Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 87 (2014) 1561 – 1564
EUROSENSORS 2014, the XXVIII edition of the conference series
Stress-unsusceptible pressure sensors embedded in fiber composite Martin Schwertera*, Christian Behrb, Monika Leester-Schädela, Peter Wierachc, Michael Sinapiusb, Stephanus Büttgenbacha, Andreas Dietzela a
b
TU Braunschweig, Institute of Microtechnology, Alte Salzdahlumer Str. 203, 38124 Braunschweig, Germany TU Braunschweig, Institute of Adaptronics and Functional Integration, Langer Kamp 6, 38106 Braunschweig, Germany c DLR, Institute of Composite Structures and Adaptive Systems, Lilienthalplatz 7, 38108 Braunschweig, Germany
Abstract This paper discusses the integration of pressure sensors into flow sensing airfoils made of fiber composite material to be used in future aircraft. An embedding procedure for damage-free integration is described, in which the sensors experience stresses by vacuum and curing during the integration at composite lamination. The mechanical characteristics and the influences of external mechanical stresses on the integrated sensor are further investigated. A sensor design unsusceptible to external mechanical stresses parallel to the air wing surface is proposed and verified by tensile stress tests. © 2014 The TheAuthors. Authors.Published Published Elsevier © 2014 byby Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the scientific committee of Eurosensors 2014. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 Keywords: MEMS; pressure sensor; influence of external stress; fiber composite; embedding; tensile stress test
Nomenclature p0 pi Ubridge R1…R4 V+ GND
ambient pressure reference pressure of integrated chamber Wheatstone bridge voltage piezoresistor positive sensor supply voltage ground
* Corresponding author. Tel.: +49-531-391-9748; fax: +49-531-391-9751. E-mail address:
[email protected]
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi:10.1016/j.proeng.2014.11.598
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1. Introduction In future aircraft the importance of control systems will further increase. The performance of such systems depends on accurate sensor signals in order to monitor, among others, the flow conditions at the wing surface. For distortion free measurements the influences of the sensor geometry on the flow should be kept as low as possible by providing a smooth and stepless surface. Therefore, two sensor integration concepts may be considered: ‚ Thin film sensors with a non-flow-affecting thickness: These may for example be flexible foils with hot film microsensors as described in [1]. ‚ Integration of more bulky microsensors in cavities provided in the wing construction (if necessary with filling the small remaining gap between sensor and wing material). More and more aircraft parts are manufactured using fiber composite materials like the new Airbus A350 [2]. The fiber composite enables a direct integration of functional elements like sensors while being processed; an intelligent composite structure is thereby created [3]. However, the advantage of a closed shape without any steps may be accompanied by perturbing influences of the fiber composite structure on the sensor. These influences are mainly tensile stress and torsion. The tensile stress test as described below helps to estimate these influences on the sensor and to find a stress independent sensor design. 2. Pressure sensor designs A microfabricated pressure sensor to be used for active flow control together with its schematic cross section is shown in Fig. 1. The sensor is made of n-doped silicon substrates. It comprises a wet etched reference chamber isolated from its surrounding by a deformable membrane that holds a thicker boss structure in the center. Piezoresistors are applied to the membrane’s (100) surface using a diffusion doping process. The electrical connection is realized by aluminum tracks. For mechanical protection and electrical passivation the sensor surface is covered with a thin layer of silicon nitride; opened at the contact pads for electrical connection. With a difference between the chamber pressure pi and the ambient pressure p0 the membrane deflects and the mechanical surface stress in the membrane leads to a resistance change of the piezoresistors. The resistors are connected as a Wheatstone bridge in order to reach high sensitivity when decreasing and increasing resistances placed at opposing locations are used. Further information on the sensor can be found in [4]. For tensile stress tests two sensor designs with sufficient pressure sensitivity but different alignments of the piezoresistors (Fig. 2) were used. In version (a), all resistors are aligned transversally to the local directions of deformation in the membrane. Increasing and decreasing resistances in the Wheatstone bridge are realized by selecting areas with compression stress (close to the boss; increasing resistance) as well as tensile stress (close to the solid frame; decreasing resistance) [5]. In version (b) longitudinal together with transversal sensor orientations were used in order to obtain both longitudinal and transversal piezoresistive effects. All resistors are placed on the tensile stress area, so the resistance of longitudinally aligned resistors increases and the resistance of transversally aligned resistors decreases with increasing ambient pressure.
5 mm
a
b Fig. 1. (a) Integrated pressure sensor; (b) cross section.
