Abstract:
A preexisting voltage across a sensor is latched to a storage capacitor prior to any excitation current being applied to the sensor. Once the excitation current is applied, the voltage on the storage capacitor is directly subtracted from a differential voltage across the sensor. The subtraction is done before a measurement is converted to a digital value and passed to a transmitter. The subtraction is performed in hardware, and a time required to sample and hold the preexisting voltage across the storage capacitor is within a settling time used for collecting any sensor measurements.

Description:
BACKGROUND 
       [0001]    The present invention relates to process variable transmitters used in process control and monitoring systems. More specifically, the present invention relates to monitoring EMF voltage across a sensor. 
         [0002]    Process variable transmitters are used to measure process parameters in a process control or monitoring system. Microprocessor-based transmitters often include a sensor, an analog-to-digital (A/D) converter for converting an output from the sensor into a digital form, a microprocessor for compensating the digitized output, and an output circuit for transmitting the compensated output. Currently, this transmission is normally done over a process control loop, such as a 4-20 milliamp control loop, or wirelessly. 
         [0003]    One exemplary parameter that is measured by such a system is temperature. Temperature is sensed by measuring the resistance of a restive temperature device (RTD), which is also sometimes called a platinum resistance thermometer (or PRT), or the voltage output from a thermocouple. Of course, these types of temperature sensors are only exemplary and others can be used as well. Similarly, temperature is only one exemplary process variable and a wide variety of other process control parameters can be measured as well, including, for example, pH, pressure, flow, etc. Therefore, while the present discussion proceeds with respect to a temperature sensor, it will be appreciated that the discussion could just as easily proceed with respect to sensing of other parameters. 
         [0004]    There are a number of connection points between a temperature sensor and a measurement transmitter, that can fail or become degraded. When the connection points or measurement lines have elevated levels of resistance, small currents can be induced on these connection points or lines that impact the sensor measurement accuracy. The sensor connection points and measurement lines can become degraded, and thus exhibit these elevated levels of resistance, due to wire fraying, corrosion, or the connections can just become loose. In any of these cases, it is possible that small voltages across the temperature sensor or in the measurement loop can begin to form, and can be sensitive to temperature changes. These voltages can create measurement inaccuracies. 
         [0005]    As one specific example, a resistive temperature detector (or RTD) ohmic measurement is generated by using up to six individual voltage points on a ratiometric calculation. All of these measurements take approximately 60 milliseconds to collect. In a typical equation for an RTD calculation, one term that can be important, and that feeds into the RTD calculation, provides significant levels of accuracy in the final output of the transmitter. This term is the residual voltage that preexists the measurement on the measurement lines, and is referred to as V emf . 
         [0006]    In order to obtain the value of V emf , up to two 60 millisecond voltage measurements are taken per sensor, when no excitation current is induced across the RTD. These measurements represent thermal voltages that can be induced on the sensor wires due to fraying, corrosion, or loose connections, among other things. These measurements can be subtracted in software from the voltage drop measured across the RTD when the excitation current is present. U.S. Pat. No. 6,356,191 is directed to this process, and the process works quite well. 
         [0007]    However, it takes time to collect the measurements for V emf . For instance, in one conventional system, the time to collect the V emf  measurements is approximately 120 milliseconds per sensor. This can negatively affect the update rate on a temperature transmitter. 
       SUMMARY 
       [0008]    A differential voltage is used to measure a parameter of a sensor which is related to a process variable. Prior to measuring the differential voltage, a preexisting voltage across a sensor is latched to a storage capacitor. The latched voltage on the storage capacitor is subtracted from the differential voltage across the sensor. The subtraction is done before a measurement is converted to a digital value. This reduces inaccuracies in the measurement of the differential voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a simplified diagram showing an industrial process control system including a temperature sensor that senses a temperature of a process fluid. 
           [0010]      FIG. 2  is a block diagram illustrating the transmitter of  FIG. 1  in more detail. 
           [0011]      FIGS. 3A and 3B  are exemplary schematic and partial block diagrams showing a transmitter with an EMF detection component in more detail. 
