PRODUCT FEATURE: Digital touch goes capacitive

It is now common for even relatively mundane devices, such as an electric drill or a vacuum cleaner, to use a programmable device for its control algorithms, but the same level of sophistication has yet to migrate to the user interface. Where there could now be a touch panel display and solid state relay, it is still more common to find conventional LEDs and discrete switches.

However, the continued price erosion of display technology has led to an increased use of LCDs in all manner of equipment, which means relatively sophisticated graphical user interfaces (GUIs) are now common. This extends from the latest portable media device to domestic household appliances, such as microwaves and washing machines.

GUIs, thanks to a supporting cast of embedded graphic software libraries, are now often cited as differentiating features for end products. This is a well deserved viewpoint because with the right graphics library and some careful design, a GUI can make the operation of any device more engaging, intuitive and, subsequently, more appealing.

These solutions rely heavily on the cost reduction in display technology, but a display on its own doesn't create the type of user interface necessary for 'leading edge' applications; that demands a comparable improvement in the way the device is controlled by the user.

With the advent of touch sensitive LCDs and, more recently, certain Apples products, the consumers' level of expectation is, arguably, on a steep, upward gradient.

The standard mouse pads used on laptops aren't sophisticated enough to allow multiple points of contact, as demonstrated by the iPhone, iPod Touch and now Mac Air. The same is true for the majority of touch sensitive technologies.

How do they do that?
The reason touch sensitive devices can't natively support multiple points of contact is because the underlying technology relies on fairly basic principles, essentially sensing either capacitive or resistive changes across a surface or membrane.

Hence as each X/Y coordinate corresponds to a specific capacitance or resistance, touching multiple points on the membrane causes a breakdown in the relationship between a relative position on the membrane and an absolute capacitance/resistance measured.

Software can go a long way towards overcoming this but unless the underlying technology can natively support multiple points of contact, the options are limited.

The technology used for the iPhone, for example, is revolutionary because it uses a technique called Frustrated Total Internal Reflection (FTIR). Here, a camera is used to track the user's finger(s).

As it doesn't rely on relative changes in resistance or capacitance, it's able to detect multiple points of contact and incorporate them into its control functions, such as zooming in and out or rotating an image, by using two fingers to 'grab' the corners of an object displayed.

Clearly this kind of technology requires greater investment and processing power, so isn't quite as accessible as a simpler touch sensitive LDC display. An LCD display, on the other had, even a touch sensitive display, can now be controlled from 8- or 16bit microcontrollers.

But even though touch sensitive technology has become more accessible and simpler to interface to, it can still represent a relatively large part of the total Bill of Materials. Because of this, there is still a host of applications that can't support the additional cost of a display, yet could benefit from more sophisticated or rugged input solutions.

One technology that is gaining ground in this class of device is capacitive touch sensing, where the input device isn't integrated with a 'sophisticated' display, but more often is used in conjunction with low cost LEDs. This technology can be implemented using a standard PCB trace and even be mounted below a fascia.

<>A light touch from Cypress
Recently Cypress Semiconductor launched its CapSense Express range of touch sensing solutions, extending its existing range and designed to provide an entry point to the technology, promising the industry's easiest and fastest button and slider replacement solution.

"Mechanical buttons and sliders are prone to wear and tear from prolonged usage and exposure to heat, moisture and other elements," said Robert Dunnigan, Vice President of Cypress' Non-Volatile and Mixed Signal Business Unit.

"In addition, they do not provide the clean, elegant styling preferred by today's consumer. The CapSense Express solution offers our customers an easy to use, cost competitive solution to button replacement that enables innovative new products with the fast time-to-market needed to beat the competition."

Any engineer will understand the potential (no pun intended) problems with capacitive sensing. It works on the principal that the introduction of any conductive material in to the proximity of sensitive analogue circuitry will capacitively couple noise in to it without making physical contact.

With this type of touch sensing, the noise is detected and registered as intended stimuli, as opposed to unintentional interference. The trick is, telling the two apart; noise can creep in to sensitive analogue circuitry from many sources, not just a nearby finger, and can affect digital circuits, too.

