Research & Development on a Traveling Wave Driver for a Liquid within a Microfluidic Chip
Date Issued
2015
Date
2015
Author(s)
Wang, Pei-Wen
Abstract
In this thesis, we present a non-traditional micro-pump with its working principle based on the same concept as that of a traditional linear ultrasonic motor. Two pieces of piezoelectric material PZT (Lead Zirconate Titanate) were bonded onto a thin steel plate to serve as the vibration source. The traveling wave generated by the phase difference of these two PZT actuators formed a structure similar to that of the stator of a traditional linear ultrasonic motor. The traveling wave generated was then transmitted to a biochip, which served the same function as that of the rotor of a traditional linear ultrasonic motor, to drive the liquid. The system working frequency was set at a frequency located between the resonant frequencies of two neighbored modes of the elastic structure formed by the steel plate and the two PZT’s. It is to be noted that this operating frequency is different from the operating frequency of the traditional linear ultrasonic motor, which is selected at one of the resonant frequency of the natural mode. According to the superposition principle, driving the two PZT actuators at the selected frequency mentioned above, the mode shape can be approximated as the superposition of the two natural modes. This excitation approach is thus properly called as two modes excitation. It is to be noted that this two modes excitation approach poses less stringent requirements on the piezoelectric materials and the structural design as that of the traditional ultrasonic motor. Over the last few decades, the low efficiency associated with linear ultrasonic motor has always been attributed to the one less optimization parameter, the radius of the stator and the rotor, when compared to the rotary type ultrasonic motor. This concept was identified to have a fundamental flaw. More specifically, generating two standing waves with 90 degree spatial and temporal phase difference to create a traveling wave within the stator of an ultrasonic motor has been identified to be the most fundamental configuration of an ultrasonic motor. This concept can be perfectly executed in rotary type ultrasonic motor as the boundary effect will not come into play. In comparison, when the traveling wave impinge on the boundary of a finite-dimensional linear ultrasonic motor, the reflected wave generated by the boundary will superimpose onto the original traveling waves to destroy the underlying driving principle mentioned above. This effect was identified to be the cause of low efficiency or even non-functional in linear ultrasonic motor during the course of this thesis study. Trying to circumvent the detrimental effects induced by the boundary mentioned above, this thesis first proposed that the requirement of having two spatial 90 degree difference standing waves can be achieved by having two orthogonal modes serve as the two standing waves with 90 degree spatial phase difference. Taking these research results, the four waves, i.e., sine, cosine, hyperbolic sine, and hyperbolic cosine waves, then can be adopted to understand and to design the spatially driving conditions needed. It is also to be noted that the traditional two-mode excitation approach then can be perfectly implemented to excite the two piezoelectric ceramic actuators so as to create the high efficiency traveling bulk wave. The boundary effect had been considered by analyzing the different mode shapes in different conditions. The proper operating frequency was first chosen by simulation done by a commercial finite element software COMSOL. The traveling wave generated by the actuator was utilized to drive the fluid in the channel and the steel plate excited accordingly was identified to be tailorable by varying the applying voltage. Changing the phase difference between the stator and the rotor that forms the actuator was found to vary the driven direction of the liquid. The operating frequency of the device developed in this thesis is 13795.5 Hz. By simulating the relationship between the applied voltage and the induced deformation, the deformation was found to increase with the increase of the externally applied voltage. This simulation results were verified by using a laser displacement sensor to measure the induced steel plate deformation with respect to different driving voltage. The speed and the direction of the driving fluid were verified both by simulation and by experiments to be controllable by varying the amplitude and the phase difference of the applied voltage and the flow rate of the liquid driven was linearly dependent to the steel plate vibration amplitude and thus linearly dependent to the externally applied voltage. It is worth noting that even though this thesis utilized biochip as the verification platform for the newly invented linear ultrasonic motor, the innovative results achieved can have applications ranging from micro-robots, ultra-high precision moving stages, self-walking high-precision machineries. The moving speed and traveling directions can all be easily controlled by inducing different vibrational modes and their corresponding eigen-frequencies.
Subjects
Linear ultrasonic motor
two modes excitation
micropump
Type
thesis
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