Patent Document

BACKGROUND OF THE INVENTION 
   The present invention relates to monitoring the condition of field devices. Specifically, the invention relates to a system for detecting the presence of fluids in the field devices. 
   In many industrial settings, control systems are used to monitor and control inventories, processes, and the like. A typical control system includes a centralized control room and a number of field devices geographically removed from the control room. The field devices communicate process data to the control room using either analog or digital communication means. 
   Traditionally, analog field devices have been connected to the control room by two-wire twisted-pair current loops, with each field device connected to the control room by a single two-wire twisted pair loop. Located within the field device housing are terminals for connecting the twisted-pair current loops to circuitry within the field device. This region is referred to as the terminal block area of the field device. Typically, a voltage differential is maintained between the two wires of approximately 20 to 25 volts, and a current between 4 and 20 milliamps (mA) runs through the loop. An analog field device transmits a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. A receiving device measures the voltage across a load resistor, typically located in the control room, in order to determine the magnitude of the modulated current. 
   While historically field devices were capable of performing only one function, recently, hybrid systems that superimpose digital data on the current loop have been used in distributed control systems. The Highway Addressable Remote Transducer (HART) and the Instrument Society of America (ISA) Fieldbus SP50 standards superimpose a digital carrier signal on the current loop signal. The HART standard employs frequency-shift keying (FSK) to transmit digital data over the current loop, and operates at frequencies of 1200 and 2400 baud. Other common protocols for communication of digital information over the current loop are Foundation Fieldbus, Profibus, and DeviceNet. Typically, these systems operate at much higher frequencies than the HART protocol. The digital carrier signal can be used to send secondary and diagnostic information. Examples of information provided over the carrier signal include secondary process variables, diagnostic information (such as sensor diagnostics, device diagnostics, wiring diagnostics, process diagnostics, and the like), operating temperatures, sensor temperature, calibration data, device ID numbers, configuration information, and so on. Accordingly, a single field device may have a variety of input and output variables and may implement a variety of functions. 
   Field devices are often located in physically challenging environments, with one potential problem being the collection of fluid within the terminal block area of the field device. The presence of fluid within the terminal block area can have a corrosive effect on the terminals and wires located within the terminal block area, ultimately causing the field device to fail. It is difficult and time-consuming, however, to periodically inspect each field device. Therefore, it would be beneficial to design a system for automatically detecting the presence of fluids with the terminal block area of field devices. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a system and method for detection of fluids within the terminal block area of a field device. An AC test signal is generated on the current loop, and a resulting AC voltage magnitude is measured. The change in impedance caused by the presence of fluid between terminals of the field device can be detected based on the measured AC voltage magnitude. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a field device opened to illustrate the location of terminals. 
       FIG. 2  is a functional block diagram of the components located within the field device. 
       FIG. 3  is a circuit diagram of a current regulator circuit employed by the field device. 
       FIG. 4  is a circuit diagram illustrating a field device connected to a control room by a current loop with the presence of terminal leakage. 
       FIG. 5  is a functional block diagram illustrating the connection of components used to generate the AC test signal and measure the resulting AC voltage magnitude within the field device. 
       FIG. 6  is a flow chart illustrating a method of detecting fluid in the terminal block of the field device. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates field device  10 , which includes housing  12 , sensor board  14 , circuit board  16 , and terminal block  17  (which includes at least two terminals labeled  18   a  and  18   b ). Sensor board  14  measures a process variable (e.g. pressure, temperature, flow, etc.) and converts the measured process variable to an electronic signal. Circuit board  16  converts the signal provided by sensor board  14  to a signal that can be communicated to a control room, using either the traditional 4-20 mA analog communication technique, or some form of digital communication protocol (e.g., HART). Wiring from the control room enters field device  10  through field conduit port  20 , and is connected to terminals  18   a  and  18   b  within terminal block  17 . 
   Terminal block  17  includes threads that allow a cover  21  to be placed over terminal block  17 . Ideally, housing  12  and cover  21  act to protect terminals  18   a  and  18   b  from environmental factors, such as fluid accumulation in terminal block  17 . Despite these efforts, fluids may infiltrate and accumulate within terminal block  17 . The presence of fluid in terminal block  17  may have a corrosive effect on terminals  18   a  and  18   b . Corrosion on terminals  18   a  and  18   b  can adversely affect communication between field device  10  and a control room (shown in  FIG. 4 ). Therefore, the ability to detect fluid within terminal block  17  would be very beneficial. 
