Abstract:
A pressure measurement device with protection from long-term loads and pressure shocks. The novel device includes a diaphragm adapted to deform in response to a pressure applied thereto, and a retractable support adapted to support the diaphragm during periods of non-operation. A strain gauge is attached to the diaphragm for measuring the deformation and the retractable support is designed to minimize stress on the strain gauge during periods of non-operation. The support is retracted during periods of operation to allow the diaphragm and strain gauge to function normally in response to the applied pressure. The diaphragm is disposed within a housing to form a first cavity, which is coupled to media to be measured, and a second cavity, which is set at a reference pressure. In an illustrative embodiment, the retractable support is a threaded plug inserted into the second cavity.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to pressure measurement devices. More specifically, the present invention relates to pressure transducers based on solid-state strain gauge devices. 
         [0003]    2. Description of the Related Art 
         [0004]    Pressure transducers are devices that convert pressure to an electrical signal, and are commonly used to measure fluid and gas pressures. For example, pressure transducers might be used to monitor ink levels in an inkjet printer or to measure the air pressure in a tire. 
         [0005]    A conventional strain gauge based pressure transducer typically includes a cylindrical housing that is separated into two chambers by a thin, flexible diaphragm. The first chamber is coupled to the medium being measured, and the second chamber is set to a known reference pressure, such as atmospheric pressure. The diaphragm flexes slightly into one chamber or the other depending on the difference in pressure between the chambers. A strain gauge (a device whose resistance changes depending on the amount of strain placed on it) is attached to the diaphragm such that the flexure of the diaphragm introduces a strain on the gauge. The strain gauge then generates an electrical output signal that has a well-defined relationship to the pressure in the medium. 
         [0006]    In certain applications, the pressure transducer may be left under load for a long time period before a measurement is needed. For example, a missile typically includes vessels charged with gas. After assembly, the missile may be held in storage for a long period of time (up to ten years) before being placed in operation. During this period, the gas may leak. If the required amount of gas is not present, the missile will fail to operate properly. It is therefore critical to accurately determine the amount of gas remaining in the missile just prior to operation, after a potentially long storage period (without disassembling the missile). 
         [0007]    Conventional pressure transducers are not suitable for applications such as the one described above because solid-state strain gauge devices—such as a semiconductor Wheatstone bridge or piezoelectric wafer—are known to be susceptible to signal errors such as drift and hysteresis when left under load for extended periods of time. This drift is due to creep of the transducer substrate and transducer element materials. These transducers are also susceptible to damage from pressure shocks or “spikes” which can permanently deform or even rupture the electronic transducer element. 
         [0008]    Hence, a need exists in the art for an improved system or method for measuring pressure that retains accuracy when subject to long term loads or pressure shocks. 
       SUMMARY OF THE INVENTION 
       [0009]    The need in the art is addressed by the pressure measurement device of the present invention. The novel device includes a diaphragm adapted to deform in response to a pressure applied thereto and a retractable support adapted to support the diaphragm during periods of non-operation. A strain gauge is attached to the diaphragm for measuring the deformation and the retractable support is designed to minimize stress and deformation of the strain gauge during periods of non-operation. The support is retracted during periods of operation to allow the diaphragm and strain gauge to function normally in response to the applied pressure. The diaphragm is disposed within a housing to form a first cavity, which is coupled to media to be measured, and a second cavity, which is set at a reference pressure. In an illustrative embodiment, the retractable support is a threaded plug inserted into the second cavity until it comes into contact with the diaphragm or strain gauge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1   a  is a simplified schematic of a pressure transducer designed in accordance with an illustrative embodiment of the present teachings. 
           [0011]      FIG. 1   b  is a cross-sectional view of an illustrative embodiment of a retractable support for a pressure transducer designed in accordance with the present teachings. 
           [0012]      FIG. 2   a  is a cross-sectional view of an alternate illustrative embodiment of a pressure transducer designed in accordance with the present teachings. 
