Method of operating an electromagnetic flowmeter

Abstract

The flowmeter comprises a flow sensor with a flow tube, two electrodes, and two field coils traversed by a first excitation current and a second excitation current, respectively, as well as control and evaluation electronics. The method serves to generate an error signal when the uniform turbulence in the liquid to be measured is disturbed. There are four quarter cycles. During each quarter cycle, a voltage is derived from the electrodes, and from these voltages, a first and a second voltage difference and a quotient using the first and the second voltage differences are formed. The latter is determined during calibration under uniformly turbulent flow conditions, and stored. In operation, values of the quotient are continuously formed and compared with the stored quotient; in case of deviations, an alarm is triggered and/or the volumetric flow rate signal represented by the first voltage difference is corrected.

What is claimed is: 
     
       1. A method of operating an electromagnetic flowmeter having a flow tube connected into a fluid-conveying line, said method comprising the steps of:
 causing the fluid to flow though the flow tube; 
 causing a first excitation current of predeterminable strength, said first excitation current being generated by means of a measuring and control circuit of the electromagnetic flowmeter, to flow through a first field coil mounted on the flow tube for producing a first partial magnetic field of predeterminable average strength which cuts through the fluid; 
 causing a second excitation current of predeterminable strength, said second excitation current being generated by means of said measuring and control circuit, to flow through a second field coil mounted on the flow tube for producing a second partial magnetic field of predeterminable average strength which also cuts through the fluid; 
 varying the strength of at least one of the excitation currents in such a manner that the average strengths of the two partial magnetic fields are at least temporarily different from each other; 
 reversing the polarity of one of the two excitation currents in such a manner that the two partial magnetic fields are at least temporarily directed opposite to each other while having different average strengths; 
 inducing a voltage in the moving fluid traversed by the two partial magnetic fields for changing potentials applied to measuring electrodes positioned at the flow tube; and 
 picking off potentials applied to the measuring electrodes for producing a measurement signal derived from the voltage induced in the moving fluid. 
 
     
     
       2. A method as set forth in  claim 1 , further comprising the step of:
 varying the strength of at least one of the two excitation currents in such a way that the average strengths of the two partial magnetic fields are temporarily essentially equal. 
 
     
     
       3. A method as set forth in  claim 2 , further comprising the step of:
 repeatedly sampling the measurement signal to obtain a sequence of discrete sample values which corresponds to a waveform of said induced voltage. 
 
     
     
       4. A method as set forth in  claim 3 , further comprising the step of:
 storing the sampling sequence section by section in a storage means of the measuring and control circuit. 
 
     
     
       5. A method as set forth in  claim 4 , further comprising the step of:
 determining a first voltage difference between said discrete sample values of said sequence of discrete sample values which were each sampled at an instant when said average strengths of the two partial magnetic fields are equal. 
 
     
     
       6. A method as set forth in  claim 3 , further comprising the step of:
 determining a first voltage difference between said discrete sample values of said sequence of discrete sample values which were each sampled at an instant when said average strengths of the two partial magnetic fields are equal. 
 
     
     
       7. A method as set forth in  claim 1 , further comprising the step of: repeatedly sampling the measurement signal to obtain a sequence of discrete sample values which corresponds to a waveform of said induced voltage. 
     
     
       8. A method as set forth in  claim 7 , further comprising the step of:
 storing the sampling sequence section by section in a storage means of said measuring and control circuit. 
 
     
     
       9. A method as set forth in  claim 8 , further comprising the step of:
 determining a first voltage difference between said discrete sample values of said sequence of discrete sample values which were each sampled at an instant when said average strengths of the two partial magnetic fields are equal. 
 
     
     
       10. A method as set forth in  claim 9 , further comprising the step of:
 determining a second voltage difference between said discrete sample values of said sequence of discrete sample values which were each sampled at a instant when said average strengths of the two partial magnetic fields are different from each other and the two partial magnetic fields are directed opposite to each other. 
 
     
     
       11. A method as set forth in  claim 7 , further comprising the step of: determining a first voltage difference between said discrete sample values of said sequence of discrete sample values which were each sampled at an instant when said average strengths of the two partial magnetic fields are equal. 
     
     
       12. A method as set forth in  claim 11 , further comprising the step of:
 determining a second voltage difference between said discrete sample values of said sequence of discrete: sample values which were each sampled at a instant when said average strengths of the two partial magnetic fields are different from each other and the two partial magnetic fields are directed opposite to each other. 
 
