Abstract:
A sensor monitoring system comprises a passive sensor, a monitored transmitter, a repeater, and a central receiver. The system provides easy installation, convenient calibration, and very low maintenance. The sensor and the monitored transmitter are battery powered and, in order the conserve battery power, the sensor and the monitored transmitter receive power only on command from their respective microprocessor.

Description:
[0001]    This application is a Continuation In Part of U.S. patent application Ser. No. 09/541,353; filed Mar. 31, 2000, which will issue on May 8, 2001 as U.S. Pat. No. 6,229,448; which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/128,868; filed Apr. 12, 1999. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to the field of remote sensor systems and, more particularly, to an intrinsically safe, wireless, battery-operated sensor system that conserves battery power.  
         BACKGROUND OF THE INVENTION  
         [0003]    Industry and safety standards require that liquid storage tanks of the type located on petroleum tank farms be given periodic checks to determine the liquid level of each tank. Level indicators for such tanks typically comprise a tape and pulley apparatus with a float at one end of the tape within the tank and a mechanical level indicator at the other end.  
           [0004]    This typical arrangement was improved on by Clarkson, U.S. Pat. No. 4,459,584, to include a transmitter for remote monitoring of liquid level, but the Clarkson system still required the use of the old tape and pulley system to monitor liquid level. This system suffered all of the common problems with the tape and pulley system in that such a system may mechanically jam, thereby disabling the movement of the float. Further, a large volume of volatile gases fills the conduit enclosing the tape and pulley, since this system is open to the tank. This feature alone presents a significant safety hazard. Further, the Clarkson system provides no means for calibration of the level detector, since it relies upon the old tape and pulley system.  
           [0005]    We began the development of the present invention with a level monitoring and alarm system sold under the trademark Fillcheck®. This system included a transmitter that was mounted in an off-the-shelf NEMA-7 explosion-proof enclosure so as to enable it to be used in the electrically hazardous environments associated with petroleum storage tanks, pipelines, oil refineries, petrochemical plants, and fuel terminals. This type of mounting solved the problems described with regard to Clarkson, but it brought about certain shortcomings, such as the attenuation of the level signal. Mounting the transmitter and its antenna inside the explosion-proof enclosure significantly reduced the system&#39;s effective range. In this system, the explosion-proof transmitter had to be aimed toward its receiver or repeater for optimum performance. Further, that system was expensive, and the transmitter had to be equipped with a safety barrier so as to permit its connection to an external level switch, which added to system cost. That system was also heavy, in that explosion-proof enclosures are quite massive and add significantly to shipping and handling costs. Finally, explosion-proof enclosures are typically constructed of aluminum which is prone to pitting and corrosion, known as exfoliation, particularly when used in marine or coastal environments in which many refining and petrochemical facilities are located.  
           [0006]    The recognition of these drawbacks of the Fillcheck® system led to the development of the system which is described and claimed in our U.S. patent application Ser. No. 09/541,353; filed Mar. 31, 2000; which will issue on May 8, 2001 as U.S. Pat. No. 6,229,448; the parent application of the present application. In that system, we disclosed an intrinsically safe transmitter which maybe used generally for sending digital signals derived from any analog sensor, including a sensor for temperature, pressure, flow rate, etc., as well as the tank level sensor for which the original system was specifically designed. The original impetus for the that system was over-fill protection for storage tanks in hazardous environments, and has proven to be successful.  
           [0007]    In systems in which that system finds applications, sensors typically require some small amount of power to develop a sensed signal, which may then be manipulated into a signal for transmission. Although the amount of power drawn by such sensors is small, it is significant when one is dealing with remote, battery powered sensors which must operate without operator attention over a long period.  
           [0008]    Thus, there remains a need for a sensor monitoring system which is intrinsically safe and operates only intermittently in order to conserve the precious resource of battery power. The present invention addresses this need in the art.  
         SUMMARY OF THE INVENTION  
         [0009]    The system of this invention comprises four components: (1) a passive sensor; (2) a transmitter module which includes a microprocessor controller, or microcontroller; (3) a repeater (if required); and (4) a receiver. It provides easy installation, convenient calibration, and very low maintenance. It is specifically designed to provide safe and power-efficient sensor operation where external electrical power and/or alarm signal wiring to a supervised point are not available. Further, it is easily adapted to other systems which include a passive sensor providing an analog signal which is then transmitted by the system of this invention to a central monitor.  
           [0010]    In a preferred embodiment of the invention, a passive sensor for a specific application is installed. Such a sensor may sense temperature, pressure, flow rate, vibration, torsion or other mechanical stress, level, or other parameters of interest which must be monitored periodically or otherwise on command. The sensor typically develops an analog or digital signal, which is coupled through an electrical conductor to the intrinsically safe transmitter. The sensor remains unpowered (dormant) until commanded by the microcontroller in the transmitter module to take a reading. The command from the microprocessor may be activated by a programmed clock signal within the microprocessor. When the transmitter module receives a sensed signal from the sensor, it transmits the sensed signal in digital form to a repeater (if necessary) and ultimately to the receiver at the central monitor.  
