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Force sensor manufactured with laser-based powder bed fusion

Chadda, Romol (2023)
Force sensor manufactured with laser-based powder bed fusion.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00024620
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

Structural health monitoring (SHM) and condition monitoring are gaining in importance due to the increasing digitalization in several fields such as industry automation, energy and aerospace industry as well as in plant construction. The main objective is to monitor the condition of a system or plant in order to facilitate predictive maintenance and prevent failures on one hand and to increase the productivity on the other hand. This requires high-quality data within relevant locations of the plant, which inevitably leads to the integration of sensors into existing structures to provide in-situ measurements. In this context, force sensors are of great importance, as they allow the detection of load peaks. However, since commercially available force sensors are designed as closed systems, it is not always possible to integrate them into existing structures, or only with high effort. Therefore, force sensors with a high degree of individualization in terms of geometry and measuring range are necessary. These requirements can be met with additive manufacturing processes such as laser-based powder bed fusion (LPBF), since they allow manufacturing of almost any geometry combined with their characteristic layer-by-layer build-up that enables the integration of a sensor at a specific layer. However, there is no systematic approach up to now for the realization of LPBF-manufactured force sensors based on strain gauges, which are the gold standard as sensing elements in conventional force sensors.

In this thesis, a disruptive manufacturing approach is presented that breaks and re-arranges the well-known state of the art assembly sequence of force sensors, and, thus, enables a systematic integration of strain gauges into LPBF-manufactured parts. This approach is based on a published patent application and involves inserting a steel plate applied with strain gauges, which serves as a measuring element carrier, into an LPBFmanufactured part during a process interruption. The LPBF-manufactured force sensors (80 mm x 25.5 mm x 14 mm) are investigated regarding two key aspects that significantly influence their performance, which are: the strain transfer from the LPBF-manufactured part to the inserted measuring element carrier and the behavior of the strain gauge in terms of linearity and sensitivity after integration in the rough manufacturing environment. Investigating the first aspect has shown that the LPBF-manufactured force sensors using the disruptive manufacturing approach are reproducible and feature linearity and hysteresis errors of ±0.1 % and ±0.2 %, respectively. The achieved results are very similar to a conventionally manufactured reference force sensor showing the competitive potential of the new approach. At this stage of investigation the strain gauges are not fully encapsulated. In addition, the temperature and creep behavior of the LPBF-manufactured force sensors are also investigated. A comparison of the sensitivity of the LPBF-manufactured force sensors with a FEA model shows excellent agreement, which provides a reliable prediction of strain transmission, allowing for a targeted design of such force sensors in advance. The comparison of the strain gauge behavior before and after being exposed to the LPBF-process shows an irreversible change of the base resistance by up to −0.15 %. This irreversible change results from the temperature load during the LPBF-process as well as a plastic deformation of the measuring element carrier and is larger the closer the location of the strain gauge to the scanned surface. However, this does not negatively affect the linearity and sensitivity of the LPBF-manufactured force sensor due to the chosen geometry of the force sensor and the location of the strain gauges, which undergo a maximum temperature of 145 ◦C.

However, a severe deformation of the measuring element carrier with a deflection of 0.5 mm in height direction is present due to large temperature gradients during the LPBF-process in combination with the chosen geometry of the force sensor. The onset of this thermally induced deformation of the measuring element carrier is visible in measurements of the resistance of the strain gauge during the LPBF process. It shows that a tensile stress acts during establishing a material connection between the measuring element carrier and the LPBF-manufactured base body, which promotes the deformation. This thermally induced deformation is reduced by up to 67 % by adapting the geometry with respect to the height of the material connection as well as some parameters of the LPBF-process, such as scanning strategy and inter layer time.

Based on these findings, a geometry is developed for complete encapsulation of the strain gauges, which is not manufacturable by conventional subtractive or forming processes. The encapsulation increases the temperature experienced by the strain gauges, which reaches a maximum of 230 ◦C. It is shown that during the manufacturing time the strain gauges ar able to withstand these temperatures and maintain their characteristic values. The subsequent characterization of the LPBF-manufactured force sensor with complete encapsulated strain gauges shows linearity and hysteresis errors of ±0.1 % and ±0.2 %, respectively, which are in very good agreement with the LPBF-manufactured force sensors mentioned previously. Furthermore, the designed geometry offers possibilities for the realization of an overload protection as well as the adjustment of the measuring range.

