WO2002021657A1 - Fiber optic light sensor system - Google Patents

Abstract

A fiber optic light sensor system employs a fiber optic cable having a core and an annular jacket, the annular jacket covers the core along a jacketed section (20) of the cable to prevent extraneous light from coupling into the cable. One or more sensing sections (16) along the length of the cable where the core is not covered by the jacket are used to receive light from areas of interest such as electrical enclosures (18). A photo detector (26) is coupled to one end of the cable and detects a light signal that enters the cable at the sensing section (16). Signal processing instrumentation (24) connected to the photo detector (26) then processes the light signal. Optionally, a Built-In-Test feature employs a light source (58) to send test signals throughout the system.

Description

TITLE OF THE INVENTION

Fiber Optic Light Sensor System

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority of U.S. Provisional Applications 60/229,958 and 60/229,959 both filed on September 1, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to light detectors and is directed more particularly to a fiber optic light sensor system where light enters a sensing section along a fiber optic cable and is coupled into the cable.

BACKGROUND OF THE INVENTION [0003] Arcs across short circuits in industrial electrical switchgear can release massive amounts of energy. In a single second, several mega watts of energy from an unintended electrical arc can vaporize equipment and generate great pressures, sometimes exploding enclosures and injuring workers. Minimizing the duration of an arc is critical to minimizing damage. High power arcs lasting under 30ms will often result in little damage; however arc durations greater than 500ms can result in extreme destruction. Unintended lower power arcs in circuits and motors should also be detected and eliminated as quickly as possible to minimize the risks of fire and equipment damage.

[0004] A typical circuit breaker will generally not trip fast enough during an arc fault to minimize damage. The high resistance of an arc fault often causes the arcing current to be below the instantaneous magnetic trip threshold of most circuit breakers. Further, the intermittent quality of an arcing fault can create an average RMS current value that is below the thermal threshold of the circuit breaker. Numerous solid state, electro-mechanical, and optical systems are therefore available to detect such arcs and trigger circuit breakers in the shortest time possible, disconnecting the power source and minimizing the magnitude and duration of the arcs.

[0005] The solid state and electro-mechanical arc detection systems use various techniques to distinguish nefarious arcing currents from innocuous arcing currents caused by equipment such as brush motors. Generally a microprocessor is used to identify electrical signature characteristics of unintended and dangerous arcs. If the system detects such characteristics, for example, random occurrences of high frequency current spikes, then a solenoid is used to immediately trip a circuit breaker.

[0006] In appropriate locations, such as inside switchboards, optical arc-detection systems that detect arcs directly at their source have proven to be the best and fastest means for detecting arcs. Some of these systems aim the end of a fiber optic cable at an area susceptible to arcing. Light from an electrical arc enters the end of the cable and triggers a solenoid that opens a circuit breaker. However, a significant limitation of these systems concerns the relatively narrow field-of-view of the individual cables. Wide-angle lenses are often attached to the ends of the cables to improve the field-of-view. Yet even with wide-angle lenses, multiple cables must often be arranged inside an equipment enclosure to ensure that at least one cable is positioned to receive light from each of the various locations where arcing may occur. The need for numerous lenses, cables and associated couplers increases the overall system costs. And the high cost of such systems has resulted in their underutilization. Many industry sites that could benefit from an effective arc-fault protection system instead elect to have no arc-fault protection at all.

[0007] Furthermore, in most present fiber optic light sensor systems there is no fast and easy way to test whether the system is working properly. It is often not possible to tell whether the absence of a signal at the detector is due to a broken cable or due to lack of an input signal. Yet in many real operating environments it is vitally important to have the ability to regularly and comprehensively test whether the sensor system is functional. SUMMARY OF THE INVENTION

[0008] The present invention, among other things, presents a solution to the aforementioned problems associated with prior art electrical arc detection systems and with general prior art light detection systems.