           [0011]    The sensed signal from the sensor may also include an alarm function by including an alarm switch associated with the sensor. When the switch is activated, the transmitter broadcasts an alarm signal to the receiver. In the alarm mode of operation, a relay output at the receiver is either opened or closed thus activating the alarm or other device attached to it. The relay output may also connect to a control apparatus, such as a signal transmitter to provide for remote operation of such devices as valves.  
           [0012]    The sensor and transmitter package are powered by the same or a secondary battery, such as for example a D cell size 3.6V lithium thionyl chloride battery. During non-alarm conditions, a supervisory signal may be transmitted by the transmitter module under the control of the microcontroller every 30 seconds or at a selected interval. The supervisory signal contains the transmitter identification and battery condition. If the receiving system fails to hear from any of its transmitters, an inactive alarm is issued. Also, low battery alarms are issued before a transmitter&#39;s battery dies.  
           [0013]    Any type of passive sensing device that provides an analog or digital electrical output signal can be used with this embodiment of the present invention. For example, storage vessels may include a magnetostrictive level sensor, such as a liquid level sensing system from Ametek Patriot Sensors in Clawson, Mich. These devices can be constructed of stainless steel, brass, polypropylene, or other appropriate materials, and are available in a vertical configuration. Other passive sensing devices for other parameters to be sensed can also be used in accordance with this invention.  
           [0014]    The system of the present invention provides reliable, low cost, wireless sensor monitoring capability which conserves battery power by activating either the transmitter module or the sensor only when needed. Many applications now require such remote monitoring. Thus, this system provides continuous, fully supervised protection against conditions which routinely require monitoring. It includes a battery powered transmitter, so it needs no external electrical power at the storage tank. The transmitter is intrinsically safe (Class I Division 1 Groups C&amp;D), and requires no FCC licensing. It has low installation and maintenance costs, and the alarms are provided with fail-safe aspects for increased reliability.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is an overall schematic diagram of the level monitoring system of this invention.  
         [0016]    [0016]FIG. 2 is a detail cross-section view of the transmitter module of this invention.  
         [0017]    [0017]FIG. 3 is a schematic diagram of the invention adapted for use with a passive sensor. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    [0018]FIG. 1 shows an exemplary level monitoring system  10  of this invention. It should be understood that the system  10  is equally applicable to any remote parameter which may require monitoring functions, so long as the parameter is sensed by a sensor, and the sensor develops an analog or digital signal for further manipulation by the system  10 . The system may also include the alarm features of our parent application, U.S. application Ser. No. 09/541,353.  
         [0019]    The system shown in FIG. 1 monitors the fluid level  12  in a storage tank  14 . The fluid level is detected by a passive level sensor  16  which moves vertically with fluid level  12 . The level sensor  16  is coupled by a wire  20  to the interior of a mast  22 , which supports a transmitter module  24 .  
         [0020]    The passive level sensor requires electrical power in order to be activated to give a level indication. The provision of electrical power is microprocessor activated, as described below with regard to FIGS. 2 and 3. The transmitter module  24  also remains dormant until activated by microcontroller command.  
         [0021]    Upon activation, the transmitter module  24  transmits a signal, either to an intermediate repeater  26 , if required, or directly to a receiver  28 . The repeater  26  is required if the distance from the transmitter module  24  to the receiver  28  is too great to transmit directly to the receiver  28 , which may be located in a distant control room. In a preferred embodiment, the receiver  28  may be dedicated to a specific transmitter module  24 , or the system can be configured so that the repeater  26  boosts signals from a number of transmitters and other repeaters. The repeater  26  is preferably mounted within a weatherproof nonmetallic (NEMA 4X) enclosure that can be purged for use in hazardous areas. It uses AC power, with an internal backup battery included to continue operation through power outages. The repeater  26  may have a specified range, for example of up to  3  km (line of sight). Additional repeaters can be used for greater distances or to clear obstructions.  
         [0022]    The receiver  28  may operate as a stand-alone receiver (discrete) or with a computer interface (serial). Discrete receivers may have an LED display indicating exact tank location, fluid level, or the value of any other sensed parameter, such as pressure in psig, temperature in degrees Fahrenheit, or variation from a specified norm or limit, etc. If the system operates with a computer interface, the receiver  28  can monitor hundreds of similar transmitter modules  24 . Receivers  28  may only “listen” for transmitters and repeaters programmed to them.  
         [0023]    Once the receiver  28  senses a received signal from the transmitter module  24 , a central station  30 , labeled in FIG. 1 with the functions of control, display, and alarm, provides means for displaying the value of the signal from the sensor. Such a display may comprise a separate gauge, but preferably comprises a selectable readout display on a computer monitor. The receiver  28  may also provide relay outputs which can be used to sound an alarm, shut off a pump, close a valve, activate a telephone auto-dialer, or operate almost any other device.  
         [0024]    It should be noted that the system of this invention is also adaptable to other applications where a passive switch can sense an out of specification condition. For example, certain systems require proper operation within a band of pressures, and such a system may include a passive switch for each of the upper and lower pressure settings. Such an application and others are within the scope and spirit of the present invention.  
