Abstract:
A process variable transmitter for measuring a temperature of a process includes a first, a second, third, and fourth terminal configured to couple to the temperature sensitive element. Measurement circuitry measures an electrical parameter between a pair of the terminals. A microprocess identifies a location of the temperature sensitive element coupled to at least two of the terminals based upon an electrical parameter measured by the measured circuitry between two terminals. In another configuration, the process variable transmitter measures temperature of a process using a thermocouple. A heating element is configured to heat terminals coupled to the thermocouple. A microprocessor determines polarity of the thermocoupled based upon a measured electrical parameter between the terminals in response to applied heat.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to process variable transmitters used in process control and monitoring systems. More specifically, the present invention relates to the termination of the configuration or orientation of a temperature sensor coupled to a process variable transmitter. 
         [0002]    Process control transmitters are used to measure process parameters in a process control system. Microprocessor-based transmitters include a sensor, an analog-to-digital converter for converting an output from a sensor into a digital format, a microprocessor for compensating the digitized output and an output circuit for transmitting the compensated output. Typically, this transmission is over a process control loop, such as a 4-20 mA current loop. One example parameter is temperature which is sensed by measuring the resistance of an RTD (Resistive Temperature Device), also called a PRT (Platinum Resistance Thermometer) sensor, or a voltage output of a thermocouple sensor. 
         [0003]    Temperature is measured by converting the sensory output (resistance to voltage) to an output indicative of temperature of the sensor. However, in order for the process variable transmitter to obtain the desired measurement, the circuitry of the transmitter must be properly configured. For example, some RTD sensors use a four wire Kelvin connection while other sensors use a three wire connection. Similarly, although thermocouples typically use only two wires, there is a polarity between the two wires. It is known to detect whether a RTD utilizes three or four wires (see, for example, the STT 3000 Smart Temperature Transmitter, model STT 350 Operator Manual). However, the RTD must still be coupled in a known manner. This can be either in a predetermined manner or in a manner which is identified using some type of user input. 
       SUMMARY 
       [0004]    A process variable transmitter for measuring a temperature of a process. The transmitter couples to a temperature sensor. The transmitter is configured to determine a manner in which the temperature sensor is coupled to the transmitter. A method is also provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1A  is a simplified diagram of a temperature transmitter coupled to an RTD sensor. 
           [0006]      FIG. 1B  is a simplified diagram of a temperature transmitter coupled to a thermocouple sensor. 
           [0007]      FIG. 2A  is a flow chart showing steps performed by the temperature transmitter. 
           [0008]      FIGS. 2B ,  2 C and  2 D show cases  1 ,  2  and  3  respectively of an orientation of an RTD. 
           [0009]      FIG. 3A  is a flow chart showing steps performed by the temperature transmitter. 
           [0010]      FIGS. 3B ,  3 C and  3 D show cases  4 ,  5  and  6  respectively of an orientation of an RTD. 
           [0011]      FIG. 4A  is a flow chart showing steps performed by the temperature transmitter. 
           [0012]      FIGS. 4B ,  4 C and  4 D show cases  7 ,  8  and  9  respectively of an orientation of an RTD. 
           [0013]      FIG. 5A  is a flow chart showing steps performed by the temperature transmitter. 
           [0014]      FIGS. 5B ,  5 C and  5 D show cases  10 ,  11  and  12  respectively of an orientation of an RTD. 
           [0015]      FIG. 6A  is a flow chart showing steps performed by the temperature transmitter. 
           [0016]      FIGS. 6B ,  6 C and  6 D show cases  13 ,  14  and  15  respectively of an orientation of an RTD. 
           [0017]      FIG. 7  is a flow chart showing steps performed by the temperature transmitter. 
           [0018]      FIG. 8A  is a flow chart showing steps performed by the temperature transmitter. 
           [0019]      FIG. 8B  shows a configuration of a thermocouple. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present invention is directed to a process variable transmitter which is configured to identify the configuration or orientation of a temperature sensor which is coupled to the device. Examples include identification of a two, three or four wire RTD, the position or location of an RTD element between the terminals of the transmitter, or the position or polarity orientation of a thermocouple. 
         [0021]      FIG. 1A  is a block diagram of temperature transmitter  10  connected to measure temperature with an RTD sensor. 
         [0022]    Transmitter  10  couples to process control loop  11  which provides power to transmitter  10  and over which information is transmitted and received. Alternatively, process control loop  11  may employ various wireless techniques or configurations. In this embodiment, transmitter  10  preferably includes terminal block  14  having terminals  1  through  4  for coupling to, for example, an RTD temperature sensor  16  or a thermocouple temperature sensor  18  (shown in  FIG. 1B ).  FIG. 1A  shows the electrical connections to RTD  16 . Sensor  16  (and sensor  18 ) can be either internal or external to transmitter  10 . Transmitter  10  includes multiplexer  20  controlled by microprocessor  22  which is coupled to control loop  11  through input/output (I/O) circuitry  24 . Multiplexer  20  multiplexes appropriate sets of analog signals, including signals from terminals  1  through  4 , to positive and negative inputs of differential amplifier  26 , which connects to high accuracy A/D converter  28 . Memory  30  stores instructions and information for microprocessor  22 , which operates at a speed determined by clock  32 . Multiplexer  20  selectively connect input pairs to the positive and negative inputs of differential amplifier  26 . A reference resistance R REF    38  couples to multiplexer  20  and is connected in series with RTD  16 . 
         [0023]    In operation, transmitter  10  measures temperature of sensor  16  and transmits a representation of temperature over control loop  11 . Transmitter  10  employs the following equation to compute the major value of temperature of RTD  16 : 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     INPUT 
                   
