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Qt applications should now access accelerometer data using the cross platform Qt Mobility Sensor API

02 May
2010

Archived: This article demonstrates how to develop a Qt Application that will to get the information from the accelerometer sensor at N900 device using native methods.

An accelerometer is a device that measures proper acceleration, the acceleration experienced relative to free-fall.Single- and multi-axis models are available to detect magnitude and direction of the acceleration as a vector quantity, and can be used to sense orientation, vibration and shock. Micro-machined accelerometers are increasingly present in portable electronic devices like mobile phones and video game controllers, to detect the orientation of the device or provide for game input.

Accelerometer Data at N900

The accelerometer data is sysfs file information. Sysfs is a virtual file system provided by Linux 2.6. Sysfs exports information about devices and drivers from the kernel device model to userspace, and is also used for configuration.

The three coordinates of accelerometer sensor is in a text file provided on one line and separated by white space. This file is at path below:

/sys/class/i2c-adapter/i2c-3/3-001d/coord

The values are in mG (milli G). 1000 = 1 G. The g-force experienced by an object is its acceleration relative to free-fall like described above.

At N900 device, the values are connected to the position showed below.

The Code

• At starts, i created a QThread. This thread will read the file with data at all the time.

class aThread : public QThread
{
  Q_OBJECT
 
public:
    aThread(QObject *parent = 0);
    void run();
 
    QString x;
    QString y;
    QString z;
 
public slots:
    void updateCoords();
    void processCoords(QString &data);
 
signals:
    void deviceOrientation(QString x, QString y, QString z);
};

• At implementation of our thread, the updateCoords() method is called with 20ms interval.

void aThread::run()
{
    QTimer timer;
    connect(&timer, SIGNAL(timeout()), this, SLOT(updateCoords()));
    timer.start(20); // 50 Hz update rate
    exec();
}

• This method open the file, read and process the string found on the file.

void aThread::updateCoords()
{
     QFile file("/sys/class/i2c-adapter/i2c-3/3-001d/coord");
     if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
         return;
     QTextStream in(&file);
     QString data = in.readAll();
     processCoords(data);
}

• Then, in the processing of information, the string found is split by the white spaces creating three different strings.These strings are the X,Y,Z coordinates of accelerometer.The signal deviceOrientation() is emitted to allowing that other class can connect yours slots to this signal.

void aThread::processCoords(QString &data)
{
    QStringList data_splited = data.split(" ");
 
    x = data_splited[0];
    y = data_splited[1];
    z = data_splited[2];
 
    emit deviceOrientation(x, y, z);
}

• At constructor of our superior class, i created the thread described above . Then i connected the signal deviceOrientation() of our thread with the slot showData of superior class.

Accelerometer::Accelerometer(QWidget *parent)
    : QWidget(parent)
{
    ...
    myThread = new aThread(this);
    connect(myThread, SIGNAL(deviceOrientation(QString, QString, QString)),
            this, SLOT(showData(QString, QString, QString)));
    myThread->start(QThread::NormalPriority);
    ...

• Then the showData method shows the three axis information of accelerometer sensor at labels at the screen.

void Accelerometer::showData(QString x, QString y, QString z)
{
    xlabel->setText(x);
    ylabel->setText(y);
    zlabel->setText(z);
}

Screenshots

Source Code

File:QtAccelerometer.zip

Note: The N900 allow that you use shortcuts with the combination of keys.