Abstract:
Disclosed is an optical sensor used for remote laser level monitoring in liquid storage vessels. The sensor is mounted directly to a standard tank nipple, located on top of the vessel, by a threaded connecting means and is linked to a laser measurement device via fiber optic cabling. The level in the vessel is measured as a function of the time required for a laser signal to be transmitted from the sensor, reflected off the liquid surface and returned to the receiver lens located in the sensor.

Description:
FIELD OF THE INVENTION AND RELATED APPLICATION 
     The present is a continuation of U.S. patent application Ser. No. 09/069,477 filed Apr. 29, 1998, now U.S. Pat. No. 6,040,897, which is assigned to the assignee hereof, and discloses an invention relates generally to instrumentation devices used to transmit level indication signals from a remote location to a local indicating station or microprocessor and, more particularly, to a remote optical sensor head connected to a laser measurement device via fiber optic cables that is used to monitor levels in liquid storage vessels. 
    
    
     BACKGROUND OF THE INVENTION 
     Description of the Related Art 
     In many industrial applications, it is common practice to monitor fluid levels in liquid storage vessels located in remote areas using instrumentation equipment that transmits a level indicating signal to a centralized location for monitoring and control purposes. In the past, several methods have been utilized in obtaining these remotely generated level indicating signals. However, most are flawed in the sense that they are susceptible to drawbacks and intrinsic characteristics that often result in inaccurate measurements. 
     Mechanical, float-type level indicating devices have been used to indicate fluid levels by placing a float in the vessel itself. As the float raises and lowers with the fluid level in the vessel, a variable resistance electronic device is manipulated, thus providing a means by which the level is measured in proportion to the variable resistance. While this method provides accurate readings, it requires frequent calibration and is limited in use to liquids that will not deteriorate the hardware. Furthermore, turbulent conditions within the vessel can also lead to inaccurate measurements. Similarly, mechanical float-type level limit switches suffer from the same limitations as the level indicators, although calibration is not as great a factor. 
     The drawbacks associated with the aforementioned devices gave rise to the development of other devices that do not rely on direct contact with the liquid. Ultrasonic level detectors utilize a transmitter/receiver to detect liquid levels by transmitting a signal from the top of the vessel and monitoring the time required for it to be reflected off the surface of the liquid and return to the receiver. While this method eliminates many of the problems associated with mechanical devices, it too suffers from inaccuracies caused by varying temperatures and densities in the area above the fluid level inside the vessel. 
     Using principles similar to those of ultrasonic indicators, microwave and radar level indicators use their respective signal types to indicate liquid levels without coming into direct contact with the liquid. However, these methods too suffer from limitations in the sense that liquids with poor dielectric constants cause inaccurate level indications. 
     Other limitations associated with the ultrasonic, radar and microwave devices results from the fact that these devices are electronic in nature and therefore suffer from intrinsic safety concerns. As with any electronic device, there are, necessarily, heat generating components that create, no matter how remote, the possibility of hazardous situations when used in areas where flammable materials are present. Furthermore, electronic devices also suffer from complications created by line noise, electromagnetic field inteference (EMF) and grounding problems. The aforementioned limitations associated with the use of these devices has lead to the development of alternative methods, including the use of laser and optical instrumentation devices. However, depending upon their configuration, these devices also suffer from inadequacies and drawbacks that identify the present invention as being a superior method of measurement. 
     In the ancillary art, several devices have been developed that utilize laser based devices to calculate and indicate fluid levels within liquid storage vessels. For example, U.S. Pat. No. 5,257,090, issued in the name of Meinzer et al, discloses a laser liquid level/distance measuring device wherein a laser beam is aimed vertically downward into the vessel and is reflected from a floating reflector back to an optical receiver. The level is then calculated as being proportional to the time required for a transmitted signal to be reflected back to the receiver. This method suffers from drawbacks because, as previously mentioned, the device requires submersion in the liquid itself which can lead to problems associated with corrosion and turbulence within the vessel. This method also suffers from the aforementioned intrinsic safety concerns related to electronic devices. 
