Abstract:
A magnetic flowmeter comprises a pipe section for process flow, a coil for generating a magnetic field across the pipe section, a current source for energizing the coil to generate the magnetic field at a coil drive frequency, and electrodes for sensing voltage induced across the process flow by the magnetic field. A processor calculates a function of the sensed voltage and generates a flow output based on the function. The processor adjusts an operating parameter of the flowmeter, such as electrode voltage sampling period, the coil drive frequency, or a phase shift, as a function of a sensed electrode-to-electrode or electrode-to-ground complex impedance.

Description:
BACKGROUND 
     This invention relates generally to fluid processing, and specifically to process flow measurement and control. In particular, the invention concerns measurement techniques for magnetic flowmeters. 
     Magnetic flowmeters (or mag meters) measure flow by Faraday induction, an electromagnetic effect. The meter energizes a coil to generate a magnetic field across a pipe section, and the magnetic field induces an electromotive force (EMF) across the process flow. The resulting potential difference (or voltage) is measured using a pair of electrodes that extend through the pipe section and into contact with the process flow, or via capacitive coupling. The flow velocity is proportional to the induced EMF, and the volumetric flow rate is proportional to the flow velocity and flow area. 
     In general, electromagnetic flow measurement techniques are applicable to water-based fluids, ionic solutions and other conducting flows. Specific uses include water treatment facilities, high-purity pharmaceutical manufacturing, hygienic food and beverage production, and chemical processing, including hazardous and corrosive process flows. Magnetic flowmeters are also employed in the hydrocarbon fuel industry, including hydraulic fracturing techniques utilizing abrasive and corrosive slurries, and in other hydrocarbon extraction and processing methods. 
     Magnetic flowmeters provide fast, accurate flow measurements in applications where differential pressure-based techniques are disfavored because of the associated pressure drop (for example, across an orifice plate or Venturi tube). Magnetic flowmeters can also be used when it is difficult or impractical to introduce a mechanical element into the process flow, such as turbine rotor, vortex-shedding element or Pitot tube. 
     Some magnetic flowmeters use field coils driven directly by AC line power. Another type of magnetic flowmeter, commonly referred to as a pulsed DC magnetic flowmeter, excites or powers the field coil periodically with a low frequency square wave. Pulsed DC magnetic flowmeters utilize a magnetic field which changes direction at a certain frequency. When the field is inverted, a spike in the electrode voltage is created by the rapidly changing magnetic field. This spike is not related to the flow rate, but to the rate of change of the magnetic field. In order to make a flow measurement, the electrode voltage measurement circuitry must be programmed to wait until this spike has completely decayed, or the voltage difference due to the decaying spike will appear to be measured flow. 
     The rate of decay of the voltage spike changes in relation to the impedance of the process fluid, which acts as a resistive-capacitive (RC) filter of the voltage spike. The impedance in turn depends on the conductivity of the process fluid, any coatings on the electrodes, and the length of the cable between the transmitter and the flow tube. Therefore, the rate of decay of the voltage spike is not known when the flowmeter leaves the factory and may change over time if the fluid conductivity changes or if coatings build up on the electrode surfaces. 
     Because the electrode-to-electrode impedance is not known in advance and may change over time, pulsed DC flowmeters are typically programmed to sample only at the very end of each half cycle where the probability of the voltage spike “leaking” into the flow measurement is very low. This measurement time may be, for example, only 20 percent of each half cycle. 
     SUMMARY 
     This invention relates to a magnetic flowmeter which makes an automatic adjustment of an operating parameter based on sensed complex impedance. The magnetic flowmeter includes a pipe section, a coil proximate an outer diameter of the pipe section, a current source, electrodes that extend to the inner diameter of the pipe section, and a processor that generates a flow output as a function of sensed voltage between the electrodes. The processor adjusts a flowmeter operating parameter as a function of a sensed complex impedance between the electrodes or between one or more of the electrodes and ground. 
     A further embodiment of the invention is a method of measuring flow. The method includes generating a magnetic field across a process flow and sensing, during an electrode voltage sensing period, a voltage induced between sensing electrodes by the magnetic field, and generating a flow output based on voltage sensed. The method further includes sensing a complex impedance between sensing electrodes or between one or more of the sensing electrodes and ground, and varying a flowmeter operating parameter as a function of the sensed complex impedance. 
     In another embodiment, a method of operating a magnetic flowmeter comprises sensing complex impedance between sensing electrodes or between one or more of the sensing electrodes and ground and automatically adjusting an operating parameter of the magnetic flowmeter as a function of the sensed complex impedance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a pulsed DC magnetic flowmeter. 
         FIGS. 2A and 2B  are block diagrams illustrating embodiments for adjusting the operating parameters of electrode voltage sampling period or coil drive pulse frequency, respectively, as a function of sensed complex impedance. 
         FIG. 3  is a graph showing electrode signal as a function of time for a pulsed DC magnetic flowmeter with a flow tube filled with water of two different conductivities. 
