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

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.

Challenge #4: The need to differentiate

Consumer electronics is one of the quickest moving industries. Therefore competition is increasing every month, so there is a continued need to find a way to stand-out from your competitor.

To do something different to set yourself apart. Implementing capacitive sensing is a great way to do this, specifically in the following ways:

Sleek design: No one likes the clunky appearance of buttons on their devices. Think of an oven, a TV, or STB. Now imagine those same devices with completely flat surfaces, where you don’t even see the buttons until you want to see them, just smooth, reflective glass or black surface.

Then when you want to adjust the temperature of the stove, turn up the volume of the TV, or pause the show you are watching the buttons ‘magically’ appear.

This is possible through a capacitive proximity sensor and capacitive button, slider, or wheel implementations. This is not just an interface differentiator, but a complete product differentiator!

Multi-functional sensors: With mechanical solutions it is a very binary result. The button is either pressed or not.

But what if you could get one type of functionality out of a button when you have your finger on it, but not pressing it and another when you actually push it down.

Through an implementation utilizing both mechanical and capacitive buttons this enables 1-button to essentially become 2 in the same board space.

Customized feeling: What if I what to give the user a different feedback depending on the button, slider, or wheel they touch? With the first button there is no feedback required, with the second a 1-second soft response, the third a medium 5-second response, and last a strong 15- response.

With a mechanical dome switch this isn’t possible to provide varied response depending on which button is pressed. With an MCU driving both capacitive touch technology and an actuator, a large variety of haptic effects are available and can be set-up to provide a completely different feedback from button to button.

Power savings: With the coming of the “Green Movement” everyone is attempting to reduce power consumption, increase battery life by making electronics work more effi- ciently, yet without sacrificing their functionality.

Take a wireless mouse for example, when you aren’t using it you want it to save power by being in a sleep or hibernate mode, yet when you go to use it you don’t want to have to shake it and wait for it to wake-up.

This is a great example of where capacitive proximity sensing can be a game changer. As my hand approaches this sleeping mouse that has a capacitive proximity sensor in it, the mouse senses the hand and wakes it up from its sleep mode so that by the time my hand actually reaches the mouse it is ready to move with my every motion.

This addition of the capacitive proximity sensor therefore saves power, and improves the previous functionality of the standard design.

Mechanical interface weaknesses

As previously referenced, mechanical solutions have their weaknesses. By definition a mechanical solution will have moving parts and air-gaps so wear and tear is inevitable.

RIM’s most common failure in their BlackBerry phones was the trackball. Kids spill juice boxes and trail mix on the center console in the car making it so the window button no longer works.

The outdoor garage opener button rusts out because of being rained on. All of these problems are preventable by using capacitive solutions where there is no movement, no air-gap, and are water resistant.

With all of these new solutions available reduce the risk of converting from the ancient mechanical solution to the more modern capacitive solution, with all these cool new differentiators, what more does a designer want to make the change?

Atmel is asking this same question as they have available microcontrollers to drive this touch technology. It also has proximity sensing capable of greater than 6”, greater than 100- haptic effects in library formats to choose from, and easy to design with software in Atmel’s QTouch Studio (Figure 1 below ).

 

Figure 1: Atmel QTouch Design Studio 4.3 allows designers to implement touch buttons, sliders, and wheels in a wide variety of applications.

Why is the software so important?

Since many designers are still unfamiliar with touch technology, they rely on the supplier’s software development tool to complete the design and meet the system requirements. Designers are looking for tools that provide easy–to-use, reliable, and flexible building blocks with which to evaluate and develop their own touch applications.

These building blocks fundamentally consist of a touch sensing device and the algorithms necessary to achieve a high quality user interface. By far the most capable and flexible building blocks are general purpose microcontrollers running the software that implements the user interface and other system features. The tool with which the software is developed is clearly critical for an efficient and successful design.

The software designer is tasked with defining the behavior of the user interface in software and executed by the microcontroller. The touch functionality is supported by the microcontroller’s sensing of the various touch channels and the processing of the capacitance data to determine if a hand is present, a touch has occurred, and/or there is movement of a finger on the touch interface – sensing and tracking movement is important for touch interfaces such as sliders, wheels, touchpads, and touchscreens.

The software development tool is the designer’s lifeline to not only implement this functionality but also to ensure its quality and reliability. The following capabilities of the tool enable the designer to meet these goals:

1. Intuitive software creation: software library supporting the touch features and automatic generation of other functional code

2. Tuning: automated tuning of the sensors

3. Debugging: Quality Analyzer providing performance information to the designer, such as signal-to-noise ratio (SNR), capacitance, noise and reference levels, and drift. A Validation Wizard identifying marginalities and providing specific feedback on how to resolve them

A designer’s ability to design the best product in the shortest period of time is greatly influenced by how well the tool supports these capabilities. One software development tool available today is Atmel’s QTouch Studio 4.3.

QTouch Studio 4.3 includes a Touch Quality Analyzer, Touch Validation Wizard, automatic selection of tuning parameters, graphical real-time tuning, data logging, automatic generation of initialization software, and many other features that allow software designers to integrate touch functionality into their microcontroller designs.

This development in touch design is causing an accelerated adoption of touch interfaces across many markets and it is changing the way we think about interfacing.

Steve Berry is director of touch technologies at Atmel Corp.

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