K.M. Kalayeh, A. Charalambides, S. Bergbreiter, P.G. Charalambides
Keywords: capacitive tactile sensor, experiments, experimental validation, microfabrication, non-linear predictive model, optimal sensor design, parametric studies
Summary:This study is motivated by the need to develop highly sensitive tactile sensors for both robotic and bionic applications. The developments of advanced technologies in robotic grasping, motion of prosthetics, and robot assisted biomedical surgery require ``real-time'' knowledge of applied forces and their locations. In response, in recent years, tactile sensor technologies received increased attention. Recent developments in microfabrication technologies have enabled the development of Micro-Electro-Mechanical Systems (MEMS) tactile sensors that have shown to have increased spatial resolution while manufactured at relatively low cost and large quantities. Since these sensors usually need to be mounted on curved surfaces, it is critical that they are flexible to bend as well as stretchable. Thus, soft polymer-based materials such as polyimide (PI), or polydimethlysiloxane (PDMS) are used as the substrate phase for flexible MEMS sensors. Among many sensors, capacitive tactile sensors are known to be characterized by better spatial resolution and higher sensitivity. In this work, a large deformation Mooney-Rivlin (M-R) incompressible material model is used to develop the non-linear mechanics of an elastomeric layer with finite thickness under uniform compression. The above large deformation layer compression model was recently combined with an enhanced capacitance model that accounts for both the parallel plate as well as the fringe electric field effects in developing a force-capacitance predictive model for pressure tactile MEMS sensors. The above model was shown to capture the sensor response over the entire sensing range from 350 mN associated with a linear sensor response to over 1.5 N registered well within the sensors non-linear regime. Exceptional agreement was found to exist between the model predictions and related experimental results over the entire sensing range. The working principle of the above sensors can be summarized as follows; conductive electrodes, which are separated by some initial gap, are embedded in a thin elastomeric layer. Upon contact the top surface of the sensor may compress by a uniform deformation. Thus, the initial layer thickness decreases, while the electrode gap increases, and the electrode thickness decreases. Then, the change in capacitance can be related to the applied force using the large deformation and force-capacitance model which utilizes the electrode gap and thickness changes during contact. In order to calibrate the above tactile sensor model, several sensors were fabricated. A test setup was used to capture capacitance and force measurements for known displacements which were then compared to predictions obtained by the sensor model.