Using proximity sensing to meet mobile device FCC SAR regulations
Capacitive sensor capabilities
A highly configurable device (Figure 4, below) best addresses the important issue of calibration. With pre-loaded calibration data, the sensor could adapt for various protective covers and start-up conditions. Such conditions include the possibility of the user powering up the device while holding it.
A safe implementation would initialize the sensor with predetermined data that is based on the absence of human or object proximity. With such a configuration, proximity events will lead to the assumption of a fitted device cover (all sensors activated) or human proximity (only some sensors activated).
The calibration values can be adapted for that which is common to all sensors (device covers), and proximity can be based on the differences (human proximity).
Capacitive sensors are subject to long-term environmental drift as well as sudden changes in environment. Automatic sensor calibration algorithms are essential for simple and accurate sensing under such conditions.
These algorithms can be forced to start only from the first perceived release condition (the human proximity taken away). This allows for a safe start-up with predetermined values. Detection of the first release is made possible by dynamic threshold value adjustment and threshold-crossing direction adjustment.
SAR regulations could not be met when including the effect of human proximity as an environmental shift. For this reason it is important to know when proximity is registered (touch event), but also when it is cleared (release event).
Hence, a need exists for knowing the direction of proximity threshold crossing. By adding hysteresis to this, the user may vary touch intensity without causing releases. Only a proper release will clear the touch event. This functionality is crucial to applications in which long-term constant proximity is mixed with the need for long-term environmental calibration.
Conclusion
Capacitive sensors in wireless mobile devices offer a cost-effective solution for enforcing FCC SAR regulations, with many possibilities for improving sensor activation accuracy. With the increase in wireless network connectivity and popularity of various protective covers for devices, accurate detection forms part of being ready for future and current demands in mobile devices.
Patented proximity sensing technology from Azoteq meets all the required sensor capabilities mentioned. The automatic tuning implementation (ATI) is intelligently used to adapt to environmental drift and to keep the sensor operation at an optimal level.
DYCAL technology from Azoteq ensures that touch and release events are accurately detected without calibration during sensor activation events. With DYCAL, environmental drift is continuously compensated for, while re-calibration is only done after release events.
Azoteq offers a comprehensive product line of single- or multi-channel devices using various sensing technologies.
The IQS253 (Figure 5, below) is a three-channel sensor that can be configured for self- or projected-capacitance. It offers I2C-compatibility for run-time configurability along with DYCALTM technology.
In addition to the IQS253, the IQS128 is a standalone solution that is used in many tablet computers across the world. The IQS128 offers simplicity with direct outputs, along with DYCAL technology. The IQS252 offers a two-channel standalone solution using projected-capacitance and DYCALM technology.
Riaan du Toit is an applications engineer at Azoteq who holds an M.Sc. degree in electronic engineering. Riaan works from the R&D headquarters in South Africa and has experience in various applications for the IQS253, along with other sensors that implement the DYCALTM intelligent hysteresis. Riaan can be reached at riaan.dutoit@azoteq.com.
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