Modeling and development of piezoelectric based tactile sensors

Humankind interacts with each other and their environment by their five senses: see, hear, touch, smell and taste. The touch sense has a significant role in humans daily life. This sense is responsible for perceiving force and its position, vibration, slip, temperature and pain. By development of robotic science, the need of interacting and collaborating between robots and humans has been increased. One of the ways to enhance the quality of this interaction between humans and robots is to produce touch sensitive robots and machines with the help of tactile sensors. Tactile sensors are the sensors with the ability of measuring and determination of physical properties such as pressure, vibration, softness, texture, shape and temperature through physical contact. Nowadays tactile sensors are used in a variety of applications in industry, robotic and healthcare fields. Tactile sensors are commonly used in automotive industry. The main purpose of utilizing tactile sensor in this industry is to measure the pressure distribution over a surface such as a seat or a brake pad to design more ergonomic components and increase the driving quality. Tactile sensors applications in the robotic field are mostly seen in robotic grippers. In these type of applications, the embedded tactile sensor is responsible to measure the applied force. With help of this sensor, the possibility of slippage or over forcing decreases. Besides all of these, tactile sensors have various applications in healthcare. Minimally Invasive Surgery (MIS) is one of the fields that using tactile sensor are getting popular. In this type of surgery, the tactile sensor is utilized in laparoscopic devices to avoid harming patient and help the surgeon to get information about the patient's body. In this thesis, two types of human-inspired flexible tactile sensors have been proposed and their properties in term of sensitivity and flexibility have been compared. Even though the design of the proposed sensors is same, they are different in their sensing element. These sensing elements are Lead Zirconate Titanate (PZT) and Polyvinylidene Fluoride (PVDF) which are piezoceramic and piezopolymer types of piezoelectric respectively. In the first chapter of this thesis, the aim of the project and the tasks defined for the sensors are explained. In literature review section, human touch mechanism, tactile sensors, and their application areas have been discussed. In the following the different sensing technologies used in the tactile sensors and their advantages and disadvantages have been described. Finally, piezoelectricity and piezoelectric constitutive relationships have been studied. In the second chapter, the design parameters are defined. In order to study the effects of these parameters on the output voltage of the sensor, finite element model (FEM) of the single sensing unit (taxels) has been prepared. In the following, manufacturing steps of the proposed sensor have been described. The third chapter theoretical model of single taxel and behavior of that to applied normal force has been investigated . In addition to state space model of taxels, transfer function of charge amplifier used in measurements are obtained. Moreover, FEM modeling of the sensor and the silicone modeling has been explained briefly. Finally, in the fourth chapter, the experimental setup and results are discussed. The response of piezoelectric taxels to impulse and step responses have experimented. Moreover, Frequency response function of the sensors has been obtained. Moreover, The effect of silicone thickness on taxel's output voltage is studied. Lastly, the force localization ability of the sensors tested and compared with each other. In conclusion, it has been observed that both of the proposed tactile sensors are capable of force localization. However, PZT based tactile sensors are more successful in this task. In PVDF type tactile sensors crosstalk noises are seen commonly which decreases the quality of force localization. These human inspired tactile sensors with their compact designs can be embedded in robotic manipulators or they can be placed on the palm of the hand. It has been observed that the force sensitivity of PZT based tactile sensor is higher than PVDF one whereas the wearability of PVDF based tactile sensor is better due to the flexibility of PVDF materials.