The subject matter disclosed herein relates generally to pressure transducers and, more specifically, to temperature-compensated digital pressure transducers for use in applications requiring a high degree of accuracy.
Pressure transducers are widely used in a myriad of applications. Among the uses of pressure transducers are the indirect measurement of other variables such as fluid/gas flow, speed, water level, and altitude. There are a variety of technologies that have been used for pressure transducers, and these technologies vary in performance, and cost. The typical analog pressure transducer is characterized by relation between input pressure and an output analog signal. As with all measuring instruments, pressure transducers must be calibrated. Calibration is defined as a set of operations that establish, under specified conditions, the relationship between the values of quantities indicated by a measuring instrument or measuring system (readings) and the corresponding values realized by standards (true value). Once the relationship between the readings and true values is known the readings may be adjusted to provide a more accurate value. However the relationship between the input pressure and the output analog signal is significantly affected by temperature. Consequently, at any given pressure, variations in temperature will cause errors to be introduced in the output signal, which if left uncompensated, will cause errors leading to inaccurate pressure readings.
Compensation for temperature variations may be accomplished in a variety of ways. For example, an analog pressure transducer may be placed in a chamber where temperature and pressure can be changed. Various known pressures are applied as transducer input and output signals are measured then temperature is changed and the process is repeated. As result of this process, tables are created that describe relation between input pressure and output signal. The relation between input pressure and output analog signal may be described by a mathematical function. There is a possibility to define a few mathematical functions that describe a relation between input pressure and output signal for various temperatures during an iterative calibration process. In general, accuracy of the mathematical function depends from the number of created tables, the size of the tables and the interpolation technique. However, with this approach temperature information has to be sent to a device that is used to select the correct function to adjust pressure values based on temperature values. This method is impractical because it is difficult to define the function if the measured temperature does not match the values of temperature for which the transducer was calibrated.
Another approach is to obtain multiple readings at multiple known pressures over a range of temperatures. Tables of these values may be created and a mathematical interpolation technique may be applied to create a correction algorithm with some coefficients. If these coefficients are known and the temperature value is known, then output signal from the pressure transducer can be measured and then by using interpolation technique the temperature compensated pressure value is calculated. In general, accuracy of the function depends from the number of created tables and their size but also from interpolation technique.
Commercially available pressure transducers include transducers that provide pressure and temperature values in analog form (e.g. voltage). For these devices, correction algorithm coefficients may be stored in EEPROM located in pressure transducer. End users of the pressure transducer have to know the mathematical function describing relation between input pressure and output signal. Usually, a pressure transducer is connected to a host device. An example of a host device is a volume corrector in gas distribution lines, or a flow computer used in gas transmission lines, or similar end user electronic hardware. After the pressure transducer is connected to the host device, correction algorithm coefficients have to be provided to the host device. Usually the host device is provided with analog/digital converter that converts analog signals into digital form. The digital information is provided to a microprocessor in the host device that calculates a temperature compensated pressure value based on mathematical function and correction algorithm coefficients.
Another type of pressure transducer provides digital outputs to the host device. These pressure transducers include an analog/digital converter. Output signals in digital form are sent directly into inputs of an end user microprocessor. Correction algorithm coefficients may be stored in pressure transducer. The user of these digital-types of pressure transducers has to know the mathematical function describing relation between input pressure and output signal. Usually, after the digital-type of pressure transducer is connected for the first time into the host device, the correction algorithm coefficients are sent from the pressure transducer to the host device. The microprocessor in the host device then calculates a temperature compensated pressure value based on an applied correction algorithm and coefficients obtained during calibration process.
One application of a digital pressure transducer is as a component of a host device comprising, for example, a volume corrector in gas distribution lines, or a flow computer used in gas transmission lines. The measurement of volume flowing through a pipeline requires correction for the effects of pressure and temperature on the gas volume passing through the measuring instrument. The degree of accuracy of volume correctors or flow computers is regulated by government authorities. Charles Law and Boyle's Law are applied to adjust for pressure and temperature effects to the gas. The gas volume is converted to “Standard Pressure and Temperature values.”
To determine the volume of gas exposed to varying conditions of temperature and pressure flowing through a pipeline, accurate temperature compensated pressure measurements are required. There are three temperatures that may be measured in this type of application. These temperatures include (a) ambient temperature (volume correctors of this type may operate over a range of ambient temperatures of about −40° C. to about +70° C.), (b) the temperature of the gas flowing through the pipe, and (c) the temperature of the pressure sensing elements.
Existing pressure transducers have a number of problems when used in connection with instruments or hardware such as volume correctors. One problem is that in compensating for temperature, the temperature of the ambient air or of the gas or fluid flowing through a pipe is measured instead of the temperature of the pressure sensing element. Another problem is that when there is a reading that indicates a malfunction, the user is unable to distinguish whether the malfunction is in the pressure transducer or in the host device. Another problem is that over time, the relationship between the inputs pressure in the sensor output may change, and consequently, the complexity, expense and sometimes technical inability to recalibrate the pressure transducer in the field poses a significant problem.
