Browsing by Subject "Equivalent Circuit Model"

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Open Access
Airborne cmut cell design
(2014) Yılmaz, Aslı
All transducers used in airborne ultrasonic applications, including capacitive micromachined ultrasonic transducers (CMUTs), incorporate loss mechanisms to have reasonably wide frequency bandwidth. However, CMUTs can yield high efficiency in airborne applications and unlike other technologies, they offer wider bandwidth due to their low characteristic impedance, even for efficient designs. Despite these advantages, achieving the full potential is challenging due to the lack of a systematic method to design a wide bandwidth CMUTs. In this thesis, we present a method for airborne CMUT design. We use a lumped element circuit model and harmonic balance (HB) approach to optimize CMUTs for maximum transmitted power. Airborne CMUTs have narrowband characteristic at their mechanical part, due to low radiation impedance. In this work, we restrict the analysis to a single frequency and the transducer is driven by a sinusoidal voltage with half of the frequency of operation frequency, without any dc bias. We propose a new mode of airborne operation for CMUTs, where the plate motion spans the entire gap. We achieve this maximum swing at a specific frequency applying the lowest drive voltage and we call this mode of operation as Minimum Voltage Drive Mode (MVDM). We present equivalent circuit-based design fundamentals for airborne CMUT cells and verify the design targets by fabricated CMUTs. The performance limits for silicon membranes for airborne applications are derived. We experimentally obtain 78.9 dB//20Pa@1m source level at 73.7 kHz, with a CMUT cell of radius 2.05 mm driven by 71 V sinusoidal drive voltage at half the frequency. The measured quality factor is 120. CMUTs can achieve a large bandwidth (low quality factor level) as they can be manufactured to have thin plates. Low-quality-factor airborne CMUTs experience increased ambient pressure and therefore a larger membrane deflection. This effect increases the stiffness of the plate material and can be considered by nonlinear compliance in the circuit model. We study the interaction of the compliance nonlinearity and nonlinearity of transduction force and show that transduction overwhelms the compliance nonlinearity. To match the simulation results with the admittance measurements we implement a very accurate model-based characterization approach where we modify the equivalent circuit model. In the modified circuit model, we introduced new elements to include loss mechanisms. Also, we changed the dimension parameters used in the simulation to compensate the difference in the resonance frequency and amplitude.
Open Access
Lumped element modeling of circular CMUT in collopsed mode
(2014) Aydoğdu, Elif
Capacitive micromachined ultrasonic transducer is a microelectromechanical device, which serves as an acoustic signal source or sensor, in a variety of applications including medical ultrasound imaging, ultrasound therapy, airborne applications. It has a suspended membrane with an electrode inside, and at the underlying substrate there is another electrode, so that the membrane can be deflected by the electrical field formed between the electrodes. Similarly, any mechanical disturbance on the membrane can be sensed as a change in the capacitance of the two electrodes. CMUT is a nonlinear device which has a distributed mechanical operation. Although, it is a mass-spring system basically, the nonlinear electrical force and the radiation force makes it impossible to solve CMUT through analytical equations. In order to predict its behavior, and design a CMUT towards the needs of a specific application, either finite element analysis or equivalent electrical circuit modeling should be utilized. Finite element analysis (FEA) can predict the distributed CMUT operation with high accuracy, but its usage is limited to designs employing low number of CMUTs because of the computation cost. Recently, advances in equivalent circuit modeling, made it possible to simulate arrays employing very high number of CMUTs, with high accuracy. These models assume uncollapsed mode operation and except collapsed mode operation as it is highly nonlinear. This thesis focuses on obtaining an accurate equivalent circuit model for a circular CMUT in collapsed mode. The outcome is a parametric circuit model, that can simulate a CMUT of any physical and material parameters, under an arbitrary electrical or mechanical excitation. In collapsed mode, the compliance of the membrane is no longer constant as in uncollapsed mode, and it increases with increasing contact radius. Similarly, the capacitance, the electrical force and the radiation impedance all have new behavior regarding the contact radius. As there is no analytical solution for those parameters, we perform numerical calculations and extract expressions for each of them. First, we calculate the collapsed membrane deflection, utilizing the exact electrical force distribution in the analytical formulation of membrane deflection. Then we use the deflection profile to calculate the capacitance, electrical force, and compliance. Performing a set of calculations for different CMUT dimensions, different pressure and voltage levels, we obtain dependencies of those parameters on rms deflection. Then we develop a lumped element model of collapsed membrane operation, expressing the parameters as functions of rms deflection. The radiation impedance for the collapsed mode is also included in the model. The model is then merged with the uncollapsed mode model to obtain a simulation tool that handles all CMUT behavior, in transmit or receive. Large- and smallsignal operation of a single CMUT can be fully simulated for any excitation regime. The results are in good agreement with FEM simulations.