[0009] The present invention is directed to a fiber optic light sensor that employs a fiber optic cable having a core and an annular jacket, the annular jacket covering the core along a jacketed section of the cable; however, unlike sensors used in the prior art that receive light through the ends of a cable, the present invention includes a bare sensing section along the length of the cable where the core is not covered by the jacket. A photo detector is coupled to one end of the cable and detects a light signal that enters the cable at the sensing section. Signal processing instrumentation connected to the photo detector then processes the light signal.

[0010] Advantages of particular embodiments of the present invention include the ability to detect light 360 degrees around the circumference of the cable. There are also many general advantages to using a bare sensing section of a fiber optic cable to transmit light back to the signal processing instrumentation rather than directly installing the instrumentation at the point where the light originates. The use of a fiber optic cable provides electrical, mechanical, thermal, and environmental isolation from the area surrounding the light source. This allows the placement of a sensing section in direct contact with high voltage circuits that could destroy signal processing instrumentation. It also allows placement of the sensing section in locations of high vibration, high or low temperature, and in corrosive or wet environments that could harm electronics. Further, use of a fiber optic cable enables placement of the sensing section in tight locations where an electronics package would not fit. Fiber optic cables are also less expensive than electronic instrumentation and can be used as a disposable item. Additionally, fiber optic cables are totally immune to electromagnetic energy and many electronics packages are not. [0011] It is therefore an object of the present invention to provide an improved light sensing system in regard to range of viewing angle effectiveness, environmental isolation, adaptability for use in tight spaces, and cost.

[0012] Another object of the present invention is to provide an improved light sensing system that includes a Built-in-Test (BIT) feature.

[0013] Other objects and advantages of the invention will become more fully apparent from the following more detailed description and the appended drawings that illustrate several embodiments of the invention. In the following description, all like reference numerals refer to like elements.

BRIEF DESCRIPTION OF THE DRAWINGS [0014] FIG. 1 is a cross-section view of a typical fiber optic cable. [0015] FIG. 2 is a schematic diagram of one embodiment of the present invention. [0016] FIG. 3 is a schematic circuit diagram of one embodiment of the signal processing instrumentation of the present invention.

[0017] FIG. 4 is a schematic diagram of one embodiment of the Built-in-Test (BIT) electronics of the present invention.

[0018] FIG. 5 is a schematic diagram of one embodiment of the present invention employing the BIT feature.

[0019] FIGS. 6 A and 6B are schematic diagrams of the BIT features of the present invention as used in conjunction with prior art fiber optic light sensors. [0020] FIGS. 7A and 7B are cross-sectional and top views, respectively, of one embodiment of the present invention that functions as a light safety curtain.

DETAILED DESCRIPTION OF THE INVENTION

[0021] FIG. 1 is a cross section view of a typical fiber optic cable. A core 10 is made of glass or plastic and surrounded by a cladding 12 and a jacket 14. Fiber optic communication is based upon injecting light into one end of the core 0 and receiving it out of the other end of the core 10. Any light that is injected into the fiber core 10 and strikes the core-to-cladding interface at greater than a "critical angle" reflects back into the core. The critical angle is defined as the angle of incidence at which a refracted light ray points along the surface of the cable, the angle of refraction being 90 degrees. At angles of incidence larger than the critical angle there is no refracted ray, a condition called "total internal reflection." Fiber optic cables are designed to exploit this phenomenon by causing light rays that are coupled into the fiber to strike the core-to-cladding interface at greater than the critical angle. The light is thus reflected repeatedly with minimal energy loss and propagates down the fiber. Stray light rays that strike the interface at less than the critical angle pass into the cladding 12 where they are attenuated and absorbed by the jacket 14. Since the primary application of fiber optic cables is to efficiently transmit light, light is normally coupled into the end of the core 10 where the coupling is most efficient; i.e., where most of the introduced light rays strike the core-to- cladding interface at greater than the critical angle.

