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Employ glove touch in capacitive touch UI

01 Jul 2015  | Joshan Abraham, Vibheesh Bharathan

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Noise: Wherever there is a signal, there is also noise. The signal is the change in capacitance that results in a meaningful change in output. Noise, on the other hand, is any disturbance that does not change the capacitance but does change the output. Noise that exceeds a threshold could potentially produce false touches. In general, a signal-to-noise ratio of 5:1 is required for a reliable touch-sensing system. This means that along with having a high sensitivity, the controller must maintain low noise. In other words, a controller tuned to 500 counts / pF sensitivity must limit the noise below 10 counts to maintain a 5:1 SNR for a touch of 100 fF.

More often than not, noise is introduced into a system through conduction effects (such as power supply switching noise or electrical fast transient (EFT) currents) or through coupling effects (such as radiated noise from a cell phone or crosstalk between signal traces). In general, capacitive sensors and the controller must be isolated from noise sources, such as a switching power supply. Careful system and PCB design are key to maintaining the isolation and avoiding noise from entering the capacitive sensing system.

Avoid "unwanted hover" and false touches
This section details design methods to reduce "unwanted hover" in touch-sensing systems that support glove touch.

Use dedicated thresholds: The amplitude of a glove touch signal is significantly smaller than that of a regular finger touch. Using dedicated thresholds, along with firmware design logic, can help to detect and differentiate between finger touch and glove touch and thereby improve the hover rejection performance.

Two dedicated thresholds can be set for finger touch and glove touch signals (FThreshold and GThreshold in figure 3). Typically, these thresholds are set at 80% of a typical finger touch or glove touch signal.

Figure 3: Two dedicated thresholds can be set for finger touch and glove touch signals.

When a user first touches the sensor, the firmware identifies if the signal is above the finger threshold or only above the glove threshold. If the signal is above the finger threshold, it assumes that the user does not have gloves on and it discards all signals below the finger threshold for a predefined amount of time (e.g. 30 sec) from the detection of the last touch. This ensures that a hovering finger is not detected as a false glove touch. The firmware decision tree is shown in figure 4.

It is reasonable to assume that it would take the user at least 30 seconds to put on a glove and re-touch the sensor. However, if the first touch produces a signal that crosses the glove threshold but not the finger threshold, the system assumes that the user has gloves on and will continue to detect glove touches. While in this mode if a user removes the glove and touches the sensor, the signal will cross the finger threshold and the system will immediately move into a mode that will only detect finger touches.

Figure 4: The firmware decision tree.

A typical touch-sensing user interface panel consists of multiple sensors. It is possible to improve the firmware decision logic of the panel such that it looks for signals on all sensors, and if a finger touch is detected on any of the sensors, all of the sensors can be made to reject glove touches for predefined amounts of time.

The primary disadvantage of this method is that if the first signal detected is that of a hovering finger, it may cause a false touch.

Use touchscreen inputs: Some products, such as mobile phones, printers or high-end home appliances, have an independently controlled touchscreen as well as touch buttons on their user interface (UI) panels as shown in figure 5. In such systems, intercommunication between the respective controllers can be helpful to efficiently manage glove and finger touches.

Figure 5: A system with touchscreen controller and capacitive touch button controllers.

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