           [0012]      FIG. 4  is a flow diagram showing one example of the operation of the circuit shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  is a simplified diagram of an industrial process control system  5 . In  FIG. 1 , process piping  7  carries a process fluid. A process variable transmitter  10  is configured to couple to the process piping  7 . Transmitter  10  includes a process variable sensor  18  which, in one embodiment, comprises a resistive temperature device or other temperature sensor. However, this is exemplary only and sensor  18  could be any of a wide variety of other sensors, including a flow sensor, a pH sensor, a pressure sensor, etc. 
         [0014]    Transmitter  10  transmits information to a remote location, such as a process control room  6 . The transmission can be over a process control loop such as a two-wire control loop  11 . The process control loop can be in accordance with any desired format, including, for example, a 4-20 milliamp process control loop, a process control loop which carries digital communications, a wireless process control loop, etc. In the example shown in  FIG. 1 , the process control loop  11  is powered by a power supply  6 A at control room  6 . This power is used to provide power to the process variable transmitter  10 . Sense resistor  6 B can be used to sense the current flowing through loop  11 , although other mechanisms can be used as well. 
         [0015]      FIG. 2  is a block diagram of a portion of industrial process control system  5 , shown in  FIG. 1 , and transmitter  10  is shown in greater detail. In  FIG. 2 , sensor  18  is illustratively a process variable transmitter that receives input  14  from a process being sensed. The input is illustratively the process fluid flowing through piping  7 , and sensor  18  is illustratively a temperature sensor, such as a resistive temperature device. Sensor  18  illustratively provides an analog output  20 , indicative of the sensed parameter (e.g., temperature), to A/D converter  22  in transmitter  10 . 
         [0016]    In one embodiment, it should be noted that the output  20  from sensor  18  can illustratively be provided to a circuit (not shown in  FIG. 2 ) that amplifies and filters the analog signal, as appropriate. This can be part of sensor  18 , or transmitter  10  or a separate circuit. In any case, the amplified and filtered signal  20  is then provided to A/D converter  22 . A/D converter  22  provides a digitized output to processor  24 , which is a digital representation of the analog signal  20  provided by sensor  18 . 
         [0017]    Processor  24  includes associated memory and clock circuitry and provides information regarding the sensed parameter over process control loop  11 . It should be noted that processor  24  can include an input/output (I/O circuit), or an I/O circuit can be provided separately, that transmits information in a digital format on loop  11 , or in an analog format by controlling current flow though loop  11 . Thus, the information related to the sensed parameter is provided over process control loop  11  by transmitter  10 . Processor  24  is shown in this embodiment as being separate from A/D converter  22 . However, it could be included in A/D converter  22 , or A/D converter  22  can have its own state machine or processor, separate from processor  24 , controlling other parts of A/D converter  22  and controlling EMF compensation as discussed below. The present description is provided by way of example only. 
         [0018]      FIG. 2  also shows that transmitter  10  includes current source  30  that is controlled by processor  24 . Current source  30  can provide excitation current (also referred to as a control signal), as needed, to sensor  18 . For instance, where sensor  18  is a resistive temperature device, current source  30  provides an excitation current across the resistive temperature device so that the voltage across the resistive temperature device can be used to provide the output  20  indicative of the sensed temperature of the fluid. 
         [0019]    The embodiment shown in  FIG. 2  also illustrates that A/D converter  22  includes EMF detection component  26 . EMF detection component  26  detects an EMF voltage preexisting on the sensor  18  prior to application of the excitation current. Component  26  can be either internal or external to A/D converter  22 . It is shown internal to A/D converter  22  in the example shown in  FIG. 2 , but this is by way of example only. EMF detection component  26  provides an output indicative of the level of the detected preexisting EMF voltage to processor  24 , and it is also configured to subtract the preexisting EMF voltage from the voltage in signal  20  when the excitation current is applied, in order to correct signal  20  for the preexisting EMF voltage. 