This is evidenced by the need to decouple the power supply lines of even the most robust digital integrated circuits, to ensure their reliable operation.

In effect, the operation of a capacitive sensor relies on one conductive stimuli (such as a finger) acting as a capacitor and being placed in parallel with an exiting capacitor (formed using PCB tracks) on the board.

Again, as any engineer will know, two capacitors placed in parallel have a net effect of increasing the overall capacitance, so it is simply a case of detecting a change in capacitance. The amount of capacitance introduced by a finger, however, is in the region of 0.05pF, while the reference capacitor will by around 100pF.

So while the technology has a lot to offer, it can also be difficult to implement. And as it relies on a very small reference capacitance, formed using PCB traces, it can also be susceptible to drift over time, humidity or temperature.

<>Successive approximation's contribution
Cypress believes it has overcome these issues through the use of successive approximation, sigma-delta conversion and switched capacitor circuits. The successive approximation method (CSA) is a new sensing method used in the CapSense (CY8C20x34) and CapSense Express (CY8C201xx) device families.

CSA enables the implementation of an array of capacitive sensors through switched capacitor circuitry, an analogue multiplexer and digital counting functions. In operation, the switched capacitor network charges an external modification capacitor.

The voltage on this capacitor is routed, through a low pass filter, to a comparator, where it is compared to a reference voltage. The output of the comparator is used to gate a counter, which is clocked by an oscillator.

The sigma-delta method (CSD) is used in the CapSense Plus (CY8C21x34 and CY8C24x94) device families. Intended for applications that require a larger number of inputs, CSD enables the implementation of an array of capacitive sensors.

This method includes a switched capacitor input stage, containing the sensor capacitor(s). The switched capacitor network is between VDD and the voltage on a fixed external modification capacitor, which is continuously charged and discharged.

When the comparator trips, a bleed resistor switch closes, discharging the modification capacitor until a new value is stored in the synchronisation latch. The output of the synchronisation latch, also referred to as the bit stream, is 'ANDed' with the output of a pulse width modulator, which enables a counter. In both technologies, the output of the counter is then processed to determine the status of the sensor.

As the devices are based on Cypress' PSoC technology, they are configured through software routines provided in PSoC Designer and PSoC Express, which compensate for environmental and physical sensor variations.

The CapSense Express devices offer up to ten capacitive and/or general purpose I/Os, allowing combinations of buttons, sliders and general purpose functions " including LED control and interrupt outputs.

No mechanics
The key to implementing simple capacitive sensing is its cost; because there are no mechanical switches needed, the Bill of Materials goes down. That model only works, of course, if the cost of silicon implementing the capacitive sensing is low. From Cypress' point of view, it offers a CapSense Express device that has just 8 pins, capable of implementing two buttons and two status LEDs.

Another provider in this area, Microchip, also offers a highly integrated capacitive sensing solutions, in its 8bit PIC product range. The company recently announced the PICDEM Touch Sense 1 Demo Board, which comes complete with a PICkit serial analyser and a software development kit. Microchip's solution is based on a relaxation oscillator to detect the small changes in capacitance that represent intentional stimuli.

The use of alternative interface solutions, from Apple's multi-touch technology, through to touch sensitive LCDs, to capacitive sensing solutions, show that user interfaces are evolving. Nintendo's Wii has broken all the 'rules' with its wireless joy-pad, which uses 3D positioning sensing, while in the world of Computer Aided Design, one of the latest innovations is the 3D mouse, which allows engineers to control their designs more intuitively by moving the mouse in free space.

Microsoft is now demonstrating its own vision of future user interfaces, in the form of Microsoft Surface. This uses similar technology to the iPod Touch, to recognise not just multiple points of contact but actual objects, such as a paintbrush, and then interact with them appropriately.

"With Surface, we are creating more intuitive ways for people to interact with technology," said Microsoft Corporation CEO, Steve Ballmer. "We see this as a multibillion dollar category, and we envision a time when surface computing technologies will be pervasive, from tabletops and counters to the hallway mirror. Surface is the first step in realising that vision."

It seems likely that, in the near future at least, touch sensitive technology will play an ever more important role in enabling humans and computers to interact more constructively