Abstract:
A method of determining a temperature and a pressure in a tank is provided. The method comprises the steps of obtaining sensor data and calculating coefficients. Then, a bridge resistance is ratiometrically determined from a bridge voltage. A temperature is calculated with the coefficients and the bridge resistance. Thereafter, a transition from a temperature measurement mode to a pressure measurement mode is made. A voltage per resistance is calculated with the coefficients and a differential voltage. Then, a normalized voltage is calculated using the voltage per resistance, the differential voltage, and the bridge resistance to indirectly compensating for temperature. A pressure is calculated with the coefficients and the normalized voltage.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This patent application claims the benefit of U.S. Provisional Patent Application No. 60/600,157, filed Aug. 10, 2004, the teachings and disclosure of which are hereby incorporated in their entireties by reference thereto. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to a piezo-resistive strain gauges, and more particularly to a pressure and temperature compensation algorithm to determine a pressure in a tank after compensating for temperature affects of the fluid.  
       BACKGROUND OF THE INVENTION  
       [0003]     A tank or similar vessel is often employed at a factory, construction site, warehouse, and the like, to receive, store, and distribute a product. These tanks are adapted to accommodate a variety of different products such as, for example, gasoline, propane, fertilizer, chemicals, fuels, and the like. The tanks that hold these products are often being drained of, and subsequently replenished with, the product. Therefore, sensors are employed to monitor a pressure within the tank so that the amount of product currently in the tank can be determined. One such sensor that have found wide use is a piezo-resistive strain gauge type pressure sensor.  
         [0004]     When a piezo-resistive strain gauge type pressure senor is used to determine the level of fluid in a tank, measurements of the pressure at the bottom of the tank and the temperature of the fluid are taken. The results of these two measurements can be a base for a fluid level, fluid volume, tank inventory control, and additional information. A silicon piezo-resistive pressure sensor can be used for both the fluid temperature and the pressure measurements. Unfortunately, the temperature of the sensor biases both the bridge resistance and the pressure reading. This temperature bias of the bridge resistance allows for a determination of the sensor temperature (and therefore fluid temperature), but the temperature bias must be removed from the pressure calculation.  
         [0005]     However, removing the temperature bias is not an easy task. The temperature changes the bridge resistance and bridge pressure sensitivity. This temperature effect is greater when measured across the entire bridge. A three-dimensional graph of pressure, temperature, and bridge voltage creates a surface bended on each end. Previous methods involve using two, multi-order polynomials. One polynomial calculates temperature and the other pressure. This method gives relatively good results for temperature measurements, but it keeps adding the temperature bias to the pressure measurement.  
         [0006]     Unfortunately, at least two difficulties exist with such conventional pressure transducers (i.e., sensors) used with tanks. First, standard sensor calibration requires eighteen measurements - six pressure measurements at three different temperatures. Second, a temperature calibration of the standard sensor requires nine measurements—three measurements of the bridge resistance at three different temperatures. Because so many measurements are needed for calibration, both time and money are wasted or, at the least, not used efficiently.  
         [0007]     The invention provides a method of using a piezo-resistive strain gauge to calculate temperature and pressure. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     In one aspect, the invention provides a method of determining a temperature and a pressure in a tank. To begin, sensor data is obtained and coefficients are calculated. Next, a bridge resistance is ratiometrically determined from a bridge voltage. A temperature is then calculated with selected ones of the coefficients and the bridge resistance. Thereafter, a transition from a temperature measurement mode to a pressure measurement mode is made. A voltage per resistance is then calculated with selected ones of the coefficients and a differential voltage. Thereafter, a normalized voltage is calculated using the voltage per resistance, the differential voltage, and the bridge resistance to indirectly compensate for temperature. A pressure is then calculated with selected ones of the coefficients and the normalized voltage.  
         [0009]     In another aspect, the invention provides a method of determining a temperature compensated pressure in a tank. To begin, sensor data is obtained and first coefficients are calculated for a voltage per resistance equation with the sensor data. Thereafter, second coefficients are calculated for a pressure equation with the sensor data. Then, a differential voltage is measured. A voltage per resistance is calculated with the voltage per resistance equation using the first coefficients and the differential voltage. Next, a normalized voltage is calculated with a normalized voltage equation using the voltage per resistance, the differential voltage, and the bridge resistance. Then, a pressure is calculated with the second coefficients and the normalized voltage.  