                   = 
                   
                     
                       
                         V 
                         RINPUT 
                       
                       
                         V 
                         RREF 
                       
                     
                      
                     
                       ( 
                       
                         R 
                         REFNOM 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0000]    where:
       R REFNOM  the nominal resistance of the reference resistance in ohms, and/or stored in memory  30 ;   V RINPUT  voltage drop across the input; and   V RREF  voltage drop across R REF .
 
Current source  50  provides current I S  through sensor  16  (via terminals  1  and  4 ) and reference resistor  38  through MUX  20 . In the configuration of  FIG. 1A , microprocessor  22  measures the voltage drop (V RINPUT ) across RTD  16  between terminals  2  and  3 , and the voltage drop (V RREF ) across resistor  38  with MUX  20 . R REFNOM  is a calculation constant and is retrieved from memory  30 . In a four-wire resistance measurement such as this, the voltage drop across the connections to terminals  2  and  3  is largely eliminated, because substantially all the current flows between terminals  1  and  4 , and has little impact on the accuracy of the measurement. R INPUT  is converted to temperature units with a look-up table or suitable equation stored in memory  30 .
       
 
         [0027]    In the configuration  FIG. 1B , transmitter  10  is connected to measure temperature with thermocouple sensor  18  which creates a voltage V TCINPUT  across terminals  1  and  2 . Multiplexer  20  couples inputs of differential amplifier  26  to terminals  2  and  1 .  FIG. 1B  shows a voltage reference (V TCREF )  36  coupled to MUX  20  and current source  50 . A heater  41  is provided for determining the orientation of thermocouple sensor  18  as discussed below. Heater  41  is controlled by microprocessor  22 . 
         [0028]    Transmitter  10  measures the temperature of thermocouple sensor  18  by determining the thermocouple voltage V TC  with the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     TC 
                   
                   = 
                   
                     
                       
                         V 
                         TCINPUT 
                       
                       
                         V 
                         TCREF 
                       
                     
                      
                     
                       ( 
                       
                         V 
                         TCREFNOM 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0000]    where:
       V TCINPUT =the measured voltage across terminals  1  and  2  of terminal block  14  sensed by amplifier  26 ;   V TCREF =the measured voltage generated by voltage reference  36  as sensed by amplifier  26 ;   V TCREFNOM =a nominal value of voltage reference  36  stored in memory  30 ;       
 