     
     
       13. A method as set forth in  claim 12 , further comprising the step of:
 determining a volumetric flow rate value by means of said two voltage differences. 
 
     
     
       14. A method as set forth in  claim 13 , further comprising the steps of:
 deriving the volumetric flow rate value from said first voltage difference; and 
 using said second voltage difference to correct flow-profile-induced deviations of said first voltage difference from the actual volumetric flow rate. 
 
     
     
       15. A method as set forth in  claim 12 , further comprising the step of:
 using said second voltage difference to trigger an alarm which signals a flow profile resulting in erroneous measurement signals. 
 
     
     
       16. A method as set forth in  claim 12 , further comprising the step of:
 forming a quotient of said two voltage differences. 
 
     
     
       17. A method as set forth in  claim 16 , further comprising the step of:
 comparing said quotient with a threshold value which represents a predetermined flow profile to be monitored. 
 
     
     
       18. An electromagnetic flowmeter for a fluid flowing in a line, comprising:
 a flow tube connectable into the line conducting the fluid; 
 a measuring and control circuit; 
 a coil assembly fed by said measuring and control circuit, said coil assembly producing a magnetic field cutting across said flow tube by means of a first field coil mounted on said flow tube and by means of a second field coil mounted on said flow tube; 
 at least two measuring electrodes for picking off potentials which are induced in the fluid flowing through said flow tube and traversed by the magnetic field; and 
 means connected at least intermittently to said measuring electrodes for producing at least one measurement signal derived from the potentials induced in the fluid, wherein:
 said first field coil is traversed at least intermittently by a first excitation current, and said second field coil is traversed at least intermittently by a second excitation current; and 
 the two excitation currents are adjusted by means of said measuring and control circuit in such a manner that at least intermittently, a first partial magnetic field, produced by means of said first field coil, has an average strength which is different from an average strength of a second partial magnetic field, produced simultaneously by means of said second field coil. 
 
 
     
     
       19. A method of operating an electromagnetic flowmeter for measuring the volumetric flow rate of an electrically conductive and moving fluid, the flowmeter having a flow sensor comprising: a flow tube for the moving fluid, of which an inner portion, which contacts the fluid, is electrically nonconductive, and which has a tube wall; a first electrode positioned with respect to the flow tube, a second electrode positioned with respect to the flow tube, which electrodes are located on a first diameter of the flow tube; and a coil assembly, mounted on the flow tube and comprising a first field coil and a second field coil, the coil assembly being located on a second diameter of the flow tube, which is perpendicular to the first diameter, and being operable to produce a magnetic field cutting across the tube wall and the fluid when a first excitation current flows in the first field coil and a second excitation current flows in the second field coil, the method comprising the step of:
 changing the amplitude and direction of the excitation currents periodically during each cycle of the excitation currents such that: during a first quarter cycle, the excitation currents are equal, have a constant value, and flow through the field coils in the same direction, a first direction; during a second quarter cycle, the first excitation current has the constant value and flows through the first field coil in an opposite direction to the first direction, the second excitation current is less than the constant value by a constant amount and flows through the second coil in the first direction; during a third quarter cycle, the excitation currents have the constant value and flow through the field coils in the opposite direction; and during a fourth quarter cycle, the first excitation current has the constant value and flows through the first field coil in the first direction, and the second excitation current is less than the constant value by the constant amount and flows through the second field coil in the opposite direction; 
 forming first, second, third and fourth voltages from the two potentials during the first, second, and fourth quarter cycles, respectively; 
 forming a first voltage difference from the first and third voltages, which serves to compute a volumetric flow rate signal, 
 forming a second voltage difference from the second and fourth voltages, 
 forming a quotient from the second and first voltage differences, said quotient is determined during a calibration step of the electromagnetic flowmeter under uniformly turbulent flow conditions and stored as a device constant in the flowmeter; 
 continuously forming instantaneous values of the quotient in operation, which are compared with the device constant; and 
 triggering an alarm when a predeterminable threshold is exceeded and/or the volumetric flow rate signal is corrected. 
 
