The skewed accelerometer

What can you do if your WP7 accelerometer is not correctly calibrated? As I found out, there seems to be no OS mechanism to calibrate the accelerometer. So if you have a production device you will have to return it for repair or replacement. But what do you do if you have a loaner preproduction device like I do? Well, you have to fix the accelerometer the only way you can: through software.

A correctly calibrated accelerometer should return the vector (0, 0, -1) when you place your device with the screen up on a flat surface (like your office floor). If you have a skewed accelerometer like I do, then you should be able to compensate the difference to the reference vector, once you know what your device is reporting when placed on the floor with the screen up.

To do just that, I wrote a very simple Silverlight application for WP7 that polls the accelerometer every second and displays the vector components, both the sampled value and an average value in order to smooth out the natural variations in the sampling.

You can get the sample application with source code here. After running this app on the floor, I noticed that the Y component of the accelerometer vector was slightly skewed to -0.15, while all other components reported the expected values. After adding 0.15 to the Y component, I got back a calibrated accelerometer.

Now I can get back to my XNA gravity ball code and make it work like it should.

Back to mobile!

I'm back to writing mobile code. After seeing renewed my MVP status, I was able to get my hands on a loaner developer Windows Phone 7 device. I have to personally thank Cristina Gonzales Herrero from Microsoft Spain for both  (MVP and the phone).

Having a WP7 phone on my hands was just the excuse I needed to get back to mobile development. The first hurdle I had to overcome was the App Hub registration. My first attempt was to use my own company account to enroll but the registration process requires two people from the company. So what happens to companies of one like mine? Well, I turned back and took the individual path only to be greeted by payment form (yes, you have to pay USD $99 to deploy to the phone), that assumed I was a US citizen. After talking to some friends, I understood that the only way to go about this was to create a live.com.pt (I live in Portugal) account, but, before I could do that, I had to change my language preferences on IE. You see, I just cannot use a development machine with a Portuguese OS. The keyboard is OK, but not the OS "translations". I started developing on computers that could hardly "speak Portuguese" so I got used to the English language for development, and this also includes the language preferences on the browser. Incidentally, if you don't have Portuguese as your first browser language selection, you cannot create a live.com.pt account. Sigh! This should me much easier, Microsoft!

After going through all these hurdles, I managed to get my developer account and wrote my first mini-app: an XNA application that explores the device's accelerometer. It merely uses the accelerometer information to move a ball around the screen as you tilt the device (something that you have already seen). The accelerometer input in not treated in any special way so the movement you get is sluggish, just as if the ball is moving through molasses...

You can get the first sample from here.

The next version of the code will include more realistic ball movement, so there is some kinetics a dynamics stuff to review.

To help me in writing this tiny piece of code, I read the following articles:

Using the Accelerometer on Windows Phone 7

Orientation and rotation on Windows Phone