Screens for Large Appliances have a New Touch

Thanks to the growth of the Internet of Things (IoT) touchscreens are finding themselves in more and more places – including our home appliances. Touchscreens have been a part of high-end appliances for a while, but the future is seeing them become a part of more and lower-cost appliances.

Driven by internet-connected applications, features, and services, a touchscreen human machine interface (HMI) solves several operating environment issues and provides aesthetic options as well. Ongoing advances have made touchscreens even more practical and addressed performance and cost issues to extend their use into more mass-market, cost-sensitive appliances. One particular new development, differential mutual (DM) technology, is key to expanding the use cases of touchscreens in appliances all over the home. The added value of DM to a touch sensor more than offsets the increased cost at many points in the appliance’s life from assembly to customer usage.

A Touchy-feely Relationship to Appliances

Let's look at a techie couple to explain what's happening with touchscreen appliances today. We'll call them Pat and Leslie:

Pat and Leslie are a couple that embrace new technology to simplify and improve their lives. They often enjoy spending quality time together in their home, especially in the kitchen and, surprisingly, even the laundry room.

Pat and Leslie both wear silicone cooking gloves or thick dishwashing gloves that have bristles. These gloves give you the dexterity to grab pots, ingredients, and utensils, and even interface with a touchscreen. The only problem is that the gloves are so thick the touchscreen can't detect their fingers.

As a second example, imagine the situation where Pat and Leslie are working in the kitchen preparing for invited guests. One of them drops a pan or heavy pot on the screen's cover glass and it cracks. With today’s designs, this could mean that they have to shut down the stove and disappoint their guests.

A third example involves cooking pasta. While handling the pasta strainer, Leslie drips the salty water onto the touchscreen causing a false touch event where the stove turns back on - even after Pat had previously turned it off. This is a potentially dangerous water immunity issue that could result in a burn with today’s touchscreens.

These are just a few examples of what can change with recently-announced touchscreen technology. However, the right touchscreen technology can address these common or soon to be common use case problems as well as manufacturing and service issues.

A Touchy Subject – New Technology to the Rescue

A touchscreen system is comprised of an array of drive electrodes, receive electrodes, and the circuitry in the touch IC to faithfully detect a user’s touch. Measuring nanocoulombs of charge, the touch IC controller is an extremely sensitive component. Simply touching the touchscreen with a finger changes the incumbent charge of the touch sensors of the screen by a tiny amount that needs to be consistently interpreted properly. Noise can inject significant charge into the sensor to confuse the controller, especially one without sufficient noise immunity.

Similar to the audio noise cancellation that occurs in noise-canceling headphones, a patented approach called differential mutual (DM) technology or DM noise cancellation allows the application of very high gain without amplifying the electrical noise in touch controllers.

In contrast, in the single-ended touch sensing that all regular touch controllers use today, gain applied to common-mode noise increases both the signal and the noise, so the signal to noise ratio (SNR) stays the same at best and in some cases reduces it – especially if the gain saturates the touch controller’s analog front end. In DM, the sense lines are treated in pairs to provide differential signaling, which is used in many communication areas such as Ethernet, USB, HDMI or anywhere high-speed data is sent over cables especially for long distances.

Common mode noise is injected onto both pairs carrying the signal as well as the negative of the signal, it affects both wires the same way. With DM, the signal of the two pairs is subtracted and since the noise is identical on both wires, it cancels, leaving just the desired signal. Since differential signaling removes the common-mode noise, very high gain can be applied to amplify the desired signal without increasing the noise. The increased gain allows the touchscreen sensor to detect valid signals through thick gloves, thick cover lenses, and even airgaps above the touch sensor.

The Advantages of Differential Mutual Technology

DM technology allows the use of bare fingers or gloves through very thick, protective cover lenses. Figure 1 shows the stack up of the glass, a cover lens between the touch sensor, and the finger – the surface that is touched. Historically, the cover lens has had a limited thickness, being made of around 4-mm glass or 2.2 mm of plastic. With DM, a much thicker cover lens can be used. Now, lenses up to 10-mm glass or 5-mm plastic can be supported with excellent performance.

This is quite important for several reasons.

First, cook tops currently are 3 to 4 mm of glass for an inductive cook top and they are quite large – up to 42 inches diagonally – and quite heavy. Sensing touch through such a thick lens consistently is very difficult, especially with the added noise from the inductive burners.

DM technology provides additional performance margin to sense touch accurately and precisely even through very thick lenses. Historically, controllers could work with bare fingers but they would struggle, especially in the presence of noise, to provide reasonable performance. The additional performance margin of DM allows the support of thick gloves on top of an inductive cooktop as discussed earlier.

