Five years ago, the provision of touch screens for kitchen appliances and laundry appliances was just a concept presented at CES to attract attendees and showcase the company's vision. Today, at least for a few high-end appliances, white goods with touch screens have become a reality. With the rise of the Internet of Things (IoT), the future trend is to gradually integrate touch screens into more low-cost appliances. For users, the touch screen Human Machine Interface (HMI) can solve a variety of work environment problems while ensuring beautiful appearance, driven by the functions of Internet connection and supporting applications.


Use capacitive touch technology in appliances
Capacitive touch technology is mainly used for touch buttons, sliders and rollers. As a replacement for mechanical buttons and knobs, it is widely used in various white goods. In addition to reducing system cost, capacitive touch technology improves reliability because mechanical buttons or knobs are more susceptible to damage over time and can stick to the water and grease commonly found in kitchens and laundry rooms. However, the touch sensor surface is covered with glass or plastic, which is easy to clean and supports a variety of stylish designs. (see picture 1)




Figure 1 Touch screen buttons and sliders (a) have been widely used in appliances; refrigerators with large displays that can be connected to the Internet (b) provide a clean and attractive look using touch screen technology.


These common advantages of touch buttons and sliders apply to capacitive touch HMI touch screens, enabling users to communicate with appliances and the Internet. In addition, Internet of Things appliances connected to the Internet bring key advantages to appliance manufacturers, including:
Remote maintenance
Suppliers use data mining to understand user patterns
Power Management - Synchronous high current appliances to avoid spikes in the power line
Firmware update for remote defect fixes and product improvements
 
At the same time, IoT connectivity brings many advantages to users, helping manufacturers to promote and sell home appliances that support touch screens, including:
Cooking instructions, recipes and best practices for some products
Washing powder and dosage, set the washing machine (even according to the type of washing powder or laundry detergent) and handle the malfunction
Firmware update for adding new features and improving performance
Weather, news and stock updates


However, in order to provide these advantages to suppliers and users, some design issues must be resolved.
 
Solve white goods application problems
Similar to modern smartphones and cars (but to varying degrees of importance), the three most common problems with touchscreen HMIs in white goods applications are noise immunity, moisture resistance, and the ability to recognize touch commands when a user wears gloves. The quality of the touch screen controller or IC to solve these problems varies from device to device.


In terms of noise immunity, our patented technology helps controllers improve their ability to withstand power line noise. This is especially important in areas outside the United States where power line noise is extremely severe due to ungrounded or poorly grounded. Noise is conducted to the power supply through the power line and then to the touch controller IC.


The touch controller is an extremely sensitive component that measures the charge at the N Coulomb level. Simply touch the touch screen with your finger and you can take a small amount of charge from the screen, which requires a consistent and correct interpretation. Noise can inject a lot of charge into the sensor, disturbing the controller, especially controllers that don't have enough noise immunity.


When a false touch event or ghost touch occurs, the button may be pressed by itself at random. This can be very dangerous for the oven. For example, a false touch event may initiate a self-cleaning process if the user does not need it, and the items stored therein may pose a safety problem to the user, posing a hazard. This is a problem with every capacitive touch controller, but our patented technology avoids conduction noise and controls problems.


To solve the noise problem, the controller filters out common mode noise and avoids noise problems through a frequency hopping scheme. This patented method utilizes self-capacitive touch and mutual capacitance touch scanning and involves differential touch sensing. Instead of treating each sensing line as its own independent element, the IC measures the difference between pairs of sensing lines; this eliminates the noise common to both lines. If the same noise occurs in a similar area of the display, the noise will be cancelled and only valid signals will be retained. This differential touch sensing provides very effective noise cancellation/suppression capabilities.


Inside the appliance, the noise radiated by the burner motor, the refrigerator compressor, and the burner on the induction cooker is within the noise cancellation band, which provides reliable, robust performance for the touch screen on these appliances. Therefore, this avoids accidental touch events. However, it is equally important to detect legitimate touches and report to the host controller to avoid missed touch events due to noise, otherwise there will be situations where the user expects an event to occur but does not actually occur.


Since water and other liquids are very common in kitchens and laundry rooms, moisture protection is required. For example, when the liquid in the pot on the stove splashes onto the touch screen due to boiling, no false touch event should occur. However, fog or droplets can also cause problems. Therefore, being able to detect touches in the presence of moisture or water should be a key requirement for each designer to study the touch screen HMI of an appliance.