           [0013]      FIG. 2   b  is a close-up cross-sectional view of the retractable support of the alternate embodiment of  FIG. 2   a.    
           [0014]      FIG. 3  is a simplified diagram of a pressure transducer with an automated retractable support designed in accordance with an illustrative embodiment of the present teachings. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0015]    Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention. 
         [0016]    While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
         [0017]      FIG. 1   a  is a cross-sectional view of a pressure transducer  10  designed in accordance with an illustrative embodiment of the present teachings. The pressure transducer  10  includes a transducer body or housing  12  that forms an internal cavity  14  and an external cavity  16 . The internal cavity  14  is coupled to the working media. The external cavity  16 , which is not exposed to the working media, is set to a known reference pressure. The external cavity  16  may be open to the atmosphere (set to atmospheric pressure), or sealed and isolated from the atmosphere. In a preferred embodiment, the transducer body  12  is made from metal. 
         [0018]    The internal cavity  14  and external cavity  16  are separated by a diaphragm  18  that flexes slightly into one cavity or the other depending on the difference in pressure between the cavities. A strain gauge  20  is attached to the diaphragm  18  such that the flexure of the diaphragm  18  introduces a strain on the gauge  20 . The strain gauge  20  then generates an electrical output signal that has a defined relationship to the pressure in the working media. In a preferred embodiment, the strain gauge  20  is a solid-state strain gauge such as a semiconductor Wheatstone bridge or piezoelectric wafer. 
         [0019]    In the illustrative embodiment of  FIG. 1   a , the strain gauge  20  is attached on the side of the internal cavity  14  and is therefore in direct contact with the working media. In operation, the working media in the internal cavity  14  loads the strain gauge  20 , and deforms the strain gauge  20  and the diaphragm  18 . This creates an electronic signal which can then be converted to a pressure reading by additional electronics (not shown in  FIG. 1   a ). 
         [0020]    In accordance with the teachings of the present invention, the pressure transducer  10  also includes a retractable diaphragm support  22  that protects the transducer  10  from long term loads and/or pressure shocks. The retractable support  22  is adapted to relieve stress on the electronic transducer element  20  during periods when measurements are not being acquired. The support  22  is then retracted during periods of use to allow the transducer  10  to function in a normal manner. 
         [0021]      FIG. 1   b  is a cross-sectional view of an illustrative embodiment of a retractable support  22  for a pressure transducer  10  designed in accordance with the present teachings. In this embodiment, the retractable support  22  is in the form of a threaded metal plug. The plug  22  is inserted into the external cavity  16  by means of a thread on the exterior of the plug  22  and a mating thread on the interior of the external cavity  16 . During periods of non-operation (i.e., when measurements are not needed), the plug  22  is inserted into the external cavity  16 , by screwing it into the transducer housing  12  until it comes into contact with the diaphragm  18 , relieving the load on the transducer element  20  and diaphragm  18 . During periods of operation (acquiring measurements), the support  22  is retracted (by unscrewing the plug  22 ) to allow the transducer  10  to function properly. 
         [0022]    The retractable diaphragm support  22  and/or the housing  12  may have Vernier scale markings to allow it to be inserted to a precise location and retracted to a precise location. Alternatively, the placement of the support  22  can be determined by monitoring the output signal of the strain gauge  20 . For example, the support  22  may be inserted until there is a null in the strain gauge output signal. At this point, when the strain gauge  20  is no longer giving any signal, the gauge  20  is fully supported. 
         [0023]    The retractable diaphragm support  22  may also have a gas relief channel  24  cut into it as a means to maintain neutral gas pressure on the external side of the diaphragm  18  when the support  22  is inserted or retracted. Thus, when the support  22  is inserted, any ambient gas in the external cavity  16  will not be trapped between the support  22  and the diaphragm  18 , but flows out through the gas relief channel  124 . When the support  22  is retracted, gas flows back into the external cavity  16  through the gas relief channel  124 . 