     U.S. Pat. No. 4,938,590, issued in the name of Ishida, similar in nature to the Meinzer invention, includes a means to compensate for varying vessel pressure. However, this invention also suffers from the corrosion and turbulence problems because it also requires submersion and is susceptible to intrinsic safety concerns due to its electronic nature. 
     U.S. Pat. No. 5,020,901, issued in the name of de Groot, discloses a laser range detector used to measure the distance between objects. Although related to the present invention in the sense that it uses a similar method for measurement, this device is not intended for use in level measurement. Furthermore, this device relies on a reflector being located at the same location as the remote object. Therefore, any adaptation for liquid level measurement would require submersion and result in the same problems associated therewith. 
     U.S. Pat. No. 5,278,426, issued in the name of Barbier,. discloses an optical liquid level limit switch-type sensor wherein the sensor module is mounted on the sidewall of the vessel facing the interior thereof. The device consists of a light transmitter and a light detecting receiver. In the absence of liquid at the sensor level, the amount of light reflected and detected by the receiver is substantially greater than when the liquid is present. Thus the device indicates when the liquid in the storage vessel reaches a level equal to that of the sensor. While this invention overcomes the problems associated with submersion previously discussed, it does rely on electronics being located at the sensor and therefore suffers from the problems associated therewith, including noise, EMF, grounding and fire hazards. 
     U.S. Pat. No. 3,995,169, issued in the name of Oddon and U.S. Pat. No. 4,051,726, issued in the name of Hastbacka, disclose optical level indicating devices for liquid storage vessels wherein transparent, light conducting probe rods are submersed in the liquid. Knowing that the amount of light transmitted through the probe is a function of the liquid level within the vessel, the level is determined by calculating the extent to which light is refracted by the surrounding liquid. By definition, these devices require submersion and are therefore susceptible to the problems associated therewith. These devices also suffer from intrinsic safety concerns due to the fact that they are electronic in nature. 
     Examples of level indicating devices that utilize ultrasonic, microwave and radar principles, and that display the aforementioned problems associated with devices of this nature are displayed by the following: 
     U.S. Pat. No. 5,406,842, issued in the name of Locke; 
     U.S. Pat. No. 5,438,867, issued in the name of van der Pol; and 
     U.S. Pat. No. 5,319,973, issued in the name of Crayton et al. 
     Finally, other devices that demonstrate methods of level detection and indication, do not necessarily relate directly to the present invention, but warrant inclusion for reference purposes include: 
     U.S. Pat. No. 4,354,180, issued in the name of Harding; 
     U.S. Pat. No. 4,670,660, issued in the name of Kuhlen et al; and 
     U.S. Pat. No. 5,478,966, issued in the name of Sugi. 
     A search of the previous art did not disclose any patents that read directly on the claims of the instant invention. Consequently, a need has been felt for providing a level indicating device for use in liquid storage vessels that provides accurate indications while avoiding the previously stated problems associated with other conventional methods. 
     SUMMARY OF THE INVENTION 
     Briefly described according to a preferred embodiment, the present invention consists of an optical sensor head used for remote laser level monitoring in liquid storage vessels. The sensor head is contained within a housing that is designed to mount directly to a standard tank nipple or flange via a threaded connecting means and is connected to a laser measurement device via fiber optic cabling. By directing the sensor perpendicular to the surface of the liquid in the vessel, the level therein can be determined by monitoring the time required for a pulsed or continuous wave phase laser to be transmitted from the sensor head, reflected off the liquid surface and returned to a receiver lens located in the sensor head. A single laser measuring device can be used to provide redundant level measuring signals within the same vessel or single level indicating signals in multiple tanks by installing several sensor heads and multiplexing them using a standard optical switching system. 
