Guest columnist Eric Itakura from IDT outlines the importance of providing user feedback in the area of touch control design
Capacitive touch technology is being widely used for user interfaces in industrial as well as consumer electronics. As these new touch inputs are implemented most often on rigid surfaces, the need for effective sensory feedback is increasing in visibility and importance.
Multiple sensory feedback methods are commonplace, but they are often so subtle that they go practically unnoticed. The three senses involved in product sensory feedback – visual, aural and touch – are used singularly and in combination.
Studies have shown that sensory feedback improves user accuracy and provides a better “emotional” response for the user.
While visual and aural feedback are well understood and have been implemented with capacitive touch from the beginning, tactile feedback has only recently started to see widespread adoption.
Tactile feedback
Sight- and sound-directed sensory feedback can be extremely important, but no touch design should overlook the importance of tactile feedback.
The most sophisticated touch response mechanism is called haptics. Haptics’ effects – supplying forces, motions or vibration to the user via their sense of touch – can be configured to provide not only simple sensory feedback in the form of basic vibration, but also through the shaping and timing of vibration effects to realistically simulate the touch sensations associated with mechanical inputs, such as buttons and sliders.
Haptics feedback can be utilised for far more than simple validation of the touch itself. It can also be tailored to convey other messages. Sensations can be presented to the user representing such things as the amount of pressure on a touch-button or to allow third-dimension (z-axis) input.
High input tactility can also improve the accuracy of using the device.
A study by Nokia examined the effect of tactility on numerical entry tasks in mobile handsets. Tactility was varied by using protruding, separated keys with less than 1mm travel (high tactility) and flat, horizontally connected keys with only about 0.5mm travel (low tactility).
The resulting error rate in the test case when subjects were not allowed to view the phone, and so had no visual feedback, shows a greater than six times improvement for the high tactility condition (see Figure 1).
Even when subjects were allowed to view the phone during numeric entry, the error rate was almost three times lower for the high tactility phone.
Sensory feedback integration
For capacitive touch, it is important to have the touch controller sensing circuits close to the sensor itself. Because the measured capacitance is relatively small, lengthy connection traces from sensor to controller may increase susceptibility to noise to the point where valid touches can become indistinguishable from noise.
Therefore, incorporating capacitive sensing into the main or application processor, which are frequently distant from the touch sensor array, may not result in reliable touch performance. For this reason, a separate capacitive touch controller is often the most practical approach.
In the case of sensory feedback, the necessary logic and control could feasibly be implemented on the main or application processor, but places another burden on the processor. Depending upon the priority level of the different processor interrupts, latencies to these touch feedback functions may be introduced, creating a jumble of asynchronous and mistimed feedback.
Furthermore, because processor responsiveness varies with activity load, feedback latencies can vary, creating unpredictable and inconsistent results.
Integrating sensory feedback functionality into the touch controller not only frees up the main processor from the additional requirements of driving LEDs or haptics devices, but also provides significant cost and performance advantages.
An integrated touch controller combines the functionality of multiple discrete components into an optimised application-specific design, with the benefits of reduced component cost and board space, lower power compared to SoCs, and well-synchronised enhanced sensory feedback.
Without question, touch control and user feedback will be a significant area for change and growth now and in the near future of consumer and commercial/industrial products.
Author is Eric Itakura, senior marketing manager for IDT user interface products
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