Robust Touch Controls Replace Mechanical Switches

Pauli Laitinen, Aito


There are many other markets that can benefit from touch controls.

Touch control is about more than just providing an interface to a display-oriented device such as a smartphone, tablet or computer. Certainly we have become very familiar with these applications in recent years and improvements in touch technology have ensured widespread adoption to the point where mobile phones with traditional keypads are almost non-existent, or at least limited to the most basic pre-pay models.

Outside of graphical user interfaces (GUIs) there are many other markets that can benefit from touch controls. These include consumer electronics and domestic appliances, industrial and medical equipment, automotive, and even the embryonic wearable device market. However for touch controls to provide an effective replacement for mechanical switches it is vital they are robust and reliable, and deliver a quality user experience at a competitive cost. This is where software like Aito’s enhanced piezo (SEP) touch control technology wins out in overcoming the problems of other touch technologies that either require significant button travel or will not operate with gloved hands or through metal panel overlays.

Software-enhanced piezo (SEP) touch control

Piezoelectric material, formed into discs or ‘pills’, generates an electrical output when subjected to the pressure applied by a finger or stylus. These discs, which typically measure 10-20mm in diameter and just 200μm thick, are sandwiched in a simple mechanical stack (see Figure 1) between the panel overlay and the printed circuit board (PCB), adding no more than 0.3mm to the height of the PCB.

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Figure 1. Piezo touch panels provide robust user interfaces for home appliances and industrial controls

Unlike a resistive touch switch the movement involved is minimal, less than 1µm, avoiding need for a flexible panel overlay. The overlay can even be a seamless metal surface, something that is not possible with capacitive controls that either require separate touch plates for each sensing circuit or simply would not work if shielded by a grounded metal case.

Capacitive sensing is also problematic for users wearing gloves or where liquids or other contaminants affect surfaces. Seamless overlays not only make it easier to avoid moisture ingress but offer considerable creative freedom in terms of design aesthetics, a major differentiator for high-end consumer products.

For consistent and reliable operation, piezo switches need to be compensated for variations in the composition of piezo materials and the effects of temperature change. They also need to be configured to suit the application: to set sensitivity, detection area and cope with different overlay stiffness, etc. This requires a dedicated interface to capture the analogue signal from the piezo disc, by applying a switching threshold that takes account of both the piezo characteristics and the system requirements, and then either communicating with a system’s host processor or providing suitable outputs for direct device control.

As a founding member of the Software Enhanced Piezo (SEP) consortium, Dutch company Aito BV pioneered the use of (SEP) touch controls by developing AitoChip, a touch controller IC that addresses these interface requirements and takes care of all the compensation and configuration issues. From this base, Aito has derived a family of products with functionality to suit differing end-use requirements, complemented by development tools, including evaluation kits and design software, as outlined below.

Touch button controllers with MCU and standalone interfaces

Devices like the ATB250 AitoTouch Controller provide a combination of input and output channels, in this case able to support up to 11 piezo touch buttons, each with drivers for LED indicators to provide visual feedback, along with a single buzzer output for audible feedback. The piezo sensing parameters for each key can be individually configured for sensitivity, detection area and other aspects of the mechanical integration, while the LED and buzzer feedback patterns can also be customized. For easy integration into existing hardware, the device offers SPI, I2C and DEO (Digital Encoded Output) mode interfaces for communication with a host processor.

For touch panel designs that don’t require an external host processor the ATB252 controller features a Digital Direct Output (DDO) mode that allows the ATB252 to be directly configured from an EEPROM to provide a standalone solution. It supports up to 6 piezo touch key inputs in combination with up to 11 LED indicators and a buzzer output. In DDO mode, six of the ATB252 pins become outputs that are directly mapped to each individual piezo input. These outputs signal touch button operation in one of several ways; so pressing a key can either cause the associated output to go high or low, or trigger an output pulse, or toggle from one state to the other with each key press.

Design software streamlines touch-panel development

With its UX Design Studio development tool Aito aims to streamline the creation of “user experience” (UX) designs for physical user interfaces based on SEP touch technology. This tool, when used in conjunction with Aito’s evaluation boards and an Arduino host platform, dramatically eases the adoption of touch controls by allowing the rapid evaluation of design concepts without needing to prototype early designs or have any detailed knowledge of the underlying technology. It provides a comprehensive capability for easily configuring the touch interface by adjusting the touch sensitivity and providing various tactile, audible and visual feedback signals. The software’s intuitive GUI (see Figure 2) provides a timeline view, with separate bars for each output, to show changes to design parameters in real-time.

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Figure 2. Aito’s UX Design Studio software enables the ultimate in “user experience” touch interfaces

Looking forward

Aito’s SEP technology allows the creation of seamless touch keys on any surface, replacing traditional mechanical switches and overcoming the problems of other touch technologies. Target markets are those where this capability adds value in design aesthetics and functionality to provide a quality user experience.

Applications include: kitchen appliances, such as ovens, hobs, hoods, refrigerators, coffee machines, toasters, as well as consumer audio/video and home automation products, but also medical equipment and industrial controls, where the benefits of sealed panels and robust, reliable operation are vital.