Electromagnetic Flow Meters: Design Considerations and Solutions

"If you don't measure it, you don't manage it." This is a common saying in the industrial world, and it is especially applicable to flow measurement. Simply put, there is an increasing demand for flow monitoring, often with higher speed and accuracy. There are several areas where industrial flow measurement is important, such as domestic waste. As people pay more and more attention to environmental protection, waste disposal and monitoring have become very important in order to make our world cleaner, healthier and less polluted. Humans consume a lot of water, and as the global population grows, water consumption will increase. Flow meters are essential, both for monitoring domestic wastewater and as an integral part of the process control system of wastewater treatment plants.

In which industries are flow meters used today?

"If you don't measure it, you don't manage it." This is a common saying in the industrial world, and it is especially applicable to flow measurement. Simply put, there is an increasing demand for flow monitoring, often with higher speed and accuracy. There are several areas where industrial flow measurement is important, such as domestic waste. As people pay more and more attention to environmental protection, waste disposal and monitoring have become very important in order to make our world cleaner, healthier and less polluted. Humans consume a lot of water, and as the global population grows, water consumption will increase. Flow meters are essential, both for monitoring domestic wastewater and as an integral part of the process control system of wastewater treatment plants.

Figure 1. Schematic diagram of a sewage treatment plant

Flow meters are also used in many industrial control processes, including chemical/pharmaceutical, food and beverage, pulp and paper, etc. These applications often require measuring flow in the presence of large amounts of solids – a requirement that most flow technologies cannot easily handle.

The field of transportation metering deals with the transfer and payment of products between two parties, requiring high-end flow meters. One example is the transportation of oil products through a large pipeline system. In this application, even a small change in the flow measurement accuracy over time can result in a significant loss or gain in profit for one party.

Why is electromagnetic induction technology very suitable for liquid flow measurement?

For liquid flow measurement, electromagnetic flow meter technology has several advantages. Its sensor is generally connected to the pipe and its diameter is consistent with the pipe diameter, so it will not interfere with or restrict the flow of the medium when measuring. Because the sensor is not directly immersed in the liquid, there are no moving parts, so there is no wear and tear.

The electromagnetic method measures volumetric flow, which means the measurement is insensitive to changes in fluid parameters such as density, temperature, pressure and viscosity. Once an electromagnetic flowmeter is calibrated with water, it can be used to measure other types of conductive fluids without further calibration. This is an important advantage over other types of flowmeters.

Electromagnetic flowmeter is particularly suitable for measuring solid-liquid two-phase media, such as mud and other high conductivity media with suspended mud, solid particles, fibers or viscous substances. It can be used to measure sewage, mud, ore pulp, paper pulp, chemical fiber pulp and other media. This makes it particularly suitable for the food and pharmaceutical industries, where it can be used to measure corn syrup, juice, alcohol, medicine, plasma and many other special media.

What is the working principle of electromagnetic flowmeter?

The working principle of electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. According to Faraday's law, when a conductive fluid flows through the magnetic field of the sensor, an electromotive force proportional to the volume flow rate will be generated between a pair of electrodes, and its direction is perpendicular to the flow direction and the magnetic field. The magnitude of the electromotive force can be expressed as:

Where E is the induced potential, k is a constant, B is the magnetic flux density, D is the inner diameter of the measuring tube, and v is the average velocity of the fluid in the measuring tube in the axial direction of the electrode cross section.

Figure 2. Working principle of magnetic flowmeter

What is the sensor output range?

The sensor provides a differential output. Its sensitivity is typically 150 μv/(mps) to 200 μv/(mps). As the direction of the excitation current is constantly alternating, the sensor output signal amplitude doubles. For the flow measurement range of 0.5 m/s to 15 m/s, the sensor output signal amplitude is between 75 μv and 4-6 mV. Figure 3 shows the sensor output signal when excited by a constant current source and with fluid flowing through the sensor. The oscilloscope plot captured on the sensor output leads shows a very low level signal located on a large common mode voltage. The purple curve corresponds to the positive electrode and the red curve corresponds to the negative electrode. The pink curve is the mathematical calculation channel that subtracts the positive and negative electrodes. The low level signal is located in the large common mode voltage.

Figure 3. Output signal of electromagnetic flow sensor

What are the traditional methods of sensor measurement?

