FEM-Analysis of 2D Micromachined Flow Transduers based on aGe-Thermistor Arrays and a Double Bridge Readout

Abstract

This paper reports on a design and simulation study aiming at high-accuracy 2D micromachined thermal flow transducers. The scope is restricted to micromachined devices featuring a square-shaped membrane incorporating central symmetric thin-film devices. A microthermistor array probed spatial excess temperature variations while the main heat supply was alternatively established by optional heating resistors or by pronounced self-heating of the thermistor devices. Proper device designs enable leading edge transducer performance without sophisticated signal conditioning schemes. We found that a high azimuthal uniformity of flow magnitude transduction is tantamount to a precise azimuthal accuracy. The most advanced result gave a maximum azimuthal aberration of 0.17 and 1.7 degrees for 1 m/s and 10 m/s, respectively, while the corresponding magnitude uniformity amounted to 0.07% and 0.5%. Such excellent specifications exceed the need of ordinary meteorological applications by far. However, they are essential for, e.g., precise non-contact measurements of 2D relative movements of two quasi-planar surfaces via the related Couette flow in intermediate air gaps. The simulations predicted significantly better device characteristics than achieved by us in first experiments. However, this gap could be attributed to imperfect control of the flow velocity field by the measurement setup.

The Impact of Surface Discontinuities on MEMS Thermal Wind Sensor Accuracy

“…In contrast to the encouraging simulation results, preliminary measurements [ 21 ] yielded a vast peak-to-peak deviation of 27.4° at 5 m/s (see Figure 5 ). As it is hardly probable that this massive mismatch is caused by the transducer element itself, further research is reasonable to find the ultimate performance of the transducer.…”

“…In a recent paper, we investigated a large number of layouts of membrane-based micromachined calorimetric flow transducers capable of 2D flow transduction [ 21 ]. The common features are resistive thin film devices for heat generation and temperature sensing, all embedded in a thin membrane that is exposed to tangentially flowing fluids.…”

“…The modeling results of a couple of designs exhibited extraordinary performance in terms of direction-independent flow magnitude transduction and the precision flow direction measurements. Preliminary experimental investigations of a specific transducer design supported the high expectations but only for low flow magnitudes [ 21 ].…”

“…As suggested by the symmetry of the device design, the simulated deviation exhibits a main period of 90°. However, the details of these deviation characteristics indicate more complex transduction behavior [ 21 ]. The minimum error is observed around multiples of 45° because both heat generation and temperature detection are symmetric with respect to the flow direction.…”

“…We focus on a specific transducer design that promises excellent specifications. Previous 3D FEM simulations of the device predicted a moderate deviation of 3° peak to peak for a surface-normal gradient of the tangential velocity of 6.33·10 4 s −1 at the transducer membrane corresponding to a mean guided flow velocity of 10 m/s in the rectangular channel measuring 12 mm in width and 1 mm in height [ 21 ].…”