Abstract:
Self-Validating Thermocouple (SVT) Systems capable of detecting sensor probe open circuits, short circuits, and unnoticeable faults such as a probe debonding and probe degradation are useful in the measurement of temperatures. SVT Systems provide such capabilities by incorporating a heating or excitation element into the measuring junction of the thermocouple. By heating the measuring junction and observing the decay time for the detected DC voltage signal, it is possible to indicate whether the thermocouple is bonded or debonded. A change in the thermal transfer function of the thermocouple system causes a change in the rise and decay times of the thermocouple output. Incorporation of the excitation element does not interfere with normal thermocouple operation, thus further allowing traditional validation procedures as well.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The application claims the benefit, under 35 U.S.C. §119(e), of U.S. Provisional Application Ser. No. 60/807,217 filed Jul. 13, 2006, the contents of which are incorporated herein by reference. 
     
    
     ORIGIN OF THE INVENTION 
       [0002]    The invention described herein was made in the performance of work under a NASA contract and by employees of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. In accordance with 35 U.S.C. §202, the contractor elected not to retain title. 
     
     TECHNICAL FIELD OF THE INVENTION 
       [0003]    The present invention relates generally to thermocouples and in particular to the thermocouple designs capable of self validation. 
       BACKGROUND OF THE INVENTION 
       [0004]    The basic concept of a sensor automatically monitoring its operational capability, i.e., self-validating performance, is generally recognized. An attempt is made to continuously monitor and self-validate the sensor&#39;s performance to determine the health of the sensor. The process of self-validation involves the continued assessment of a combination of: 1) reviewing physical parameters obtained real-time by means of electronic circuitry to obtain actual measurement data; and 2) utilizing a combination of statistical tools to estimate and predict a measurement value at a given time in the process and compare the predicted measurement value to the actual measurement data. Self-validation processes used by others include ARMA (Auto Regression Moving Average), LCSR (Loop Current Step Response), and Power Spectrum Density determination. The failure or success of any of these processes presupposes properly functioning sensor circuitry. 
         [0005]    However, in many sensors, and particular thermocouples, the actual cause for failure is directly related to the physical bonding between the thermocouple sensor element and the attachment surface. As a consequence, conventional self-validating techniques may fail to reliably identify the bonded/debonded condition that directly leads to sensor failure. 
         [0006]    For the reasons stated above, and for other reasons that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative approaches to thermocouple validation. 
       SUMMARY OF THE INVENTION 
       [0007]    The various embodiments provide a Self-Validating Thermocouple (SVT) System capable of detecting sensor probe open circuits, short circuits, and unnoticeable faults such as a probe debonding and probe degradation. The various embodiments provide such capabilities by incorporating a heating or excitation element into the measuring junction of the thermocouple. By heating the measuring junction and observing the decay time for the detected DC voltage signal, it is possible to indicate whether the thermocouple is bonded or debonded. A change in the thermal transfer function of the thermocouple system causes a change in the decay time for the DC voltage signal. The various embodiments are further capable of traditional validation procedures as the excitation elements in accordance with the various embodiments do not interfere with the normal operation of the thermocouple. 
         [0008]    The invention includes methods and apparatus of varying scope. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic of a basic thermocouple design. 
           [0010]      FIGS. 2A and 2B  depict portions of two thermocouple circuits having measuring junction excitation elements for use with the various embodiments. 
           [0011]      FIG. 3  is a block schematic of a thermocouple system in accordance with an embodiment of the invention. 
           [0012]      FIG. 4  is a flowchart of a method of validation in accordance with one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that process, mechanical, and electrical changes may be made without departing from the spirit and scope of the present invention. It is noted that the drawings are not to scale unless a scale is provided thereon. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof. 
         [0014]    It is well known that a metal or other conductor subjected to a thermal gradient will generate a voltage. To measure the voltage, a closed circuit must be provided, thus requiring a return conductor. If the same material were used for the return conductor, its temperature-generated voltage would cancel out the voltage of the first conductor. However, the voltage response is dependent upon the conductor itself. By using a dissimilar metal for the return conductor, a measurable voltage differential will be developed that is related to the temperature gradient experienced by both conductors. 
         [0015]      FIG. 1  is a schematic of a basic thermocouple design. The thermocouple  100  includes a first conductor  102  and second conductor  104 . Two junctions  106  and  108  are formed where the two conductors are joined, and the voltage differential can be read across nodes  110  and  112 . One junction, such as junction  106 , is a measuring junction while the remaining junction, such as junction  108 , is the reference junction. 