If there are mists or small drops on the screen, multi-touch operation should be supported. Electrical applications typically support two-touch, but electrical designers can provide support for ten or more touches on a large display so that multiple users can simultaneously touch. If the water is brought together when the user touches the screen or a large water drop falls on the horizontal screen, the false touch caused by the water should be suppressed and the normal single-finger operation is supported. Designers should avoid the use of high-conductivity liquids such as salt water and even touch-sensitive cleaning solutions such as bleach. Figure 2 outlines the situation behind the display panel.


figure 2


The touch IC behind the touch screen of FIG. 2 has a series of driving electrodes, receiving electrodes and corresponding circuits, which can accurately and reliably detect the user's touch.


Another advantage of touch screen HMI technology is support for gloved operation. In the kitchen, you usually wear thin or thick gloves. Among the appliances currently delivered, several functions are not used for touch screen ICs, which may provide significant value to end users, but are often overlooked and not adopted.


When opened and adjusted during development, the optional glove-enabled operation in the controller provides multi-touch (up to 10 touches) for gloves commonly used in the kitchen (approximately 1.5 mm thick). If the user needs to connect to the refrigerator or stove touch screen while wearing gloves standing next to the kitchen sink, glove support may be used.


More commonly, thick (up to 5 mm) gloves or oven gloves, usually made of silicone, are used for cooking, and the controller can still provide accurate input to the HMI due to the glove-resistant operation. This can happen automatically without going into a separate mode and can return to normal sensing levels when no gloves are used, so the system is not overly sensitive, avoiding false touch events. In contrast, some controllers require the user to choose between wet, bare fingers, stylus, and gloves, and cannot automatically detect and adjust settings in real time to perform natural and intuitive user interaction in all environments.


In these applications, the user interface is often simpler when using larger buttons, and support gloves should also be considered when designing the touch screen of the appliance.
 
Choose the right touch controller / sensor / screen
For kitchen appliances and laundry appliances, various screen sizes will be used depending on the size of the appliance. For example, the coffee machine display is 3 inches, the display for microwave ovens, stoves and washing machines is 5 inches, and the display for refrigerators and freezers is 22 inches or larger.


In the appliance supply chain, chip suppliers work with sensor suppliers to provide chip and sensor designs. Together they complete system integration: Typically, module/display manufacturers integrate systems that include touch sensors and touch interfaces and then provide them to appliance manufacturers (see Figure 3). This example illustrates how today's semiconductor suppliers can provide chips and provide services that adapt and improve the system, making the chip easier to use throughout the supply chain.


image 3


In addition to the touch IC, the touch IC provider additionally performs a number of functions to successfully bring the touch screen to market.


Standard ICs operate over the industrial temperature range of -40 to 85 ° C and come standard with firmware to meet a wide range of display sizes and different appliance manufacturers' requirements. The family of touchscreen controllers offers a variety of appropriate screen size options to increase the scalability of electrical touchscreen designs, ultimately reducing design time and system and development costs.


In addition to the screen size in the table, other parameters may also affect the selection of the selected controller, especially if the screen sizes overlap.


The last consideration is electromagnetic compatibility (EMC). Obviously, the design must support EMC. Subsequently, tests must be performed to verify that the design achieves the desired conducted and radiated emissions results.
 
Touch all aspects of design
In order to understand and improve the understanding of touch screen functions as early as possible, a designated evaluation kit is provided for each controller in the electrical touch screen series. The kit includes a printed circuit board (PCB) with a touch screen controller and a rear end of a passive flexible printed circuit (FPC) that connects the touch IC to a touch sensor on a glass/plastic lens. The kit connects to the host PC via USB and includes all necessary cables, software and documentation.


The evaluation kit is used with a complete software development environment (available for free download from the web), the maXTouch Studio Development System Integrated Development Platform (IDP), which enables electrical designers to develop and debug electrical touch controllers. Figure 4 shows what the appliance manufacturer can find in the evaluation kit.




Figure 4
Figure 4 Microchip's ATEVK-MXT2952T2 evaluation kit includes a dedicated sensor with a flexible connector and an electronic control board.
 
Touch summary
Appliance manufacturers plan to take advantage of the capabilities of the Internet of Things. To do this, you must provide a way to read and enter information, and a touch screen is the ideal solution. In order to successfully transition from today's methods to advanced touch screen technology, appliance manufacturers need to work with IC suppliers, or with touch screen or module suppliers working with IC suppliers, to design touch controllers specifically for electrical applications. Using a suitable touch controller, the appliance provides an internet connection and provides noise immunity, moisture resistance, and glove operation.