         FIG. 4  is a graph showing electrode signal as a function of time for a pulsed DC magnetic flowmeter with flow tube filled with water of two different conductivities, and further illustrating the automatic adjustment of the electronic voltage sampling period based upon sensed complex impedance. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary pulsed DC magnetic flowmeter  10 , which includes primary section (or flow tube)  10 A and secondary section (or transmitter)  10 B. Flow tube  10 A includes pipe  12 , insulating liner  14 , electrodes  16 A and  16 B, and field coils  18 A and  18 B. 
     The primary function of flow tube  10 A is to produce a voltage proportional to the velocity of the fluid being measured. Field coils  18 A and  18 B are energized by passing a current through them to develop a magnetic field. In a pulsed DC magnetic flowmeter, the direction of the coil drive current is reversed periodically so that the magnetic field produced by field coils  18 A and  18 B changes direction. The process fluid flowing through the interior of flow pipe  10 A functions as a moving conductor inducing a voltage in the fluid. The flush mounted electrodes  16 A,  16 B inside flow tube  10 A are in direct electrical contact with the conductive process fluid, thereby picking up voltages present in the fluid. To prevent the voltage from being shorted, the fluid must be contained in an electrically insulating material. When pipe  12  is a metal tube, the insulation is provided by liner  14 , which is a non-conducting material such as polyurethane, polytetrafluoroethylene (PTFE), or an insulating rubber material. 
     Transmitter  10 B interprets the voltage generated at electrode  16 A and  16 B and transmits a standardized signal to a monitoring or control system. Secondary section  10 B is commonly referred to as a transmitter or signal converter. 
     Transmitter  10 B typically includes signal processor  20 , digital processor  22 , coil driver  24 , and communication interface  26 . Signal conversion, conditioning, and transmission are the principal functions of transmitter  10 B. 
     Digital processor  22  controls the pulse frequency of the pulsed DC coil drive current supplied by coil driver  24  to coils  18 A and  18 B. The current waveform provided by coil driver  24  is a square wave having a frequency referred to as the pulse frequency. The coil drive may be periodically interrupted to turn coils  18 A and  18 B off. This allows periodic measurement of the voltage between electrodes  16 A and  16 B or between electrode  16 A or electrode  16 B and ground. These measurements may be used to sample and correct for noise induced voltages within flow tube  10 A. The measurements may also be used to sense complex impedance, which can then be used to automatically adjust an operating parameter of magnetic flowmeter  10 , such as electrode voltage sampling period or coil drive pulse frequency. 
     It is also possible to sense complex impedance without turning off the coils. A current can be applied through electrodes  16 A and  16 B at a frequency that is away from the drive frequency and/or asynchronous to the drive signal to coils  18 A and  18 B so that the flow rate can continue to be measured while the complex impedance sensing function is being performed. 
     Signal processor  20  is connected to electrodes  16 A and  16 B and to ground. The ground connection may be to pipe  12 , or may be to a flange or pipe section upstream or downstream of pipe  12 . 
     During the electrode voltage sampling period defined by digital processor  22 , signal processor  20  monitors potential VA at electrode  16 A and potential VB at electrode  16 B. Signal processor  20  produces a voltage representing the difference in potential between electrode  16 A and  16 B and converts that voltage into a digital signal representing the electrode voltage during the electrode voltage sampling period. Digital processor  22  may perform further signal processing and grooming of the digital signals received from signal processor  20 . Digital processor  22  supplies a flow measurement value to communication interface  26 , which communicates that value to a read out or control system (not shown). The communication by communication interface  26  can be in the form of an analog current level which varies between 4 and 20 mA; a HART® communication protocol in which digital information is modulated upon a 4-20 mA current; a communication protocol over a digital bus such as, for example, Fieldbus (IEC 61158); or wireless communication over a wireless network using a wireless protocol such as, for example, WirelessHART® (IEC 62951). 
     Signal processor  20  can also monitor complex impedance within flow tube  10 A, which will vary depending on conductivity of the fluid flowing through flow tube  10 A, the presence of any coatings built up on the surfaces of electrode  16 A and  16 B, and the length of cable between flow tube  10 A and transmitter  10 B. In this sensing function, signal processor  20  applies a current through electrodes  16 A and  16 B. The voltage between electrode  16 A and  16 B or between either electrode  16 A or  16 B and ground can be used by digital processor  22  to derive the value of a sensed complex impedance, which can then be used to automatically adjust an operating parameter of magnetic flowmeter  10  as shown in  FIGS. 2A and 2B .  FIGS. 2A and 2B  are block diagrams illustrating embodiments for adjusting the operating parameters of electrode voltage sampling period or coil drive pulse frequency, respectively, as a function of sensed complex impedance. As shown in the embodiment of  FIG. 2A , a complex impedance is sensed (Step  100 ), then an electrode voltage sampling period is determined as a function of the sensed complex impedance (Step  102 ). Finally, the electrode voltage sampling period is adjusted to the sampling period determined as a function of the sensed complex impedance to enhance the signal-to-noise ratio (Step  104 ). Similarly, in the embodiment of  FIG. 2B , a complex impedance is sensed (Step  200 ), and then a coil drive frequency is determined as a function of the sensed complex impedance (Step  202 ). Finally, the coil drive frequency is adjusted to the coil drive frequency determined as a function of the sensed complex impedance to enhance the signal-to-noise ratio (Step  204 ). In some embodiments, sensed complex impedance may be employed to adjust both electrode voltage sampling period and coil drive pulse frequency. 