         [0010]     In yet another aspect, the invention provides a method of determining a temperature in a tank. To begin, sensor data is obtained and first coefficients are calculated for a temperature equation with the sensor data. Then, a bridge voltage is measured. A bridge resistance is ratiometrically determined with the bridge voltage. Thereafter, a temperature is calculated using the first coefficients and the bridge resistance.  
         [0011]     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0013]      FIG. 1  is a simplified schematic view of an exemplary embodiment of an a pressure and temperature measurement apparatus constructed in accordance with the teachings of the present invention; and  
         [0014]      FIG. 2  is a simplified schematic view of the pressure and temperature measurement apparatus of  FIG. 1  after switches have been actuated.  
         [0015]     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     Referring to  FIG. 1 , a temperature and pressure and temperature measuring circuit  10  is illustrated. The circuit  10  provides an inexpensive and economically desirable alternative to more expensive circuits or devices used to measure temperature and pressure within a tank. Moreover, as will be more fully explained below, since the circuit  10  includes only passive components, the circuit is safe to operate within the tank, even when the tank receives, stores, and dispenses a flammable product.  
         [0017]     As shown in  FIG. 1 , the circuit  10  comprises a transducer  12 , an analog-to-digital (A/D) converter  14 , a reference resistive device  16 , a first switch  18 , and a second switch  20 . The transducer  12  is a completely uncompensated transducer adapted to measure both a temperature and a pressure of a product in a tank. The transducer  12  can be a silicon piezo-resistive sensor, a strain gauge, a Wheatstone bridge, a sensor in a Wheatstone bridge type configuration, and the like.  
         [0018]     In a preferred embodiment, the transducer  12  is a piezo-resistive Wheatstone bridge pressure sensor commercially available from Druck, Inc., of New Fairfield, Conn., a subsidiary of the General Electric Company of Fairfield, Conn.  
         [0019]     As depicted in  FIG. 1 , the transducer  12  includes a first bridge node  22 , a second bridge node, a third bridge node  26 , and a fourth bridge node  28 . In such an embodiment, a first resistive device  30  is disposed between the first and third bridge nodes  22 ,  26  and a second resistive device  32  is disposed between the second and third bridge nodes  24 ,  26 . Likewise, a third resistive device  34  is disposed between the second and fourth bridge nodes  24 ,  28  and a fourth resistive device  36  is disposed between the first and fourth bridge nodes  22 ,  28 .  
         [0020]     Preferably, each of the resistive devices  30 ,  32 ,  34 ,  36  has an electrical resistance that fluctuates and/or varies based on a change in one or both of a temperature in the tank and a pressure in the tank. Therefore, when either or both of the temperature and pressure rise or fall proximate one of the resistive devices  30 ,  32 ,  34 ,  36 , the electrical resistance of that particular resistive device correspondingly changes.  
         [0021]     The A/D converter  14  comprises first and second reference inputs  38 ,  40 , first and second differential inputs  42 ,  44 , and a ground pin  46  coupled to a ground  48 . In a preferred embodiment, the A/D converter  14  is a sigma-delta A/D converter commercially available from a variety of manufacturers. In an exemplary embodiment, the A/D converter  14  is an integrated circuit (IC) 16-bit sigma-delta A/D converter such as, for example model AD7709, commercially available from Analog Devices, Inc., of Norwood, Mass.  
         [0022]     The A/D converter  14  is configured to provide a gain within a gain range (e.g., a gain of 2 to a gain of 128). The gain of the A/D converter  14  is adjustable such that the gain can be set low (i.e., around a gain of 2), set high (i.e., around a gain of 128), or set somewhere in between low and high settings. If an A/D converter such as model AD7709 is selected, the gain range noted above is available. Worth noting, the lower the gain such as, for example, a gain of 1 or 0.5, the easier it is to scale the resistance values for of the resistance device  16  and/or the resistive devices  60 ,  62 ,  64  in the protection network  58  to measure extremes in the temperature when measuring the overall resistance of the bridge ( FIG. 1 ). Therefore, a lower gain can provide certain advantages (e.g., better resolution at low pressures) and inhibit or prevent undesirable effects (e.g., saturation of inputs).  
         [0023]     The first reference input  38  of the A/D converter  14  is coupled to a first reference node  50  and the second reference input  40  is coupled to a second reference node  52 . In between the first and second reference nodes  50 ,  52 , the reference resistive device  16  is disposed. Since the reference resistive device  16  is a conventional fixed resistor having a known and/or predetermined resistance, the A/D converter  14  can make ratiometric comparisons and/or take ratiometric measurements by comparing a voltage drop between the first and second reference inputs  38 ,  40  to a voltage drop between the first and second differential inputs  42 ,  44 .  
         [0024]     The first reference node  50  receives a voltage from a voltage source  54 . Since the first reference node  50  is coupled to the first reference input  38  of the A/D converter  14 , the voltage source  54  provides voltage (i.e., excitation) to the A/D converter. The voltage source  54  can be a variety of voltage sources known in the art but, because of the ratiometric capabilities of the A/D converter  14 , the voltage source need not be a precision voltage source for the circuit  10  to operate properly.  