         [0024]    As shown in  FIG. 1   b , a wiring channel  26  may be cut into the transducer housing  12  and/or the diaphragm  18  to provide a means for making electrical connections with the strain gauge  20 . Wires can then be placed through the channel  26  to couple the strain gauge  20  to additional electronics such as an output amplifier and/or a bias voltage source (not shown). 
         [0025]      FIG. 2   a  is a cross-sectional view of an alternate illustrative embodiment of a pressure transducer  10 ′ designed in accordance with the present teachings.  FIG. 2   b  is a close-up cross-sectional view of the retractable support  22  of the alternate embodiment of  FIG. 2   a . The pressure transducer  10 ′ shown in  FIGS. 2   a  and  2   b  is identical to that of  FIGS. 1   a  and  1   b , except that the strain gauge  20  is attached to the diaphragm  18  on the side of the external cavity  16  (instead of on the side of the internal cavity  14  as in the embodiment of  FIGS. 1   a  and  1   b ). This placement serves to protect the strain gauge  20  from exposure to the working media and eliminates the need for hermetically sealed electrical connections through the diaphragm  18 . 
         [0026]    As in the first embodiment, the pressure transducer  10 ′ includes a retractable support  22  to protect the strain gauge  20  from long-term loads and shocks. During periods of non-operation, the support  22  is inserted into the external cavity  16  until it comes into contact with the strain gauge  20 . During periods of operation, the support  22  is retracted. The working media in the internal cavity  14  loads the diaphragm  18  and deforms it and the strain gauge  20  affixed to it. This creates an electronic signal, which can then be converted to a pressure reading. 
         [0027]    As shown in  FIG. 2   b , a wiring channel  26  may be cut into the transducer housing  12  to provide a means for making electrical connections with the strain gage  20 . In this embodiment, the wiring channel  26  does not need to pass through the diaphragm  18  to provide access to the strain gauge  20 . 
         [0028]    A threaded metal plug as described above is a simple implementation of the retractable support concept of the present invention. Other implementations may also be used without departing from the present teachings. For example, the pressure transducer may include an automated retractable support such as a solenoid, electric motor or piezoelectric wafer. 
         [0029]      FIG. 3  is a simplified diagram of a pressure transducer  10 ″ with an automated retractable support  22  designed in accordance with an illustrative embodiment of the present teachings. In this embodiment, the retractable support  22  is a metal plug coupled to a mechanism  30  for moving the plug  22  towards or away from the diaphragm  18  and strain gauge  20  in response to an electronic control signal. The retracting mechanism  30  may be implemented using, for example, a solenoid or an electric motor. A control system  32  generates the control signal for the retracting mechanism  30 . When the pressure transducer  10 ″ is to begin acquiring measurements, the control system generates a control signal instructing the mechanism  30  to move the support  22  away from the diaphragm  18  by some predetermined amount (such that the transducer  10 ″ can operate properly). When the pressure transducer  10 ″ is finished acquiring measurements, the control system generates a control signal instructing the mechanism  30  to move the support  22  towards the diaphragm  18  by some predetermined amount (such that the diaphragm  18  and strain gauge  20  are fully supported by the plug  22 ). Depending on the application, the periods of operation and non-operation may be programmed into the control system  32 , or they may be input by the user (or by another system). 
         [0030]    Optionally, the pressure transducer  10 ″ may use the output from the strain gauge  20  to control the precise placement of the retractable support  22 . As shown in  FIG. 3 , the output of the strain gauge  20  is passed through an amplifier  34  and input to the control system  32 . The control system  32  converts the strain gauge measurement to a pressure measurement, which is output to the user. The strain gauge measurement is also used to generate the control signal. Instead of moving the support  22  towards the diaphragm  18  by some predetermined amount, the control system  32  is adapted to instruct the mechanism  30  to stop moving the support  22  when the strain gauge measurement is at a predetermined level such as, for example, when the strain gauge output is at a null (at this point the strain gauge  20  is flat, minimizing the strain on the strain gauge  20 ). 
         [0031]    Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof. 
         [0032]    It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
         [0033]    Accordingly,