         [0032]    The temperature sensors  16  and  18  illustrated in  FIGS. 1A and 1B  are typically connected to the transmitter  10  when the transmitter  10  is located in the field. Thus, the operator that connects the temperature sensor  16 ,  18  to transmitter  10  may couple the leads of the sensor  16 ,  18  to any one of the four terminals  1 ,  2 ,  3  and  4 . However, for the microprocessor  22  to obtain accurate temperature measurements, the orientation and configuration of the leads of the sensor  16 ,  18  must be known. This can, for example, require that the sensor be connected to the transmitter at a particular orientation. Alternatively, information can be input to the transmitter  10  which instructs the microprocessor  22  regarding the orientation of the sensor  16 ,  18 . With the present invention, the microprocessor  22  performs a number of tests on the sensor  16 ,  18  in order to determine the configuration and orientation of the sensor. 
         [0033]      FIG. 2A  is a flow chart  100  showing steps in accordance with the present invention and  FIGS. 2B ,  2 C and  2 D are show example configurations of the sensor  16  of the RTD sensor  16  coupled to transmitter  10 . Further, the following table, Table 1 shows three cases regarding the resistance measured between pairs of terminals T 12  is between terminals  1  and  2 , T 13  is between terminals  1  and  3  and T 14  is between terminals  1  and  4 . 
         [0000]                                                          TABLE 1                       Case 1   Case 2   Case 3                                        T12   L   L + S   L + S           T13   L + S   L   L + S           T14   L + S   L + S   L                       L = Leadwire (ohms)           S = Sensor (ohms)           Assumption: S &gt;&gt; L            
Flow chart  100  is initiated by measuring the resistance between terminals  1  and  2 . If an open circuit is determined, microprocessor  22  determines that the sensor  16  is not a four-wire sensor and that terminal  1  or terminal  2  is not used. Alternatively, the resistance between terminals  1  and  2  are stored in memory and another measurement between terminals  1  and  3  is obtained. If this measurement indicates that the connection is an open circuit, then the microprocessor  22  determines that the sensor is not a four-wire sensor and that terminal  3  is not in use. Alternatively, the resistance between terminals  1  and  3  is stored in memory. Next, the resistance between terminals  1  and  4  is measured. If this resistance indicates an open circuit, then the microprocessor  22  determines that the sensor  16  is not a four-wire sensor and that terminal  4  is not in use. Alternatively, the resistance between terminals  1  and  4  is stored in memory. Based upon the measured resistances, the location of the sensor  16  (Case  1 , Case  2  or Case  3  as illustrated in  FIGS. 2B ,  2 C and  2 D, respectively) is determined based upon information in Table 1 above.
 
         [0034]      FIG. 3A  is a flow chart  120  showing subsequent steps performed by microprocessor  22  in determining the location of sensor  16  and  FIGS. 3B ,  3 C and  3 D show cases  4 ,  5  and  6  respectively, of the orientation sensor  16  as set forth in Table 2: 
         [0000]                                                          TABLE 2                       Case 4   Case 5   Case 6                                        T13   L   L + S   L + S           T14   L + S   L   L + S                       L = Leadwire (ohms)           S = Sensor (ohms)           Assumption: S &gt;&gt; L            
Flow chart  120  begins with microprocessor  22  measuring determining that the terminal  1  or terminal  2  is not used. Next, the resistance between terminals  1  and  3  is measured. If this is an open circuit, then terminal  1  is not used or sensor is not a three wire sensor. Alternatively, the resistance between terminals  1  and  3  is stored in memory. Next, microprocessor  22  measures the resistance between terminals  1  and  4 . If this is an open circuit, microprocessor  22  determines that the sensor  16  is a two wire sensor connected between terminals  1  and  3 . Alternatively, the resistance between terminals  1  and  4  is stored in memory. Finally, microprocessor  22  identifies the location of sensor  16  based upon the data collected and the information shown in Table 2 above.
 
         [0035]      FIG. 4A  is a flow chart  140  showing subsequent steps performed by microprocessor  22  and  FIGS. 4B ,  4 C and  4 D show cases  7 ,  8  and  9  respectively, of the orientation of the sensor  16  based upon data contained in Table 3: 
         [0000]                                                          TABLE 3                       Case 7   Case 8   Case 9                                        T23   L   L + S   L + S           T24   L + S   L   L + S                       L = Leadwire (ohms)           S = Sensor (ohms)           Assumption: S &gt;&gt; L            
In flow chart  140  begins with microprocessor  22  having determined that terminal  1  is not used. Next, the resistance between terminals  2  and  3  is measured. If this indicates an open circuit, then the sensor is not a three wire sensor but is a two wire sensor coupled between terminal  2  and  4  or between terminals  3  and  4 . Alternatively, data related to the resistances is stored in memory. Next, microprocessor  22  measures the resistance between terminals  2  and  4 . If this resistance indicates an open circuit, then the sensor is a two wire sensor connected between terminals  2  and  3 . Alternatively, the resistance between terminals  2 ,  3 , and  4  is stored in memory. Finally, the microprocessor identifies the location of the sensor  16  based upon information contained in Table 3 above.
 
