Sunday, June 6, 2010


Well, anybody can draw up frightening equations like I did in the acceleration signal analysis report in my previous post and claim that they work just because of some diagrams made by a mathematical analysis program. You'd better not believe this sort of bullshit before you experience it yourself. To help you along that path, I implemented a simple application that let you try out, how these methods work in real life.

Click here to download the example program.

If you open this simple application, you can enable the sampling with a checkbox. Once the sampling is switched on, the service behind the app constantly samples the acceleration sensor and tries to detect motion patterns. It is able to distinguish 3 patterns now: shake, walk and run. Not only it distinguishes these patterns, it also tries to extract shake, step and run count. Unfortunately I tested it only on myself so the rule base probably needs some fine tuning but it works for me. The walk detector uses the w3 wavelet (read the report if you are curious, what w3 wavelet can be) and is therefore a bit slow, it takes about 2-3 seconds before it detects walking and starts counting the steps but that delay is consistent - if you stop walking, it continues counting the steps it has not counted yet.

The moral for this application is that the tough part in detecting motion patterns is the rule base behind the signal processing algorithms. It is OK that wavelets separate frequency bands and signal power calculation allows to produce "gate" signals that enable/disable counting for certain motion patterns. But where are the signal power limits for e.g. shaking and how to synchronize "gate" signals with the signals to be counted? This example program is a good exercise as you can observe, how it uses delayers and peak counters synchronized with signal power calculators to achieve the desired functionality.

Some notes on the implementation. The machinery behind the background service that sends events to an activity is described in this and this blog posts. Also, observe the debug flags in SamplingService, if you set one of these, the service spits out signal data that you can analyse with Sage later. I did not include the keep-awake hack into this implementation because it was not necessary on my Nexus One with the leaked Android 2.2. I put a public domain license on the download page because somebody was interested in the license of the example programs.

I have to think a bit, where I want to go from here. The most attractive target for me is to formalize all these results into a context framework for Android applications but I may fiddle a bit more on signal processing.


Anonymous said...

This does not work in my HTC hero with Android 1.5. Do you think Android 1.5 is the main reason for that?

Gabor Paller said...

Anonymous, uses the new Service interface, available from 2.0. The changes to backport it to pre-2.0 would be minimal, see onStartCommand documentation.

Anonymous said...

this doesnt work as expected.
if you move you phone only in 1 direction, it also counts as walking steps.

any chance you'll fix this problem or help me on fixing it?