Design innovations in AC-excited magmeters offer the fast response and noise-free outputs with the zero-drift performance of pulsed DC magmeters
March 2008
By Greg Livelli
The pulp and paper, metals and mining industries hold some of the most aggressive and challenging applications for electromagnetic flowmeter (magmeter) measurement. Changing operating conditions, higher consistency and solid contents, and tighter process control are pushing the boundaries of magmeter performance.
Conventional electromagnetic flowmeters using DC excitation have been used to measure flow in these noisy and difficult applications, but they are typically saddled with long damping times to smooth their output. This results in slow response times and ultimately waste and quality issues.
Recent improvements in AC electromagnetic flowmeters can now provide these users with better performance, quicker response, and noise-free outputs not possible with DC technology. These improvements can lead to increased efficiency, improved quality, and conservation of raw materials and energy, reducing direct costs and increasing profits. The new AC magmeters can perform well in the simplest to the most extreme applications, making them a universal solution to conductive liquid flow measurement.
Reviewing the Principles
The principle of operation for magnetic flowmeters is based on the principle of Michael Faraday’s law of electromagnetic induction. Magnetic coils are excited by an AC or DC current creating a magnetic field within a meter body through which a conductive liquid passes. This voltage is extracted through a pair of electrodes that are installed on opposite sides of the pipe. The voltage developed is proportional to the density of the magnetic field, the length of the conductor, and the velocity of the conductor moving through the field. There is nothing in the function of magnetic flowmeters that depends on pressure, temperature, density or viscosity because the magmeter develops its signal independent of these parameters.
The volumetric flow rate through the magmeter is an easily derived function of this velocity signal and the known cross-sectional area of the meter body. The raw voltage signal (on the order of micro-volts) then goes to a transmitter for processing and conversion to a signal more suitable for process control or for simple totalization.
Internal noise signals accompany the raw flow velocity signal. Appropriate measures such as shielding, insulation and capacitance neutralization can eliminate internal noise. Signals from sources external to the flow meter (such as electrically charged fluids, large particles and electrochemical potentials at the electrode interface) also introduce unwanted electronic noise into the system. In addition, electromagnetic fields at line frequency present in virtually all installations contribute to the external noise.
Electronic noise does not affect AC and DC excited systems in the same way. Noise effects must be considered in the context of both zero shift and flow signals respectively.
AC vs DC Excitation Systems
Early electromagnetic flowmeters were all excited by AC. The magmeter coils were energized by line voltage (120 VAC +/- 10%) at 60 Hz line frequency. Figure 2 shows the resulting sinusoidal (sine) waveform developed by the signal electrodes of an AC magmeter. The amplitude of the waveform varies directly as the flow rate through the magmeter.
The AC magmeter proved to be a significant advancement when compared to flow measurement by differential pressure techniques. It offered no obstruction to the flow and could measure flow rates over a much wider range. Additionally, in cases of rapidly changing flow conditions, and, most significantly, when the pipe became empty, the AC magmeter provided fast full-scale response time. It recovered quickly from a non-full state.
However, the zero flow signal of conventional AC meters tended to drift, requiring repeated recalibration. The primary weakness of conventional AC systems is this zero-shift. The phenomenon is best understood by appreciating that Faraday’s Law cuts two ways. As already noted, the measured voltage is proportional to the velocity of the fluid passing through a magnetic field created by excitation. It is equally true that non-moving conductors, such as electrode signal wires near the varying magnetic field, also generate voltage.
In conventional AC-excited magnetic flowmeters, the continuous alternating current in the presence of a stationary conductor (the fluid at zero flow rate) creates a varying non-flow induced voltage, which is electronic noise. Typically, noise created by fixed conductive electrode wires can be “zeroed out” by operators during start-up (or even eliminated by circuitry if the voltages are out of phase with each other). However, non-moving conductive coatings that accumulate on the electrodes during normal process conditions (after zeroing the system) often cause an apparent shift in zero in conventional AC systems.