     It is therefore an object of the present invention to provide an improved level indicating means for use with liquid storage vessels that provides an accurate indication of the fluid level within the vessel. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that utilizes an optical sensor head contained within a housing that is designed to mount directly to a standard tank nipple or flange via a threaded connecting means and is connected to a laser measurement device via fiber optic cabling. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels in which a single laser measuring device can be used to monitor redundant level measuring signals within the same vessel or single level indicating signals in multiple tanks by installing several sensor heads and multiplexing them using an optical switching system. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that does not require submersion in the liquid stored in the vessel in order to operate. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that isolates the optical measuring components from the harmful effects of substances within the vessel. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that does not require vessel draining or cause leakage during maintenance or replacement. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that is designed to withstand high pressure exerted on the sensor head by the liquid within the vessel and/or pressure within a sealed vessel. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that is easy to install and calibrate. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels whose optical transmitting and receiving lenses are easily focused. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that can be easily adjusted so as to aim perpendicular to the surface of the liquid within the vessel. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that does not require frequent re-calibration. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that incorporates the use of a reference signal by using prisms to reflect a portion of the transmitted laser to the receiver. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that incorporates the use of a reference signal by using fiber optic couplers to direct a portion of the transmitted optical pulse to the receiver. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that improves accuracy by the incorporation of a reference signal. 
     It is another object of the present invention to provide an improved level indicating means for use with liquid storage vessels that reduces or eliminates the effects of fiber length, and the linear expansion/shrinkage thereof, by the incorporation of a reference signal. 
     Finally, it is an object of the present invention to provide an improved level indicating means for use with liquid storage vessels that is sealed in a manner such that it is waterproof and can withstand installation in locations exposed to outdoor weather conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages and features of the present invention will become better understood with reference to the following more detailed description and claims taken in conjunction with the accompanying drawings, in which like elements are identified with like symbols, and in which: 
     FIG. 1 is a side cutaway view of a remote sensor head for laser level measurement in liquid storage vessels according to the preferred embodiment of the present invention; 
     FIG. 2 is a top view of a remote sensor head for laser level measurement in liquid storage vessels according to the preferred embodiment of the present invention; 
     FIG. 3 is a bottom view of a remote sensor head for laser level measurement in liquid storage vessels according to the preferred embodiment of the present invention; 
     FIG. 4 is a perspective view of a remote sensor head for laser level measurement in liquid storage vessels according to the preferred embodiment of the present invention; and 
     FIG. 5 is a profile diagram depicting an operational implementation of the remote sensor head for laser level measurement in liquid storage vessels according to the preferred embodiment of the present invention. 
    
    
     LIST OF REFERENCE NUMBERS 
       10  Sensor Head 
       11  Base 
       12  Sleeve 
       13  Leveling Screw 
       14  Rubber Gasket 
       15  Fastening Threads 
       20  Window 
       21  Window Securing Ridge 
       22  Window Retaining Ring 
       23  O-Ring 
       30  Transmitter Lens 
       31  Transmitter Lens Assembly 
       32  Transmitter Lens Retaining Ring 
       33  Transmitter Lens Locking Screw 
       35  Receiver Lens 
       36  Receiver Lens Assembly 
       37  Receiver Lens Retaining Ring 
       40  Receptacle Base Fastener 
       41  Transmitter Receptacle Base 
       42  Transmitter Receptacle 
       43  Receiver Receptacle Base 
       44  Receiver Receptacle 
       45  Circular Bubble Level 
       50  Liquid Storage Vessel 
       51  NPT Nipple-Type Connector 
       52  Liquid Surface 
       55  Transmitter Fibers 
       56  Receiver Fibers 
       57  Fiberoptic Terminating Connectors 
       58  Laser Measurement Device 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to describe the complete relationship of the invention, it is essential that some description be given to the manner and practice of functional utility and description thereof. Accordingly, the best mode for carrying out the invention is presented in terms of its preferred embodiment, herein depicted within the figures. 
