3D multi-touch FSR application

Hardware

You need an MCU with multiple digital and analogue I/O’s. You connect all the rows to the digital I/O's and columns to the analogue I/O’s or vice versa (typically there are a limited number of analogue I/O’s available so if not a square matrix you would connect the analogue to the lowest number). Standard chips are available with 8,16 or 32 analogue I/O’s. The digital lines would be used to ‘drive' the voltage into the sensors and the analogue lines used to ‘read’ the output. The read lines are connected to ground via a reference resistor (typically 10k or 4.7k) and the voltage between the sensor and the reference resistor is measured by the analogue I/O. Usually a reference resistor is required for each analogue line although if suitable isolation is available between the analogue lines in the MCU it may be possible to use just one reference resistor. Again depending on the MCU impedance and isolation additional diodes and or high resistance buffers may be required. Typically 8 bit AtoD resolution is suitable for most HMI applications but if high position and/or pressure accuracy is require 10bit may be advantageous.

 

Scanning method

The first digital line is set to High and all others set to Low or Ground or Floating (differing ghosting and/or cross talk effects will be seen depending on how the additional digital lines are set) then each analogue line is measured sequentially while the others are set to to low or ground or floating (as above different results will be seen depending on the choice). Once all analogue lines are read the next digital line is set to high and all others as above and the analogue lines read again. This is repeated till all sensor cells are read.