Employ glove touch in capacitive touch UI01 Jul 2015 | Joshan Abraham, Vibheesh Bharathan
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However, implementing glove touch on capacitive touch interfaces is not easy, and most implementations tend to offer unreliable and inconsistent performance. This article focuses on the challenges in implementing glove touch on capacitive touch buttons and how these challenges can be overcome to design a robust and reliable touch-sensing interface with glove touch capability.
There are two primary challenges to implementing reliable glove touch, they are:
Detecting low signals produced by a gloved hand
Ignoring false touches from a finger hovering above the sensors
Why glove touch produces a low signal
Capacitive touch sensing works on the principle that a finger introduces a change to the capacitance of a sensor when the finger touches the overlay covering the sensor. This change in capacitance is measured and converted to the digital domain (A to D conversion) by a touch-sensing controller. When the measured value exceeds a pre-defined threshold, a touch is registered.
The change in digitized capacitance due to a finger touch is known as signal and the unintentional change in digitized capacitance without a finger touch is known as noise. A signal-to-noise ratio (SNR) of 5:1 is recommended for a reliable touch-sensing system. Figure 1 shows how capacitance is measured in a touch-sensing system.
In simple terms, capacitance introduced by a finger can be viewed as a parallel plate capacitor, where the finger and the sensor are the two conductive plates and the overlay is the dielectric medium between the plates. The finger-introduced change in capacitance is proportional to factors such as the size of the sensor and the finger (i.e. area of plates) and the dielectric constant of the overlay material; and inversely proportional to the thickness of the overlay on top of the sensor (i.e. the distance between the plates). A thicker overlay increases the distance of separation between plates, thus producing a smaller change in capacitance. This leads to a lower signal-to-noise ratio.
Figure 1: How capacitance is measured in a touch-sensing system.
Wearing a glove on a finger adds a new overlay proportional to the glove thickness on top of the existing overlay increasing the overall overlay thickness. This decreases the strength of the signal below the pre-defined threshold and a touch with a gloved hand is typically not detected. This is the reason why most users have to remove their gloves to effectively touch a button on a capacitive touch-sensing user interface.
Unwanted hover and false touches
A touch sensor can be tuned to work with thicker overlays by increasing its sensitivity. Similarly, a touch sensor can be tuned to detect a touch, even when touched by a gloved hand. Increasing the sensitivity of a sensor means that it requires a smaller change in capacitance to detect a touch.
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