IoT Building Block: Touch Interface HMI

Many IoT devices have started to include touch screen interfaces as part of their UI to make the devices more intuitive and easier to use. A great interface can make customers really love and want to use your IoT product, but a poor HMI interface makes the device unusable damaging the brand value.

Given this impact a touch interface can have on your IoT device, it made me wonder what are the traits that make a touch interface great to use. From consulting with experts at Infineon, I learned that a touch interface that is reliable, responsive, and has low power consumption are some of the traits that make for a great interface for any device.

Reliability

For a touch interface to be considered reliable, it must consistently and accurately detect when a user’s finger has touched the screen. To achieve this result a touch interface has a capacitive touch sensor that detects changes in the electric field and capacitance caused by a user’s finger touch. The challenge is that this operating principle makes the sensor detect not only human touches, but also any electrically conductive objects which influence the sensor in the form of noise. Noise can come from electromagnetic sources or physical objects like insects, animals, or liquids if the touch interface gets wet. If the noise is severe enough it can cause the interface to behave erratically with false triggers and even stop working.

Noise from electromagnetic sources can come from:

• Radiated energy from electronics like smart phones and Wi-Fi routers
• Line power where noise from heavy home appliances is conducted through the device wall power cord
• Electrostatic discharge that is physically coupled into the sensor through the product during the user touch event.

To mitigate noise from these sources requires a system level approach include laying out the board to minimize the impact of electromagnetic sources. Also selecting a touch controller inherently immune to common noise sources can significantly simplify the design process. Failing to account for noise at the system level can lead to complex, time-consuming troubleshooting that will often require multiple rounds of changes to the board hardware.

To deal with interference from liquids, the touch interface requires liquid tolerance. Liquid tolerance is the measure of a touch interface’s ability to work reliably when subjected to liquids. When liquids are near the capacitive sensor, they can cause the touch interface to become unusable due to false positives on the touch interface. Applications like kitchen appliances, smart watches, and wireless earbuds require some level of liquid tolerance as they often need to operate in the presence water and sweat.

Responsiveness

Responsiveness for a touch interface can be measured by how quickly a user action is detected. When it comes to responsiveness, the important design factors for a touch interface include the refresh rate, low power optimization, capacitive sensor design, and wake up time.

The refresh rate determines how quickly the display will update and check for new user touches. A device will usually need to update or refresh every second so that a user does not perceive it as slow. Higher refresh rates (40Hz to 120Hz) allow quicker responses to user actions and can create a better, more fluid user experience. For batter powered devices a lower refresh rates help designers balance responsiveness and power optimization.

Capacitive sensor design also affects the responsiveness of the interface. One cause of slow responsiveness is poor capacitive sensor design where the change in sensor capacitance caused by a finger touch is not large enough to be easily detected. A finger touch on the user interface will change the capacitance of the sensor directly proportional to the overlap between the user’s finger and the sensor (hundreds of attofarads to several picofarads). The larger the change in sensor capacitance the faster the touch controller will detect the user touch.

Smaller touch sensors on devices like earbuds will produce a smaller change in sensor capacitance from a user’s touch due to the smaller the overlapping area between the sensor and the user’s finger. For earbuds the touch sensor often needs to fit into an area 2mm to 4mm in size. This level of capacitance change is sometimes too small to be reliably distinguished from noise sources. For these smaller touch sensors, it is important to use a high-performance controller that can detect capacitance changes of a few hundred attofarads.

Another element of responsiveness is how quickly the device can respond to a user touch after the device has been inactive for a period. To keep the touch interface responsive, it will need to quickly wake up when a user touch is detected. One way to speed of the wake-up touch detection is to design the sensor so that it can easily determine the capacitance changes caused by a user touch relative to the parasitic capacitance. Parasitic capacitance is the base level capacitance of the sensor that is always present when the interface is turn on. Minimizing the sensor parasitic capacitance while increasing the touch capacitance level will increase the responsiveness of the interface.

Low power

Touch interfaces need to be low power since it is a subsystem that remains active to checks for user touches. When the touch interface is actively checking for user touches it can consume up to milliwatts of power. This amount of power consumption can be small compared to other subsystems, but the requirement to always be active makes low power consumption a key feature of any touch interface. This power savings is important for devices such as wearables or earbuds that must make the most of the limited battery capacity.

To minimize the power consumption of touch interfaces you can use techniques like keeping the interface in a low power sleep mode for as long as possible and using ganged or proximity sensing to quickly wake up the interface in response to user touches. When the touch interface does not detect any user touches it can be placed into a low power deep sleep state that can lower the power consumption to just a few microwatts.

Techniques like ganged and proximity sensing allow a touch interface to spend less time in active mode by reducing the number of sensors inputs that need to be checked for a wake-up event. Ganged sensing is when all of the physical sensors which can wake up the system from standby mode are connected together to form a single virtual “ganged sensor”. The capacitive controller remains in sleep mode longer since it only needs to scan the ganged sensor rather than each sensor individually, which reduces the time the device is active and consuming power. Proximity sensing is like a ganged sensor but involves using a capacitive proximity sensor instead of the virtual ganged sensor. This capacitive proximity sensor can detect the presence of a hand when it is near the sensor without it touching the sensor.

Get started

With all these techniques required to make a touch interface reliable, responsive, and low power it can be difficult to know where or how to get started amongst the long list of technical challenges that must be overcome to build any IoT device.

To make it easier to get started on your IoT design Infineon has created a CAPSENSE Design Guide and Making IoT Easy Guide. The guides offer system level solutions to make IoT designs easier and faster than ever before. In the guides you will find practical design tips for designing a touch interface along with solutions for consumer and industrial controls. All the solutions in the guides will speed up your IoT product development journey and help you get to market faster.