     1. Detailed Description of the Figures 
     Referring now to FIG. 1, FIG.  2  and FIG. 3, illustrated is a remote sensor head for laser level measurement in liquid storage vessels  10 , hereinafter sensor head. The sensor head  10  houses equipment, described in detail hereinafter, used to obtain accurate level measurements when used in conjunction with a laser measurement device (not shown). The housing portion of the sensor head  10  consists of a base  11  secured to the end of a sleeve  12  by several leveling screws  13  with a waterproof seal created therebetween by a rubber gasket  14 . The sleeve is generally cylindrical in shape and hollow throughout with an interior cavity contoured so as to accept and support various components housed therein and described in further detail below. Fastening threads  15  are located on the outer surface of the sleeve  12  at the end opposite the base  11  and are designed to mate with a standard NPT nipple-type connector (not shown) located on the top of a liquid storage vessel, used to secure the sensor head  10  to the liquid storage vessel and providing fluid connectivity with the interior cavity thereof. In an alternate embodiment, the fastening threads  15  are replaced with a connecting means designed to mate with standard flange connector (not shown) rather than a NPT nipple-type connector. 
     A window  20  spans the interior cross-section of the sleeve  12  at approximately the midpoint of the length thereof, perpendicular to the longitudinal axis of the sleeve  12 , and is secured against an annular window securing ridge  21  contoured on the interior wall of the sleeve  12  by a window retaining ring  22 . A window sealing O-ring  23  creates a water tight seal that isolates the interior of the liquid storage vessel from the optical components housed in the base  11 . The window  20  is constructed of a high strength material such as borosilicate that, in conjunction with the seal created by the O-ring  23  and the window retaining ring  22 , allows the sensor head  10  to withstand high pressure forces created by the liquid within the storage vessel and/or pressure within a sealed vessel. 
     The base  11  houses the optical components that facilitate the transmission and reception of optical signals from a laser measurement device (not shown). A transmitter lens  30  is secured in a transmitter lens assembly  31  by a transmitter lens retaining ring  32 . The transmitter lens  30  is focused by rotating the transmitter lens assembly  31 , which is threaded so that the transmitter lens  30  travels linearly in a direction parallel to the longitudinal axis of the sensor head  10 . The transmitter lens assembly  31  can be locked in place by a transmitter lens locking screw  33 . A receiver lens  35  is secured in a receiver lens assembly  36  by a receiver lens retaining ring  37 . The receiver lens  35  is focused it by rotating the receiver lens assembly  36 , which is threaded so that the receiver lens  35  travels linearly in a direction parallel to the longitudinal axis of the sensor head  10 . The receiver lens assembly  36  can be locked in place by a receiver lens locking screw (not shown). The transmitter lens  30  and the receiver lens  35  are oriented in a manner such that, when the sensor head  10  is assembled, they are directed approximately parallel to the longitudinal axis of the sleeve  12  and aim through the window  20 . 
     Receptacle base fasteners  40  are used to secure a transmitter receptacle base  41 , housing a transmitter receptacle  42 , to the base  11 . Receptacle base fasteners  40  are used to secure a receiver receptacle base  43 , housing a receiver receptacle  44 , to the base  11 . The transmitter receptacle  42  and the receiver receptacle  44  are designed to accommodate standard fiber optic cable connectors (not shown), thus allowing a standard laser measurement device to transmit and receive optical signals through the transmitter lens  30  and the receiver lens  35 , respectively. 
     In an alternative embodiment (not shown), a coaxial arrangement requiring the use of a single transmitting/receiving lens and a single fiber optic cable is used to reduce the number and length of cabling and the number of lenses required to provide the measurements, reducing material and installation costs, without sacrificing accuracy, and allowing for a more compact configuration. 
     2. Operation of the Preferred Embodiment 
     In accordance with a preferred embodiment of the present invention, as shown in FIG. 1, FIG. 2, FIG. 3, FIG.  4  and FIG. 5, the sensor head  10  is used in the following manner: 
     The optics of the sensor head  10  are factory calibrated in the following steps: The transmitter lens  30  is focused to infinity by adjusting the threaded transmitter lens assembly  34  and locking it into place with the transmitter lens locking screw  33 . The receiver lens  35  is then focused to infinity by adjusting the threaded receiver lens assembly  38  and locking it into place with the receiver lens locking screw (not shown). The base  11  is leveled by monitoring the circular bubble level  45 . Light exiting the transmitter is aimed vertically down by adjusting the transmitter receptacle base  41  and is locked into place using the receptacle base fasteners  40 . Light is sent through the receiver lens  35  is aimed vertically down by adjusting the receiver receptacle base  43  and is locked into place using the receptacle base fasteners  40 . 
