QT Compass/Magnetometer/Accelerometer

BTSTOnline

Ok... LG V50 running Android 9.

Been working on an app that needs use of the compass.

First issue was that the compass was always returning bad readings at a very high rate (around 100hz). I set up a timer to grab the readings every 300-500ms.. which helped stabilize the readings somewhat... and then added in code to force the returned values to a 360 degree range... no problems... -ish.

Now... Why does the compass readings float so much. I'm getting values ranging +/- 30 degrees randomly when the phone is sitting still on my desk (away from other magnetic devices).

I then moved to try using the magnetometer and accelerometer to get a good bearing... again, that works OK, except for the same issue... Still getting +/- 30 degrees, randomly.

Ideas?

mvuori

Just one idea: Qt seems to have broken sensor related things in 5.12.4, and I wouldn't be surprised if they had broken the compass too. So if you use later Qt, I'd suggest trying version 5.12.3.

(Showing code is also always a good idea, as everybody makes silly mistakes they don't see themselves.)

BTSTOnline

Compiling with 5.12.3 instead of 5.12.4 helps...
Marking this as Solved.

BTSTOnline

Reopened this...
My Compass code:

mapCompass = new QCompass(this);
mapCompass->setDataRate(100);
connect(mapCompass,SIGNAL(readingChanged()), this, SLOT(compassReadingChanged()));

And the slot that processes the output:

void Functionsqt::compassReadingChanged() {
    QCompassReading *myReading = mapCompass->reading();
    double myAngle = myReading->azimuth();
    if( myAngle < 0.0 ) {
       myAngle = myAngle + 360.0;
    }

   azArray.append(myAngle);

    if( azArray.count() == 100 ) {
        float tAz = 0;
        for( int i = 0; i< 100; i++ ) {
            tAz = tAz+azArray[i];
        }
        tAz = tAz / 100.0;
        myCurrentAz = QString::number( tAz, 'f', 1).toFloat();

        qDebug() << "new AZ: " << tAz;

        azArray.clear();
    }
    myCurrentCal = QString::number( myReading->calibrationLevel(), 'f', 1).toFloat();
}

And the output from the qDebug():

new AZ:  221.512
new AZ:  220.078
new AZ:  221.048
new AZ:  227.811
new AZ:  223.733
new AZ:  196.193
new AZ:  192.071
new AZ:  213.953
new AZ:  208.428
new AZ:  208.155
new AZ:  208.688
new AZ:  208.456
new AZ:  209.089
new AZ:  210.445
new AZ:  210.164
new AZ:  210.134
new AZ:  209.201
new AZ:  208.077
new AZ:  208.99
new AZ:  209.358
new AZ:  210.152
new AZ:  209.229
new AZ:  209.664
new AZ:  221.767
new AZ:  221.634

Trying to 'smooth' the data via averaging... Problem is, with or without the averaging, the 'floating' of the data happens... and this is with the phone laying down, not moving. As you can see, this dataset ranges from 221 to 192...

I've also tried various data rates (1hz, 10hz, 50hz... ) with the same results.

I get the same results anywhere: plugged into my laptop, out in the yard, in the car, etc. Always 'floating' 30+ degrees.

Any ideas why?

--Sam

Pablo J. Rogina

@BTSTOnline said in QT Compass/Magnetometer/Accelerometer:

Any ideas why?

Could it be the sensor sensitivity or precision and not Qt framework?
Is it possible you run another application to validate you don't have such "floating" range, in order to rule out hardware or software?

BTSTOnline

Yes, I have tried other compass apps on my 'Droid and none exhibit this behavior.