Interlink Electronics Ring Sensor - Manual (Page 10)

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8
Ring Sensor
Integration Guide
Measuring the Angle of Touch
Once the zone of touch is known, the actual angular touch position can be measured. The angle
can be measured with a single measurement.
If zone 1 is being touched, then drive lines two and three will be used for the angle measurement.
Similarly for zones 2 and 3; the rule is to drive the two lines that are furthest from the touch point to
perform the angle measurement. Here are steps for measuring the angle if zone 1 is being
touched:
1. Configure driveline D1 as an input, effectively disconnecting it from the sensor.
2. Set D3 low and D2 high (so that the potential increases clockwise).
3. Measure the sense line using the ADC. Call this Vtheta.
One could improve angular resolution by measuring Vtheta several times and accumulating the
results. The calculation of the touch angle from Vtheta will be described in a later section.
Because the ADC could have a few counts of noise on it, a second measurement could be
performed to “sanity check” the results. This would be exactly the same as the measurement
described above, but with the drive lines set so that the potential increases counterclockwise. In
the zone 1 example, this means that D2 would be set low and D3 would be set high. Call the
voltage from this measurement Vtheta_inverse. One can then check that Vtheta and
Vtheta_inverse are complements, as in this code example:
if(abs(255-v_theta-v_theta_inv)<10)
sensor_active_flag=1;
If Vtheta and the complement of Vtheta_inverse are not equal to within some tolerance, then no
valid touch was found.