         [0025]     In both  FIGS. 1 and 2 , a wire resistance  56  between the transducer  12  and the A/D converter  14  is depicted. Even though measurable, the wire resistance  56  is small enough that it is generally considered negligible. Therefore, for purposes of clarity in explaining the invention, the wire resistance  56  is generally ignored. As such, the third and fourth bridge nodes  26 ,  28  are considered to extend though and past the illustrated wire resistances  56 . In other words, the third and fourth bridge nodes  26 ,  28  are for all intents and purposes directly coupled to the first and second differential inputs  42 ,  44  when the first and second switches  18 ,  20  are configured as shown in  FIG. 1 . Additionally, a wire resistance (not shown) in the lead extending from node  22  toward differential input  42  and in the lead extending from node  24  and differential input  44  would be present. Since the wire resistances are small and the A/D converter has a very high input impedance, the wire resistances can be ignored.  
         [0026]     The first switch  18  is adapted to selectively couple the first differential input  42  to either the third bridge node  26  as shown in  FIG. 1  or the first bridge node  22  as shown in  FIG. 2 . Likewise, the second switch  20  is adapted to selectively couple the second differential input  44  to either the ground  48  as shown in  FIG. 1  or the second bridge node  24  as shown in  FIG. 2 . The switches  18 ,  20  are actuatable and/or movable such that the circuit  10  can be transitioned between the temperature measurement mode or configuration shown in  FIG. 1  and the pressure measurement mode or configuration shown in  FIG. 2 .  
         [0027]     In a preferred embodiment, each of the first and second switches  18 ,  20  is a single pole, double throw (SPDT) switch. However, the first and second switches  18 ,  20  can also be other switches known in the art such as, for example, single pole change over or single pole center off (SPCO) switches. Also, in one embodiment, the first and second switches  18 ,  20  are replaceable with a double pole double throw (DPDT) switch or a double pole changeover or double pole center off (DPCO) switch.  
         [0028]     As shown in  FIG. 1 , with the first switch  18  positioned to couple the first differential input  42  and the third bridge node  26  and the second switch  20  positioned to couple the second differential input  44  and the fourth bridge node  28 , a bridge voltage (Vb) is experienced between the first and second differential inputs. Based on this total bridge voltage, a bridge resistance can be determined and/or calculated. When in this configuration, a temperature of a fluid in the tank can be determined as discussed more fully below and, as such, the circuit  10  is said to be in a temperature measurement mode.  
         [0029]     As shown in  FIG. 2 , with the first switch  18  positioned to couple the first differential input  42  and the first bridge node  22  and the second switch  20  positioned to couple the second differential input  44  and the second bridge node  24 , a differential output voltage (Vo) is experienced between the first and second differential inputs. Based on this differential output voltage, a differential resistance can be determined and/or calculated. When in this configuration, a pressure in the tank can be measured and, as such, the circuit  10  is said to be in a pressure measurement mode.  
         [0030]     As shown in both  FIGS. 1 and 2 , the circuit can include a protection network  58  comprised of first, second, and third protective resistive devices  60 ,  62 ,  64 . As depicted in  FIGS. 1 and 2 , each of the protective resistive devices  60 ,  62 ,  64  is a conventional fixed resistor. In a preferred embodiment, the first protective resistive device  60  is disposed between the third bridge node  26  and the second reference node  52 , the second protective resistive device  62  is disposed between the fourth bridge node  28  and the ground  48 , and the third protective resistive device  64  is disposed between the first reference node  50  and the voltage source  54 . In addition to the first, second, and third resistive devices  60 ,  62 ,  64 , the protection network  58  can be supplemented with limiting diodes (not shown) and other components to afford the circuit  10  further protection against, for example, over-voltage and over-current conditions.  
         [0031]     In operation, the circuit  10  employs a pressure and temperature compensation algorithm of the present invention. The formation of the algorithm begins by obtaining a variety of data from a manufacturer of the transducer  12 . This data typically includes, for example, a first temperature (t 1 ), a first bridge resistance (Rb 1 ), a second temperature (t 2 ), a second bridge resistance (Rb 2 ), a third temperature (t 3 ), and a third bridge resistance (Rb 3 ) for differential voltages (Vo 1 , Vo 2 , Vo 3 ) at pressures (p 1 , p 2 , p 3 ). In other words, the manufacturer provides data on how a temperature corresponds to a bridge resistance when the differential voltage and pressure are held constant. Additionally, an initial normalized bridge voltage (Vno) and a standard bridge resistance (Ro) at room temperature (i.e., ambient temperature) are given by the manufacturer.  
         [0032]     Using the data provided by the manufacturer, a first set of coefficients is calculated (e.g., by curve fitting) for a temperature equation. Specifically, from the manufacturer data a set of three linear equations with three unknowns can be generated and solved for the coefficients of a second order polynomial that characterizes the temperature for a given bridge resistance. Such calculations can be performed with the assistance of a personal computer. In the disclosed embodiment, three coefficients, namely a, b, and c are calculated using the data obtained from the manufacturer and a second order polynomial temperature equation: 
 
