Abstract:
A battery powered wireless fluid pressure sensor has a sealed chamber which can be vented to the outside atmosphere through a re-sealable reference port to allow a user to set the reference atmosphere inside the pressure sensor enabling the pressure sensor to provide absolute, gauge and true gauge pressure readings. The sensor calculates and transmits the fluid pressure taking into account the temperature of the pressure transducer, the temperature of the electronic devices and the barometric pressure inside the sealed chamber to provide accurate pressure measurements over a wide range of operating conditions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/381,420, filed on May 3, 2006 now U.S. Pat. No. 7,389,695, which is a continuation-in-part of International Application PCT/US2006/00570, with an international filing date of Jan. 9, 2006. 
     This application claims the benefit of U.S. Provisional Application No. 60/642,365 filed on Jan. 7, 2005, which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to fluid pressure sensors (for both liquids and gases), and, more particularly, to wireless fluid pressure sensors. 
     BACKGROUND OF THE INVENTION 
     Accuracy, versatility, ease of use, durability, and cost of manufacturing are important parameters for fluid pressure sensors. In the past hermetically sealed sensors have been used to provide a reference atmosphere for the pressure transducer (the pressure transducer providing an output indicative of a pressure differential on two surfaces of the transducer). A hermetic seal requires a container that is rigid and sealed well enough to withstand the normal wear and tear of a component which may be used in relatively instrument unfriendly industrial environments such as in chemical refineries and oil wells. 
     Such hermetically sealed pressure sensors provide a pressure measurement that is with respect to the environment inside the sensor package when the sensor was sealed. Sealing the sensor package in a vacuum increases the cost of manufacturing the sensor, while sealing the package at the factory ambient pressure prevents the accuracy of any direct absolute pressure measurement since moving the sensor to a different altitude will cause a pressure differential between the reference pressure of the sensor and the ambient air pressure. Either reference environment does not allow simple, direct measurement of both absolute and gauge pressure. 
     The use of a wireless pressure sensor allows easy relocation of the sensors and the easy addition of additional sensors as compared to more conventional wired pressure sensors. 
     What is needed is a fluid pressure sensor that is of high accuracy in an industrial operation while also being versatile, easy to set up and use, durable, and cost effective to manufacture. 
     It is a principal object of the present invention to a fluid pressure sensor that provides these needed parameters. 
     SUMMARY OF THE INVENTION 
     Briefly described, a fluid pressure sensor has a pressure transducer and a closable passage between the air outside of said pressure sensor for the pressure transducer and the reference atmosphere inside said pressure sensor. 
     Also described is a method of improving the performance of a pressure sensor by opening a fluid passageway between the interior of a housing of the pressure sensor and the outside of said housing and closing the passageway prior to measuring a fluid pressure. 
     In a further aspect of the invention the pressure is transmitted using IEEE standard 802.15.4 with a ZigBee type of data structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings: 
         FIG. 1  is a perspective view of a fluid pressure sensor in accordance with the present invention; 
         FIG. 2  is an exploded view of the pressure sensor shown in  FIG. 1 ; 
         FIG. 3  is a perspective view of the wired pressure transducer with the temperature detection device in the pressure sensor shown in  FIG. 1 ; 
         FIG. 4  is a sectional view of a portion of the fluid pressure sensor shown in  FIG. 1 ; 
         FIG. 5  is a flow diagram for the calibration procedure of the fluid pressure sensor shown in  FIG. 1 ; and 
         FIG. 6  is a flow diagram of the operation of the fluid pressure sensor shown in  FIG. 1  in a customer application. 
     
    
    
     It will be appreciated that for purposes of clarity and where deemed appropriate, reference numerals have been repeated in the figures to indicate corresponding features, and that the various elements in the drawings have not necessarily been drawn to scale in order to better show the features of the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings  FIG. 1  shows a perspective view of a wireless pressure sensor  10  in accordance with one embodiment of the present invention. The sensor  10  has a pressure cap  12  with a pressure port  14  for receiving a fluid, a pressure equalizing or reference port  16  in the pressure cap  12 , a sleeve-like enclosure or body  18 , and an antenna  20 . 
       FIG. 2  is an exploded view of the sensor  10  showing various components of the sensor  10 . The body  18  is manufactured from two parts, a case  30  and an end cap  32  which is press fit into the case  30  to provide a flat surface for an O-ring  34  located between the body  18  and the antenna  20 . Female threads in the antenna  20 , manufactured by Antennex of Glaendale Heights, Ill., mate with male threads formed on a high tension aluminum frame  36 . The high tension aluminum frame  36  provides a rigid structure for the pressure gauge  10 . The frame  36  is fastened to the pressure cap  12  by screws  38 . A second O-ring  40  fits into a groove  42  in the pressure cap  12 , and the body  18  fits over the frame  36  and onto a lip  44  in the pressure cap  12 . The O-ring  40  forms an airtight and moisture tight seal between the body  18  and the pressure cap  12 . When the antenna is screwed onto the frame  36 , the O-ring  34  also forms an airtight and moisture tight seal between the antenna  20  and the body  18  such that the interior of the body  18  is sealed from the outside atmosphere when a reference port screw  46  is screwed into the reference port  16 . 
