Sensors

Ever since I heard that Android devices come with a wide array of sensors, I have been excited about the possibilities. I am a firm believer of the ubiquitous computing vision and all the consequences it brings, including sensors that can be accessed wirelessly. Some parts of the vision (e.g. self-powering microsensors embedded into wallpaint) are still futuristic but mobile phones can be equipped with such sensors easily. Google has a strategy about sensor-equipped mobile devices where the sensor values are processed by powerful data centers. As I did not have an Android device before, I could not play with those sensors. Not anymore! (again, many thanks to those gentle souls who managed to get this device to me).

Sensors are integral part of the Android user experience. Acceleration sensor makes sure that the screen layout changes to landscape when you turn the device (open an application that supports landscape layout, e.g. the calendar, keep the device in portrait mode, move the device swiftly sideways to the right and if you do it quick enough, you can force the display to change to landscape mode). Proximity sensor blanks the screen and switches on the keylock when the user makes a phonecall and puts the device to his or her ear so that the touchscreen is not activated accidentally. In some devices, temperature sensor monitors the temperature of the battery. The beauty of the Android programming model is that one can use all these sensors in one's application.

Click here to download the example program.

A very similar application called SensorDump can be found on Android Market. Our example program is inferior to SensorDump in many respect but has a crucial feature: it can log sensor values into a CSV file that can be analysed later on (use the menu to switch the capture feature on and off). Update: from the 0.2.0 version of SensorDump, sensor data logging into CSV file is available. This is not much important with e.g. the proximity sensor which provides binary data but I don't believe one can understand at a glance, what goes on with the e.g. accelerator sensor during a complex movement just by looking at the constantly changing numbers on the device screen.

I can see the following sensors on my Nexus One.

BMA150 - 3-axis accelerometer
AK8973 - 3-axis Magnetic field sensor
CM3602 - Light sensor

Some sensors are projected as more than one logical sensor, for example the AK8973 is also presented as an orientation sensor and the CM3602 as the proximity sensor. This is just software, however, these duplicate logical sensors use the same sensor chip but present the sensor data in different format.

Let's start with the most popular sensor, the accelerometer. This measures the device's acceleration along the 3 axis. A logical but somewhat unintuitive property of this sensor is that the zero point is in free fall - otherwise the Earth's gravity acceleration is always present. If the device is not subject to any other acceleration (the device is stationary or moves with constant speed), the sensor measures the gravity acceleration that points toward the center of the Earth. This is commonly used to measure the roll and the pitch of the device, try the excellent Labyrinth Lite game on Android Market if you want a demonstration.

The graph below shows sensor data in two scenarios (note that all the data series can be found in the download bundle under the /measurements directory). The red dots show the value of the accelerometer when the device was turned from horizontal position to its side, right edge pointing to the Earth. The green dots show the sensor values when the device was tilted toward its front edge so that at the end the upper edge pointed toward the Earth.



This is all beautiful but don't forget that the acceleration sensor eventually measures acceleration. If the device is subject to any acceleration other than the gravity acceleration (remember the experiment with the portrait-landscape mode at the beginning of the post), that acceleration is added to the gravity acceleration and distorts the sensor's data (provided that you want to measure the roll-pitch of the device). The following graph shows the accelerometer values when the device was laying on the table but I flicked it. The device accelerated on the surface of the table and the smaller blue dot shows the value the accelerometer measured when this happened. As if the device was tilted to the right.


The second sensor is the magnetic field sensor, the compass. As far as I know, this sensor is not used for anything by the base Android applications, it is all the more popular for all sorts of compass applications. The magnetic sensor measures the vector of the magnetic field of the Earth, represented in the device's coordinate system. In 3D, this points toward the magnetic north pole, into the crust of the Earth. The following graph shows the scenario when the device was laying on the table but was rotated in a full circle on the surface of the table.



Even though the magnetic sensor is not subject to some unwanted acceleration like the accelerometer, it is subject to the influence of metal objects. The following graph shows the values of the magnetic sensor when the device was laying on the table but after a while I put a small pair of scissors on top of the device. You can see that there are two clusters of sensor values: one with the scissors, one without.


The third sensor is the light sensor that doubles as proximity detector. The light sensor is more evident but the proximity detector deserves some explanation. The proximity detector is really a light sensor with binary output. If blocked, it emits 0.0, otherwise it emits 1.0. The photo belows demonstrates the location of the sensor and how to block it.