 t=a×Rb   2   +b×Rb+c    [1.1]
 
 where t is the temperature, Rb is the resistance of the bridge (i.e., transducer  12 ), and a, b, and c are the coefficients. 
 
         [0033]     Those skilled in the art will recognize that polynomial equations of higher orders can be employed to achieve greater accuracy and/or higher resolution. In those instances, further data must be provided by the manufacturer or otherwise obtained by, for example, testing to obtain values for the added number of coefficients. Additionally, to correct for any errors, additional polynomials, preferably second order polynomials, can be employed.  
         [0034]     In a preferred embodiment, after each of the coefficients a, b, and c have been determined, the coefficients are stored, along with the standard bridge resistance Ro, in the memory of measurement electronics (e.g., a monitor) for later recall and use. In an exemplary embodiment, the memory is a flash memory.  
         [0035]     Now that coefficients a, b, and c have been determined and stored, the temperature in the tank is calculable. To calculate the temperature in the tank at any particular moment, the bridge voltage is measured while the circuit  10  is in the temperature measurement mode ( FIG. 1 ). Based on the bridge voltage that is observed, the A/D converter  14  ratiometrically determines the bridge resistance (Rb) as follows:  
         Vb   Vref     =     Rb   Rref         
 
 where Vb is the bridge voltage, Rref is the resistance value of the reference resistor, and Vref is the voltage across the reference resistor Rref. At this point, if desired, the temperature can be stored, reported to a user, relayed to a data center, and the like. 
 
         [0036]     Still employing the data provided by the manufacturer, a second set of coefficients (e.g., by curve fitting) is calculated. Again, such calculations can be performed with the assistance of a personal computer. In the disclosed embodiment, four additional coefficients, namely d, e, f, and g, are calculated using the data obtained from the manufacturer and the following two equations:  
                 ⅆ   v       ⅆ   r       =       d   ×   Vo     +   e             [   1.2   ]               p   =       f   ×   Vno     +   g             [   1.4   ]             
 
 where dv/dr is the change in bridge voltage per one ohm of the bridge resistance (i.e., a slope), Vo is bridge voltage, p is pressure, Vno is the normalized bridge voltage (i.e., bridge voltage that has been corrected to compensate for the effect of temperature), and d, e, f and g are the coefficients. 
 