[0022] However, if the jacket 14 is removed or if a clear jacket 14 is used, exterior light can penetrate into the core 10 through the cladding 12. The cladding 12 will attenuate the light entering the cable and much of the remaining light will pass completely through the cable. But a percentage of the light will penetrate the core 10 at an angle such that it will hit the far sidewall of the core 10 at greater than the critical angle and propagate down the cable. The present invention exploits the fact that light may be coupled in this manner into a "bare" fiber optic cable (i.e., fiber optic cable with its jacket 14 removed or with a clear jacket 14) by penetrating through the cladding 12.

[0023] With the advent of more sensitive light detectors, the amount of light that can couple into the side of a fiber optic cable is enough to be useful. FIG. 2 is a schematic diagram of one embodiment of the present invention where the jacket 14 of a continuous fiber optic cable is selectively removed to create one or more bare sensing sections 16 to admit light and act as a sensor/detector in multiple desired locations. In this embodiment the sensing sections 16 are routed around the inside of metal enclosures 18 where electrical switchgear is housed. The sensing sections 16 are intended to detect electrical arcs inside the enclosures 18 that can destroy equipment and start fires. The cable outside of the enclosures 18 include jacketed sections 20 to shield the core 10 from extraneous light signals. Light 27 from a sensed event, such as from an electrical arc, is then transmitted over a distance to a photo detector 26 and related signal processing instrumentation 24. The signal processing instrumentation 24 may in turn send a signal to an interface such as a computer 25 or an alarm panel.

[0024] In addition to selectively removing the jacket 14 from jacketed fiber optic cable, various alternatives may be used to create the sensing sections 16. For example a cable with a clear jacket 14 can be used in the sensing section 16 and an opaque jacket 14 can be applied to create the jacketed section 20. Also, a fiber with a clear jacket 14 can be used as the sensing section 16 in the desired location and optical connectors can connect the sensing section 16 to an opaque jacketed section 20. Further, bare optical fiber can be used throughout the system and sleeved with tubing to form the jacketed sections 20.

[0025] The present invention is applicable to fiber optic cables having either glass or plastic cores 10, and may be used with various types of cladding 12 and jackets 14. Further, the cables may operate in single mode or multimode transmission.

[0026] Prior art fiber optic sensors admit light through one end of the cable. As mentioned above, this limits both the amount of light that can be admitted and the angle-of-view. The use of a sensing section 16 along the length of the cable allows one to detect light continuously along the length of the cable or at any given point where the cladding 12 is exposed. A single fiber optic cable can then be routed around objects to detect light over broad areas with a 360-degree field-of-view. This eliminates the need for multiple expensive electronics packages to support multiple fiber optic detectors. The sensing sections 16 are also effective in tight quarters where the prior art sensors would not work. [0027] Following is a detailed description of the signal processing instrumentation 24 according to one embodiment of the present invention. Referring again to FIGS. 1 and 2, light 27 from a sensed event is transmitted down the core 10 of the receiving fiber optic cable to the photo detector 26. The photo detector 26 can be a phototransistor, a photo Darlington, a photodiode, or other type of light detector. Referring now to the schematic circuit diagram of FIG. 3, one side of the detector 26 is connected to a positive DC voltage and the other side to ground through a resistor 28. The resistor 28 is set to bias detector 26 in its operating range. The value of resistor 28 can vary depending upon the type of detector 26 used and the magnitude of the DC voltage. The DC voltage is typically 5 NDC, but could be any value. The change in light on the surface of the detector 26 causes the amount of current passing through the detector 26 to vary. This causes a varying voltage across resistor 28 that is fed into the input of amplifier chip 30. The ratio of resistor 32 and resistor 34 set the gain of amplifier chip 30 by the textbook op-amp equation that is well known in the art. The gain value is selected based upon the fiber optic cable type, cable length, and the amplitude of the light to be detected. The capacitor 36 sets the cutoff frequency of amplifier chip 30. This is useful to prevent high frequency oscillations of the output and to filter noise. While the amplifier chip 30 shown is a non-inverting amplifier, an inverting amplifier works just as well and adjustments to the circuit may be made accordingly.

