The location and force of the user's touch can be quickly and accurately determined by placing four force sensors (S1, S2, S3, S4) near the corners of the touch surface (Figure 1). Touching the surface at point A imposes equal force on all four sensors while touching the surface at point B imposes greater force on sensor S4 and the least amount of sensor S1.
Figure 1 InfiniTouch force panel sensor layout.
Figure 2 Slotted Plate Touch System.
X-Y location is easily determined by using relatively simple math equations:
X = (S3 + S4) / (S1 + S2 + S3 + S4)
Y = (S1 + S3) / (S1 + S2 + S3 + S4)
±z-axis force easily determined with another simple equation:
Force (finger) = -(S1 + S2 + S3 + S4)
The fundamental invention requires taking a rigid plate and cutting some slots in it (Figure 2). The slots create four beams, onto which the force sensors are installed. When the inner plate is touched, the four beams absorb the force exactly. Based on typical off-axis forces induced by typical touch profiles, 5% accuracy across the X and Y dimensions of the unit is achieved.
A Relatively inexpensive DSP device can do the processing necessary to detect the four force waveforms, calculate appropriate measurement points and generate an accurate touch location.