Abstract:
Methods and apparatus to detect leakage current in a resistance temperature detector are disclosed. An example method includes providing a resistance temperature detector circuit with a first resistance circuit and a second resistance circuit, measuring a first voltage at the first resistance circuit in response to applying a first current to the first resistance circuit, measuring a second voltage at the second resistance in response to applying a second current to the second resistance circuit, comparing the first and second voltages to determine a difference value, and determining that a current leak exists in the resistance temperature detector circuit when the difference value is not within a first range.

Description:
RELATED APPLICATIONS 
     This patent claims priority to U.S. Provisional Application No. 61/643,516, filed on May 7, 2012, the entirety of which is hereby incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure relates generally to temperature detection and, more particularly, to methods and apparatus to detect leakage current in a resistance temperature detector. 
     BACKGROUND 
     In a process control system, when calculating the flow of gas in a pipeline using an orifice plate method, it is important to have an accurate temperature measurement for use in calculation. RTD (resistance temperature detector) circuits are used to accurately determine temperature. 
     SUMMARY 
     An example method includes providing a resistance temperature detector circuit with a first resistance and a second resistance, measuring a first voltage across the first resistance in response to applying a current to the first resistance, measuring a second voltage across the second resistance in response to applying a second current to the second resistance, comparing the first and second voltages to determine a difference value, and determining that a current leak exists in the resistance temperature detector circuit when the difference value is not within a first range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a known 4-wire resistance temperature detector. 
         FIG. 2  is a circuit diagram of an example resistance temperature detector constructed in accordance with the teachings of this disclosure. 
         FIG. 3  is a flowchart illustrating an example method to detect leakage current in a resistance temperature detector circuit. 
         FIG. 4  is a flowchart illustrating another example method to detect leakage current in a resistance temperature detector circuit. 
         FIG. 5  is a block diagram of an example processor system that may be used to implement the example detector of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Although the following discloses example systems including, among other components, software and/or firmware executed on hardware, it should be noted that such systems are merely illustrative and should not be considered as limiting. For example, it is contemplated that any or all of these hardware, software, and firmware components could be embodied exclusively in hardware, exclusively in software, or in any combination of hardware and software. Accordingly, while the following describes example systems, persons of ordinary skill in the art will readily appreciate that the examples provided are not the only way to implement such systems. 
     The accuracy of RTD circuits may be compromised by electrical current leakage into or out of the circuit measuring the resistance. In such cases, the measurements are inaccurate and, thus, the calculated temperature is inaccurate. A change in temperature by one degree Celsius can result in a 0.5% error in gas flow calculated. Some applications require temperature measurement accuracies equal to or better than one degree Celsius. For example, custody transfer stations are a prime application for this kind of accuracy requirement. 
     If an RTD wire is shorted and/or there is water in the wiring conduit, known approaches may only indicate a change in resistance, and may not indicate a failure until an output is off-scale (e.g., there is a gross error). However, example methods and apparatus disclosed herein detect leakages as small as one microampere (μA), which represents an error of approximately 0.04%. Thus, example methods and apparatus described below reduce or prevent errant calculations of gas flow before the errors become significant. Example methods and apparatus disclosed herein may also be used to detect water present on the circuitry or connections. The example methods and apparatus may also enable errant leakage current to be measured and, thus, to be used to correct a faulty gas flow measurement in real time (e.g., without physical correction). 
       FIG. 1  is a circuit diagram of a known 4-wire resistance temperature detector (RTD) circuit  100 . The RTD circuit  100  includes a resistor  102  having temperature-variable resistance. The resistor  102  is placed into an environment  104  to be measured, and the resistor  102  assumes substantially the same temperature as the environment. A current source  106  generates a current through the resistor  102  (e.g., via resistors  108 ,  110 ). A voltage across the resistor  102  may then be measured (e.g., via resistors  112 ,  114 ) to determine the resistance of the resistor  102  and, thus, the temperature of the resistor  102  and the environment  104 . 
