QTouch® Sensor Parameters

The PTC QTouch® library provides several sensor specific parameters which can be used to tune each sensor individually. The parameters that are common for each button, slider, and wheel include the following:

  • Detect Threshold
  • Detect Hysteresis
  • Adjacent Key Suppression (AKS®)

Sliders and wheels have two additional parameters for tuning of reported position values:

  • Position Resolution
  • Position Hysteresis (mutual capacitance technology only)

Detect Threshold

The Detect Threshold parameter of a sensor defines how much its Signal must increase above its Reference level to be qualified as a potential touch event. Larger threshold values will desensitize sensors since the signal must change more (i.e. requires larger touch) to exceed the threshold level. Conversely, lower threshold values will make sensors more sensitive.

Threshold setting depends on the amount of signal swing when a sensor is touched. Usually, thicker front panels or smaller electrodes have a smaller signal swing on touch, thus requiring lower threshold levels. The amount of air from the sensor up to the touch panel will also have an effect on the strength of the touch signal.

It is possible to use different types of materials between the touch surface and the panel with a better dielectric constant than air to get a better signal. It is also not uncommon to use various types of springs in order to get the sensor surface closer to the panel.

Detect Hysterisis

This setting is the sensor detection hysteresis value and is expressed as a percentage of the sensor detection threshold setting. When a sensor goes into detect, its threshold level is reduced (by the hysteresis value), to avoid the sensor dithering in and out of the detect state if the signal level is close to the original threshold level. There are four different default hysteresis levels to choose from:

  • Setting 0 = 50% of detect threshold value (HYST_50)
  • Setting 1 = 25% of detect threshold value (HYST_25)
  • Setting 2 = 12.5% of detect threshold value (HYST_12_5)
  • Setting 3 = 6.25% of detect threshold value (HYST_6_25)

Detect Hysteresis only has an effect when a touch delta is varying around the threshold and the threshold has been exceeded.

Hysteresis.png

Adjacent Key Suppression (AKS®)

In designs where the sensors are placed close together or configured for high sensitivity, multiple sensors might report a detection simultaneously. To allow applications to determine the intended single touch, the QTouch library makes it possible to configure multiple sensors in an AKS® group. When a group of sensors is in the same AKS group, only the first strongest sensor will report detection. The sensor continues to report detection until its delta falls below its detection threshold. As long as this sensor is in detect, no other sensor in the same AKS group will report detection even if another sensor's delta becomes stronger in the same AKS group.

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