The present disclosure describes pressure transducers with features that improve accuracy of pressure measurements of fluids over a wide range of temperatures. These features allow precise measurement of standardized fluid volumes delivered by, for example, pipelines and storage tanks As set forth more below, embodiments of the pressure transducers can utilize a sophisticated mathematical approach that employs a second order polynomial to correct measured pressures across a variety of temperatures. These embodiments can use a plurality of coefficients from tests on a large number of temperature and pressure sample environments. In one implementation, the tests generate six coefficients for use with the second order polynomial in a calculated correction. This feature allows the pressure transducer's rated operational range to be mapped with high resolution over both its temperature and pressure spans. For example, eight different sets of coefficients may be generated and stored in the pressure transducer so that a different set of coefficients may be selected depending on the current pressure being measured.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
Reference is now made briefly to the accompanying drawings, in which:
Where applicable like reference characters designate identical or corresponding components and units throughout the several views, which are not to scale unless otherwise indicated.
As set forth in more detail herein below, systems for measuring pressure can utilize a number of calibration points during a calibration phase for generating a detailed resolution of the pressure transducer's operational range. This phase generates a customized set of correction coefficients, or compensation coefficients, for subsequent use when correcting pressure measurements in the field. In one embodiment, specific field measurements may determine that one of eight different compensation coefficient sets are to be employed in a polynomial correction function. Each compensation coefficient set maps to a particular sector of temperature and pressure in the pressure transducer's operational range. In one embodiment, each coefficient set contains six coefficients which allow a high degree of correction accuracy. A second order polynomial provides the compensation function and is evaluated using the selected coefficient set.
Inputs to the polynomial correction function may include a temperature measurement from a pressure transducer temperature sensor. Additional accuracy in pressure measurement can be obtained by employing a temperature compensation function for this measured temperature. In one embodiment, a temperature compensation function may utilize one of two sets of temperature compensation coefficients to be employed in correcting a temperature measurement. An increased number of calibration points, whether for pressure compensation or temperature compensation, improves accuracy of the pressure transducer measurements and, thereby, of standardized fluid volume measurements.
Illustrated in
The digital pressure transducer 11 also includes a processor 21 and computer readable medium 23. Depending upon the particular embodiment, examples of the processor 21 may embody a general purpose processor, a microprocessor, a content addressable memory, a digital signal processor, an application specific integrated circuit, a field programmable gate array, any suitable programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof, designed to perform the functions described herein. The processor 21 may also be realized as a microprocessor, a controller, a microcontroller, or a state machine. The processor 21 may also be implemented as a combination of computing devices, e.g., a combination of a digital signal processor and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a digital signal processor core, or any other such configuration. Memory 23 is preferably an EEPROM but may be any type of memory including, RAM, ROM, or flash memory.
Firmware 25 may also be provided to control certain aspects of the functionality of the digital pressure transducer 11, such as the implementation of an adjustment application 27 that adjusts outputs to correct for temperature variations. The adjustment application 27 may implement a variety of mathematical interpolation procedures that might be applied to create a mathematical function that describes relation between input pressure and temperature and output signal. In some cases, the mathematical function may be described by polynomials with the set of coefficients.
The digital pressure transducer 11 may also include a power supply 29, which in one embodiment may be in the form of a battery or power supplied from a host device 37.
The digital pressure transducer 11 may have a read/write port 31 and a read only port 33. The read/write port 31 may be used to communicate with a computer (user) terminal 35 and/or with a host device 37. The read only port 33 may be used to communicate with the host device 37. Computer terminal 35 may be a digital processor such as a microprocessor. The host device 37 may be an instrument such as for example a volume corrector used in gas distribution lines, or a flow computer used in gas transmission lines. In one embodiment the read/write port 31 maybe a USB port. The read-only port 33 may be used to communicate with the host device 37 using an electronic component level protocol. Various methods are typically used for communication between devices at the electronic component level using protocols such as USB, IEEE 1394, RS 232, I2C, etc. The I2C protocol was developed for communication between integrated circuit (IC) chips through two bus lines. Computer terminal 35 may access application software 36 that may include applications for data interpolation and interrogation of the digital pressure transducer 11. Computer terminal 35 may be used to provide data and programs to the digital pressure transducer 11. Exemplary data and programs include calibration data, value adjustment applications, and recalibration data.