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Fig. 2. Pressure sensor designs with (a) only transversally oriented piezoresistive paths and (b) transversally oriented paths mixed with longitudinally oriented paths
3. Embedding process The pressure sensors are embedded in a laminate of unidirectional glass fiber reinforced plastic (GFRP) prepreg (HexPly 913). Several specimens for tensile tests are produced whose dimensions are matched to the geometry of the pressure sensors. The laminate is made up of 22 individual layers with a 0°/90° layout (orthogonal fiber orientation of subsequent layers), where the thickness of 7 layers corresponds to the thickness of the pressure sensor. Within these 7 layers an area at the desired location is cut out matching the lateral dimensions of the sensor in order to avoid adverse surface steps. The samples with sensors are processed under vacuum in an autoclave and cured at 120 °C for 2 h at a pressure of 3 bar. The proper function of the embedded sensors is verified after the lamination process by static pressure tests. 4. Tensile stress test The test setup is designed to obtain the sensor signals as well as the mechanical deformation of the sensor and the composite material itself. The samples are clamped in a tensile testing machine and loaded to fracture. Simultaneously to force and elongation the electrical signals of the sensors are recorded. The sensor voltage (V+ to GND) is set to 1 V. Using an optical 3D measuring system (ARAMIS) the areal deformation of the specimen is captured, which is not possible with traditional measuring systems such as strain gages. With a suitable choice of the resolution the true 3D stress-strain behavior of the samples can be determined and the plain strain tensor for each measurement point can be received. Measurements on all devices are started with a comparable time base. The tests are performed for both sensor designs up to visible fracture of the sensor. 5. Results The sensor signal output Ubridge with p0 = const is shown in Fig. 3 (a). It can be seen that the signal of the sensor having only transversal piezoresistors remains nearly unchanged for all four Wheatstone bridges (compare with Fig. 2) up to its damage indicated by sudden and steep output changes (blue line). In contrast, the signal of the sensor with the combination of longitudinal and transversal piezoresistors (red line) shows a continuous decrease or increase depending on strain direction versus bridge orientation with increasing external strain. Note that the different critical elongations for the two curves in Fig. 3 do not indicate a trend because the point of damage varies already within the same design. The different signal behaviors are a consequence of the different alignments of the piezoresistors in combination with their interconnection within the Wheatstone bridge circuitry (Fig. 2).
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0,1
sensor output [V]
0,05
transversal resistor alignment
0 0,0
0,2
0,4
0,6
0,8
1,0
1,2
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1,6
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-0,05 -0,1 -0,15 longitudinal and transversal resistor alignment -0,2
sensor damage
strain [%]
a
b Fig. 3. Results: (a) sensor output signal sequence; (b) mechanical deformation at 0.98 kN
Having the same alignment of all sensors within one bridge, all resistances are changed by the external stress with the same rate; the effect of external mechanical stresses is therefore compensated. In contrast, with the combination of longitudinal and transversal piezoresistors, external mechanical stresses in the directions parallel to the membrane surface have a similar result as pressure driven out-of-plane deflection of the membrane. The sensor is therefore not able to distinguish between variations of ambient pressure and external stresses parallel to the membrane. Fig. 3 (b) shows the areal strain of the test sample captured with the optical 3D measuring system. It can be seen that high differences occur between the low strains within the sensor area (square in the middle) and the stresses in surrounding fiber composite. The material interfaces perpendicular to the external tensile stress direction experience local strain concentrations and finally the sensor begins to lose its mechanical connection. 6. Discussion and Outlook Provided that suitable cut-outs are prepared in the prepregs MEMS pressure sensors can be integrated without damage and without perturbing surface steps during composite lamination processes. Tensile tests with thereby produced materials with embedded sensors revealed that sensor signals unsusceptible to external tensile stress can be obtained with designs where four piezoresistors within a Wheatsone bridge configuration are all aligned in one direction. Furthermore, orthogonally orientated piezoresistors in one bridge allow quantifying the in-plane tensile stress due to external tensile forces. At tensile load undesired stress concentrations occur in the vicinity of the interfaces between fiber composite and embedded sensors which ultimately lead to device damage. To reduce local stress accumulation sensor outer geometries with sharp 90°corners should be avoided and the transition zone material should be optimized. References [1] M. Schwerter, T. Beutel, M. Leester-Schädel, S. Büttgenbach, A. Dietzel, Flexible hot-film anemometer arrays on curved structures for active flow control on airplane wings, Microsystem Technologies 20 (2014), 4, 821–29. [2] Sebastian Steinke, Airbus unveils a 350 XWB, FLUG REVUE (2006), 9, 26. [3] A. Weder, S. Geller, A. Heinig, T. Tyczynski, W. Hufenbach, W.-J. Fischer, A novel technology for the high-volume production of intelligent composite structures with integrated piezoceramic sensors and electronic components, Sensors and Actuators A: Physical 202 (2013), 106–10. [4] T. Beutel, M. Leester-Schädel, S. Büttgenbach, Design and evaluation process of a robust pressure sensor for measurements in boundary layers of liquid fluids, Microsystem Technologies 18 (2012), 7-8, 893–903. [5] F. Völklein, T. Zetterer, Praxiswissen Mikrosystemtechnik, Grundlagen, Technologien, Anwendungen, Vieweg, Wiesbaden, 2006.
