Q&A: AC vs DC Magmeter Excitation

Dec. 14, 2007

Ronald W. DiGiacomo Ronald W. DiGiacomo is ABB”s business development manager for flow technologies in the United States. Ron has worked in the process instrumentation field for 25 years,

Ronald W. DiGiacomo

Ronald W. DiGiacomo is ABB”s business development manager for flow technologies in the United States. Ron has worked in the process instrumentation field for 25 years, primarily in flow measurement and control. Mr. DiGiacomo also spent 15 years with Emerson and four years with Invensys. He can be reached at [email protected] or 215 589-4350.

Q: How do electromagnetic flowmeters work?

A: The basic principle of operation for magnetic flowmeters is the same for all types, regardless of manufacturer. Faraday’s Law states that the induced voltage across a conductor moving at a right angle through a magnetic field is directly proportional to the velocity of the conductor. Accordingly, a conductive fluid that passes through a magnetic field at a right angle to the magnetic field generates a voltage directly proportional to the fluid velocity. Given a velocity measurement and a known free cross-sectional area (ID of the meter body), a volumetric flow can easily be derived.

A pair of electrodes on either side of the flowmeter detects the voltage proportional to fluid velocity. The voltage goes to a transmitter that processes the raw flow signal and converts it to a scaleable useable signal for process control or simple totalization.

Q: What might introduce electronic noise into a system and does noise affect AC and DC magnetic flowmeters in the same way?

A: Besides the flow velocity signal, internal electronic noise sourced to capacitive and inductive couplings for example can be present in metering systems, which can be addressed by sound cabling practices; appropriate measures such as shielding, insulation, and capacitance neutralization are recommended. In addition to internal noise components, process-induced noise sourced to electrically charged fluids, large particles, and electrochemical potentials at the electrode interface can be introduced into the system. Such externally induced noise does not affect AC and DC excited systems in the same way with respect to zero shift and continuous flow signal.

Conventional AC systems remain more susceptible to zero shift than DC systems. This phenomenon is best understood by appreciating that Faraday’s Law cuts two ways. As previously noted, the voltage proportional to velocity is created by a conductor (i.e., conductive process fluid) passing through a magnetic field. It is equally true that nonmoving conductors — such as electrode wires — near to a changing magnetic field also produce voltage.

In AC-excited magnetic flowmeters the continuous alternating current in the presence of a stationary conductors or couplings between the magnetic coils and electrode wires can create a varying non-flow induced voltage, which is electronic noise. In AC systems, if the noise and flow signals are out of phase with each other they can be distinguished with circuitry or software, enabling the pure flow signal to be processed and understood. Field affects that are peculiar to the installation and not present during factory calibration must, of course, be dealt with in the field. Accordingly, nonmoving insulating coatings that accumulate on the electrodes during normal process conditions (i.e. after zeroing the system in the field) can cause an apparent shift in zero in AC systems.

DC magnetic flowmeters excite magnetic coils with a pulsating direct-current, making it possible to subtract-out noise signals that would otherwise be generated by the continuously changing magnetic field in traditional AC excitation systems. Within "square wave" excitation technology there is an inherent settling time for noise that would otherwise be induced in continuous alternating current systems.
In DC systems, depending upon the frequency of excitation, remaining noise that affects zero can be easily detected in isolation from the flow signal when the magnetic coil has no current flowing through it, enabling the noise component to be subtracted from the aggregate of the noise and flow signal that is present when the magnet is turned on. Accordingly, a DC system has the potential of continuously compensating for zero-shift without the need of having to manually re-zero the system.

Q: What are the key pros and cons of traditional AC and pulsed-DC excitation methods for magmeters?

A: As discussed above, due to the changing magnetic field in AC systems, traditional AC systems are inherently prone to zero-shift, whereas DC signals are not. In addition, power requirements are less for DC systems. The coils in DC systems are energized intermittently, being a pulsed system; and the power consumption in AC magnetic flowmeters is a function of meter size, which is not the case with DC-excited systems.

Originally DC magnetic flowmeters were driven at low frequencies, on the order of 3.75 Hz, which afforded excellent zero stability, but were also susceptible to 1/f noise, which is electronic noise that is inversely proportional to excitation frequency. By increasing the excitation frequency of DC meters, certain kinds of process noise can be addressed but at the cost of zero stability since at higher operating frequencies the square wave begins to take on the characteristics of AC meters and, therefore, the inherent weaknesses of traditional AC-driven magnetic flowmeters, as discussed above. Moreover, the zero-stability advantage of DC meters is diminished through attempts to filter out 1/f noise. Distortion of the square waveform occurs, which in turn affects how cleanly and instantaneously the signal changes from its high to low state, directly limiting the meter’s ability to distinguish and subtract out noise from the noise plus flow signal.

AC systems, because they are continuous (not pulsed) systems, have faster response times than DC systems, making them more suitable when dynamic response is critical. However, the main advantage of AC systems is with respect to process-generated noise. AC systems have higher signal-to-noise ratios and operate in more ideal noise spectrums, due to higher signal strength and excitation frequency respectively. Accordingly, AC systems are more robust in noisy applications (e.g. slurries and pulps). In addition, inherent to a DC-excited (square wave) system is a large bandwidth, which reacts to all noise components at the excitation frequency and at each odd harmonic of the excitation frequency, making the system less suitable in noisy applications. Whereas conventional AC (sinusoidal) excited systems have a narrower bandwidth and, therefore, are affected by noise voltages primarily in the range of the excitation frequency.

Q: Why does ABB believe its new FSM4000 magmeter offers the best of AC and DC excitation in a single device?

A: The FSM4000 magmeter from ABB uses a continuous nonpulsed AC excitation, which allows for faster dynamic response yielding tighter accuracies (most notably during pulsating flow conditions) and higher signal-to-noise ratios. The FMS adjustable noise damping facility enables response times of down to 50 milliseconds.
The FSM4000 also operates at an optimal 70 Hz, a frequency that is significantly higher than line frequency and in a low part of the noise spectrum. Operating at this frequency eliminates virtually all need for output signal dampening used in noisy applications.

FSM4000 employs digital signal processing (DSP), which is more flexible and effective than hardware in separating flow signal from unwanted noise. Digital filters with sharp drop-offs enable the system to eliminate hydraulic and line noise from the overall signal. DSP allows for faster A/D conversions from the sensor signal and allows for greater numbers of sampling points when compared to non-DSP technologies.

The FSM4000, through use of a search coil, directly measures the strength of the magnetic field. As opposed to utilizing a constant-coil drive current, the measured value of the magnetic field is fed back into the coil drive circuit in order to control the coil drive so that a constant magnetic field is maintained. Accordingly, losses to the magnetic field that would otherwise affect meter performance are dramatically reduced. The improved signal quality produced by DSP technology eliminates the need to routinely reset zero. The processing power of the DSP Converter increases zero stability and low-flow performance, enabling the meter to function with extreme accuracy over higher turndowns and a wide range of process conditions. Tangible advantages include improved measurements in applications involving vibration, hydraulic noise, and temperature fluctuation.

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