1.2.5 Surface Sensor Design
A self-capacitance touch surface consists of ‘row’ and ‘column’ electrodes whose measurements are used to implement slider functionality in both the horizontal and vertical directions. The results can be combined to accurately resolve touch coordinates and detect 2D gestures, including dual-touch gestures like pinch-and-zoom.
The simplest pattern is the ‘diamond’ pattern shown below. In this example, sensors H0 to H5 provide the horizontal location of a touch contact, while V0 to V4 provide the vertical location.
| Type | Min. | Typical | Max. |
|---|---|---|---|
| Electrode Pitch (P) | 4 mm | 6 mm | 10 mm |
| Electrode Separation (S) | 0.25 mm | 0.5 mm | 1 mm |
- Horizontal and vertical sensor pitch is the distance between column and electrode centers, respectively
- Sensor separation is the perpendicular distance between the parallel edges of adjacent diamonds
Each sensor electrode forms a chain of squares (symmetrical node pitch) or diamonds (asymmetrical pitch), which are turned 45° to improve interpolation in the horizontal and vertical directions.
Electrode Pitch
The ideal electrode pitch is approximately 5 mm for a user touch area of 8 mm. This ensures that a contact placed anywhere on the surface will include an overlap area with at least two sensor electrodes in each dimension and thus enables the best interpolation of the touch position.
For larger touch surface designs, this means a high number of sensor electrodes is required to maintain optimum linearity. However, more sensors mean longer acquisition time, which may lead to reduced response time. In many cases, the designer must compromise between sensor linearity and the number of sensors.
Extended Interpolation
As with sliders and wheels, it is possible to design electrodes for a surface sensor with increased interpolation between adjacent sensors. This allows the designer to increase the electrode pitch while maintaining linearity.
One example is the ‘flower’ pattern, where each element of the sensor array has increased spatial interpolation with its neighbors.
| Type | Min. | Typical | Max. |
|---|---|---|---|
| Electrode pitch (P) | 4 mm | 6 mm | 10 mm |
| Electrode separation (S) | 0.5 mm | 1 mm | 1.5 mm |
As with other sensors, sharp corners in the electrodes have to be rounded to minimize susceptibility to ESD. The points of the triangles forming the interpolation must be truncated to a rounded end of ~2 mm diameter.