TI technical article: How to add capacitive touch capabilities to your stove

Under the market call of "combining design and function", capacitive touch technology has been rapidly applied in the field of electronic devices. Imagine that there is an "invisible" touch interface on your stove, and when you get close, the backlight will automatically turn on and the touch button will appear in front of you.

Many cooktops are moving away from traditional knobs and buttons in favor of capacitive touch, enabling modern designs and addressing functionality concerns including reliability, usability, environmental impact, design freedom, conformity to complex appliance shapes, manufacturability, price, and ease of use.

Adding capacitive touch functionality to a cooktop presents a unique set of challenges. First, the touch keys are located on a flat surface very close to the work area, and the normal operating conditions for the touch keys include:

● Requires cleaning.

● Kitchen items may rest on the sensor.

● Liquid splashing may occur, and even the entire surface may be wetted by liquid.

● Users are required to wear gloves when operating.

● The buttons are located under a thick glass overlay (>4 mm).

TI’s CapTIvate™ technology provides the tools and methods to address these issues and enables stove systems to easily and flexibly adapt to new designs (and their associated challenges).

CapTIvate technology is integrated as a peripheral module in some MSP430™ microcontrollers (MCUs). CapTIvate microcontrollers (MCUs) enable automatic capacitive touch detection without the need for a large central processing unit. After detecting proximity or valid touch, the CPU runs the corresponding application and performs tasks such as recalibration and controlling tactile, auditory or visual feedback. This makes the modular touch concept a reality that can be reused in different versions of the application or in new designs independent of the host controller.

CapTIvate technology provides the flexibility to easily change configuration and functionality through software, minimizing time to market. The technology supports the design of buttons, sliders, wheels, and proximity sensors, as well as the use of self-capacitance and mutual-capacitance touch technologies in the same design.

For stoves, it is very important to avoid false touch operations caused by liquids, objects or noise. CapTIvate technology provides hardware and software solutions to solve these problems. In normal operation, users can only touch one or two buttons at a time according to the prescribed process. In any other case, such as touching multiple buttons at the same time during cleaning, or when someone leans on the touch area, the software will detect these unprescribed touch actions and prevent the system from false triggering.

To protect against moisture, CapTIvate technology also uses a guard channel. The guard channel is an independent electrode with higher sensitivity that surrounds all buttons on the printed circuit board (PCB). This guard can detect any unspecified touch operations, and the software can also use it to prevent false triggers.

In addition to simplifying the production process, it is also important to keep the assembly tolerance between the PCB and the glass surface small and stable. In the stove, metal springs or conductive fillers can fill the air gap between the PCB and the cover. Figures 1 and 2 show the use of metal springs to lift the sensing electrode from the PCB to the cover material.

Figure 1: Example of a typical stack used in a stove

Figure 2: Example of a touch interface with a 4 mm transparent glass surface and metal springs

In addition, electrical noise robustness is also a key requirement for capacitive touch applications in stoves. According to the IEC61000-4-3 and IEC61000-4-6 standards, the MSP430FR2675 and MSP430FR2676 microcontrollers can simplify the electromagnetic compatibility certification process.

Learn how CapTIvate technology can transform your human machine interface applications.