Page 387 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

APPLICATIONS 367

13.4 APPLICATIONS

In this section, we present in detail some examples of the applications of SAW-IDT
devices as temperature, strain, pressure, torque, rotation rate (gyroscope), humidity, and
so forth sensors. In the next chapter, the applications are extended to include micro-
electromechanical system (MEMS) IDT structures along with IDTs for remote sensing of
acceleration.

13.4.1 Strain Sensor

In this section, a remote MEMS-IDT strain sensor system is employed to study the
deflection and strain of a 'flex-beam' type structure of a helicopter rotor (Varadan et al.
(1997)). The system is based on the fact that the phase delay is changed because of the
strain in the sensor substrate. The system consists of a remote passive SAW sensor read
by a fixed microwave system station.
The FM signal sent by the system antenna is expressed as

S(t) = A COS(O)Q + fit/2)t (13.9)

where COQ is the initial frequency of the FM signal, /JL is 2n times the rate of modulation,
and t is time.
The echoes from the two reflectors, S\(t} and 82(1), are the same as the transmitted
signal S(t) but with different amplitudes and time delays t\ and , respectively. These
t 2
may be written as
Si(0 = A 1 cos(w 0 + V>t/2)(t - fi) (13.10)

and
S 2(t) = A 2cos(o) 0 + l^t/2)(t -t 2) (13.11)

with

+ T e (13.12)
(13.13)
+ r e
where v is the SAW velocity, d\ and d 2 are the distances from the IDT transducer to
the two reflectors, and r e is the total of other delays (such as the delay in the electronic
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circuit and devices and the traveling time of the electromagnetic wave ) that is the same
for both echoes.
Through the mixer that uses the transmitted signal as a reference and low-pass filter,
frequency differential signals are obtained as

Ei(t) = BI COS\JJLtit + (cOQti - fJLti)] = BI COS[(i>it + <pi] (13.14)
and
E 2(t) = B 2 cos[/Ltf 2f + (co 0t 2 - nt] = B 2 cos[co 2t + (p 2] (13.15)

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For short distances, this time is negligible.