In the last part of this thesis, LPBF manufactured-threads are investigated. Until now, the high surface roughness of LPBF-manufactured parts has prevented the fabrication of usable threads by means of LPBF, causing screws to break off during tightening. Therefore, the flank angle and the clearance of threads of the sizes M3 to M8 are adapted such that usable threads can be fabricated by means of LPBF. For this purpose, a torque test bench is used to insert screws in a defined manner into the LPBF-manufactured threads while measuring the tightening torque. It is found that LPBF-manufactured threads with an additional clearance of 60 μm for flank angles from 80◦ to 100◦ do not require any significant torque when tightening the screws. The possibility of manufacturing threads during fabricating the part using LPBF eliminates the need for subsequent processing steps, and, thus, significantly reduces the production time while increasing the cost-effectiveness.

This work proves that a defined integration of strain gauges into LPBF-manufactured parts is possible using the disruptive approach presented. Hence, custom additively manufactured parts can be fabricated that allow for in-situ measurements. In addition to the promising performance of the LPBF-manufactured force sensors introduced in this work, which is comparable with the conventionally manufactured force sensors, the design freedom allowed by the LPBF-pocess makes their integration in complex geometries a characteristic feature that can not be achieved otherwise. This work provide the basis for developing structurally integrated force sensors and shows the high potential for individualization.

Typ des Eintrags: Dissertation
Erschienen: 2023
Autor(en): Chadda, Romol
Art des Eintrags: Erstveröffentlichung
Titel: Force sensor manufactured with laser-based powder bed fusion
Sprache: Englisch
Referenten: Kupnik, Prof. Dr. Mario ; Weigold, Prof. Dr. Matthias
Publikationsjahr: 13 Oktober 2023
Ort: Darmstadt
Kollation: xvi, 138 Seiten
Datum der mündlichen Prüfung: 11 August 2023
DOI: 10.26083/tuprints-00024620
URL / URN: https://tuprints.ulb.tu-darmstadt.de/24620
Kurzbeschreibung (Abstract):

Structural health monitoring (SHM) and condition monitoring are gaining in importance due to the increasing digitalization in several fields such as industry automation, energy and aerospace industry as well as in plant construction. The main objective is to monitor the condition of a system or plant in order to facilitate predictive maintenance and prevent failures on one hand and to increase the productivity on the other hand. This requires high-quality data within relevant locations of the plant, which inevitably leads to the integration of sensors into existing structures to provide in-situ measurements. In this context, force sensors are of great importance, as they allow the detection of load peaks. However, since commercially available force sensors are designed as closed systems, it is not always possible to integrate them into existing structures, or only with high effort. Therefore, force sensors with a high degree of individualization in terms of geometry and measuring range are necessary. These requirements can be met with additive manufacturing processes such as laser-based powder bed fusion (LPBF), since they allow manufacturing of almost any geometry combined with their characteristic layer-by-layer build-up that enables the integration of a sensor at a specific layer. However, there is no systematic approach up to now for the realization of LPBF-manufactured force sensors based on strain gauges, which are the gold standard as sensing elements in conventional force sensors.

In this thesis, a disruptive manufacturing approach is presented that breaks and re-arranges the well-known state of the art assembly sequence of force sensors, and, thus, enables a systematic integration of strain gauges into LPBF-manufactured parts. This approach is based on a published patent application and involves inserting a steel plate applied with strain gauges, which serves as a measuring element carrier, into an LPBFmanufactured part during a process interruption. The LPBF-manufactured force sensors (80 mm x 25.5 mm x 14 mm) are investigated regarding two key aspects that significantly influence their performance, which are: the strain transfer from the LPBF-manufactured part to the inserted measuring element carrier and the behavior of the strain gauge in terms of linearity and sensitivity after integration in the rough manufacturing environment. Investigating the first aspect has shown that the LPBF-manufactured force sensors using the disruptive manufacturing approach are reproducible and feature linearity and hysteresis errors of ±0.1 % and ±0.2 %, respectively. The achieved results are very similar to a conventionally manufactured reference force sensor showing the competitive potential of the new approach. At this stage of investigation the strain gauges are not fully encapsulated. In addition, the temperature and creep behavior of the LPBF-manufactured force sensors are also investigated. A comparison of the sensitivity of the LPBF-manufactured force sensors with a FEA model shows excellent agreement, which provides a reliable prediction of strain transmission, allowing for a targeted design of such force sensors in advance. The comparison of the strain gauge behavior before and after being exposed to the LPBF-process shows an irreversible change of the base resistance by up to −0.15 %. This irreversible change results from the temperature load during the LPBF-process as well as a plastic deformation of the measuring element carrier and is larger the closer the location of the strain gauge to the scanned surface. However, this does not negatively affect the linearity and sensitivity of the LPBF-manufactured force sensor due to the chosen geometry of the force sensor and the location of the strain gauges, which undergo a maximum temperature of 145 ◦C.

