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Author Topic: Measuring tilt with an MMA7260Q accelerometer  (Read 2926 times)
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Hi,im trying to measure tiltangles with a MMA7260Q accelerometer. I'm using the following code:

Code:
float x, y, z;  // X and Y in-plane sensing
int i = 0;
float Ax, Ay, Az;
float angleX, angleY, angleZ;
float sens = 800; // mV/g
float Voff = 1650; // mV
const float pi = 3.14;


void setup()
{
  Serial.begin(9600);      // sets the serial port to 9600
}

void loop()
{
  if (i == 10)
  {
    x = x/10;
    y = y/10;
    z = z/10;
    Ax = x-Voff/sens;
    Ay = y-Voff/sens;
    Az = z-Voff/sens;
    angleX = atan(Ax/sqrt(pow(Ay,2)+pow(Az,2)));
    angleY = atan(Ay/sqrt(pow(Ax,2)+pow(Az,2)));
    angleZ = atan(Az/sqrt(pow(Ay,2)+pow(Ax,2)));
    angleX = angleX*180/pi;
    angleY = angleY*180/pi;
    angleZ = angleZ*180/pi;
    Serial.print("Angles: ");
    Serial.print(angleX);    // print the rotational rate in the X axis
    Serial.print(" ");       // prints a space between the numbers
    Serial.println(angleY);  // print the rotational rate in the Y axis
    Serial.print(" ");       // prints a space between the numbers
    Serial.println(angleZ);  // print the rotational rate in the Y axis
    delay(100);              // wait 100ms for next reading
    i = 0;
    x = 0;
    y = 0;
    z = 0;
  }
  else
  {
    x = x + analogRead(0);
    y = y + analogRead(1);
    z = z + analogRead(2);
    i = i + 1;
    delay(50);
  }
    
}

I use time-averaging over 10 samples.. Offset and Sensitivity came from the datasheet..

When I align the axis of the accelerometer with the ground, I receive angles of x: 27°, y:28° and z: 47° when i accept them to be 0°...

Now when I start tilting the accelerometer, the angles change (dont think correctly). The problem is that the highest angle is 50° and the lowest 20°. Behind those values the angles start moving in the other direction again.. Must be something wrong in the code. Can someone please help?!

Thx
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Code:
Ax = x-Voff/sens;

You are using the read value (x above) as it would be expressed in mV as the other numbers. But this is not the case. analogRead() will return a value from 0 to 1024 proportional to the voltage on the associated pin.

The following holds true:
read value from analogRead() : 1024 = read value in mV : 3.3 V (provided that you connected the AREF to 3.3V)

so if you want x expressed in mV..
x = (analogRead() / 1024) * 3300mV

Hope this helps,

Fabio Varesano
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analogRead() will return a value from 0 to 1023 proportional to the voltage on the associated pin
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ops.. sure.. replace 1024 with 1023 in my post above.
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Besides the problems pointed out already, could you link to the spec sheet of the part? Some accelerometers have on board 5V-3.3V converter and only output 3.3/2=1.65V when there is no acceleration. I want to make sure you're not overlooking this possibility.
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Thx for the replies!

I'll try this as soon as possible and post the result here.

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@liudr he is actually offsetting 1650 mV from the read value so it seems that he implemented what you suggested.
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hi,

i'm also working on a tilt sensor but with a MMA7361.
the code must be quite the same, as my code works more or less the same also with an ADXL335. just changing constant like sensibility.

for the moment, the result of my code is really far from perfection. but maybe together we will find a good compromise.

actually i'm working on the "tilt sensing", but it's just a first step of what i'm trying to do...

http://arduino.cc/forum/index.php/topic,52514.0.html

the posted code, here, is really bad, i will post my last one tonight, please get a look on it....

regards

Benjamin


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Code:
Ax = x-Voff/sens;

You are using the read value (x above) as it would be expressed in mV as the other numbers. But this is not the case. analogRead() will return a value from 0 to 1024 proportional to the voltage on the associated pin.

The following holds true:
read value from analogRead() : 1024 = read value in mV : 3.3 V (provided that you connected the AREF to 3.3V)

so if you want x expressed in mV..
x = (analogRead() / 1024) * 3300mV

Hope this helps,

Fabio Varesano

Hi, I tried this but with no good results.. The outputs in mV from x = (analogRead() / 1024) * 3300mV are (when flattened):

x ~ 1020
y ~ 1050
z ~ 1650

When using the same code for calculating the angles:

x ~ 27.5°
y ~ 28.3°
z ~ 48.5°

What would be the right output when the accelerometer is flattened? Im using range of 1.5g. So does this mean that I can't measure big angles (no problem for my project, just asking)?

Thx!
« Last Edit: March 09, 2011, 05:59:51 am by deviukk » Logged

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hi,

i'm also working on a tilt sensor but with a MMA7361.
the code must be quite the same, as my code works more or less the same also with an ADXL335. just changing constant like sensibility.

for the moment, the result of my code is really far from perfection. but maybe together we will find a good compromise.

actually i'm working on the "tilt sensing", but it's just a first step of what i'm trying to do...

http://arduino.cc/forum/index.php/topic,52514.0.html

the posted code, here, is really bad, i will post my last one tonight, please get a look on it....

regards

Benjamin





Hi, if you have a working code for this kind of accelerometer, this would really help me smiley
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Ideally, when the accelerometer is flat you should read x=0g, y=0g, z=1g.

Do you have your AREF connected to a 3.3Volts source? You have also to set this in the code http://arduino.cc/en/Reference/AnalogReference
analogReference(EXTERNAL)
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Code:
Ax = x-Voff/sens;

This calculation will not produce the value you want as the order of operations is off.  Division will occur before subtraction, but you want the subtraction to occur first.  IE, offset the x value, then divide it by the sensitivity (which is not the same as dividing the offset by the sensitivity, and subtracting that value from the x value, which basically means you're just subtracting 2 from all your values).

Code:
Ax = (x-Voff)/sens;


Paranthesis have the highest order of operations, so operation inside paranthesis will always be performed before operations outside them.
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Do you have your AREF connected to a 3.3Volts source? You have also to set this in the code http://arduino.cc/en/Reference/AnalogReference
analogReference(EXTERNAL)

Whats the use of the doing that? So its not possible to use the sensor with only your laptop and an Arduino board?

Code:
Ax = x-Voff/sens;

This calculation will not produce the value you want as the order of operations is off.  Division will occur before subtraction, but you want the subtraction to occur first.  IE, offset the x value, then divide it by the sensitivity (which is not the same as dividing the offset by the sensitivity, and subtracting that value from the x value, which basically means you're just subtracting 2 from all your values).

Code:
Ax = (x-Voff)/sens;


Paranthesis have the highest order of operations, so operation inside paranthesis will always be performed before operations outside them.

Off course! Stupid mistake. Thanks!
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Quote
Whats the use of the doing that? So its not possible to use the sensor with only your laptop and an Arduino board?

Any voltage applied to the AREF pin after calling analogReference(EXTERNAL) will be taken as the reference voltage for the analog-digital converter.. this means that if you have a 3.3V voltage on the AREF pin, when you use the analog input you will get 1023 when the voltage read on it is 3.3V.

Without doing so, the default voltage (on any 16Mhz arduino) would be 5 Volts. So if you don't do the above and you expect your analog voltages to be 3.3V maximum you are loosing almost 1/3 of precision.
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Ok,

But normally this should also work without doing that thing with Aref. Just with less accuracy. But I'm just receiving crap.. I can't see what wrong in the code.. Maybe I need to do some sort of calibration?

Thx
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