[0028] A reference voltage for the comparator 38 is supplied by a voltage divider, made up of resistor 40, variable resistor 42 and resistor 44. The reference voltage sets the threshold level at which the comparator 38 output changes state. The trip point is easily varied by selecting different values for the resistors or by the adjustment of variable resistor 42. The combination of resistor 46 and resistor 48 add hysteresis to the comparator 38. The output of the comparator 38 is pulled up to the plus voltage by resistor 50. If light is present at the detector 26, the input to the + pin on the comparator 38 will be high. If the voltage input to the + pin is higher than that to the - pin then the output of the comparator 38 will be high. Since the output of the comparator 38 will toggle between on and off, based upon the presence or absence of light, it can be used to drive an alarm device 52, for triggering an appropriate response to a light signal, such as a visual or audible alarm or a relay that can control some external device such as a circuit breaker.

[0029] Referring still to FIG. 3, embodiments of the invention can also include a node 54 that furnishes an analog output that is proportional to the amount of light seen by the detector 26. This output can be fed to a meter or other analog devices. The analog output from amplifier chip 30 is connected to the input of an analog to digital (A/D) converter 56. The A/D converter 56 will convert the varying analog output of the node 54 into a varying digital output that can be connected to the computer 25 shown in FIG. 2. This varying output can be used to detect different types of sensed light events. For example, referring again to FIG. 2, different types of light events could be indicative of specific and important parameters of this system. Perhaps the inside of the electronic enclosure 18 is dark when the door is properly closed. If there is some light leakage into the enclosure 18 it can be biased or calibrated out of the readings and ignored. If the door of the enclosure 18 is opened there will be a change in background light that can be recognized. That information might be significant and can be indicated to an operator. If a fire occurs the light will be stronger than the background light and the fire can be recognized. If an electrical arc occurs in the enclosure 18 the light will be much brighter and the computer can perform an emergency shutdown of the power to the enclosure 18.

[0030] Other embodiments of the present invention include a Built-in-Test (BIT) feature that provides a simple, fast and holistic system test. Referring to FIG. 4, the BIT electronics 57 include a light source 58, such as but not limited to a laser diode or Light Emitting Diode (LED), for performing a BIT to determine if the system is functional. The light source 58 can be located at a distal end of the fiber optic cable. An AND gate 60 toggles the light source 58 output under manual or automatic control to send an on/off signal to the signal processing instrumentation 24 that provides a pass/fail test of the cable from end-to-end. [0031] More specifically, if the two inputs 64 and 66 of the AND gate 60 are caused to go high then the output will go high which will turn on the light source 58. The resistor 68 is used to set the current and govern the brightness of the light source 58. When either of the two inputs 64 and 66 go low the light source 58 will turn off. The signals to AND gate 60 can be furnished by a switch or by computer control.

[0032] Alternatively, a digital to analog (D/A) converter 62 with a variable input can be coupled to the system to help determine a level of contamination (e.g., dirt, dust or grease) on the sensing section 16 of the cable. When the light source 58 is turned on some light leaks from the sensing section 16. If the light source 58 output is varied in a known fashion then the analog or digital output of the signal processing instrumentation 24 will also vary in a known fashion. Any deviation from a predicted output can be used to calculate the contamination on the sensor section 16 and thus predict system degradation as well as perform a go/no go test. The system can also alert an operator as to when cleaning of the sensor section 16 is necessary.

[0033] FIG. 5 is a schematic of one embodiment of the present invention employing the BIT feature shown in FIG. 4 where the distal end of the fiber optic cable is routed back near the signal processing instrumentation 24. This enables the BIT electronics 57 to be packaged together with the photo detector 26 and the signal processing instrumentation 24. [0034] The above described BIT features of the present invention may also be used in conjunction with prior art fiber optic sensors. Referring to FIG. 6A, when used with prior art sensors that employ a lens 70 on the end of a sensing fiber optic cable 72, the BIT feature of the present invention may be used by adding a second BIT fiber optic cable 74. Light from the light source 58 enters the BIT fiber optic cable 74 and is reflected from the back surface of the lens 70. The reflected light then enters the sensing fiber optic cable 72 and is detected. Similar to the description above, a fixed input AND gate 60 or a variable input D/A converter 62 may be used to provide a pass/fail test of the prior art sensor system from end-to-end. Using the D/A converter 62 the amount of light reflected from the lens 70 will vary with the amount and type of contamination on the outside surface of the lens. It is presumed that the inside surface is in a sealed environment and will not change.