     
     
       20. A method of operating an electromagnetic flowmeter for measuring the volumetric flow rate of an electrically conductive and moving fluid, the flowmeter including a flow sensor comprising: a flow tube for the moving fluid, the flow tube including a nonconductive inner surface which contacts the fluid, a first electrode, a second electrode, both electrodes being located on a first diameter of the flow tube, and a coil assembly, mounted on the flow tube and comprising a first field coil and a second field coil, the coil assembly being located on a second diameter of the flow tube, which is perpendicular to the first diameter, and being operable to produce a magnetic field cutting across the tube wall and the fluid when a first excitation current flows in the first field coil and a second excitation current flows in the second field coil, the method comprising the steps of:
 feeding a current to said first and second field coils, whereby the amplitude, and direction of the current changes periodically;   generating a voltage as a function of the periodically changing current and said moving fluid;   forming a quotient defined as a ratio of a first and a second generated voltage; and   comparing said quotient with a given device constant value for triggering an alarm when said quotient exceeds a predetermined threshold value.    
     
     
       21. The method as claimed in claim 20, wherein the device constant is determined during calibration of the electromagnetic flowmeter.  
     
     
       22. The method as claimed in claim 21, further comprising the step of:
 correcting an error in computing the volumetric flow rate based on said quotient.    
     
     
       23. The method as claimed in claim 21, further comprising steps of:
 deriving from said at least one measurement signal a digital sampling sequence corresponding to a waveform of a voltage induced in the fluid, and   computing from said digital sampling sequence a volumetric flow rate of the, fluid flowing in said line.    
     
     
       24. The method as claimed in claim 20, further comprising the step of:
 deriving from said at least one measurement signal a digital sampling sequence corresponding to a waveform of a voltage induced in the fluid.    
     
     
       25. The method as claimed in claim 24, further comprising the step of:
 using said digital sampling sequence for monitoring the flow profile of said flowing fluid.    
     
     
       26. A method of operating an electromagnetic flowmeter for measuring the volumetric flow rate of an electrically conductive fluid, the flowmeter including a flow tube for the fluid to be measured, a first electrode and a second electrode, a coil assembly including a first field coil and a second field coil, and a measuring and control circuit coupled to said first and second electrodes and to said coil assembly, said method comprising the steps of:
 feeding a first excitation current to said first field coil and a second excitation current to said second field coil for producing magnetic field cutting across the tube wall and a fluid within the flow tube;   reversing a polarity of each of said first and second excitation currents;   changing a strength of at least one of said first and second excitation currents;   picking off potentials applied to said first and second electrodes for producing measurement signal derived from a voltage induced in the fluid; and   using said measurement signal for detecting an event reducing the accuracy of said flowmeter.    
     
     
       27. The method as claimed in claim 26, wherein the strength of at least one of said first and second excitation currents is changed such that said at least one of said first and second excitation currents has a first current value temporary and a second current value temporary, said second current value is less than said first current value.  
     
     
       28. The method as claimed in claim 27, further comprising the steps of:
 deriving from said at least one measurement signal a digital sampling sequence corresponding to a waveform of a voltage induced in the fluid, and   computing from said digital sampling sequence a volumetric flow rate of the fluid flowing in said line.    
     
     
       29. The method as claimed in claim 27, further comprising the step of:
 correcting an error in computing a volumetric flow rate.    
     
     
       30. The method as claimed in claim 29, further comprising the step of: triggering an alarm indicating said error in computing the volumetric flow rate.  
     
     
       31. The method as claimed in claim 26, wherein the event reducing the accuracy of said flowmeter is detected based on a device constant determined during calibration of the electromagnetic flowmeter.  
     
     
       32. The method as claimed in claim 26, further comprising the step of:
 deriving from said at least one measurement signal a digital sampling sequence corresponding to a waveform of a voltage induced in the fluid.    
     
     
       33. The method as claimed in claim 32, further comprising the step of:
 using said digital sampling sequence for monitoring a flow profile of said flowing fluid.    
     