         [0016]    The various embodiments include a heating or excitation element at the measuring junction.  FIGS. 2A and 2B  depict portions of two thermocouple circuits  200 A and  200 B having measuring junction excitation elements for use with the various embodiments. In  FIG. 2A , the thermocouple  200 A includes a first capacitor  220 , a resistor  222  and a second capacitor  224  coupled in series at the measuring junction  206 . The thermocouple  200 A further includes a first inductor  228  and a second inductor  230  coupled in series with the measuring junction  206 . The resistor  222  acts as an excitation element. Elements located above the dashed line in  FIG. 2A  may generally be located on a circuit board of a thermocouple system while elements below the dashed line would be located at the sensing element. The excitation element  222  is in thermal contact with the measuring junction  206 . That is, the excitation element  222  is sufficiently coupled to the measuring junction to cause a temperature rise in the measuring junction  206  upon application of the alternating current (AC) stimulation signal. The excitation element  222  need not be in physical contact, and may be separated by a thermal compound capable of thermal transfer. 
         [0017]    By applying an AC signal from the excitation and signal conditioning circuitry  226 , such as a pulse width modulated signal, to resistor  222  the measuring junction  206  will heat up. The AC stimulation signal, by itself, does not affect the thermocouple measuring junction  206  because inductors  228  and  230  act as an open circuit to the AC signal. In a similar manner, the DC voltage generated by the thermocouple will not affect the resistor  222  voltage since the capacitors  220  and  224  act as an open circuit to the DC signal. While two capacitors  220  and  224  and two inductors  228  and  230  are depicted in the embodiment of  FIG. 2A , one capacitor and one inductor would suffice in that the path to the excitation element  222  could still act as an open circuit to a DC signal with one capacitor in the loop to the excitation and signal conditioning circuitry  226  and the path to the measuring junction  206  could still act as an open circuit to an AC signal with one inductor in the loop to the excitation and signal conditioning circuitry  226 . Other circuit configurations can also be used to satisfy these criteria. For one embodiment, the same lead could be used to supply the AC signal to the resistor  222  and to read the measuring junction  206 . For example, capacitor  220  and inductor  228  could both be coupled to a single lead in the excitation and signal conditioning circuitry  226 , and capacitor  224  and inductor  230  could both be coupled to a single lead in the excitation and signal conditioning circuitry  226  such that a circuit path containing the resistor  222  would be coupled in parallel with a circuit path containing the measuring junction  206 . 
         [0018]    In  FIG. 2B , the thermocouple  200 B includes one inductor  228  coupled in parallel with series-coupled capacitor  220  and resistor  222  between the excitation and signal conditioning circuitry  226  and the measuring junction  206 . The resistor  222  acts as an excitation element. Elements located above the dashed line in  FIG. 2B  may generally be located on a circuit board of a thermocouple system while elements below the dashed line would be located at the sensing element. The excitation element  222  is in thermal contact with the measuring junction  206 . The excitation element  222  need not be in physical contact, and may be separated by a thermal compound capable of thermal transfer. For a further embodiment, the same lead could be used to supply the AC signal to the resistor  222  and to read the measuring junction  206 . For example, capacitor  220  and inductor  228  could both be coupled to a single lead in the excitation and signal conditioning circuitry  226  such that a circuit path containing the resistor  222  would be coupled in parallel with at least a portion of a circuit path containing the measuring junction  206 . 
         [0019]    By applying an alternating current (AC) signal, such as a pulse width modulated signal, to resistor  222  the measuring junction  206  will heat up. The AC stimulation signal, by itself, does not affect the thermocouple measuring junction  206 . In a similar manner, the DC voltage generated by the thermocouple will not affect the resistor  222  voltage since the capacitor  220  acts as an open circuit to the DC signal. Other designs may be utilized with the various embodiments, provided that the resulting excitation element provides one path inhibiting an AC signal and another path providing an open circuit to a DC signal. The embodiment of  FIG. 2A  adds improved noise immunity to the thermocouple circuit using a four-wire configuration while the embodiment of  FIG. 2B  reduces physical interfacing by using a three-wire configuration. As shown in  FIG. 2B , a circuit path containing the resistor  222  may also include the measuring junction  206 . 
         [0020]    Thermocouples including excitation elements in accordance with embodiments of the invention are compatible with traditional thermocouple systems. Typical systems would provide instrumentation such as a cold junction compensator, signal conditioner circuitry, analog/digital (A/D) converter, processor, power section, and system interface, e.g., a universal serial bus (USB) interface or the like. However, the various embodiments would further include thermocouple excitation means and a pulse wave modulator (PWM). 
         [0021]      FIG. 3  is a block schematic of a thermocouple system  350  in accordance with an embodiment of the invention. The thermocouple system includes a measuring junction  306  and reference junction  308 . The measuring junction  306  includes an excitation element  322  in accordance with an embodiment of the invention. The excitation element  322  is coupled to receive an AC stimulation signal from PWM  354  through excitation circuitry  352 . A cold junction compensator  356  and signal conditioner circuit  358  are coupled to receive the detected DC signal from the measuring junction  306 . An A/D converter  360  is coupled to receive the compensated and conditioned signal and provide a digital signal representative of the expected temperature of the measuring junction  306  to the processor  362 . Interface (I/F)  364  is coupled to the processor  362  to provide input/output (I/O) capabilities to receive commands at the processor  362  to perform various validation methods in accordance with the embodiments, and to provide data output of the detected temperature and of detected health of the system  350 . Power section  366  may provide power to the various elements of the system  350 . Alternatively, power may be received through the I/F  364 . 
