Years ago, touch screens on kitchen and laundry appliances were concepts shown at CES to attract attendees and demonstrate the visionary leadership of the company. Today, white goods appliances with touch screens are a reality, at least for a few high-end appliances. With the Internet of Things (IoT), the vison for the future is touch screens expanding to more and lower-cost appliances. Driven by internet connectivity and app-enabled features for users, a touch screen human machine interface (HMI) solves several operating environment issues and provides aesthetic options as well.
Using Capacitive Touch Technology in Appliances
Capacitive touch technology, mainly for touch buttons, sliders and wheels, has already been widely adopted in white goods applications to replace mechanical buttons and knobs. In addition to providing a system cost reduction, capacitive touch technology provides reliability improvements since mechanical buttons or knobs break easier over time or stick when confronted with water, grease or fat, all substances usually seen in kitchens and laundry rooms. Also, touch sensors located underneath or behind a glass or plastic surface provide an easy way of cleaning the surface and enable a variety of stylish design options. (See Figure 1.)
Figure 1. Touch screen buttons and sliders (a) are already popular on appliances, but a refrigerator (b) with a large display for interfacing to the internet provides a clean, attractive look using touch screen technology.
These same advantages of touch buttons and sliders apply to capacitive touch HMI touch screens that enable the user to communicate to the appliance as well as to the internet. In addition, IoT appliances linked to the internet provide key benefits to appliance manufacturers, including:
1. Remote maintenance
2. Data mining by the provider to know the user's patterns
3. Power management - syncing high current appliances to avoid spikes on the power line
4. Firmware updates for remote bug fixes and product enhancements
At the same time, IoT connectivity provides benefits to the user, that the manufacturer can promote to sell the touch screen enabled appliance, including:
1. Cooking instructions for certain products, recipes and best practices downloads
2. Washing machine powder and usage, programming the washing machine, even based on the type of watching powder or liquid and dealing with malfunctions
3. Firmware updates to add new features and improve performance
4. Weather, news and stock updates
However, with all of these benefits for suppliers and users, there are a few design issues that must be addressed.
Addressing White Goods Application Issues
Similar to modern smartphones and cars (but to varying degrees of significance), three of the most common white goods application issues for touch screen HMIs are noise immunity, moisture immunity and recognizing touch commands when the user wears gloves. How well a touch screen controller or IC addresses these issues differentiates one from the others.
For noise immunity, patented technology allows the controller to be more immune to noise coming over the power lines. This is especially important to regions outside of the U.S. that have extremely noisy power lines, generated because of the lack of an earth ground or poorly connected grounds. That noise conducts into the power supply through the power cable and gets routed to the touch controller IC.
The touch controller is an extremely sensitive component that measures nanocoulombs of charge. Pulling a tiny amount of charge away from the screen by simply touching the touch screen with a finger needs to be consistently interpreted properly. Noise can inject significant charge into the sensor to confuse the controller, especially one without sufficient noise immunity.
With a false touch event, or ghost touch, random buttons start pressing themselves. In an oven, this could be quite dangerous. For example, a false touch event could initiate the self-clean cycle without the user’s intent and stored items could create a safety issue and dangerous situation for the user. This is an issue for every capacitive touch controller, but patented technology circumvents conducted noise and brings the problem under control.
To solve the noise problem, the controller filters common-mode noise and avoids noise issues through a frequency hopping scheme. The patented approach utilizes self-capacitive touch as well as mutual capacitive touch scanning and involves differential touch sensing. Instead of treating each sense line as its own individual element, the IC measures the differences between pairs of sense lines; this way any noise common to both lines is eliminated. If the same noise occurs from a similar region of the display, that noise will be cancelled, so only the valid signal remains. This differential touch sensing provides a very effective level of noise cancellation/rejection.
Within the appliance, motors in washing machines or refrigerator compressors as well as burners on an inductive cooktop stove radiate noise that is within the noise cancelling frequency band to provide reliable, robust performance for touch screen displays on these appliances. As a result, false touch events are avoided. However, it is also important that a legitimate touch is detected and reported to the host controller to avoid a missed touch event from noise, where nothing happens when the user expects something to happen.
Moisture immunity is required because water and other liquids are common in the kitchen and in the laundry room. For example, if a pot on the stove boils over and its contents drip onto the touch screen, a false touch event should not occur. However, mist or droplets could also cause problems. As a result, the ability to detect touches with moisture or water is should be a key requirement of every designer working on a touch screen HMI for appliances.
