Hossain Saboonchi

MEMS acoustic emission transducers designed with high aspect ratio geometry

In this paper, micro-electro-mechanic systems (MEMS) acoustic emission (AE) transducers are manufactured using an electroplating technique. The transducers use a capacitance change as their transduction principle, and are tuned to the range 50?200?kHz. Through the electroplating technique, a thick metal layer (20??m nickel?+?0.5??m gold) is used to form a freely moving microstructure layer. The presence of the gold layer reduces the potential corrosion of the nickel layer. A dielectric layer is deposited between the two electrodes, thus preventing the stiction phenomenon. The transducers have a measured quality factor in the range 15?30 at atmospheric pressure and are functional without vacuum packaging. The transducers are characterized using electrical and mechanical tests to identify the capacitance, resonance frequency and damping. Ultrasonic wave generation using a Q-switched laser shows the directivity of the transducer sensitivity. The comparison of the MEMS transducers with similar frequency piezoelectric transducers shows that the MEMS AE transducers have better response characteristics and sensitivity at the resonance frequency and well-defined waveform signatures (rise time and decay time) due to pure resonance behavior in the out-of-plane direction. The transducers are sensitive to a unique wave direction, which can be utilized to increase the accuracy of source localization by selecting the correct wave velocity at the structures.

Accurate Source Localization Using Highly Narrowband and Densely Populated MEMS Acoustic Emission Sensors

The real time source detection and localization are the fundamental advantages of Acoustic Emission (AE) method. The multi-dimensional localization requires an array of sensors distributed and positioned strategically on the structure. The sensor positions are controlled by the attenuation characteristics of the structure in order to have minimum required wave arrivals to sensors (e.g., minimum three sensors detecting an event for 2D source localization) for an AE event. While minimum three sensors are sufficient for 2D source localization, redundant data collection increases the accuracy of source location, and cleans the data set from non-relevant signals. Additionally, accurate prediction of wave velocity results in better localization result. Current AE sensors are piezoelectric type, and while they are designed to exhibit resonant behavior, they have a certain bandwidth that opens the response to a range of frequencies. In this study, highly narrowband Micro-Electro-Mechanical Systems (MEMS) sensors are presented with more accurate source localization ability due to detecting single frequency, and potential to densely populate on structures due to lightweight, and lowcost properties. The MEMS AE sensors are tuned to a range of frequencies between 60 kHz – 200 kHz for different structural applications (e.g., 60 kHz for concrete and 150 kHz for steel). The accurate prediction of source localization is tested on aluminum plate, and wave velocity of particular frequency is obtained using the dispersion curves. The source localization ability of the MEMS AE sensors is compared with conventional piezoelectric sensors. The efficiency of MEMS AE sensors with densely positioned scheme is discussed to monitor large-scale structures. doi: 10.12783/SHM2015/372

The design, characterization, and comparison of MEMS comb-drive acoustic emission transducers with the principles of area-change and gap-change

Comb-drive transducers are made of interdigitized fingers formed by the stationary part known as stator and the moving part known as rotor, and based on the transduction principle of capacitance change. They can be designed as area-change or gap-change mechanism to convert the mechanical signal at in-plane direction into electrical output. The comb-drive transducers can be utilized to differentiate the wave motion in orthogonal directions when they are utilized with the outof- plane transducers. However, their sensitivity is weak to detect the wave motion released by newly formed damage surfaces. In this study, Micro-Electro-Mechanical System (MEMS) comb-drive Acoustic Emission (AE) transducer designs with two different mechanisms are designed, characterized and compared for sensing high frequency wave propagation. The MEMS AE transducers are manufactured using MetalMUMPs (Metal Multi-User MEMS Processes), which use electroplating technique for highly elevated microstructure geometries. Each type of the transducers is numerically modeled using COMSOL Multiphysics program in order to determine the sensitivity based on the applied load. The transducers are experimentally characterized and compared to the numerical models. The experiments include laser excitation to control the direction of the wave generation, and actual crack growth monitoring of aluminum 7075 specimens loaded under fatigue. Behavior and responses of the transducers are compared based on the parameters such as waveform signature, peak frequency, damping, sensitivity, and signal to noise ratio. The comparisons between the measured parameters are scaled according to the respective capacitance of each sensor in order to determine the most sensitive design geometry.

MetalMUMPs-Based Piezoresistive Strain Sensors for Integrated On-Chip Sensor Fusion

In this paper, polysilicon-based microelectromechanical system (MEMS) piezoresistive strain sensors manufactured with acoustic emission sensors and accelerometers on the same device are introduced. Three strain sensors are placed in horizontal, vertical, and angled directions to extract the principle strains, and manufactured using MetalMUMPs. The influences of sensor position on the silicon substrate and trenching to the strain transfer from structure under loading to polysilicon layer are numerically demonstrated and experimentally validated. The characterization experiments include monotonic, cyclic, and fatigue mechanical loading and thermal loading. The performance of MEMS strain sensors is compared with conventional metal gauges. While the strain transfer from structure to polysilicon is limited due to the stiffness of package, and the location of strain sensors on the substrate, the gauge factor of MEMS strain sensors is about twice of metal gauges. Combining strain sensors on the same package of other structural health monitoring (SHM) sensors can tackle several limitations of SHM methods, such as the need of redundant measurement to increase the reliability and define idle/active mode of acoustic emission sensor using strain sensor to reduce the power consumption, and enable integrating energy harvesting devices.