     FIG. 2  is a functional block diagram that illustrates how a monitored process variable is processed within field device  10  before being communicated to the control room. As shown in  FIG. 2 , sensor board  14  includes sensor device  22  and analog-to-digital converter  24 , and circuit board  16  includes microprocessor  26  and communication chipset  28 . Sensor device  22  measures a process variable, such as pressure or temperature, and converts the measured process variable to an analog signal. Sensor device  22  provides the analog signal representing the sensed process variable to A/D converter  24 , which converts the analog signal to a digital signal that is provided to microprocessor  26 . Microprocessor  26  (also referred to as a microcontroller) refers broadly to a device capable of performing calculations and communicating with other components. Microprocessor  26  may include a memory device for storing input provided by connected devices. At the request of microprocessor  26 , communication chipset  28  converts a signal received from microprocessor  26  to a signal that can be communicated to the control room. 
   Communication chipset  28  communicates with the control room, in one embodiment, by regulating current provided between terminals  18   a  and  18   b  between 4-20mA, wherein the magnitude of the current provided by communication chipset  28  represents the magnitude of the sensed process variable. In addition, communication chipset  28  may communicate with the control room by superimposing a digital signal over the standard 4-20 mA signal (i.e., using a protocol known as the HART protocol). In yet another embodiment, communication chipset  28  may communicate all data digitally using other digital communication protocols such as Foundation Fieldbus or Profibus. 
     FIG. 3  is a circuit diagram illustrating an embodiment of current regulator circuit  30 , located within communication chipset  28  (shown in  FIG. 2 ), that converts input provided by microprocessor  26  (also shown in  FIG. 2 ) to either an analog signal that is provided to the control room using the 4-20 mA loop current or a digital signal. Current regulator circuit  30  includes input terminal V TXA  and input terminals V MSB , capacitors C 1 , C 2 , C 3 , C 4  and C 5 , resistors R 0  (also referred to as current regulator resistor R 0 ), R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , and R 9 , operational amplifier OpAmp 1 , transistors Q 1  and Q 2 , and output terminals that connect to terminals  18   a  and  18   b . Input is provided by microprocessor  26  at input terminals V TXA  and V MSB , and output is provided to the control room via output terminals  18   a  and  18   b.    
   Current regulator circuit  30  regulates the current generated through resistor R 0  based on the inputs received at input terminals V TXA  and V MSB . The signal provided by microprocessor  26  to input terminal V MSB  represents the sensed process variable, and the magnitude of the signal provided to input terminal V MSB  dictates the magnitude of the 4-20 mA current provided through resistor R 0 . That is, current regulation circuit  30  varies the current provided through resistor R 0  between 4 mA and 20 mA based on the signal provided at input terminal V MSB . 
   In addition to the 4-20 mA analog current regulation provided by current regulator circuit  30  based on an input representative of the sensed process variable, current regulator circuit  30  may also regulate the current through resistor R 0  to communicate a digital signal to the control room. In this example, the digital signal is provided to current regulation circuit by microprocessor  26  at terminal V TXA . 
   In one embodiment, the digital signal is communicated to the control room using the HART communication protocol. This protocol employs frequency-shift keying (FSK) to transmit digital data over the current loop. In HART communications, the input provided at V TXA  modulates the 4-20 mA current approximately ±0.5 mA at either 1200 Hertz (Hz) or 2400 Hz. Modulating the current at 1200 Hz represents a low or “0” digital signal, and modulating the current at 2400 Hz represents a high or “1” digital signal. In another embodiment, instead of analog communication using a 4-20 mA current regulation, field device  10  communicates digitally with the control room using a protocol known as Foundation Fieldbus. Much of this disclosure describes an embodiment in which field device  10  communicates with the control room via a standard 4-20 mA analog signal, although the present invention is applicable to embodiments that employ digital communication as well. As described in more detail with respect to  FIG. 4  below, the present invention makes use of the digital communication capabilities of field device  10  to detect the presence of fluid in the terminal block. 
     FIG. 4  is a circuit diagram illustrating the connection of field device  10  to monitoring station or control room  32 . For purposes of this circuit diagram, only the elements responsible for communicating with control room  32  are shown. Therefore, field device  10  is represented by current regulation circuit  30  (modeled here as an ideal current source), and terminals  18   a  and  18   b . Control room  32  is modeled here as including a DC power supply V CC , measuring resistor R M1 , measuring device  36 , and display unit  38 . Field device  10  is connected to control room  32  by twisted cable or wire pair  40 . Capacitor C C  represents the capacitance created by twisted wire pairs  40 . 
   For analog communication with field device  10 , DC power supply V CC  maintains a voltage differential between the two wires of approximately 20 to 25 volts, and a current between 4 and 20 milliamps (mA) runs through the current loop. Current regulator circuit  30  regulates the amplitude of the current provided to terminals  18   a  and  18   b  to a value proportional to the sensed process variable. The magnitude of the current provided by current regulator circuit  30  to terminals  18   a  and  18   b  is approximately equal to the magnitude of the current provided to control room  32  by twisted wire pair  40 . Receiving device  36  is DC coupled to measure the resulting voltage across a measuring resistor R M1 . The measured voltage indicates the amplitude of the current provided by field device  10  and therefore the value of the measured process variable. 