However, a severe deformation of the measuring element carrier with a deflection of 0.5 mm in height direction is present due to large temperature gradients during the LPBF-process in combination with the chosen geometry of the force sensor. The onset of this thermally induced deformation of the measuring element carrier is visible in measurements of the resistance of the strain gauge during the LPBF process. It shows that a tensile stress acts during establishing a material connection between the measuring element carrier and the LPBF-manufactured base body, which promotes the deformation. This thermally induced deformation is reduced by up to 67 % by adapting the geometry with respect to the height of the material connection as well as some parameters of the LPBF-process, such as scanning strategy and inter layer time.

Based on these findings, a geometry is developed for complete encapsulation of the strain gauges, which is not manufacturable by conventional subtractive or forming processes. The encapsulation increases the temperature experienced by the strain gauges, which reaches a maximum of 230 ◦C. It is shown that during the manufacturing time the strain gauges ar able to withstand these temperatures and maintain their characteristic values. The subsequent characterization of the LPBF-manufactured force sensor with complete encapsulated strain gauges shows linearity and hysteresis errors of ±0.1 % and ±0.2 %, respectively, which are in very good agreement with the LPBF-manufactured force sensors mentioned previously. Furthermore, the designed geometry offers possibilities for the realization of an overload protection as well as the adjustment of the measuring range.

In the last part of this thesis, LPBF manufactured-threads are investigated. Until now, the high surface roughness of LPBF-manufactured parts has prevented the fabrication of usable threads by means of LPBF, causing screws to break off during tightening. Therefore, the flank angle and the clearance of threads of the sizes M3 to M8 are adapted such that usable threads can be fabricated by means of LPBF. For this purpose, a torque test bench is used to insert screws in a defined manner into the LPBF-manufactured threads while measuring the tightening torque. It is found that LPBF-manufactured threads with an additional clearance of 60 μm for flank angles from 80◦ to 100◦ do not require any significant torque when tightening the screws. The possibility of manufacturing threads during fabricating the part using LPBF eliminates the need for subsequent processing steps, and, thus, significantly reduces the production time while increasing the cost-effectiveness.

This work proves that a defined integration of strain gauges into LPBF-manufactured parts is possible using the disruptive approach presented. Hence, custom additively manufactured parts can be fabricated that allow for in-situ measurements. In addition to the promising performance of the LPBF-manufactured force sensors introduced in this work, which is comparable with the conventionally manufactured force sensors, the design freedom allowed by the LPBF-pocess makes their integration in complex geometries a characteristic feature that can not be achieved otherwise. This work provide the basis for developing structurally integrated force sensors and shows the high potential for individualization.
Alternatives oder übersetztes Abstract:
Alternatives AbstractSprache