     The sensor head  10  is attached to a liquid storage vessel  50  using the fastening threads  15  on the sleeve  12  to connect it to a standard NPT nipple-type connector  51  located on the top of the liquid storage vessel  50  and oriented in a manner such that, when connected, the sensor head  10  is aimed in a direction perpendicular to the liquid surface  52 . The angle of the sensor head  10  can be fine tuned by adjusting the leveling screws  13  and monitoring a circular bubble level  45  located on the base  11 . The transmitter fibers  55  and the receiver fibers  56  are connected to the transmitter receptacle  42  and the receiver receptacle  44 , respectively, using special fiber optic terminating connectors  57  that provide a weather-proof seal. The transmitter fibers  55  and receiver fibers  56  are thereby used to connect the sensor head  10  to a laser measurement device  58 . 
     The laser measurement device  58  is used to monitor and report the level of the liquid contained in the liquid storage vessel  50  by generating pulsed or continuous wave phase lasers and recording the time needed for laser signal to travel from the laser measurement device  58 , through the transmitter fibers  55 , through the transmitter lens  30  to the liquid surface  52  where it is reflected back to the receiver lens  35 , through the receiver fibers  56  and back to the laser measurement device  58 . Knowing that the speed that the laser travels is essentially constant and measuring the travel time accurately, the level of the liquid is calculated, automatically, by the laser measuring device  58 . This routine is performed in a highly repetitive manner so as to provide a more accurate reading. Of course, calibration is required to compensate for the distance that the laser signal travels through the transmitter fibers  55  and the receiver fibers  56 . Furthermore, periodic re-calibration is required in order to account for linear expansion and/or shrinkage of the transmitter fibers  55  and the receiver fibers  56 . 
     In an alternate embodiment, the calibration procedures used to compensate for fiber lengths and linear expansion/shrinkage is avoided by the incorporation of two prisms (not shown) in the sensor head  10 . Mounted in the base  11 , the prisms are used to reflect a portion of the laser signal from the transmitter lens  30  to the receiver lens  35 . The reflected portion of the pulse is used to initiate the timing cycle within the laser measurement device  58 , rather than starting the cycle upon generating the laser signal, thus removing the fiber length, and any linear expansion/shrinkage thereof, from the timing cycle. 
     In an alternate embodiment, several sensor heads  10  can be multiplexed in order to provide redundant level measuring signals within the same vessel or single level indicating signals in multiple tanks to a single laser measurement device  58  by using a standard optical switching device (not shown). 
     In an alternate embodiment, the calibration procedure used to compensate for fiber lengths and linear expansion/shrinkage is avoided by the incorporation of two 1×2 fiber optic couplers/splitters (not shown) attached to the sensor head  10 . One fiber optic coupler/splitter is connected to the transmitter receptacle  42 , transmitter fiber  55 , and to the second fiber optic coupler/splitter. The second fiber optic coupler/splitter is connected to the receiver receptacle  44 , and the receiver fiber  56 . This configuration completes a closed loop which allows a portion of the transmitted pulse to be directed back through the receiver fiber  56  to act as a reference signal. The reference signal is used to initiate the timing cycle within the laser measurement device  58 , rather than starting the cycle upon generating the pulse, thus removing the fiber length, and any linear expansion/shrinkage thereof, from the timing cycle. 
     While the preferred embodiments of the invention have been shown, illustrated, and described, it will be apparent to those skilled in this field that various modifications may be made in these embodiments without departing from the spirit of the present invention. It is for this reason that the scope of the invention is set forth in and is to be limited only by the following claims.