Hannes Sturm

Boundary Layer Separation and Reattachment Detection on Airfoils by Thermal Flow Sensors

A sensor concept for detection of boundary layer separation (flow separation, stall) and reattachment on airfoils is introduced in this paper. Boundary layer separation and reattachment are phenomena of fluid mechanics showing characteristics of extinction and even inversion of the flow velocity on an overflowed surface. The flow sensor used in this work is able to measure the flow velocity in terms of direction and quantity at the sensors position and expected to determine those specific flow conditions. Therefore, an array of thermal flow sensors has been integrated (flush-mounted) on an airfoil and placed in a wind tunnel for measurement. Sensor signals have been recorded at different wind speeds and angles of attack for different positions on the airfoil. The sensors used here are based on the change of temperature distribution on a membrane (calorimetric principle). Thermopiles are used as temperature sensors in this approach offering a baseline free sensor signal, which is favorable for measurements at zero flow. Measurement results show clear separation points (zero flow) and even negative flow values (back flow) for all sensor positions. In addition to standard silicon-based flow sensors, a polymer-based flexible approach has been tested showing similar results.

Modeling of the Response Time of Thermal Flow Sensors

This paper introduces a simple theoretical model for the response time of thermal flow sensors. Response time is defined here as the time needed by the sensor output signal to reach 63.2% of amplitude due to a change of fluid flow. This model uses the finite-difference method to solve the heat transfer equations, taking into consideration the transient conduction and convection between the sensor membrane and the surrounding fluid. Program results agree with experimental measurements and explain the response time dependence on the velocity and the sensor geometry. Values of the response time vary from about 5 ms in the case of stagnant flow to 1.5 ms for a flow velocity of 44 m/s.

Thermoelectric Flow Sensor Integrated Into an Inductively Powered Wireless System

A prototype system has been designed, characterized and built to perform wireless in-situ flow measurements in pipes or other environments where wired connections are not reliable or cannot be realized. The wireless sensor consists of a printed circuit board board inductor, a flow sensor and electronics. An induced electromotive force in the coil by a time varying magnetic field powers the flow sensor and the electronics. The thermal flow sensor uses high temperature processes and chemically inert materials which allow the measurement of a wide array of mediums, even at high temperatures. The prototype system is presented along with characterization of the rigid flow sensor which was used. A flexible flow sensor on a 10-μm polyimide substrate was developed parallel to the wireless system and is currently being integrated into a fully flexible version of the wireless sensor.

Thermoelectric flow sensors on flexible substrates and their integration process

A thermoelectric mass flow rate sensor on a 10 µm thick polyimide foil and its integration process on planar as well as non-planar surfaces has been developed. Flow sensors, fabricated by MEMS technology, usually have a height which is directly coupled to the wafer thickness (typically 525 µm or 380 µm). They are not bendable and an integration process is always complex when steps in flow channels have to be avoided. A fabrication process has been developed where the functional layers were removed from the silicon substrate and transferred onto a 10 µm thick polyimide foil. The resulting flexible flow sensor has been integrated on different materials by means of flip chip mounting and tested on its electrical and mechanical characteristics. It could be shown that thermoelectric flow sensors on polymer foils have comparable characteristics to flow sensors on silicon substrates but higher diversity in system integration.

Thin Chip Flow Sensors for Nonplanar Assembly

A thermoelectric mass flow rate sensor on a 10-μm thick polyimide foil for nonplanar assembly has been developed. For principal operation of thin film sensors a few hundred micrometres thick substrate (usually a 525-μm or 380-μm thick silicon wafer) is not necessary and might be detrimental for system integration, e.g., it could cause turbulences due to step in flow channel. A fabrication has been developed by removing the functional layers from the silicon substrate and transferring them onto a 10-μm thick polyimide foil. The sensor foil has been integrated on different materials by means of flip chip mounting and tested on its electrical and mechanical characteristics with air as flowing medium. Here, it was shown that thermoelectric flow sensors on polymer foils have comparable characteristics to flow sensors on silicon substrates. The outcome of this development is a highly integrated flow sensor on a polymer foil, which can be placed on planar, nonplanar as well as flexible surfaces and still has a comparable performance to thermoelectric flow sensors on silicon substrate.

New electrical connection technology for microsystems using inktelligent printing ® and functional nanoscaled INKS

A new electrical connection technology for highly miniaturized systems based on functional printing has been studied. INKtelligent printing® with its ability to print structures in micrometer size uses electrically conductive inks with functional nanoparticles to provide MEMS devices with electrical contacts. Due to large dimensions of conventional packaging technologies like flip chip or wire bonding, INKtelligent printing® will dramatically increase size of assembled systems. Here, an aerosol beam technology - also known as Maskless Mesoscale Material Deposition (M3D®) or Aerosol Jet® - is presented. Tests have been done to characterize conductivity, contact resistance, long term stability and temperature dependency as also applications with flow sensors.