Product How-To: The mechanics of capacitive touch sensor interfaces

Steve Barry

March 01, 2011

Steve BarryMarch 01, 2011

Mechanical buttons.

You know, those things you push on your alarm clock, your computer, your TV, your microwave. They are the single most common user interface in application today.

So common, in fact, that we hardly think twice about them. They have become a part of our subconscious – and yet, they are a source of frustration to most of us when they break, pop off, get stuck, or just stop working.

Why do we tolerate this? The answer is because there is a general unawareness about alternatives. In recent years, capacitive touch sensing technologies have matured to the extent that they now provide better robustness, better reliability, and a far greater capability to enhance the user experience than mechanical buttons.

Touch interfaces have already been adopted in a wide variety of applications ranging from: home appliances, consumer electronics, industrial systems, and to medical equipment. Adoption of this technology continues to improve as people around the world (and the many markets that service them) become more comfortable with this user interface and confident in its reliability.

This increased comfort has stemmed from technological improvements that have reduced the challenges that have plagued designers with this technology historically and new market needs.

Challenge #1: Complexity of capacitive touch interface design

There is an inherit risk about changing from a known working solution to something that hasn’t been proven, at least not in your product, market, or specific design. The tried and true mechanical solution is easy, no electrical design occurs, it is essentially a drop-in-and-go approach.

But with this lack of initial effort, it should not be surprising that the end result is not revolutionary (you get what you pay for).

But what if the design process, the risk, of creating a capacitive interface wasn’t a painful, largely time-consuming effort? What if the design effort - tuning, sensor lay-out, and code written (if required) - could be completed in less than a week?

Through the dramatic improvement in touch software tools, sensor lay-out tools, and the expanded portfolio of products available (both application specific and library capable) a designer can complete a user interface design from start to finish in under 1-week.

Challenge #2: Lack of interest in user interface

In general, the designer’s focus is on the core functionality of the product – what’s going on under the hood or what is the heart of the product. While the core functionality is absolutely important, how the user interacts with the product has an undeniable affect on the experience that person has with the product.

There are examples of this virtually everywhere today: the iPod’s click-wheel interface and the touch screen of nearly every new smart phone on the market today. These are all examples of capacitive user interfaces.

Increasingly, the success of new products is greatly affected by the design of the user interface and capacitive sensing offers a sleek interface that makes using the product more natural, intuitive, and fun.

Challenge #3: Lack of tactile feedback

As objects in this world, we abide by Newton’s 3-Laws: 1. Objects in motion tend to stay in motion unless acted on by outside forces

2. Force equals mass x acceleration (F = ma)

3. For every action there is an equal and opposite reaction

So it is an unnatural, event if when I push on something and I don’t feel like there is any response from it. It doesn’t move, give, push back, react. We have been trained from mechanical solutions our entire life that when the button moves, we have registered a touch with it.

So with capacitive touch solutions, how do I know I pushed it? This has been an on-going and very common complaint with capacitive sensing to-date.

As I push the elegantly smooth front panel of my new STB to play a movie, I notice that the box starts to think. How am I to know that the play button request was accepted?

Historically there was the option of lighting an LED or playing a quick sound. But what if I want to feel the ‘push’?

Through the implementation of haptics it is possible to feel when a capacitive touch has been registered. It is also possible to feel a variety of haptic effects that can be selected from an effect library to fit your desired needs.

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