Piezoresistive MEMS sensor arrays for tactile applications

Abstract

As common human computer interfaces(HCIs), touchscreen panels (TSPs) have been ubiquitously adopted in our daily life especially in the consuming electronics, and numerous industrial applications. However, force information has been a missing part in conventional TSPs, drawing limitations in the gesture strategy and user experiences. This void has led to weaknesses such as complex gestures, multi-level menus, waiting, etc. To fill this gap, the objective of this thesis was to study the feasibility of building MEMS-based multi-axis force sensor arrays for tactile applications. Firstly, this work investigated the feasibility of the MEMS-based tactile sensor array for sensing the normal tactile force. The structural configuration of the sensor array has been studied by the numerical models using the finite element method. The outputs are corelated with the distance between the location where the force is applied and the tactel (tactile cell). The geometric parameters have been examined for the behavior of the sensor array. The sensor array for the normal force has been built in a 2 à 2 configuration. Tactels in the sensor array have been optimized and fabricated for this tactile application. The fabrication process flow has been introduced, including the challenges encountered in the development. The sensor array for the normal force has four tactile cells packaged inside. By applying a known normal force on the sensor array, the sensor array has been tested. The test results agreed with the trend of the numerical simulation data. The tactile force and its location were quantified by a lookup table based on a least square method. A 2-mm location resolution has been accomplished in the force range of 0.01 - 0.25 N. The prototype-of-concept shed light on reducing the number of tactile cells. Further numerical analysis demonstrated the scalability of the sensor array for larger-area applications with the same number of tactile cells. Moreover, the tangential forces can be introduced as complementary input gestures. To enrich touchscreen functions, the multi-axis tactile sensor array with the unique layered structure has been developed, sensing both the multi-axis tactile force and its location. This prototype has a functional area of 60 mm ⨠60 mm, packaged by only four tactels. The development of the multi-axis tactile sensor array has been addressed in this work including the design, fabrication, packaging, and tests. Qualitative and quantitative tests have been performed to study the responses of the tactile sensor array with the force range of 0.1 - 0.5 N. The results of the proposed sensor array demonstrated promising potentials for future tactile applications. Generally, the MEMS development entails different phases including fundamental design, mask preparation, process flow, packaging and tests. To finalize the solution for one specific application, engineers must go through several iterations of the development, which is time-consuming and high-cost. To shorten the development time/cost and explore broader applications, tuning the sensitivity of concurrent devices is merited for accelerating the development progress. Since two types of tactels for the sensor arrays have been developed, the sensitivity tunability of these devices has been introduced in their packaging phase. The force ranges of the original tactels have been altered to be more than 10-folds and 3000-folds by packaging the assembly with PDMS and PU, respectively. The tuned force sensors with different force ranges can be used for physiological signal monitoring applications.