         [0020]      FIG. 3A  is a more detailed diagram of transmitter  10 , and similar items are numbered the same as in  FIG. 2 .  FIG. 3A  also specifically shows more detail for A/D converter  22  and EMF detection component  26 . In the embodiment shown in  FIG. 3A , A/D converter  22  illustratively includes differential amplifier  32  and a sigma delta converter  34 . Of course, sigma delta converter  34  is shown by way of example only and other conversion mechanisms can be used as well. 
         [0021]      FIG. 3A  also shows that EMF detection component  26  illustratively includes level detector  26 , switches S 1 , S 2  and S 3 , and capacitor C 1 .  FIG. 3A  shows that sensor  18  has two leads  38  and  40  which can be coupled to input terminals  42  and  44 , respectively. In one embodiment, the voltage across terminals  42  and  44  is indicative of the temperature sensed by sensor  18 , in addition to the EMF voltage represented by voltage source  46 . Sensor  18  can illustratively be a four lead sensor with two additional leads coupled to two additional terminals, respectively. This is shown in greater detail with respect to  FIG. 3B  described below. However, for the sake of the present example, the description will proceed with respect to sensor  18  having two leads connected to terminals  42  and  44 . 
         [0022]    A more detailed operation of EMF detection component  26  is described below with respect to  FIG. 4 . Briefly, however, the voltage across terminals  42  and  44  is first latched across capacitor C 1 , before the excitation current I rtd  is provided across sensor  18 . This effectively causes storage capacitor C 1  to store the preexisting voltage on sensor  18  (i.e., the EMF voltage  46 ). Then, switch S 2  is opened and switches S 1  and S 3  are closed. Processor  24  controls current source  30  to apply excitation current I rtd  across sensor  18  to develop a voltage across sensor  18  to take a temperature measurement. This circuit configuration operates to subtract the voltage on capacitor C 1  from the voltage across terminals  42  and  44 , before it is input to differential amplifier  32 . That is, the voltage difference at the input to differential amplifier  32  between terminal  42  and circuit node  60  has the EMF voltage  46  effectively removed from it because it was previously stored on capacitor C 1 . At the same time, level detector  36  detects the voltage level across capacitor C 1 , which is indicative of EMF voltage  46 . Level detector  36 , in one embodiment, is a comparator that compares the EMF voltage to one or more thresholds that can be set empirically or otherwise. If the EMF voltage exceeds any of the thresholds, detector  36  outputs an indication of this to processor  24 . Processor  24  can then determine whether the EMF voltage is excessive and requires further action. 
         [0023]    Therefore, differential amplifier  32  provides an output to sigma delta converter  34  that is indicative of the voltage across sensor  18 , but not EMF voltage  46 , because that has been subtracted from the voltage across terminals  42  and  44 . The output from level detector  36 , and the output from converter  34 , are provided to processor  24  for further processing. 
         [0024]      FIG. 4  illustrates the operation of transmitter  10  in more detail. The operation of transmitter  10  will now be described with respect to  FIGS. 2 ,  3 A and  4  in conjunction with one another. 
         [0025]    Processor  24  first provides a control signal to close switch S 2  and open switches S 1  and S 3 . This is indicated by block  80  in  FIG. 4 , and it is done before processor  24  controls current source  30  to apply the excitation current I rtd  across resistor  18 . Therefore, this has the effect of latching the preexisting EMF voltage across storage capacitor C 1 . This is indicated by block  82  in  FIG. 4 . 
         [0026]    Processor  24  then opens switch S 2  and closes switches S 1  and S 3 . This is indicated by block  84  in  FIG. 4 . Processor  24  then turns on current source  30  to apply excitation current I rtd  across sensor  18 . This is indicated by block  86  in  FIG. 4 . 
         [0027]    Therefore, the voltage across sensor  18 , along with the EMF voltage  46 , is applied across terminals  42  and  44 . Capacitor C 1  acts to subtract the EMF voltage from that input voltage, so that the voltage applied to the inputs of differential amplifier  32  (across terminal  42  and node  60 ) is substantially only the voltage across sensor  18 . Subtracting the EMF voltage from the sensor input voltage at the input of the differential amplifier  32  is indicated by block  88  in  FIG. 4 . 
