An Introduction to Fiber Optic Lever Displacement Transducers

Topics Covered

Introduction
Principle
Variations
Applications
Conclusion
About MTI Instruments

Introduction

Technological advances in fiber optic sensors have resulted in the development of a wide range of next-generation devices that use techniques such as polarization, interference and wavelength modulation.

The fiber optic lever displacement transducers are intensity modulated devices that are cost-effective, easy to use, and offer excellent versatility and performance. These features make it suitable for a wide range of industrial and laboratory applications.

Principle

In the fiber optic lever displacement transducer, an adjoining pair of fiber optic elements are used: one to transmit light from a source to a target or object whose motion has to be determined, and the other to receive the light that is reflected from the object and to transmit it back to a photo sensitive detector placed at a distant level.

Figure 1. Fiber optic filament

A fiber optic filament is a flexible strand of plastic or glass that can transfer light along its length by sustaining near total internal reflection of the light received at its input end (Figure 1).

Step index is a type of fiber which is extensively used in a number of applications. It contains an inner core to transmit the outer cladding and the light flux. In order to realize total internal reflection, the glass’ refractive index in the core (Nl) must be higher than the refractive index of the glass cladding (N2).

The sine of the light’s half angle is referred as the numerical aperture (N.A.). This is the highest angle at which a ray of light incident on the fiber face can be captured in the core and then reflected along its length. The light rays coming out from the other end of the fiber are similarly restricted to the same angle.

Individual fibers generally measure 0.001" to 0.010" in diameter; however, this size has been increased to approximately 0.060" in diameter, thanks to new developments in the fiber optic manufacturing techniques.

Additionally, transmission efficiency depends on the quality of the optical finish on the fibers’ end surfaces, and also on the purity and composition of the glass utilized in the cladding and core.

Figure 2. Target moving away from probe - reflected light intensity increases.

Figure 2 shows the interaction of receive and transmit fibers as the light is reflected from an object or target. At zero gap, the light contained in the transmit fiber is reflected back into itself, with little or no light transmitted to the receive fiber.

As and when the gap increases, the receive fiber captures some of the reflected light and carries it to the remote photo-sensitive detector. A distance will be reached at which point most of reflected light is transmitted to the receive fiber. When the gap is further increased, the light at the face of the receiver fiber will diminish and subsequently reduce the output signal from the photo detector.

The fiber optic lever transducer is capable of operating with different surfaces, ranging from diffuse and specular through to insulators and conductors. This type of commercial devices utilize a number of receive and transmit fibers (Figure 3) to achieve improved levels of intensity at the photo detectors so that preferred levels of performance are realized.

Figure 3. Fiber optic probe

The gap at which the zero slope or maximum output occurs offers an easy calibration reference position that can be readily used, and at this point the output signal can be standardized to achieve a reliable sensitivity factor. This sensitivity factor will be independent of the finish or color of the surface of the target being measured.

Figure 4. Output vs. later edge position

The highest output position provides an operating point at which point the output signal becomes independent of gap across a certain range. It also provides users with an extra mode of operation that allows measurement of displacement and lateral position of an object having a defined reflective interface or edge (Figure 4).

Therefore, with the help of advanced signal conditioning devices, dynamic or static measurements of displacement or position can be obtained. These measurements are rectified for reflectance differences that would cause potential errors in the data.

Variations

Upon placing a focusing lens system close to the sensing end of the fiber optic probe, a remarkable variation on this device can be realized. Figure 5 shows the outcome of one such combination of fiber optics and lenses.

Figure 5. KD-LS-1 lens with fiber optic probe

Another variation can be obtained by using a fiber optic sensor equipped with two receive channels provided by a transmit bundle. The size and distribution of the fiber with the receive channels are made to produce varied optical lever responses and hence can be ratioed to render an output signal that is independent of surface reflectivity.

Applications

Fiber optic lever transducers are used in a wide range of applications such as performance examination of rolling element bearings, bearing health monitoring of rotating devices used in liquid oxygen environment, development of fast response pressure transducers, and modal analysis of small parts such as flexure assembler, and hard disk drive, to name a few. These fiber optic sensors are available in a wide range of fiber distribution patterns to provide a choice of resolution, sensing range, physical shape, frequency response, and the likes.

Conclusion

Fiber optic lever displacement transducers come in different sizes and configurations. These devices are simple, user-friendly, low-cost, and offer excellent performance, making it suitable for both lab and industrial applications.

About MTI Instruments

MTI Instruments is a worldwide leader in the design, manufacture and engineering of non-contact measurement systems and sensors.

MTII’s main products consist of computerized general gauging instruments for position, displacement, thickness and vibration applications based on laser triangulation, fiber-optic and capacitance measurement technologies.

The Semiconductor Products sensor group manufactures manual, semi-automated and fully automated wafer characterization tools designed to measure wafer thickness, total thickness variation (TTV), bow, warp and flatness of semi-insulating and semiconducting materials.

MTII’s Aviation Balancing Instruments group provides state-of-the-art portable balancing and vibration analysis systems for turboprop and jet aircraft engines.

This information has been sourced, reviewed and adapted from materials provided by MTI Instruments.

For more information on this source, please visit MTI Instruments.

Ask A Question

Do you have a question you'd like to ask regarding this article?

Leave your feedback
Submit