1.2.3 Slider Sensor Design

Sensor Pitch

With large sensors and no spatial interpolation, the consistency of the reported touch position vs. actual position is very poor. As a contact moves across the slider, most of the time, there is a touch contact only on one of the four electrodes. Position interpolation can only occur while the contact is crossing from one sensor to the next.

This may be improved by reducing the sensor size and increasing the number of sensors. If the sensor pitch is reduced to ~ ½, the width of a touch contact (i.e., sensor pitch ~4 to 5 mm), then there will always be two to three sensor electrodes under the touch contact area, and several touch deltas are available for interpolation wherever the contact is placed.

However, this is not always the optimal solution as it requires more sensor electrodes than necessary. This will reduce the availability of general-purpose I/O pins, complicate the PCB routing, or require more touch channels than are available on the microcontrollers. Also, the touch acquisition time is proportional to the number of electrodes, meaning that, for a long slider, the required number of sensor electrodes can lead to an unacceptable touch latency.

An alternative is to use spatial interpolation to ‘stretch’ the crossover position from one slider electrode to the next. One example is the electrode shape illustrated below. This design has tapered overlapping edges to ensure that a touch contact anywhere along the length of the slider will always have contact area with at least two sensor electrodes.

Spacing Between Slider Electrodes

Each element of the slider is loaded by its default capacitance and by the capacitance between it and its neighboring electrodes as other electrodes are usually driven to a static DC level while a particular sensor is being measured.

The recommended separation between the sensor electrodes depends on the size of the electrodes and their overlap lengths.

A slider consisting of small keys with no extended interpolation must have a separation of 0.5 mm, or smaller, between electrodes. This improves touch delta consistency as the contact moves from one element to the next, without the occurrence of reduced touch delta in between. See Table 1-2 for recommended separation distances.

A slider consisting of large electrodes with long overlap lengths must have increased separation between the sensor electrodes to avoid excess sensor load capacitance. In such a design, the separation can be increased to 1 mm or more.

The electrodes must be close together for continuous sensitivity, but too little separation can cause increased loading capacitance, as each sensor electrode has a parasitic load against its neighboring electrodes. The spacing must be increased to a maximum of 1.5 mm in the cases when there are long parallel edges between the electrodes due to extensive interpolation. See Table 1-3 for recommended separation distances.

As with the button sensor design, sharp corners in the slider electrodes must be rounded to minimize susceptibility to ESD. The points of the triangles forming the interpolated slider must be truncated to a rounded end with a ~2 mm diameter.