In my Droidcon 2011 presentation I tried to highlight the battery cost of the continuous sensor sampling which is necessary for detecting motion patterns. While the general case still requires some sort of improvement over the current Android sensor architecture e.g. the use of the "wake on motion" feature of the acceleration sensors, it is possible to decrease the battery consumption if the motion to be detected is longer than 5-10 seconds. This is still not suitable for recognising very short events like fall, tap, shake, etc. but can be suitable to "wake up" a step counter when the motion starts. Some steps would be missed but this may be acceptable if the battery life is increased.
Click here to download the example program.
The essence of the workaround is to use the AlarmManager and to generate periodic wakeup events. When the wakeup event is received, sensor sampling is started and some samples are collected. Then we have to figure out using some heuristics whether there is some movement. This implementation calculates the average value of the acceleration sample absolute values and looks for values deviating from the average. If there is movement detected, the sampling continues. If there is no movement detected, the program gives up the wake lock and goes back sleeping.-
I have made some measurements using my Nexus 1. I switched the phone into airplane mode and ran the program during the night, for 7-8 hours. The battery percentage consumed during an hour can be seen below as a function of wakeup period (check out WAKEUP_PERIOD constant in AlarmSleep.java).
5 sec - 1.14%/hour
10 sec - 1.10%/hour
20 sec - 0.56%/hour
30 sec - 0.27%/hour
The battery consumption with the last timeout value - 30 sec - is very close to the standby consumption, 0.25%. If you can tolerate that long "dead period", then you can bring the battery consumption in "no motion" state very close to the phone's normal standby consumption. For a general "tap" or "shake" detector, however, this is not an adequate solution. I have received encouraging mails that a proper solution relying on the sensor's low-power mode may be deployed soon.