Struktur- und Zustandsüberwachung gewinnen durch die zunehmende Digitalisierung in den Bereichen Industrieautomatisierung, Energie- und Luftfahrtindustrie sowie im Anlagenbau immer mehr an Bedeutung. Das Ziel besteht dabei darin, den Zustand einer Anlage oder Systems zu überwachen, um einerseits eine vorausschauende Wartung zu ermöglichen und somit Fehlerfälle bzw. Ausfälle vorzubeugen und andererseits die Produktivität zu erhöhen. Voraussetzung hierzu ist die Bereitstellung aktueller Zustandsdaten an den kritischen Stellen des zu überwachenden Systems. Dies erfordert die Integration von Sensoren in eine bestehende Struktur, um sogenannte ïn-situ Messungenßu ermöglichen. In diesem Kontext spielen Kraftsensoren eine große Rolle, da diese die Erkennung von Lastspitzen ermöglichen. Da kommerziell erhältliche Kraftsensoren jedoch als Allzwecksensoren ausgelegt werden, ist deren Integration nicht immer oder nur erschwert in bestehende Strukturen möglich. Vielmehr sind Kraftsensoren mit einem hohen Grad an Individualisierbarkeit hinsichtlich Geometrie und Messbereich sowie vernachlässigbarer Rückwirkung auf das Bauteil erforderlich. Diese Eigenschaften lassen sich mit additiven Fertigungsverfahren, wie beispielsweise dem pulverbettbasierten Laserstrahlschmelzen (LPBF), realisieren, da sich einerseits nahezu jede Geometrie damit herstellen lässt und andererseits der typische Schichtauffbau die Intergration von Sensorik an beliebiger Stelle des Bauteils ermöglicht. Aktuell besteht allerdings kein systematischer Ansatz zur Realisierung von LPBF-gefertigten Kraftsensoren basierend auf Dehnungsmessstreifen (DMS), die bei konventionellen Kraftsensoren das Standard-Sensorelement darstellen.

Daher wird in dieser Arbeit ein neuartiger, disruptiver Fertigungsansatz vorgestellt, der die übliche Fertigungsreihenfolge von konventionellen Kraftsensoren aufbricht und neu anordnet und somit eine systematische Integration von DMS in LPBF-gefertigte Bauteile ermöglicht. Dieser Lösungsansatz wurde in einer eigenen bereits veröffentlichten Patentanmeldung fixiert. Dabei wird eine mit Dehnungsmessstreifen (DMS) applizierte Stahlplatte (Messelementträger), die als Verformungskörper dient, in dem LPBF-gefertigten Bauteil während der Unterbrechung des additiven Fertigungsprozesses eingesetzt. Mit den in LPBF-gefertigten Demonstratoren als Kraftsensoren werden die zwei wichtigsten Sensor-Qualitätskriterien, die Dehnungsübertragung vom LPBF-gefertigten Bauteil zum eingelegten Messelementträger und das geforderte statische Sensor-Übertragungsverhalten, welches die Empfindlichkeit und Linearität einschließt, unter realen Prozessbedingungen untersucht. Hierbei zeigt sich, dass die LPBF-gefertigten Sensoren (80 mm x 25.5 mm x 14 mm) nach disruptivem Herstellungsansatz reproduzierbar mit einem Linearitätsfehler von ±0.1 % und einem Hysteresefehler von ±0.2 % aufgebaut werden können. Diese Werte entsprechen den typischen Werten konventioneller Kraftsensoren. Außerdem werden das Temperatur- und Kriechverhalten der LPBF-gefertigten Kraftsensoren untersucht. Der Vergleich der Empfindlichkeit der LPBF-gefertigten Kraftsensoren mit einem FEM-Modell zeigt eine sehr gute Übereinstimmung, sodass eine zuverlässige Vorhersage der Dehnungsübertragung und damit eine gezielte Auslegung solcher Kraftsensoren im Vorfeld möglich ist. Der Vergleich des DMS-Verhaltens vor und nach Durchführung des LPBF-Prozesses liefert lediglich eine Abweichung des DMS-Grundwiderstandes von maximal 0.15 %. Diese Änderung resultiert aus der thermischen Belastung während des LPBF-Prozesses sowie einer plastischen Verformung des Messelementrägers und ist umso größer, je näher der DMS der belichteten Fläche ist. Dies wirkt sich aufgrund der gewählten Geometrie des Verformungskörpers und der Positionierung des DMS, die eine Temperatur von maximal 145 ◦C ausgesetzt sind, jedoch nicht negativ auf die Linearität und Empfindlichkeit des LPBF-gefertigten Kraftsensors aus.

Nichtsdestotrotz ist eine starke Verformung des Messelementträgers mit einer Auslenkung von 0.5 mm in Höhenrichtung aufgrund auftretender großer Temperaturgradienten während des LPBF-Prozesses in Kombination mit der gewählten Geometrie des Verformungskörpers vorhanden. Der Beginn dieser Verformung des Messelementträgers ist bei Messungen des DMS-Widerstands während des LPBF-Prozesses ersichtlich und zeigt, dass zum Zeitpunkt der stoffschlüssigen Verbindung des Messelementträgers mit dem LPBF-gefertigten Bauteil eine Zugspannung wirkt und es somit zur Verformung des Messelementträgers kommt. Diese Verformung wird jedoch durch Anpassung der Geometrie hinsichtlich der Höhe der Materialanbindung sowie einiger Prozessparameter, wie beispielsweise die Belichtungsstrategie und Belichtungszeit, um bis zu 67 % reduziert.