   For digital communication between field device  10  and control room  32 , current regulation circuit  30  superimposes a digital signal on the current loop. For example, as discussed above with respect to  FIG. 3 , the HART digital communication protocol modulates the 4-20 mA current approximately ±0.5 mA at either 1200 Hertz (Hz) or 2400 Hz to represent digital signals. For digital communication, measuring device  36  would also be AC coupled to receive digital information transmitted by field device  10 . That is, the current modulation of ±0.5 mA (used in HART applications) is detected by measuring device  36 , which measures the resulting AC voltage magnitude across resistor R M1 . In other embodiments, a hand-held measuring device is connected across terminals  18   a  and  18   b  to receive digital information transmitted by field device  10 . 
   The presence of fluid between terminals  18   a  and  18   b  can be modeled by a leakage resistor R L , leakage capacitor C L , and fluid resistor R F . Leakage resistor R L  and leakage capacitor C L  model the interface between a fluid and a terminal, and are typically large. For example, leakage resistor R L  may have a value of approximately one megohm (MΩ), while leakage capacitor may have a value of approximately one microfarad (μF). Fluid resistance varies depending on the fluid, but is typically lower than the leakage resistance (for example, one kilohm (kΩ)). The large leakage resistance and leakage capacitance result in DC signals being relatively unaffected by the presence of fluid in terminal block area  17 . However, the overall impedance (i.e., the combination of resistance and capacitance) created by the presence of fluid between terminals  18   a  and  18   b  can be detected using the AC signal generated by current regulation circuit  30  for digital communication. 
   For example, if field device  10  communicates using the HART standard, then current regulation circuit  30  generates a ±0.5 mA AC test current at either 1200 Hz or 2400 Hz. For the situation in which no fluid is present within terminal block area  12  (i.e., no terminal leakage), the leakage resistor R L , leakage capacitor C L  and fluid resistor R f  are removed from the circuit. If cable capacitance C C  is not taken into account, then a ±0.5 mA AC test current generated at 2400 Hz by current regulation circuit  30  results in ±125 mV signal being generated across measurement resistor R M1  (assuming resistor R M1  has a value of approximately 250 ohms). If cable capacitance C C  is taken into account, a ±0.5 mA AC test current generated by field device 10 at 2400 Hz results in the amplitude of the AC voltage signal generated across the measurement resistor R M1 , being reduced to approximately ±117 mV. 
   If there is fluid present in terminal block area  17 , then leakage resistor RL, leakage capacitance C L , and fluid resistance R F  are connected between terminals  18   a  and  18   b  as shown in  FIG. 4 . With the change in impedance caused by fluid in terminal block area  17 , a ±0.5 mA AC test current generated by current regulation circuit  30  at 2400 Hz results in the magnitude of the AC voltage signal generated across the measurement resistor R M1  being reduced to approximately ±90 mV. Thus, the change in impedance caused by the presence of fluid in terminal block area  17  results in a detectable decrease in the AC voltage magnitude measured across measurement resistor R M1 . This detectable decrease in the magnitude of the AC voltage signal allows the present invention to determine whether there is fluid present in terminal block area  17  of field device  10 . 
     FIG. 5  illustrates a functional block diagram of an embodiment in which the components necessary to detect fluid in terminal block area  17  are located locally within field device  10  (in contrast with the embodiment discussed with respect to  FIG. 4 , in which the resulting AC voltage magnitude was measured within control room  32 ). The components include terminal block  17 , terminals  18   a  and  18   b , current regulation circuit  30 , AC coupled measurement device  42 , and power supply  44 . 
   As discussed with respect to  FIG. 4 , control room  32  provides approximately 20-25 volts between terminals  18   a  and  18   b . Power supply  44  is connected between terminals  18   a  and  18   b  and uses the 20-25 volts provided by control room  32  to provide regulated power (labeled as PWR) to devices and components located within field device  10  (for example, microprocessor  26 , current regulator circuit  30 , and AC coupled measurement device  42 ). 
   Current regulator circuit  30  is connected to receive input from microprocessor  26 , and to regulate the current provided to terminals  18   a  and  18   b  (as described with respect to  FIG. 3 ). As discussed with respect to  FIG. 3 , the input received from microprocessor  26  may include a signal representative of a sensed processor variable as well as a digital signal. In the present invention, microprocessor  26  may periodically instruct current regulator circuit  30  to generate an AC test signal at a determined frequency to determine whether fluid is present in the terminal block. In the alternative, field device  10  may receive a request from control room  32  instructing field device  10  to generate an AC test signal and measure the resulting AC voltage magnitude. 