[0035] Alternatively, the BIT features of the present invention may be used with prior art sensors without the addition of the BIT cable 74. As shown in FIG. 6B, in this embodiment the light source 58 is coupled to the sensing cable 72 by adding a beam splitter 76 at the photo detector 26 end of the sensing cable 72. The beam splitter 76 allows the light source 58 to be coupled into the sensing cable 72 upon command. When the light is reflected back from the lens 70 it is detected by the photo detector 26 as described above.

[0036] Selective coatings can also be applied to the lens 70 that allow all wavelengths of light to pass through the lens 70 except the narrow wavelengths of the light source 58. This does not impede the ability of the photo detector 26 to detect light from the event of interest; rather it enhances the reflection of the BIT light source 58 into the sensing cable 72 and gives the BIT feature a much higher signal-to-noise ratio. [0037] The above-described techniques of the present invention can be used wherever changes in light need to be sensed. For example, the present invention can be used to sense excessive arcing on the brushes of motors that may be an indication of premature wear. It can also be used to sense the presence of arcing in electrical distribution system components such a transformers, power supplies, switchboards, substation panels, load centers, switch panels, and circuit breaker panels. The system can also be imbedded in any critical electrical enclosure such as a large computer. Further, the system is useful wherever ultra reliable electrical power is needed. This includes the main power feed switchboards for hospitals, semiconductor production lines, computer server farms, etc.

[0038] Furthermore the use of large multi-pole solid-state motor controllers is becoming wide spread in both military and civilian applications. High voltages and currents are switched by closely packed solid-state electronics. These are so closely packed as to prevent direct installation of photo detectors. The sensing sections 16 of the present invention can be routed to protect these electronics. Also, large sealed transformers can contain a bare sensing section 16 to detect winding fiashovers. The signal processing instrumentation 24 can be mounted accessibly outside of the transformer and only the fiber optic cable would be inside the transformer.

[0039] The BIT features of the present invention allow end-to-end testing of the system without need for access to each component. Security systems that use the presence of light to determine a breach in security can also benefit from the BIT features. Equipment where the opening of a cover would present a substantial hazard might benefit from the use of the present invention with the BIT features. For example, a single sensing section 16 could have five functions: 1) no light would mean the cover is closed; 2) a medium light would mean the cover is open; 3) a pulsed light would mean the BIT feature is active; 4) pulsing different amplitudes of BIT light source 58 would determine if the sensing section 16 is clean; and 5) a strong light would mean an arc fault is present.

[0040] Still another application of the present invention concerns use of the sensing section 16 to create a "light safety curtain." Prior art light safety curtains are presently used to shut off equipment such as drills, mills, lathes, etc., if an operator tries to reach toward the equipment while it is running. Such prior art light safety curtains require an array of individual light sources and individual detectors. If an object, such as a hand, is inserted between an individual light source and its corresponding detector, a safety switch is tripped. However such systems can be simplified using the techniques of the present invention. For example, referring to FIG. 7A, if the sensing section 16 (shown in cross-section) of the present invention is laid in the bottom of a reflecting channel 78, and covered with a matte finished plastic diffusion cover 84, it will amplify the light received from the direction the channel 78 is facing and prevent the reception of light from other directions. If a single longitudinal light source 80 is then positioned some distance in front of the channel 78, the sensing section 16 can detect objects located in a safety zone 82 between the light source 80 and the sensing section 16. FIG. 7B shows a top view of the same embodiment shown in FIG. 7 A. Thus, to create a continuous light safety curtain, such an embodiment of the present invention requires only one sensing section 16, whereas the systems of the prior art require numerous individual sensors. The sensing section 16 can also be used with existing light safety curtain light sources that employ an array of individual light sources. [0041] In summary, the present invention provides for a fiber optic light sensor that is durable, non-invasive, compact and inexpensive. Furthermore different embodiments of the present invention are adaptable to numerous and diverse applications. While the above description contains many specifics, the reader should not construe these as limitations on the scope of the invention, but merely as examples of specific embodiments thereof. Those skilled in the art will envision many other possible variations that are within its scope. Accordingly, the reader is requested to determine the scope of the invention by the appended claims and their legal equivalents, and not by the specific embodiments given above.

