PVDF nanofiber sensors for real time impact measurements in string structures

• Main Author: Rahul Kumar Singh
• Other Authors: Lye Sun Woh
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Nanotechnology; Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/144159
• Institution: Nanyang Technological University

Advances in the Internet of Things (IoT) have led to a surge in the development of smart devices for various key application areas such as human healthcare, sports performance tracking, soft robotics, smart homes and energy monitoring. The backbones of these smart devices are flexible sensors that can measure important mechanical properties such as strain, vibration and displacement. The use of these flexible sensors is growing and they are being employed in new areas of applications. One of the potential areas of use is strings which are the building blocks of many important structures such as human ligaments, robotic arms, sports rackets, string musical instruments and cable bridges. The determination of strain and vibration in the strings can be used to evaluate some key properties such as string tension and impact location of a force on the string. The measurement of these properties is crucial to the performance of the strings and string structures. The sensing methods to measure string properties are still in their infancy till now and in most cases inadequate especially for real-time measurement for sports applications. This thesis presents the development of a new lightweight Polyvinylidene fluoride (PVDF) nanofiber sensor that can be attached to a string and measure the real-time impact characteristics on the string upon the impact of a force. The developed sensor is able to gather the required strain and vibration data for impact characterization without the need for an external voltage source. Liquid crystal polymer (LCP) has been chosen as the substrate for the sensor fabrication due to its qualities of being highly bendable and light-weight. High elongation to break ratio and a high surface area to volume ratio are some of the properties of PVDF nanofibers that make them suitable for their use in the development of the current sensor. The electrospinning process is chosen to fabricate PVDF nanofibers, as the nanofibers formed do not need any additional poling for their use in piezoelectric sensors. A study is made into the effect of the electrospinning process parameters on the morphology, fraction of β-phase and crystallinity of the PVDF nanofibers for enhancing their piezoelectric and mechanical properties. The parameters affecting the viscosity of the solution, the availability of the solution for electrospinning and the stretching of the nanofibers have been investigated using one-factor variation experiments. The important parameters of spin distance, electric field and drum collector speed have been identified from these experiments contributing significantly to the fraction of β-phase of the electrospun PVDF nanofibers. A new methodology for an electrospinning process based on the combined effect of the process parameters with the use of a factorial design has also been developed. This had led to the formation of electrospun PVDF nanofibers with a large fraction of β-phase (80.3 %) and a higher degree of crystallinity (50.3%). The development of the sensor for the measurement of strain and vibration on a string with the deposition of electrospun PVDF nanofibers on an LCP substrate has been achieved. A comparison between two electrode designs, a comb drive design and a parallel electrode design, has suggested the latter to be superior in terms of voltage output (approximately double) and ease of fabrication. The chosen sensor is tested in the range of strains (200 µε to 2400 µε) and vibration frequencies (0 to 180 Hz) used in string applications. The developed sensor displays a good sensitivity (0.2 mV/µε) to strain and (±0.4%) maximum error for frequency measurements in comparison with a commercial laser displacement sensor (LDS) sensor. The voltage output from the sensor demonstrates a dependence on the deposition of different thicknesses of PVDF nanofiber samples and the deposition time of 1 hour is chosen where a larger voltage output is obtained. Two sets of experimental tests are conducted where the sensor is attached first to a single string and then to a meshed racket string bed. For the sensor that is attached to a single string, results show that good repeatable output voltage response (25-35 mV) can be obtained when subject to an impact hammer. The developed sensor is also able to measure and identify the various frequencies (0-180Hz) that the string is subjected to. The testing of the sensor on the ball impact on a single string under a tension of 40 lbs reveals a high sensitivity of 150 mV/ε (%) indicating its ability to successfully measure strain from the string. A comparison is then made between the hit profile results obtained by the sensor and those from the high-speed camera pictures. Good adherence can be achieved between the comparative results suggesting the ability of the sensor to detect different instants of ball impact made on a single string. A positive relationship between the velocity of the ball and the voltage output is also established. From the relationship, it indicates that the sensor can be used to predict the velocity of the impact object on the string. A new theoretical model based on strain inputs has been developed that can be used to calculate the impact location of a force and the corresponding deflection of the string. The theoretical model with the strain inputs from the sensor was compared with the actual drop readings. A maximum ±10 % error variance between them in terms of deflection and the impact location for a ball drop on the string is recorded. For attachment of the sensors to a mesh string in a tennis racket, a new predictive model based on the output voltage ratio approach for prediction of impact location on ball drops has been developed. Based on the model, the sensors can be used to identify the zone along the vertical direction for the ball drop on the racket. This new approach of using piezoelectric strain and vibration sensors in the measurement of the impact location and impact characteristics in string structures offers a new way of gathering more accurate real-time measurements such as for racket sports in a game situation.