One of the advanced multi-touch sensing techniques is based on pressure sensing. Resistive multi-touch screens from Stantum use a slightly modified version of the pressure sensing scheme. In general, pressure sensitive multi-touch surfaces include two patterned flexible sheets coated with parallel electrodes and separated by a pressure sensitive material. In the Touchco design (below), three layers – two force sensing resistor (FSR) layers and an air gap – collectively act as a pressure sensitive layer. Since the resistance of FSR decreases when compressed, it is ideal for pressure sensing.
When contact is made to the multi-touch surface, the two sheets are pressed and the resistance of the middle layers is changed. A sensing circuit checks the change of the resistance and gets pressure information. To generate pressure signals, an electrode of the top (bottom) layer is connected a voltage source and the voltage is at each of the electrodes on the bottom (top) layer is measured one at a time. Remaining electrodes at the top and bottom layers are switched off. This measurement process is repeated for all the crossings between the top and the bottom electrodes. Tekscan provides a good illustration of the sensing mechanism. If you need more detailed schematics, Tech Note 3 and 4 in FSR Tech Notes might be for you.
The pressure sensitive multi-touch sensing technique is not new. Many researchers and companies have worked with the technique. For example, Kitronyx, Tekscan, Pressure Profile Systems, and XSENSOR have done business mainly focusing on industrial and medical measurement instruments. And many researchers have used pressure sensing to build their own tactile sensing systems.