       FIG. 2  is a circuit diagram of an example resistance temperature detector circuit  200  to detect leakage current. In contrast to the known RTD circuit  100  of  FIG. 1 , the example RTD circuit  200  of  FIG. 2  may be used to identify current leaks into or out of the circuit (e.g., in a process control environment). 
     The example RTD circuit  200  of  FIG. 2  includes a resistor  202  located in an environment  204  to be measured. The example RTD circuit  200  further includes a first sense resistor  206 , a second sense resistor  208 , and a comparator  210 . To monitor both sides of the example resistor  202 , the example sense resistors  206 ,  208  are in circuit with the resistor  202  on opposite ends of the resistor  202 . The example comparator  210  measures the voltage across the first sense resistor  206  in response to a known current (e.g., from current sources  212 ,  214 ) flowing through the resistor  206 . The comparator  210  also measures the voltage across the second sense resistor  208  in response to the same known current flowing through the second sense resistor  208 . 
     To take the measurements, example switches  216  and  218  (and test current switches  220  and  222 ) are closed to cause a test current to flow through the example first sense resistor  206 . The example comparator  210  measures the output via an amplifier  224 . The example switches  216  and  218  are then opened and switches  226  and  228  are closed to cause a test current to flow through the example second sense resistor  208 . The comparator measures the output via the amplifier  224 . The comparator  210  may then compare the measurements. 
     After taking the measurements, the example comparator  210  compares the measurements to determine whether a difference between the measurements is within an expected range (e.g., whether the measurements are substantially equal). For example, the first and second sense resistors  206 ,  208  may be high-precision resistors having the same target (e.g., nominal) resistance value. In that case, if the currents flowing through the first and second sense resistors  206 ,  208  are equal or substantially equal, the measurements taken by the comparator  210  should have a difference not greater than a threshold corresponding to the potential compound error in the resistance values and/or the applied current(s). 
     In some other examples, the first and second sense resistors  206 ,  208  may be high precision resistors having different target (e.g., nominal) resistance values. In such examples, the comparator  210  determines whether the difference in the measurement is within a range of an expected difference. The range may be based on, for example, the potential compound error in the resistance values and/or the applied current(s). 
     If the comparator  210  determines that the difference between the measurements is not within an expected range (or is greater than a threshold), the example comparator  210  outputs an alert (e.g., a flag) signaling the presence of a potential electrical shorting or leakage condition in the RTD circuit  200 . In some examples, the comparator  210  controls the switches  216 - 222 ,  226 , and  228 , the amplifier  224 , and/or the current sources  212 ,  214 . 
     The example comparator  210 , the example switches  216 - 222 ,  226 , and  228 , and the example amplifier  224  of  FIG. 2  may be implemented by hardware, software, firmware and/or any combination of hardware, software and/or firmware. Thus, for example, the example comparator  210 , the example switches  216 - 222 ,  226 , and  228 , and/or the example amplifier  224  of  FIG. 2  could be implemented by one or more circuit(s), programmable processor(s), application specific integrated circuit(s) (ASIC(s)), programmable logic device(s) (PLD(s)) and/or field programmable logic device(s) (FPLD(s)), etc. Further still, the example comparator  210 , the example switches  216 - 222 ,  226 , and  228 , and/or the example amplifier  224  may include one or more elements, processes and/or devices in addition to, or instead of, those illustrated in  FIG. 2 , and/or may include more than one of any or all of the illustrated elements, processes and devices. 
     A flowchart representative of an example method to implement any of the example comparator  210 , the example switches  216 - 222 ,  226 , and  228 , and/or the example amplifier  224  are shown in  FIGS. 3-4 . In this example, the method may be implemented using machine readable instructions comprising a program for execution by a processor such as the processor  512  shown in the example computer  500  discussed below in connection with  FIG. 5 . The program may be embodied in software stored on a tangible computer readable medium such as a computer readable storage medium (e.g., a CD-ROM, a floppy disk, a hard drive, a digital versatile disk (DVD), a Blu-ray disk, or a memory associated with the processor  512 ), but the entire program and/or parts thereof could alternatively be executed by a device other than the processor  512  and/or embodied in firmware or dedicated hardware. Further, although the example program is described with reference to the flowchart illustrated in  FIGS. 3-4 , many other methods of implementing the example comparator  210 , the example switches  216 - 222 ,  226 , and  228 , and/or the example amplifier  224  may alternatively be used. For example, the order of execution of the blocks may be changed, and/or some of the blocks described may be changed, eliminated, or combined. 
