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Sensors/MEMS??

ToF measurements pave the way for user-interaction

Posted: 30 May 2013 ?? ?Print Version ?Bookmark and Share

Keywords:time-of-flight? proximity sensors? Single Photon Avalanche Diode?

STMicroelectronics recently launched an all-in-one module embedding both a wide dynamic ambient light sensor and a ranging sensor along with an infra-red light emitter. The first member of ST's FlightSense product family, the VL6180, uses direct time-of-flight (ToF) technology to measure the time the light takes to travel to the nearest object and reflect back to the sensor located right next to the emitter. The speed of light in air has a well-known value and can be used to reliably convert time into distance regardless of the target object's properties such as reflectance, unlike conventional amplitude-based optical proximity sensors.

The fundamental sensing technology underlying this sensor is a "Single Photon Avalanche Diode" (or SPAD), which is integrated on a single chip along with everything else except the light emitter. The single photon avalanche diode must be reverse biased beyond its breakdown voltage, which puts it in a very sensitive state, called Geiger mode. When an incoming photon impacts the sensing area, it splits an electron-hole pair. These are subsequently accelerated due to the high electric field and go on to cause a chain reaction, generating an avalanche current in a very short amount of time. This very fast response time, combined with extreme sensitivity, make SPADs a perfect match for time-of-flight constraints and allow them to output two independent measurements: the amplitude of the light reflected back from a target is calculated by counting the photons, and the distance of a target is based on the time-of-arrival information from each photon detected.

Simply detecting a single photon that has traveled from the module to a target object and back to the SPAD detector is not enough to determine the distance. This is because the emitted pulse of light is not infinitely small. We use very short optical pulse, which is essentially a stream of photons, whose arrival time follows a Poisson distribution. When the SPAD detector is triggered, it is not possible to know whether the event detected was due to a photon on the leading edge of the emitted pulse, or from the middle or end of the pulse. To complicate matters further, it is also not possible to know whether an event was detected due to a photon emitted by the module, or whether it was simply a photon from background ambient lighting that triggered the system. To understand whether a photon is correlated to the emitter, or is simply background noise, we need to repeat the optical pulse many times, and essentially build up a histogram to separate the signal from the noise.

The primary application intended for the VL6180 product is a replacement of existing proximity detection technology, which is amplitude-based and cannot measure absolute distance. These proximity sensors are used in nearly all smartphones to detect the user's head during a phone call, for example. Unfortunately, the amplitude of the reflected light varies according to the distance but also with the reflectance level of the target, which can be as low as 3 per cent for dark black hair. This leads to very ambiguous resultsquite frustrating to some users: when the amplitude of the light level is low, the amplitude-based proximity sensor may "think" the user's head is far away, when in fact it is very close, but the user's black hair is not reflecting enough light. As a result, the touchscreen is not disabled, and the user's cheek may brush up against any number of buttons and functions (Search "face hang-up" and any smartphone brand on the Internet to find example of frustrated users!). On the other hand, smartphones equipped with the VL6180 will detect the user's head, irrespective of hair colour or hats/glass frames/etc., and shut off the touchscreen to avoid any unwanted touch interactions.

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