     The pressure port  14  connects to the other end of the pressure cap  12  at an opening  48 . The pressure inlet side of a pressure sensing element  50 , a model number P571 manufactured by Strain Measurement Devices of Meriden, Conn., is electron-beam welded to the opening  48 . The opposite side of the pressure sensing element  50  has a sputtered metal strain gauge formed on the pressure sensing element  50  in the form of a Wheatstone bridge thereby providing four electrical contacts to the strain gauge. 
     As shown in  FIG. 3  a flexible wire harness  60  is attached to these four electrical contacts. A temperature measuring device  62  is mounted on the wire harness  60  in close proximity to the pressure sensing element  50  and connections to the temperature measuring device  62  are included in the wire harness  60 . In the preferred embodiment the temperature measuring device  62  is a model PCS 1.1302.1 platinum RTD temperature sensor manufactured by Jumo Process Control, Inc. of Canastota, N.Y. 
     Returning to  FIG. 2 , an electronics board  70  is attached to the frame  36  by four bolts  72 , and a battery holder  74  is attached to the back of the electronics board  70  so that it projects through an opening  76  in the frame  36 . A battery  78 , which in the preferred embodiment is a lithium thionyl chloride battery, is mounted in the battery holder  74 . Other battery chemistries, such as lithium manganese, can also be used. The electronics board has four major components, a Zero Insertion Force (ZIF) connector  80  which receives one end of the flexible wire harness  60 , a barometric pressure sensor  82  for measuring the absolute pressure inside the pressure sensor  10 , a microcontroller  84  for controlling the operation of the pressure sensor  10 , and a ZigBee/IEEE 802.15.4 RF data modem  86 . The microcontroller  84  has an internal temperature sensor  87 . The RF data modem  86  is mounted onto sockets  88 , and the microcontroller  84  is located under the RF data modem  86 . The RF data modem  86 , which in the preferred embodiment is either a XBee or a XBee-Pro RF Module manufactured by MaxStream of Lindon, Utah, has an RF connector  89  attached to a coaxial cable  90  to connect the RF data modem  86  to a connecting conductor  92  held in a connecting insulator  96  of an RF feedthru system  94  which provides consistent characteristic impedance required for effective coupling of the RF data modem  86  to the antenna  20 . 
       FIG. 4  is a sectional view of the feedthru system  94 . The coaxial cable  90  has an outer insulator  110 , a braided shield  112 , an inner conductor  114 , and an inner insulator  116  between the shield  112  and the inner conductor  114 . The coaxial cable  90  passes through the frame  36  into a cavity  108  formed in the top of the frame  36 . The outer insulator  110  extends to the bottom of the cavity  108 , and the braided shield  112  is flattened onto the bottom of the cavity  108 . The inner insulator  116  is trimmed back a short distance from the end of the inner conductor  114 . The connecting insulator  96  of the feedthru system  94  is placed in the cavity  108 . The inner conductor insulator  116  and the inner conductor  114  pass through an opening  118  in the bottom of the connecting insulator  96 , and the bottom of the connecting insulator  96  presses the braided shield  112  against the bottom of the cavity  108 . A pipe structure  120  formed at the bottom of the connecting conductor  92  fits into the opening  118  and the inner conductor  114  is pressed into an opening  122  of the pipe structure  120 . The rest of the connecting conductor  92  sits in an opening  126  of the connecting insulator  96  and projects beyond the top of the connecting insulator  96  to make contact with the inner connector on the antenna  20 . 
     As shown in  FIG. 2 , a 20 micron filter  100  is inserted in the passageway between the reference port  16  and the interior of the pressure sensor  10  to prevent dirt and other debris from entering the pressure sensor  10 . 
       FIG. 5  is a flow diagram  128  for the calibration procedure of the fluid pressure sensor  10 . The microcontroller  84  is first recalibrated by issuing a recalibration command to the microcontroller  84  as indicated by box  130 . Then the fluid pressure gauge  10  has zero gauge pressure applied to it at 21° C. (ambient temperature) as indicated by box  132 . The resistance of the temperature sensing device  62  is stored as indicated by box  134 . The pressure data from the pressure sensing element  50  is then read and stored as indicated by box  135 . The pressure applied to the pressure port  14  is then incremented by steps of 20% of the maximum pressure of the fluid pressure sensor  10 , and the data from the pressure sensing element  50  is read and stored until 100% of the maximum pressure applied to the pressure port  14  has been reached as indicated by boxes  136  and  138 . The temperature is changed to −40° C. as indicated by boxes  140  and  142 , and the above operations indicated by boxes  134  and  135  are repeated. The temperature is then changed to 85° C. as indicated by boxes  140 ,  144 , and  146 , and the above operations indicated by boxes  134  and  135  are repeated. After the high temperature calibration process is completed, the calibration process ends as indicated by box  148 . 
