Guide for Surface Sensor Design using Modular QTouch® Library

This article provides guidelines to design a surface sensor. This surface design needs to be used with a surface project created using the START-based QTouch® Modular Library.

Sensor Arrangement

The surface sensor is achieved using the rhombus-shaped sensor pattern shown in the following image. This pattern is also known as the diamond pattern. The surface sensor is formed as a superimposition of vertical and horizontal channels.

sensorarrangement.png

The vertical channels are counted on the Y-axis and are stacked one above another. The following image shows the vertical channels in a 5x5 surface sensor. Each channel contains more than one diamond electrode which are physically connected using PCB trace. The highlighted channel shows how the sensors are connected.

verticalsensor.png

The horizontal channels are counted on the X-axis and are stacked one after another. The following image shows the horizontal channels in a 5x5 surface sensor. Like vertical channels, horizontal channels contain more than one diamond which are physically connected using PCB traces. The highlighted channel shows how the diamond electrodes are connected.

horizontalsensor.png

Diamond Size and Gap

  • The size of the rhombus can vary between 3 mm and 5 mm.
  • The edge-to-edge gap between each rhombus should be 0.5 mm.
Nodedimension.png

PCB Stackup

A surface sensor can be implemented in two or four-layer PCB. When using the two-layer PCB, an optional shield layer can be used. In a four-layer PCB, a shield layer must be used next to the sensor layer.

  • The shield layer can be either be driven or hatched-ground shield.
  • Using driven shield is better as it provides better sensitivity and moisture performance.
  • Using driven shield on sensor-layer (surrounding the sensor) improves moisture performance.
  • If hatched-ground is used, sensitivity drops significantly. When hatched-ground shield is used, it is recommended not to use more than 1 mm front panel.

Two Layer PCB Stackup Example

twolayerstackup.png

Four Layer PCB Stackup Example

fourlayerstackup.png

Driven Shield examples

drivenshield.png

Front Panel

  • Any nonconducting materials like ABS, acrylic, polycarbonate can be used.
  • Recommended thickness: between 0.1 mm and 2 mm.
  • If hatched-ground is used as shield, restrict the panel thickness to less than or equal to 1 mm.
  • No conductive coating should be used.

Surface Orientation and Position

The finger touch position is indicated by means of X and Y position. The X position varies when the finger moves on horizontal channels and Y position varies when the finger moves on vertical channels.

The following image shows the expected X and Y position on each corner. The maximum value depends on the resolution. For example, if 10-bit resolution is used, the maximum value is 1023.

Example5x5.png

Note: It is possible to use the gesture library with the surface library. Orientation is important to have proper gesture detection.

For example, the gesture module detects:

  • UP swipe if the Y-position increases.
  • DOWN swipe if the Y-position decreases.
  • RIGHT swipe if the X-position increases.
  • LEFT swipe if the X-position decreases.

Avoid Saturation

The sensor capacitance of each channel should be less than the PTC's internal compensation capacitance to avoid saturation. There are two ways to avoid saturation:

  1. Design with driven shield: The capacitance of a surface sensor will be relatively lower if a driven shield is used.
  2. Series Capacitance: Having a capacitance in series between the MCU pin and sensor electrode reduces the overall capacitance. Series capacitance has an effect on sensitivity. It would be good to have the option to connect series capacitor during prototype. The starting value of the series capacitor can be 100 pF.

Example of 5x6 Surface

Example5x6.png
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