     As mentioned above, the example method of  FIGS. 3-4  may be implemented using coded instructions (e.g., computer readable instructions) stored on a tangible computer readable medium such as a hard disk drive, a flash memory, a read-only memory (ROM), a compact disk (CD), a digital versatile disk (DVD), a cache, a random-access memory (RAM) and/or any other storage media in which information is stored for any duration (e.g., for extended time periods, permanently, brief instances, for temporarily buffering, and/or for caching of the information). As used herein, the term tangible computer readable medium is expressly defined to include any type of computer readable storage and to exclude propagating signals. Additionally or alternatively, the example method of  FIGS. 3-4  may be implemented using coded instructions (e.g., computer readable instructions) stored on a non-transitory computer readable medium such as a hard disk drive, a flash memory, a read-only memory, a compact disk, a digital versatile disk, a cache, a random-access memory and/or any other storage media in which information is stored for any duration (e.g., for extended time periods, permanently, brief instances, for temporarily buffering, and/or for caching of the information). As used herein, the term non-transitory computer readable medium is expressly defined to include any type of computer readable medium and to exclude propagating signals. 
       FIG. 3  is a flowchart illustrating an example method  300  to detect leakage current in an RTD circuit. The example method  300  may be implemented by the comparator  210  of  FIG. 2  to detect leakage in the RTD circuit  200  of  FIG. 2  and/or by a user (e.g., a technician, an installer) of the RTD circuit  200 . The example method  300  may be used if, for example, substantially equal resistances are installed in the RTD circuit to implement the first and second resistances. 
     The example method  300  begins with providing a first resistance (e.g., the sense resistor  206  of  FIG. 2 ) in circuit with the RTD circuit  200  (e.g., in circuit with the resistor  202 ) (block  302 ). A second resistance (e.g., the sense resistor  208  of  FIG. 2 ) is also provided in circuit with the RTD circuit  200  (e.g., in circuit with the resistor  202 ) (block  304 ). In the example method  300 , the first and second resistances may be provided on opposite sides of the RTD circuit  200 . 
     A current is applied to the first resistance (e.g., the sense resistor  206 ) (block  306 ). The example comparator  210  measures a voltage drop across the first resistance (block  308 ). A current is applied to the second resistance (e.g., the sense resistor  208 ) (block  310 ). The example comparator  210  measures a voltage drop across the second resistance (block  312 ). 
     The example comparator  210  determines whether a difference between the first and second voltage drops is less than a threshold (block  314 ). If the difference is less than a threshold (block  314 ), the example comparator  210  determines that the RTD circuit  200  does not have current leakage (block  314 ). Conversely, if the difference between the voltage drops is not less than the threshold (block  314 ), the example comparator  210  determines that the RTD circuit  200  has a possible current leakage or other issue, and raises a flag or alert for maintenance (block  318 ). 
     After determining that the RTD circuit  200  does not have leakage (block  316 ) or does have leakage (block  318 ), the example method  300  of  FIG. 3  ends. In some examples, the comparator  210  proceeds to measure a temperature via the resistor  202  after determining in block  316  that the RTD circuit  200  does not have leakage. 
       FIG. 4  is a flowchart illustrating another example method  400  to detect leakage current in an RTD circuit. The example method  400  may be implemented by the comparator  210  of  FIG. 2  to detect leakage in the RTD circuit  200  of  FIG. 2  and/or by a user (e.g., a technician, an installer) of the RTD circuit  200 . The example method  400  may be used if, for example, different resistances are installed in the RTD circuit to implement the first and second resistances. 
