Accelerometer: get a value as angle (pitch and roll)

danybr · October 22, 2018, 5:20pm

Hi there!

I need to have the information of the the angle of inclination of the RuuviTag+ in degree (0-360).
In order to make that, I’m trying to use a past code made for ADXL335 accelerometer. It has coded to get the pitch by using Z and X values and some transformation based on Vreference and so on… I don’t have a clear idea on how it works. The code that I made until now is that:

setInterval(function() {
  on = !on;
  //console.log(Ruuvitag.getAccelData().x);
  accValues = Ruuvitag.getAccelData();
  var x_g_value = ((((accValues.x * 5)/1024) - 1.65 ) / 0.330 ); 
  var y_g_value = ( ( ((accValues.y * 5)/1024) - 1.65 ) / 0.330 ); 
  var z_g_value = ((((accValues.z * 5)/1024) - 1.80 ) / 0.330 ); 

  var pitch = ( ( (Math.atan2(z_g_value,x_g_value) * 180) / 3.14 ) + 180 );
  //console.log(pitch);
  //LED1.write(on);
}, 500);

Obviously however the values are not good for Ruuvitag+. Someone could help me?
Thanks in advance.

otso · October 23, 2018, 6:44am

Hi,

Check public static RuuviMeasurement calculateAllValues

github.com

Scrin/RuuviCollector/blob/master/src/main/java/fi/tkgwf/ruuvi/utils/MeasurementValueCalculator.java

package fi.tkgwf.ruuvi.utils;

import fi.tkgwf.ruuvi.bean.RuuviMeasurement;

public class MeasurementValueCalculator {

    /**
     * Calculates and populates values that can be calculated based on other
     * values, such as total acceleration and absolute humidity
     *
     * @param measurement the measurement to populate
     * @return the supplied measurement, for convenient use in streams
     */
    public static RuuviMeasurement calculateAllValues(RuuviMeasurement measurement) {
        measurement.accelerationTotal = totalAcceleration(measurement.accelerationX, measurement.accelerationY, measurement.accelerationZ);
        measurement.absoluteHumidity = absoluteHumidity(measurement.temperature, measurement.humidity);
        measurement.dewPoint = dewPoint(measurement.temperature, measurement.humidity);
        measurement.equilibriumVaporPressure = equilibriumVaporPressure(measurement.temperature);
        measurement.airDensity = airDensity(measurement.temperature, measurement.humidity, measurement.pressure);
        measurement.accelerationAngleFromX = angleBetweenVectorComponentAndAxis(measurement.accelerationX, measurement.accelerationTotal);

This file has been truncated. show original

danybr · October 23, 2018, 11:10am

Hi Otso,

Thanks for answer.
I tried, but it doesn’t work. Here my code:

var Ruuvitag = require("Ruuvitag");
Ruuvitag.setAccelOn(true);

function totalAcceleration(accelerationX,accelerationY,accelerationZ) {
    if (accelerationX === null || accelerationY === null || accelerationZ === null) {
        return null;
    }
    return Math.sqrt(accelerationX * accelerationX + accelerationY * accelerationY + accelerationZ * accelerationZ);
}

function angleBetweenVectorComponentAndAxis(vectorComponent, vectorLength) {
    if (vectorComponent === null || vectorLength === null || vectorLength === 0) {
        return null;
    }
    var angle = Math.acos(vectorComponent / vectorLength);
    //return Math.toDegrees(angle);
    return toDegree(angle);
}

function toDegree(radians){
  var pi = Math.PI;
  return radians * (180/pi);
}
setInterval(function() {
  accValues = Ruuvitag.getAccelData();
  accelerationX = accValues.x;
  accelerationY = accValues.y;
  accelerationZ = accValues.z;
  accelerationTotal = totalAcceleration(accelerationX,accelerationY,accelerationZ);
  accelerationAngleFromX = angleBetweenVectorComponentAndAxis(accelerationX,accelerationTotal);
  accelerationAngleFromY = angleBetweenVectorComponentAndAxis(accelerationY,accelerationTotal);
  accelerationAngleFromZ = angleBetweenVectorComponentAndAxis(accelerationZ,accelerationTotal);
  
  console.log("X: " + accelerationAngleFromX + "\tY: " + accelerationAngleFromY + "\tZ: " + accelerationAngleFromZ);
}, 500);

Theoretically, it should show an angle between 0 - 360 and instead also shows negative values and strange behavior.
Thanks.

otso · October 23, 2018, 11:53am

Hello,

The accelerationAngleFromX should correspond to your x_g_value, assuming that the RuuviTag is stationary and only acceleration is the gravity. You can then use var pitch = ( ( (Math.atan2(z_g_value,x_g_value) * 180) / 3.14 ) + 180 ); (or similar, I’ve not checked what the result would be).

