Let your finger do the (optical) navigating - Embedded.com

Let your finger do the (optical) navigating

The explosive advances in today’s consumer electronic devices have changed the way we interact with them and what features we expect. What we once used to make calls on the go and send text messages with is now a mobile multimedia center with the ability to access the internet, keep up with email, listen to your favorite songs, watch a movie, and download and run applications to do almost anything. Today’s latest smartphones are not only getting better, smaller, and faster, but the way consumers interact with them is also changing.

A simple set of buttons to navigate the user interface of a cell phone is now a sophisticated touch interface, full keyboard and/or navigation piece functioning much like a computer mouse. As the functionality of cell phones evolved to support functionality such as email and internet browsing, the original navigation interfaces were no longer adequate to support users’ needs.

This demand saw the development of mechanical navigation tools (side wheels, track balls, multi-direction buttons and joy-sticks, etc.) as well as some of the more advanced systems we see today such as touch screen displays and optical finger navigation (OFN) track pads.

As smartphones continue to develop and offer additional functionality and productivity, the demand for advanced navigation tools to support these features and benefits becomes critical for consumers. For the business professional, the ability to edit and review documents on a smartphone, for example, requires precise pointer control for text editing.

The consumer now demands advanced internet browsing, multimedia support, and gaming capabilities from their mobile device, all of which require detailed control functionality to better the gaming experience or ease internet browsing. The user interface is now becoming an important factor when customers decide on their next smartphone purchase. This demand has lead top smartphone manufacturers to consider OFN as a first choice technology to meet users’ needs.

There are many benefits to an OFN module that makes it a superior navigation system for mobile devices. As smartphones continue to develop, the need for precise pointer control and high resolution accuracy are becoming increasingly important for editing documents, selecting portions of text, navigating web pages, and gaming functionality.

OFN modules enable one-handed operation and are resistant to dust and other malfunctions that mechanical navigation systems are prone to, all while being a weatherproof and offering highly reliable navigation.

Table 1: Technology comparison summary
In addition, OFN systems do not degrade over time because they are fully enclosed subsystems, unlike some of the alternative mechanical navigation options that experience usage malfunctions over time. These are key advantages to OFN solutions when compared to touch screen and mechanical navigation alternatives. Table 1 above compares a variety of navigation solutions across various parameters.

It is important to consider the technological differences among current OFN implementations on the market today. Although all OFN modules offer the same functionality, manufacturers utilize unique technologies to arrive at the same goal. There are two main technologies: image correlation and spatial frequency detection.

Image correlation technology uses a light emitting diode to shine light onto a surface (a finger, in this case). The light reflects off microscopic features in the area and a system of lenses collects the reflected light to form an image on a sensor.

An illustration of a typical setup for image correlation systems can be seen in Figure 1 below. As the user’s finger moves, new images are formed on the sensor and the set of images are compared to determine the movement in the horizontal and vertical directions. The movement values are then sent to the processor in the mobile device which reflects a movement of the cursor on the screen.

Figure 1: Image Correlation Technology
Spatial frequency detection, on the other hand, is a laser based technology. Coherent light from a Vertical Cavity Surface Emitting Laser (VCSEL) is reflected off microscopic textural features on the user’s finger. The reflected light creates different spatial frequencies which is a signature of the surface texture and motion. A 2-D comb array sensor detects the spatial frequency and processes the motion into horizontal and vertical movements which are communicated to the mobile device’s main processor. Figure 2 below provides an illustration of spatial frequency technology.

Figure 2: Spatial frequency detection technology
A closer investigation into the two technological approaches to OFN reveals some advantages and trade-offs. The high-level ideas are very similar in that both approaches shine a light source on a surface and then analyze, compare, and process the reflection into horizontal and vertical movements that can be sent to a processor.One of the major differences in the technologies is that image correlation requires a complex system of lenses to gather and focus the light reflection so that is can be processed. In contrast, spatial frequency detection requires no additional parts or lenses beyond the silicon die itself.

The additional lenses required in image correlation technologies complicate assembly, add process control and quality issues during manufacturing, and increase the bill of materials. Furthermore, image correlation utilizes a light emitting diode as its light source which can be affected by ambient light sources and result in tracking errors.

Because spatial frequency detection use a laser light source which is not part of the visible light spectrum, ambient light is not a factor and does not impact navigation reliability. In addition, because ambient light has no effect on performance, spatial frequency detection is able to offer external lighting features to provide halo or glowing effects to OFN modules for esthetic appeal and to improve visibility in darkness.

Finally, because of the comparison techniques required for image correlation, spatial frequency is able to operate with lower overall power consumption, a critical factor for mobile devices relying on a battery source. This being said, image correlation systems have the flexibility to reduce the resolution of tracking by reducing the amount of information captured in each frame in order to lower the power consumption below that of spatial frequency detection.

Spatial frequency detection does not have the flexibility. However, it is important to remember that while reducing resolution reduces power consumption in image correlation, it also reduces tracking performance. Table 2 below shows a technology comparison between image correlation and spatial frequency detection.

Table 2: Technology comparison summary
With ever increasing advances in smartphone and mobile device development, the challenge of integrating navigation tools to meet consumer demands has become a significant issue. OFN modules are becoming one of the most popular interfaces but in order to reap the benefits of both worlds, some smart phone manufacturers are turning to designs that incorporate OFN as well as touch screen functionality.

It is crucial for developers of smartphones to understand the technology behind OFN and that not all designs are equal. Spatial frequency detection designs offer significant advantages over image correlation solutions that include higher reliability, simple assembly, and less power consumption.

The applications for OFN are continually expanding. They can be seen today in top smartphone designs by RIM, Samsung, and HTC. They are also beginning to reveal themselves in cutting edge netbooks and ultra-mobile PCs. Consumers will begin to see OFN in a growing number of mobile electronic products and it is clear that this new technology is here to stay.

Mike McCauley is currently working as a Product Manager at Cypress Semiconductor. Prior to this, he worked in the mobile consumer electronics and semiconductor industries.

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