handbook of modern sensors

8.7 Piezoelectric Cables 319
(A)
(B)
Fig. 8.14. (A) Fiber-optic ring resonator; (B) fiber-optic analog coil gyro. (Adapted from
Ref. [9].)
In practice, optic gyros are designed with either a fiber ring resonator, or a fiber
coil where the ring has many turns of the optical fiber [10]. The optic ring resonator is
shown in Fig. 8.14A. It consists of a fiber loop formed by a fiber beam splitter that has
a very low cross-coupling ratio. When the incoming beam is at the resonant frequency
of the fiber ring, the light couples into the fiber cavity and the intensity in the exiting
light drops. The coil fiber gyro (Fig. 8.14B) contains a light source and the detector
coupled to the fiber. The light polarizer is positioned between the detector and the
second coupler to ensure that both counterpropagating beams traverse the same path
in the fiber-optic coil [11]. The two beams mix and impinge onto the detector, which
monitors the cosinusoidal intensity changes caused by rotationally induced phase
changes between the beams. This type of optical gyro provides a relatively low-cost,
small-size, rotation-sensitive sensor with a dynamic range up to 10,000. Applications
include yaw and pitch measurements, attitude stabilization, and gyrocompassing. A
major advantage of optical gyros is their ability to operate under hostile environments
that would be difficult, if not impossible, for the mechanical gyros.
8.7 Piezoelectric Cables
A piezoelectric effect is employed in a vibration sensor built with a mineral-insulated
cable. Such a cable generates an electric signal in its internal conductor when the
outer surface of the cable is compressed. The piezoelectric Vibracoax TM cables 5
have been used in various experiments to monitor the vibration in compressor blades
5 Philips Electronic Instruments, Norcross, GA.