caused by the above factors. One

usually uses the feedback oscillator method to measure the change in the

velocity. The operating principle

s

shown in Figure 1.

n p u t r a n s d u c e r O u t p u t t r a n s d u c e r

c o u n t e r

A m p l i f i e r

Figure

1.

The

SA W

Oscillator Sensor

An electric amplifier connects two inter-digital transducers on a

piezoelectric wafer

so

that oscillations result because of the feedback of the

surface wave propagating from the input transducer to the output

transducer. The oscillation frequency satisfies the condition that the total

phase sh ift of the loop equals an in teger multiple of 271 and varies with the

surface wave velocity. In the SAW-oscillator sensor, wires

are

needed to

connect the transducers to the amplifier. The fact that the frequency range of

0090-5607/87/ooo0-0583

1.00

987 IEEE

SAW devices m atches microwave frequencies

suggests

the idea of utilizing

microwa ves instead of w ires, i. e., exciting the interdig ital transduc ers with

a remote radar in order to meet the special requirements of certain

applications. Unfortunately, the feedback oscillation method cannot be used

directly because the output signal and the input signal will be mixed in the

microwave channel. As is well known radar systems, based on microwave

technology, range

targets

by meas uring the time delay of the echo

[ 6 ]

f

the

target is fixed, the time delay should be only dependent on the wave

velocity.

So

it

is

passible

to

remotely read the SAW sensor by an operating

principle similar to the radar. A remote SAW sensor system developed by

X-Cyte Co. for monitoring personnel and merchan dise has been adapted and

calibrated as a temperature sensor. The details

of

the operating principle and

the results of experimental calibration are described in the following

sections.

11.OPERATING PRINCIPLE

The diagram in Figure 2 shows the basic operating principle of the system.

An inter-digital transducer (IDT) and two reflectors are on the surface of a

YZ ut Lithium Niobate wafer. The transducer connects

A n t e n n a ,

d Tramducer

generater

9 7

Figure 2.Remote Tem perature Reading SAW System

directly to a small antenna. In the remote reading system, a Fh4 generator

sends a linear frequency modulated signal to an antenna and to a mixer. The

signal transmitted by the system antenna is received by the small antenna

connected

to

the LiN b0 3 wafer and converted into a surface acoustic wave

by

the transducer. The echo es from the reflectors are received by the IDT and

transmitted back to the system antenna and mixed with the original FM

signal in the mixer. The echoes are delayed copies of the original FM

signal. The time delays depe nd on the acous tic surface wave velocity. which

is a sensitive function of the ambient temperature. The difference frequency

signals, which are usually called IF, are output by the mixer. The

frequencies and the phase shifts of the I S vary with the time delays. Since

the changes in time delay with temperature are very small, the phase shifts

are used instead, since they are mor e sensitive than the frequen cy. In

order

to

avoid the effects o f time delay variations other than temperature chang es

(for example, the changes of distance between two antennas), the

temperature

is

determined by the differen ce in the phase shifts of the two IFS

corresponding to the two reflectors.

1987

ULTRASONICS

SYMPOSIUM

83



The original FM signal is expressed as

S

(t ) = A COS[ t )I

=

A

COS[ oo + pt 2) t +

eo]

The ech o from the first reflector input S1)o the mixer is the same as the

original FM signal but with a time delay t 1 and a different amplitude,

so

that it is written as

tl

= re 7 1

(3)

where

c1

is the time delay correspon ding to the surfac e wave travelling from

the transduc er to the first reflector and back. The delay can be written as

'1 = 2dl I

V,

(4)

in terms of dl , the distance between the IDT and the fr st reflector and v is

the surface acoustic wave velocity, and the time delay due to the

electronic circuit and signal propagation.

The

IF

corresponding to S,(t

)

is expressed as

The frequency ptl and the phase shift

1=

cootl- p1?12 both dep end on

the time delay tl. Since

oo

s usually much greater than ptl, the phase

shift is more sensitive than variation of the frequency.

From Eq. (3) we know that the total delay tl depends not only on the travel

time of the surface wave, which is a function of the temperature, but also

on the microwave propagation path. Th e latter varies with the distance

between the excitation transmitted and SA W device. To eliminate the error

f ro m th e v ar ia ti on of T ~ ,second reflector is put on the wafer. The

corresponding time delay is 7 . Similar to the first reflector, we have the

IF

correspondingto the second reflector

as

~ ~ ( t

= B~ COS [

e(t 2(t)

1

=

B2 cos[

pty wo -

p$/21

where

= T~

+ T~

and ~

=

2d$v

(7)

and s the distance between the the IDT and the second reflector. The

difference of the

two

phase sh ifts can be written as

where

K = ~ , - 1 J J 2 ( 5 + t 1 ) 0 0 (9)

since mo < pl2

(5

tl) as can be seen from the numbers given later;

and

T =

2 d I v

(10)

where is the total travel time of the surface wave from the fxst reflector

to

the second and back. This time being inversely proportional to the surface

wave speed is very sensitive to the temperature in the vicinity of the SAW

device and we propose the following relationship between the travel time 5

and

the

emperature T

T =

r0

[ 1

+ CY (T To)]

(11)

where a

s

the temper ature coefficient of time delay of the SAW device and

To is the ambient temperature.

