Browsing by Subject "Equivalent circuit"

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Open Access
Deep collapse mode capacitive micromachined ultrasonic transducers
(2010) Olçum, Selim
Capacitive micromachined ultrasonic transducers (CMUTs) are suspended microelectromechanical membrane structures with a moving top electrode and a rigid substrate electrode. The membrane is actuated by electrical signals applied between the electrodes, resulting in radiated pressure waves. CMUTs have several advantages over traditional piezoelectric transducers such as their wider bandwidth and microfabrication methodology. CMUTs as microelectromechanical systems (MEMS), are fabricated using CMOS compatible processes and suitable for batch fabrication. Low cost production of large amount of CMUTs can be fabricated using already established integrated circuit (IC) technology infrastructure. Contrary to piezoelectrics, fabricating large 2-D arrays populated with transducer elements using CMUTs is low-cost. The technological challenges of CMUTs regarding the fabrication and integration issues were solved during the past 15 years, and their successful operation has been demonstrated in many applications. However, commercialization of CMUTs is still an overdue passion for CMUT community. The bandwidth of the CMUTs are inherently superior to their piezoelectric rivals due to the nature of the suspended membrane structure, however, their power output capability must be improved to achieve superior signal-to-noise ratio and penetration depth. In this thesis, we gave a comprehensive discussion about the physics and functionality of CMUTs and showed both theoretically and experimentally that their power outputs can be increased substantially. Using the conventional uncollapsed mode of CMUTs, where the suspended membrane vibrates freely, the lumped displacement of the membrane is limited. Limited displacement, unfortunately, limits the power output of the CMUT. However, a larger lumped displacement is possible in the collapsed state, where the membrane gets in contact with the substrate. By controlling the movement of the membrane in this state, the power output of the CMUTs can be increased. We derived the analytical expressions for the profile of a circular CMUT membrane in both uncollapsed and collapsed states. Using the profiles, we calculated the forces acting on the membrane and the energy radiated to the medium during an applied electrical pulse. We showed that the radiated energy can be increased drastically by utilizing the nonlinear forces on the membrane, well beyond the collapse voltage. Using the analytical expressions, we developed a nonlinear electrical equivalent circuit model that can be used to simulate the mechanical behavior of a transmitting CMUT under any electrical excitation. Furthermore, the model can handle different membrane dimensions and material properties. It can predict the membrane movement in the collapsed state as well as in the uncollapsed state. In addition, it predicts the hysteretic snap-back behavior of CMUTs, when the electric potential across a collapsed membrane is decreased. The nonlinear equivalent circuit was simulated using SPICE circuit simulator, and the accuracy of the model was tested using finite element method (FEM) simulations. Better than 3% accuracy is achieved for the static deflection of a membrane as a function of applied DC voltage. On the other hand, the pressure output of a CMUT under large signal excitation is predicted within 5% accuracy. Using the developed model, we explained the dynamics of a CMUT membrane. Based on our physical understanding, we proposed a new mode of operation, the deep collapse mode, in order to generate high power acoustic pulses with large bandwidth (>100% fractional) at a desired center frequency. We showed both by simulation (FEM and equivalent circuit) and by experiments that the deep collapse mode increases the output pressure of a CMUT, substantially. The experiments were performed on CMUTs fabricated at Bilkent University by a sacrificial release process. Larger than 3.5 MPa peak-to-peak acoustic pulses were measured on CMUT surface with more than 100% fractional bandwidth around 7 MHz using an electrical pulse amplitude of 160 Volts. Furthermore, we optimized the deep collapse mode in terms of CMUT dimensions and parameters of the applied electrical pulse, i.e., amplitude, rise and fall times, pulse width and polarity. The experimental results were compared to dynamic FEM and equivalent circuit simulations. We concluded that the experimental results are in good agreement with the simulations. We believe that CMUTs, with their high transmit power capability in the deep collapse mode can become a strong competitor to piezoelectrics.
Open Access
Design of a birdcage-like radio frequency transmit array coil for the magnetic resonance imaging using equivalent circuit model
(2016-05) Tarakameh, Alireza Sadeghi
One of the conditions to have a good magnetic resonance (MR) image is applying a homogeneous radio-frequency (RF) excitation (magnetic field) with effciently high intensity to the region of interest. However, there are some limitations such as specific absorption rate (SAR) which is not allowed to exceed some standard levels. Since SAR level directly depends on the electric field and the electric field is coupled to the magnetic field, there is a trade-off between high-intensity RF-excitation and low SAR level. Moreover, in conventional RF coils (birdcage) for the MRI, the magnetic field profile is almost constant so that its intensity is pretty high at the center of the coil and decreases toward the coil. In such a coil, it is not possible to aim an off-center small region of interest and make the homogeneity concentrated at that region. Transmit array (Tx-array) coils provide high controllability on both electric and magnetic field, so, they would be good solutions for all of these issues, although, they come across the effciency problem at the center when the same performance of a conventional RF coil is required. This problem has been already handled using a birdcage-like Tx-array coil, however, there are some diffculties to design and tune such a coil. In this thesis, we proposed a novel design method for birdcage-like Tx-array coil; an eight-channel birdcage-like Tx-array coil is designed using the equivalent lumped-element circuit model. This design profits controllability feature of an array and high transmit effciency of a birdcage coil at the center, simultaneously. A capacitive decoupling method is utilized in order to get rid of reactive interactions between channels of the array. Then, an optimization (the steepest-descent method) with constraints based on minimizing the electric field and smoothing the magnetic field is applied to the voltage-excitations of the Tx-array coil. The proposed decoupling method provides 15dB matching for each channel and higher than 12dB decoupling between adjacent channels and at least 19dB for nonadjacent channels. This Tx-array coil provides only 3% less effciency versus the birdcage coil at the center of the coil, while, at the regions close to the surface of the phantom we achieved more than 72% better effciency in comparison to the birdcage coil. Furthermore, we demonstrated that the Tx-array is capable to produce a homogeneous magnetic field at an arbitrary (off-center) region of interest. This adjustment can be performed for the electric field as well such that the electric field and so the SAR can be minimized locally. Consequently, the proposed configuration of the Tx-array coil provides an ef- ficient excitation while capability of local RF shimming and local electric-fieldreduction can be achieved.