Abstract:
A magnetic flow meter includes a flowtube arranged to receive a flow of process fluid and a coil proximate the flowtube arranged to apply a magnetic field to the process fluid. A sense electrode is arranged to sense a voltage potential in the flowtube in response to the applied magnetic field. The sensed voltage is indicative of flow rate of process fluid through the flowtube. Diagnostic circuitry provides an output related to an electrical path between the coil and electrical ground.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to magnetic flowmeters. More specifically, the present invention relates to magnetic flow meters with coil ground path detection. 
     A magnetic flowmeter measures the volumetric flow rate of a conductive fluid by detecting the velocity of the fluid passing through a magnetic field. Magnetic flowmeter systems typically include a flowtube assembly and a transmitter assembly. The flowtube assembly is installed in a process piping line, either vertically or horizontally, and includes a pipe section, a coil section and electrodes. The coils are located on opposite sides of a cross section of the pipe. The coils, energized by a coil drive current from the transmitter, develop a magnetic field along the cross section of the pipe. Two electrodes are located across the pipe from each other along a line which is perpendicular to the magnetic field. Fluid passing through the pipe is electrically conductive. As a result of the conductor movement through the magnetic field, an electric potential or electromotive force (EMF) is induced in the fluid which can be detected across the electrodes. Operation is thus based on Faraday&#39;s law of electromagnetic induction. 
     The coils in the magnetic flowmeter flowtube may be compromised by process fluid leaking into the coil compartment. This can cause an electrical path between the coil and electrical ground. Electrical paths between the coil and electrical ground can also arise from other sources including age or components fatigue. The electrical ground path causes the drive signal applied to the coil to be reduced because a portion of the coil drive signal flows to electrical ground. This leads to a reduction in the applied EMF and a corresponding reduction in the output from the sense electrodes. This will lead to inaccurate flow measurements. 
     The loss of the coil drive signal typically cannot be detected by simply measuring the coil drive current. This is because output of the coil drive control circuitry is fixed at a set current level, regardless of any current leakage to ground. 
     SUMMARY 
     A magnetic flow meter includes a flowtube arranged to receive a flow of process fluid and a coil proximate the flowtube arranged to apply a magnetic field to the process fluid. A sense electrode is arranged to sense a voltage potential in the flowtube in response to the applied magnetic field. The sensed voltage is indicative of flow rate of process fluid through the flowtube. Diagnostic circuitry provides an output related to an electrical path between the coil and electrical ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a magnetic flowmeter in a two-wire communication loop. 
         FIG. 2  is a schematic diagram showing a bridge pulse controlled current driver for a magnetic flowmeter. 
         FIG. 3A  is a simplified diagram showing drive circuitry coupled to the coils in one configuration. 
         FIG. 3B  is a simplified diagram showing the drive circuitry coupled to the coils in a second configuration. 
         FIG. 4  is a simplified diagram illustrating measurement of the coil to ground resistance. 
         FIG. 5  is a simplified block diagram showing example steps of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , magnetic flowmeter system  2  connects to two-wire communication 4-20 mA loop carrying current I and an AC power line (not shown). Flowtube  4  carries a fluid flow. Transmitter  9  supplies coil drive current I L  to coils  26  adjacent flowtube  4  which generate a magnetic field in the fluid. Electrodes  6 , 8  mount in flowtube  4  along a line perpendicular to the magnetic field in the fluid for sensing EMF induced by the fluid flow. Transmitter  9  senses the EMF between electrodes  6 , 8  and controls a DC output current I representative of the sensed EMF which is, in turn, proportional to fluid flow. Transmitter  9  transmits current I over a 4-20 mA current loop to a remote receiving station  11 . Transmitter  9  can also transmit the flow output digitally using HART digital protocol, a Fieldbus protocol, a wireless protocol, or other technique. 
       FIG. 2  shows driver circuitry  10  in transmitter  9 . H-bridge flowtube driver  10  of magnetic flowmeter system  2  generates alternating drive current I L  to a load (coils)  26 . In H-bridge driver  10 , power source  12  energizes a transistor bridge circuit  14 . In bridge circuit  14 , control circuits  28  and  30  connect to the gates of field effect transistor (FET)  16 , FET  18 , FET  20  and FET  22  to switch them on in pairs to provide alternating current to load  26 . Lines from power source  12  connect to drain terminals of FETs  16  and  18 , and to source terminals of FETs  20  and  22 . The source terminal of FET  16  and the drain terminal of FET  20  connect to one side of the load  26 . Control circuits  28  and  30  convert input HIGH and LOW logic levels to desired voltage bias levels compatible to the gates of transistors  16 ,  18 ,  20 ,  22  for switching between ON and OFF states. 