The traditional approach is largely analog—a preamplifier with high input impedance and high input common-mode rejection to deal with the effects of sensor leakage current, followed by a third- or fourth-order analog bandpass filter and sample-and-hold stage, and finally an analog-to-digital converter. A typical analog front-end approach is shown in Figure 4. The sensor output signal is first amplified by an instrumentation amplifier. The desired signal must be amplified as much as possible while avoiding unwanted DC common-mode voltages that would saturate the amplifier output. This typically limits the gain of the first instrumentation amplifier to a maximum of 10 times. The bandpass filter stage further removes DC effects and amplifies the signal again before going to a sample-and-hold circuit—it is this difference signal that represents the flow rate—and then to an analog-to-digital converter.

Figure 4. Traditional analog front-end approach

What are the market trends that influence changes in electromagnetic flowmeter architecture?

There are multiple industry trends that are calling for new architectures. One of these is the increasing demand for data. For liquids, the ability to monitor other properties besides flow is becoming increasingly valuable. For example, to determine what contaminants may be in the liquid, or to determine if the liquid has the correct density/viscosity for a specific application. There are many such requirements and benefits to adding this diagnostic capability. This information is not easily accessible using traditional analog methods because much of the sensor information is lost during the synchronous demodulation stage.

In addition, manufacturing processes continue to require improved productivity and efficiency. For example, in liquid injection/filling applications, more and more filling nodes are added; the expansion of manufacturing process scale and the increase in filling speed require faster and more accurate flow monitoring.

Figure 5. Liquid injection/filling

Traditionally, mechanical or weighing techniques have been used to determine the correct amount of liquid to add during the filling process, or the precise fill amount in the production process. These methods are often very expensive and difficult to scale. To meet this need, flow meters (especially electromagnetic flow meters for liquids) have become the technology of choice.

What does the new electromagnetic flow meter architecture look like?

The oversampling approach greatly simplifies the analog front-end design. The analog bandpass filter and sample-and-hold circuit are no longer required. The preamplifier in the circuit is the only instrumentation amplifier stage—in our case, the AD8220 JFET input stage rail-to-rail output instrumentation amplifier, which can be directly connected to a high-speed Σ-Δ converter.

Figure 6. Oversampling architecture analog front end using AD8220 and AD717x-x.

What is important when it comes to analog front ends and how does it affect my design?

The amplifier and ADC are the two most important blocks in this type of application. The first stage amplifier has several key requirements.

One requirement is the common mode rejection ratio (CMRR). The ions in the liquid electrolyte move in a directional manner, so a potential is generated between the electrode and the fluid, which is called polarization. If the two electrodes are exactly aligned, the potentials on the electrodes should be equal to each other. The polarization voltage of different metals varies from a few hundred millivolts to ±2 volts. This is the DC common mode voltage that appears at the output of the sensor and the input of the preamplifier. The preamplifier is key to rejecting this common mode voltage.

Figure 7. Common-mode rejection of the preamplifier.

A 100 dB common-mode rejection ratio attenuates the 0.3 V DC common mode to 3 microvolts, which appears as a DC offset at the amplifier output and can be calibrated out. Ideally, the common-mode voltage across the sensor remains constant, but in reality it varies over time and is affected by other factors such as liquid quality or temperature. The higher the common-mode rejection ratio, the less need for continuous background calibration and the better the flow stability.

Table 1. Effect of common-mode rejection on actual flow rate

The metal material of the electrodes is in contact with the electrolyte liquid. The friction between the liquid electrolyte and the electrodes generates a higher frequency AC common mode voltage. Although the amplitude is usually small, the AC common mode appears as completely random noise and is more difficult to suppress. This requires the preamplifier to have not only good DC common mode rejection ratio, but also excellent higher frequency common mode rejection ratio. The AD8220 amplifier has excellent common mode rejection ratio from DC to 5 kHz. For the AD8220 B grade, the minimum common mode rejection ratio is 100 dB from DC to 60 Hz and 90 dB below 5 kHz, which can well suppress common mode voltage and noise to microvolt levels. When the common mode rejection ratio is 120 dB, 0.1 volt peak-to-peak is reduced to 0.1 microvolt peak-to-peak. Table 2 shows the impact of poor CMRR on the output sensor signal.