       FIG. 6  is a flow diagram  150  of the operation of the fluid pressure sensor  10  in a customer application. The fluid pressure sensor  10  waits for a command for a pressure reading as indicated by box  154 . When a command for a pressure sensor reading is received, the microcontroller  84  then reads the temperature of the temperature sensor  87  to determine if the temperature of the microcontroller  84  has changed more than 20° C. since the microcontroller  84  was last calibrated as indicated by box  156 . If the temperature has changed more than 20° C., the microcontroller  84  is recalibrated by issuing a recalibration command to the microcontroller  84  as indicated by box  158 . Then the voltage of the battery  78  is measured by the microcontroller  84  as indicated by box  160 . The temperature of the pressure sensing element  50  is determined using electrical measurements from the temperature sensing device  62  which are then normalized using the measured battery voltage to compensate for decaying battery voltage and converted to a corresponding temperature using the ITS-90 table as indicated by box  162 . The pressure at the pressure port  14  is calculated as indicated by box  164  by interpolating the reading from the pressure sensing element  50  and the resistance of the temperature sensing device  62  based on the stored pressure and the resistance of the temperature sensing device  62  generated in the calibration process described above with respect to  FIG. 5 . The internal barometric pressure of the pressure sensor  10  is calculated as indicated in box  166  from the output of the barometric pressure sensor  82  using data from the sensor manufacturer. The temperature of the microcontroller  84  is read using the microcontroller  84  as indicated by box  168 . Finally, the pressure at the pressure port  14 , the internal barometric pressure of the pressure sensor  10 , the temperature of the pressure sensing element  50 , the battery voltage, and the temperature of the microcontroller  84  are transmitted to a base station as indicated by box  170 . The fluid pressure sensor  10  returns to the state of waiting for another pressure reading command as indicated in box  154 . 
     The sending of the internal barometric pressure of the pressure sensor  10  along with the pressure at the pressure port  14  allows the user to determine the gauge pressure, the absolute pressure, and the true gauge pressure. The temperature of the temperature sensing device  62  provides an indication of the temperature of the fluid at the pressure port  14 , while the temperature of the microcontroller  84  provides the temperature of the interior of the pressure sensor  10 . The battery voltage provides an indication of the remaining effective life of the battery  78 . 
     In a customer application, the microcontroller  84  puts itself and the RF data modem  86  into a sleep mode for 10 second intervals in the preferred embodiment, although the sleep time can be changed by the customer at any time. At the end of the 10 seconds, the RF data modem  86  interrogates a base station located remote from the pressure sensor  10  for any requests or instructions for the pressure sensor  10 . If no data is to be transmitted and no action is to be performed by the pressure sensor  10 , the RF data modem  86  goes into the sleep mode for another sleep interval. If pressure data is requested from the pressure sensor, the RF data modem  86  wakes up the microcontroller  84  and the microcontroller  84  calculates the pressure of the fluid at the pressure port  14  and sends the data to the RF data modem  86  which transmits the data to the base station using the procedure described above with respect to  FIG. 6 . Depending upon the instructions received by the RF data modem from the remote base station, the microcontroller  84  may perform other tasks such as configuration changes. Subsequent to any activity, both the RF data modem  86  and microcontroller  84  return to a sleep mode until the next wake-up event. 
     The reference port  16  can be opened by the customer at the site where the pressure sensor is to be used and the pressure sensor  10  can then provide absolute pressure, gauge pressure, or true gauge pressure which cannot be provided accurately by hermetically sealed pressure sensors. 
     The non-hermetically sealed pressure sensor of the preferred embodiment of the invention is less expensive to manufacture since a hermetic seal which will remain hermetic during normal use in industry requires specialized packaging materials and production steps that are not required by a non-hermetic sealed pressure sensor. The compensation of the pressure reading from the pressure sensing element  50  based on the temperature of the pressure sensing element  50 , the internal pressure inside the pressure sensor  50 , and the temperature of the electronic components provides greater accuracy in the pressure measurement than without taking into account these additional factors. The pressure sensor  10  is easy to use since only a wireless connection is needed to use the pressure sensor  10 . The compensation of the pressure sensing element  50  data by the effects of the temperature of the pressure sensing element  50 , the pressure inside the pressure sensor  50  and the temperature of the electronics is invisible to the user. The duration of the guaranteed accuracy of the pressure sensor  10 , and the life of the battery is one year in the preferred embodiment using the XBee RF module. This RF module has a data transmission range of about 100 feet indoors. If a greater range is required, the XBee-Pro can be used to provide about 300 feet of transmission indoors, but at a corresponding greater use of battery power during non-sleep operation of the RF data modem  86 . The use of the sockets  88  allows easy mounting of the type of RF data modem  86  needed by the customer. 
     During construction of the pressure sensor  10  one of several types of pressure transducers  50  are selected depending on the maximum pressure which will be applied to the pressure sensor  10  as specified by the customer. 
     Other embodiments according to the present invention include embodiments with an LCD display for visual reading of the pressure, the use of a larger battery to provide longer unattended service for the pressure sensor  10 , and modifying the preferred embodiment of the pressure sensor  10  to measure only the temperature of the fluid. 
     The embodiments described are chosen to provide an illustration of principles of the invention and its practical application to enable thereby one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, the foregoing description is to be considered exemplary, rather than limiting, and the true scope of the invention is that described in the following claims.