   AC coupled measurement device  42  is connected to monitor the AC voltage magnitude generated in response to an AC test signal generated by current regulator circuit  30 . In one embodiment, AC coupled measuring device is incorporated onto the same application specific integrated circuit (ASIC) as current regulation circuit  30 . 
   As shown in  FIG. 5 , AC coupled measurement device  42  is electrically connected between terminals  18   a  and  18   b . AC measurement device  42  may include a measurement resistor (not shown) and an AC coupled measuring device (not shown). The resulting AC voltage magnitude generated across the measurement resistor in response to the AC test signal is measured by the AC coupled measuring device. 
   As shown in  FIG. 5 , AC coupled measurement device  42  is also connected to provide the measured AC voltage magnitude to microprocessor  26 , which can either store the measured value locally or communicate the measured value digitally to control room  32  via current regulation circuit  30 . Additionally, microprocessor  26  may determine based on the measured AC voltage magnitude whether to initiate an alarm or notification to control room  32  indicating the presence of fluid detected in terminal block  17 . The determination may be made based on previous AC voltage magnitudes measured with respect to an AC test signal, or may be based on some preprogrammed threshold level that microprocessor  26  uses to determine whether fluid is present in the terminal block. 
     FIG. 6  is a flowchart of one method used to detect fluid in terminal block area  17  of field device  10 . At step  50 , an initial AC test signal is initiated at a selected frequency. The initial test signal may be initiated at the request of control room  32 , or may be done automatically upon installation of field device  10  (i.e., when no fluid is present in terminal block  17 ). 
   At step  52 , an initial AC voltage magnitude is measured in response to the initial AC test signal. As discussed above, the AC voltage magnitude measurement may be made either internally within field device  10  (as shown in  FIG. 5 ) or in control room  32  (as shown in  FIG. 4 ). 
   At step  54 , the initial AC voltage magnitude is stored to memory. In one embodiment, the initial AC voltage magnitude measured locally within field device  10  is communicated to microprocessor  26 , which stores the measured initial AC voltage magnitude locally. In another embodiment, microprocessor  26  instructs communication chipset  28  to communicate the measured AC voltage magnitude to control room  32 , which proceeds to store the measured value to memory located within control room  32 . If the AC voltage magnitude measurement is made by control room  32 , then control room  32  stores the measured value to memory located within control room  32 . 
   At step  56 , a subsequent AC test signal is initiated at the same frequency as the initial AC test signal. Once again, the subsequent AC test signal may be initiated at the request of control room  32 , or may be initiated internally by field device  10 . For example, field device  10  may periodically initiate a subsequent AC test signal to determine whether fluid is present in terminal block  17 . 
   At step  58 , a subsequent AC voltage magnitude is measured in response to the subsequent AC test signal. The subsequently measured AC voltage magnitude may be measured either locally by field device  10  (as shown in  FIG. 5 ) or at control room  32  (as shown in  FIG. 4 ). 
   At step  60 , the subsequently measured AC voltage magnitude is stored to memory. The subsequently measured AC voltage magnitude may be stored locally within memory located within field device  10 , or may be communicated via communication chipset  28  to control room  32 . 
   At step  62 , the presence of fluid in terminal block area  17  is detected by comparing the initial AC voltage magnitude (representing a situation in which no fluid is present in terminal block area  17 ) with the subsequently measured AC voltage magnitude. The subsequently measured AC voltage magnitude may be compared directly to the initial AC voltage magnitude, or may be compared to a threshold value determined based on the initial voltage magnitude. 
   At step  64 , based on the determination made at step  62 , a notification or alarm regarding the presence of fluid in the terminal block area of field device  10  is generated. If the determination was made within field device  10 , then the notification is communicated to control room  32  using the digital communication capabilities of field device  10 . 
   The method described with respect to  FIG. 6  is one method of detecting the presence of fluid in the terminal block  17  of field device  10 . In another embodiment, rather than generating an initial AC test signal and measuring the resulting AC voltage magnitude, a customer characterizes the impedance of the current loop upon initialization of the system, and defines a threshold AC voltage value that is used to detect the presence of fluid in terminal block area  12 . After field device  10  is installed and operational, a command from control room  32  causes field device  10  to generate an AC test signal at a particular frequency and to measure the resulting AC voltage amplitude (either within control room  32  or locally within field device  10 ). If the measured AC voltage amplitude drops below the selected threshold value, then a determination is made that fluid is present within terminal block  17  of field device  10 . Based on the determination, a notification is communicated to control room  32 . 
   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. For instance, the location of the measured AC voltage magnitude may be conducted at control room  24 , at a handheld device, or within field device  10 . The AC voltage magnitude provides insight into impedance changes to field device  10 , regardless of the location of the measurement of the AC voltage magnitude. Likewise, in other embodiments the AC test signal is not generated by current regulation circuit  30 , but is generated independently either internally within field device  10  or external to field device  10 .

Technology Category: 5