How to Calibrate Android Accelerometer & Reduce Noise, Eliminate Gravity
So, I've been struggling with this problem for some time, and haven't had any luck tapping the wisdom of the internets and related SO posts on the subject.
I am writing an Android app that uses the ubiquitous Accelerometer, but I seem to be getting an incredible amount of "noise" even while at rest, and can't seem to figure out how to deal with it as my readings need to be relatively accurate. I thought that maybe my phone (HTC Incredible) was dysfunctional, but the sensor seems to work well with other games and apps I've played.
I've tried to use various "filters" but I can't seem to wrap my mind around them. I understand that gravity must be dealt within some way, and maybe that's where I am going wrong. Currently I have tried this, adapted from a SO answer, which refers to an example from the iPhone SDK:
accel[0] = event.values[0] * kFilteringFactor + accel[0] * (1.0f - kFilteringFactor);
accel[1] = event.values[1] * kFilteringFactor + accel[1] * (1.0f - kFilteringFactor);
double x = event.values[0] - accel[0];
double y = event.values[1] - accel[1];
The poster says to "play with" the kFilteringFactor value (kFilteringFactor = 0.1f in the example) until satisfied. Unfortunately I still seem to get a lot of noise, and all this seems to do is make the readings come in as tiny decimals, which doesn't help me all that much, and it appears to just make the sensor less sensitive. The math centers of my brain are also atrophied from years of neglect, so I don't completely understand how this filter is working.
Can someone explain to me in some detail how to go about getting a useful reading from the accelerometer? A succinct tutorial would be an incredible help, as I haven't found a really good one (at least aimed at my level of knowledge). I get frustrated because I feel like all of this should be more apparent to me. Any help or direction would be greatly appreciated, and of course I can provide more samples from my code if needed.
I hope I'm not asking to be spoon-fed too much; I wouldn't be asking unless I've been trying to figure it our for a while. It also looks like there is some interest from other SO members.
Answer
Partial answer:
Accuracy. If you're looking for high accuracy, the inexpensive accelerometers you find in handsets won't cut the mustard. For comparison, a three-axis sensor suitable for industrial or scientific use runs north of $1,500 for just the sensor; adding the hardware to power it and turn its readings into something a computer can use doubles the price. The sensor in a handset runs well below $5 in quantity.
Noise. Cheap sensors are inaccurate, and inaccuracy translates to noise. An inaccurate sensor that isn't moving won't always show zeros, it will show values on either side within some range. About the best you can do is characterize the sensor while motionless to get some idea how noisy it is and use that to round your measurements to a less-precise scale based on expected error. (In other words, If it's within ±x m/s^2 of zero, it's safe to say the sensor's not moving, but you can't be precisely sure because it could be moving very slowly.) You'll have to do this on every device, because they don't all use the same accelerometer and they all behave differently. I guess that's one advantage the iPhone has: the hardware's pretty much homogeneous.
Gravity. There's some discussion in the SensorEvent documentation about factoring gravity out of what the accelerometer says. You'll notice it bears a lot of similarity to the code you posted, except that it's clearer about what it's doing. :-)
HTH.