Abstract:
A two-wire temperature transmitter performs thermocouple diagnostics on a thermocouple attached to the transmitter to determine if, and the extent to which, the thermocouple has degraded. Various methods of obtaining thermocouple resistance are also provided.

Description:
BACKGROUND OF THE INVENTION 
     The process industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process fluid temperature transmitter provides an output related to a sensed process substance temperature. The temperature transmitter output can be communicated over a process control loop to a control room, or the output can be communicated to another process device such that the process can be monitored and controlled. In order to monitor a process fluid temperature, the transmitter includes a temperature sensor, such as a thermocouple. 
     A thermocouple is fabricated by joining two dissimilar metals, such as bismuth and antimony. The junction of the two dissimilar metals produces a small voltage that is related to its temperature. This is known as the Seebeck effect. Process fluid temperature transmitters that employ thermocouple sensors, thus measure the small voltage of the thermocouple, and then calculate process fluid temperature based upon the thermocouple voltage. Although a thermocouple&#39;s primary variable of interest is its voltage (indicative of temperature) it is generally known that the thermocouple&#39;s resistance is indicative of its condition. As thermocouples age, or otherwise degrade, thermocouple resistance changes. Thus, thermocouple resistance measurement can be used to evaluate the condition of the thermocouple. In order to measure the resistance, a test current is generally passed through the thermocouple, and the resulting voltage is measured and used to calculate the resistance. 
     In two-wire process control installations, process measurement devices, such as temperature transmitters can receive all required electrical power through the same two wires that are used for data communication. Generally, the amount of power available on the loop is limited in order to facilitate compliance with intrinsic safety requirements. Typically, the loop current varies between 4 and 20 mA to indicate a process variable. Thus, a device powered by the loop must be operable on 4 mA or less. Such minimal electrical power generally limits the computational capacity of a given process device, as well as the amount of power that can be used for diagnostics. Thus, there is a tradeoff between the convenience of two-wire temperature transmitters, and the ability to provide suitable amounts of diagnostic current through a thermocouple to achieve accurate diagnostic information. 
     As process control becomes more accurate, there is an increasing need to provide process devices that not only provide process variables, but also indicate their own health. By providing enhanced process device diagnostics, process variable information can be relied upon to a greater or lesser extent, depending upon the state of the process device. Providing such devices will enhance process control and potentially increase the efficiency of predictive maintenance. 
     SUMMARY 
     A two-wire temperature transmitter performs thermocouple diagnostics on a thermocouple attached to the transmitter to determine if, and the extent to which, the thermocouple has degraded. The transmitter passes a diagnostic current through a thermocouple to obtain the resistance of the thermocouple. The resistance is then used to calculate a diagnostic output that is related to thermocouple degradation. Various methods of obtaining thermocouple resistance are also provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of the environment of a process fluid temperature transmitter. 
     FIG. 2 is a diagrammatic view of process fluid temperature transmitter  12 . 
     FIG. 3 is a system block diagram of a process fluid temperature transmitter. 
     FIG. 4 is a system block diagram of a process fluid temperature transmitter. 
     FIG. 5 is a system block diagram of a process fluid temperature transmitter. 
     FIG. 6 is a schematic representation of a portion of the transmitter shown in FIG.  5 . 
     FIG. 7 is a block diagram of a method of measuring thermocouple degradation with a two-wire temperature transmitter. 
    
    
     DETAILED DESCRIPTION 
     Although the present invention will be described with reference to embodiments of two-wire process fluid temperature transmitters, and the manner in which thermocouple degradation is assessed, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention, which are defined by the appended claims. 
     FIGS. 1 and 2 illustrate an environment of a two-wire process fluid temperature transmitter in accordance with embodiments of the invention. FIG. 1 shows process fluid control system  10  including process fluid temperature transmitter  12 . FIG. 2 illustrates process control system  10  including process fluid temperature transmitter  12  electrically coupled to control room  14  (modeled as a voltage source and resistance) over a two-wire process control loop  16 . Transmitter  12  is mounted on and coupled to a process fluid container such as pipe  18 . Transmitter  12  monitors the temperature of process fluid in process pipe  18  and transmits temperature information to control room  14  over loop  16 . Transmitter  12  is couplable to loop  16  through terminals  17  (shown in FIG.  3 ). 
     FIG. 3 is a system block diagram of process fluid transmitter  12  in accordance with an embodiment of the invention. Transmitter  12  includes power module  20 , loop communicator  22 , thermocouple input  24 , measurement circuitry  26 , current source  28 , and controller  30 . Transmitter  12  is couplable to thermocouple  32  (modeled as a voltage source) such that transmitter  12  can obtain a voltage measurement from thermocouple  32 , and relate the measurement to a calculated process fluid temperature. Transmitter  12  then provides the calculated process fluid temperature on two-wire process control loop  16 . 