In one embodiment, some and/or all of the components of the system for measuring pressure 10 may be embodied in a single chip. Examples of chips that may be used in such cases are “system on a chip” integrated circuits and programmable “system on a chip” integrated circuits. A typical system on a chip may include a microcontroller, microprocessor or one or more digital signal processor cores. The system on a chip may also incorporate memory (e.g. ROM, RAM, EEPROM and flash memory), peripherals, external interfaces including, analog interfaces and the like. Additionally, the system on a chip may include software. A programmable system on a chip may provide integrated configurable analog and digital peripheral functions, memory, and a microcontroller on a single chip.
In one embodiment, the method 50 includes an initial element to determine the correction algorithm that will be applied to the data and to install the correction algorithm application (method element 52). The determination of the correction algorithm involves the balancing of data quality and performance factors such as transducer battery power and life.
Exemplary correction algorithms are often used as part of a calibration process. One of the simplest approaches to calibration of transducers is a one-point correction approach. Examples of this approach assume that the response of a sensor is linear. However, for the pressure and temperature ranges that the digital pressure transducer 11 is subjected to, the response of the analog pressure and temperature sensor 15 is a non-linear multidimensional function. This response requires, in one example, at least a two-point correction algorithm using a higher order (2 or higher) polynomial is desired. A correction algorithm can be derived for non-linear data sets by the use of polynomial interpolation. A set of data points may be replaced with an approximate polynomial function. This function may require the storage of a reduced number of polynomial correction algorithm coefficients and a curve-fitting computation that can be implemented by digital processing devices. Some embodiments may use a second, third or fourth order polynomial. A correction algorithm application may be programmed and installed in the computer terminal 35 (method element 52) and/or in the transducer (method element 53).
The method 50 may also include initiating the calibration process (method element 54) disclosed in more detail below. In one implementation, as shown in
In one embodiment, the method 73 may include a determination as to whether the temperature being tested is the end temperature Tn (method element 85). If the temperature at which the digital pressure transducer 11 is being tested is not Tn, then the temperature is changed by the predetermined increment (method element 87) and measurements are made and values for the analog signal and digital values may be recorded. If the temperature at which the digital pressure transducer 11 is being tested is the end temperature Tn then a determination of whether the pressure being tested is the end pressure Pm (method element 89) is made. If the pressure being tested is not the final pressure Pm, then the pressure is changed by the predetermined increment (method element 91) and the temperature is reset to the initial temperature T1 (method element 93). If the pressure being tested is the final pressure Pm then the calibration process ends (method element 95). The results of this method are tables correlating analog signal output at a certain temperature with pressure. These tables may be used to identify the coefficients for the correction algorithm to be used with the digital pressure transducer 11.
In one embodiment, another method may be used such as two point recalibration (bracketing calibration) where the two calibration points are used to bracket the range of values that will be measured. Two point recalibration may require some interpolation function to generate adjustment values. The adjustment value(s) is/are sent to the transducer read/write port 31 (method element 139) and is/are received by the transducer (method element 141). The adjustment value(s) may then be stored into memory 23 (method element 143). Optionally, the adjustment values may be calculated by the processor 21 in the digital pressure transducer 11. Additionally, other recalibration reference information such as user identification, and date and time of recalibration are may be identified (method element 145) and instructions provided to the digital pressure transducer 11 to store the recalibration reference information into a recalibration log (method element 147). The instructions would be received by the digital pressure transducer 11 wherein the recalibration reference information will be stored in a recalibration log (method element 149). Thereafter the digital pressure transducer 11 would apply the adjustment value (method element 151). Additional optional elements may be employed in this method. For example, the processor 21 may be programmed to allow a limited number of entries (recalibrations). If the limit is reached then recalibrations will not be permitted until this log is downloaded into the computer terminal 35. In that case the event of downloading is registered in a recalibration log for traceability.
In light of the foregoing embodiments describing various configurations and uses of the system of
Moreover, embodiments of the digital pressure transducer 11 serves to maintain the digital pressure transducer 11 separate from the host device 37. Correction coefficients may be stored in the digital pressure transducer 11 and the correction processing may be implemented in the digital pressure transducer 11. In a situation where the digital pressure transducer 11 fails, there is no need to replace the host device.
The digital pressure transducer 11 provides additional functionality such as for example the ability to inspect the digital pressure transducer 11 by connecting the computer terminal 35 through the read/write port 31. The temperature compensated pressure value may then be presented on the screen of the computer terminal 35. Additionally, the digital pressure transducer 11 provides the ability to periodically check if the correction algorithm coefficients have been changed (intentionally or by not controlled reasons e.g. electromagnetic radiation). One of the methods could be cyclic redundancy check (CRC). Any other method such as a checksum function may also be used. The checksum function takes a value generated from an arbitrary block of data and compares it to a recomputed value. If the checksums do not match then the data has been altered. CRC values may be stored in the digital pressure transducer 11 and if newly calculated value does not match previously stored values then the digital pressure transducer fault is set. Checking of the calibration may be done on a periodic basis (e.g. every one hour).