     Thus, based upon the sensed complex impedance, digital processor  22  can vary electrode voltage sampling period or the coil drive pulse frequency, or both in a way that will enhance the signal-to-noise ratio. In particular, electrode voltage sampling period may be adjusted in an inverse relationship to sensed complex impedance, so that as the sensed complex impedance decreases, the electrode voltage sampling period may be increased. Adjustment of coil drive pulse frequency is also according to an inverse relationship with sensed complex impedance. As complex impedance decreases, the coil drive pulse frequency may be increased. 
       FIG. 3  is a graph showing electrode signal as a function of time for a pulse DC magnetic flowmeter. This graph is taken from a 6 inch Rosemount 8705 flow tube filed with water at two different conductivities. The coil drive pulse frequency is set to 37 Hz. The first sample was deionized water (DW) with a conductivity adjusted to 5 μS/cm, the minimum specified conductivity for the Rosemount 8705 flow tube. The second sample was standard tap water (TW), for which the conductivity measured 220 μS/cm. 
     As shown in region A in  FIG. 3 , the amplitude of the spike produced by the reversal of magnetic field direction is lower in the deionized water (DW) waveform than in the tap water (TW) waveform. 
     Area B shows the falling edge of the spike in the DW waveform lags the falling edge of the TW waveform spike. Area C shows that the spike of the DW waveform decays more slowly than the TW waveform. 
       FIG. 4  again shows a deionized water (DW) waveform and a tap water (TW) waveform. The coil frequency was again 37 Hz. The deionized water had a conductivity of 5 μS/cm, and the tap water had a conductivity of 220 μS/cm. 
     In  FIG. 4 , the TW and DW signals have been superimposed with a  15  period moving average labeled DWRA and TWRA. These rolling average waveforms make it easier to see the spike decay through the noise. 
     The graph in  FIG. 4  is marked with the standard 20% electrode voltage sampling period used in pulse DC magnetic flowmeter. This standard 20% period is labeled STD in  FIG. 4 . 
     Also shown by dashed line in  FIG. 4  is an increase to the electrode voltage sample time, labeled ADD. This represents additional sampling time that is possible for higher conductivity fluids. 
     The deionized water sample does not decay to near zero until shortly before the standard (20%) electrode voltage sampling period STD begins. The sampling period for deionized water, therefore, is approximately optimal. 
     The tap water (TW) spike, in contrast, has completely decayed long before standard electrode voltage sampling period STD begins. This means a longer sampling period can be used, which results in a higher signal-to-noise ratio. In the example shown in  FIG. 4 , the sampling period (STD+ADD) can be nearly triple the STD sampling period. 
     The ability to use a longer sampling period depends on being able to measure impedance of the process fluid, which permits a prediction of the decay time. Since the majority of process fluids that flow through magnetic flowmeters have a conductivity equal to or greater than tap water, most flow measurements using a magnetic flowmeter may benefit from increased sampling time. 
     Signal-to-noise ratio increases can lead to improvements in magnetic flowmeter performance. Alternatively, a reduction in power to the field coils could be realized without compromising performance. Fluids with conductivities higher than that of tap water would settle even faster, so they could see even larger improvements. 
     The benefits achieved by an automatic adjustment of electrode voltage sampling interval as a function of sensed complex impedance can be obtained without significant additional cost. In general, the changes to circuitry in the transmitter are minimal, and the determination of complex impedance and determination of the adjustment to the sampling period can be accomplished with software changes in digital processor  22 . 
     In another embodiment of the invention, coil drive frequency can also be automatically adjusted based on the impedance of the fluid. Generally, a higher frequency coil drive improves the signal-to-noise ratio, because process noise tends to be lower at higher frequencies. However, operating at higher frequencies has a limitation of a less reliable zero, because the voltage spike may not have completely decayed, and the decay time is not known because the fluid impedance is not taken into account. By sensing complex impedance of the fluid and adjusting the coil drive frequency accordingly, transmitter  10 B can operate magnetic flowmeter  10  at a maximum possible frequency without compromising zero stability. 
     Varying of an operating parameter of the magnetic flowmeter based on sensed complex impedance is also applicable to AC driven magnetic flowmeters. For example, sensed complex impedance could be used to adjust phase shift of the electrode signal based on electrode impedance. This provides better quadrature alignment, thereby reducing zero drift and may improve noise reduction depending on the measurement technique employed. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.