Technical Articles

In the last few years fiber optics have become very popular in the sensor market. Products such as fiber optic switches, fiber optic temperature sensors and fiberoptic fluid level sensors are now being produced and used in many applications. There is also a lot of activity in the fiber optic position sensing market. This article will discuss the different approaches being considered for position sensing and the technology employed by the Fiber Optics Group of Computer Optical Products, Inc.

History

Several years ago the aircraft industry issued a request to various sensor companies to develop fiber optic components for use in commercial aircraft. Their goal was to develop practical fly-by-light technology. This technology addresses some important concerns in the aircraft industry such as weight, radar detection and electro-magnetic interference. From this effort two technologies emerged and until now have dominated the position sensor field, namely, wavelength division multiplexing(WDM) and time division multiplexing (TDM).

Wavelength Division Multiplexing

WDM is a technique that uses one light emitting diode (LED) and one photodetector for each bit of resolution. Therefore, for a ten bit encoder, WDM requires ten LED's and ten photodetectors.

The light from the ten LED's must be of ten different wavelengths which are mixed together and sent down one fiber from the electronics system. When the light reaches the sensing element the light is bent by a prism (different wavelengths of light bend at different angles) and is projected onto ten channels of a commutator. The light that is reflected back by the commutator pattern returns to the electronics system via the same fiber.

When the returning light reaches the electronics system, it goes through another prism and is split into ten rays of light. There is a photodetector for each ray of light which produces a corresponding output.

WDM Disadvantages

The disadvantages of WDM are cost and complexity. For each bit of resolution an LED of another wave length is required. The optics for splitting and combining, and the prisms all require critical and stable alignment. WDM is also sensitive to amplitude variations and reflections which impose stricter requirements for the fiber optic connectors.

However, new developments in WDM show promise that wide band LED's can be made, thus requiring fewer LED's. Furthermore, internally generated connector reflections can be minimized if the system uses a separate fiber for the returning light.

WDM Advantages

The advantage is that the electronic module and sensing element interface together with one (or two) fiber(s). Therefore connector costs are minimal and so is weight.

Time Division Multiplexing

TDM uses one LED pulsed at a high periodic rate and it's light is sent down a single fiber. When the light reaches the sensing element it is split (ten times for a 10-bit encoder) and sent through long coils of fiber. Each coil is a different length, thus causing the pulse of light to reach the end of its respective fiber at a different time (light travels at approximately one nano-second per foot). The light from the fiber is then reflected off the commutator pattern and travels the same path back picking up more time delay through the coils. The result is a stream of light pulses. One photodetector is used to detect the stream of light pulses and a shift register organizes the data for output.

Disadvantages

Relatively large delay coils using hundreds of feet of fiber along with many expensive and bulky optical splitters, all of which are difficult to miniaturize.

However, new developments include an integrated splitter/readhead which will reduce the bulk of the sensing element.

Advantages

A single fiber between the electronics and the sensing element. Lower cost associated with single LED and photodetector. Not as sensitive to amplitude variations as WDM.

COPI, Fiber Optics Group's Technique

We have taken a totally different approach to provide customers with a practical and cost effective method of sensing position with fiber optics. We use two mature technologies, namely, conventional optical encoders and bundled fiber optics, and package them to create a small, cost effective, EMI immune, rugged, hi-temp, intrinscally safe and reliable position sensor.

Bundled fiber optics have been around for a long time. They are used as light pipes in various industries. Any application that needed the brilliance of incandescent light without the associated heat has made use of a light pipe (such as microscope illumination). Bundled fibers are used in remote viewers that are used, for example, by doctors during orthoscopic surgery. Card readers for computers and lotteries use this technology to form an appropriate illumination pattern to detect punched holes or pencil marks.

We use similar bundled fibers to form light pipes to remotely illuminate a conventional commutator through a conventional mask. This makes it possible to modify any existing encoder to use fiber optics.