The introduction of pulsed DC electromagnetic flow meters went a long way towards solving the problem of maintaining a stable zero flow signal. Pulsed DC magmeters were originally energized by low voltage (~10 VDC) at 3.75 Hz. Instead of continuous power to the coils, they are both energized and deenergized during a voltage cycle. Figure 3 shows the square pulse waveform developed by the signal electrodes of a pulsed DC magmeter. During the "off" part of the cycle, any measured voltage constitutes electric noise. The measuring system can automatically and continuously compensate for this noise, greatly reducing the zero signal drift. As a result, pulsed DC magnetic flowmeters can maintain a stable signal for zero flow that is less affected by coating deposits on the signal electrodes.
Dealing with Process Signal Noise
Despite this advantage, pulsed DC magnetic flowmeters have often proved unsuccessful in certain applications because of process noise, which comes in two forms: 1/f noise and electrode impingement.
The 1/f noise decreases with excitation frequency. More noise occurs at low frequencies, especially at 10 Hz and less. The major problem with this process noise is that the low levels of the magnetic coil excitation result in a greater chance for disruption of the flow measurement. Figure 4 shows this 1/f noise function in graphical terms.
The magnitude of 1/f noise typically depends on the severity of conditions in a field installation. For example, in low consistency (0 to 3%) pulp stock flows, the 1/f noise increases as stock consistency increases. Also, chemical additions upstream and near a magmeter often produce 1/f noise as a function of rapidly changing fluid conductivities, which disturb the electrode interface.
The second type of process noise is a phenomenon known as electrode impingement. This is a mechanical disturbance of the electrode/electrolytic interface of the process fluid with the electrode surfaces. These disturbances cause the electrode signal to temporarily spike several times its normal magnitude. This noise is typical of heavy liquors that contain a variety of solids particles. In addition, most slurries represent a potential to display electrode impingement characteristics to various degrees.
Reducing Noise Content
The original pulsed DC magmeters operated at 3.75 Hz (waveform cycles of 3.75 times per second). This low frequency coil excitation produced excellent zero stability, but was susceptible to 1/f noise. In recent times, pulsed DC magmeter vendors have attempted to reduce the effect of 1/f noise by increasing the signal filter's low frequency cutoff value as well as by increasing the coil excitation frequency.
But two problems arise. First, the signal filter can also eliminate part of the live process signal being measured, adversely affecting magmeter accuracy. Second, as coil excitation frequency increases, the square waveform begins to deform, taking on the some of the characteristics of the AC magmeter. The waveform begins to “droop,” becoming distorted.
Because the pulsed DC magmeter measures process noise when the coils are de-energized, a droopy waveform makes it increasingly difficult for the signal converter to accurately separate process noise from process signal. For example, a 30 Hz coil drive will experience 1% droop in the square waveform when the low frequency cut-off is 0.3 Hz, and a 10% droop if the cut-off is raised to 3 Hz. Ultimately, this can lead to pulsed DC magmeter inaccuracies and non-repeatabilities. Pulsed DC noise suppression algorithms have had limited success in reducing the problems associated with mechanical disturbances.
Process noise, because its magnitude is a relatively large portion of the pulsed DC magmeter’s flow signal, also tends to distort the flow measurement. The historic fix is to increase the damping constant until the signal converter’s analog output ceases to be erratic. Because of the resulting long response times, this fix greatly reduces the ability to control production processes by means of magmeter flow measurement. Typically, pulsed DC magmeters in these cases serve only for monitoring purposes.
The AC magmeter, on the other hand, has a high signal-to-noise ratio in noisy flow applications. The AC magmeter delivers more power to the magmeter coils, so the measured electrode signal is a higher voltage compared to a DC meter. Most specifications for power consumption call for pulsed DC magmeters 24 in. and smaller to consume no more that 23 VA, while AC magmeters of the same size consume no more than 50 VA. The higher power requirements of the AC magmeter help to establish a measuring system that is much less susceptible to the ill effects of process noise.