     
       34. An electromagnetic flowmeter for a fluid flowing in a line, comprising:
 a flow tube connectable into the line conducting the fluid;   a coil assembly for producing a magnetic field, which induces a voltage in said flowing fluid, said coil assembly including a first field coil mounted on said flow tube and a second field coil mounted on said flow tube, said first field coil is traversed at least intermittently by a first excitation current, and said second field coil is traversed at least intermittently by a second excitation current,   at least two measuring electrodes for picking off potentials, which are induced in the fluid flowing through said flow tube and traversed by the magnetic field; and   a measuring and control circuit feeding said coil assembly with said first and second excitation currents and producing at least one measurement signal derived from the potentials picked-off by said at least two measuring electrodes, wherein:   the first and second excitation currents are adjusted by means of said measuring and control circuit in such a manner that a polarity of each of said first and second excitation currents changes periodically and that a strength of at least one of said first and second excitation currents varies at least intermittently; and   the measuring and control circuit monitors, based on said potentials picked-off by said at least two measuring electrodes, a flow profile of said flowing fluid.    
     
     
       35. The electromagnetic flowmeter as claimed in claim 34, wherein:
 the measuring and control circuit monitors the flow profile of said flowing fluid based on a device constant determined during calibration of the electromagnetic flowmeter.    
     
     
       36. The electromagnetic flowmeter as claimed in claim 34, wherein:
 said measuring and control circuit computes from said at least one measurement signal a volumetric flow rate of the fluid flowing in said line.    
     
     
       37. The electromagnetic flowmeter as claimed in claim 36, wherein:
 said measuring and control circuit correct an error in computing the volumetric flow rate based on said monitoring of the flow profile of said flowing fluid.    
     
     
       38. The electromagnetic flowmeter as claimed in claim 34, wherein:
 said measuring and control circuit derives from said at least one measurement signal a digital sampling sequence corresponding to a waveform of a voltage induced in the fluid.    
     
     
       39. The electromagnetic flowmeter as claimed in claim 38, wherein:
 said measuring and control circuit monitors the flow profile of said flowing fluid based on said digital sampling sequence.    
     
     
       40. A method of operating an electromagnetic flowmeter for measuring the volumetric flow rate of an electrically conductive fluid, the flowmeter including a flow tube for the fluid to be measured, a first electrode and a second electrode, a coil assembly including a first field coil and a second field coil, and a measuring and control circuit coupled to said first and second electrodes and to said coil assembly, said method comprising the steps of:
 feeding a first excitation current to said first field coil and a second excitation current to said second field coil for producing magnetic field cutting across the tube wall and a fluid within the flow tube;   reversing a polarity of each said first and second excitation currents;   changing a strength of at least one of said first and second excitation currents;   picking off potentials applied to said first and second electrodes for producing a measurement signal derived from a voltage induced in the fluid;   using said measurement signal for computing said volumetric flow rate; and   using said measurement signal for triggering an alarm indicating an error in computing the volumetric flow rate.    
     
     
       41. A method of operating an electromagnetic flowmeter for measuring the volumetric flow rate of an electrically conductive fluid, the flowmeter including a flow tube for the fluid to be measured, a first electrode and a second electrode, a coil assembly including a first field coil and a second field coil, and a measuring and control circuit coupled to said first and second electrodes and to said coil assembly, said method comprising the steps of:
 feeding a first excitation current to said first field coil for and a second excitation current to said second field coil for producing magnetic field cutting across the tube wall and a fluid within the flow tube;   changing each of said first and second excitation currents;   picking off potentials applied to said first and second electrodes for producing a measurement signal derived from voltages induced in the fluid during cycles defined by periodical changing said first and second excitation currents, and using said measurement signal for computing said volumetric flow rate; and   wherein said step of changing each of said first and second excitation currents comprises reversing a polarity of each of said first and second excitation currents, and varying a strength of at least one of said first and second excitation currents.    
     
     
       42. The method as claimed in claim 41, wherein:
 an event reducing the accuracy of said flowmeter in computing said volumetric flow rate is detected based on said measurement signal.    
     
     
       43. The method as claimed in claim 41, wherein:
 a flow profile of said flowing fluid is monitored based on said measurement signal.    
     
     
       44. The method as claimed in claim 43, wherein:
 said volumetric flow rate is computed based on a first voltage difference derived from said potentials applied to said first and second electrodes and on a second voltage difference derived from said potentials applied to said first and second electrodes, each of said first and second voltages does correspond with different strengths of at least one of said first and second excitation currents.    
     
     
       45. The method as claimed in claim 41, wherein:
 said volumetric flow rate is computed based on a first voltage difference derived from said potentials applied to said first and second electrodes and on a second voltage difference derived from said potentials applied to said first and second electrodes, each of said first and second voltages does correspond with different strengths of at least one of said first and second excitation currents.