Fiber Gyros Provide Low-Cost Rotation Sensing

Incorporating the Sagnac effect, optically-based gyros offer economical, robust, reliable performance.

By: Sid Bennett, KVH Industries Inc.

Traditional gyroscopes, based on the momentum of a spinning rotor, are used in a wide range of applications in which orientation and guidance data are required. Although they are time-tested and well understood, these mechanical devices are susceptible to damage from shock and vibration and exhibit cross-axis acceleration sensitivity. Lower cost versions may also suffer from reliability problems. In an attempt to solve these problems, engineers have developed alternative technologies such as ring laser gyros (RLGs), fiberoptic gyros (FOGs), and Coriolis vibratory gyros (CVGs).

Figure 1: A ruggedized fiber optic gyro (FOG) is designed to withstand heavy motion and impact when deployed in vehicles such as military tanks. Applications for other FOGs in the product line include stabilization and positioning for positive train control, precision farming, navigation systems, and robotics systems. (Photo courtesy of KVH)

Both the RLG and FOG employ the Sagnac effect, discovered in 1913. In a Sagnac interferometer, two light beams traverse a ring-configuration in opposite directions. Rotating the interferometer effectively shortens the optical path traveled by one of the beams, while lengthening the other. The interference pattern produced by the two beams displays a fringe shift proportional to the angular speed of rotation.

Ring laser gyros are now used extensively in inertial navigation systems for aircraft. Since RLGs require high vacuum and precision mirror technology, however, cost issues have limited their penetration of lower-end applications such as robotics, stabilization, and land navigation.

Fiberoptic gyros
By employing a coil of special-purpose optical fiber, FOGs perform the same tasks as RLGs at considerably less cost. The physical principal of operation is analogous to the Doppler effect, but in this instance it involves determination of the phase shift between the two counter-propagating light beams.

Interferometric fiber optic gyros can be configured as either closed-loop or open-loop, but the cost of the former presently restricts it to avionics and inertial navigation grade applications. The open-loop configuration consists of a fiber coil, one or two directional couplers, and a polarizer, all made from single-mode, polarization-maintaining optical fiber (see Figure 2). In addition, the system requires an optical source and detector, and a piezoelectric (PZT) device wound with a small length of one end of the fiber coil to apply a non-reciprocal phase modulation.

Figure 2: Diagram of open-loop fiber optic gyro shows sensing loop, polarizer, and couplers, all made of polarization-maintaining fiber.

Light in the coil
The laser-generated light traverses the first directional coupler, passes through the polarizer, and into the second directional coupler, where it is split into two signals of equal intensity that travel around the fiberoptic coil in opposite directions. The wavefronts recombine at an optical interferometer at the coil coupler and returns through the polarizer. The second coupler directs a portion of the light into a photodetector.

Remarkably this configuration is capable of measuring the phase difference between the two signals to one part in 10^-16. This is possible due to the principal of reciprocity. Light passing from the laser through the polarizer is restricted to a single state of polarization. The directional couplers and coil, made of polarization-maintaining fiber, ensure a singlemode path. Both directions of travel follow the same physical path; thus, almost all environmental effects except rotation cancel one another.

Rotation detection
The light intensity returning from the coil to the polarizer is in the form of a raised cosine function that has a maximum value when there is no rotation and a minimum value when the optical phase difference is ±l/2. The gyro is therefore sensitive only to rotation about the axis perpendicular to the plane of the coil.

Due to the cosine shape, the change in interferometer output would be small for small input rotation rates; moreover, it is not sensitive to the sense of the rotation, as the decrement in amplitude is equal for both directions. These problems can be solved by applying a dynamic phase bias to the light path. The phase bias is applied by modulating the PZT with a sinusoidal voltage to impress a differential phase shift between the two light beams at the modulating frequency. In addition to solving the directionality problem, the phase bias also shifts the demodulation to a frequency well-removed from the DC value, eliminating bias drifts associated with offsets in low-level DC amplifiers.

Fiber gyro output
When there is no rotation of the coil, the interferometer output exhibits periodic behavior whose frequency spectrum comprises Bessel harmonics of the modulation frequency (see Figure 3). Since the phase modulation is symmetrical, only even harmonics are present; the ratio of the harmonic amplitudes depends on the amplitude of the phase modulation.

Figure 3: In the absence of rotation, Sagnac interferometer response for open-loop configuration consists of even Bessel harmonics of the modulation frequency.

When the coil is rotated, the modulation takes place about the shifted position of the interferometer response. The modulation is unbalanced, so the fundamental and odd harmonics will also be present (see Figure 4). The amplitudes of the fundamental and odd harmonics are proportional to the sine of the angular rotation rate, while the even harmonics have a cosine relationship. The simplest demodulation scheme synchronously detects the signal at the fundamental frequency.

Figure 4: Rotation introduces the fundamental and odd Bessel harmonics into the Sagnac interferometer response.

The open-loop FOG is sometimes criticized for the sinusoidal relationship between the input and output characteristics. However, since this is a well-known analytic function, it can be dealt with by subsequent signal processing or modeling, which are inexpensive and relatively easy solutions.

Performance
KVH Industries is using its proprietary optical technology to develop a family of FOG products for a range of factory automation, robotics, land navigation, and optical and antenna stabilization applications. Advantages include resistance to shock and vibration, high reliability, long-term durability, low-noise, and low cost. A number of these FOG designs are now in volume production as replacement technology for mechanical gyros and in new product design.

A typical KVH Industries FOG operates at 820 nm and incorporates a 75-m coil of elliptical-core, polarization-maintaining fiber. The elliptically-shaped core region provides polarization-preserving transmission, and the short coil length ensures operation in the linear portion of the sine response curve, without the need for further linearization.

The gyro bias drift versus time at a constant temperature is less than the angle random walk. The bias varies slowly with temperature due to offsets in DC amplifiers following the detector, but the effect is repeatable and can be calibrated with the aid of the internal temperature sensor. The input-output rate relationship demonstrates that good linearity can be achieved by a simple demodulation technique.

In the 20 years since the use of the Sagnac effect for a fiber optic gyro was first proposed, the technology has undergone continual development. A wide range of gyros based on this principle now exists. With modest performance requirements, the low-price versions are particularly suitable for commercial applications.

About the author…
Sid Bennett is vice president, Fiber Optic Group, KVH Industries Inc., 8412 W. 185th St., Tinley Park, IL 60477. Phone: 708-444-2800; fax: 708-444-2801; e-mail: info@kvh.com.