         [0025]    It is particularly noteworthy in the system shown in FIG. 1 that the transmitter module and the sensing device are intrinsically safe. No spark producing components are exposed to volatile gases, and such gases are retained entirely within the tank  14 , unlike known systems. It should also be noted that the receiver  28  may be dedicated to many such sensor monitoring systems, and may passively monitor all of the sensor monitoring systems assigned to it. Further, as shown in FIG. 1, the system is independent of external power requirements, and it is independent of communication wiring from the transmitter module  24  to the receiver  28 .  
         [0026]    Referring to FIG. 2, the transmitter module  24  includes a sealed outer shell  32 , such as fiberglass or other durable, weather resistant material. The transmitter module  24  mounts onto the mast  22 , such as for example with a cavity  34  to receive the top of the mast, and may be secured with a set screw  36 . Also through the mast is a multi-strand electrical conductor  38 , which is coupled at the tank  14  to the level sensor  16 .  
         [0027]    The conductor  38  is directed into a sealed chamber  40 , which is sealed at the bottom with a bottom wall  42  and at the top with a top cap  44 . The top of the chamber  40  is further sealed with an O-ring seal  46 . The conductor  38  penetrates the bottom wall  42  through a stuffing tube, or is otherwise potted to seal the penetration with the wall  42 .  
         [0028]    The chamber  40  includes a battery  48 , a transmitter  50 , and a microcontroller  68 ′. The battery is preferably a lithium thionyl chloride battery, available from Tadiran in Port Washington, N.Y., selected because of its high power density and long life. The system may also include a solar charging system (not shown) in order to extend the useful life of the battery even further between replacement cycles. The battery is mounted into the transmitter module  24  with battery clips, and is electrically coupled to the transmitter  50  with wires  52  and a battery connector  54 . The battery  48 , transmitter  50 , and microcontroller  68 ′ are preferably assembled as a unit onto a backbone  56 , for ease of installation and maintenance. The battery  48  may also provide power to the sensor by way of the electrical conductor  38  through the wall  42 , if desired.  
         [0029]    The transmitter  50  is self-contained, intrinsically safe, and very reliable. Once the level sensor  16  is activated, the transmitter module  24  sends a radio frequency signal, which includes a unique identifying code to distinguish the transmitter from any other transmitter included in the system  10 . It should be noted that the microcontroller  68 ′ may include buffer memory so that the sensed signal need not be transmitted immediately. The onboard battery  48  (which also powers the sensors) has a 2-3 year life, and provides fully supervised operation. A supervisory signal is transmitted by the transmitter  50  periodically, for example every 30 seconds, which contains the ID Code of the specific transmitter module  24  and battery condition. If the receiver  28  or the central station  30  the fails to hear from any of its associated transmitters, such as transmitter module  24 , an inactive alarm is issued. Also, low battery alarms may be issued about 30 days before a transmitter&#39;s battery dies.  
         [0030]    The transmitter  50  may comprise a 900 MHz spread spectrum device that contains an onboard processor and non-volatile ROM. The transmitter  50  is preferably awakened from its dormant state by its own onboard processor, but it may alternatively be controlled by the microcontroller  68 ′. The transmitter  50  is enclosed in a fiberglass housing, and contains the components shown in FIG. 3.  
         [0031]    Referring now to FIG. 3, an embodiment including the features of the invention is disclosed. The transmitter module  24  includes the sealed outer shell  32  as previously described. In this case, a passive sensor  60 , which may detect temperature, pressure, flow rate, level, or any other appropriate parameter which may vary over time, is placed where it is exposed to the parameter. The transmitter module  24  may also be dedicated to more than one such sensor, such as for example a sensor  60 ′. The sensor  60  or  60 ′ provides an analog signal over a signal line  62  to an analog to digital (A/D) converter  64 . The A/D converter digitizes the analog signal, and conducts that digital signal over a communication line  66  to a microprocessor  68 . If the sensor  60  or  60 ′ provides a digital form of the sensed signal, the A/D converter is bypassed. The microprocessor is programmed to monitor the incoming digital signal for a limit condition or a band of specific values. The digitized signal is processed by the microprocessor  68  into a transmitter signal over a communication line  70 , and the transmitter signal may be periodically transmitted by the transmitter module  24  to periodically monitor the predetermined parameter, or the transmitter may only be activated if an out of specification condition develops.  
         [0032]    The sensors  60  and  60 ′ may also be provided with command lines  72  and  72 ′, respectively. The microprocessor  68  issues a command to turn on the sensors; otherwise the sensors remain de-energized, in order to conserve energy. The command signal to energize the sensors originates from a timed periodic programmed command from the microprocessor  68 .  
         [0033]    The principles, preferred embodiment, and mode of operation of the present invention have been described in the foregoing specification. This invention is not to be construed as limited to the particular forms disclosed, since these are regarded as illustrative rather than restrictive. Moreover, variations and changes maybe made by those skilled in the art without departing from the spirit of the invention.