2.2.3 Slider Sensor Design
A slider may be implemented as a row of two or more sensors placed together. The measurements of the sensor group are combined to determine the position of a touch contact with an increased resolution by interpolation between the sensors.
The slider must be between 8-20 mm wide. To avoid excess XY capacitance – and an associated increase in acquisition time – the electrode dimensions and spacing should be increased for larger keys.
As noted in the previous section, large sensors without interpolation lead to poor linearity.
This may be mitigated by increasing the sensor count and decreasing sensor width, but at the cost of total measurement time and touch channel usage. Capacitive or resistive interpolation may be implemented to improve slider linearity.
Interleaved Slider
Spatial interpolation ensures good linearity with fewer sensors and can be implemented using an interleaved layout, where the sensor nodes are formed by alternating X and Y electrodes.
Typically, a single Y line is used with multiple X lines as this allows for the easiest sensor routing, as the Y line sensor trace must be routed with more care to avoid capacitive loading and increased time constant.
| Min. | Typical | Max. | |
|---|---|---|---|
| Slider Height (H) | 8 mm | 12 mm | 20 mm |
| Segment Width (SW) | 8 mm | 12 mm | 30 mm |
| X Electrode Width (XW) | 0.25 mm | 0.5 mm | 2 mm |
| Y Electrode Width (YW) | 0.25 mm | 0.5 mm | 1 mm |
| X-Y Separation (S) | 0.25 mm | 0.5 mm | 1.5 mm |
The interleaved slider may be formed as a coplanar sensor, with X and Y electrodes on the same layer or split to different layers with X on the layer further from the touch surface.
Flooded X Slider
A flooded X slider provides improved linearity as the X electrodes are on a separate PCB layer. Spatial interpolation may be extended without complex routing around the Y electrodes. The X-layer pattern for a flooded X slider is identical to the interpolated self-capacitance slider presented in the previous section.
| Min. | Typical | Max. | |
|---|---|---|---|
| Slider Height (H) | 8 mm | 12 mm | 20 mm |
| Height Segment (HS) | 4 mm | 5 mm | 6mm |
| X Electrode Pitch (P) | 8 mm | 12 mm | 30mm |
| X Segment Separation (XS) | 0.25 mm | 0.5 mm | 1 mm |
| X Electrode Overlap (XO) | 1 mm | 2 mm | 3 mm |
| Deadzone (D) | - | 2 mm | 4 mm |
| Y Electrode Width (YW) | 0.25 mm | 0.5 mm | 1 mm |
| Y Gap (YG) | 3 mm | 4 mm | 5 mm |
Resistive Interpolation
In both interdigitated and flooded X slider designs, it is possible to reduce the number of sensor node measurements while maintaining linearity by resistive interpolation of some sensor nodes.
A minimum of at least two directly routed X electrodes is required, placed at either end of the slider. Intermediate nodes are joined with a series of resistors, forming a resistive divider driving each intermediate node at a fraction of the X drive voltage.
A touch contact on an intermediate node XA causes a proportional touch delta on each of the direct nodes X0 and X1. Reduced pulse amplitude at XA and XB causes proportionally less touch delta at these locations, allowing interpolation between nodes (X0,Y) and (X1,Y).
In this example, with two intermediate nodes, the delta measured at (X0,Y) is 2/3 while that at (X1,Y) is 1/3.
Segment interpolation resistors Rxi must be selected so that the total series combination between each pair of directly connected X lines is in the range of 10-20 kOhm.