Claims

WHAT IS CLAIMED IS: 1. A fiber optic light sensor system comprising: a fiber optic cable having a proximal end, a distal end, a core and an annular jacket, said jacket extending over a section of said core forming a jacketed section of said cable, said cable further including a sensing section, between said proximal end and said distal end of said cable where said core is not covered by said jacket, for receiving a light signal; a photo detector, coupled to said proximal end of said cable, for detecting the light signal that enters said cable at said sensing section; and signal processing instrumentation, operatively connected to said photo detector, for processing the light signal.

2. A fiber optic light sensor system according to claim 1, wherein said fiber optic cable further comprises more than one sensing section and more than one jacketed section between said proximal end and said distal end.

3. A fiber optic light sensor system according to claim 1, further comprising alarm means, connected to said signal processing instrumentation, for triggering an appropriate response to the light signal.

4. A fiber optic light sensor system according to claim 1, further comprising a Built-In- Test light source, operatively connected to said distal end of said fiber optic cable, for outputting a signal.

5. A fiber optic light sensor system according to claim 4, wherein: the output signal from said Built-in-Test light source traveling through said fiber optic cable interacts with said sensing section of said cable to form an altered output signal, said altered output signal traveling to said photo detector and being processed by said signal processing instrumentation, said altered output signal being indicative of a level of contamination of said sensing section of said fiber optic cable.

6. A fiber optic light sensor system according to claim 1, further comprising a Built-In- Test light source, operatively connected to said proximal end of said fiber optic cable, for outputting a signal.

7. A fiber optic light sensor system according to claim 1, further comprising: a channel, positioned adjacent said sensing section, for reflecting light originating from one general direction into said sensing section and for shielding light originating from other general directions from said sensing section; and a light source, positioned near said sensing section, for creating a light safety curtain between said light source and said sensing section, that detects objects disposed there between, whereby light from said light source reflects off said channel and into said sensing section unless an object blocks a path of light between said light source and said sensing section.

8. A fiber optic light sensor system, comprising: a fiber optic cable having a cladding; sensing section means, operatively connected to said fiber optic cable, for receiving light signals through said cladding of said fiber optic cable; a photo detector, coupled to said sensing section means, for detecting said light signals from said sensing section means; and signal processing instrumentation means, operatively connected to said photo detector, for interpreting said light signals detected by said photo detector.

9. A fiber optic light sensor system according to claim 8, further comprising: Built-in-Test means, operatively connected to said fiber optic cable, for determining whether said system is functioning properly.

10. A fiber optic light sensor system comprising: light inputting means, for sending a test light signal; a first fiber optic cable having a proximal end and a distal end, said proximal end operatively connected to said light inputting means, for receiving the test light signal; a lens, having a front surface and a back surface, said back surface operatively connected to said distal end of said first fiber optic cable, for reflecting the test light signal, said front surface positioned so as to receive a light signal from a sensed light event; a second fiber optic cable having a proximal end and a distal end, said distal end of said second fiber optic cable positioned near both said distal end of said first fiber optic cable and said lens, for receiving both the reflected test light signal and the light signal from the sensed light event; a photodetector, operatively connected to said proximal end of said second fiber optic cable, for detecting both the reflected test light signal and the light signal from the sensed light event.