Measuring viscosity of liquid using ultra-sonic with digital signal processor
Tsui, Wai-Toi Alex
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For more than a century, the method of measuring viscosity of liquid has not changed despite the advancement of other measurement techniques. A new technique was invented by Argonne National Laboratory in 1996. A prototype system was designed and built to measure viscosity using high-frequency ultra-sonic. The prototype is completely analog which is typical in all ultra-sonic instruments and equipment. The system does have its limitations and inaccuracies because of its ultrasonic signal characteristics such as short signal duration and large signal loss. Due to recent advances in high-speed analog-to-digital converter technology, together with improvements'in real-time digital signal processing schemes, a new digital approach is proposed and implemented in this thesis. Scientists and engineers still use the old classic way to measure viscosity today. A metal ball is dropped in a tube filled with fluid. The viscosity is calculated by timing how much time the metal ball takes to sink to the bottom of tube. A newly modified technique is to use a motor to stir fluid. Viscosity is measured by the resistive force in this case. Both methods measure the viscosity by determining the resistance (or impedance) of the liquid. Both methods suffer the same problem: they are done off-line and require large quantities of sample fluid. In this thesis, a new real-time, portable system based on the TMS320C54x digital signal processor (C54x DSP) is designed and implemented. Similar to the previous methods, viscosity is determined by measuring impedance of the liquid. A high-speed custom mixed-signal hardware and specialized algorithmic software are designed and implemented to handle the high-frequency, short-duration multi-echo ultra-sonic signal. The DSP generates a high-frequency signal and excites the ultrasonic transducers. The reflected signal or the response of the excitation is digitized by high-speed ADC and processed in DSP. The viscosity of the liquid could be calculated from the amplitude and time delay of each echo. The amplitude of echo represents the signal loss from one media to another which is directly related to the impedance of the samples. Similarly, the time delay of echo represents the traveling velocity of the ultra-sonic wave.