Using the Accelerometer on Windows Phone 7
Hola quiero compartirles este excelente artículo de cómo usar el acelerómetro en Windows Phone 7, aqui el detalle:
Using the Accelerometer on Windows Phone 7
When Windows Phone 7 is released, users will be able to enjoy the fact that the hardware requirements for the phone include an accelerometer. Some applications are naturally inclined to work well with accelerometer input. Imagine a “Labyrinth” style game where you can now just rotate or tilt the device instead of simulating the tilting of the table using some buttons or sliders? Just rotating the device from portrait to landscape can re-flow the screen in the right orientation without the user selecting their preferred orientation.
In this blog post we will talk about the different ways you can use the accelerometer API in Windows Phone 7. We also include a class called AccelerometerHelper that uses the Microsoft supplied managed API and provides a variety of data for you to experiment with and find the right solution for your application. There are two major usages of the accelerometer:
1. Orientation relative to our planet Earth (assuming the device is being held steady)
2. Detecting movement of the device relative to the initial point (assuming you know the orientation).
The AccelerometerHelper class adds two high level functions for filtering of the data and calibration of the accelerometer.
Before we jump into the helper class, let’s take a look at the Windows Phone 7 managed accelerometer API. It’s pretty simple. There is a class called Accelerometer that is in the Microsoft.Devices.Sensors namespace. You create a new instance of the Accelerometer and then set up an event handler for the ReadingChanged event. Call the Start() method and you will start getting these events:
private void SetupAccelerometer()
{
_sensor = new Accelerometer();
_sensor.ReadingChanged += OnSensorReadingChanged;
_sensor.Start();
}
private void OnSensorReadingChanged(
object sender,
AccelerometerReadingEventArgs e
)
{
}
The AccelerometerReadingEventArgs contains the X, Y, and Z accelerometer readings from the sensor on the phone and a timestamp. The values are expressed in G-forces (1G = 9.81 m/s2), so a value of 1.0 means that the corresponding axis is being pulled with the same force as the gravity in Paris (the oh-so obvious center of the gravitational universe, hi Greg!). For example, if Z is -1.0, then the device is lying flat, face up, on a perfectly flat surface. If Z is 1.0, then the device is lying flat, faced down. Here is a diagram showing all the corresponding values:
There is something very important to note, however. Getting a value of 1.0 is not going to happen all the time. Earth’s gravity doesn’t roll that way. You may be at the Mystery Spot, you may be on top of a very tall mountain, or your hand may be trembling enough that there are some extra forces on the device. The accelerometer has some error tolerance, so you will want to experiment around with the cumulative margin of errors for the max/min values if your application needs to know these sorts of things. The AccelerometerHelper does not provide this functionality.
Now keep in mind that you will get those events often. Fifty times per second, to be exact. That means you are going to be getting a lot of data. The data you get will be noisy because of the nature of the accelerometer sensor on the device. Even with the device sitting on a table, minding its own business, the numbers coming in will have some variance. Additionally, there will be some calibration problems due to the nature of manufacturing tolerances (and users possibly dropping their phones a few times). Some devices will also come with edges that are not exactly flat. This means setting the device on a table may not result in a “level” reading. For applications using orientation, they need to really know that when the X and Y values are 0, the device is “leveled” (if “leveled” means the device is lying flat. If “leveled” means on edge then Z is 0, and either X or Y is 0). A level application is a perfect example of this.
With this short preamble, let us now look at the AccelerometerHelper class. It provides methods to calibrate the accelerometer and smooth the noisy 50 Hz data stream that the underlying Accelerometer class is generating. The AccelerometerHelper class provides a ReadingChanged event, which uses AccelerometerHelperReadingEventArgs. I think the class speaks for itself. Here is the class’s signature:
Saludos.
Fernando García Loera (MVP Lead – Latin America Region)