     Microprocessor  40  produces control outputs a and a′ at the desired operating frequency, typically 37.5 Hz as a function of the sensed current. Outputs a and a′ provide logic levels to circuits  28  and  30 , respectively. Microprocessor  40  is connected to memory  44 , clock  46 , operator input/output (I/O) circuitry  48  and loop I/O circuitry  49 . Memory  44  contains programming instructions to control operation of microprocessor  40 . Microprocessor  40  operates at a speed determined by clock  46  and receives operator command inputs through input/output circuitry  48 . Input/output circuitry  49  is used to provide an output connection over the 4-20 mA current loop. Alternatively, or in addition to the loop connection, I/o circuitry  49  can be used for wireless communication. 
     In one embodiment, supply  12  is a switching power supply. As described below, bridge circuit  14  periodically alternates, or commutates power source  12  through load  26 . 
     During a first alternation or condition period, signal a goes HIGH and a′ goes LOW. Control circuits drive signal b HIGH and b′ LOW causing transistors  16  and  22  to conduct and transistors  18  and  20  to turn off, thereby supplying current I L  in the direction shown by the arrow. Similarly, during a second alternation or conduction period, signal a goes LOW and a′ goes HIGH. Control circuits  28  and  30  drive signal b to LOW and b′ to HIGH causing transistors  18  and  20  to turn on and  16  and  22  to turn off thereby supplying current I L  in a direction opposite that shown by the arrow. During normal operation, this alternation is at 37.5 Hz and, in some cases, 6 Hz. However, other frequencies may be used. 
     Current I S  from power source  12  flows to return path  50  through a sense resistor R SENSE    52 . Resistor  52  also connects to signal ground  54 . Analog to digital converter  58  connects to sense resistor  52  and provides an output representative of the current through load (coil)  26  to microprocessor  40 . The output of A/D circuitry  58  is representative of the magnitude of current I S  flowing through sense resistor  52 . Microprocessor  40  monitors the magnitude of I S  as discussed below. 
       FIGS. 3A and 3B  are simplified diagrams showing the conditions of field effect transistors  16 - 22  during normal operation. Transistors  16 - 22  are illustrated as switches. In  FIG. 3A , field effect transistors  16  and  22  are in a closed condition while transistors  18  and  20  are in an open condition. This allows current from power supply  12  to flow through coils  26  in the direction indicated. In contrast, in  FIG. 3B , field effect transistors  16  and  22  are open while transistors  18  and  20  are in a closed position. This allows the current from power source  12  to flow in the direction indicated. An electrical ground  61  which corresponds to the process ground is also shown. 
     In a typical flowtube, the resistance between the coils  26  and electrical ground is substantially infinite. However, if even a highly resistive path to ground is formed, the resulting flow measurements will be inaccurate. The present invention provides a technique to measure a leakage resistance between the coil  26  and electrical ground  61 . This can be used to provide an alert to the operator and provide an indication that a flowtube needs servicing. The resistance can also be used to calculate a percentage of the current drive signal which is lost and/or can be used to correct the error caused by the current leakage. 
       FIG. 4  is a simplified electrical schematic diagram showing one example of a configuration of the present invention in which the microprocessor  40  and the analog digital converter  58  are configured to operate as diagnostic circuitry to measure a ground path resistance  70  between coil  26  and electrical ground  61 . As illustrated in  FIG. 4 , field effect transistor  16  is closed. This provides an electrical path for current from power source  12  to coils  26 . However, field effect transistors  18 ,  20  and  22  are open in this configuration. Thus, any current flowing through sense resistor  52  measured by analog digital converter  58  is related to the current through the ground path resistance  70 . The resistance  70  of the ground path can be calculated as R=V/I, where R is the ground path resistance, V is the voltage across the coil  26  and I is the current through the coil  26 . In another example configuration, both field effect transistors  16  and  18  are in a closed condition while transistors  20  and  22  are open. In a further example configuration, only field effect transistor  18  is in a closed state. 
     Through experimental testing, it was determined that the accuracy of the flow measurements was not significantly affected (i.e., less than 0.01%) if the electrical resistance  70  was 100 kOhms or smaller. The signal loss can be calculated by placing the coil resistance in parallel with the coil to ground resistance  70  and solving for the percent of the drive current that will not pass through the coils  26  to generative an electric field in the moving fluid. For example, a typical coil resistance of 10 ohms can be assumed. This leads to table 1: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Resistance 
               