Not only can thick gloves be used with a single finger (which was possible with some advanced touch controllers in the past), but now thick glove multi-finger operation is possible thanks to DM. As a result, Pat and Leslie can use multi-finger gestures like pinch, zoom, and rotate. These are convenient actions while searching through a recipe containing small images and text. It also provides better watersplash immunity, as well as improved noise immunity because the signal levels are so much higher that they can be sensed through the thick material.

Meanwhile in the laundry room, laundry machines tend to have plastic cover lenses as shown in Figure 2a because curved surfaces enhance the aesthetic appeal. The rounded front panels differ considerably from the flat boxy shape of kitchen appliances, where glass is more common. The plastic lenses have been limited to 2 to 3 mm using materials like poly methyl methacrylate (PMMA) or polyethylene terephthalate (PET) between the touch sensor and the finger.

The thickness limitation was determined by what the previous generation of touch controllers could sense. With DM, industrial designers have more flexibility to use an even thicker material if it is desired. Now, the thickness can be extended up to 5 mm to provide more rounded shapes and the use of different materials. See Figure 2b.

Another aspect of DM that impacts the industrial design of appliances is the ability to add an air gap to the display. Adding an air barrier between the touch sensor and the cover lens avoids gluing the cover lenses to the touch sensors, today’s common design approach. Optically clear adhesive (OCA) glue is used in a bonding process where the sensor is physically glued to the lens. This allows a thinner stack up with very good quality optics. However, the process is expensive since it is difficult to achieve without incurring air bubbles between the display and the sensor. Minimizing air bubbles adds to the process cost for either glass or plastic lenses. A third-party bonding expert usually performs this process which adds several steps to the appliance manufacturing operation.

With DM and the added air gap above the touch sensor, the appliance OEM can self-assemble the display with the touch sensor onto the front panel in their own factory. In-house assembly avoids sending the front panel and display module to a third-party optical bonding expert to perform the special gluing, since this type of gluing is rarely performed in the OEM’s factory.

This can significantly change an OEM’s process, especially when dealing with large heavy glass panels such as an inductive cook top. In low-cost assembly regions, it is not unusual for an OEM to ship a large 42 inch diagonal, 4-mm-thick panel to another country to perform the bonding and then shipping the glued assembly to the factory. In addition to the shipping costs which can be significant, breakage can occur during the shipping process. Figure 3 shows the process.

Eliminating the shipping cost, breakage and the time it takes for the processing outside of the appliance manufacturer, allows the OEM to offset any additional cost for a more capable touchscreen controller. In fact, these savings may totally cover the cost of the touch controller. Among the problems that can occur with external processing are issues related to who owns the yield loses when breakage occurs - the display vendor with inadequate packaging, the carrier, or another? Also, regardless the size of the lens, even manufacturers with smaller displays, such as those in microwave ovens, can benefit by eliminating the external manufacturing process steps and improve their control over the supply chain. Increased freedom and flexibility also allow the use of a second touchscreen display or cover lens sources and ability to easily substitute when required.

A final benefit of the air gap between the touch sensor and the cover lens allowed by a DM controller, and perhaps the most significant, is improved field serviceability. With the display module no longer glued to the front panel, if the front panel gets scratched or broken, the service technician only has to replace the cover glass. Since the display and touch sensor electronics are not replaced as they currently are, the customer has a much lower service expense. Alternatively, if the touch sensor display module fails, just this portion of the appliance can be replaced.

These same benefits occur during the manufacturing process. Currently, once they are glued together, breakage of the display or failure of the touch controller module anywhere in the manufacturing process means replacing the entire control panel assembly. With the air gap above the touch sensor, only the failed portion must be replaced. This increases yield and reduces production costs.

Finishing Touches

The development of DM technology was driven by a combination of customer inputs to solve a specific problem and analysis of the “as is” use cases to provide a more desirable situation through innovative controller design. Customer feedback helped to refine DM and has shaped the timing of implementing DM in further controller updates.

The patented technology already exists, so appliance makers can start considering how it will impact their next designs and future products. It provides flexibility, freedom, and options to industrial designers as well as cost reduction and manufacturing efficiency and improved service in the actual operation.

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Chad Solomon is a member of Microchip Technology’s human machine interface division. He focuses on business development for emerging touchscreen markets such as home appliances and manages a global network of ecosystem partners who develop touchscreens and displays, using Microchip’s touch technology.