In this paper, a novel torsional resonator is proposed to measure the viscosity of fluids. The proposed measurement system is based on the torsional mode vibration of a metallic cylindrical tube bonded with specially designed twisting-motion-d33-piezo actuators. Two piezoelectric patches are bonded nearer to the fixed end of the cylindrical tube. The one bonded on the upper semicircle of the cylindrical tube is used as a twisting actuator and the other on the bottom semicircle is used as a sensor. The entire system is maintained at its torsional mode resonance frequency by using simple closed loop resonator electronics connected between the two piezo patches. The free end of the resonating cylindrical tube is half immersed into a viscous medium of which the viscosity is intended to be measured in this proposed paper. The torsional resonator experiences an additional viscous drag force-Fd due to the viscosity of the fluid medium which alters its torsional mode resonance frequency (ωT). The value of drag force Fd acting on the torsional resonator will vary depending on the type of viscous fluids in use. The closed loop resonant circuit tracks the change in torsional resonance frequency due to Fd and vibrates the tube with the new resonance frequency whenever the force generated by the viscous fluid changes. The shift in torsional resonant frequency is related to the viscosity of the fluid medium. The analytical model of the vibrating cylindrical tube with viscous fluid is derived and the results are validated with numerical simulation and experimentation. The key enabling concept of this proposed paper is the benefit of torsional mode resonator over the flexural mode resonator i.e., the torsional mode resonator experiences much less viscous damping at its resonance frequency. For the real-time laboratory experimentation, the hollow cylindrical tube with 30cm length and 2.5cm diameter is used. For the actuation and sensing, the MFC M-8528-F1 type (450 fiber orientation) d33 twisting actuator is used. The torsional resonance frequency of the tube in air is 2.5KHz. The proposed fluid viscosity measurement concept is novel and it is found to have better sensitivity and linearity than the flexural mode viscosity measurement system.
Suresh Kaluvan, Chen Zhang, Celena Lipscomb, and Haifeng Zhang, "Design of piezo actuated torsional resonator for fluid viscosity measurement (Conference Presentation)," Proc. SPIE 10595, Active and Passive Smart Structures and Integrated Systems XII, 105951U (Presented at SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring: March 08, 2018; Published: 3 April 2018); https://doi.org/10.1117/12.2296637.5763073634001.
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