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Design, fabrication and characterisation of a force sensitive sensor from inhomogenous 3D printed material

Laagland, M.G.T. (2018) Design, fabrication and characterisation of a force sensitive sensor from inhomogenous 3D printed material.

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Abstract:The objective of this report is developing a 3D printed capacitive force sensitive sensor made from flexible conductive carbon based thermoplastic polyurethane (ETPU). In earlier research a resistance between printed layers in a 3D printed material is found, which could be due to an inhomogeneous distribution of conductive carbon particles. These inhomogeneous properties are unwanted in existing 3D printed applications, but can also be used to create new applications, as is done in this research. The functioning of the capacitive force sensor is be based on the use of the inhomogeneous properties of the material. For the capactive sensor to function as a capacitor, two conducting plates should be close to each other separated with a dielectric material. Because the sensor only consists of ETPU it can not function solely as a capacitor, it will have a parallel and serial resistance as well. So, ETPU should function as both the conductive plates as the dielectric. The conductive plates are the areas with the highest concentration of carbon particles, which is assumed is in the center of the material. What results in the surrounding material containing a lower concentration carbon particles. Here is where the percolation theory comes along. The percolation theory states that there is a turning point in the relation of percentage of carbon particles and conductivity of the material. This turning point is the percentage of carbon particles where the conductivity substantially increases. Because carbon particles make the material more brittle, the least possible amount of carbon particles is infused in the material, which caused that the percentage carbon particles is probably around the turning point. Areas with a slightly smaller concentration of carbon particles, could cause a lot less conductivity than areas with a slightly higher concentration of carbon particles. This causes the areas with a slightly smaller percentage carbon particles to function as the dielectric, which is the reason two layers of 3D printed material could function as a capacitor, with a serial and parallel resistor. These inhomogeneous properties could be caused by the fabrication process of the filament or during the printing process. The cause is not yet known. The biomedical relevance is in prosthetics. When an external force is applied to the sensor, the capacitance will change. This change could be linked to a specific force change, which would make the force sensor measure the amount of force that is applied. The use of this force sensor could be in thimbles for prosthetic hands to give feedback to the person wearing the prosthetic to what kind of force is exerted. The force sensor will be printed with use of a technique called fused deposition modeling (FDM). In FDM a spool of filament is loaded into the printer where it is heated up and deposit on a platform in a pre-set pattern. After filament is printed, it immediately hardens. When one layer is finished, the next layer will be printed on top. The filament will not blend with the previously printed filament, which causes the filament to keep its inhomogeneous properties. The printer used is a FlashForge Creater Pro with a flexion extruder head which allows the printer to print two materials at the same time. The technology behind the capacitive sensing relays on a measured change in capacitance between the printed layers when external force is applied. The force sensor is compared to the functioning of a parallel plate capacitor, but with a few alterations. The surface area used in the formula is replaced by the total volume of the sensor and a parallel and serial resistor is added in the electrical circuit of the sensor. The impedance, capacitance and resistance are measured with an LCR meter. For the transition from impedance to resistance and capacitance a frequency response model is created, this model is be fitted over the measured data and helps determining the capacitance and resistance of the test structures and force sensors. After all the requirements were gathered, it was clear that for the transition from a idea to a prototype research is required. The impacts of printing orientations will be compared, the contact resistances of contact interfaces and the functioning of the two-layered force sensor. The parallel resistance of different types of printing orientation will be compared to gain insight about the differences originating in the printing process and the contact resistances of the contact interfaces will be measured. These will be tested on one-layered test structures. For the infill patterns a four point measurement is used to exclude the influence of the contact resistance and to solely measure the parallel resistance. The three infill patterns compared are perpendicular, parallel with a brim and parallel without a brim relative to the contact interfaces. Every type of infill is at least printed and measured 5 times. The prediction is that the parallel resistance of the perpendicular oriented infill is the lowest, because it would be the easiest for the current to follow the path with the Page 2 highest concentration of carbon particles. The parallel oriented infill patterns will be more difficult for the current to flow through. This is confirmed by the results of the measurements. The perpendicular oriented infill patterns causes the lowest parallel resistance, parallel oriented infill patterns with a brim the second lowest parallel resistance and the parallel oriented infill patterns without a brim caused the highest parallel resistance. So the perpendicular oriented infill is used while printing the two-layered sensor. The measurements regarding the contact resistances used two and four point measurements on the same test structure to calculate the contact interface. Four types of contact interfaces were compared, two which required heat in the mounting process, namely a header pin and stripped copper wire. The other two did not affect the material in the mounting process. These connections are a copper clip and a braid structure which was printed with the sensor. Every contact interface is at least printed and measured 3 times. The measured data showed that the copper clips have the lowest contact resistance. The braid structure has the highest contact resistance. The recommendations when using contact interfaces in further research is to use clips, it will not damage the material because no heat is used in the mounting process and the connection is easier to mount. The two-layered force sensor are two times fabricated, one consisting of two layers with the same printing orientation and one consisting of two layers with a different printing orientation. A four point measurement is applied to solely focus on the change in resistance due to the applied force. The contact interfaces used to connect the sensor to a electrical source in both layers is mounted in a way that only one layer is connected. A surface of 10mm by 10mm is the real sensor, this consists of two electrically loaded layers. A tip of a linear actuator applies an external force to this surface. The expectation was an increase in capacitance when external force was applied but the results of the force sensor with the same printing orientation show a decrease in capacitance with steps of 50 pF by a force difference of 2N. The results of the sensor made of two layers with a different orientation also show a decrease in capacitance but on a smaller, about scale5 pF by a force difference of 2N. Based on the observations and measurements the two-layered force sensor with the same printing orientation show potential to function as a capacitive force sensor. However, further research needs to be done.
Item Type:Essay (Bachelor)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Programme:Electrical Engineering BSc (56953)
Link to this item:http://purl.utwente.nl/essays/80951
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