Gerrit Dumstorff

Integration Without Disruption: The Basic Challenge of Sensor Integration

The basic challenge in embedding sensors in materials is to meet simultaneously two conflicting requirements; on one hand, we want to retrieve sensor data from the material and thus we have to integrate sensors, electronics, and interconnections. On the other hand, sensors are foreign bodies in the material, which may deteriorate its macroscopic properties. This paper discusses several possibilities to integrate sensors in material in a minimal invasive way, avoiding subsequent deterioration of its macroscopic performance. It is our idea to adapt the integrated sensor to the surrounding matrix and to reduce sensors volume to the minimum, which is needed to guarantee the function. This approach is called function scale integration.

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.

Investigations on the Impact of Material-Integrated Sensors with the Help of FEM-Based Modeling

We present investigations on the impact of material-integrated sensors with the help of finite element-based modeling. A sensor (inlay) integrated with a material (matrix) is always a foreign body in the material, which can lead to a “wound effect”, that is degradation of the macroscopic behavior of a material. By analyzing the inlays impact on the material in terms of mechanical load, heat conduction, stress during integration and other impacts of integration, this wound effect is analyzed. For the mechanical load, we found out that the inlay has to be at least as stretchable and bendable as the matrix. If there is a high thermal load during integration, the coefficients of the thermal expansion of the inlay have to be matched to the matrix. In the case of a high thermal load during operation, the inlay has to be as thin as possible or its thermal conductivity has to be adapted to the thermal conductivity of the matrix. To have a general view of things, the results are dimensionless and independent of the geometry. In each section, the results are illustrated by examples. Based on all of the results, we present our idea for the fabrication of future material-integrated sensors.

Strain gauges — Volume embedding vs. surface application

We present a strain gauge, which is directly embedded in a tensile test specimen made out of epoxy resin for minimal stress invasive embedding. Reducing the stress due to the embedding process is done by fabricating the sensor on a substrate which has approximately the same mechanical and thermal characteristics as the epoxy resin. We will show that strain gauge shows higher sensitivity it it is embedded in the specimen, compared to be placed on the specimen with adhesive. It is our goal to transfer these results from epoxy resin to other materials like steel, aluminum, carbon fiber reinforced plastics or elastomers and integrate sensors in these materials.

3D printed pressure sensor with screen-printed resistive read-out

By combining different additive manufacturing techniques, we can merge various functionalities in single, custom designed printed components. In this work we coupled a 3D printed pressure chamber with a screen-printed resistive strain gauge to form a robust, fully printed pressure sensor. This demonstrator illustrates the suitability of using printed, polymer-based materials not only for rapid prototyping but also for rapid manufacturing of application specific sensor devices. Evaluating the change in resistance of a half-bridge layout, we characterized the circular cylindrical measurement chamber with integrated fluidic connectors for different membrane thicknesses in a pressure range of 0 to 1000 mbar.

Online monitoring of the curing of adhesives with a miniaturised interdigital sensor using impedance spectroscopy

Abstract Monitoring adhesives during manufacturing and their lifetime has become increasingly important due to the variety of materials and applications. Impedance spectroscopy is a suitable method for online monitoring of the curing process. We present a miniaturised interdigital structure to monitor the curing process of the adhesives using impedance spectroscopy. Compared to other sensors, our sensor is ultrathin, so that it disappears in the adhesive joint. Therefore, it can remain in the joint and be used for lifetime monitoring. In addition it is suitable for thin adhesive layers due to its fine grid. We demonstrate that the impedance of the sensor embedded in the adhesive gives insight into the curing mechanisms. Therefore monitoring of a dispersion and an epoxy is shown. In addition, the curing cycles can be reliably controlled using this monitoring method. The permittivity of the adhesives is extracted from the impedance measurements, applying analytical models of the electrical field of the interdigital structure.

Sensor Integration in Castings Made of Aluminum - New Approaches for Direct Sensor Integration in Aluminum High Pressure Die Casting

The use of sensors for detection, measurement and evaluation of mechanical and thermal loads is well known and essential for the implementation of »Structural Health Monitoring« (SHM). For this, sensors are mainly used for condition monitoring of mechanical loads and their impact to the castings state, which is a decisive advantage for safety-related components. The use of sensors on the surface of metallic components, in particular of cast metal components made of aluminum, is still limited to the use of strain gauges. They are usually applied on the surface of the cast metal components and get fixed by adhesives. The idea is, to integrate the sensors directly during the aluminum casting process. Since integrated sensors are naturally protected against chemical and mechanical influences, furthermore the load can be measured directly at the point of interest inside the component. Measurement data can be recorded and provide a good data basis for future calculations and dimensioning of components, which is known as »Data Mining« or »Industrial Data Space«. New technology and material combinations, which allow the fabrication of sensors capable of withstanding force and temperature during the integration process in aluminum casting, are investigated.In this paper, the design and fabrication of a strain gauge printed on an aluminum sheet is shown. These sensor sheets get integrated in aluminum during high pressure die casting (HPDC) in a way that a specimen is build up. The specimen is characterized in a fatigue bending test and the sensor data was read permanently during this test. It is shown that the new approach with printed thick film sensors on aluminum substrate sheets works properly to withstand the heavy thermal conditions during high pressure die casting. The fabricated sensor is able to sense the mechanical tiredness and detects the fatigue of the metal matrix. This a first step to use such material integrated sensors in structural health monitoring applications.

Strain gauge printed on carbon weave for sensing in carbon fiber reinforced plastics

A new and innovative approach to integrate a printed strain gauge in carbon fiber reinforced plastics (CFRP) is presented. Therefore the carbon weave is prepared by locally applying epoxy resin and curing it. Thus a plateau is created on the carbon weave on which a strain gauge is screen printed. Within this step the electrical interconnections are printed too and no cables have to be integrated. The final “sensorial” carbon weave is integrated in a CFRP beam by common prepreg technology. Characterization is done in a three point bending test by bending the CFRP beam and measuring the change in resistance of the strain gauge. It is shown that there is a correlation between the deflection of the beam and the sensor response. The generated data of the strain gauge can be used in applications like condition monitoring or structural health monitoring.

Resistive silicon microstructure for embedding in aluminium during casting

A resistive silicon microstructure embedded in aluminium is presented. For the resistive structure a boron-doped poly-silicon on a silicon substrate has been used. The passivation of the structure was done by thick film processing. Half of the sensor was integrated in aluminium in a casting process. Due to the thick film passivation the sensor could successfully be integrated in aluminium. Sensors had been characterized by measuring the temperature dependent resistance of the microstructure. Beside embedding the sensor can also be used for other harsh environments.

Experimental and Numerical Investigations in Shallow Cut Grinding by Workpiece Integrated Infrared Thermopile Array

The purpose of our study is to investigate the heat distribution and the occurring temperatures during grinding. Therefore, we did both experimental and numerical investigations. In the first part, we present the integration of an infrared thermopile array in a steel workpiece. Experiments are done by acquiring data from the thermopile array during grinding of a groove in a workpiece made of steel. In the second part, we present numerical investigations in the grinding process to further understand the thermal characteristic during grinding. Finally, we conclude our work. Increasing the feed speed leads to two things: higher heat flux densities in the workpiece and higher temperature gradients in the material.