danybr · October 23, 2018, 3:08pm

Hello otso,

The angle actually shown is a value between 0 and 360, but it is not the true value. It has a strange behavior.
Is that strange behaviour maybe due to an not full compatibility with Espruino on this specific case?
What means the value that we get from accValues? Like accValues.x?
For me it is really important to take the value of the angle instant by instant with a certain reliability, as it will be the basis of my project.
Thanks for help!

otso · October 24, 2018, 6:29am

Hello,

Could you be more specific?
Where your RuuviTag is installed, what kind of values you’re expecting and what you’re getting?

danybr · October 24, 2018, 12:05pm

The code is:

var Ruuvitag = require("Ruuvitag");
Ruuvitag.setAccelOn(true);

function totalAcceleration(accelerationX,accelerationY,accelerationZ) {
    if (accelerationX === null || accelerationY === null || accelerationZ === null) {
        return null;
    }
    return Math.sqrt(accelerationX * accelerationX + accelerationY * accelerationY + accelerationZ * accelerationZ);
}

function angleBetweenVectorComponentAndAxis(vectorComponent, vectorLength) {
    if (vectorComponent === null || vectorLength === null || vectorLength === 0) {
        return null;
    }
    var angle = Math.acos(vectorComponent / vectorLength);
    //return Math.toDegrees(angle);
    return toDegree(angle);
}

function toDegree(radians){
  var pi = Math.PI;
  return radians * (180/pi);
}
setInterval(function() {
  accValues = Ruuvitag.getAccelData();
  accelerationX = accValues.x;
  accelerationY = accValues.y;
  accelerationZ = accValues.z;
  accelerationTotal = totalAcceleration(accelerationX,accelerationY,accelerationZ);
  accelerationAngleFromX = angleBetweenVectorComponentAndAxis(accelerationX,accelerationTotal);
  accelerationAngleFromY = angleBetweenVectorComponentAndAxis(accelerationY,accelerationTotal);
  accelerationAngleFromZ = angleBetweenVectorComponentAndAxis(accelerationZ,accelerationTotal);
  pitch = (((Math.atan2(accelerationAngleFromZ,accelerationAngleFromX) * 180) / 3.14 ) + 180 );
  console.log("Pitch: " + pitch);
}, 500);

Here are the images of the axes with respect to the sensor:
ruuvitag

At this point I made my measurements and I got (angle in degree):
Standing on the table, FaceUp to the ceiling: 180 - 182 / 358-359
FaceUp to me: 92 - 93 / 263 - 264
FaceUp down: 2 - 5 / 173 -174
FaceUp outwards: 271 / 88

Moreover, having the FaceUp towards the ceiling and tilting it towards me, the angle should be 135° in reality the one measured is 225. In fact, the values that should be between 180 and 90 but those measured are 180 and 270.

This should be because of how the sensor feels the gravity. What I need is a unique angle value following the counterclockwise fencing (see this picture).

Angle

I hope the problem is now clearer. Thank you

otso · October 25, 2018, 7:49am

Just to clarify:

Do you get 180 degree “jumps” on the measurement while sensor is still?

This sounds like you’d have 90 degree offset in the results. The Z-axis positive direction is to “FaceUp”.

danybr · October 26, 2018, 3:21pm

No, the value is fixed. From 182 pass to 358 only if I tilt it a few degrees manually and then fix it in that position.

otso · October 26, 2018, 4:52pm

Hello,

Maybe this STMicro article about 3-axis sensing, especially equations 6. and 7. would help?

Or maybe you could drop the angleBetweenVectorComponentAndAxis as it seems that the original code meant “Acceleration of axis x in g”, which is actually the same raw value which RuuviTag gives

danybr · October 28, 2018, 8:28pm

Hello,

My problem is not solved.

accValues = Ruuvitag.getAccelData();
console.log("X: " + accValues.x + "\tY: " + accValues.y + "\tZ: " + accValues.z);

Let’s start again from the beginning: what do these data mean? What is the unit of measurement?
I don’t understand what these values are. Could you please explain?

My problem is to get, in some way, instant by instant the angle of inclination measured by the RuuviTag, measured in degrees between 0 and 360, so I can perform some operations such as counting the laps of the device.
Thanks for help.

otso · October 29, 2018, 8:42am

Hello,

The raw values are in milli-g, thousandths of a g.

I’m not too familiar with the mathematics on how to convert the acceleration to roll and yaw, essentially all I can do there is to read through application notes and check if they have an algorithm that would work

danybr · October 29, 2018, 7:23pm

Hello,

I noticed that the RuuviTag takes values from the accelerometer every 900 - 950 ms. In fact, the value does not change within that interval of time. Why this limit? Is it possible to change it to increase the refresh rate through Espruino?
I hope at least it can be modified via firmware (I bought the devkit that has to arrive).

Thanks.

otso · October 30, 2018, 7:53am

I’m not sure how Espruino configures the sample rate, but I’m sure it’s configurable as the Espruino sets it to 1 Hz. LIS2DH12 supports at least 1, 10, 25, 50, 100, 200 and 400 if I recall correctly.