         [0022]    A memory  368  may be included to store historical data on rise and/or decay times of the DC signal of the measuring junction  306  during validation. Preferably, the memory  368  is a non-volatile memory, such as flash memory or EEPROM (electrically erasable programmable read-only memory), so that historical data is retained in case of a power failure. 
         [0023]    During operation of a self-validating thermocouple in accordance with the various embodiments, the following occurs. 
         [0024]    Temperature measurement: The A/D converter measures the very small (μV to mV) voltage of the thermocouple and the cold junction compensators. Since the output voltage of the thermocouple is between μV and mV, it is generally necessary to use the internal gain of the A/D converter. The A/D converter also monitors the output of the cold junction compensator. Depending on the type of thermocouple used, the processor compensates the thermocouple output to obtain an accurate reading as is well understood in the art. The temperature may be calculated by using the following equation: Ttip=A 0 +A 1 Vout+A 2 Vout 2 + . . . +AnVout n . Alternatively, the temperature could be generated from a look-up table. Software in processor  362  can assist the user to operate in learning mode to automatically gather historical data of the thermocouple system during operation (monitoring and diagnostic mode). The user can also manually enter historical data. 
         [0025]    Thermocouple Validation: To observe if the thermocouple is short or open, each differential line of the thermocouple is measured as being single ended to estimate the common mode. The leakage resistance of the capacitors of the AC-coupled PWM will either pull high or low any lead as the result of an open circuit. This condition can be detected by the processor, which then flags the condition as one of the failure modes. The thermocouple is slightly biased to have a common mode offset, which will change in the case of a short circuit. This condition can also be detected by the processor and flagged as another failure mode. 
         [0026]    Bonding/Debonding Detection: Debonding of the thermocouple is evaluated based on a departure from a known thermal transfer function of the bonded system. When debonding occurs, the reduction in thermal mass translates into a different temperature rate of change, resulting in different rise and decay times. The processor sends a PWM excitation signal for the length of time needed to heat up the thermocouple. The difference in temperature (d[temp]/dt) and the time it takes to return to the original temperature before the excitation of the thermocouple indicates the health of the thermocouple and whether the thermocouple is bonded or debonded. For example, the thermocouple in a bonded condition will have faster decay in temperature, and thus detected DC voltage, than if it were in an unbonded condition. In addition, historical values of the rise and decay times can be compared with current values to indicate degradation of the thermocouple. 
         [0027]    An operator may commence operation by selecting to start a diagnosis/monitoring sequence, wherein the PWM is used to estimate the time constants corresponding to the correct configuration. The user has the further option of using previous diagnostic values, which are stored in memory and readily available upon each commencement of operation. 
         [0028]      FIG. 4  is a flowchart of a method of validation in accordance with one embodiment of the invention. The method of  FIG. 4  may be initiated by an operator request, or the processor of the thermocouple system may be configured to periodically initiate the validation method, such as daily, weekly, or monthly. At  480 , an AC excitation signal is applied to the thermocouple. At  482 , the rise time and/or decay time of the DC signal of the thermocouple are observed. A thermocouple that is bonded to an object of interest, i.e., the object whose temperature is desired to be measured, will exhibit differing rise and decay times of its DC signal during and after, respectively, AC excitation. Optionally, the rise and/or decay times can be compared to historical data at  484 . Historical comparisons can be especially useful in detecting degradation of the thermocouple measuring junction where trends in the times can be observed. Values that are trending in one direction or the other, as opposed to random variation, can be indicative of degradation of the thermocouple. This failure mode may be used to indicate a need for calibration, repair, or replacement. 
         [0029]    If the raw observations for rise and/or decay times at  482 , of the trend observations at  484 , indicate a failure at  486 , the resulting failure mode may be transmitted to the user or host system at  488 . If no failure is indicated at  486 , the validation may end at  490 . 
         [0030]    The Self-Validating Thermocouple (SVT) System in accordance with the various embodiments not only facilitate detection of open or short faults, but also facilitates identification of degradation of the thermocouple as well as its bonded or debonded state. The SVT system may provide signal conditioning and data acquisition capability in-situ to each thermocouple. It is capable of interfacing and processing signals from the most commonly used thermocouple types (J, K, E, and T) as well as other thermocouple types. The SVT can periodically evaluate the health of the thermocouple and the measurement capability. The circuit is capable of detecting failures and notifying the user/operator of the failure mode. The SVT may automatically provide a stream of data to be analyzed, or the SVT may respond to individual requests at any time, i.e., on demand. 
         [0031]    SVTs in accordance with the various embodiments will be valuable for anyone using thermocouples as temperature sensors that require highly reliable measurements. The invention could allow elimination of the need for redundant thermocouple measurements which, in turn, translates into savings in operating and maintenance costs. Finally, the present invention facilitates increased failure detection capabilities as well as improved dating validity and reliability. 
         [0032]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the embodiments shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.