As a result of noise issues, AC magmeters are often selected over pulsed DC magmeters on difficult, highly noisy applications. They more rapidly respond to changing flow conditions and recover more quickly from empty pipe conditions.
Closing the Gap
Although the pulsed-DC method of magnetic field excitation now predominates, it has not replaced AC technology entirely. AC magmeters are more resistant to process generated noise created by varying fluid conductivities and slurries, making them more suitable for many demanding applications. Various innovations have reduced the functionality gap between DC and AC magnetic flowmeter, namely:
- The introduction of hard electrode tips
- Higher operating frequencies
- Microprocessor techniques in noise reduction
But in the final analyses, conventional AC magnetic flowmeters still outperform DC designs with respect to process noise; whereas DC magnetic flowmeters are inherently better with respect to zero stability.
ABB’s new FSM4000 AC magmeter contains more innovations to close the gap with pulsed DC. For example, the FSM4000 operates at an optimal 70 Hz, a frequency in the low part of the noise spectrum. Operating at this frequency eliminates virtually all need for output signal dampening used in noisy applications.
In addition, the FSM4000 eliminates zero-shift through use of a field search coil, which allows the processor to subtract electronic noise signal from the aggregate of the noise and flow signal. To accomplish this, the signal and reference values are stored after filtering, and are available in their respective results buffer at a sample rate of 4 kHz. The 71.43 Hz magnetic field excitation frequency is a multiple of the sampling frequency. As a result, the number of measured values per period has an integer value, producing exactly 56 samples per excitation period.
In this way, exactly one period of the measured signal can be stored in the respective results buffers. To determine the measured flowrate, the ratio of the signal to reference must be known. For this purpose, the areas of the signal and reference are integrated and inserted in the ratio computation. The start signal for the calculation of the flowrate is the zero crossing of the reference: easily determined by waiting for the change in the sign of the measured values sent.
The system determines the phase shift between the signal and the reference by examining the zero crossing of the signal and reference using samples and partial samples. The phase shift constitutes the zero adjustment for the flowrate measurements, enhancing zero stability.
The result is a magmeter with faster dynamic response, yielding tighter accuracies and higher signal-to-noise ratios.
Digital Signal Processing
Advances in digital signal processing (DSP) have made it easier for users to develop data acquisition systems (signal converters) and analysis systems. The term DSP is misleading because it is usually associated with Fast Fourier Transforms, digital filters, and spectrum analysis. DSP, however, involves processing of an analog signal in the digital domain. Real-world signals, such as voltages, pressures and temperatures are converted to their digital equivalents for processing by the digital microprocessor.
Digital signal processing relies more heavily on the software processing of signals in the digital mode rather than using analog hardware and filtering to do the job. Processing a digital signal offers more alternatives, and software is much more flexible than hardware. These benefits lead to more effective methods of separating the real signal from the process noise. Tangible advantages include: improved measurements in applications involving vibration, hydraulic noise and temperature fluctuation.
As with any electromagnetic flowmeter, unwanted frequency signals can be generated caused by hydraulic noise and line noise. DSP provides faster A/D conversions for the sensor signal, providing a greater number of sample points compared to prior technologies. Digital filters, with sharp drop-offs, eliminate signal frequencies created by hydraulic noise and line noise that are outside the targeted measurement range. Advanced filtering techniques, such as automatic filter adaptation and frequency weighting, further allow the processor to accurately extract the flow signal from a potentially noisy sensor signal. By employing the power of the DSP Converter, improvements are made to zero stability, low flow performance, and measurement accuracy over a wide range of conditions.
Summary
Advanced AC electromagnetic flowmeters such as the FSM4000 magmeter now outperform pulsed-DC field instrumentation in noisy and difficult applications. Varieties of technical innovations combine to provide higher accuracies, better noise correction, faster response, and reduced zero drift. Noise reduction comes from an operating frequency appreciably higher than line frequency coupled with narrow bandwidth digital filters. Use of digital signal processing and ample computing power further enhances performance.
Greg Livelli is senior product manager, ABB Instrumentation, Warminster, PA.
