A high frequency digital induction system for condutive flow level measurements
Introduction
The wide-spread application of gas–liquid two-phase flow systems in many industries such as chemical, pharmaceutical, petroleum, and ocean engineering calls for reliable and advanced measurement techniques. Many techniques have been used in gas–liquid flow measurements, including radioactive (e.g. Gamma ray and X ray), electrical, optical and other mechanical principles based methods (e.g. ultrasonic methods) [1], [2]. Electrical method is one that has attracted much attention due to its low cost, portability and no radioactive hazard. In particular, conductive method has been much researched due to its simplicity in implementation and effectiveness for measuring conductive fluids. It has been known that the equivalent conductivity of gas–liquid two-phase flow is an effective indicator of phase distribution [3], [4], [5].
However, most conventional conductivity detection techniques rely on contact conductivity detection. This is not ideal due to the fact that the electrode has to be in direct contact with the fluid, which suffers from polarization effect and electrochemical effect. Further, the contact electrodes are subject to biological growth (biofouling) or the buildup of sludge in some cases, which leads to unreliable results. In particular, in some high temperature, chemically aggressive environments, non-contact measurement is an essential requirement. These drawbacks limit the practical applications of the conventional conductivity detection technique and call for new non-contact methods. Capacitive method offers a way of non-contact measurements of gas–liquid flow involving either conductive or non-conductive liquids [5], [6].
In this paper, we report a flow measurement system based on inductive principles. Magnetic field can operate through air and inductive measurements do not require being contact with the flow. It works on the electromagnetic induction principle; by establishing a sinusoidal current in the transmitting coil, induction signal is set up across the terminal of the receiving one, which is proportional to the local magnetic field strength. Due to eddy current effects, conductive objects put between the coils cause a disturbance to the original magnetic field, and therefore cause change in the induced voltage across the receiver coil.
Induction measurement systems have been widely reported for measuring high conductivity materials such as monitoring metal production [7], [8], thickness/coating inspection [9], and crack imaging [10], two phase liquid flow [11], [12]. Due to strong eddy current effects of highly conductive objects, low and medium excitation frequencies (in the kilohertz range) are normally adequate.
However, for liquids with much lower conductivities, higher excitation frequencies (above 1 MHz) are required to achieve sufficient sensitivity. These applications include multiphase flow imaging, level measurement, and biomedical applications [13], [14], [15], [16], [17], [18] (e.g., determining body composition, imaging human thorax and head, and imaging brain edema, etc.). At these frequencies, challenges arise due to capacitive stray effect [19], requirements for high speed ADCs and fast demodulation. Some progress has been made in these areas [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. In terms of the computational aspects of the problem, i.e. forward and inverse problems, some workers have also contributed in this area [32], [33], [34].
Overall, currently, there still lacks suitable non-ironizing radiation techniques to address multi-phase measurement challenges. A typical example is gas/oil/water separation where the water is the range of 5 S/m and capacitance techniques are not able to cope. In this paper we report an inductive measurement system that explores a real time direct digital sampling and demodulation scheme without analog down conversion and all main functionalities have been implemented in an integrated FPGA system. This system aims to move a step closer to providing a solution in multiphase measurement applications where a low cost electrical method can play a part in.
Overall, this paper presents the hardware and software design aspects of a high frequency digital measurement system based on electromagnetic induction principle, which is sensitive to objects with conductivity below 5 S/m.
Section snippets
System design
The hardware comprises three parts, i.e. the sensor, the front end circuit, the FPGA main board as the data acquisition board (DAQ) and control unit. Fig. 1 shows the hardware structure of the system.
Experiments on the induction system
To evaluate the performance of the induction system as whole (FPGA digitization, demodulation, front end electronics and coils), tests were performed using two channels of the measurement system.
The first test case simulates the measurement setup in a typical Magnetic Inductance Tomography (MIT) situation, where two coils (one as transmitter and the other as receiver) are positioned opposite the cylindrical vessel to test the sensitivity of MIT.
The second test involves measuring flow level in a
Conclusions
In this paper we have presented a high frequency, highly integrated FPGA-based induction measurement system, which is suitable for detecting low conductivity objects. This system operates in real time, exploiting direct digital conversion and IQ demodulation. Details were discussed in both hardware and software aspects. 8.3 MHz excitation frequency is also possible using the under-sampling strategy. Experimental results have confirmed that the system is sensitive to the low conductivity objects.
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