The development and the corresponding evaluation of a multidirectional thermal flow sensor are presented in this work. The sensor was fabricated on a flexible substrate, allowing for new applications, since it provides the possibility of installation in nonplanar surfaces such as pipelines. Furthermore, the sensing elements are not in direct contact with the fluid, which increases the device reliability, extends its application range, and allows the noncontact monitoring of fluids. This was achieved by utilizing the substrate as a protective layer between the sensing elements and the fluid under measurement. The operation principle is based on the determination of the flow-induced temperature profile variations. A dedicated experimental setup was designed and used for the device evaluation. Both flow velocity value and direction were successfully extracted, while the results were consistent with the predicted theoretical values. A single-layer back propagation neural network that correlates the sensors’ readouts to the angle of rotation was implemented, which leads to a mean absolute direction estimation error in the order of 2.7 degrees independent to the training procedure datasets.
A thermal flow sensor was printed on a flexible plastic substrate using exclusively screen-printing techniques. The presented device was implemented with custom made screen-printed thermistors, which allows simple, cost-efficient production on a variety of flexible substrates while maintaining the typical advantages of thermal flow sensors. Evaluation was performed for both static (zero flow) and dynamic conditions using a combination of electrical measurements and IR imaging techniques in order to determine important characteristics, such as temperature response, output repeatability, etc. The flow sensor was characterized utilizing the hot-wire and calorimetric principles of operation, while the preliminary results appear to be very promising, since the sensor was successfully evaluated and displayed adequate sensitivity in a relatively wide flow range.
This work presents the development and evaluation of a fully printed multi-directional thermal flow sensor on PET substrate. The device consists of conductive Ag tracks and printed thermistors based on BaTiO 3 , activated carbon and a solvent based thermoset polymeric system. Each element presents similar temperature-electrical resistance behavior (in terms of normalized values), thus enabling utilization as sensing and active elements for thermal two-dimensional flow sensing. A custom experimental setup was used for evaluating the device under two modes of operation, namely constant current and constant resistance. It was observed that in the range of 0 to 25 standard liters per second (SLPM) of air flow, constant resistance provided better input dynamic range than constant current (2.75 and 0.12 mW respectively), while constant resistance mode exhibited a sensitivity of approximately two orders of magnitude greater than that of constant current mode. For constant resistance mode, sensitivities of 0.46 mW/SLPM for zero flow and 0.08 mW/SLPM for flow greater than 6 SLPM were extracted, while for constant current mode the corresponding sensitivity values were 0.018 mW/SLPM for zero flow and 0.0036 mW/SLPM for flow greater than 6 SLPM. The device was capable of successfully detecting the flow direction throughout the target flow range. The influence of the flow magnitude on detecting flow angle was found to be negligible. The proposed device can be mounted to various non-planar surfaces, after the necessary re-calibration. It can be mass produced with various printing technologies for applications as a flexible two-dimensional flow sensor with low fabrication cost.
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