     Power module  20  is disposed within transmitter  12 , and is couplable to two-wire process control loop  16 . Module  20  suitably conditions power received from loop  16  for the various components of transmitter  12 . Utilizing power module  20 , transmitter  12  is able to operate solely upon power received from process control loop  16 . Module  20  can comprise, for example, known electronics such as a DC-DC power regulation device. On loop  16 , which in some embodiments employs analog signaling between 4 and 20 mA, module  20  operates to condition four or less milliamps for provision to other components within transmitter  12 . Additionally, module  20  can be adapted to prevent electrical noise received from loop  16  to reach the other components. 
     Loop communicator  22  is couplable to two-wire process control loop  16 , and is configured to communicate over loop  16 . Communicator  22  can be of the type generally known in the art. For example, communicator  22  can be suitably selected to provide analog communication, digital communication, or a combination of the two. One such combination of analog and digital communication is known as the Highway Addressable Remote Transducer (HART®) protocol. One version of the HART® protocol superimposes a digital signal upon a 4-20 mA analog signal. With such a protocol, the primary variable of interest can be provided in one mode, such as the analog mode, while a diagnostic signal is provided in the other mode. However, the present invention can be practiced with purely analog communications, as well as purely digital communications (such as provided by FOUNDATION™ Fieldbus). 
     Transmitter  12  also includes thermocouple input  24 . Input  24  provides a removable electrical coupling to thermocouple  32 . Additionally, input  24  can, if desired, be configured to accommodate a second thermocouple to allow transmitter  12  to provide cold junction compensation. Further, the actual temperature of input  24  can be sensed, in any known manner, to provide cold junction compensation through known mathematical algorithms. Thermocouple  32  can be any appropriate thermocouple, such as Type J or Type K, or the like. As will be described in more detail later in the specification, the resistance of thermocouple  32  is sensed to provide an indication of thermocouple viability. However, since the thermocouple&#39;s primary variable of interest is its voltage, and since virtually no current flows through the thermocouple circuit during voltage sensing, thermocouples typically utilize only two wires. However, to provide more accurate resistance measurements, it is contemplated that four-wire thermocouples could be used, in which case input  24  is suitably adapted to receive the four wires and create a Kelvin connection. 
     Measurement circuitry  26  is disposed within transmitter  12 , and is adapted to measure a voltage across thermocouple  32 . Circuitry  26  can be any circuitry capable of providing a suitable electrical indication of thermocouple voltage. In one embodiment, circuitry  26  comprises a known analog to digital converter. Circuitry  26  is coupled to input  24 , power module  20  and controller  30 . Circuitry  26  provides an output to controller  30 , typically in digital form, that is indicative of a voltage sensed across thermocouple  32 . 
     Current source  28  is coupled to input  24 , power module  20 , and controller  30 . Current source  28  can be any suitable circuitry capable of passing a known diagnostic current through a thermocouple connected to input  24 . Diagnostic currents as low as one microamp can be used. For example, source  28  can be a precision semiconductor current device, or the like. Source  28  can be adapted to pass direct current (DC) or alternating current (AC) through thermocouple  32 . Additionally, source  28  can be circuitry that provides an unknown current through a known resistance, such that the current can be measured, optionally with measurement circuitry  26 . During a diagnostic mode, source  28  passes a diagnostic current through thermocouple  32 . The diagnostic current can be passed in either direction through thermocouple  32 , and can also be alternately passed through thermocouple  32  in opposite directions. While the diagnostic current passes through thermocouple  32 , measurement circuitry  26  provides a signal to controller  30  that is related to the voltage across thermocouple  32 , and thus is related to the resistance of thermocouple  32 . As will be described later, the voltage measured during the diagnostic mode can be compensated to reduce or eliminate the voltage component due to the Seebeck effect, thus providing a diagnostic signal that is indicative substantially solely of thermocouple resistance. 
     Controller  30  is disposed within transmitter  12 , and is coupled to power module  20 , loop communicator  22 , measurement circuitry  26 , and current source  28 . Controller  30  can be any suitable circuitry capable of relating voltage information received from measurement circuitry  26  to process fluid temperature, and capable of providing thermocouple diagnostics. Specifically, controller  30  can be a microprocessor or the like. During normal operation, current source  28  does not pass any current through thermocouple  32 , and thus the signal received from measurement circuitry  26  is indicative solely of thermocouple voltage. Controller  30  relates the information received from measurement circuitry  26  to process fluid temperature through suitable equations or a look-up table. Controller  30  then passes process variable output information to loop communicator  22 , such that the process variable is communicated over two-wire process control loop  16 . 
     During the diagnostic mode, controller  30  commands current source  28  to pass the diagnostic current through thermocouple  32 . In some embodiments, the diagnostic current can be alternately passed in opposite directions, and voltage information received from measurement circuitry  26  (indicative of voltage across the thermocouple in each direction) can be used to calculate thermocouple resistance independent of the Seebeck voltage. In other embodiments, the Seebeck voltage can simply be subtracted from the voltage measured while the diagnostic current passed through the thermocouple. Various other techniques for eliminating the Seebeck voltage from diagnostic measurements are set forth below. 
