Nithin Raghunathan

Arrays of silicon cantilevers for detecting high-G rapidly varying acceleration profiles

This work presents the first experimental study on the effectiveness of single-crystal silicon (SCS) cantilever arrays as contact-based high-G sensors in digital MEMS accelerometers. Unlike conventional designs, a digital scheme is employed where detection of a specific acceleration level is associated with a group of silicon cantilevers, which deflect and make solid-to-solid silicon contacts with the substrate. This scheme is especially useful in applications where a high-G rapidly changing acceleration profile needs to be detected with a high confidence level. The proposed designs have been successfully demonstrated up to 45,000 g, which is commonly found in impact and pyroshock phenomena, such as in multistage rocket launches and earth penetrating weapons. The arrays of beams offer high redundancy in the measured data, which is critical when used in events with severe consequences. The fabricated devices were tested using a modified Kolksy bar setup and found to have contact resistances in the order of ∼3.2±3 kΩ. Depending on the applied acceleration profile, contact bouncing is observed during testing.

Near-Contact Gas Damping and Dynamic Response of High-g MEMS Accelerometer Beams

This paper introduces and experimentally validates a new model for near-contact gas damping of microbeams. The model is formulated based on numerical simulations of rarefied gas dynamics using the Boltzmann Ellipsoidal Statistical Bhatnagar-Gross-Krook (ES-BGK) equation. The result is compared with existing models by simulating the motion of beams under high-g acceleration. To experimentally validate the damping models, single crystal silicon MEMS g-switches with cantilever microbeams of various lengths were utilized. The experimental measurements of beam dynamics under peak accelerations of approximately 50,000 g and acceleration ramp rates from 600 to 3,000 g/μs are compared with simulations. Additionally, the damping coefficients are extracted from existing vibrational mode data, and the resulting values are compared to the various models. The new near-contact model was found to predict contact and release times within a root-mean-square deviation from experiment below 9 and 7 for contact and release events, respectively. The damping values for the vibrational modes away from contact were predicted within 33% error, showing a more consistent predictive capability than provided by earlier models.

Near-contact damping model and dynamic response of μ-beams under high-g loads

This paper presents the first near-contact aerodynamic damping model based on rarefied flow modeling for use in dynamic simulations of large-displacement motion and contacting behavior of microbeams. The damping model is constructed based on high-fidelity simulations of rarefied gas flow around microbeams based on the Boltzmann kinetic equation with the Ellipsoidal Statistical Bhatnagar-Gross-Krook (ES-BGK) collision relaxation model. The predictions using the new model and previously published models are compared with experimentally measured responses of silicon microbeams under a high-g dynamic load. The new model is validated by measuring the near-contact behavior of silicon microbeams under loads up to 52,500 g and with ramping rates up to 2,750 g/µs. The model and experiments were found to be in close agreement with a maximum variation of less than 13.1%.

Wireless low-power temperature probes for food/pharmaceutical process monitoring

This work presents the design, fabrication, and characterization of a wireless, low-power, multi-point temperature sensor system for process monitoring applications. This is particularly useful for low-temperature applications commonly found in pharmaceutical and food processing plants where accurate monitoring of temperature over long time intervals (e.g. days) is necessary. Using capacitive temperature elements, a wireless three-point temperature measurement in liquid has been successfully demonstrated. The system has been tested down to 55°C and achieves a resolution of <;0.01°C. The developed system utilizes the low power ANT wireless protocols for data transmission. This allows operation in the 2.4 GHz ISM band and supports large-scale sensor network topologies (232 nodes). The entire system occupies a small footprint of 13 by 33 mm2 with a power consumption less than 1 mW.

MOS-capacitor-based ionizing radiation sensors for occupational dosimetry applications

Presented is the first accumulating capacitive radiation sensor for low-dose, long-term exposures observed in occupational dosimetry. The sensors capacitance-voltage curve undergoes a semi-permanent negative shift due to ionizing radiation. By measuring the change in capacitance at a given voltage in the depletion region, the ionizing radiation that has been present on the sensor can be extracted. In order to achieve the low dose resolution required (less than 100 μGy) for occupational dosimetry, parameters such as the oxide thickness and annealing conditions are optimized. The result for a 2 mm × 2 mm sensor is 1.6 fF per 100 μGy, a capacitance shift detectable with commercial electronics.

