Using proximity sensing to meet mobile device FCC SAR regulations
Capacitive sensing
As opposed to IR solutions, capacitive sensing technology is preferred because of lower power consumption, the fact that an aperture is not essential and that it is not sensitive to ambient light conditions. Additional advantages include the small real estate required for the sensor and the low cost of such a solution.
Capacitive sensing can be done between a single electrode and the circuit common ground (self-capacitance), or between two electrodes (mutual capacitance). Each method has its advantages, but because of the largely variable parasitic capacitance between the circuit common ground and earth in mobile devices, mutual capacitance techniques are preferred.
The sensitivity of capacitive sensors is highly adjustable. This allows for a large variety of electrode sizes and overlay materials. Conductive overlays have a degrading effect on the operation of the sensors and may cause loss of sensitivity or even unstable operation. As with RF antennas, capacitor sensors should not be covered with a conductive overlay.
Mobile device covers are commonly used and may be regarded as an undetermined additional overlay. With capacitive sensing, calibration could be critical to the accurate triggering of proximity sensors.
Detection of the cover becomes possibly by pre-fitting the cover with a metal strip. This would enable an automatic variable calibration for accurate sensing, whether or not the cover is fitted. Such implementation also allows for detecting the changes caused by opening and closing a screen cover.
Distinguishing human vs.non-human objects
It is relatively simple to prove that it is impossible to distinguish between human and non-human triggers by observing capacitance in one dimension only. Figure 1 shows an example where a sensor is placed on a human lap and then on an earthed metal sheet.
The difference in capacitive effect is much too small to distinguish, especially with the variance in proximity and human body characteristics.
Click on image to enlarge.
By placing multiple sensors in a manner that would reflect user behavior and capability, one is able to distinguish between human and non-human proximity triggers. For example, as shown in Figure 2 below, it would be very unlikely for the user of a tablet computer to cover four sensors at the same time for a few minutes, keeping in mind that FCC regulations dictate a 6-minute SAR average value.
Click on image to enlarge.
On the other hand, it would be easy to trigger all four sensors by placing the tablet computer on a metal or glass table. On this basis, multiple sensors are proposed to solve issues of false triggers with proximity sensors. A three-sensor solution is also shown in Figure 3 below.
Figure 3: Example sensor placement for the use of a three-channel device
Such solution is aimed at a single IC sensor implementation using three channels.
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