     Controller  30  is adapted to relate thermocouple resistance to a diagnostic output. Such relation is typically in the form of a comparison of present thermocouple resistance to initial thermocouple resistance (measured during the commissioning of transmitter  12 ). However, the relation can also be in the form of a comparison with a pre-selected threshold resistance, or comparison with a running long-term average. Additionally, the long-term average can be used by controller  30  for trend analysis to provide lifetime estimation. In embodiments where various diagnostic measurements are stored, controller  30  can utilize optional memory  34  for such storage. 
     The diagnostic output is provided to loop communicator  22  for communication across loop  16 . The diagnostic output can take many forms. The output can simply be an alarm indicating thermocouple failure, or impending failure. However, the output can also be in the form of a lifetime estimation indicating an estimated time at which the thermocouple output will no longer suitably indicate process fluid temperature. 
     In addition to providing the diagnostic output, controller  30  can be adapted to utilize knowledge of the degradation condition of thermocouple  32  while providing the process variable output. Such adaptation can be in the form of hardware, software or a combination of both. In this manner, as thermocouple  32  degrades, and the relationship between thermocouple voltage and process fluid temperature changes, controller  30  can compensate, to some extent, for the degradation when providing the process variable output. The relationship between degradation, process fluid temperature, and voltage can be determined experimentally and provided to controller  30  in the form of compensation equations, or look-up tables. For example, if the input impedance of the measurement circuitry is known, and thermocouple resistance is measured as discussed above, then measurement error caused by voltage divider action between the input impedance and the thermocouple resistance can be calculated and used to compensate the actual Seebeck voltage. 
     FIG. 4 is a system block diagram of transmitter  40  in accordance with another embodiment of the invention. Transmitter  40  bears many similarities to transmitter  12 , and like components are numbered similarly. Transmitter  40  differs from transmitter  12  in that transmitter  40  includes thermocouple  32 . Since thermocouple  32  is disposed within transmitter  40 , an input, such as input  24 , is not included. Instead, thermocouple  32  is coupled directly to measurement circuitry  26  and current source  28 . Although single lines are used to denote such coupling, such lines are provided for clarity and can, in fact, comprise multiple conductors. 
     FIG. 5 is a system block diagram of transmitter  50  in accordance with another embodiment of the invention. Transmitter  50  is similar to transmitter  12  and like components are numbered similarly. The main difference between transmitter  12  and transmitter  50  is that transmitter  50  does not include a current source, but instead includes known resistance load  52 . Load  52  is coupled to controller  30 , and is selectively shunts the thermocouple circuit in response to a control signal received from controller  30 . A schematic illustration of load  52  in the thermocouple circuit is shown in FIG.  6 . In embodiments where measurement circuitry  26  is suitably accurate, and has an appropriate input impedance, use of load  52  can provide diagnostics without necessarily passing the a diagnostic current through the thermocouple. Since load  52  is of known resistance, the effect of load  52  shunting the thermocouple circuit is used to provide an indication of thermocouple resistance. 
     FIG. 7 is a system block diagram of a method  60  of measuring thermocouple degradation in a two-wire temperature transmitter. The method begins at block  62  where the two-wire transmitter obtains an initial resistance of a thermocouple, such as thermocouple  32 . Transmitter  60  can obtain the initial resistance in various ways. For example, the initial resistance can be measured by the transmitter during commissioning. Alternatively, the initial resistance value can be sent to the transmitter through the two-wire process control loop, after the resistance is measured elsewhere (such as at the thermocouple manufacturer). 
     At block  64 , a subsequent thermocouple resistance is measured. Such measurement is effected in the manner described above. Optionally, effects of the Seebeck voltage can be removed or reduced from the subsequent resistance measurement to enhance accuracy. Such compensation can be done by reversing the direction that diagnostic current passes through the thermocouple and measuring the average absolute value of the resultant voltage for each current direction. The compensation can also be done by simply subtracting the Seebeck voltage from the voltage measured while the diagnostic current passed through the thermocouple. Additionally, the compensation can also be done by ensuring that the diagnostic current creates a voltage drop across the thermocouple that is significantly larger that the Seebeck voltage, thus reducing the effect of the Seebeck voltage. For example, diagnostic current as high as one millamp or more can be used. 
     At block  66 , a diagnostic output is generated that is related to a comparison between the initial thermocouple resistance and the subsequent resistance. The diagnostic output can be in any of the various forms given above. After the diagnostic output has been generated it can optionally be transmitted across a two-wire process control loop. Block  66  can optionally pass control to block  64  such that multiple iterations are provided. The iterations can occur after a pre-selected delay, such as every ½ hour. The delay can also be random, or it can be a function of the last known thermocouple degradation value. Thus, as the thermocouple degrades, diagnostic method  32  can be invoked more frequently. Additionally, method  62  can also be invoked by a suitable command received by the transmitter (either locally, or through process control loop  16 ) to enter the diagnostic mode. 
     Although the invention has been described with reference to specific modules and functional blocks, such description is for clarity. It is contemplated that any or all of the various blocks can be combined, such as in an Application Specific Integrated Circuit (ASIC).