A parametrized three-dimensional model for MEMS thermal shear-stress sensors

This paper presents an accurate and efficient model of MEMS thermal shear-stress sensors featuring a thin-film hotwire on a vacuum-isolated dielectric diaphragm. We consider three-dimensional (3-D) heat transfer in sensors operating in constant-temperature mode, and describe sensor response with a functional relationship between dimensionless forms of hotwire power and shear stress. This relationship is parametrized by the diaphragm aspect ratio and two additional dimensionless parameters that represent heat conduction in the hotwire and diaphragm. Closed-form correlations are obtained to represent this relationship, yielding a MEMS sensor model that is highly efficient while retaining the accuracy of three-dimensional heat transfer analysis. The model is compared with experimental data, and the agreement in the total and net hotwire power, the latter being a small second-order quantity induced by the applied shear stress, is respectively within 0.5% and 11% when uncertainties in sensor geometry and material properties are taken into account. The model is then used to elucidate thermal boundary layer characteristics for MEMS sensors, and in particular, quantitatively show that the relatively thick thermal boundary layer renders classical shear-stress sensor theory invalid for MEMS sensors operating in air. The model is also used to systematically study the effects of geometry and material properties on MEMS sensor behavior, yielding insights useful as practical design guidelines

A micromachined flow shear-stress sensor based on thermal transfer principles

Microhot-film shear-stress sensors have been developed by using surface micromachining techniques. The sensor consists of a suspended silicon-nitride diaphragm located on top of a vacuum-sealed cavity. A heating and heat-sensing element, made of polycrystalline silicon material, resides on top of the diaphragm. The underlying vacuum cavity greatly reduces conductive heat loss to the substrate and therefore increases the sensitivity of the sensor. Testing of the sensor has been conducted in a wind tunnel under three operation modes-constant current, constant voltage, and constant temperature. Under the constant-temperature mode, a typical shear-stress sensor exhibits a time constant of 72 μs

Experiments and simulations of MEMS thermal sensors for wall shear-stress measurements in aerodynamic control applications

MEMS thermal shear-stress sensors exploit heat-transfer effects to measure the shear stress exerted by an air flow on its solid boundary, and have promising applications in aerodynamic control. Classical theory for conventional, macroscale thermal shear-stress sensors states that the rate of heat removed by the flow from the sensor is proportional to the 1/3-power of the shear stress. However, we have observed that this theory is inconsistent with experimental data from MEMS sensors. This paper seeks to develop an understanding of MEMS thermal shear-stress sensors through a study including both experimental and theoretical investigations. We first obtain experimental data that confirm the inadequacy of the classical theory by wind-tunnel testing of prototype MEMS shear-stress sensors with different dimensions and materials. A theoretical analysis is performed to identify that this inadequacy is due to the lack of a thin thermal boundary layer in the fluid flow at the sensor surface, and then a two-dimensional MEMS shear-stress sensor theory is presented. This theory incorporates important heat-transfer effects that are ignored by the classical theory, and consistently explains the experimental data obtained from prototype MEMS sensors. Moreover, the prototype MEMS sensors are studied with three-dimensional simulations, yielding results that quantitatively agree with experimental data. This work demonstrates that classical assumptions made for conventional thermal devices should be carefully examined for miniature MEMS devices

Mass flowmeter using a multi-sensor chip

We report here a novel mass flowmeter using a multisensor chip that includes a 1-D array of pressure, temperature and shear stress sensors. This shear stress sensor based flowmeter is capable of high sensitivity and wide measurement range. Our study also shows that the mass flowmeter using shear-stress sensors produces better resolution than that from pressure sensors in the laminar flow regime. Extensive tests have been carried out to evaluate the effects of overheat ratio, channel height and gas properties. We also find the V^2 ∝ τ^(1/3) law for conventional hot film sensors does not hold for our micromachined shear stress sensor

Underwater shear-stress sensor

This paper reports the development of a micromachined, vacuum-cavity insulated, thermal shear-stress sensor for underwater applications. We focus on two major challenges for underwater shear-stress sensors: the waterproof coating and pressure sensitivity. It is found that thin-film CVD Parylene is a good waterproof material and sensors coated with 2 µm Parylene N can survive in water for at least one month at 55°C. It is also found that reducing the size and increasing the thickness of the sensor diaphragm are effective in minimizing the pressure sensitivity

Analog VLSI system for active drag reduction

We describe an analog CMOS VLSI system that can process real-time signals from surface-mounted shear stress sensors to detect regions of high shear stress along a surface in an airflow. The outputs of the CMOS circuit are used to actuate micromachined flaps with the goal of reducing this high shear stress on the surface and thereby lowering the total drag. We have designed, fabricated, and tested parts of this system in a wind tunnel in laminar and turbulent flow regimes

Fluidic shear-stress measurement using surface-micromachined sensors

A poly-silicon hot-film shear-stress sensor insulated by a vacuum-chamber underneath has been designed and fabricated by the surface micromachining technology. The sensor is operated at both constant current and constant temperature modes. The dynamic performance (including time constant and cut-off frequency) measurement, calibration, and temperature compensation of the sensor have been realized

A wafer-scale MEMS and analog VLSI system for active drag reduction

We describe an analog CMOS VLSI system that can process real-time signals from integrated shear stress sensors to detect regions of high shear stress along a surface in an airflow. The outputs of the CMOS circuit control the actuation of integrated micromachined flaps with the goal of reducing this high shear stress on the surface and thereby lowering the total drag. We have designed, fabricated, and tested components of this system in a wind tunnel in both laminar and turbulent flow regimes with the goal of building a wafer-scale system

A surface-micromachined shear stress imager

A new MEMS shear stress sensor imager has been developed and its capability of imaging surface shear stress distribution has been demonstrated. The imager consists of multi-rows of vacuum-insulated shear stress sensors with a 300 /spl mu/m pitch. This small spacing allows it to detect surface flow patterns that could not be directly measured before. The high frequency response (30 kHz) of the sensor under constant temperature bias mode also allows it to be used in high Reynolds number turbulent flow studies. The measurement results in a fully developed turbulent flow agree well with the numerical and experimental results previously published

An integrated MEMS system for turbulent boundary layer control

The goal of this project is a first attempt to achieve active drag reduction using a large-scale integrated MEMS system. Previously, we have reported the successful development of a shear-stress imager which allows us to "see" surface vortices (1996). Here we present the promising results of the interaction between micro flap actuators and vortices. It is found that microactuators can actually reduce drag to values even lower than the drag associated with pure laminar flow, and that the microactuators can reduce shear stress values in turbulent flow as well. Based on these results, we have attempted the first totally integrated system that consists of 18 shear stress sensors, 3 magnetic flap-type actuators and control electronics for use in turbulent boundary layer control studies