Click on the drawing to open a larger image in a new window.

The fiber bundles are composed of many .002" (50 micron) diameter fibers. Depending on how many channels and how large the active (illuminated) area is for each channel determines total number of fibers. Each active area is filled with as many fibers as will fit. A typical active area might be .030" diameter which will allow more than 150 fibers to fit. Since a typical high resolution bi-directional with index, incremental encoder has 12 channels (differential reading - 6 detectors and 6 emitters) the total number of fibers can be as high as 1800. If one fiber breaks, or even as many as 30%, the encoder will still work (built in redundancy). Compare this to WDM and TDM, if one fiber breaks the whole unit stops working. Is a single fiber a real advantage?

One technique that can be implemented with this technology is to integrate the light from many LED's. In the example above we would combine the 6 emitter channels and randomize the fibers together into one channel. This does not reduce the total number of fibers, but it does allow illumination of each channel with the combined and integrated light from one or more LED's. This results in improved reliability. If one LED out of several fail, all channels of the encoder will still function. WDM, TDM and even conventional optical encoders lack this capability.

These bundled fibers have many other advantages. They are fabricated differently than communication grade fiber, like those used for WDM and TDM. The bundled fibers are compound glass (radiation hardened) and can withstand temperatures from -55¡C to as high as 480¡C (-67¡F to 900¡F). The limiting factors in high temperature applications are the adhesive and bearing lubricants used. We have developed various assembly techniques to either eliminate adhesives and lubricants or to use suitable alternates.

Another advantage these fibers exhibit is their ruggedness. These fibers are not as brittle as communication grade fibers and they do not break as easily under shock and vibration. The bend radius is less than 1/8", which allows us to design low profile readheads to keep the sensor as small as possible.

Smart Encoder

Being able to disconnect the light path from the sensor and/or to interchange optical parts presented a new problem for us to deal with. Encoders are precision built, sensitive, electro-mechanical sensors that are mechanically, electrically, and optically matched and trimmed. This means inter-changing parts requires re-trimming and/or re-calibration. We have developed a technique which allows the customer to mix and match opto-electronic parts without having a skilled technician re-trim the unit. There is also a built in function that compensates for aging of electronics, temperature and changes in the optical path.

We can make the sensing element incredibly small and light weight because we do not use large delay coils as in TDM nor do we use prisms or complicated optics as in WDM. Using our reflective technology commutators can be put on almost any flat surface, or on the circumference of a drum or ring. Line widths, or sectors of a circle, of .001" can be encoded (over one part in 3000 on a 1" dia.). The gap between the commutator and the readhead is typically .008" to .040".

Summary

This unique blend of technology gives our fiber optic encoders many advantages that no other position sensor can offer. The fact that these technologies are mature and reliable is probably the most important aspect. We use conventional LED's and photodetectors which gives our product greater reliability, longer life, and higher MTBF.

The passive sensing element eliminates the possibility of an electrical failure in inaccessible places. This can be very important in some systems when maintenance costs are considered. Lower total system costs can be realized when considering cabling and common electronics in multiple axis systems.

A lot of flexibility is offered by this technology. A customer can change the form factor of the sensing element, or make electronic changes without affecting the sensor geometry.

Combining the above with EMI/RFI immunity, high temperature capability and hazardous environment compatibility, these sensors offer solutions never before available. These sensors can replace resolvers, LVDT's, RVDT's, potentiometers and conventional optical encoders in a wide variety of tough applications.

About the Author

Lou Rosinski is Group Manager of the Fiber Optics Group of Computer Optical Products, Inc., a supplier of sine/cosine analog and digital encoders for the computer peripheral, industrial, and military markets. He earned his B.S. degree in electrical engineering from Brigham Young University in Provo, Utah. Mr. Rosinski has nearly 20 years experience in encoder design and applications and has been developing fiber optic sensors since 1987. He has three U.S. patents.