         [0036]      FIG. 5A  is a flow chart  160  showing subsequent steps performed by microprocessor  22  and  FIGS. 5B ,  5 C and  5 D show cases  10 ,  11  and  12 , respectively regarding the configuration of sensor  16  as illustrated in Table 4: 
         [0000]                                                          TABLE 4                       Case 10   Case 11   Case 12                                        T12   L   L + S   L + S           T14   L + S   L   L + S                       L = Leadwire (ohms)           S = Sensor (ohms)           Assumption: S &gt;&gt; L            
Flow chart  160  is initiated when it is determined that terminal  3  is not used. Next, the resistance between terminals  1  and  4  is measured. If this is an open circuit, then microprocessor  22  determines that sensor  16  is a two-wire sensor connected between terminals  1  and  2 . Alternatively, the resistance is stored and microprocessor  22  calculates the sensor location based upon the data contained in Table 4 above.
 
         [0037]      FIG. 6A  is a flow chart  180  illustrating steps in accordance with the present invention when terminal  4  is not used.  FIGS. 6B ,  6 C and  6 D illustrate possible cases  13 ,  14  and  15  regarding the location of sensor  16  as set forth in Table 5: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Case 13 
                 Case 14 
                 Case 15 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 T12 
                 L 
                 L + S 
                 L + S 
               
               
                   
                 T13 
                 L + S 
                 L 
                 L + S 
               
               
                   
                   
               
               
                   
                 L = Leadwire (ohms) 
               
               
                   
                 S = Sensor (ohms) 
               
               
                   
                 Assumption: S &gt;&gt; L 
               
             
          
         
       
     
       In flow chart  180 , the microprocessor  22  calculates the location of sensor  16  based upon data stored in Table 5 above. 
       [0038]      FIG. 7  is a flow chart  200  illustrating steps performed by microprocessor  22  if it is determined that the sensor  16  is a two wire sensor between terminals  2  and  4  or between terminals  3  and  4 . Microprocessor  22  measures the resistance between terminals  2  and  4 . If this is not an open circuit, then the sensor is a two wire sensor connected between terminals  2  and  4 . Alternatively, microprocessor measures the resistance between terminals  3  and  4 . If this is an open circuit, then microprocessor  22  determines that a sensor has not been connected. Alternatively, microprocessor  22  determines that the sensor is a two wire sensor coupled between terminals  3  and  4 . 
         [0039]      FIGS. 8A and 8B  all relate to the determination of the polarity of a thermalcouple  18  coupled to transmitter  10 .  FIG. 8A  is a flow chart showing steps executed by microprocessor  22  and  FIG. 8B  is an example configuration of the thermocouple. In flow chart  220 , microprocessor  22  measures the voltage across terminals  1  and  2 . This is recorded as a baseline value, V base . Next, terminals  1  and  2  (or the cold junction formed by thermocouple  16 ) are heated using heater  41 . Heater  41  can be configured to operate under the control of the microprocessor  22 . The microprocessor  22  then records the heated voltage value V heat . Next, if V heat  is greater than V base , microprocessor  22  determines that terminal  2  is the positive lead of the thermocouple. Alternatively, microprocessor  22  determines that terminal  1  is the positive lead. 
         [0040]    The various steps set forth in the flow charts can be arranged as desired. Typically, the steps are performed using program instructions stored in memory  30  and executed by microprocessor  22 . 
         [0041]    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. Although the measurement circuitry is described as measuring resistance or voltage, any appropriate electrical parameter can be measured. As used herein, “measurement circuitry” can optionally include a microprocessor, or steps performed by a microprocessor, and/or additional digital or analog circuitry.