Embodiments of the digital pressure transducer 11 provide an additional functionality with regard to the re-use of the host device 37. For example, an end user may decide that the host device 37 (e.g. volume corrector or flow computer) should be used in another installation. In such an event, a new digital pressure transducer may be installed with the re-used host device 37 without the need of metrological verification, even if the pressure in the other installation is different. The reason for this is that the accuracy of the digital pressure transducer 11 was verified prior to installation.
An exemplary portion of one embodiment of a correction algorithm will now be described. Referring to
As an exemplary reading of the table 600, the controlled temperature and pressure prepared for “environment 1” is designated by the column and row headers corresponding to the table entry labeled “1”. Thus, environment “1” was set at a known temperature of about −40° C. and the controlled pressure for that environment was set at a known pressure of about 0 bar. Each of the table entries shown in
As an example, coefficient a3 is calculated as follows. The formula for calculating a3, as shown in
a
3=(M3−2*M2+M1)/(0.5*Wp2) (1)
This formula of Equation 1 requires the stored values from table 600 designated as M1, M2, and M3. These refer to the digital pressure output value provided by the pressure transducer 11 for table entry “1” (or calibration point 1) corresponding to environment 1 (e.g., 0 bar at −40° C.), table entry “2” corresponding to environment 2 (e.g., 17.5 bar at −40° C.), and table entry “3” corresponding to environment 3 (e.g., 35 bar at −40° C.), respectively. The terms Wt and Wp in the formulas 700 are equal to half the temperature span (in Celsius) and half the pressure span (in bar), respectively, of the pressure transducer 11. In the example pressure transducer span depicted in
In one implementation, the compensation coefficients can be stored in the memory 23 of the pressure transducer 11, e.g. via the method element 67 of the procedure illustrated in
M
p
=a
0
+a
1
P+a
2
T+a
3
P
2
+a
4
T
2
+a
5
PT (2)
In Equation 2, Mp is the measured digital pressure value output by the pressure transducer 11 in the field and T is the corresponding digital temperature value output by the pressure transducer at the same time. As described above, the temperature value may be the temperature of the pressure sensor 15. The remaining unknown variable P represents the temperature compensated pressure to be determined, and may be finally determined (solved) using a variety of computational techniques. In one embodiment, P is determined using a standard Newton-Raphson method, an example of which follows as Equation 3:
Another coefficient set is also calculated in order to determine which compensation coefficient set of the plurality of coefficient sets 700 to use for a particular temperature compensated pressure measurement in the field. This coefficient set is referred to herein as an approximation coefficient set, the “S” set comprising coefficients S0 through S5, and is illustrated in
In one implementation, the approximation coefficient set is used as follows below. An initial approximate pressure measurement in the field is first obtained, using the approximation coefficients, the general equation, Equation 1, and solving for P as explained above, together with its corresponding temperature measurement. Using the solution for P thus obtained and its corresponding temperature measurement, these values are used as orthogonal coordinates to identify a sector in the chart 900 illustrated in
Thus, one embodiment of a correction algorithm disclosed herein is a two step method utilizing and solving the general equation (Equation 1) twice—one time by calculating an approximate compensated pressure in the field using the approximating coefficients of
An alternative addition to the algorithm described above may include a temperature measurement compensation algorithm, as follows. Two sets of three temperature measurement compensation coefficients are generated according to the formulas shown in
In light of the foregoing discussion, embodiments of the pressure transducer generate a more precise volume correction mechanism. Once the relationship between the readings and true values is known, as determined in a calibration phase, the readings may be adjusted according to a correction algorithm to provide a more accurate value. A technical effect is to improve the evaluation of fluid volumes according to standardized metrics.
The methods of the various embodiments may be embodied as one or more computer programs. However, it would be understood by one of ordinary skill in the art that the invention as described herein could be implemented in many different ways using a wide range of programming techniques as well as general-purpose hardware systems or dedicated controllers. In addition, many, if not all, of the steps for the methods described above are optional or can be combined or performed in one or more alternative orders or sequences without departing from the scope of the present invention and the claims should not be construed as being limited to any particular order or sequence, unless specifically indicated.
The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied in hardware, in firmware, in a software module executed by a processor, or in any practical combination thereof. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CDROM, or any other form of computer readable medium known in the art. In this regard, memory 23 can be coupled to processor 21 such that processing unit processor 21 can read information from, and write information to, memory 23. In the alternative, memory 23 may be integral to processor 21.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
This application is a continuation-in-part of U.S. patent application Ser. No. 13/401,050, filed on Feb. 21, 2012 and entitled “Temperature Compensated Pressure Transducer.” The content of this application is incorporated herein by reference in its entirety.
Number | Date | Country | |
---|---|---|---|
Parent | 13401050 | Feb 2012 | US |
Child | 13853441 | US |