With a thin mist or small droplet on the screen, multitouch operation should be supported. While two touches are typical in the appliance application, support for ten or more touches is available for appliance designers on larger displays so that multiple users can touch at the same time. If the water is pooling when the user touches the screen or if a larger drop falls onto a horizontal screen, false touches induced by water should be suppressed and normal single finger operation is supported. Designers should expect to avoid false touch events from highly conductive liquids like salt water and even cleaning solutions like bleach.
Another aspect of touch screen HMI technology is glove support. In the kitchen, thin and thick gloves are commonly worn. There are several capabilities that are not being used on touch screen ICs in appliances being shipped today that are often overlooked and not used when they could provide significant value to the end user.
When turned on and tuned during development, the optional glove support feature in the controller provides multi-touch (up to 10 touches) for gloves typically used in kitchens (about 1.5-mm thick). The use of gloves could occur when someone wearing gloves at the kitchen sink needs to interface to a refrigerator or stove touch screen.
More commonly, for cooking, thick (up to 5 mm) gloves or oven mitts often made of silicone are used, and with glove support, the controller can still provide accurate input to the HMI. This can occur automatically without the need to enter a separate mode and return to the normal sensing level when a glove is not used, so the system is not overly sensitive to avoid false touch events. In contrast, some controllers require users to select between modes like moisture, bare finger, stylus and glove rather than automatically detecting and adjusting settings on the fly for natural, intuitive user interactions in all environments.
In these applications the user interface tends to be simpler with larger buttons and needs to be taken into account in the design of the touch screen for appliances that take advantage of glove support.
Choosing the Right Touch Controller/Sensor/Screen
For kitchen and laundry room appliances, a variety of screen sizes will be used depending on the size of the appliance. For example, screen sizes range from 3-inch displays for coffee makers to 5-inch on microwave, stovetops and washing machines up to 22-inch and larger for refrigerators and freezers.
In the appliance supply chain, the chip supplier provides the chip and the design of the sensor in partnership with a sensor supplier. Together they perform the system integration and typically a module/display manufacturer integrates the system including the touch sensor and the touch interface that is then provided to the appliance manufacturer (See Figure 2). This is an example of how today’s semiconductor supplier provides more than just the chip but also a service to tune and complete the system to make the chip easier to use in the entire supply chain.
Table 1. In addition to screen size, other parameters can impact the choice of the chosen controller, especially if there is an overlap in screen sizes.
One final consideration is electromagnetic compliance (EMC). Obviously, the design must support EMC. Then, testing must verify that it achieves the desired results for both conducted and radiated emissions.
Touching on Design-in Aspects
For early engagement and improved awareness of touch screen capabilities, a designated evaluation kit is available for each of the controllers in the appliance touch screen family. The kit includes a printed circuit board (PCB) with a touch screen controller with a passive flexible printed circuit (FPC) tail 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.
Used with the maXTouch Studio Development System Integrated Development Platform (IDP), a full software development environment (a free download from the web), the eval kit enables appliance designers to develop and debug appliance touch controllers. Figure 3 shows what appliance manufacturers can expect to find in an evaluation kit.
Figure 2. In addition to the touch IC, the touch IC provider performs many functions behind the scene to bring a successful touch screen to market.
Operating in the industrial -40 to 85°C temperature range, the standard IC ships with standard firmware and is able to address a variety of display sizes and different appliance manufacturer requirements. With the right range of screen size options in the appliance touch screen controller family, appliance touch screen designs are more scalable, which ultimately shortens design time, and lowers system and development cost. Table 1 shows some of the design parameters for industrial grade appliance displays.
Figure 3. Microchip’s ATEVK-MXT2952T2 evaluation kit contains a dedicated sensor with a flex connector and an electronic control board.
Appliance manufacturers plan to take advantage of the IoT. To do this, there must be something to read and a means to input information, so a touch screen display provides an ideal solution. For a successful transition from today’s approach to advanced touch screen technology, appliance manufacturers need to work with an IC supplier, or touch screen or module supplier that works with an IC supplier, with touch controllers designed specifically for appliance applications. With the right touch controller, appliances can provide internet connectivity and have noise immunity, moisture immunity and glove support.