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.
Highlights
After more than 40 years of research, miniaturized thermal flow transducers based on thermopiles as well as on PTC or NTC thermistors are mature for crucial applications in, e.g., industrial process control [1,2,3,4,5,6,7,8]
Micro-machined thermal flow transducers are favorable for spot measurements of the flow velocity field since, in general, shrinking their dimensions improves sensitivities and response time on the one hand and reduces power requirements on the other hand
Calorimetric transduction typically comprises several transduction steps: (i) Electrical power is dissipated as heat by means of a heating resistor; (ii) conjugated heat transfer takes place in case of a passing fluid, leading to a modification of the temperature distribution within the transducer membrane; (iii) various thermoelectric or resistive transducers can be incorporated in the membrane which convert local excess temperatures into electrical
Summary
After more than 40 years of research, miniaturized thermal flow transducers based on thermopiles as well as on PTC or NTC (positive or negative temperature coefficient of resistivity) thermistors are mature for crucial applications in, e.g., industrial process control [1,2,3,4,5,6,7,8]. This work deals with design aspects of membrane-based, micro-machined 2D calorimetric flow transducers that sense magnitude and direction of fluid flow velocities in close vicinity to their membrane surfaces. Temperature transducers in form of microthermistors made of thin-film amorphous germanium (aGe) are best suited for demanding flow sensing applications [9,10,11] These devices offer high temperature resolution, small heat capacity, and fast response. Pronounced bidirectional flow transducers have been developed based on such thermistors for flow rate measurements in tubes [9] They usually exhibit approximate sinusoidal dependences on the azimuth of the flow velocity direction with respect to the alignment of the thin-film elements on the sensor membrane. Based on a novel design, we will present simulation results on these key characteristics and compare them with preliminary experimental results
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