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Sensors - A clever touch

Driven by the influence of products such as the iPod, capacitive touch sensing is finding wider application and support. By Graham Pitcher.

Like them or not – and there are many people who don't – the Apple iPod, iPhone and, more recently, the iPad have brought a major change in the way interfaces are now perceived. That change is, of course, touch sensing.

While the concept of touch sensing isn't new, the arrival of these consumer devices has impacted how industrial designers look at interfacing. Now, traditional buttons and switches are being replaced by touch panels in products ranging from white goods to industrial controls. And this move is being supported by microcontroller manufacturers, some of whom have recently introduced devices designed specifically to enable low cost, low power touch interfaces.

Shahram Tadyon is mcu product manager with Silicon Laboratories. He believes the 'iPod' factor is important. "Since the iPod came out, everyone has wanted to do touch buttons and similar. Not only are touch interfaces cheaper and less intrusive, they are easier to develop than mechanical systems."

Chris MacCallum, Microchip's European field applications manager, agreed with this view. "Capacitive touch sensing is going into a wide range of applications, so there is no single solution. This means you end up with many algorithms and ways of doing things.

"Today," he continued, "most capacitive sensing applications are through glass or plastic, but there are many applications where these materials aren't the best solution."

Both companies have recently introduced microcontroller based touch sensing technology, not only to help designers to get their products to market more quickly, but also to address what they see as a large business opportunity. Tadyon said: "I believe the market for capacitive touch sensing is worth at least $500million."

While companies such as Microchip and SiLabs are taking a microcontroller based route, others are looking to develop solutions that work alongside a microcontroller. Fujitsu and Freescale are just two companies to have recently launched such devices, which can enable retrofits.

Christian Harders, a Fujitsu applications engineer, said: "It is comparatively easy to develop this type of product using devices such as Fujitsu's FMA1127 capacitive touch sensor controller. This performs the functions required for touch recognition and makes it possible to construct a touch control panel on the host mcu with minimum software work. The FMA1127 is connected to the host mcu via the i2c bus, so existing systems fitted with mechanical keys can be optically and functionally upgraded, for example as part of a 'facelift'."

Freescale, meanwhile, is not only looking to support touch interfaces, but also the various sensing technologies being integrated – or at least considered – for forthcoming smartphones.

Its offering is Xtrinsic sensing solutions, devices with built in intelligence and the ability to make decisions within their environment. One of the first parts of the Xtrinsic family is the MMA9550L, a motion sensing control system. But the company has also developed the Xtrinsic Touch Sensing Software Suite 2.0, which it says extends capacitive touch sensing beyond the 8bit mcu market to 32bit devices.

The rationale behind the moves is Freescale's belief that sensing is becoming more complex and, to solve the problems, new levels of innovation are needed. Discussing the development, Stephane Gervais-Ducouret, Freescale's director of global sensor marketing for consumer products, said it is no longer about what each sensor does. Rather, it is about unlocking a sensor's potential. No longer is it acceptable, he points out, to have a sensor without decision making capability; neither is it acceptable to simply integrate multiple transducers.

Xtrinsic sensors, he believes, offer the right combination of integration, logic and customisable software to allow smarter, differentiated applications to be developed.

The MMA9550L (see fig 1) can manage multiple sensor inputs, performing intelligent, system-level decisions within the application. The MMA9550L has a three axis accelerometer, a ColdFire V1 processor, power management and connectivity blocks. Boasting 14bit resolution, the MMA9550L is described as a decision engine, with the ability to take inputs from up to 12 sensors of varying type. By using this part, calibration, compensation and sensor functions can be offloaded from the host processor.

SiLabs' contends that its F990 range has the lowest power wake on touch performance, drawing less than 1µA, and a power consumption of 150µA/MHz over its operating range of 1.8 to 3.6V. Other features include a patent pending capacitance to digital converter which features an acquisition time of 40µs.

The part can accept input from up to 14 capacitive sensing elements, allowing more complex interface schemes to be created. The company says that, by using an F99x mcu with its Si11xx infrared and ambient light sensors, touchless proximity sensing interfaces can be developed that support gesture based control (see fig 2).

SiLabs' capacitive touch sense mcus are being used to replace buttons in equipment as varied as set top boxes, notebook pcs and industrial control keypads. In one innovative application, the mcus are being used to sense contact at the opposite end of an audio wire – which can be up to 1m long – without being susceptible to conductive material approaching the transmission line and without shielding. This application has the potential be used in a variety of consumer audio applications that require awareness of end user touch to audio wires, SiLabs claims.

Fujitsu is also looking at gesture based control. Harders noted: "By using particularly large electrodes in the form of a grid with a low cover, it is possible to create a reaction when a hand is approaching, and hence trigger an action. Since the sensitivity of the channel being used is normally set as high as possible for this application, further hardware functions in the FMA1127 – filter period and filter threshold – are useful in increasing the range. This makes it possible to perform time based filtering in the FMA1127 itself.

Microchip, meanwhile, has made the mTouch projected capacitive touch sensing technology available across its mcu range. Projected capacitive touch sensing extends resistive and existing capacitive touch sensing technology to include multiple touch and gesture sensing. This, says the company, allows designers to implement robust glass front user interfaces that simplify interaction.

MacCallum pointed to the PIC16F707 as a suitable vehicle for this technology. "It has the ideal mix of peripherals," he claimed. The part features two 16 channel capacitive sensing modules and, because these modules can run in parallel, two touches can be tracked in real time.
Designers are supported by the mTouch development kit, which eases the design and integration of projected capacitive touch applications. Because only one mcu is required, costs are reduced. According to Microchip, the F707 will cost less than $1 in volume.

One thing is fairly certain: the influence of the consumer market on industrial product design will only increase. With everyone becoming accustomed to keyless and gesture based interfaces, it's only a matter of time before they become standard in the industrial world.

Graham Pitcher

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