Calculate sample interval

[bknill]

Forgive my igorance.

I have a sample rate of 4.93 millis from my GoPro.

How do I covert that to Hz?

Thanks

[psiphi75]

Frequency in Hertz (Hz) = 1 / Sample Rate (seconds)
                   Freq = 1 / (Sample Rate (milliseconds) / 1000)
                   Freq = 1 / (4.93 / 1000)
                   Freq = 202.8 Hz

[bknill]

Thanks. Weirdly it works well when sampleRate = 4.93 and does crazy stuff when it's at 202.8

I'm using this with extracted data from Go Pro telemetry. i.e. looping through the samples and updating AHRS every sample.

  const sampleInterval = 202.8
    const converttoG = ms2 => ms2/9.80665
    const times = Object.keys(samples)
    const madgwick = new AHRS({sampleInterval, beta: 0.5, doInitialisation: false})

      const data = Object.keys(samples).reduce((prev, key, index) =>  {
            const {gyro, accel, magn} = samples[key]
        
            if(magn){
                init.magn = magn
                return prev
            }
            //return {...prev, [key]: [gyro[0], gyro[1],gyro[2]]}
            init.gyro = gyro || init.gyro
            init.accel = accel || init.accel
            //Data order Z,X,Y
            madgwick.update(
                init.gyro[1],init.gyro[2], init.gyro[0], 
                converttoG(init.accel[1]), converttoG(init.accel[2]), converttoG(init.accel[0])
                )
            const {heading, pitch, roll} = madgwick.getEulerAngles()
            // heading is from north, going west (about z-axis).
            // pitch is from vertical, going forward (about y-axis).
            // roll is from vertical, going right (about x-axis).
            return {...prev, [key]: [heading, pitch, roll]}
        }, {} )  

I'm then using this data in Three.js to rotate a sphere with the footage projected onto for a VR stabalisation effect.

When the sample rate is 4.93 this works well but is a bit jerky.

Am I going about this the right way?

Another question, the magnetmeter data comes back with 4 values, any idea how to convert this into a compass direction!?

[bknill]

ps. Hi from Ben in Wellington!

[psiphi75]

Hi Ben from Wellington.

I don't know much about Go Pro telemetry. Yes, the sampleInterval in this case should be 4.93, although 5.0 will suffice. You may want to use the following:

madgwick.update(
                init.gyro[1],init.gyro[2], init.gyro[0], 
                converttoG(init.accel[1]), converttoG(init.accel[2]), converttoG(init.accel[0]),
                undefined, undefined, undefined, 
                deltaTimeSec
                )

Where deltaTimeSec is the time since the last update. This can help when data is not arriving exactly at sampleInterval, or you skip data points.

Regarding the magnetometer, what does the documentation say it returns? Does it give you the units?

[bknill]

Thanks!

I'm using https://github.com/JuanIrache/gopro-telemetry which returns the units in µT

A sample looks like this [-550, -71, -41, 6991]

[psiphi75]

Have a look to find more information about what each value in the array is.

[bknill]

Looks like a problem on the MAX gopro/gpmf-parser#82

[bknill]

The values are three axis x,y,z and rhall in microteslas

[psiphi75]

You can just plug those magnetometer x, y and z values straight into madgwick.update.