From Eq. 8).

$d

=

Kro [ 1 CY.(T

-

To)]

= a K r o T + K z , , ( l - T o )

= a T + b

a = K a . r o (13)

If the resolution of phase shift difference in degrees is A , then the

resolution of the temperature rrading w ill be as

A T = A q l a (1 4)

The wafer is made

of YZ

ut LithiumN iobate with

a =

9 4 ~ 1 0 - ~ / O C (15)

The two reflectors are located such that the time delay at room temperature

To is

r l = 1ps and z2 = 1.1

Then,

16)

o = 0.1

ps

The transmitted FM signal is pulse modulated with a time duration of

1/60

second. The carrier frequency varies linearly from 905 MHz to 925 MHz

during the

period.

The parameters in Eq.

(1)

for the

FM

signal are as

m0 2 a

=

905 MHz

(18)

(19)

2x = 1.2 x 10-3 M H ~

ps

In

operation, the distance between the two antennas is within one

or

two

meters

so

that re can be neglected compared to r 1 r T ~ .he temperature

variation can be in the range 0-200°C. The first and second terms in Eq. (9)

are given by

KO =

-

N 2 ( t l+p

= 2 a x 90 5 x 1 0

-

1.2 x x 1.05 2

=

2rr

x 905 x lo6

(20)

so

that the approximation in Eq.(9) is justified. From Eqs.(l3), (15) and

17), he constant 'a' is

a = 3.06 angular degrees I C

(21)

The resolution of the phase shift is

1' so

that the resolution of the

tempera ture reading is given by Eq.(14) as

AT

= 0 . 33

C (22)

3. C A L I B R A T I O N OF THE SY STEM

The experimental calibration is done in a Delta 9023 which is a

temperature-controlled chamber. The apparatus is shown in Figures 2 and 3.

A

digital thermometer RTDHand held Thermometer, Keruco Instruments

Co.)

with an accuracy of ? 0.2 C is taken as a standard. The temperature

range in the experiment is from room temperature near 20°C

to

140'C. The

584

987 ULTRASONICS SYMPOSIUM



Chamber

/

wi th

antenna

4

rl

i g i t a l thermometer

Figure3. Calibration of the Remote Reading System

highest temperature is limited by the melting point of the plastic piece

supporting the small antenna connected

to

the

SAW

device. Because

the IF

signal is a periodic function, the system can only give the phase shift

differences in the

range

from -179' to 180 . This

is

the reason for

the

ump

of 360 near a temperature of 80°C (see Figure 4). After correcting for this

-100

Figure4. Phase Shift Difference

Vs

Temperature

jump, the experimental points can

be

connected by a straight line

as

shown

in Figure

5

The line in Figure

5

is obtained by the least mean square

method.

The

equation of the line is

(23)

2.89 T 9.1

The value of the coefficient a

=

2.89 is in agreement with that estimated in

Eq.(21). The root mean square of the phase difference is equal to

0.78 ,

which corresponds to an RMS of 0.27OC.

Figure 5 Phase S hift Difference

Vs

Temperature after Jump Correction

There is a problem associated with obtaining multiple values of the

temperature corresponding to one phase shift difference in the current

system. A possible method to overcome this problem is to roughly

determine the temperature range by th e frequency of the IF signal. We plan

to do this in the near future.

REFERENCES

1. T. M. Reeder, et al., IEEE Ultrason. Symp. Roc., 26, 1975.

2. K. Toda, e t al.,

J.

Acoust. Soc. Am., 74,677-679, 1983.

3.

D.

Hauden, et al., Annual Freq. Control Symp., 312-319, 1980.

4. A. Arthur and H. Wohltjen, Anal. Chem., 56, 1411-1416, 1984.

5. R.

M

hite,

E E E

Ultrason. Symp. hoc. 90, 1985.

6.

N.

. zannes, Comm unication

and

Radars, Prentice-Hall, Inc., 1985.

1987

ULTRASONICS SYMPOSIUM

85

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SAW Temperature Sensor