Wheel Sensor Design

A wheel sensor consists of a row of three or more sensor electrodes, which are arranged in a circle.

Note: A minimum of three electrodes is needed as position calculation requires unique crossover regions.

A wheel sensor operates in the same way as a slider sensor, with the single exception being that it is wrapped around from Channel n to Channel 0, so there are no end electrodes in the design.

As with a slider, a wheel can be made with discrete non-overlapping sensors.

Figure 1. Three-Channel Button Wheel

A larger wheel may be implemented by increasing the number of sensor keys used or by increasing the segment interpolation, as in the case of the slider.

Figure 2. Button Wheel Dimensions

Table 1. Button Wheel Dimensions
	Min.	Typical	Max.
Wheel Height (H)	8 mm	12 mm	20 mm
Electrode pitch (P)	4 mm	6 mm	8 mm
Electrode Separation (S)	0.25 mm	0.5 mm	1 mm

As with other sensors, sharp corners in the electrodes need 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.

Wheel 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 should be increased up to a maximum of 1.5 mm in the cases when there are long parallel edges between electrodes due to extensive interpolation.

Figure 3. Interpolated Wheel Dimensions

Table 2. Interpolated Wheel Dimensions
	Min.	Typical	Max.
Slider Height (H)	8 mm	12 mm	20 mm
Height Segment (HS)	4 mm	6 mm	8 mm
Electrode Pitch (P)	8 mm	16 mm	30 mm
Electrode Separation (S)	0.5 mm	1 mm	1.5 mm
Deadzone (D)	—	—	4 mm