The process of designing and validating a touch sensor depends on the level of experience and expertise of the designer. However, no matter the case, a design begins from the high level (system-level) product specifications of the touch sensor. Those specifications describe the desired outcome and what is expected from the touch sensor when it is ready for production. For example, a product specification table of mutual and self-capacitive touch sensors is shown in Figure below.
In this figure we can see that the capacitance touch sensor is expected to have a response time of 30ms, to be able to support 2 concurrent touches with a minimum diameter of 7mm and many more. Other high level specs can be the size of the screen, restrictions on the controller and material choices, the thickness of the protective cover lens and many others.
Once the specifications are clearly defined, the designer can depict them in an equivalent circuit, or a Target Schematic, which ideally transforms those specifications into the electrical circuit of the product.
Finally, when the target schematic is ready, the designer is ready to produce the sensor, that is the sensor layout. The physical layout should incorporate all the product specifications and remain consistent with the target schematic in terms of the typical requirements that the product definition describes. For example, you cannot have a target schematic for a 5 inch touchscreen, but design a 5.5 inch Sensor Layout, because that better sensitivity, cheaper materials etc.
The most common practice of doing so is to design the layout by following generic or specific guidelines and experience. There is a very large list of guidelines for the design of a capacitive touch sensor. For example, in order to design a typical touchscreen using the common diamond double layer pattern, we learn that there is a typical row/column pitch, a typical XY separation, but also minimum and maximum values that should be considered:
The next step is the product validation. The sensor layout is going to be compared against the product specifications. This can be done by performing a series of measurements and tests on the actual prototype model or by building a virtual prototype and test it against the target through a round of simulations.
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