Darauf aufbauend wird mit den optimierten Prozessparametern eine Geometrie für eine vollständige Verkapselung der DMS entwickelt, die mit konventionellen subtraktiven oder umformenden Verfahren nicht fertigbar ist. Durch die Verkapselung nimmt die von den DMS erfahrene Temperatur zu, die in diesem Fall bei maximal 230 ◦C liegt. Dabei zeigt sich, dass die DMS auch diesen Temperaturen standhalten und ihre Kennwerte beibehalten. Die anschließende Charakterisierung des LPBF-gefertigten Kraftsensors mit vollständig verkapselten DMS zeigt einen Linearitätsfehler von ±0.1 % sowie einen Hysteresefehler von ±0.2 %, die in sehr guter Übereinstimmung mit den restlichen in dieser Arbeit aufgebauten Kraftsensoren sind. Darüberhinaus bietet die Geometrie Möglichkeiten zur Realsierung eines Überlastschutzes sowie die Anpassung des Messbereichs.

Im letzten Teil der Arbeit werden zur Gewährleistung einer zweiseitigen Einbindung des Messelementes LPBF-gefertigte Gewinde untersucht. Aufgrund der Eigenschaft der hohen Oberflächenrauheit von LPBFgefertigten Bauteilen, sind bisher keine nutzbaren Gewinde mittels LPBF herstellbar, sodass sogar Schrauben beim Anziehen abbrechen. Dazu werden in dieser Arbeit der Flankenwinkel und die Clearence von Gewinden der Größen M3 bis M8 so angepasst, dass sich nutzbare Gewinde mittels LPBF herstellen lassen. Hierfür wird ein vorhandener Drehmomentmessstand genutzt, um Schrauben definiert in die LPBF-gefertigten Gewinde einzubringen. Die Ergebnisse zeigen, dass LPBF-gefertigte Gewinde mit einer zusätzlichen Clearence von 60 μm für Flankenwinkel von 80◦ bis 100◦ kein nennenswertes Drehmoment beim Anziehen der Schrauben benötigen. Die Möglichkeit Gewinde direkt bei der Fertigung des Bauteils im LPBF-Verfahren mitzudrucken, erspart nachträgliche Bearbeitungsschritte und reduziert damit die Fertigungszeit sowie Produktionskosten eines LPBF-gefertigten Bauteils.

Insgesamt bestätigt die Arbeit, dass eine definierte Integration von DMS-Messelementen in LPBF-gefertigte Bauteile mit Hilfe des vorgestellten disruptiven Ansatz reproduzierbar möglich ist. Die mit LPBF gefertigten Kraftsensoren entsprechen in ihren Kennwerten denen konventionell gefertigter Kraftsensoren. Sie bilden daher einen Lösungsansatz für die Entwicklung von strukturintegrierten Kraftsensoren. Zusätzlich weisen diese neuartigen Kraftsensoren ein hohes Potential für individuelle Adaption in Konstruktionsbauteile auf.

Deutsch
Status: Verlagsversion
URN: urn:nbn:de:tuda-tuprints-246200
Sachgruppe der Dewey Dezimalklassifikatin (DDC): 600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften und Maschinenbau
600 Technik, Medizin, angewandte Wissenschaften > 621.3 Elektrotechnik, Elektronik
Fachbereich(e)/-gebiet(e): 18 Fachbereich Elektrotechnik und Informationstechnik
18 Fachbereich Elektrotechnik und Informationstechnik > Mess- und Sensortechnik
TU-Projekte: DFG|KU3498/4-1|Strukturintegration
Hinterlegungsdatum: 13 Okt 2023 11:43
Letzte Änderung: 16 Okt 2023 13:18
PPN:
Referenten: Kupnik, Prof. Dr. Mario ; Weigold, Prof. Dr. Matthias
Datum der mündlichen Prüfung / Verteidigung / mdl. Prüfung: 11 August 2023
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