             
          
           
               
                 Actual 
                 Measure 
                 Signal Loss 
               
               
                   
               
               
                 Infinity/Open 
                 3.2 M 
                 0.000% 
               
               
                 900k 
                 685k 
                 0.001% 
               
               
                 800k 
                 637k 
                 0.001% 
               
               
                 700k 
                 538k 
                 0.001% 
               
               
                 600k 
                 483k 
                 0.002% 
               
               
                 500k 
                 424k 
                 0.002% 
               
               
                 400k 
                 344k 
                 0.002% 
               
               
                 300k 
                 277k 
                 0.003% 
               
               
                 200k 
                 185k 
                 0.005% 
               
               
                 100k 
                 94.2k  
                 0.010% 
               
               
                  50k 
                 47.3k  
                 0.020% 
               
               
                  10k 
                  9.6k 
                 0.100% 
               
               
                  5k 
                  4.8k 
                 0.200% 
               
               
                  1k 
                  1.0k 
                 0.990% 
               
               
                   
               
             
          
         
       
     
     Although the above description illustrates measuring the electrical resistance of the coil to ground path using the sense resistance, any appropriate configuration may be utilized. For example, a sense resistance can be positioned in other locations, or other current resistance measurement techniques may be utilized. The measurement techniques are not limited to a sense resistor and/or analog to digital converter. 
     The present invention allows the coil to ground resistance  70  to be checked without using external circuitry and can be performed automatically by the microprocessor  40 . This avoids a lengthy process of shutting down the industrial process or otherwise taking the magnetic flowmeter offline. Manual testing is also prone to operator error. The testing may be performed remotely, for example, from a control room. Microprocessor  40  can be instructed to perform a test by communicating over the two-wire process control loop or through other techniques including radio or other wireless communication techniques. The testing can also be performed as a background operation. For example, a test can be periodically performed during a half cycle of the drive current. This automatic testing can be performed periodically, for example, every few minutes or otherwise as desired. 
     If the resistance of the signal path between the coil and ground is measured, this information can be used to correct the flow measurements for the resultant loss in drive signal. For example, microprocessor  40  can compensate the flow measurements using a compensation algorithm, for example, a polynomial curve fit, linear offset, or the like. This allows the magnetic flowmeter to continue operation prior to replacement or repair of the damaged components. Further, upon determination of a ground path, the flowmeter can be configuring to signal the error to an operator, for example, using an audible alert, by transmitting data over the two-wire process control loop, by wireless transmission, etc. 
     The particular steps used to perform the diagnostics can be stored as programming instructions in memory  44  for use by microprocessor  40 .  FIG. 5  is a simplified flowchart  100  showing steps in accordance with the present invention. Flowchart  100  begins at start block  102 . At block  104 , the field effect transistors  16 - 22  are controlled as described above. A drive current is applied at block  106  by power source  12  and the resultant current I S  is measured at block  108 , for example, using analog to digital converter  58 . At block  110 , the measured current I S  is compared with a threshold level. The threshold can be set as desired. For example, it may be desirable to have the threshold sufficiently high to avoid falsely identifying a current leakage path. If the current I S  is less than the threshold, operation continues without any fault identifying. However, if the sensed current is greater than the threshold, an output warning can be provided at block  112 . Additionally, or alternatively, at block  114  the flow measurements can be compensated based upon the amount of current leaking through the ground path and the compensated flow measurement can be output at block  116 . In addition to comparing to a threshold, as illustrated at block  110 , other diagnostic techniques can be utilized including monitoring the leakage current for trends over a period of time, sudden change in the leakage current, or other techniques. Further, the output  112  can include additional information including the value of the leakage current, the resistance of the path to ground, a residual life estimate before which service will be required, a percent error in the flow measurements due to the leakage current, etc. 
     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. The particular measurement or drive circuitry can be selected as desired. Other types of diagnostic circuitry and algorithms may be employed. The circuitry can be configured to operate automatically, or in response to a recorded instruction.