Abstract:
A diagnostic device for coupling to a process control loop includes digital communication circuitry configured to receive a digital communication signal from the process control loop. The digital communication signal is a digitally modulated analog signal on the process control loop which is modulated to a plurality of discrete analog signal levels representative of digital values. Diagnostic circuitry diagnoses operation of the process control loop which may include field devices of the process control loop based upon the digitally modulated analog signal.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to industrial process control and monitoring systems. More specifically, the present invention relates to diagnostics of industrial process control and monitoring systems which utilize two-wire process control loops to transmit information. 
   Industrial process control and monitoring systems are used in many applications to control and/or monitor operation of an industrial process. For example, an oil refinery, chemical processing plant, or paper manufacturing facility may have numerous processes which must be monitored and controlled. 
   In such industrial processes, process variables are measured at remote locations across the process. Example process variables include temperature, pressure, flow and the like. This information is transmitted over a two-wire process control loop to a central location, for example, a control room. Similarly, process variables can be controlled using controllers placed in the process. The controllers receive control information from the two-wire process control loop and responsively control a process variable, for example by opening or closing a valve, heating a process fluid, etc. 
   Various protocols have been used to communicate on two-wire process control loops. One protocol uses a 4-20 mA signal to carry information on the loop. The 4 mA signal can represent a zero or low value of a process variable while the 20 mA signal can represent a high or full scale value. The current can be controlled by a process variable transmitter to values between 4 and 20 mA to represent intermediate values of the process variable. A more complex communication technique is the HART® communication protocol in which digital information is superimposed onto a 4-20 mA signal. Typically, in such configurations a separate two-wire process control loop is required for each field device. 
   A more complex communication technique used on two-wire process control loops is generally referred to as fieldbus-based protocols, such as Foundation™ fieldbus. In a Fieldbus protocol, all information is transmitted digitally and the analog current level on the process control loop is not required to carry information. One advantage of such a configuration is that multiple process variable transmitters or controllers can be coupled in series on a single process control loop. Each device on the loop has an address such that it can identify messages which are addressed to it. Similarly, messages transmitted by a field device can include the address of the device so that the sender can be identified. 
   SUMMARY 
   A diagnostic device for coupling to a process control loop includes digital communication circuitry configured to receive a digital communication signal from the process control loop. The digital communication signal is a digitally modulated analog signal on the process control loop which is modulated to a plurality of discrete analog signal levels representative of digital values. Diagnostic circuitry diagnoses operation of the process control loop based upon the digitally modulated analog signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified diagram of a process control or monitoring installation which includes a two-wire process control loop. 
       FIG. 2  is a simplified block diagram of a process control loop diagnostic device. 
       FIGS. 3A and 3B  are graphs of loop current I versus time. 
       FIG. 4  is a more detailed diagram showing digital communication circuitry of  FIG. 2 . 
       FIG. 5  is a more detailed diagram of the process control loop diagnostic device. 
   

   DETAILED DESCRIPTION 
   The present invention is directed to diagnostics in a process control loop including diagnostics of the wiring used in a two-wire process control loop itself, as well as other devices connected to the process control loop. In particular, the present invention provides diagnostics including detection of a failed or potentially failing component in a two-wire process control loop operating in accordance with a fieldbus based protocol in which multiple devices can be connected to a single two-wire process control loop. 
     FIG. 1  is a simplified diagram showing a process control or monitoring system  10  including field devices  12  and  14  coupled to process piping  16 . Devices  12  and  14  are coupled to a single two-wire process control loop  18  which in turn couples to a control room  20 .  FIG. 1  also illustrates a two-wire process control loop diagnostic device  22  coupled to loop  18 . The loop  18  carries a current I which can be used to provide power to all of the field devices on loop  18  and can be generated at control room  20 . Information is transmitted digitally on loop  18  by modulating a digital signal on top of the loop current I. For example, devices  12  and  14  can include unique addresses such that they are able to uniquely identify messages which they transmit, as well as identify which received messages are addressed to them. Devices  12  and  14  can comprise any type of field device including process variable transmitters and controllers. The process control loop  18  terminates at a segment terminator  24 . The term “segment” refers to a portion of or all of two-wire process control loop  18 . 
     FIG. 2  is a simplified block diagram of a two-wire process control loop diagnostic device  50  in accordance with the present invention, similar to device  22  shown in  FIG. 1 . Diagnostic device  50  couples to two-wire process control loop  18  and includes digital communication circuitry  52  and diagnostic circuitry  54 . Two-wire process control loop diagnostic device  50  can, in some configurations, be implemented in field device  12 , field device  14 , stand-alone diagnostic device  22  and/or control room  20 . 