In-Plane MEMS Acoustic Emission Sensors Development and Experimental Characterization

Damage initiation and growth in materials releases elastic waves, which can be detected by surface mounted acoustic emission (AE) transducers. In this paper, new MEMS comb-drive AE transducers, responsive to in-plane motion, manufactured using electroplating technique for highly elevated microstructure geometries are presented. The transduction principle is capacitance change achieved by area/gap change in two separate designs. Mechanism of spring orientation, dimensions and mass have been selected in such a way that they satisfy three design criteria as the frequency range of 100–200 kHz, 2–30 pF capacitance and the functionality under atmospheric pressure. The challenge of coupling the microstructure vibration in out-of-plane and in-plane directions is addressed with differential mode approach and frequency domain responses. The squeeze film damping is reduced with 8 μm gap between moving electrodes so that the transducers are operational under atmospheric pressure. The directional independence of the transducers to two orthogonal directions is demonstrated using laser source as the excitation signal. The Nd: Yag Q switch laser has 3 mm beam diameter and is focused on the top and the edge of the transducer package. The results show a distinct output signal for in-plane and out-of-plane motions due to the directional sensitivity of the MEMS transducers.

Numerical demonstration of MEMS strain sensor

Silicon has piezoresistive property that allows designing strain sensor with higher gauge factor compared to conventional metal foil gauges. The sensing element can be micro-scale using MEMS, which minimizes the effect of strain gradient on measurement at stress concentration regions such as crack tips. The challenge of MEMS based strain sensor design is to decouple the sensing element from substrate for true strain measurement and to compensate the temperature effect on the piezoresistive coefficients of silicon. In this paper, a family of MEMS strain sensors with different geometric designs is introduced. Each strain sensor is made of single crystal silicon and manufactured using deposition/ etching/oxidation steps on a n- doped silicon wafer in (100) plane. The geometries include sensing element connected to the free heads of U shape substrate, a set of two or more sensing elements in an array in order to capture strain gradients and two directional sensors. The response function and the gauge factor of the strain sensors are identified using multi-physics models that combine structural and electrical behaviors of sensors mounted on a strained structure. The relationship between surface strain and strain at microstructure is identified numerically in order to include the relationship in the response function calculation.

Numerical and experimental characterizations of piezoresistive MEMS strain sensors

In this paper, new MEMS strain sensors are introduced. The transduction principle of the sensors is the resistance change due to piezoresistive property of polysilicon. Five different sensors are designed on the same device and tuned to resistance values of 350 Ω and 120 Ω. The sensors are aligned in horizontal, vertical and 45° directions in order to extract the principle strains. The geometry of the sensing element is a rectangular bar anchored at two ends and suspended above silicon substrate. The sensors are numerically modeled using COMSOL Multiphysics software. The model consists of all the micromachining layers, including silicon substrate, 0.7 μm thick polysilicon layer (sensing element) sandwiched between two layers of 0.35 μm thick silicon nitride layers and trenching under polysilicon layer, in order to estimate the strain that piezoresistive element is exposed to. The MEMS strain sensors are manufactured using MetalMUMPs process. The sensors are attached to aluminum and steel plates, and their gauge factors are compared with conventional foil gauges under uniaxial and biaxial loading. It is demonstrated that the MEMS strain sensors can detect both static and dynamic strains with the gauge factor reaching significantly high values. High gauge factor occurs because of unique geometry design and trenching, which amplify the strain that the polysilicon layer senses. The MEMS strain sensor can be fused with other sensing elements on the same device such as accelerometer, acoustic emission in order to have redundant measurement from a single point.

Numerical and experimental characterizations of low frequency MEMS AE sensors

In this paper, new MEMS Acoustic Emission (AE) sensors are introduced. The transduction principle of the sensors is capacitance due to gap change. The sensors are numerically modeled using COMSOL Multiphysics software in order to estimate the resonant frequencies and capacitance values, and manufactured using MetalMUMPS process. The process includes thick metal layer (20 μm) made of nickel for freely vibration layer and polysilicon layer as the stationary layer. The metal layer provides a relatively heavy mass so that the spring constant can be designed high for low frequency sensor designs in order to increase the collapse voltage level (proportional to the stiffness), which increases the sensor sensitivity. An insulator layer is deposited between stationary layer and freely vibration layer, which significantly reduces the potential of stiction as a failure mode. As conventional AE sensors made of piezoelectric materials cannot be designed for low frequencies (<300 kHz) with miniature size, the MEMS sensor frequencies are tuned to 50 kHz and 200 kHz. The each sensor contained several parallel-connected cells with an overall size of approximately 250μm × 500 μm. The electromechanical characterizations are performed using high precision impedance analyzer and compared with the numerical results, which indicate a good fit. The initial mechanical characterization tests in atmospheric pressure are conducted using pencil lead break simulations. The proper sensor design reduces the squeeze film damping so that it does not require any vacuum packaging. The MEMS sensor responses are compared with similar frequency piezoelectric AE sensors.

Numerical Simulation of Novel MEMS Strain Sensor for Structural Health Monitoring

This paper presents the numerical simulation of novel Micro-Electro-MechanicalSystem (MEMS) based piezoresistive strain sensor in order to monitor the localized strain in structures such as crack tips. The MEMS strain sensor developed in this study is made of single crystal silicon, and has 100 μm width and 400 μm length which prevents the strain gradient effect at the vicinity of stress concentration regions. The device has a novel U shape configuration where the open side of the U shape faces the crack tip and provides additional strain amplification. The silicon has piezoresistive property, which can have a gage factor reaching up to 165. Rigid substrate effect of silicon package on the strain sensor has been minimized based on the geometrical feature which has been used to amplify the stress in the sensor. The MEMS strain sensor has been optimized using COMSOL Multiphysics software, which enables the combination of multiple physics solutions in a single model, which are structural and electrical in this study.