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Sensor combinations enable smarter mobile devices

28 Apr 2015  | Ernst Haselsteiner

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Sometimes, smartphones can be remarkably dumb. Take this example: an office is full of employees, each quietly concentrating on his or her own tasks. Suddenly, the silence is broken by a loud burst of pop music – the ringtone of the phone of a colleague who happens to be away on a lunch break. Left on a desk, the phone vibrates, and as it does so it shuffles towards the edge, forcing a colleague to jump up and move it to a safe place.

It's irritating for the people in the office, and it breaks their concentration.

On the face of it, it seems odd that a smartphone could behave in such an unsmart way – after all, smartphones are normally very aware of their relationship to their environment.

In fact, it requires some additional components, and a carefully thought-out combination of various sensors' outputs, to enable a mobile device to automatically perform appropriate notifications. This article sketches out a hardware and software framework that could provide this capability while harnessing functions already available in many mobile devices.


A smartphone's rich array of sensors
A smartphone already contains many sensors. Some may directly support the provision of smart notifications:

 • A proximity sensor embedded in the display detects when the phone is held to the user's ear. This enables the phone to turn off the display and its touch sensitivity.
 • An ambient light sensor enables the system to adjust the brightness of the display in response to the intensity of the ambient light. Advanced ambient light sensors combine digital colour sensing with proximity sensing, to enable more sophisticated display management.
 • An accelerometer determines the orientation of the screen, so that the contents may rotate as appropriate to fit a landscape or portrait configuration.
 • A microphone may sense the volume of ambient noise.

As currently deployed, each sensor is assigned to one main function, and is generally not used for any other. The one exception is the accelerometer, which is also used in gaming apps to support the player's user interface.


Partial awareness of the environment
The array of sensors in a smartphone today provides a partial ability to support smart notification of events such as incoming calls, messages and e-mails. For instance, the device's ambient light sensor and accelerometer can detect whether the smartphone is still and in a bright environment (because of sunlight or artificial light). This means that the device is not on the user's person (if it were in a pocket, there would be no ambient light incident on the display). This means that the vibration function can automatically be disabled.

But user notifications can be made far smarter than this, with the addition of a few extra components.

For instance, the ambient light sensor cannot function when the device is lying face-down on a desk or table. In order for the smartphone to see in both directions, it requires two ambient light sensors, one on the front and one on the back. Now it can use ambient light information no matter what position it is in.

The same applies to proximity detection: a second proximity sensor is required on the back of the phone. Sensor modules can combine proximity and ambient light sensing, so only one extra device needs to be added to the back of the phone. If a colour sensor/proximity sensor module is used, this device can actually measure the colour temperature of the ambient light, allowing it to distinguish between artificial light (indicating the phone is indoors) and sunlight (indicating that it is outdoors).

Smart notifications also require information about the ambient noise levels. This calls for sense inputs from the microphone, combined with digital signal processing, to determine the volume of sound in the vicinity of the device. Conversion of the amplitude of the microphone's output to a noise volume measurement may be implemented in a dedicated sensor interface to avoid the need to use the computing power of the device's main applications processor.

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