     The example method  400  begins with providing a first resistance (e.g., the sense resistor  206  of  FIG. 2 ) in circuit with the RTD circuit  200  (e.g., in circuit with the resistor  202 ) (block  402 ). A second resistance (e.g., the sense resistor  208  of  FIG. 2 ) is also provided in circuit with the RTD circuit  200  (e.g., in circuit with the resistor  202 ) (block  404 ). In the example method  400 , the first and second resistances may be provided on opposite sides of the RTD circuit  200 . 
     A current is applied to the first resistance (e.g., the sense resistor  206 ) (block  406 ). The example comparator  210  measures a voltage drop across the first resistance (block  408 ). A current is applied to the second resistance (e.g., the sense resistor  208 ) (block  410 ). The example comparator  210  measures a voltage drop across the second resistance (block  412 ). 
     The example comparator  210  determines whether a difference between the first and second voltage drops is within a range (block  414 ). If the difference is within a range (block  414 ), the example comparator  210  determines that the RTD circuit  200  does not have current leakage (block  414 ). Conversely, if the difference between the voltage drops is not within the range (block  414 ), the example comparator  210  determines that the RTD circuit  200  has a possible current leakage or other issue, and raises a flag or alert for maintenance (block  418 ). 
     After determining that the RTD circuit  200  does not have leakage (block  416 ) or does have leakage (block  418 ), the example method  400  of  FIG. 4  ends. In some examples, the comparator  210  proceeds to measure a temperature via the resistor  202  after determining in block  416  that the RTD circuit  200  does not have leakage. 
       FIG. 5  is a block diagram of an example processor system  510  that may be used to implement the example comparator  210 , the example switches  216 - 222 ,  226 , and  228 , and/or the example current sources  212 ,  214  of  FIG. 2 . As shown in  FIG. 5 , the processor system  510  includes the processor  512  that is coupled to an interconnection bus  514 . The processor  512  includes a register set or register space  516 , which is depicted in  FIG. 5  as being entirely on-chip, but which could alternatively be located entirely or partially off-chip and directly coupled to the processor  512  via dedicated electrical connections and/or via the interconnection bus  514 . The processor  512  may be any suitable processor, processing unit or microprocessor. Although not shown in  FIG. 5 , the system  510  may be a multi-processor system and, thus, may include one or more additional processors that are identical or similar to the processor  512  and that are communicatively coupled to the interconnection bus  514 . 
     The processor  512  of  FIG. 5  is coupled to a chipset  518 , which includes a memory controller  520  and an input/output (I/O) controller  522 . As is well known, a chipset typically provides I/O and memory management functions as well as a plurality of general purpose and/or special purpose registers, timers, etc. that are accessible or used by one or more processors coupled to the chipset  518 . The memory controller  520  performs functions that enable the processor  512  (or processors if there are multiple processors) to access a system memory  524  and a mass storage memory  525 . 
     The system memory  524  may include any desired type of volatile and/or non-volatile memory such as, for example, static random access memory (SRAM), dynamic random access memory (DRAM), flash memory, read-only memory (ROM), etc. The mass storage memory  525  may include any desired type of mass storage device including hard disk drives, optical drives, tape storage devices, etc. 
     The I/O controller  522  performs functions that enable the processor  512  to communicate with peripheral input/output (I/O) devices  526  and  528  and a network interface  530  via an I/O bus  532 . The I/O devices  526  and  528  may be any desired type of I/O device such as, for example, a keyboard, a video display or monitor, a mouse, etc. The example switches  218 - 222 ,  226 , and/or  228  and/or the example current sources  212 ,  214  of  FIG. 2  may be implemented and/or controlled by the I/O devices  526  and  528 . The network interface  530  may be, for example, an Ethernet device, an asynchronous transfer mode (ATM) device, an 802.11 device, a DSL modem, a cable modem, a cellular modem, etc. that enables the processor system  510  to communicate with another processor system. 
     While the memory controller  520  and the I/O controller  522  are depicted in  FIG. 5  as separate functional blocks within the chipset  518 , the functions performed by these blocks may be integrated within a single semiconductor circuit or may be implemented using two or more separate integrated circuits. 
     Although certain example methods, apparatus and articles of manufacture have been disclosed herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the claims of this patent.