Capacitive touch expands switch technology options in industrial and household equipment application

Touch control can enhance the high-tech appearance of smart home appliances, but when considering sensor technology, touchpad design, and control strategies, there are some considerations that should be kept in mind.

Nowadays, homes and workplaces are increasingly populated with smart home appliances. They are attractive and reliable. Ideally, they should support interaction and be easy to operate through the smart phone in the user's pocket.

As new smart home appliances such as TVs, set-top boxes, audio equipment, white goods, PC peripherals, security panels, and industrial controls are almost constantly entering the market, the design of user interfaces and the quality of user experience have a key impact on market attractiveness. , Affecting sales and revenue. Creating a good impression through the user interface is crucial, and designers can choose a variety of technologies such as traditional push-button switches or various types of touch sensors.

Switch or touch, pros and cons: Mechanical switches such as membrane or tactile switches are mature technologies that provide advantages including low cost, ease of use, and simple integration, requiring minimal electronic design or software engineering. These types of switches also provide the user with assurance that a button press has been detected by providing tactile feedback. However, some disadvantages of mechanical switches include relatively slow response time, poor reliability due to reliance on moving parts, and additional requirements for noise suppression or bounce cancellation circuits.

On the other hand, touch sensors can create a more modern impression and also simplify the structure and assembly. There is no need to install the panel opening required for the mechanical switch, which can simplify the mechanical design of the control panel. The panel can also be sealed, making it easier to prevent liquid penetration, such as waterproofing. This is a valuable feature for many types of equipment, such as kitchen appliances such as coffee machines, while enhancing reliability and ease of cleaning.

Among the touch sensing technologies used today, including capacitive, resistive or piezoelectric sensing, capacitive sensors are widely used in products in today's market. Capacitive touch sensors can be easily placed on glass or plastic, using processes such as screen printing to create various shapes such as buttons, sliders or rotary selectors. The backlight is also easy to arrange. In the absence of any mechanical response confirming the actuation provided by the tactile switch, the designer can use light to confirm the detection of the touch. This can be used to create stylish effects, such as keeping the panel dark until touched, using color changing effects or high-tech icons. A deep beep or jingle can also be introduced to make the touch control more attractive. Keep all these effects with a sophisticated look and feel that a touch-sensitive user interface can create.

Sensing principle: Generally speaking, capacitive sensing works by detecting changes in the basic capacitance of the sensor pad when the user's fingertip approaches. In self-sensing detection, a close finger effectively increases the capacitance of the sensor. The change increases the time constant of the touch panel circuit, which is detected by the charging time measurement circuit used in the touch sensor controller.

Mutual differential sensing is an alternative technology that detects the decrease in capacitance between the sensor pad and a separate excitation pad when the user's fingertip is close to the touchpad. The change in capacitance (ΔC) is detected and converted to voltage (Vout). Figure 1 shows the differential amplifier circuit used to generate Vout. This has a linear relationship with ΔC, compared to the threshold voltage (Vt). When Vout exceeds Vt, the touch is detected.

Cross-differential sensing provides an advantage, providing a large dynamic range through increased sensitivity. Practical benefits include expanded flexibility in sensor pattern design, and increased tolerance for sensor long tracking lengths due to better elimination of parasitic capacitance (Cp) between the pad and ground. In practice, the track length has been successfully verified up to 500 mm.

Implement a touch controller: The touch sensor must have an electronic controller with enough channels to support the total number of touch buttons required. In the past, the controller has been implemented as an independent application-specific IC or integrated into a microcontroller. Microcontroller-based solutions can use analog peripherals and software in combination, while other solutions provide dedicated on-chip touch control functions, such as a charging time measurement unit (CTMU) for self-sensing detectors. Microcontroller vendors can provide free capacitive touch software IP, which can help simplify code development.

Implementing an independent controller brings many advantages. ICs such as ON Semiconductor’s LC717A00 do not require a host controller and have some built-in features, including noise cancellation and water detection to prevent spurious responses. In addition, a patented automatic noise environment change compensation technology is built-in to prevent noise-related failures and compensation effects that can change sensitivity, such as humidity changes or dirt accumulation in the sensor area.

The built-in circuit of the controller has a unique C/V conversion amplifier, which converts the monitored differential capacitance into an output voltage. No programming is required, which can significantly shorten development time and no need to add external components, thus helping to minimize bill of materials costs. Figure 2 shows that the LC717A00 adopts an independent mode and controls up to 8 capacitive touch sensors. The device has a communication port, I2C or SPI, and can be easily connected to the host controller as needed.

The controller adopts the differential signal method to provide high sensitivity and expand the freedom for designers to create various shapes of control panels, and provide long connection lengths as needed. Figure 3 shows an example touchpad design with a very long track length. The data rate can be configured to exceed 200 Hz, which supports the use of devices that require fast touch response.

The high signal-to-noise ratio of the controller supports extremely reliable touch sensing, even if the user is wearing gloves. This is usually not possible with self-inductive detectors. In addition, the design regularly controls the shape and size of the sensor electrode, and the air gap effect and the thickness and material of the cover layer can be relaxed. Figure 4 shows an example of signal and noise levels using real sensor data. In this example, the signal is a difference of 17,000 and the noise is a difference of 160, so the signal-to-noise ratio (SNR) is 106:1.

As a user interface technology, capacitive touch has many advantages for modern smart home appliances. Mutual differential sensing ensures reliable touch detection and supports greater design freedom when laying out control panels and selecting materials. The independent application-specific touch control IC supports convenient and fast implementation. Based on this mutual differential sensing technology, it can provide advantages such as no software design, saving external components, and speeding up project completion.