         [0028]    The differential amplifier  32  then provides an input to sigma delta converter  34  that is indicative of a measurement of the voltage across sensor  18 . This is indicated by block  90  in  FIG. 4 . 
         [0029]    Converter  34  then digitizes the measurement signal output by differential amplifier  32  and provides a digital representation of the sensor voltage to processor  24 . This is indicated by block  92  in  FIG. 4 . 
         [0030]    Level detector  36  detects the voltage level across capacitor C 1 , which is substantially equivalent to EMF voltage  46 . As discussed above, this can be done by comparing the EMF voltage on capacitor C 1  to one or more thresholds. Of course, it can be digitized as well. Detecting the EMF voltage level is indicated by block  94  in  FIG. 4 . 
         [0031]    The EMF voltage level is provided to processor  24  so that processor  24  can determine whether the EMF voltage is high enough to indicate a warning condition, or other problem that the user should be made aware of. For instance, when the voltage level exceeds a predetermined threshold value, this can indicate undue wear or corrosion of leads  38  and  40 , or it can indicate a loose connection at one of terminals  42  and  44 , or it can indicate fraying or corrosion of wires used to connect sensor  18  to A/D converter  22 , or any of a wide variety of other conditions. For instance, there may also be thermocouple junctions at connection points that add to the EMF voltage when exposed to thermal gradients. This will be captured in the EMF voltage  46  as well. The particular voltage threshold can be set empirically or otherwise, and more than one can be set as well. In one embodiment, it is set to approximately +/−12 mV, although any other desired voltage level can be used as well. Detecting whether the EMF voltage level is excessive is indicated by block  96  in  FIG. 4 . 
         [0032]    If processor  24  determines that the EMF voltage level is excessive, it generates an excessive EMF indicator, that can be detectable by the user, so the user knows of the condition. This is indicated by block  98  in  FIG. 4 . In one embodiment, for instance, processor  24  simply sets a status bit to indicate that the EMF voltage is excessive, and that information is transmitted to control room  6  using loop  11 . Of course, other types of indicators can be used as well. 
         [0033]    Again, it will be noted that while  FIG. 3A  shows that sensor  18  is only connected to terminals  42  and  44 , this is exemplary only.  FIG. 3B  shows an embodiment in which sensor  18  is a four lead sensor with additional leads  48  and  50  coupled to terminals  52  and  54 , respectively. The excitation current I rtd  is applied from current source  30  at terminal  52  and lead  50  to sensor  18 . Processor  24  controls multiplexor  53 , which receives inputs from terminals  42 ,  44 ,  52  and  54  so the desired voltages are input to differential amplifier  32  and EMF detection component  26 . The voltage drop in the connections to terminals  42  and  44  can largely be eliminated because substantially all of the excitation current I rtd  flows between terminals  52  and  54  and across resistor R 1 . This improves accuracy. However, there still may be undesired preexisting voltage  46  in the circuit, and this can be detected and compensated for as set out above with respect to  FIGS. 3A and 4 . 
         [0034]    In an embodiment where a thermocouple (or other voltage sensor) is used, there will be a voltage from the sensor, but resistance will also be present on the sensor loop due to the high resistivity of the wires and junctions connecting to the sensor. When this resistance changes, it can indicate some type of degrading condition such as those mentioned above in the other embodiment. In such an embodiment, the resistance on the sensor loop can be measured by applying an excitation current in the same way as the resistance of an RTD is sensed. The thermocouple (or other sensor) voltage is compensated for (like the preexisting EMF voltage  46  discussed above) to obtain a measure of the loop resistance. This can be done intermittently to monitor the loop resistance. 
         [0035]    It can thus be seen that the present system automatically compensates for residual (preexisting) EMF voltage that exists on any of the measurement lines or terminals in the system. While it is described with respect to a resistive temperature device, it can be applied to thermocouples as well in order to measure their loop resistance. Of course, it can be applied to other sensors to sense other parameters and temperature is described by way of example only. It can also be seen that the compensation is done in hardware, very quickly, prior to digitizing the sensor measurement. Thus, it can be performed well within the normal settling time of a measurement circuit. 
         [0036]    Although the present invention has been described with reference to preferred embodiments, 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.