Chaouki Rouaissia, Senior Applications Engineer of consumer analogue products at Semtech explores improving the touchscreen experience with proximity detection with standard four-wire analogue resistive touchscreens

Today, touchscreens have become widespread and are progressively replacing mechanical buttons in applications where an interactive, easy-to-use interface is desirable.

While it is becoming increasingly natural for people to interact with a system via touchscreen, there are still considerable advancements that can be made to make touch interfaces even more intuitive and comfortable. With that in mind, there are now solutions being developed such as Semtech’s ultra low power multi-touch 4/5-wire resistive touchscreen controllers with integrated proximity sensing and haptics for an enhanced user experience.

Proximity detection allows the system to know when the user wants (or doesn’t want) to interact with the screen. This information can then be used to control LCD backlight intensity, system wake-up, and button/menu visibility thereby improving display legibility, ease-of-use, and comfort.

The proprietary touch-sensing circuitry is also capable of supporting various common multi-touch gestures such as pinch, stretch and rotation on any four-wire analogue resistive touchscreen, giving manufacturers the opportunity to upgrade their existing platforms with the popular zoom-in/zoom-out gesture in picture viewer, internet browsing, and gaming.

Haptics feedback can be used together with audio (i.e. audible feedback via buzzer) to enhance the touch acknowledgement or to simulate mechanical button tactile feedback.

Capacitive sensing is the art of measuring a small variation of capacitance in a noisy environment. When designed properly, it can be used to perform not only touch sensing but also proximity sensing.

Self-capacitance sensing

When no conductive object (finger/ palm/face, etc.) is present, the sensor only sees an inherent capacitance value CEnv created by its electrical field’s interaction with the environment, in particular with ground areas.

When a conductive object (finger/palm/face, etc.) approaches, the electrical field around the sensor will be modified and the total capacitance seen by the sensor increases by the user capacitance CUser.

The challenge of capacitive sensing is to detect this relatively small variation of CSensor (CUser usually contributes only a few percents) and differentiate it from environmental noise (CEnv also slowly varies together with environmental characteristics such as temperature, etc.).

For this purpose, the controller integrates an auto offset compensation mechanism, which dynamically monitors and removes CEnv to extract and process CUser only.

When performing proximity sensing the dielectric relative permittivity is roughly equal to that of the air as the overlay is relatively thin compared to the detection distance targeted.

It is clear that the most robust and efficient design will be the one that minimises CEnv value and variations while improving CUser.

A four-wire resistive touch screen consists in two (resistive) conductive sheets separated by an insulator when not pressed. Each sheet is connected through two electrodes at the border of the sheet. When pressure is applied on the top sheet, a connection with the lower sheet is established.

All four electrodes are connected to the touchscreen controller, which performs an ADC-based measurement to determine touch location. Capacitive sensing is performed using nothing more than what is already there for the standard resistive operation.

For obvious cost and power consumption optimisation reasons the touchscreen controller embeds an ADC, which automatically switches between resistive and capacitive measurements depending on the detected touchscreen status.

Proximity detection feature

Proximity detection can also be used to bring up a special/hidden menu when the user’s hand approaches the panel for a more comprehensive list.

Typical applications include navigation and video players where the screen’s area can be used more efficiently (no permanent ‘Menu’ button needed anymore) and the interaction from the user is reduced (no first touch required anymore to open the ‘Menu’).

Of course proximity sensing using the touchscreen is not limited to these two usages. Its innovative technology can open paths to new concept ideas only limited by designers’ creativity.

Capacitive proximity sensing using the touchscreen can significantly contribute in making touch interfaces even more intuitive and comfortable. 

By enabling built-in proximity sensing using any resistive panel,  adds a previously, particularly expensive high-end feature at the fraction of the cost of traditional IR proximity sensing solutions.

Proximity, multi-touch and haptics feedback enable OEMs to upgrade a generally lower cost resistive touchscreen panel with high-end features.