Dual range capacitive MEMS force sensor for touch screen applications

A touch interface, comprising:

• a) a flexible cover glass that seals the touch interface;
• b) an LCD display that provides an interaction medium;
• c) an array of force sensors interposed between the flexible cover glass and the LCD display, each force sensor comprising:
• i) a moving upper panel having a lower surface and an upper touch surface to receive a touch force;
• ii) a fixed substrate having a top surface and an overload protection surface to protect the sensor from excessive applied touch forces,
• iii) a first set of planar conductive electrodes mounted on the lower surface of the moving upper panel;
• iv) a second set of planar conductive electrodes mounted on the top surface of the fixed substrate;
• v) the first and the second planar conductive electrodes are separated by a predetermined distance defining an initial capacitance value of the force sensor;
• vi) a first set of elastic spacers comprising of a set of flexible pillars, each flexible pillar having a low stiffness, a long height and a first surface area;
• vii) a second set of elastic spacers comprising of a set of stiff pillars, each stiff pillar having a high stiffness, a short height and a second surface area;
• wherein the first and the second set of elastic spacers are interposed between the first and the second set of planar conductive electrodes to control the changes in a distance between the first and the second set of planar conductive electrodes according to an applied external force, and
• wherein the set of flexible pillars have low stiffness to respond to small applied forces while the set of stiff pillars have high stiffness to respond to large applied forces, and
• wherein the long heights of the set of flexible pillars are longer than the short heights of the set of stiff pillars for tolerating higher applied forces,
• d) a flexible circuit board for monitoring changes in a capacitance at each force sensor,
• whereby applied touch forces squeeze the cover glass and lead to a capacitance change at one or more of the force sensors indicating the position of the touch within the LCD display panel and the magnitude of the touch force, whereby the change of capacitance is monitored at all force sensors, and the exact location is determined using an interpolation algorithm.