A System-Based Approach for Flow Sensors

System-capable sensors offer features like self test, auto-calibration, auto-ranging, or "plug and play"- interchangeability. Therefore, they are favorable for embedding sensors in complex measurement systems. Of course, such system capabilities are based on appropriate electronic means as well. The novel scope of the project is to design sensors for smooth implementation of system features. The main advantage of this strategy a wide range of applications becomes accessible with few sensor versions will be demonstrated with thermal flow sensors based on known physical effects. However, the intended innovative sensor structures enable new conversion schemes, which especially offer significant advantages to the embedding of the sensors in a measurement system. Novel architectures of mixed-signal electronic circuits will be developed to deal with sensor signal conditioning, data acquisition, and networking or system integration of sensors. To save costs and time, all planned electronic developments are based on programmable electronic components rather than application specific monolithic integrated solutions. The chosen approach enables close integration of the sensing element and the signal processing circuits. This allows on-board closed-loop interaction with minimal delays, which is a major advantage in distributed systems of autonomous sensors. The ability of locally integrated intelligence opens the way to very flexible sensors suited for employment in vastly different applications as medical technology, environmental monitoring, automotive applications, and flow measurement in technical as well as fundamental research. The new flow sensors are at first produced with already proven technologies. Subsequently, technological improvements are scheduled to meet the demanding operation conditions present, e.g., in automotive applications. The primary goal is to develop applicable strategies for the realization of auto-start functions, advanced self-diagnostic functions, error recognition (e.g., soiling), and adaptive sensing range. The optimization of sensor layout and sensor operation, measurement setup, the assessment of systematic errors (like free convection caused by the heat sources of the sensor) is attempted with the assistance of modeling and numeric simulation for typical applications. Rough conditions, as found in the automotive field, will be made accessible by exploring alternative technologies. The dynamics will be improved with modifications of the sensor design. Realization of system-ready prototypes is performed by considering the connectivity to common measurement systems. Wherever possible, the solutions to be developed are supposed to bear an exemplary character for the development of other sensors.

The research project "system capable flow sensors" was dedicated on research of novel thermal transduction methods that convert flow velocity or flow rate into electrical signals that are specifically suited for further processing by large sensor networks. A main goal was the development of suitable constructions that enable features with high relevance for large systems, e.g., fault tolerant operation or autonomous self checks. These goals are achieved through novel transducer designs that feature redundancy of all functional elements of flow conversion. It turns out, that these new transducers deliver superior conversion efficiency but with significantly reduced thermal stress due to sensor operation. Furthermore, transducers featuring ultra-low operating power were designed utilizing a sophisticated combination of calorimetric and anemometric flow conversion. These sensors improved the state of the art by more than an order of magnitude. Furthermore the drawback of ambiguous transduction characteristics of calorimetric transducers can be circumvented with the new method. As low power is tantamount with moderate excess temperatures, these sensors will be well suited for measurements on, e.g., biological fluids that may not endure significantly elevated temperatures. Distributed long-term flow measurements, e.g., for environmental investigations, are a further field of potential applications of such sensors. Here ultra-low operating power enables lightweight, easily portable devices with long periods of operational time. Low-power operation can be achieved, at least from a time-average point of view, with a further development of the project: the miniaturized thermal time of flight transducer. Time of flight flow transduction evaluates the propagation of heat pulses in the fluid under consideration, which is of course dependent on the flow. Here power consumption can be significantly reduced by selecting a low duty cycle of operation. As the measurement phase last only for a fraction of a second, this transduction method is well suited for the power aware monitoring of slowly varying flows. Establishing a comprehensive simulation environment for modelling conjugated heat transfer in laminar flows formed a solid expertise for the successful developments. Also the development of new production methods, the characterization of decisive material properties as well as the perfection of known technological processes were essential prerequisites. Moreover, a fully compatible PC mouse based on electro-calorimetric flow sensors that gives an impressive impression of the powerful technology was developed in the course of a student diploma thesis.