   During operation, digital communication circuitry  52  receives a digital communication signal from the two-wire process control loop  18 . This digital communication signal comprises an analog signal which has been digitally modulated. Such modulation is in accordance with known techniques. For example, the loop current I can be caused to vary periodically such that a variation above a certain threshold represents a binary 1 and a variation below a particular threshold represents a binary 0. Such a configuration is illustrated in  FIG. 3A  which is a graph of the loop current I versus time. In  FIG. 3A , the time axis of the graph has been divided into five time periods: t 0 , t 1 , t 2 , t 3  and t 4 . During period to, the current level I is undetermined and represents neither a 0 or a 1. During periods t 1  and t 4 , the current level I represents a binary 0. Similarly, during periods t 2  and t 3 , the loop current level I represents a binary 1. Another data encoding technique could break each bit period into two equal parts as shown in  FIG. 3B . A binary 1 is represented by the current level being above threshold value during first half of the bit time and below a threshold during the second half. A binary 0 is represented by the first half being below the threshold and the second half being above. 
   The diagnostic circuitry  54  illustrated in  FIG. 2  performs diagnostics based upon the digitally modulated analog signal I. More specifically, the diagnostic circuitry  54  performs diagnostics based upon analog properties of the digitally modulated analog signal including signal amplitude, wave shape, current, bit error rate (BERT), segment impedance, or other parameters obtained by monitoring current on loop  18 . Further, by monitoring which device transmitted a particular signal, the diagnostic circuitry  54  can identify a particular device on the loop  18  which has failed or may fail in the future. 
     FIG. 4  is a more detailed diagram of diagnostic device  50  and illustrates one configuration of digital communication circuitry  52  in greater detail. Digital communication circuitry  52  includes a sense resistor  60  coupled in series with Input/Output (I/O) circuitry  62  and other devices on the two-wire process control loop  18 . A signal sense circuit  64  is coupled across sense resistor  60  and provides an output to diagnostic circuitry  54 . Diagnostic circuitry  54  optionally connects to I/O circuitry  62 . I/O circuitry  62  is configured to digitally communicate over process control loop  18  and, in some configurations, is configured to provide power to diagnostic device circuitry which is generated from the loop current I through loop  18 . Signal sense circuitry  64  receives a voltage signal generated across sense resistor  60  which is related to the loop current I. Signal sense circuitry can optionally amplify this signal, digitize this signal, and optionally perform additional preprocessing before providing a digital presentation of the voltage signal to diagnostic circuitry  54 . Signal sense circuitry  64  can comprise, for example, a digital signal processing (dsp) integrated circuit and associated hardware. 
     FIG. 5  is a simplified diagram of a diagnostic device configured as a process variable transmitter or process controller. In  FIG. 5 , diagnostic circuitry  54  is shown as implemented in a digital controller  70  and memory  72 . Controller  70  can comprise, for example, a microprocessor or the like which operates in accordance with programming instructions in memory  72 . A process interface  76  can comprise a process variable sensor for sensing a process variable, or can comprise a control element for controlling a process, for example by positioning a valve. When configured as a process variable sensor, element  74  comprises an analog to digital converter and related circuitry which provides a digital signal representation to controller  70 . Controller  70  is configured to transmit information related to the sensed process variable over loop  18 . Similarly, if process interface  76  is configured as a control element, element  74  comprises a digital to analog converter and related circuitry which converts a digital signal from controller  70  to an analog value for controlling the process. 
   The diagnostic device can be implemented in any of the example devices illustrated in  FIG. 1  including a process variable transmitter or controller, a stand-alone diagnostic device  22 , or in control room circuitry  20 . In one configuration, an optional display  78  is provided which can be used to display diagnostic information to an operator. The display can provide diagnostic help status, and a local display is an indication of all devices on a loop segment. In an intrinsically safe configuration, the diagnostics can be located on the intrinsically safe side of the intrinsic safety barrier thereby providing more detailed and accurate diagnostics, including diagnostics of the intrinsically safe barrier itself. 
   The diagnostics performed by diagnostic circuitry  54  can be tailored to each individual two-wire process control loop segment by having the ability to characterize the segment. When the diagnostic device is initially installed on a new or existing segment, the device can analyze the communications from each field device, as each field device performs normal process communications. This information can be saved, for example in memory  72 , for future reference conditions for each device individually. This saved data can be used to identify normal operation and provide a baseline for use in subsequent diagnostics. Characterization of each device in this manner allows for more precise diagnostics. Additionally, each device can be compared to standards in accordance with specific communication protocols, such as Fieldbus protocols, to ensure that the device is conforming to appropriate standards. 
   One example measurement performed by a diagnostic circuitry  54  is based on the amplitude of the digitally modulated analog signal from individual field devices. In such a configuration, the amplitude can be compared with stored threshold values (or amplitude signatures) and if the amplitude is outside of those thresholds a failure indication can be provided. If a single device is failing the test, this can be an indication of a possible failure of the device that transmitted the signal. On the other hand, if multiple devices are failing such a test, this can indicate a problem with something other than a particular device. For example, wiring within a specific segment of loop  18  or a failure of a power supply located in the control room  20 , etc. The advantage of such diagnostics includes the detection of an impending failure in a particular two-wire loop segment prior to its actual failure. This allows the two-wire loop segment to be repaired with minimal down time. Additional diagnostics can include the detection of a clipped wave form which may indicate a possible increase in quiescent current of a field device thereby causing unbalanced modulation. Another potential cause of a clipped signal is inadequate terminal voltage at the field device. This may be due to a power supply voltage or, in an intrinsically safe configuration, a faulty intrinsic safety barrier. 