Multi-Point Wireless Temperature Sensing System for Monitoring Pharmaceutical Lyophilization

This work presents the design and evaluation of a fully wireless, multi-point temperature sensor system as a Process Analytical Technology (PAT) for lyophilization. Each sensor contains seven sensing elements which measure the product temperature at various positions of the contents of a glass vial. The sensor performance was studied by freeze drying experiments with sensor placement in both center and edge of full shelf of 6R glass vials with 4 ml fill volume. Product temperature profile and primary drying time measured at the bottom center position in the glass vial by the wireless sensor as well as the primary drying time are in close comparison with the thermocouple data. The drying times during primary drying were determined at the top, higher middle, lower middle and bottom positions which are 3.26 mm apart vertically in the vial by the wireless sensor based on the temperature profile measured at different positions. For a center vial, the drying time from the start of primary drying to each layer was measured at 3.9, 9.3, 14.2, and 21 h respectively, allowing to track the sublimation interface during primary drying phase. In addition, sublimation rate at each layer was calculated based on the drying time and theoretical weight loss of ice in the product. The sublimation rate at the beginning of the primary drying was similar to the sublimation rate by gravimetric method. Furthermore, the vial heat transfer coefficient (Kv) was also calculated based on the sublimation rate. Thus, allowing the use of the multi-point wireless sensor to rapidly monitor the sublimation rate and Kv for every batch as continuous process verification. Similar tests were also conducted with 3% w/v mannitol solutions and the results were consistent demonstrating potential for real-time monitoring, process verification and cycle optimization for pharmaceutical lyophilization.

An ANT-based low-power battery-free wireless cryogenic temperature probes for industrial process monitoring

This paper presents an ultra low-power battery-free, wireless, multi-point temperature sensor for vacuum and cryogenic applications. This is applicable to low-temperature industrial processes such as lyophilisation (i.e. freeze-drying) where accurate temperature monitoring over long durations is necessary. The developed sensors are wirelessly powered using a 915 MHz RF source. The sensors utilize a low power 2.4 GHz ANT communication protocols and operate in a shared channel topology accommodating ∼ 232 nodes. The battery-free nature and measurement capabilities have been successfully demonstrated in typical freeze-drying processes with close agreement to thermocouple data. The sensors occupies a small footprint of 10.3 by 28.75 mm2 with a maximum power consumption of less than 1 mW.

Testing Techniques for Shock Accelerometers below 10,000 g

Research in the fields of impact science and penetration mechanics is often focused on the acceleration of projectiles during launch and penetration. In recent studies, various researchers have started using the Hopkinson bar to evaluate the performance of shock accelerometers for 10,000–100,000 g (1 g = 9.81 m/s2) accelerations. However, very little work has focused on shock environments below 10,000 g using the Hopkinson bar setup, although it has been reported that there may exist the peak amplitudes of sub 10,000 g acceleration during the penetration of concrete targets. Consequently, the presence of these acceleration levels in projectile penetration yields a definite need to create well-controlled accelerations below 10,000 g in a laboratory setting. In this study, two types of experimental techniques were investigated for accelerometer performance evaluation. The first was to modify the Hopkinson bar setup to test shock accelerometers with special attention to the acceleration level below 10,000 g. The second was to investigate a drop tower tester for the evaluation of shock accelerometers below 1,000 g.

A single crystal silicon low-g switch tolerant to impact accelerations up to 24,000 g

This paper presents a novel MEMS single-crystal silicon low-g switch surviving acceleration loads 200 times greater than its designed trigger load. In accordance with beam dynamics theory for survivability to high-g acceleration levels (on the order of 10,000-gs), low-g (<;150 g) switches were designed, fabricated and tested. Experiments have confirmed that the fabricated devices trigger in the ranges of 60-131 g and survive acceleration impacts of 24,000 g. Results were found to be in close agreement with the dynamic simulations of the fabricated device with the same acceleration profiles.

Real-time in situ electronic monitoring of dynamic contact behavior of MEMS high-g switches

This paper presents for the first time real-time contact monitoring of packaged high-g switches under acceleration loads up to 50,000 g. Such loads are typical in impact and pyroshock phenomena such as multistage rocket launches and earth penetrating weapons. Contact monitoring is performed using a fully electronic methodology utilizing an ultra low-power (<;60 μW) CMOS interface that is directly integrated to the MEMS chip and accurately senses the capacitance change around the contact region at a sampling rate greater than 500 kHz. Experimental and modeling results agree to within 5% for the switch closing time under high-g condition, confirming the validity of the measurement technique.