Overcoming The Challenges Of Capacitive Touch HMI Design

The capacitive touch human-machine interface (HMI) is a critical aspect of modern electronics. It is how we interact with our devices, making it an integral part of wearables, smart home products and IoT products. The primary goal of an HMI is to provide an intuitive and responsive interface that supports human interaction. However, designing a touch HMI system that meets the consumer’s expectations can be challenging. In this blog series we will discuss these challenges and go more in-depth on how PSoC 4000T is addressing them.

The power and size challenge

One of the most significant challenges of touch HMI design is ensuring that the system is energy-efficient without compromising on performance. Traditional HMI designs consume a significant amount of power, which is a major issue for portable and wearable devices. A power-hungry HMI system can significantly impact the battery life of these devices, making it difficult for end-users to use them for long periods. This issue has become even more pressing with the rise of wearable devices, which require long-lasting batteries to function correctly.

Another challenge of touch HMI design is the size of the system. As technology advances, end-users expect smaller and more compact devices. Traditional HMI designs are bulky and consume a lot of real estate. The size constraint is a challenge, especially for wearable devices that require a compact design.

Power consumption of HMI systems

A traditional touch HMI system consumes substantial amounts of power due to architectural limitations. To understand why this is so, it’s worth studying the operation of a touch sensor.

Most touch controllers have a microcontroller-like architecture, consisting of a CPU, non-volatile and volatile memory resources, an analog front-end, digital logic functions, and I/Os. Scanning operations require the operation of the CPU to initialize the touch-sensing system, configure the sensing element, scan the sensors, and process the results to determine whether a touch event has occurred. The CPU and its associated memory resources are heavy power consumers, making scanning operations consume high power in both the touch detect and active modes.

Fig. 1: Reducing the refresh rate in touch detect mode reduces average power consumption.

Therefore, power saved greatly depends on how slow the sensors can be refreshed and reducing the refresh rate reduces average power consumption. A common solution to power consumption problem is to reduce refresh rate (slow the HMI sub-system) for lower power, which is not an ideal solution. So it’s always a tradeoff between power consumption and quick response to user touches.

Infineon’s solution for HMI design

To address all these challenges in HMI design, Infineon has developed the PSoC 4000T touch-sensing controller, a solution that offers improved touch sensor functionality, power efficiency, and compact design.

Fig. 2: Block diagram of the PSoC 4000 series touch controllers from Infineon.

The PSoC 4000T is based on the 5th generation of CAPSENSE capacitive sensing technology, which enables designers to reduce the touch sensor’s size while increasing its power efficiency. The controller reduces power consumption by up to 10 times compared to previous generations, thanks to ultra-low power always on autonomous sensing and multi-sense sensing technology, which allow the circuitry to be autonomous. As a result, the CPU can stay dormant, leading to much lower power consumption without compromising the sensor’s performance or reducing the refresh rate.

The PSoC 4000T uses a ratio-matric architecture with differential sensing frontend, which provides enhanced signal-to-noise ratio by 10x compared to previous generation. Thanks to the improved SNR, designers can now shrink the sensor size to optimize the product form factor without losing on SNR performance. The improvement in the SNR also enables improved reliability for touch sensing, enables new intuitive user interfaces such as proximity gesture detection, air gesture detectivity and proximity sensing and many more.

The PSoC 4000T also supports multi-sense capability, which means same touch controller can perform several sensing functions, including touch buttons and sliders, force, proximity, and touchscreen sensing and several sensing methods including self-capacitance, mutual-capacitance, inductive sensing and touch over metallic surface. Additionally, the touch technology improvements result in liquid tolerance, making the HMI performance superior in wet conditions.

In conclusion, the challenges of HMI design present a significant obstacle for devices’ development, especially in wearables. The PSoC 4000T touch-sensing controller offers solutions to these challenges by enabling designers to achieve substantial improvements in touch system functionality, reducing device size and weight, and maintaining high performance in the presence of liquids.