   In another example configuration, the signal sense circuitry  64  digitizes the digitally modulated analog signal such that the complete signal wave shape is available to diagnostic circuitry  54 . In such a configuration, diagnostic circuitry  54  can perform diagnostics on the complete wave shape such that, for example, the rise and fall times of transitions in the signal can be measured. Further, the communication signal can be characterized over time at a particular installation and used as a reference to continually compare a live signal and detect changes in amplitude over time. By comparing the signals from each device to an initial reference, an indication of component failure or damage to the field device can be detected. A change in rise and fall times can also indicate a change in two-wire process control loop  18 . Using a combination of amplitude and rise/fall times of the individual field devices, in comparison of the changes to all field devices on the segment, allows for a detailed device and bus analysis. On a normally operating segment, if a single device provides a change in amplitude, that device could be flagged as potentially having an impending failure. If a comparison is done to the other devices on the segment, and those other devices all indicate similar changes in amplitude, then a mechanical/wiring fault, power supply or intrinsic safety barrier fault may be indicated. 
   In another example configuration, the diagnostic circuitry  54  monitors the current I created in loop  18  using, for example, the sense resistor  60  and an analog to digital converter which measures the voltage drop across the sense resistor  60 . By monitoring the DC value of the current I, the diagnostic circuitry  54  can detect improper variations in the DC current. For example, a variation in the DC current can indicate that a device connected to the loop has an increase in its shunt set current which could indicate a pending fault in the media access unit (MAU) circuitry for that particular field device. It may also indicate an electrical short in the two-wire loop wiring. Similarly, a reduction in the segment current can also indicate an impending fault. 
   The signal sense circuitry  64  and diagnostic circuitry  54  can be implemented in a single component or across a number of components and may share individual components. Preferably, the circuitry should have adequate processing bandwidth to perform the diagnostics in substantially real time. This can be accomplished with a single microprocessor or through the use of a digital signal processor (DSP) or other type of secondary microprocessor. One example of a diagnostic that requires substantial processing bandwidth is monitoring the signal noise on the loop  18  from, for example, the two terminals which are used to connect to loop  18 , or between one of the connections to loop  18  and the housing or other electrical ground. With sufficient processing speed, analysis calculations such as a standard deviation, a Root Mean Square (RMS), or a Fast Fourier Transform (FFT) can be performed and used to detect differences in noise characteristics. An increase in noise, for example at 60 Hz from one of the terminals to ground can indicate a possible fault in the electrical grounding. 
   Another example diagnostic can be through the monitoring of the bit error rate (BERT) of each device connected to the two-wire loop  18 . If a single device on the loop  18  shows a trend towards a higher bit error rate than a baseline for a particular installation, this can be an indication that the device is failing and may require service. Depending upon the rate at which the bit error rate increases, an indication can be provided to an operator as either a warning of degradation or an indication of imminent failure. Prediction of this impending failure allows the device to be repaired at the next scheduled maintenance interval. 
   In another example configuration, I/O circuitry  62  is configured to apply a high frequency pulse to loop  18 . This high frequency pulse can be measured by the signal sense circuitry  64  in another device and used to determine electrical impedance on loop  18  between the two devices. The high frequency pulse can be placed during normal bus communications so as to not disrupt communications over the loop  18 . By measuring the rise and fall times in amplitude of the received pulse, an impedance measurement can be performed. A comparison of this measurement to a baseline measurement for the installation can be used to provide diagnostics. In one configuration, the high frequency pulse is generated by a simple device, for example, a device which is included in the terminator  24  for the end of the segment of the loop  18  as shown in  FIG. 1 . 
   As the total number of devices which can be placed on a segment  18  is limited by the current consumption of all of the devices coupled to the segment, preferably the circuitry of the present invention operates using techniques to reduce power consumption. For example, the diagnostics can be performed during periods when other circuitry in a particular field device does not require additional power. 
   Although aspects of the diagnostics of the present invention are illustrated as discrete components, various functions can be implemented by a single component or shared between components. Aspects of the present invention can be implemented in software programming (stored in, for example, memory  72 ), can be implemented in hardware, or can be shared between hardware and software including a Link Active Scheduler (LAS). A Link Active Scheduler (LAS) is a deterministic, centralized bus scheduler that maintains a list of transmission times for all data buffers in all devices that need to be cyclically transmitted. Only one Link Master (LM) device on an H1 fieldbus Link can be functioning as that link&#39;s LAS. 
   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. As used herein, a two-wire process control loop includes field devices coupled to the loop in addition to loop wiring.