         [0037]     Like before, in a preferred embodiment, after each of the above coefficients are calculated, the coefficients are stored in the flash memory of the measurement electronics. Therefore, the coefficients can be easily recalled later and used. If desired and/or convenient, all of the coefficients, a-g, can be calculated and placed in memory simultaneously or at the same time.  
         [0038]     With the newly determined coefficients stored, the switches  18 ,  20  are manipulated and/or actuated to place the circuit  10  in the pressure measurement mode ( FIG. 2 ). Such an orientation permits a differential voltage (Vo) to be measured and/or observed as shown in  FIG. 2 .  
         [0039]     Once the differential voltage is known, the coefficients d and e are recalled from the flash memory of the measurement electronics. Thereafter, the differential voltage and appropriate coefficients are employed to calculate the change in voltage with respect to resistance, dv/dr, using the formula:  
                 ⅆ   v       ⅆ   r       =       d   ×   Vo     +   e             [   1.2   ]             
 
         [0040]     Now that dv/dr has been determined, the standard bridge resistance at room temperature, Ro, is recalled from memory and the normalized bridge voltage, Vno, is calculated using the following formula:  
             Vno   =     Vo   +         ⅆ   v       ⅆ   r       ×     (     Ro   -   Rb     )                 [   1.3   ]             
 
 where Vno is the normalized bridge voltage, Vo is bridge voltage, dv/dr is the change in bridge voltage per one ohm of the bridge resistance (i.e., a slope), Ro is the standard bridge resistance at room temperature, and Rb is the resistance of the bridge. 
 
         [0041]     Finally, knowing the calculated normalized bridge voltage, the coefficients f and g are recalled from memory and the pressure, p, is calculated using the following equation: 
 
 p=f×Vno+g    [1.4]
 
         [0042]     This final pressure in the tank, which represents a level of the product, can then be relayed and/or reported to a user, sent to a data center, and the like. If not already done, the temperature that was previously calculated can also be dispatched along with the newly calculated pressure.  
         [0043]     As the above explanation and equations illustrate, the pressure that has been calculated is not directly compensated with the either the calculated or a measured temperature. In fact, it should be noted that none of equations [1.2], [1.3], and [1.4] include a temperature variable. Instead, the relationship of bridge resistance to output bridge voltage over different temperature points provided by the sensor manufacturer is used in order to get the corrected pressure shown in equation 1.4.  
         [0044]     In a preferred embodiment, the gain of the A/D converter  14  is set low or near a lower end of the gain range when the temperature is determined ( FIG. 1 ) and high or near a high end of the gain range when the pressure ( FIG. 2 ) is determined. Also, in a preferred embodiment, the A/D converter  14  operates to filter out external noise.  
         [0045]     Finally, the reference resistive device  16 , the gain selected during particular measurements, and the maximum tolerance of the resistive devices  30 ,  32 ,  34 ,  36  should be carefully considered to avoid overflowing or saturating the inputs  38 ,  40 ,  42 ,  44  of the A/D converter  14 . This is particularly true when a temperature measurement ( FIG. 1 ) is taken for a high temperature (i.e., when the resistive values of the resistive devices  30 ,  32 ,  34 ,  36  are high) so as not to sacrifice resolution at low pressures.  
         [0046]     As those skilled in the art will appreciated, the circuit  10  provides a pressure transducer  12  (i.e., a sensor) that is adapted for use within a tank, is less expensive than commercially available sensors, and employs only passive components.  
         [0047]     Further, for typical sensor calibration, eighteen measurements (six different pressure measurements at three different temperatures) must be obtained. In contrast, circuit  10  calibration needs only four measurements because linear functions are employed. By having to take fewer readings, both time and money are saved when calibrating a sensor.  
         [0048]     Additionally, temperature calibration typically requires three measurements of bridge resistance at three temperatures. However, since the shape produced by the temperature equation [1.1] is generally the same for a particular type of sensor, only two temperature measurements are needed to calculate coefficients for a particular sensor.  
         [0049]     All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirely herein. In particular, co-pending U.S. patent application Ser. No. ______, entitled PRESSURE AND TEMPERATURE MEASUREMENT OF A PIEZO-RESISTIVE DEVICE USING DIFFERENTIAL CHANNEL OF A RATIOMETRIC ANALOG TO DIGITAL CONVERTER, filed on even date herewith, is incorporated by this reference.  
         [0050]     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.  
         [0051]     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.