Specific absorption rate (SAR) is the measure of the rate at which energy is absorbed by the human body when exposed to the radio frequency (RF) electromagnetic field (EMF) of an electronic device. The Federal Communications Commission (FCC) has strict limits regarding safe SAR levels in mobile devices such as smartphones and tablets. This tends to force manufacturers to sacrifice performance for safety. Localized movement sensing limits the amount of radiation to the human body by detecting whether the mobile device is placed on a stationary object or in use by a human. Capacitive proximity sensor (P-sensor) technology offers increased performance by differentiating between typical human use and stationary materials (e.g. placement on a table) – whilst complying with FCC SAR standards.
SAR Proximity sensor application
P-sensors are favored for limiting SAR because of the omnidirectional nature of their E-fields, which effectively cover the area around the RF antenna. When a mobile device is used near a human body, the sensor is triggered in order to reduce the RF output to an acceptable level.
P-sensors are subject to unique challenges related to the sensor core technology. These are listed below:
- P-sensors rely on detecting changes in the E-field. Any type of material/object can cause change in an E-field, although only detection of the human body is required in this case.
- Materials used for “transporting” the sensing signal to the area of interest are known to cause capacitive changes (dielectric property changes) with changes in temperature and humidity. Polyimide, FR4 and coax cables are all known to cause significant capacitance changes with changes in temperature and humidity. Capacitance changes are also dependent on the design, and are more noteworthy than capacitance changes related to proximity sensing.
- On-chip sensor “engine” hardware is normally influenced by temperature. The degree of influence depends on the chosen P-sensor.
Normal proximity detection is dependent on the environmental and material state at the time of crossing the proximity threshold. During the time of proximity-based “activation”, it is complex and more costly to compensate for the environmental and material state. Localized movement sensing is a solution that addresses all of these challenges through the detection of human behavior, rather than stationary objects and slow-changing environmental effects, to finally increase sensing performance.
Introducing localized movement sensing
Movement sensing is generally understood to rely on accelerometer (‘G-sensor’) data and can only sense the movement of the device unit as in Figure 1. Localized movement sensing senses the movement between the unit and another object (e.g. a human or a table), as in Figure 2. The sensing is done around a confined area of the unit and movement cannot be sensed without another object being close to the confined area.